Method for changing a number of channels in an optical communication network

BACKGROUND OF THE INVENTION

1. Field of the Invention

0001The This invention relates to a fiber optic communication system, the wavelength division multiplexing used to provide a wavelength division multiplexed optical signal transferred to. More particularly, the present invention relates to a control device, an optical attenuator or an optical amplifier controls to the power level of wavelength multiplexed optical signal, to change when the number of channels is varied.

2. Description of Related of the technique

0002Wavelength division multiplexing is in fiber optic communication systems used a relatively large amount of data at a high transfer speed.

0003<figref idrefs="S54">1</figref> is a diagram showing a conventional represents fiber optic communication system comprising a wavelength division multiplexing used to, for example, four channels through a single optical fiber transferred to. Referring now to <figref idrefs="S54">1</figref>Transmitted transmission units <figref>20-1</figref>. <figref>20-2</figref>. <figref>20-3</figref> and <figref>20-4</figref> single Carrier, respectively wavelengths X1-X4 have. Each carrier is modulated with information and represents a single channel . The different carrier together by an optical multiplexer <figref>22</figref> in a Wavelength division multiplexing Optical signal multiplexed. The wavelength-multiplexed optical signal is through an optical fiber <figref>24</figref> to an optical demultiplexer <figref>26</figref> transfer. The optical demultiplexer <figref>26</figref> branches the wavelength-multiplexed optical signal separated into four optical signals, each having the wavelengths λ1-λ4. The four separate branched optical signals are then respectively by receiving units <figref>28-1</figref>. <figref>28-2</figref>. <figref>28-3</figref> and <figref>28-4</figref> detected.

0004While the above optical fiber communication system multiplexes four carriers together, is it a common practice to multiplex more than four carriers. More accurate said can many different carriers are multiplexed together. In this manner, a relatively high amount of data is transmitted through an optical fiber.

0005It is typically an optical amplifier (not shown) or an optical repeater (not shown) between the optical multiplexer <figref>22</figref> and optical demultiplexer <figref>26</figref> used to the wavelength-multiplexed optical signal reinforce which through the optical fiber <figref>24</figref> runs. On Such an optical amplifier is typically a rare earth doped (eng. rare-earth doped) optical fiber amplifier, wherein the wavelength division multiplexing Optical signal directly amplified. This means, that a rare-earth-doped optical fiber amplifier, the Wavelength division multiplexing amplified optical signal, without the wavelength division multiplexing convert optical signal to an electrical signal.

0006The caused using a rare-earth-doped optical fiber amplifier several problems when the number of channels in the wavelength-division multiplex Optical signal is varied. More specifically, during the Variation (ie, before the variation in the number of channels completed is) the optical power to vary undesirably from each channel, whereby a non-linear degradation or S / N attenuation of the wavelength-multiplexed Optical signal is caused.

SUMMARY OF THE INVENTION

0007correspondingly it is an object of the present invention, an optical device provide that non-linear degradation and S / N of a slowdown Wavelength division multiplexed optical signal, reduced when the number of channels is varied.

0008additional Objects and advantages of the invention will in part in the following Description set forth and part will be obvious from the description, be, or may be learned by practice of the invention.

0009The foregoing objects of the present invention are achieved by providing achieved an apparatus, an optical amplifier and a control device contains. The optical amplifier amplifies a Light signal having a variable number of channels. The control device controls a power level of the amplified light signal in response to variations in the number of channels in the light signal.

0010More accurate the objectives of the present invention are achieved by providing a control device which (a) before, and then, varying the number of channels in the light signal, the amplified Light signal with a varying light transmissivity passes, so that a power level of the amplified light signal at an approximately constant Level in accordance with the number of channels is maintained in the light signal, and (b) while the Number of channels in is the light signal varies, the amplified light signal with a constant light transmittance passes.

0011destinations the present invention are also by providing a reached device an optical amplifier, a control device, includes a demultiplexer and an automatic level control unit. Of the optical amplifiers reinforced a light signal having a variable number of channels. The control device controls the amplified light signal in response to variations in the number of channels in the light signal. Of the Demultiplexer demultiplexes the controlled amplified light signal into individual Signals. The automatic level control unit controls the power level of a respective individual signal so that the power level of the individual signal about is maintained constant.

0012destinations the present invention are also by providing a achieved device, an automatic level control unit and an optical fiber amplifier contains. The automatic level control unit maintains a power level of a Light signal about constant and produces a corresponding output signal. The optical fiber amplifier reinforced the output of the automatic level control unit with a constant gain.

0013destinations of the present invention will be further by providing a optical amplifier and reaches a control device. The optical amplifier amplifies a Light signal having a variable number of channels. Before, and then, the Varying the number of channels maintains in the light signal the control device a power level of the amplified light signal on an approximately constant level in accordance with the number of channels in the light signal. While the Number of channels is varied in the light signal, the controller amplifies the amplified light signal with about constant gain.

0014In addition, Objects of the present invention by providing an apparatus reached, and an optical amplifier, an optical attenuator a control device contains. Of the optical amplifiers reinforced a light signal having a variable number of channels. Of the optical attenuator outputs the amplified Light signal on and has a variable light transmittance. In front varying the number of channels in the light signal, the controller varies the light transmissivity of the optical attenuator, so that a power level of the amplified light signal at an approximately constant Level is maintained, which depends of the number of channels in the light signal prior to varying the number of channels. While the Number of channels is varied in the light signal, the controller maintains the Light transmittance the optical attenuator constant. subsequently varies to the varying the number of channels in the light signal the control device controls the light transmittance of the optical attenuator so that a power level of the amplified Light signal at an approximately constant level is maintained, which depends on the number of channels in the Light signal then to the varying the number of channels.

0015destinations the present invention are also by providing a Method for controlling a light signal having a variable number of channels and strengthened by an optical amplifier reached. The method includes the steps of: (a) before, and then, varying the Number of channels in the light signal, passing the amplified light signal with a varying light transmissivity, so that a power level of the amplified light signal at an approximately constant Level in accordance with the number of channels is maintained in the light signal, and (b) while the number of channels in the Light signal is varied, passing the amplified light signal with a constant light transmissivity.

0016destinations the present invention are also by providing a Method for controlling a light signal having a variable number of channels and strengthened by an optical amplifier achieved, the method which includes the steps of: prior to (a), and subsequently, varying the number of channels in the light signal maintaining a power level at an approximately constant Level in accordance with the number of channels in the light signal, and (b) during the number of channels is varied in the light signal, amplifying the amplified light signal with about constant gain.

0017citation <patcit><text>US 5510926</text></patcit> shows a device according to the preamble of Claim. 1

BRIEF DESCRIPTION OF THE DRAWINGS

0018These and other objects and advantages of the present invention are obvious and easily recognized from the following description of the preferred embodiments, which is taken in conjunction with the accompanying drawings, in which are:

0019<figref idrefs="S54">1</figref> (Was standing art) is a graph showing a conventional fiber optic communication system represents.

0020<figref idrefs="S55">2</figref> (Was standing art) is a graph showing an optical amplification means for a Fiber optic communication system represents uses wavelength division multiplexing.

0021<figref idrefs="S56">3</figref> on Graph showing an optical amplifying apparatus according to a executionform of the present invention is illustrated.

0022<figref idrefs="S57">4 (A)</figref> and <figref idrefs="S57">4 (B)</figref> Curves which the operation of the in <figref idrefs="S56">3</figref> shown optical amplifying apparatus represent, the number of channels N in an optical signal will be changed.

0023<figref idrefs="S58">5</figref> on Graph showing an automatic gain control circuit according to a embodiment of the present invention.

0024<figref idrefs="S59">6</figref> on Graph showing an automatic level control circuit in accordance with a embodiment of the present invention.

0025<figref idrefs="S60">7</figref> on Graph showing a switching circuit of the in <figref idrefs="S59">6</figref> shown illustrating automatic level control circuit, according to an embodiment of the present invention.

0026<figref idrefs="S61">8th</figref> and <figref idrefs="S62">9</figref> Diagrams which according to an additional automatic level control circuit embodiments the present invention represent.

0027<figref idrefs="S63">10</figref> on Graph showing an optical amplifying apparatus according to a additional embodiment of the present invention.

0028<figref idrefs="S64">11</figref> on Graph showing an optical amplifying apparatus according to a another embodiment of the present invention.

0029<figref idrefs="S65">12</figref> on Graph showing an optical amplifying apparatus according to a embodiment of the present invention.

0030<figref idrefs="S66">13</figref> on Graph showing an optical amplifying apparatus according to a additional embodiment of the present invention.

0031<figref idrefs="S67">14</figref> on Graph showing an optical amplifying apparatus according to a additional embodiment of the present invention.

0032<figref idrefs="S68">15</figref> on Graph showing an optical amplifying apparatus according to a another embodiment of the present invention.

0033<figref idrefs="S69">16</figref> on Graph showing an optical amplifying apparatus according to a still another embodiment of the present invention.

0034<figref idrefs="S70">17</figref> on Graph which shows a modification to the optical amplifying apparatus in the <figref idrefs="S69">16</figref> is shown, according to a embodiment of the present invention.

0035<figref idrefs="S71">18 (A)</figref> a graph showing a gain-to-wavelength characteristic a rare-earth-doped optical fiber (EDF) in an optical amplifying means represents, according to a embodiment of the present invention.

0036<figref idrefs="S71">18 (B)</figref> a graph showing the transmissivity of a optical filter in an optical amplification device represents, according to a embodiment of the present invention.

0037<figref idrefs="S71">18 (C)</figref> a graph showing the overall gain rare-earth doped optical fiber (EDF) in <figref idrefs="S71">18 (A)</figref> and the optical filter in <figref idrefs="S71">18 (B)</figref> represents, according to an embodiment of the present invention.

0038<figref idrefs="S72">19</figref> on Graph showing an optical amplifying apparatus according to a embodiment of the present invention.

0039<figref idrefs="S73">20</figref> on Graph showing an optical amplifying apparatus according to a additional embodiment of the present invention.

0040<figref idrefs="S74">21</figref> on Graph showing an optical amplifying apparatus according to a another embodiment of the present invention.

0041<figref idrefs="S75">22</figref> on Graph showing an optical amplifying apparatus according to a still another embodiment of the present invention.

0042<figref idrefs="S76">23</figref> on Graph showing an optical amplifying apparatus according to a embodiment of the present invention.

0043<figref idrefs="S77">24</figref> on more detailed diagram of a portion of the in <figref idrefs="S76">23</figref> shown optical amplification means, according to a embodiment of the present invention.

0044<figref idrefs="S78">25</figref> on Diagram illustrating a fiber optic communication system which an optical amplifying apparatus according to a embodiment the current invention.

0045<figref idrefs="S79">26</figref> on more detailed diagram showing the optical amplifying apparatus of <figref idrefs="S78">25</figref> represents, according to a embodiment of the present invention.

0046<figref idrefs="S80">27</figref> on Graph showing a transmission line represents that a plurality of optical amplification means uses, according to one embodiment of the present invention.

0047<figref idrefs="S81">28</figref> on Timing diagram illustrating the operation of an optical amplifying device, according to a embodiment of the present invention.

0048<figref idrefs="S82">29</figref> on Graph showing a portion of an optical communication system represents, according to a embodiment of the present invention.

EMBODIMENTS OF THE INVENTION

0049It will detail to the presently preferred embodiments of the present invention, of which example in the accompanying Drawings are shown, where like numerals throughout like elements refer.

0050<figref idrefs="S55">2</figref> is a graph showing an example of an optical amplification means for a Fiber communication system represents uses wavelength division multiplexing, and similar is to that in the <patcit><text>US Patent Application 08/655 027</text></patcit> is disclosed.

0051Referring now on <figref idrefs="S55">2</figref> includes the optical amplification means a first part <figref>1000</figref> (Which here sometimes as a "rare-earth doped Optical fiber amplification portion "hereinafter) and a second part <figref>2000</figref> (Which here sometimes as an "electrically is called controlled optical device part ").

0052the first part <figref>1000</figref> contains a rare-earth-doped optical fiber (EDF) <figref>34</figref>, optical branching <figref>36₁</figref> and <figref>36₂</figref>, Optical isolators <figref>38₁</figref> and <figref>38₂</figref>. photodiodes <figref>40₁</figref> and <figref>40₂</figref>An optical wavelength division multiplexing coupler <figref>42</figref>, A pump laser diode (LD) <figref>44</figref> and a automatic optical gain control circuit (AGC) <figref>46</figref>,

0053the second part <figref>2000</figref> contains an optical branching <figref>36₃</figref>. an electrically controlled variable optical attenuator (ATT) <figref>48</figref>a Photodiode (PD) <figref>40₃</figref> and an automatic Level control circuit (ALC) <figref>50</figref>, Optical attenuator<figref>48</figref> is for instance comprised of a magneto-optical element. however can many different types of uses of variable optical attenuators will.

0054On Wavelength division multiplexing Optical signal is a rare-earth-doped optical fiber <figref>34</figref> a branching <figref>36₁</figref>, Optical isolator <figref>38</figref> and optical Wavelength division multiplexing coupler <figref>42</figref> supplied. A pump light beam is controlled by a pump laser diode <figref>44</figref> over a optical wavelength division multiplexing coupler <figref>42</figref> the rare-earth-doped optical fiber <figref>38</figref> supplied. the Wavelength division multiplexing Optical signal by the rare-earth-doped optical fiber <figref>34</figref> strengthened and is over the optical isolator <figref>38₂</figref> and optical branching <figref>36₂</figref> optical attenuator <figref>48</figref> entered.

0055On Portion of the wavelength-multiplexed optical signal, which by optical branching <figref>36₁</figref> branched is, is determined by the photodiode <figref>40₁</figref> in an electrical signal converted and the automatic optical Gain control circuit <figref>46</figref> entered. A portion of the amplified Wavelength division multiplexing Optical signal by optical branching <figref>36₂</figref> is branched, is by the photodiode <figref>40₂</figref> converted into an electrical signal and the automatic optical gain control circuit <figref>46</figref> entered. The pump laser diode <figref>44</figref> becomes is controlled to provide a ratio between a level of the input wavelength-multiplexed optical signal and a level the amplified Wavelength division multiplexing Optical signal to maintain a predetermined level.

0056More accurate said control circuit controls the optical amplifying <figref>46</figref> the Pump laser diode <figref>44</figref> such as to the ratio between the level of input wavelength-multiplexed Optical signal once by the photodiode <figref>40₁</figref> in an electrical signal converted and the level of the amplified wavelength-multiplexed optical signal, once by the photodiode <figref>40₂</figref> in an electrical signal is converted at a constant level maintain. In this way, retains the first part<figref>1000</figref> the wavelength dependence at by the optical amplification is controlled at a constant level.

0057On Portion of an output wavelength-multiplexed optical signal, which by optical branching <figref>36₃</figref> branched is, is determined by the photodiode <figref>40₃</figref> in an electrical signal converted and the automatic level control circuit <figref>50</figref> entered. Optical attenuator <figref>48</figref> becomes is controlled by the wavelength division multiplexing Optical signal to maintain a predetermined level.

0058More accurate said controls automatic level control circuit <figref>50</figref> the optical attenuator <figref>48</figref> among Use of the photodiode <figref>40₃</figref> derived electrical signal from said wavelength-multiplexed optical signal, to the output level of the wavelength-multiplexed Optical signal to maintain at a constant level.

0059Unfortunateli, when an optical amplifying apparatus, as in <figref idrefs="S55">2</figref> illustrated in a fiber optic communication system is used, which wavelength-multiplexed used, a variation in the number of channels welche in wavelength division multiplexing used optical signal, causing significant problems.

0060For example , a predetermined output optical power of an amplifier in General for each wavelength (Channel) requires, by a desired S / N ratio in ensure a receiver. Assuming that there are a total of N channels, the total optical Output Pc of a rare-earth-doped optical fiber amplifier for amplifying a Wavelength division multiplexing Optical signal at N × P controlled. In the presence of a variation of + α or -α in the number of channels N is a switching control effected such that the total optical Performance equal (N +/- α) P. Because the optical power for individual wavelengths (Channels) varies due to the switching control, a non-linear degradation or a signal-to-noise (S / N) resulting slowdown.

0061Further is in <figref idrefs="S55">2</figref> the optical output of first part <figref>1000</figref> by the second part <figref>2000</figref> maintain at a constant level. Therefore, when the optical output of first part <figref>1000</figref> a predetermined exceeds level, reserves the second part <figref>2000</figref> the optical output at a constant level at. Consequently, the use of optical attenuator<figref>48</figref> a additional measure to gain by the first part <figref>32</figref> require, and the optical power the pump laser diode <figref>44</figref> for maintaining the optical gain at a constant level should be controlled so that they in an exponential relationship with a variation in the level of the input Wavelength division multiplexing Optical signal is. Therefore, it is necessary to provide a pump laser diode<figref>44</figref> With relatively high capacity provide.

0062<figref idrefs="S56">3</figref> is a graph in accordance with an optical amplifying apparatus of a embodiment of the present invention. The optical amplifying apparatus contains a first part <figref>1000</figref> and a second part <figref>2000</figref>, The first part <figref>1000</figref> contains a rare-earth-doped optical fiber (EDF) <figref>52₁</figref>, optical branching <figref>54₁</figref> and <figref>54₂</figref>, Optical isolators <figref>55₁</figref> and <figref>55₂</figref>. an optical wavelength division multiplexing coupler <figref>56₁</figref>, Photodiodes (PD) <figref>58₁</figref> and <figref>58₂</figref>. a pump laser diode (LD) <figref>59₁</figref> and an automatic gain control circuit (AGC) <figref>60₁</figref>, The first part<figref>1000</figref> amplifies a wavelength-multiplexed optical signal, while the wavelength dependence is maintained.

0063As Example is a wavelength-multiplexed optical signal typically 1.5 microns Tape. An erbium-doped optical fiber is known, optical signals to strengthen in this volume, and therefore as a rare-earth doped optical fiber (EDF) <figref>52₁</figref> used. In addition, it is known a wavelength division multiplexing Optical signal in the 1.5 .mu.m Band traveling through an erbium-doped optical fiber, suitably reinforce a pump light of 0.98 microns or 1.48 microns Pump-band to be used. Therefore, the pump laser diode (LD)<figref>59₁</figref> a pumping light in the 0.98 micron or 1.48 micron pumping band ready.

0064Furthermore shows <figref idrefs="S56">3</figref> a forward pumping configuration, in which a pump light beam which passes through a pump laser diode <figref>59₁</figref> is emitted by a rare-earth-doped optical fiber <figref>52₁</figref> in the same Direction as the wavelength-multiplexed Optical signal passes. However, also can be a backward pumping structure are used, wherein a laser diode provides a pump light beam, which by the rare-earth-doped optical fiber <figref>52₁</figref> in the opposite direction as the wavelength-multiplexed optical signal passes. Furthermore, can be used, a bidirectional pumping construction in provide which two laser diodes, a pump-light passing through the rare-earth-doped optical fiber <figref>52₁</figref> in both directions by the rare-earth-doped optical fiber <figref>52₁</figref> passes. Thus, the present Invention not intended to in any specific Type of directional pumping to be limited.

0065the second part <figref>2000</figref> contains an electrically controlled variable optical attenuator (ATT) <figref>64</figref>An automatic Level control circuit (ALC) <figref>66</figref>, An optical branching <figref>54₃</figref> and a photodiode (PD) <figref>58₃</figref>, The second part<figref>2000</figref> controls the total optical output of the wavelength-multiplexed optical signal so as to be at a constant level, without that a wavelength dependency is maintained. More specifically, automatic level control circuit varies the<figref>66</figref> the attenuation or the light transmissivity of optical attenuator <figref>64</figref> so that the power of the wavelength-division multiplex Optical signal as soon as the first part <figref>1000</figref> issued on a constant power level corresponding to the number of channels in the wavelength-division multiplex maintaining optical signal.

0066Furthermore, if the number of channels in wavelength division multiplexing Optical signal is being varied, a monitor signal processing circuit <figref>70</figref>. that the damping or the light transmissivity of optical attenuator <figref>64</figref> constant be maintained. Thus, by monitoring the signal-processing circuit<figref>70</figref> the operation of optical attenuator <figref>64</figref> temporarily "frozen". After the number of channels was changed, allows the monitor signal processing circuit <figref>70</figref>. that the damping or the light transmissivity of optical attenuator <figref>64</figref> so is varied so that the power of the wavelength-multiplexed optical signal to a constant Pegel according to the new Number of channels is maintained.

0067More accurate said that is the optical amplification device input Wavelength division multiplexing Optical signal through an optical branching <figref>68₁</figref> branched. The branched portion is a photodiode (PD) <figref>58₄</figref> provided. The Photodiode (PD) <figref>58₄</figref> converts the branched portion into an electrical signal and provides the electrical signal a monitor signal processing circuit <figref>70</figref> ready.

0068On Control signal which via a variation in the number of channels in the wavelength-division multiplex Optic transmission system warning is superimposed on the wavelength-multiplexed optical signal, preferably as a signal at a low speed by an amplitude modulation process. However, more Method may be used to superimpose the control signal. The monitor signal processing circuit <figref>70</figref> extracted and identifies the control signal. The monitor signal processing circuit<figref>70</figref> controls then the optical attenuator <figref>64</figref> or automatic level control circuit <figref>66</figref> according to the extracted Control signal. When an amplitude modulation is used, it is relatively simple, the control signal by demodulating the electrical signal to extract, which is obtained by the photodiode <figref>58₄</figref> is obtained.

0069alternative , the control signal to a monitor signal processing circuit <figref>70</figref> on a dedicated control channel (wavelength) to be transmitted. When a assigned control signal is used, should an optical branching filter (Not shown) the control signal from the wavelength division multiplexing Optical signal (once by optical branching <figref>68₁</figref> branched) extract. It is, for example, by supplying of the optical signal through the optical branching filter is extracted, to the photodiode <figref>58₄</figref>. so as to be converted into an electrical signal, it is possible that to extract the control signal.

0070That's why a portion of the wavelength-multiplexed Optical signal by optical branching <figref>68₁</figref> is branched, through the photodiode <figref>58₄</figref> converted into an electrical signal and the monitor signal processing circuit <figref>70</figref> supplied. The monitor signal processing circuit<figref>70</figref> becomes an operation of optical attenuator <figref>64</figref> "Freeze" when a control signal, which over warns a variation in the number of channels is extracted and identified is.

0071Around ensure that the power level of the attenuated wavelength-multiplexed Optical signal matches the number of channels, causing the monitor signal processing circuit <figref>70</figref> a be selected Set voltage (reference voltage). The power level can then such be controlled so that it is at a constant level, which the setting voltage corresponds.

0072in the Generally, there are two approaches for monitor signal processing circuit <figref>70</figref>, around optical attenuator <figref>64</figref> to control. In one approach, optical attenuator<figref>64</figref> directly by monitor signal processing circuit <figref>70</figref> controlled, such as by the control signal <figref>69</figref> in <figref idrefs="S56">3</figref> shown. In an alternative approach, optical attenuator <figref>64</figref> indirectly by monitor signal processing circuit <figref>70</figref> controlled, as indicated by the control line <figref>71</figref> in <figref idrefs="S56">3</figref> shown.

0073The Number of channels can even after a warning a change the number of channels elevated be or reduced. In this case, a control signal which the completion of the change the number of channels indicates the wavelength division multiplexing Optical signal superimposed. A monitor signal processing circuit <figref>70</figref> extracted then the control signal. Alternatively, the control signal to a dedicated control channel (wavelength) to the monitor signal processing circuit <figref>70</figref> transfer will. Upon extracting and identifying the control signal allows the monitor signal processing circuit <figref>70</figref> it optical attenuator <figref>64</figref>. its control for maintaining the power level of wavelength multiplexed resume optical signal at a constant level.

0074alternative can, instead of providing monitor signal processing circuit <figref>70</figref> With a control signal indicating the completion of the change in the number of channels, such completion will be accepted after a predetermined Time period has elapsed. More specifically, the number of channels even after a lapse of a predetermined time period elevated be or reduced because given the warning of a change in the number of channels is. In this case, after the control signal for outputting warning of a Variation in the number of channels by monitor signal processing circuit <figref>70</figref> extracted and is identified by a timekeeper (not shown) is activated will. When a predetermined time period has elapsed, optical attenuator <figref>64</figref> once again driven to the power level of the wavelength-multiplexed optical signal maintain at a constant level.

0075Independently of, whether a control signal or a predetermined period of time used to becomes, at the completion of a variation in the number of channels indicate is the set voltage (reference voltage) for controlling the power level switched from one level to another in accordance with the information, which refers to how many channels are added or removed. These Information is preferably in the control signal for warning a Variation in the number of channels contain. Therefore, by resuming the control for Maintaining the total optical output power at a constant Level, the optical power is maintained at a constant level, which coincides with the number of channels.

0076Therefore<figref>64</figref> in response to a change in the number of channels a radical variation in the optical output power by its attenuation is frozen to a constant level. operates at this time the second part <figref>2000</figref> no longer because the performance of the Wavelength division multiplexing Optical signal to maintain at a constant level. after the Number of channels changed is, optical attenuator <figref>64</figref> once again controlled to the power of the wavelength-multiplexed optical signal at a constant Level to maintain. Optical attenuator<figref>64</figref> can are gradually driven such that a total output power, which the number of channels corresponds, is maintained. By this arrangement, it is possible to moderate variation in the optical output and a non-linear attenuation and S / N ratio to slowdown avoid.

0077<figref idrefs="S57">4 (A)</figref> and <figref idrefs="S57">4 (B)</figref> are Curves which the operation of the optical amplification means in <figref idrefs="S56">3</figref> represent, the number of channels N in an optical signal, for example, of 4 channels channels is changed to. 8 Referring now to <figref idrefs="S57">4 (A)</figref> and <figref idrefs="S57">4 (B)</figref>, Has the optical attenuator <figref>64</figref> a variable Light transmissivity or an attenuation, which by an automatic level control circuit <figref>66</figref> a monitor signal processing circuit <figref>70</figref> is controlled.

0078In <figref idrefs="S57">4 (A)</figref> and <figref idrefs="S57">4 (B)</figref> becomes a warning about a change in the number of channels received at time t1, and the number of channels is increased at the time t2.

0079Before a warning about a change in the number of channels is received (that is, before time t1), automatic varies Level control circuit <figref>66</figref> the light transmissivity of the electrically controlled variable optical attenuator <figref>64</figref>To a substantially constant optical signal power at the output of optical attenuator <figref>64</figref> provide. Therefore leads the second part <figref>2000</figref> before time t1, an automatic Level control (ALC) by.

0080If a warning about a change in the number of channels is received (ie, at time t1), automatic level control circuit maintains the <figref>66</figref> the Light transmissivity of electrically controlled variable optical attenuator <figref>64</figref> to substantially constant at. In this case, the output of optical attenuator<figref>64</figref> so are considered to have a constant gain, which, for example, by the first part <figref>1000</figref> or by a subsequent stage (Not shown) is provided that the signal further strengthened. Therefore, after time t1, automatic gain control (AGC), not automatic level control (ALC) performed.

0081To the Time t3 varies the automatic level control circuit <figref>66</figref>. subsequent to a change in the number of channels, the light transmissivity the electrically controlled variable optical attenuator <figref>64</figref>To a substantially constant optical signal power at the output of optical attenuator <figref>64</figref> provide. More specifically, the leads second part <figref>2000</figref> after the time t3 again an automatic Level control (ALC) by.

0082As based on <figref idrefs="S57">4 (A)</figref> and <figref idrefs="S57">4 (B)</figref> to recognize is the optical attenuator <figref>64</figref> to controlled to provide ALC. However, if the number of channels is changed, ALC is interrupted. If the number of channels is changed, optical attenuator<figref>64</figref> instead controlled to maintain a constant light transmissivity, or an attenuation provide. The operation of optical attenuator<figref>64</figref> can be described as a "frozen", if the number of channels in <figref idrefs="S57">4 (A)</figref> and <figref idrefs="S57">4 (B)</figref> in between Times t1 and t3 is changed.

0083As described above, the output of optical attenuator <figref>64</figref> in between the time points t1 and t3, a constant gain, which, for example, by the first part <figref>1000</figref> or by a subsequent stage (Not shown) is provided that the signal further strengthened. Alternatively, as in additional embodiments of the present invention, which are described in more detail below, disclosed, the second part <figref>2000</figref> modified to , so that it provides a constant gain (instead of the provision of an automatic level control) while the Number of channels changed is. In this case, the second part<figref>2000</figref> a gain controlled amplifier included to a constant gain for AGC between times T1 and T3 provide.

0084Therefore, as in <figref idrefs="S57">4 (A)</figref> and <figref idrefs="S57">4 (B)</figref> Dargerepresents, contains an optical amplifying device has a optical amplifier (Such as the first part <figref>1000</figref>), Which light signal a strengthened that a variable number of channels has. Prior to and subsequent to the varying the number of channels in the light signal, passes through a controller (such as second part <figref>2000</figref>) the amplified light signal with a varying light transmissivity, such that a line level of the amplified Light signal at an approximately constant level according to the number of channels is maintained in light signal. Further, while the number of channels in the light signal is varied, the controller passes the amplified light signal with a constant light transmissivity.

0085<figref idrefs="S58">5</figref> is a diagram showing an automatic gain control circuit <figref>60₁</figref> represents an optical amplification in such a way to control that it is at a constant level. Referring now on <figref idrefs="S58">5</figref>Containing the automatic gain control circuit <figref>60₁</figref> a divider <figref>72</figref>, An operational amplifier <figref>74</figref>. a transistor <figref>76</figref> and resistors R1-R6. V_CC is a power supply voltage, V_ref is a reference voltage, and G is the earth or ground.

0086As in <figref idrefs="S58">5</figref> shown, converts the photo diode (PD) <figref>58₁</figref> a portion of the wavelength-multiplexed Optical signal into an electrical signal which the divider <figref>72</figref> provided is. The photodiode (PD)<figref>58₂</figref> converts a portion of the amplified wavelength-multiplexed Optical signal into an electrical signal which the divider <figref>72</figref> provided. In this way the divider obtains <figref>72</figref> a relationship between the input and the output of the rare-earth-doped optical fiber (EDF) <figref>52₁</figref>, Of the pump laser diode<figref>59₁</figref> emitted pump light beam can then be controlled so that it generates a constant ratio, whereby a constant gain provided. The structure of the automatic gain control circuit<figref>60₁</figref> in <figref idrefs="S58">5</figref> is only one example of many possible constructions for a automatic gain control circuit.

0087<figref idrefs="S59">6</figref> is a diagram illustrating an automatic level control circuit <figref>66</figref> represents, an optical output control at a constant level. Referring now to <figref idrefs="S59">6</figref>Containing the automatic level control circuit <figref>66</figref> the resistors R7-R9, an operational amplifier <figref>78</figref>. a transistor <figref>80</figref>A switch controller (SWC) <figref>82</figref> and a Reference voltage circuit <figref>84</figref>, V_CC is the power supply voltage, V_ref is a reference voltage, G is the earth or ground, and cs1 and cs2 are Control signals, represented by the monitor signal processing circuit <figref>70</figref> provided are. A control<figref>86</figref> is a control of an optical attenuator <figref>64</figref> to Controlling the transmissivity of optical attenuator <figref>64</figref>,

0088If For example, the optical attenuator <figref>64</figref> by a magneto-optical effect is operated, the control can <figref>86</figref> a be coil for applying a magnetic field. Moreover, when the optical attenuator, for example, is operated by an opto-electrical effect, the control can <figref>86</figref> a be electrode, wherein the voltage applied to the electrode is is controlled. If a semiconductor optical amplifier instead of of optical attenuator <figref>64</figref> used is, a bias voltage for controlling the gain of Semiconductor optical amplifier being controlled.

0089On Portion of the optical signal from the optical attenuator <figref>64</figref> (please refer <figref idrefs="S56">3</figref>) is output is determined by the optical branching <figref>54₃</figref> branched and through the photodiode (PD) <figref>58₃</figref> into an electrical signal converted. Then compares the optical amplifier<figref>78</figref> in <figref idrefs="S59">6</figref> the electric Signal with the reference voltage (set voltage) V_ref. represented by the reference voltage circuit <figref>84</figref> according to the control signal CS1 supplied is. A difference which obtained as a result of the comparison is, is used around the transistor <figref>80</figref> to drive. By passing a current, which control the <figref>86</figref> is supplied, is controlled, the by optical attenuator <figref>64</figref> provided damping controlled such that the optical output to a constant Level is maintained.

0090<figref idrefs="S60">7</figref> is a diagram showing a switching circuit <figref>82</figref> represents. Referring now to <figref idrefs="S60">7</figref>Includes the switching circuit <figref>82</figref> capacitors C1 and C2, which are individually selected by a switch SW, which is controlled by the control signal CS2. Therefore controls the switching circuit <figref>82</figref> the frequency characteristic of automatic Level control circuit <figref>66</figref>, In addition, the switch circuit controls<figref>82</figref> the optical attenuator <figref>64</figref> by a controlling transistor <figref>80</figref>By the level of the output Wavelength division multiplexing Optical signal followed by a predetermined frequency characteristic is. The control signal cs2 from monitor signal processing circuit<figref>70</figref> changes the Frequency characteristic by between capacitors C1 and C2 of the switching <figref>82</figref> is switched. The control signal cs1 switches between different levels of the reference voltages according to the number of channels around.

0091More accurate said switching circuit forms the <figref>82</figref>Which with the operational amplifier <figref>78</figref> (please refer <figref idrefs="S59">6</figref>) and resistors R7 (see <figref idrefs="S59">6</figref>) And R9 (see <figref idrefs="S59">6</figref>) is coupled to a primary low pass filter. The cut-off frequency f_c this primary low-pass filter is: <st32:df xmlns:st32="http://lighthouseip.com/">f<st32:sub>c</st32:sub> = 1 / (C × 2πR9<st32:sub>SWC</st32:sub>9)</st32:df>where C_SWC the selected capacitor C₁ or C₂ is. Therefore, by the value of the capacitance C_SWC is increased, in the <figref idrefs="S59">6</figref> Control circuit shown at a lower operated frequency. It means that the answer of which is reduced.

0092Therefore can function of the capacity the selected Capacitor C1 or C2 of switching circuit <figref>82</figref> the filter cut-off frequency changed in the high frequency range will.

0093As an example may be such a preferable arrangement that the Cut-off frequency which is in the normal ALC operation in the range 10-100 kHz, is switched to 0.01 Hz when optical attenuator <figref>64</figref> controlled is to maintain a constant damping provide (so as to provide, for example, a constant gain, when the channels are switched). Ideally takes the control of the switching circuit<figref>82</figref> gradually on, but requires a gradual controller that the switch circuit <figref>82</figref> out a plurality of capacitors, instead of simply two capacitors is constructed.

0094Referring on <figref idrefs="S59">6</figref> is the cut-off frequency high before a warning about a change in the channels Will be received. When a signal is received, in which of a change the number of channels warns the switching circuit <figref>82</figref> is controlled that the cut-off frequency is reduced. Accordingly, the damping, which by optical attenuator <figref>64</figref> provided is set to a medium level. After the change in the channels is completed, the switching circuit <figref>82</figref> is controlled that the cut-off frequency is again switched to be high.

0095If for example, the monitor signal processing circuit <figref>70</figref> on extracted control signal and identifies which a Variation in the number of channels warns the control signal CS2 of the switching circuit is <figref>82</figref> supplied, so that the frequency characteristic of automatic level control circuit <figref>66</figref> on a low frequency range is switched. This is following the following performance for following a variation in the signal, which by the photodiode (PD) <figref>58₃</figref> detected is reduced. This means, that the constant-level control of the optical output temporarily is frozen (for example, the light transmissivity of optical attenuator <figref>64</figref> on maintained constant). Further, the control signal cs1 corresponds to the Number of channels, which are included in the optical signal, and monitor signal processing circuit <figref>70</figref> leads the Control signal cs1 the reference voltage circuit <figref>84</figref> to. The reference voltage circuit <figref>84</figref> then performs a reference voltage V_ref corresponding to the number of channels. Therefore, the total optical output power assumes a level, with which the number of channels corresponds to the variation in the number of channels. For example the reference voltage V_ref is modified in that, when a total of α channels of Total of N original channels added is the total optical output is equal to (N + α) × P.

0096Referring again <figref idrefs="S59">6</figref> and <figref idrefs="S60">7</figref>, can the value of the capacitance C_SWC be large enough to control the operation of optical attenuator <figref>64</figref> freeze. in the Generally can be achieved this purpose, for example, when the cut-off frequency f_c of 10 kHz to 0.01 Hz drops whereby a drop in the cut-off frequency f_c around is a factor of 10,000 to 100,000 is required. It can be difficult be to achieve such a high drop.

0097Usually varies by optical attenuator <figref>64</figref> provided damping from moment to moment to provide an ALC function and to provide a Polarization variation to compensate. Therefore, an abrupt setting the attenuation of the optical attenuator <figref>64</figref> on a certain level (such as when the number of channels is changed) cause problems. Instead, the attenuation is preferably maintained at a average level maintained.

0098More accurate said, are <figref idrefs="S61">8th</figref> and <figref idrefs="S62">9</figref> Diagrams which an automatic level control circuit <figref>66</figref> according to additional embodiments the present invention represent. Referring now to<figref idrefs="S61">8th</figref> is a filter <figref>90</figref> for cutting off high frequencies (f_cLeft: 10 kHz), and which consists of a capacitor and a resistor is constructed between a switch <figref>92</figref> and transistor <figref>80</figref> provided, so that the response of the automatic level control adequately is. For example, the time constant which typically is in the range of a few milliseconds, the time constant in the range of 10-100 amended milliseconds will.

0099If the cut-off frequency f_c on the high frequency range is switched, the filter response is fast, so that a comparatively high-speed variation, such as a polarization variation, can be deleted, and the output of optical attenuator <figref>64</figref> constant is maintained.

0100More accurate said memory circuit stores a signal <figref>94</figref> in <figref idrefs="S61">8th</figref>. which has a low-pass filter has (f_c: ~ 0.01 Hz), a voltage corresponding to an average level of the current in control <figref>86</figref>, While an ALC operation occurs a switching of the control loop, so that the control loop for controlling the drive current to a constant level is initiated. That is, when the switchover of the control loop occurs, the voltage corresponding to the mean Level of current in the latch circuit <figref>94</figref> saved is to serve as a reference voltage. The term "average Level "is therefore used because the bias current has a time-dependent variation, at the Level of the beam which the photodiode (PD) <figref>58₃</figref> entered is to maintain at a constant level. More specifically, is the voltage which is obtained by integrating, under Using a more extended integral time than those obtained by the time constant of the normal control loop is provided, in the latch circuit <figref>94</figref> stored.

0101The Latch circuit <figref>94</figref> may be a circuit to the value of the drive (which by transistor <figref>80</figref> provided is) a A / D converters to read, to register the read value and the registered value over output a D / A converter.

0102<figref idrefs="S62">9</figref> is a combination of <figref idrefs="S59">6</figref> and <figref idrefs="S61">8th</figref>, Referring now on <figref idrefs="S62">9</figref> the capacitance C_SWC by the switching circuit <figref>82</figref> switched to cause the cut-off frequency f_c on a low frequency range is shifted, thereby the filter response to slow down. It controls the latch circuit<figref>94</figref> the attenuation the mean based on a monitored value.

0103More accurate specifically, in <figref idrefs="S62">9</figref> switching of the control loop made such that it after increasing the time constant of the normal control loop according to the in <figref idrefs="S59">6</figref> illustrated control occurs, so as to reduce an effect, which result in the ALC characteristic from the switching of the control loop is not caused.

0104As described above, monitor signal processing circuit <figref>70</figref> on received control signal to a completion of a variation in the number of channels to report, after it receives a control signal which is a warning about a Variation in the number of channels are. Alternatively, the processing circuit Überwachungssignal-<figref>70</figref> however receive no control signal when the variation in the number of channels is completed. In this case, would a timer (not shown) is activated after the control signal for giving warning of a variation in the number of channels is extracted and identified.

0105the Control signal cs2 returns switching circuit <figref>82</figref> on the original Frequency characteristic to, after the control signal for notifying a received completion of a variation in the number of channels is, or after a predetermined time period has elapsed. This will be the constant optical output control according to the new Reference voltage V_ref resumed, which by the reference voltage circuit <figref>84</figref> is set.

0106The Control for maintaining the total optical output to a constant level which corresponds to the number of channels, can to a gradual way be resumed. For example, the Output signal of the photodiode (PD) <figref>58₃</figref> the Operationsve rstärker<figref>78</figref> a time constant circuit <figref>96</figref> entered are, or the reference voltage V_ref can are gradually varied to assume a level which is the number of channels equivalent.

0107While the Above-described arrangement ensures that the frequency characteristic, is a result of the control switch, which by the switching circuit <figref>82</figref> is effected, so that the constant Level control of the optical output is frozen, it is also possible, the by the photodiode (PD) <figref>58₃</figref> output hold signal when the control signal for giving warning of a Variation in the number of channels is extracted and identified. In this case, the held Value to the operational amplifier <figref>78</figref> entered, so that the constant level control of the optical output frozen is. Other arrangements for freezing the constant level control of the optical output are also possible. While it is believed that the electrically controlled optical device part using an optical attenuator <figref>64</figref> built up is a semiconductor optical amplifier can be used instead of optical attenuator <figref>64</figref> used will. The semiconductor optical amplifier should have a small wavelength dependence have. By the semiconductor optical amplifier is controlled, the total optical output can be controlled at a constant level.

0108<figref idrefs="S63">10</figref> is a diagram illustrating an optical amplifying apparatus and an optical transmission system according to a embodiment of present invention. Referring now to<figref idrefs="S63">10</figref>. contains optical amplification means a first part <figref>1000</figref>, A second part <figref>2000</figref> and a third part <figref>3000</figref>, The third part<figref>3000</figref> contains a rare-earth-doped optical fiber (IDF) <figref>52₂</figref>. an optical branching <figref>54₄</figref>. an optical wavelength multiplexing coupler <figref>56₂</figref>, Optical isolators <figref>55₃</figref> and <figref>55₄</figref>. a photodiode (PD) <figref>58₅</figref>, A pump laser diode (LD) <figref>59₂</figref> and an automatic Gain control circuit (AGC) <figref>60₂</figref>, The third part<figref>3000</figref> uses also the optical branching <figref>54₃</figref> and the photodiode (PD) <figref>58₃</figref> together with the second part <figref>2000</figref>,

0109As the first part <figref>1000</figref>, Controls the third part <figref>3000</figref> a optical amplification so as to be at a constant level. Specifically, controls the second part <figref>2000</figref> the power level of wavelength multiplexed Optical signal by the third part of <figref>3000</figref> receive is to be at a constant power level. Consequently, it is the optical output power level of the third part <figref>3000</figref> also maintained at a constant power level. Even when the optical signal level by the optical attenuator <figref>64</figref> of the second part <figref>2000</figref> is attenuated, provides through the third part <figref>3000</figref> provided reinforcement ensure that a desired total optical output is obtained.

0110Therefore can the pump laser diode <figref>59₁</figref> of the first Part <figref>1000</figref> and the pump laser diode <figref>59₂</figref> the third Part <figref>3000</figref> each having a relatively small capacity, whereby the costs are reduced and the amplification device is stabilized.

0111Although <figref idrefs="S63">10</figref> the second part <figref>2000</figref> and third part <figref>3000</figref> such shows to optical branching <figref>54₃</figref> and the photodiode (PD) <figref>58₃</figref> together use, it is also possible, a separate optical branching and a separate Photodiode in each of the second part <figref>2000</figref> and third part <figref>3000</figref> provide.

0112The automatic gain control circuits <figref>60₁</figref> and <figref>60₂</figref> can have the same structure. About that addition, the optical reinforcements which by the first part <figref>1000</figref> and third part <figref>3000</figref> provided will be identical. Alternatively, the gains can according to the characteristics of a transmission optical fiber which the third part <figref>3000</figref> is used, can be varied.

0113in the Case of varying the number of channels the optical attenuation, which by optical attenuator <figref>64</figref> provided is, directly through the monitor signal processing circuit <figref>70</figref> or by monitor signal processing circuit <figref>70</figref>. which the automatic level control circuit <figref>66</figref> controls, frozen. Similar the in <figref idrefs="S56">3</figref> embodiment shown, it is ensured that a variation in the optical output in response to a variation in the number of channels is restricted so that a non-linear degradation and an S / N ratio slowdown reduced will.

0114<figref idrefs="S64">11</figref> is a graph in accordance with an optical amplifying apparatus of a another embodiment of the present invention. Referring now to<figref idrefs="S64">11</figref>. contains optical amplification means the first part <figref>1000</figref>, The second part <figref>2000</figref> and the third part <figref>3000</figref>Which are the same as those which in <figref idrefs="S63">10</figref> are shown. However, including the optical amplification means in <figref idrefs="S64">11</figref> also an automatic level control (ALC) correction circuit <figref>98</figref> to the Controlling and correcting automatic level control circuit <figref>66</figref> of second part <figref>2000</figref>,

0115More accurate specifically, a portion of the wavelength-multiplexed optical signal which is carried optical attenuator <figref>64</figref> output is determined by the optical branching <figref>54₃</figref> branched, by the photodiode (PD) <figref>58₃</figref> in an electrical signal converted and the automatic level control circuit <figref>66</figref> entered. The automatic level control circuit <figref>66</figref> controls the optical attenuator <figref>64</figref> such that the total optical output power of the wavelength-multiplexed optical signal is maintained at a constant level. however is the optical output power of the output wavelength-division multiplex Optical signal in the third part <figref>3000</figref> not automatic Level control circuit <figref>66</figref> supplied. Therefore, no assurance are that the total optical output in the third part <figref>3000</figref> within a predetermined range is maintained.

0116Accordingly, a Portion of the output wavelength-multiplexed optical signal in third part <figref>3000</figref> by the photodiode (PD) <figref>58₅</figref> in an electric Signal is converted and the ALC correction circuit <figref>98</figref> as Also the automatic gain control circuit <figref>60₂</figref> entered.

0117The ALC correction circuit <figref>98</figref> determines whether the entire optical maintain output within the predetermined range is or not. If the total optical output power is not is within the predetermined range, ALC correction circuit controls the <figref>98</figref> the automatic level control circuit <figref>66</figref>Which, in turn, the optical attenuator <figref>64</figref> controls, the total optical output power within the predetermined Area maintain. If a semiconductor optical amplifier instead of of optical attenuator <figref>64</figref> used is, controls automatic level control circuit <figref>66</figref> the gain of the Semiconductor optical amplifier such that the total optical output in third part <figref>3000</figref> within the predetermined level is kept.

0118<figref idrefs="S65">12</figref> is a graph in accordance with an optical amplifying apparatus of a embodiment of the present invention. The optical amplifying apparatus in <figref idrefs="S65">12</figref> is a combination of the optical Verstärkungseinrichtions in <figref idrefs="S63">10</figref> and <figref idrefs="S64">11</figref>,

0119Referring now on <figref idrefs="S65">12</figref>, In the case of a variation in the number of channels monitor signal processing circuit <figref>70</figref> temporary the freeze control, which by means of the second part <figref>2000</figref> to the causes controlling the optical output at a constant level is such that a variation in the optical output is reduced. Further, ALC correction circuit controls <figref>98</figref> the automatic level control circuit <figref>66</figref> such that the entire optical output power in the third part <figref>3000</figref> within a predetermined range is maintained.

0120<figref idrefs="S66">13</figref> is a graph in accordance with an optical amplifying apparatus of a additional embodiment of the present invention. The optical amplifying apparatus in <figref idrefs="S66">13</figref> operates in a similar manner as previously described embodiments the present invention includes, but also an optical branching <figref>54₅</figref>, A photodiode (PD) <figref>58₆</figref>, A dispersion compensation fiber (DCF) <figref>100</figref> and a dispersion compensation fiber (DCF) loss correction circuit <figref>102</figref>, The optical branching coupler <figref>54₅</figref> and the photodiode (PD) <figref>58₆</figref> can such are believed to be in the third part <figref>3000</figref> contain are.

0121The Dispersion compensation fiber <figref>100</figref> is between the second part <figref>2000</figref> and the third part <figref>3000</figref> connected. The DCF loss correction circuit <figref>102</figref> controls the automatic Level control circuit <figref>66</figref>, In a wavelength division multiplexing Optic transmission system with great Distance and high capacity is a dispersion compensation in relation to the dispersion level the transmission optical fiber and the wavelength-multiplexed Optical signal needed. For this reason, a dispersion compensation fiber<figref>100</figref> provided.

0122however an insertion loss due to a dispersion compensation optical fiber cause problems. More specifically, causes a variation in the loss due to the Dispersion compensation optical fiber is a variation in the optical Output of repeaters which include wavelength-multiplexed optical fiber amplifier.

0123Therefore is the optical attenuator <figref>64</figref> by measuring a loss due to the dispersion compensation fiber <figref>100</figref> and adjusting the automatic level control circuit <figref>66</figref> to the Compensate for the loss, so controlled to a constant provide visual output. The loss due to the dispersion compensation optical fiber<figref>100</figref> becomes probably depending vary from a level of dispersion compensation. Accordingly, despite the constant optical output control which by the automatic level control circuit <figref>66</figref> is effected, the Level of the wavelength-division multiplex Optical signal, which the third part <figref>3000</figref> entered will vary.

0124Therefore a portion of the wavelength-multiplexed Optical signal through the dispersion compensation optical fiber <figref>100</figref> is output, and by optical branching <figref>54₅</figref> branched is, by the photodiode (PD) <figref>58₆</figref> in a electrical signal converted. The electrical signal is the DCF loss correction circuit <figref>102</figref> as well as the automatic Gain control circuit <figref>60₂</figref> entered. DCF loss correction circuit<figref>102</figref> certainly, if the level of the wavelength-division multiplex Optical signal through the dispersion compensation fiber <figref>100</figref> is output, is or not within a predetermined range. If the level outside the predetermined range, performs the DCF loss correction circuit <figref>102</figref> the automatic level control circuit <figref>66</figref> a correction signal to. For example, the reference voltage (set voltage) for constant control of the optical output corrected such that the optical output power within the predetermined range is. Therefore, a variation in insertion loss, which is due a structure originates, wherein the dispersion compensation fiber <figref>100</figref> the Dispersion in the transmission optical fiber offset, corrected, and it becomes a predetermined output level of the amplified wavelength-multiplexed Optical signal obtained.

0125<figref idrefs="S67">14</figref> is a graph in accordance with an optical amplifying apparatus of a additional embodiment of the present invention. Referring now to<figref idrefs="S67">14</figref>. Next, the operation of optical attenuator <figref>64</figref>. when monitor signal processing circuit <figref>70</figref> on Control signal for giving warning of a variation in the Number of channels extracted and identified, frozen (ie, the transmissivity or the damping is maintained at a constant), so that a rapid variation limited optical signal level is. DCF loss correction circuit<figref>102</figref> controls automatic level control circuit <figref>66</figref> such a to correct loss which, depending on the level of dispersion compensation which by the dispersion compensation fiber <figref>100</figref> provided is varied. Thus, the level of the wavelength-multiplexed optical signal, that the third part <figref>3000</figref> is entered, within a predetermined range maintained.

0126<figref idrefs="S68">15</figref> is a graph in accordance with an optical amplifying apparatus of a another embodiment of the present invention. Referring now to<figref idrefs="S68">15</figref>. compensates for the dispersion compensation fiber <figref>100</figref> a Disdispersion in the transmission optical fiber, DCF loss correction circuit <figref>102</figref> corrects a Variation in the loss of function the level of the compensation represented by the dispersion compensation fiber <figref>100</figref> provided is, and the ALC correction circuit <figref>98</figref> controls the automatic Level control circuit <figref>66</figref> such that the level of the output Wavelength division multiplexing Optical signal in the third part <figref>3000</figref> within a predetermined Range is maintained. Thus, the wavelength division multiplexing is Optical signal in the wavelength-multiplexed optical transmission system on a increasingly stable manner, and relaying.

0127<figref idrefs="S69">16</figref> is a graph showing an optical amplifying apparatus according to a still another embodiment of the present invention. Referring now to<figref idrefs="S69">16</figref> controls monitor signal processing circuit <figref>70</figref> the optical attenuator <figref>64</figref> or automatic level control circuit <figref>66</figref> Upon extracting and identifying a control signal for giving warning of a Variation in the number of channels, thereby freezing the constant level control of the optical output. In this way, a rapid variation in the level of the optical output limited.

0128Further controls the DCF loss correction circuit <figref>102</figref> automatic Level control circuit <figref>66</figref>So as a variation in the loss correct, which depends on the level of dispersion by the dispersion compensation optical fiber <figref>100</figref> provided is. ALC correction circuit<figref>98</figref> controls the automatic Level control circuit <figref>66</figref> so as to output the wavelength division multiplexing Optical signal in the third part <figref>3000</figref> within a predetermined Area maintain.

0129<figref idrefs="S70">17</figref> is a diagram in a modification to the <figref idrefs="S69">16</figref> illustrated optical amplification means according to a embodiment of present invention. More specifically, in<figref idrefs="S70">17</figref> on optical filter A1 between the output of the optical isolator <figref>55₂</figref> and optical branching <figref>54₂</figref> at the input of photodiode (PD) <figref>58₂</figref> provided. Also, a optical filter A2 between the output of the optical isolator <figref>55₄</figref> and optical branching <figref>54₄</figref> at the input of photodiode (PD) <figref>58₅</figref> provided. The optical filters A1 and A2 are optical filters, such as in the <patcit><text>US Patent Application No. 08 / 655.027</text></patcit> disclosed a wavelength dependency the gain to correct.

0130<figref idrefs="S71">18 (A)</figref> is a graph showing a characteristic by a reinforcement the wavelength a rare-earth-doped optical fiber (EDF) <figref>52₂</figref> in <figref idrefs="S70">17</figref> represents, <figref idrefs="S71">18 (B)</figref> is a graph showing the transmissivity against wavelength an optical filter A2 in <figref idrefs="S70">17</figref> represents, and <figref idrefs="S71">18 (C)</figref> is a graph, which a total gain a rare-earth-doped optical fiber (EDF) <figref>52₂</figref> and an optical filter A2 in <figref idrefs="S70">17</figref> according to a embodiment of the present invention.

0131If For example, the rare-earth-doped optical fiber (EDF) <figref>52₂</figref> a dependent in the wavelength gain characteristic, as in <figref idrefs="S71">18 (A)</figref> shown, wherein the reinforcement in the field of long wavelength is higher, a gain correction optical filter A2 at Input of the photodiode (PD) <figref>58₅</figref> provided, which ensures that the amplifier with respect to the wavelength of a uniform amplification has. The provision of the optical filter A2 ensures that the photodiode (PD) <figref>58₅</figref> the corrected Multi-wavelength signal receives, so that the adverse sensitivity characteristic, the Signal sensitivity low in the range of a short wavelength and in the field of long wavelength is high, is corrected. The optical filters A1 and / or A2 may, depending on the use the rare-earth-doped optical fibers (EDF) <figref>52₁</figref> and <figref>52₂</figref> be provided or not.

0132<figref idrefs="S72">19</figref> is a graph in accordance with an optical amplifying apparatus of a embodiment of the present invention. Referring now to<figref idrefs="S72">19</figref> becomes the positioning of the first part <figref>1000</figref> and second Part <figref>2000</figref> considerably switched. Therefore, a wavelength-multiplexed Optical signal by the second part <figref>2000</figref> is controlled that it has a constant power level, and then by the first part <figref>1000</figref> is controlled so that there is a constant reinforcement has.

0133More accurate specifically, an input wavelength-multiplexed optical signal to the optical attenuator <figref>64</figref> transfer.

0134the Wavelength division multiplexing Optical signal from the optical attenuator <figref>64</figref> output is, is applied to the rare-earth-doped optical fiber <figref>52₁</figref> about the optical isolator <figref>55₁</figref> and the optical wavelength-multiplexed coupler <figref>56₁</figref> transfer. The amplified wavelength-multiplexed optical signal is over the optical isolator <figref>55₂</figref> and optical branching <figref>54₂</figref> output.

0135On Portion of the wavelength-multiplexed optical signal, which by optical branching <figref>54₁</figref> branched is, is determined by the photodiode <figref>58₁</figref> in an electrical signal converted and the automatic level control circuit <figref>66</figref> and automatic Gain control circuit <figref>60₁</figref> supplied. The automatic level control circuit<figref>66</figref> controls the by optical attenuator <figref>64</figref> provided optical damping such that the wavelength-multiplexed Optical signal has its level controlled so that it is within a predetermined range, and it is then applied to the first part <figref>1000</figref> transfer.

0136On Portion of the wavelength-multiplexed optical signal, which by optical branching <figref>54₂</figref> branched is, is determined by the photodiode <figref>58₂</figref> in an electrical signal converted and is then sent to the automatic Gain control circuit <figref>60₁</figref> transfer. The automatic gain control circuit <figref>60₁</figref> controls the pump laser diode <figref>59₁</figref> such that a relationship between a level of the wavelength-multiplexed Optical signal optical in the rare-earth-doped fiber <figref>52₁</figref> is inputted and outputted therefrom is, is maintained at a constant level.

0137Therefore causes the second part <figref>2000</figref>That the power level of the Wavelength division multiplexing optical signal is constant, and indeed even then when a signal, which over entered a transmission optical fiber is largely varied. Consequently, a wavelength division multiplexing Optical signal having a constant level, the first part <figref>1000</figref> entered. Accordingly, the automatic gain control circuit <figref>60₁</figref> a small control area and a have relatively simple structure. Further, since it is prevented that the power level of the optical signal, which rare-earth-doped into the optical fiber <figref>52₁</figref> is inputted, exceeds a predetermined level, it is not necessary, the level of the pumping laser beam, which by the pump laser diode <figref>59₁</figref> is supplied, raise. This means, that the pump laser diode <figref>59₁</figref> have a low capacity can.

0138<figref idrefs="S73">20</figref> is a graph in accordance with an optical amplifying apparatus of a additional embodiment of the present invention. In the<figref idrefs="S73">20</figref> illustrated optical amplification means is similar the optical amplification means in <figref idrefs="S72">19</figref>contains but also an optical branching <figref>54₃</figref>, A photodiode (PD) <figref>58₃</figref> and a monitor signal processing circuit <figref>70</figref>,

0139Referring now on <figref idrefs="S73">20</figref>Is a wavelength-multiplexed optical signal, which is supplied via a transmission optical fiber, the variable optical attenuator <figref>64</figref> entered and has a portion which passes through the optical branching <figref>54₃</figref> is branched, through the photodiode <figref>58₃</figref> converted into an electrical signal and monitoring the signal-processing circuit <figref>70</figref> entered.

0140On Control signal for giving warning of a variation in the Number of channels can the wavelength division multiplexing optical signal to be superposed by an amplitude modulation or a transmit dedicated control channel will. Upon extracting and identifying the control signal after a warning issued a variation in the number of channels controls the monitor signal processing circuit <figref>70</figref> the automatic level control circuit <figref>66</figref> and retains the optical Damping, which by optical attenuator <figref>64</figref> provided is, on the current level at (whereby the operation of optical attenuator <figref>64</figref> frozen is) so that the optical output power is no longer on a constant level is maintained.

0141If the change in the number of channels is completed, allowing the monitor signal processing circuit <figref>70</figref> the optical attenuator <figref>64</figref>. its control on to maintain the optical output power a constant level resume. By this arrangement, Is it possible, a rapid variation in the power level of the optical signal to reduce or eliminate.

0142<figref idrefs="S74">21</figref> is a graph in accordance with an optical amplifying apparatus of a another embodiment of the present invention. In the<figref idrefs="S74">21</figref> illustrated optical amplification means is similar the optical amplification means in <figref idrefs="S72">19</figref>contains However, an ALC correction circuit <figref>98</figref>,

0143The ALC correction circuit <figref>98</figref> determines whether the power level the output wavelength-division multiplex or not the optical signal within a predetermined range. If the power level is not within the predetermined range, controls the ALC correction circuit <figref>98</figref> the automatic level control circuit <figref>66</figref> such that the through optical attenuator <figref>64</figref> provided optical Attenuator causes the output wavelength-multiplexed optical signal having a power level within a predetermined range.

0144<figref idrefs="S75">22</figref> is a graph showing an optical amplifying apparatus according to a still another embodiment of the present invention. In the<figref idrefs="S75">22</figref> illustrated optical amplification means is a combination of the in <figref idrefs="S73">20</figref> and <figref idrefs="S74">21</figref> illustrated optical amplification means.

0145Referring now on <figref idrefs="S75">22</figref> controls the ALC correction circuit <figref>98</figref> the automatic level control circuit <figref>66</figref> such that the power level of the the output wavelength-division multiplex Optical signal within a predetermined range. After a Extracting and identifying a control signal for dispensing a warning about a Variation in the number of channels, freezes the monitor signal processing circuit <figref>70</figref> automatic Level control function so that the optical output power is not longer is maintained at a constant level.

0146<figref idrefs="S76">23</figref> is a graph in accordance with an optical amplifying apparatus of a embodiment of the present invention. Referring now to<figref idrefs="S76">23</figref> becomes instead of a controller (freezing) the optical attenuator <figref>64</figref>, Thus stable damping provide, when the number of channels is varied, the overall optical amplifier is changed to the AGC mode when the number of channels varies. Such a change can be achieved by adjusting the ratio between the input the optical attenuator <figref>64</figref> and output thereof is at a constant level. Such an operation is equivalent with the retention of the reinforcement G (0 ≤ G ≤ 1) of optical attenuator <figref>64</figref> or Light transmissivity of optical attenuator <figref>64</figref> on a constant level.

0147Therefore is in <figref idrefs="S76">23</figref> a switch <figref>104</figref> by a monitor signal processing circuit <figref>70</figref> is controlled to between an automatic level control, which by automatic level control circuit <figref>66</figref> is provided, and an automatic gain control, which by an automatic gain control circuit <figref>60₃</figref> is provided to switch. More specifically, causing the monitor signal processing circuit <figref>70</figref>. such as in <figref idrefs="S57">4 (A)</figref> shown, that the switch <figref>104</figref> automatic level control circuit <figref>66</figref> in front and subsequently selecting a variation in the number of channels. While the Number of channels is being varied, monitor signal processing circuit <figref>70</figref>that the desk <figref>104</figref> the automatic gain control circuit <figref>60₃</figref> selects.

0148<figref idrefs="S76">23</figref> provides also a laser diode (LD) <figref>105</figref> is represented by the monitor signal processing circuit <figref>70</figref> so is controlled to information located on downstream optical components transferred to, such as downstream optical repeater. For example, the laser diode (LD)<figref>105</figref>. as described in further detail, by monitor signal processing circuit <figref>70</figref> to be used to transmit information to downstream optical components.

0149<figref idrefs="S77">24</figref> is a detailed diagram of the optical amplification means in <figref idrefs="S76">23</figref>, Referring now to<figref idrefs="S77">24</figref> is the operation as follows: <ul><li>(1) Normally (that is, if the number of channels is not varied) selects the desk <figref>104</figref> automatic level control circuit <figref>66</figref> from so that the power level of the light output from the optical attenuator <figref>64</figref> monitors and is maintained at a constant level.</li><li>(2) When monitor signal processing circuit <figref>70</figref> on Signal, which over a change warns in the number of channels, is a gain monitoring signal <figref>107</figref> the automatic gain control circuit <figref>60₃</figref> read, so that an average gain (attenuation) is determined with respect to a time constant in the range of 10-100 ms.</li><li>(3) A reference voltage V_AGC according to the average gain as determined in (2), is supported by the monitor signal processing circuit <figref>70</figref> at the automatic gain control circuit <figref>60₃</figref> output.</li><li>(4) The switch <figref>104</figref> then selects the automatic gain control circuit <figref>60₃</figref> out.</li><li>(5) The monitor signal processing circuit <figref>70</figref> receives information, which indicates that the new number of channels in the wavelength division multiplexing add optical signal is.</li><li>(6) The monitor signal processing circuit <figref>70</figref> provides automatic level control circuit <figref>66</figref> a reference voltage V_ALC prepared according to the new number of channels.</li><li>(7) Monitor signal processing circuit <figref>70</figref> receives a Signal indicating that the variation in the number of channels is complete. Alternatively elapses a predetermined time period from the reception of the signal over the change in the number of channels warns.</li><li>(8) The switch <figref>104</figref> selects automatic level control circuit <figref>66</figref> out.</li></ul>

0150The Relationship between a damping which by optical attenuator <figref>64</figref> provided, and a driving current of a control <figref>86</figref>, which one through a transistor <figref>80</figref> is provided, can of a parameter depend on how For example, an operating temperature, however, it is generally in a 1-to-1 relationship. Therefore, (2) above by a process be replaced, will be monitored in which the drive current (namely with With respect to the time constant in the range of 10-100 ms), thus an average gain (Damping), based on the monitored to determine driving. The drive current can be controlled in such a way be that its average level is maintained constant.

0151<figref idrefs="S78">25</figref> is a diagram illustrating a fiber optic communication system represents that pursuant to an optical amplifying apparatus embodiments of the present invention. Referring now to<figref idrefs="S78">25</figref> transmits a transformer (Tx) <figref>108</figref> an SV light beam to a receiver (Rx) <figref>110</figref>Wherein an SV light beam is a light which with a main signal a Welwavelengths multiplex forms. The main signal is used to transmit information downstream. An optical amplifier (O-AMP) <figref>112</figref> reinforced the SV light beam. There are a main signal control <figref>114</figref> and monitor signal processing <figref>116</figref> performed.

0152<figref idrefs="S79">26</figref> is a more detailed graph showing an optical amplification means represents that an optical amplifier <figref>112</figref>, A main signal control <figref>114</figref> and a monitor signal processing <figref>116</figref> from <figref idrefs="S78">25</figref> contains. The optical amplification means in <figref idrefs="S79">26</figref> is similar to the optical amplification means in <figref idrefs="S56">3</figref>contains However, a laser diode (LD) <figref>105</figref> for sending an SV light beam downstream.

0153More accurate said continues to monitor signal processing circuit <figref>70</figref> in the SV light beam information a indicating when the attenuation, or the light transmissivity of the optical attenuator <figref>64</figref> constant or is kept "frozen". The SV light beam, carrying that information, is by the laser diode (LD) <figref>105</figref> transmitted to the transmission line.

0154<figref idrefs="S80">27</figref> is a graph showing a transmission line represents that a plurality of optical amplification means used in accordance with embodiments of the present invention. Referring now to<figref idrefs="S80">27</figref> includes a wavelength division multiplexing Optic communication system transformer Tx <figref>120</figref>Wavelength-multiplexed optical fiber amplifiers / repeaters OAMPS <figref>122</figref> and receiver Rx <figref>124</figref>, When a variation in the number of channels processed will be all OAMPS <figref>122</figref> in the upstream (or stromabwärts-) Line in the system at a constant optical gain control set.

0155On Wavelength division multiplexing Optics Postamplifier (Not shown) provided in each transmitter Tx <figref>120</figref> provided may be, and a wavelength division multiplexing Optical preamplifier (Not shown) provided in each receiver Rx <figref>124</figref> provided may be, are also on a constant gain control set. If all OAMPS<figref>122</figref> in a constant gain control state are, the power of an optical signal which is a light receiving element in the receivers Rx <figref>124</figref> supplied will vary.

0156In a transmission line, said optical amplification means as in <figref idrefs="S78">25</figref>-<figref idrefs="S80">27</figref> illustrated has, it is possible To determine whether all the optical fiber amplifiers in the path defined by a receiving end (Rx) on the transmission line is managed, its attenuation have established or not, and its optical gain on have maintained a constant level or not. Once determined is that all the optical fiber amplifiers its optical gain to have maintained a constant level, is information which This indicates over the return path to the transmitting end (Tx) is transmitted, whereupon a variation in the number of channels started can be.

0157in the Below is an example of the operation flow in a transmission line explained, said optical amplification means as in <figref idrefs="S78">25</figref>-<figref idrefs="S80">27</figref> illustrated has to handle a variation in the number of channels. <ul><li>(1) A signal of a variation in the number of channels warns is from the upstream lying SV transmitting end (SVTx) is output.</li><li>(2) Monitor signal processing circuit <figref>70</figref> from receives each OAMP the signal via the variation in the number of channels warns.</li><li>(3) Each OAMP starts "freezing" the operation of the associated Optical attenuator.</li><li>(4) Each OAMP completes a freezing operation of the associated optical attenuator and sends downstream information, indicating that the constant optical gain control is started by this information is conveyed on the monitoring signal (An identification number to identify individual OAMPS is also in the monitoring signal used).</li><li>(5) The upstream SV receiving end (SVRx) confirmed that all upstream OAMPS the constant optical gain state are. </li><li>(6) The downstream SV transmitting end (SVTx) indicates that all upstream OAMPs in constant optical gain state.</li><li>(7) The downstream SV receiving end (SVRx) confirmed that all upstream OAMPS the constant optical gain state are.</li><li>(8) The upstream transmitting end (Tx) varies currently the number of channels.</li><li>(9) The upstream SV transmitting end (SVTx) outputs information indicating that the variation in the Number of channels completely is.</li><li>(10) The monitor signal processing circuit <figref>70</figref> in receives each OAMP the information indicating that the variation in the number of channels completely is.</li><li>(11) Each OAMP cancels the freezing operation for freezing the Operation of the associated optical attenuator and travels to the constant optical output control continues.</li><li>(12) Each OAMP sends downstream information indicating that a Shift to the constant optical output control is complete, namely in the shape of the monichung signal (An identification signal identifying the individual OAMPs, is also sent).</li><li>(13) The upstream SV receiving end (SVRx) receives the information indicating that all the OAMPs the variation in the number of channels have processed.</li><li>(14) The information indicating that all the OAMPs the variation processed in the number of channels who is sent to the transmitting end.</li></ul>

0158<figref idrefs="S81">28</figref> is a timing, which is the operation flow described above.

0159Therefore is one in processing the variation in the number of channels Wavelength division multiplexing Optical fiber amplifier temporarily during execution stopped an automatic level control function, and instead he leads a constant gain control function through, or it is effected that the optical amplification means total a constant gain function performs.

0160however it is necessary in an optical communication system usually, the power of an optical signal which is a light receiving element supplied is to maintain at a constant level. Although a variation in the input power due to polarization variation under conventional circumstances occurs, causing the control for maintaining the optical gain of the the optical fiber amplifier to a constant level, that the power of the optical signal, which is supplied to the light receiving element varies.

0161This Problem can by demultiplexing the optical signal into individual channels Fixed and controlling the power level of the individual demultiplexed channels will.

0162More accurate specifically, <figref idrefs="S82">29</figref> a diagram showing a Portion of an optical communication system according to a embodiment of the present invention. Referring now to<figref idrefs="S82">29</figref> demultiplexes a demultiplexer (DEMUX) <figref>125</figref> a wavelength division multiplexing Optical signal into individual channels, order by individual recipients <figref>126</figref> receive to become. An optical preamplifier<figref>127</figref> and a automatic level control unit <figref>128</figref> are for everyone provided channel so that the associated receiver <figref>126</figref> an optical Signal at a constant power level receives.

0163According to the above embodiments of the present invention, an optical attenuator or an optical amplifier in such a way are controlled to provide a constant gain while the Number of channels in a wavelength division multiplexing Optical signal is varied. In this case, the gain G in the range of (0 ≤ G ≤ 1) be. Thus, an optical attenuator be controlled so as to provide a constant gain by a constant ratio is maintained between the input and the output of optical attenuator.

0164According to the above embodiments of the present invention, a rare-earth-doped optical Fiber in an optical amplifier used, the dopant Erbium (Er). However, The present invention is not to an erbium (Er) doped optical Fiber limited. Instead, can more rare-earth-doped optical fibers, such as a doped neodymium (Nd) -doped optical fiber or a praseodymium (Pd) optical fiber, also in dependence on the wavelength involved be used. Further, For example, the numerous photodiodes which disclosed herein are to be replaced by phototransistors.

0165According to the above embodiments of the present invention are specific embodiments of automatic Gain control circuits and automatic level control circuits are disclosed. However, it is not intended that the present invention on any specific circuitry for these circuits or for other circuits described herein is limited. Instead, many different circuit designs are used.

0166Furthermore is according to the above embodiments the present invention uses an optical attenuation to a variable damping provide. There are many different types of known Optical attenuators, and it is not intended that the embodiments of the present Invention are not limited to any specific type of optical attenuator.

0167Although some preferred embodiments of the present invention have been shown and described, would by identified a skilled person that changes in these embodiments accomplished can be, without departing from the principles of the invention, the scope in to claims and their equivalents is defined.