Optical communication system with optical amplification

0001The present invention relates to an apparatus according to claim 1 and to an optical transmission system comprising such an apparatus.

0002An optical amplification apparatus according to the preamble part of claim 1 is known from publication XP002918851 (OAA '95 paper FC3, June 16, 1995, Davos, Switzerland), relating to Er-doped fiber amplifier for WDM systems. It shows the preamble of claim 1.

0003<patcit id="pcit0001" dnum="US5510926A"><text>US-A-5 510 926</text></patcit> discloses an optical amplifier for amplifying WDM optical signal having a variable number of channels associated with different wavelengths and for outputting the amplified WDM optical signal, said optical amplifier including an optical attenuator for controlling a level of the amplified WDM optical signal, and a controller for controlling the WDM optical signal.

0004From <patcit id="pcit0002" dnum="DE19516439A"><text>DE-A-195 16 439</text></patcit> it is known to provide an optical amplifying apparatus including a first multiplexing/demultiplexing unit for multiplexing or demultiplexing a signal light and a pumping light, a first optical amplifying medium coupled to the first multiplexing/demultiplexing unit for amplifying the signal light, a second multiplexing/demultiplexing unit coupled to the first optical amplifying medium for multiplexing or demultiplexing the signal light and a pumping light, a light signal characteristic compensation unit coupled to the second multiplexing/demultiplexing unit for compensating a characteristic of the signal light, and a second optical amplifying medium coupled to the second multiplexing/demultiplexing unit for amplifying the signal light to be amplified with a constant gain. More specifically, therein it is disclosed that an optical component may form a band pass filter which may be arranged between first and second amplifying stages, said band pass filter shall be provided for equalizing only the light in the vicinity of the signal light and for suppressing amplified light outside of the signal band to enter the next (or preceding) stage optical amplifying medium.

0005Further, it is known to use wavelength division multiplexing in fiber optic communication systems to transfer a relatively large amount of data at a high speed, as it is explained hereinbelow with reference to Fig. 1 of the drawings.

0006FIG. 1 is a diagram illustrating a conventional fiber optic communication system which uses wavelength division multiplexing to transmit, for example, four channels through a single optical fiber. Referring now to FIG. 1, transmitting units 20-1, 20-2, 20-3 and 20-4 transmit individual carriers having wavelengths K1 - X4, respectively. Each carrier is modulated with information and represents an individual channel. The different carriers are multiplexed together by an optical multiplexer 22 into a wavelength-multiplexed optical signal. The wavelength-multiplexed optical signal is transmitted through an optical fiber 24 to an optical demultiplexer 26. Optical demultiplexer 26 branches the wavelength-multiplexed optical signal into four separate optical signals having the wavelengths X1 - X4, respectively. The four separate branched optical signals are then detected by receiving units 28-1, 28-2, 28-3 and 28-4, respectively.

0007While the above optical fiber communication system multiplexes four carriers together, it is common practice to multiplex more than four carriers. More specifically, many different carriers may be multiplexed together. In this manner, a relatively large amount of data can be transmitted through an optical fiber.

0008An optical amplifier (not illustrated) or an optical repeater (not illustrated) is typically inserted between optical multiplexer 22 and optical demultiplexer 26, to amplify the wavelength-multiplexed optical signal traveling through optical fiber 24. Such an optical amplifier is typically a rare-earth doped optical fiber-amplifier which directly amplifies the wavelength-multiplexed optical signal. That is, a rare-earth doped optical fiber amplifier amplifies the wavelength-multiplexed optical signal without converting the wavelength-multiplexed optical signal into an electrical signal.

0009Use of a rare-earth doped optical fiber amplifier causes several problems when the number of channels in the wavelength-multiplexed optical signal is varied. More specifically, during the variation (that is, before the variation in the number of channels is complete), the optical power of each channel can undesirably be varied, thereby causing non-linear degradation or S/N degradation of the wavelength-multiplexed optical signal.

0010It is an object of the present invention to provide an optical apparatus including means for amplifying a WDM optical signal and to provide an optical transmission system transmitting a WDM optical signal, each having improved applicability and efficiency.

0011According to the present invention, this object is attained by an apparatus according to claim 1 and by an optical transmission system as specified in claim 4 and 5, respectively. Further improvements are specified in respectively related sub-claims.

0012Preferred embodiments according to the present invention and of features and advantages thereof as well as examples contributing to further improving the understanding of features and functions in connection with the present invention are described hereinbelow in more detail with reference to the drawings.

0013In the drawings, <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a diagram illustrating a conventional fiber optic communication system.</li><li>FIG. 2 is a diagram illustrating an optical amplifying apparatus for a fiber optic communication system which uses wavelength division multiplexing.</li><li>FIG. 3 is a diagram illustrating an example of an optical amplifying apparatus.</li><li>FIGS. 4(A) and 4(B) are graphs illustrating the operation of the optical amplifying apparatus shown in FIG.3, wherein the number of channels, N, in an optical signal is changed.</li><li>FIG. 5 is a diagram illustrating an example of an automatic gain control circuit.</li><li>FIG. 6 is a diagram illustrating an example of an automatic level control circuit.</li><li>FIG. 7 is a diagram illustrating a switching circuit of the automatic level control circuit shown in FIG. 6.</li><li>FIGS. 8 and 9 are diagrams illustrating examples of an automatic level control circuit.</li><li>FIG. 10 is a diagram illustrating an embodiment of an apparatus and optical transmission system according to the present invention.</li><li>FIG. 11 is a diagram illustrating a further embodiment of an apparatus and optical transmission system according to the present invention.</li><li>FIG. 12 is a diagram illustrating a further embodiment of an apparatus and optical transmission system according to the present invention.</li><li>FIG. 13 is a diagram illustrating a further embodiment of an apparatus and optical transmission system according to the present invention.</li><li>FIG. 14 is a diagram illustrating a further embodiment of an apparatus and optical transmission system according to the present invention.</li><li>FIG. 15 is a diagram illustrating a further embodiment of an apparatus and optical transmission system according to the present invention.</li><li>FIG. 16 is a diagram illustrating a further embodiment of an apparatus and optical transmission system according to the present invention.</li><li>FIG. 17 is a diagram illustrating a further embodiment of an apparatus and optical transmission system according to the present invention, forming a modification of that what is illustrated in FIG. 16.</li><li>FIG. 18(A) is a graph illustrating gain versus wavelength characteristics of a rare-earth-doped optical fiber (EDF) in an optical amplifying apparatus.</li><li>FIG. 18(B) is a graph illustrating the transmissivity of an optical filter in an optical amplifying apparatus.</li><li>FIG. 18(C) is a graph illustrating overall gain of the rare-earth-doped optical fiber (EDF) in FIG. 18(A) and the optical filter in FIG. 18(B).</li><li>FIG. 19 is a diagram illustrating a further example of an optical amplifying apparatus according.</li><li>FIG. 20 is a diagram illustrating a further example of an optical amplifying apparatus.</li><li>FIG. 21 is a diagram illustrating a further example of an optical amplifying apparatus.</li><li>FIG. 22 is a diagram illustrating a further example of an optical amplifying apparatus.</li><li>FIG. 23 is a diagram illustrating a further example of an optical amplifying apparatus.</li><li>FIG. 24 is a more detailed diagram of a portion of the optical amplifying apparatus as shown in FIG. 23.</li><li>FIG. 25 is a diagram illustrating a fiber optic communication system employing an optical amplifying apparatus.</li><li>FIG. 26 is a more detailed diagram illustrating the optical amplifying apparatus of FIG. 25, according to an embodiment of the present invention.</li><li>FIG. 27 is a diagram illustrating a transmission line employing a plurality of optical amplifying apparatuses.</li><li>FIG. 28 is a timing diagram illustrating the operation of an optical amplifying apparatus.</li><li>FIG. 29 is a diagram illustrating a portion of an optical communication system.</li></ul>

EMBODIMENTS AND EXAMPLES

0014Reference will now be made in detail to preferred embodiments of the present invention and to examples for further explanation.

0015Fig. 2 is a diagram illustrating an example of an optical amplifying apparatus for a fiber communication system which uses wavelength division multiplexing, and is similar to that disclosed in U.S patent application 08/655 027.

0016Referring now to FIG. 2, the optical amplifying apparatus includes a first part 1000 (sometimes referred to herein as a "rare-earth-doped optical fiber amplifier part") and a second part 2000 (sometimes referred to herein as an "electrically-controlled optical device part").

0017First part 1000 includes a rare-earth-doped optical fiber (EDF) 34, optical branching couplers 36₁ and 36₂, optical isolators 38₁ and 38₂, photodiodes 40₁ and 40₂, an optical wavelength multiplexing coupler 42, a pump laser diode (LD) 44 and an automatic optical gain control circuit (AGC) 46.

0018Second part 2000 includes optical branching coupler 36₃, an electrically-controlled variable optical attenuator (ATT) 48, a photodiode (PD) 40₃ and an automatic level control circuit (ALC) 50. Optical attenuator 48 is, for example, constructed of a magnetooptical element. However, many different types of variable optical attenuators can be used.

0019A wavelength-multiplexed optical signal is fed to rare-earth-doped optical fiber 34 via optical branching coupler 36₁, optical isolator 38 and optical wavelength multiplexing coupler 42. A pump light beam is supplied by pump laser diode 44 to rare-earth-doped optical fiber 38 via optical wavelength multiplexing coupler 42. The wavelength-multiplexed optical signal is amplified by rare-earth-doped optical fiber 34 and input to optical attenuator 48 via optical isolator 38₂ and optical branching coupler 36₂.

0020A portion of the wavelength-multiplexed optical signal branched by optical branching coupler 36₁ is converted into an electrical signal by photodiode 40₁ and input to automatic optical gain control circuit 46. A portion of the amplified wavelength-multiplexed optical signal branched by optical branching coupler 36₂ is converted into an electrical signal by photodiode 40₂ and input to automatic optical gain control circuit 46. Pump laser diode 44 is controlled so as to maintain a ratio between a level of the input wavelength-multiplexed optical signal and a level of the amplified wavelength-multiplexed optical signal at a predetermined level.

0021More specifically, optical gain control circuit 46 controls pump laser diode 44 so as to maintain, at a constant level, the ratio between the level of the input wavelength-multiplexed optical signal as converted into an electrical signal by the photodiode 40₁ and the level of the amplified wavelength-multiplexed optical signal as converted into an electrical signal by the photodiode 40₂. In this manner, first part 1000 conserves the wavelength dependence by controlling the optical gain at a constant level.

0022A portion of an output wavelength-multiplexed optical signal branched by optical branching coupler 36₃ is converted into an electrical signal by photodiode 40₃ and input to automatic level control circuit 50. Optical attenuator 48 is controlled so as to maintain the wavelength-multiplexed optical signal at a predetermined level.

0023More specifically, automatic level control circuit 50 controls optical attenuator 48 using the electrical signal derived by photodiode 40₃ from the wavelength-multiplexed optical signal, so as to maintain the output level of the wavelength-multiplexed optical signal at a constant level.

0024Unfortunately, when an optical amplifying apparatus, as illustrated in FIG. 2, is used in a fiber optic communication system which uses wavelength division multiplexing, a variation in the number of channels used in the wavelength-multiplexed optical signal can cause significant problems.

0025For example, a predetermined output optical power of an amplifier is generally required for each wavelength (channel) so as to ensure a desired S/N ratio in a receiver. Assuming there are a total of N channels, the total optical output Pc of a rare-earth-doped optical fiber amplifier for amplifying a wavelength-multiplexed optical signal is controlled to be N x P. In the presence of a variation of +α or -α in the number of channels N, switching control is effected so that the total optical power is (N ± α)P. Because the optical power for individual wavelengths (channels) varies due to the switching control, non-linear degradation or signal-to-noise (S/N) degradation may result.

0026Further, in FIG. 2, the optical output of first part 1000 is to be maintained.at a constant level by second part 2000. Therefore, when the optical output of first part 1000 exceeds a predetermined level, second part 2000 maintains the optical output at a constant level. As a result, the use of optical attenuator 48 will require an extra measure of amplification by first part 32, and the output power of pump laser diode 44 for maintaining the optical gain at a constant level should be controlled to be in an exponential relation to a variation in the level of the input wavelength-multiplexed optical signal. Therefore, it is necessary to provide a relatively high-capacity pump laser diode 44.

0027FIG. 3 is a diagram illustrating an optical amplifying apparatus, according to an embodiment of the present invention. The optical amplifying apparatus includes a first part 1000 and a second part 2000. First part 1000 includes a rare-earth-doped optical fiber (EDF) 52₁, optical branching couplers 54₁ and 54₂, optical isolators 55₁ and 55₂, an optical wavelength multiplexing coupler 56₁, photodiodes (PD) 58₁ and 58₂, a pump laser diode (LD) 59₁, and an automatic gain control circuit (AGC) 60₁. First part 1000 amplifies a wavelength-multiplexed optical signal while conserving wavelength dependance.

0028As an example, a wavelength-multiplexed optical signal is typically in the 1.5 µm band. An erbium-doped optical fiber is known to amplify optical signals in this band, and is therefore used as rare-earth-doped optical fiber (EDF) 52₁. Moreover, to appropriately amplify a wavelength-multiplexed optical signal in the 1.5 µm band travelling through an erbium-doped optical fiber, it is known to use pump light of a 0.98 µm or 1.48 µm pump band. Therefore, pump laser diode (LD) 59₁ provides pump light in the 0.98 µm or 1.48 µm pump band.

0029Moreover, FIG. 3 shows a forward pumping construction in which a pump light beam emitted by pump laser diode 59₁ travels through rare-earth-doped optical fiber 52₁ in the same direction as the wavelength-multiplexed optical signal. However, a backward pumping construction could also be used, where a laser diode provides a pump light beam which travels through rare-earth-doped optical fiber 52₁ in the opposite direction as the wavelength-multiplexed optical signal. Further, a bidirectional pumping construction could be used, where two laser diodes provide pump light which travels through rare-earth-doped optical fiber 52₁ in both directions through rare-earth-doped optical fiber 52₁. Thus, the present invention is not intended to be limited to any specific type of directional pumping.

0030Second part 2000 includes an electrically-controlled variable optical attenuator (ATT) 64, an automatic level control circuit (ALC) 66, optical branching coupler 54₃ and a photodiode (PD) 58₃. Second part 2000 controls the total optical output of a wavelength-multiplexed optical signal to be at a constant level, without conserving wavelength dependence. More specifically, automatic level control circuit 66 varies the attenuation, or light transmissivity, of optical attenuator 64 so that the power of the wavelength-multiplexed optical signal, as output from first part 1000, is maintained at a constant power level corresponding to the number of channels in the wavelength-multiplexed optical signal.

0031Moreover, when the number of channels in the wavelength-multiplexed optical signal is being varied, a monitor signal processing circuit 70 causes the attenuation, or light transmissivity, of optical attenuator 64 to be maintained constant. Thus, monitor signal processing circuit 70 temporarily "freezes" the operation of optical attenuator 64. After the number of channels has been changed, monitor signal processing circuit 70 allows the attenuation, or light transmissivity, of optical attenuator 64 to be varied so that the power of the wavelength-multiplexed optical signal is maintained at a constant level in accordance with the new number of channels.

0032More specifically, the wavelength-multiplexed optical signal input to the optical amplifying apparatus is branched by an optical branching coupler 68₁. The branched portion is provided to a photodiode (PD) 58₄. Photodiode (PD) 58₄ converts the branched portion into an electrical signal and provides the electrical signal to monitor signal processing circuit 70.

0033A control signal, which warns of a variation in the number of channels in the wavelength-multiplexed optical transmission system, is superimposed on the wavelength-multiplexed optical signal preferably as a low-speed signal through an amplitude modulation process. However, other methods can be used to superimpose the control signal. Monitor signal processing circuit 70 extracts and identifies the control signal. Monitor signal processing circuit 70 then controls optical attenuator 64 or automatic level control circuit 66 in accordance with the extracted control signal. If amplitude modulation is used, it is relatively easy to extract the control signal by demodulating the electrical signal obtained by photodiode 58₄.

0034Alternatively, the control signal may be transmitted to monitor signal processing circuit 70 on a dedicated control channel (wavelength). If a dedicated control channel is used, an optical branching filter (not illustrated) should extract the control signal out of the wavelength-multiplexed optical signal (as branched by optical branching coupler 68₁). For example, by feeding the optical signal extracted by the optical branching filter to photodiode 58₄ so as to be converted into an electrical signal, it is possible to extract the control signal.

0035Therefore, a portion of the wavelength-multiplexed optical signal branched by optical branching coupler 68₁ is converted into an electrical signal by photodiode 58₄ and fed to monitor signal processing circuit 70. Monitor signal processing circuit 70 "freezes" an operation of optical attenuator 64, when a control signal warning of a variation in the number of channels is extracted and identified.

0036In order to ensure that the power level of the attenuated wavelength-multiplexed optical signal matches the number of channels, monitor signal processing circuit 70 causes a set voltage (reference voltage) to be selected. The power level can then be controlled to be at a constant level corresponding to the set voltage.

0037Generally, there are two approaches for monitor signal processing circuit 70 to control optical attenuator 64. In one approach, optical attenuator 64 is directly controlled by monitor signal processing circuit 70, as illustrated by control signal 69 in FIG. 3. In an alternative approach, optical attenuator 64 is indirectly controlled by monitor signal processing circuit 70, as illustrated by control line 71 in FIG. 3.

0038The number of channels may actually be increased or decreased after a warning for a change in the number of channels. In this instance, a control signal, which indicates the completion of the change in the number of channels, is superimposed on the wavelength-multiplexed optical signal. Monitor signal processing circuit 70 then extracts the control signal. Alternatively, the control signal may be transmitted to monitor signal processing circuit 70 on a dedicated control channel (wavelength). Upon extracting and identifying the control signal, monitor signal processing circuit 70 allows optical attenuator 64 to resume its control for maintaining the power level of the wavelength-multiplexed optical signal at a constant level.

0039Alternatively, instead of providing monitor signal processing circuit 70 with a control signal indicating the completion of the change in the number of channels, such completion can be assumed after a predetermined period of time elapses. More specifically, the number of channels may actually be increased or decreased after lapse of a predetermined period of time since the warning for a change in the number of channels is given. In this instance, after the control signal for giving warning of a variation in the number of channels is extracted and identified by monitor signal processing signal 70, a timer (not illustrated) is activated. When a predetermined period of time has passed, optical attenuator 64 is driven again to maintain the power level of the wavelength-multiplexed optical signal at a constant level.

0040Whether a control signal or a predetermined period of time is used to indicate the completion of a variation in the number of channels, the set voltage (reference voltage) for controlling the power level is switched from one level to another in accordance with information relating to how many channels are added or removed. This information is preferably included in the control signal for warning of a variation in the number of channels. Therefore, by resuming the control for maintaining the total optical output power at a constant level, the optical output is maintained at a constant level that matches the number of channels.

0041Therefore, in response to a change in the number of channels, optical attenuator 64 prevents a radical variation in the optical output power, by having its attenuation frozen at a constant level. At this time, second part 2000 no longer operates to maintain the power of the wavelength-multiplexed optical signal at a constant level. After the number of channels is changed, optical attenuator 64 is again controlled to maintain the power of the wavelength-multiplexed optical signal at a constant level. Optical attenuator 64 may gradually be driven so that a total output power corresponding to the number of channels is maintained. With this arrangement, it is possible to moderate a variation in the optical output and avoid non-linear degradation and S/N ratio degradation.

0042FIGS. 4(A) and 4(B) are graphs illustrating the operation of the optical amplifying apparatus in FIG.3, wherein the number of channels, N, in an optical signal is changed from, for example, four channels to eight channels. Referring now to FIGS. 4(A) and 4(B), optical attenuator 64 has a variable light transmissivity, or attenuation, which is controlled by automatic level control circuit 66 an monitor signal processing circuit 70.

0043In FIGS. 4(A) and 4(B), a warning of a change in the number of channels is received at time t1, and the number of channels are increased at time t2.

0044Before a warning of a change in the number of channels is received (that is, before time t1), automatic level control circuit 66 varies the light transmissivity of electrically-controlled variable optical attenuator 64 to provide a substantially constant optical signal power at the output of optical attenuator 64. Therefore, before time t1, second part 2000 performs automatic level control (ALC).

0045When a warning of a change in the number of channels is received (that is, at time t1), automatic level control circuit 66 maintains the light transmissivity of electrically-controlled variable optical attenuator 64 to be substantially constant. In this case, the output of optical attenuator 64 can be seen has having a constant gain which is provided, for example, by first part 1000 or by a later stage (not illustrated) which further amplifies the signal. Therefore, after time t1, automatic gain control (AGC), not automatic level control (ALC), is performed.

0046At time t3, subsequent to a change in the number of channels, automatic level control circuit 66 varies the light transmissivity of electrically-controlled variable optical attenuator 64 to provide a substantially constant optical signal power at the output of optical attenuator 64. More specifically, after time t3, second part 2000 again performs automatic level control (ALC).

0047As can be seen from FIGS. 4(A) and 4(B), optical attenuator 64 is controlled to provide ALC. However, when the number of channels is being changed, ALC is halted. Instead, when the number of channels is being changed, optical attenuator 64 is controlled to provide a constant light transmissivity, or attenuation. The operation of optical attenuator 64 can be described as being "frozen" when the number of channels is being changed between times t1 and t3 in FIGS. 4(A) and 4(B).

0048As described above, between times t1 and t3, the output of optical attenuator 64 has a constant gain which is provided, for example, by first part 1000 or by a later stage (not illustrated) which further amplifies the signal. Alternatively, as disclosed in additional embodiments of the present invention described in more detail below, second part 2000 can be modified so that it provides a constant gain (instead of providing automatic level control) while the number of channels is being changed. In this case, second part 2000 could include a gain controlled amplifier to provide a constant gain for AGC between times t1 and t3.

0049Therefore, as illustrated in FIGS. 4(A) and 4(B), an optical amplifying apparatus includes an optical amplifier (such as first part 1000) which amplifies a light signal having a variable number of channels. Prior to, and subsequent to, varying the number of channels in the light signal, a controller (such as second part 2000) passes the amplified light signal with a varying light transmissivity so that a power level of the amplified light signal is maintained at an approximately constant level in accordance with the number of channels in the light signal. Further, while the number of channels in the light signal is being varied, the controller passes the amplified light signal with a constant light transmissivity.

0050FIG. 5 is a diagram illustrating automatic gain control circuit 60₁, for controlling an optical gain to be at a constant level. Referring now to FIG. 5, automatic gain control circuit 60₁ includes a divider 72, an operational amplifier 74, a transistor 76 and resistors R1 - R6. V_cc is a power supply voltage, V_ref is a reference voltage, and G is the earth or ground.

0051As illustrated in FIG. 5, photodiode (PD) 58₁ converts a portion of the wavelength-multiplexed optical signal into an electrical signal which is provided to divider 72. Photodiode (PD) 58₂ converts a portion of the amplified wavelength-multiplexed optical signal into an electrical signal which is provided to divider 72. In this manner, divider 72 obtains a ratio between the input and the output of rare-earth-doped optical fiber (EDF) 52₁. The pump light beam emitted by pump laser diode 59₁ can then be controlled to produce a constant ratio, thereby providing a constant gain. The configuration of automatic gain control circuit 60₁ in FIG. 5 is just one example of many possible configurations for an automatic gain control circuit.

0052FIG. 6 is a diagram illustrating automatic level control circuit 66, for controlling an optical output at a constant level. Referring now to FIG. 6, automatic level control circuit 66 includes resistors R7 - R9, an operational amplifier 78, a transistor 80, a switching circuit (SWC) 82 and a reference voltage circuit 84. 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 provided by monitor signal processing circuit 70. A control element 86 is a control element of optical attenuator 64 for controlling the transmissivity of optical attenuator 64.

0053For example, if optical attenuator 64 is operated by a magnetooptical effect, control element 86 may be a coil for applying a magnetic field. Moreover, for example, if optical attenuator is operated by an opto-electrical effect, the control element 86 may be an electrode, where the voltage applied to the electrode is controlled. If a semiconductor optical amplifier is used instead of optical attenuator 64, a bias voltage for controlling the gain of the semiconductor optical amplifier can be controlled.

0054A portion of the optical signal output from optical attenuator 64 (see FIG. 3) is branched by optical branching coupler 54₃ and converted into an electrical signal by photodiode (PD) 58₃. Then, in FIG. 6, operational amplifier 78 compares the electrical signal with the reference voltage (set voltage) V_ref supplied by reference voltage circuit 84 in accordance with control signal CS1. A difference obtained as a result of the comparison is used to drive transistor 80. By controlling a current supplied to control element 86, the attenuation provided by optical attenuator 64 is controlled so that the optical output is maintained at a constant level.

0055FIG. 7 is a diagram illustrating switching circuit 82. Referring now to FIG. 7, switching circuit 82 includes capacitors C1 and C2 which are individually selected with a switch SW that is controlled by the control signal CS2. Therefore, switching circuit 82 controls the frequency characteristic of automatic level control circuit 66. Moreover, switching circuit 82 controls optical attenuator 64 by controlling transistor 80 by following the level of the output wavelength-multiplexed optical signal with a predetermined frequency characteristic. The control signal cs2 from monitor signal processing circuit 70 changes the frequency characteristic by switching between capacitors C1 and C2 of switching circuit 82. The control signal cs1 switches between different levels of the reference voltages in accordance with the number of channels.

0056More specifically, switching circuit 82, coupled with operational amplifier 78 (see FIG. 6) and resistors R7 (see FIG. 6) and R9 (see FIG. 6), forms a primary low-pass filter. The cut-off frequency, f_c, of this primary low-pass filter is: <maths id="math0001"><math display="block"><msub><mi mathvariant="normal">f</mi><mi mathvariant="normal">c</mi></msub><mo mathvariant="normal">=</mo><mn mathvariant="normal">1</mn><mo mathvariant="normal">/</mo><mfenced><mn mathvariant="normal">2</mn><mo>⁢</mo><mi mathvariant="normal">πR</mi><mo>⁢</mo><mn mathvariant="normal">9</mn><mo mathvariant="normal">⋅</mo><msub><mi mathvariant="normal">C</mi><mi>SWC</mi></msub><mo>⁢</mo><mn mathvariant="normal">9</mn></mfenced><mo mathvariant="normal">,</mo></math><img file="EP0902565B1_D0001.tif" /></maths> where C_SWC is the selected capacitor C₁ or C₂. Therefore, by increasing the value of the capacitance C_SWC, the control circuitry shown in FIG. 6 is operated at a lower frequency. That is, the response thereof is slowed down.

0057Therefore, depending on the capacitance of the selected capacitor C1 or C2 of switching circuit 82, the filter cut-off frequency in the high-frequency zone can be changed.

0058As an example, a preferably arrangement may be that the cut-off frequency, which is on the order of 10 - 100 kHz in the normal ALC operation, be switched to 0.01 Hz when optical attenuator 64 is controlled to provide a constant attenuation (for example, to thereby provide a constant gain when the channels are being switched). Ideally, the control of switching circuit 82 occurs gradually, but a gradual control requires that switching circuit 82 be constructed of a number of capacitors, instead of simply two capacitors.

0059Referring to FIG. 6, the cut-off frequency is high before a warning of a change in channels is received. When a signal warning of a change in the number of channels is received, switching circuit 82 is controlled so that the cut-off frequency is lowered. Accordingly, the attenuation provided by optical attenuator 64 is fixed at an average level. After the change in channels is completed, switching circuit 82 is controlled so that the cut-off frequency is switched again to be high.

0060For example, when monitor signal processing circuit 70 extracts and identifies a control signal which warns of a variation in the number of channels, control signal cs2 is supplied to switching circuit 82 so that the frequency characteristic of automatic level control circuit 66 is switched to a low frequency zone. As a result, the following performance for following a variation in the signal detected by photodiode (PD) 58₃ is lowered. That is, the constant-level control of the optical output is temporarily frozen (for example, the light transmissivity of optical attenuator 64 is maintained to be constant). Further, control signal cs1 corresponds to the number of channels to be included in the optical signal, and monitor signal processing circuit 70 supplies the control signal cs1 to reference voltage circuit 84. Reference voltage circuit 84 then supplies a reference voltage V_ref corresponding to the number of channels. Therefore, the total optical output power assumes a level matching the number of channels after the variation in the number of channels. For example, the reference voltage v_ref is changed such that, when a total of α channels are added to the total of N original channels, the total optical output becomes (N + α) x P.

0061Referring again to FIGS. 6 and 7, the value of the capacitance C_SWC may be large enough to freeze the operation of optical attenuator 64. Generally, this purpose may be achieved if, for example, the cut-off frequency f_c is dropped from 10 kHz to 0.01 Hz, thereby requiring a drop in the cut-off frequency f_c by the factor of 10,000 to 100,000. Such a large drop can be difficult to achieve.

0062Normally, the attenuation provided by optical attenuator 64 is varying from moment to moment to provide an ALC function and to compensate for a polarization variation. Therefore, abruptly fixing the attenuation of optical attenuator 64 at a certain level (such as when the number of channels are being changed) may cause problems. Instead, the attenuation is preferably maintained at an average level.

0063More specifically, FIGS. 8 and 9 are diagrams illustrating automatic level control circuit 66, according to additional embodiments of the present invention. Referring now to FIG. 8, a filter 90 for cutting off high frequencies (f_c:~10 KHz) and constructed of a capacitor and a resistor is provided between a switch 92 and transistor 80 so that the response of the automatic level control becomes adequate. For example, the time constant, typically on the order of sub-milliseconds, may be changed to the time constant on the order of 10-100 milliseconds.

0064When the cut-off frequency f_c is switched to the high-frequency zone, the filter response becomes quick so that a comparatively high-speed variation, such as a polarization variation, can be cancelled and the output of optical attenuator 64 is maintained constant.

0065More specifically, in FIG. 8, a latch circuit 94 which has a low-pass filter (f_c:~0.01 Hz) stores a voltage corresponding to an average level of the current in control element 86. During an ALC operation, switching of.the control loop occurs so that the control loop for controlling the drive current at a constant level is initiated. That is, when the switching of the control loop occurs, the voltage corresponding to the average level of the current is latched in latch circuit 94 so as to serve as a reference voltage. The term "average level" is used because the bias current has a time-dependent variation in order to maintain the level of the beam input to photodiode (PD) 58₃ at a constant level. More specifically, the voltage obtained by integration using a more extended integral time than that provided by the time constant of the normal control loop is latched in latch circuit 94.

0066Latch circuit 94 may be a circuit for reading the value of the driving current (provided by transistor 80) via an A/D converter, registering the read value and outputting the registered value via a D/A converter.

0067FIG. 9 is a combination of FIGS. 6 and 8. Referring now to FIG. 9, the capacitance C_SWC is switched by switching circuit 82 to cause the cut-off frequency f_c to be shifted to a low-frequency zone, to thereby slow the filter response. Thereupon, latch circuit 94 controls the attenuation to the average based on a monitored value.

0068More specifically, in FIG. 9, switching of the control loop is made to occur after increasing the time constant of the normal control loop according to the control illustrated in FIG. 6, so as to reduce an effect caused in the ALC characteristic as a result of the switching of the control loop.

0069As has been described above, monitor signal processing circuit 70 may receive a control signal for reporting completion of a variation in the number of channels after it receives a control signal for giving warning of a variation in the number of channels. Alternatively, however, monitor signal processing circuit 70 may not receive a control signal when the variation in the number of channels is complete. In this case, a timer (not illustrated) would be activated after the control signal for giving warning of a variation in the number of channels is extracted and identified.

0070The control signal cs2 returns switching circuit 82 to the original frequency characteristic after the control signal for reporting a completion of a variation in the number of channels is received, or after a predetermined period of time has passed. Thereupon, the constant optical output control is resumed in accordance with the new reference voltage V_ref set by reference voltage circuit 84.

0071The control for maintaining the total optical output at a constant level that corresponds to the number of channels may be resumed in a gradual manner. For example, the output signal of photodiode (PD) 58₃ may be input to operational amplifier 78 via a time constant circuit 96, or reference voltage V_ref may be gradually varied to assume a level that corresponds to the number of channels.

0072While the above-described arrangement ensures that the frequency characteristic is switched as a result of the control effected by switching circuit 82 so that the constant-level control of the optical output is frozen, it is also possible to hold the signal output by photodiode (PD) 58₃ when the control signal for giving warning of a variation in the number of channels is extracted and identified. In this instance, the held value is input to operational amplifier 78 so that the constant-level control of the optical output is frozen. Other arrangements for freezing the constant-level control of the optical output are also possible. While it is assumed that the electrically-controlled optical device part is constructed using optical attenuator 64, a semiconductor optical amplifier can be used instead of optical attenuator 64. The semiconductor optical amplifier should have a small wavelength dependence. By controlling the semiconductor optical amplifier, the total optical output may be controlled at a constant level.

0073FIG. 10 is a diagram illustrating an optical amplifying apparatus and optical transmission system according to an embodiment of the present invention. Referring now to FIG. 10, the optical amplifying apparatus includes first part 1000, second part 2000 and a third part 3000. Third part 3000 includes a rare-earth-doped optical fiber (EDF) 52₂, an optical branching coupler 54₄, an optical wavelength multiplexing coupler 56₂, optical isolators 55₃ and 55₄, a photodiode (PD) 58₅, a pump laser diode (LD) 59₂ and an automatic gain control circuit (AGC) 60₂. Third part 3000 also shares optical branching coupler 54₃ and the photodiode (PD) 58₃ with second part 2000.

0074As with first part 1000, third part 3000 controls an optical gain to be at a constant level. More specifically, second part 2000 controls the power level of the wavelength-multiplexed optical signal received by third part 3000 to be at a constant power level. As a result, the optical output power level of third part 3000 is also maintained at a constant power level. Even when the optical signal level is attenuated by optical attenuator 64 of second part 2000, amplification provided by third part 3000 ensures that a desired total optical output is obtained.

0075Therefore, pump laser diode 59₁ of first part 1000 and pump laser diode 59₂ of third part 3000 can each have a relatively small capacity, thereby reducing the cost and stabilization of the amplifying apparatus.

0076Although FIG. 10 shows second part 2000 and third part 3000 sharing optical branching coupler 54₃ and photodiode (PD) 58₃, it is also possible to provide a separate optical branching coupler and a separate photodiode in each of the second part 2000 and the third part 3000.

0077Automatic gain control circuits 60₁ and 60₂ may have the same configuration. Moreover, the optical gains provided by first part 1000 and third part 3000 may be identical. Alternatively, the gains may be varied according to the characteristics of a transmission optical fiber used in third part 3000.

0078In the event of a variation in the number of channels, the optical attenuation provided by optical attenuator 64 is frozen directly by monitor signal processing circuit 70, or by monitor signal processing circuit 70 controlling automatic level control circuit 66. Similar to the embodiment shown in FIG. 3, it is ensured that a variation in the optical output in response to a variation in the number of channels is restricted so that non-linear degradation and S/N ratio degradation are reduced.

0079FIG. 11 is a diagram illustrating an optical amplifying apparatus, according to a further embodiment of the present invention. Referring now to FIG. 11, the optical amplifying apparatus includes first part 1000, second part 2000 and third part 3000, which are the same as that show in FIG. 10. However, the optical amplifying apparatus in FIG. 11 also includes an automatic level control (ALC) correction circuit 98 for controlling and correcting automatic level control circuit 66 of second part 2000.

0080More specifically, a portion of the wavelength-multiplexed optical signal output by optical attenuator 64 is branched by optical branching coupler 54₃, converted into an electrical signal by photodiode (PD) 58₃ and input to automatic level control circuit 66. Automatic level control circuit 66 controls optical attenuator 64 so that the total optical output power of the wavelength-multiplexed optical signal is maintained at a constant level. However, the optical output power of the output wavelength-multiplexed optical signal in third part 3000 is not fed to automatic level control circuit 66. Therefore, it cannot be ensured that the total optical output in the third.part 3000 is maintained within a predetermined range.

0081Accordingly, a portion of the output wavelength multiplexed optical signal in the third part 3000 is converted into an electrical signal by photodiode (PD) 58₅ and input to ALC correction circuit 98 as well as to automatic gain control circuit 60₂. ALC correction circuit 98 determines whether or not the total optical output power is maintained within the predetermined range. If the total optical output power is not within the predetermined range, ALC correction circuit 98 controls automatic level control circuit 66 which, in turn, controls optical attenuator 64 to maintain the total optical output power within the predetermined range. If a semiconductor optical amplifier is used in place of optical attenuator 64, automatic level control circuit 66 controls the gain of the semiconductor optical amplifier so that the total optical output in third part 3000 is maintained within the predetermined level.

0082FIG. 12 is a diagram illustrating an optical amplifying apparatus, according to an embodiment of the present invention. The optical amplifying apparatus in FIG. 12 is a combination of the optical amplifying apparatuses in FIGS. 10 and 11.

0083Referring now to FIG. 12, in the event of a variation in the number of channels, monitor signal processing circuit 70 temporarily freezes the control effected by second part 2000 for controlling the optical output at a constant level, so that a variation in the optical output is reduced. Further, ALC correction circuit 98 controls automatic level control circuit 66 so as to maintain the total optical output power in third part 3000 within a predetermined range.

0084FIG. 13 is a diagram illustrating an optical amplifying apparatus, according to an additional embodiment of the present invention. The optical amplifying apparatus in FIG. 13 operates in a similar manner as previously described embodiments of the present invention, but also includes an optical branching coupler 54₅, a photodiode (PD) 58₆, a dispersion compensation fiber (DCF) 100 and a dispersion compensation fiber (DCF) loss correction circuit 102. Optical branching coupler 54₅ and photodiode (PD) 58₆ can be considered to be included in third part 3000.

0085Dispersion compensation fiber 100 is connected between second part 2000 and third part 3000. DCF loss correction circuit 102 controls automatic level control circuit 66. In a long-distance, high-capacity, wavelength-multiplexing optical transmission system, dispersion compensation in relation to the dispersion level of the transmission optical fiber and the wavelength-multiplexed optical signal is necessary. For this reason, dispersion compensation fiber 100 is provided.

0086However, insertion loss due to a dispersion compensation optical fiber can cause problems. More specifically, a variation in a loss due to the dispersion compensation optical fiber causes a variation in the optical output of repeaters which include wavelength-multiplexed optical fiber amplifiers.

0087Therefore, by measuring a loss due to dispersion compensation fiber 100 and setting automatic level control circuit 66 so as to compensate for the loss, optical attenuator 64 is controlled to provide a constant optical output. The loss due to dispersion compensation optical fiber 100 is likely to vary depending on a level of dispersion compensation. Accordingly, even with the constant optical output control effected by automatic level control circuit 66, the level of the wavelength-multiplexed optical signal input to third part 3000 may vary.

0088Therefore, a portion of the wavelength-multiplexed optical signal output by dispersion compensation optical fiber 100 and branched by optical branching coupler 54₅ is converted into an electrical signal by photodiode (PD) 58₆. The electrical signal is input to DCF loss correction circuit 102 as well as to automatic gain control circuit 60₂. DCF loss correction circuit 102 determines whether or not the level of the wavelength-multiplexed optical signal output by dispersion compensation fiber 100 is within a predetermined range. If the level is outside the predetermined range, DCF loss correction circuit 102 supplies a correction signal to automatic level control circuit 66. For example, the reference voltage (set voltage) for constant control of the optical output is corrected such that the optical output power is within the predetermined range. Therefore, a variation in insertion loss that results in a construction where dispersion compensation fiber 100 compensates for the dispersion in the transmission optical fiber is corrected, and a predetermined output level of the amplified wavelength-multiplexed optical signal is obtained.

0089FIG. 14 is a diagram illustrating an optical amplifying apparatus, according to an additional embodiment of the present invention. Referring now to FIG. 14, when monitor signal processing circuit 70 extracts and identifies a control signal for giving warning of a variation in the number of channels, the operation of optical attenuator 64 is frozen (that is, the transmissivity or the attenuation is maintained to be constant), so that a rapid variation in the optical signal level is restricted. DCF loss correction circuit 102 controls automatic level control circuit 66 so as to correct a loss that varies depending on the level of dispersion compensation provided by dispersion compensation fiber 100. Thus, the level of the wavelength-multiplexed optical signal input to third part 3000 is maintained within a predetermined range.

0090FIG. 15 is a diagram illustrating an optical amplifying apparatus, according to a further embodiment of the present invention. Referring now to FIG. 15, dispersion compensation fiber 100 compensates for dispersion in the transmission optical fiber, DCF loss correction circuit 102 corrects a variation in the loss depending on the level of compensation provided by dispersion compensation fiber 100, and ALC correction circuit 98 controls automatic level control circuit 66 so as to maintain the level of the output wavelength-multiplexed optical signal in third part 3000 within a predetermined range. Thus, the wavelength-multiplexed optical signal in the wavelength-multiplexed optical transmission system is amplified, relayed and transmitted in a stable manner.

0091FIG. 16 is a diagram illustrating an optical amplifying apparatus, according to a still further embodiment of the present invention. Referring now to FIG. 16, monitor signal processing circuit 70 controls optical attenuator 64 or automatic level control circuit 66 upon extracting and identifying a control signal for giving warning of a variation in the number of channels, so as to freeze constant-level control of the optical output. In this manner, a rapid variation in the level of the optical output is restricted.

0092Further, DCF loss correction circuit 102 controls automatic level control circuit 66 so as to correct a variation in the loss that depends on the level of dispersion provided by dispersion compensation optical fiber 100. ALC correction circuit 98 controls automatic level control circuit 66 so as to maintain the output wavelength-multiplexed optical signal in third part 3000 within a predetermined range.

0093FIG. 17 is a diagram illustrating modification to the optical amplifying apparatus illustrated in FIG. 16, according to an embodiment of the present invention. More specifically, in FIG. 17, an optical filter A1 is provided between the output of optical isolator 55₂ and optical branching coupler 54₂, at the input of photodiode (PD) 58₂. Also, an optical filter A2 is provided between the output of optical isolator 55₄ and optical branching coupler 54₄, at the input of photodiode (PD) 58₅. Optical filters A1 and A2 are optical filters as disclosed, for example, in <patcit id="pcit0003" dnum="US655027A" dnum-type="L"><text>U.S. patent application 08/655,027</text></patcit>, for correcting wavelength dependency of the gain.

0094FIG. 18(A) is a graph illustrating gain versus wavelength characteristics of rare-earth-doped optical fiber (EDF) 52₂ in FIG. 17, FIG. 18(B) is a graph illustrating the transmissivity versus wavelength of optical filter A2 in FIG. 17, and FIG. 18(C) is a graph illustrating overall gain of rare-earth-doped optical fiber (EDF) 52₂ and optical filter A2 in FIG. 17, according to an embodiment of the present invention.

0095If, for example, rare-earth-doped optical fiber (EDF) 52₂ has a wavelength-dependent gain characteristic as shown in FIG. 18(A), wherein the gain is higher in the long wavelength range, providing a gain correction optical filter A2 at the input of photodiode (PD) 58₅ ensures that the amplifier has an even gain with respect to wavelength. Providing optical filter A2 ensures that photodiode (PD) 58₅ receives the corrected multi-wavelength signal so that the unfavorable sensitivity characteristic, wherein the signal sensitivity is low in the short wavelength range and high in the long wavelength range, is corrected. Optical filters A1 and/or A2 may or may not be provided, depending on the use- of rare-earth-doped optical fibers (EDF) 52₁ and 52₂.

0096FIG. 19 is a diagram illustrating an optical amplifying apparatus, according to an embodiment of the present invention. Referring now to FIG. 19, the positioning of the first part 1000 and the second part 2000 are essentially switched. Therefore, a wavelength-multiplexed optical signal is controlled to have a constant power level by second part 2000, and is then controlled by first part 1000 to have a constant gain.

0097More specifically, an input wavelength-multiplexed optical signal is transmitted to optical attenuator 64. The wavelength-multiplexed optical signal output from optical attenuator 64 is transmitted to rare-earth-doped optical fiber 52₁ via optical isolator 55₁ and optical wavelength multiplexing coupler 56₁. The amplified wavelength-multiplexed optical signal is output via optical isolator 55₂ and optical branching coupler 54₂.

0098A portion of the wavelength-multiplexed optical signal branched by optical branching coupler 54₁ is converted into an electrical signal by photodiode 58₁ and fed to automatic level control circuit 66 and automatic gain control circuit 60₁. Automatic level control circuit 66 controls the optical attenuation provided by optical attenuator 64 so that the wavelength-multiplexed optical signal has its level controlled to be within a predetermined range and is then transmitted to first part 1000.

0099A portion of the wavelength-multiplexed optical signal branched by optical branching coupler 54₂ is converted into an electrical signal by photodiode 58₂ and transmitted to automatic gain control circuit 60₁. Automatic gain control circuit 60₁ controls pump laser diode 59₁ so that a ratio between a level of the wavelength-multiplexed optical signal input to, and output from, rare-earth-doped optical fiber 52₁ is maintained at a constant level.

0100Therefore, second part 2000 causes the power level of the wavelength-multiplexed optical signal to be constant even when a signal input via a transmission optical fiber varies greatly. As a result, a wavelength-multiplexed optical signal having a constant level is input to first part 1000. Accordingly, automatic gain control circuit 60₁ may have a small control zone and a relatively simple construction. Further, since the power level of the optical signal input to rare-earth-doped optical fiber 52₁ is prevented from exceeding a predetermined level, it is not necessary to raise the level of the pump laser beam supplied by pump laser diode 59₁. That is, pump laser diode 59₁ may have a small capacity.

0101FIG. 20 is a diagram illustrating an optical amplifying apparatus, according to an additional embodiment of the present invention. The optical amplifying apparatus illustrated in FIG. 20 is similar to the optical amplifying apparatus in FIG. 19, but also includes optical branching coupler 54₃, photodiode (PD) 58₃ and monitor signal processing circuit 70.

0102Referring now to FIG. 20, a wavelength-multiplexed optical signal supplied via a transmission optical fiber is input to variable optical attenuator 64 and has a portion branched by optical branching coupler 54₃, converted into an electrical signal by photodiode 58₃ and input to monitor signal processing circuit 70.

0103A control signal for giving warning of a variation in the number of channels may be superimposed on the wavelength-multiplexed optical signal by amplitude modulation or transmitted on a dedicated control channel. Upon extracting and identifying the control signal for giving warning of a variation in the number of channels, monitor signal processing circuit 70 controls automatic level control circuit 66 and retains the optical attenuation provided by optical attenuator 64 at the current level (thereby freezing the operation of optical attenuator 64) so that the optical output power is no longer maintained at a constant level.

0104When the change in the number of channels is completed, monitor signal processing circuit 70 allows optical attenuator 64 to resume its control for maintaining the optical output power at a constant level. With this arrangement, it is possible to reduce or eliminate a rapid variation in the power level of the optical signal.

0105FIG. 21 is a diagram illustrating an optical amplifying apparatus, according to a further embodiment of the present invention. The optical amplifying apparatus illustrated in FIG. 21 is similar to the optical amplifying apparatus in FIG. 19, but includes ALC correction circuit 98.

0106ALC correction circuit 98 determines whether or not the power level of the output wavelength-multiplexed optical signal is within a predetermined range. If the power level is not within the predetermined range, ALC correction circuit 98 controls automatic level control circuit 66 so that the optical attenuation provided by optical attenuator 64 causes the output wavelength-multiplexed optical signal to have a power level within a predetermined range.

0107FIG. 22 is a diagram illustrating an optical amplifying apparatus, according to a still further embodiment of the present invention. The optical amplifying apparatus illustrated in FIG. 22 is a combination of the optical amplifying apparatuses illustrated in FIGS. 20 and 21.

0108Referring now to FIG. 22, ALC correction circuit 98 controls automatic level control circuit 66 so that the power level of the output wavelength-multiplexed optical signal is within a predetermined range. Upon extracting and identifying a control signal for giving warning of a variation in the number of channels, monitor signal processing circuit 70 freezes the automatic level control function so that the optical output power is not longer maintained at a constant level.

0109FIG. 23 is a diagram illustrating an optical amplifying apparatus, according to an embodiment of the present invention. Referring now to FIG. 23, instead of controlling (freezing) the optical attenuator 64 so as to provide a constant attenuation when the number of channels is varied, the optical amplifier as a whole is changed to the AGC mode when the number of channels is varied. Such a change can be achieved by controlling the ratio between the input to, and the output from, optical attenuator 64, to be at a constant level. Such an operation is tantamount to maintaining the gain G (0 ≤ G ≤ 1) of optical attenuator 64 or the light transmissivity of optical attenuator 64 at a constant level.

0110Therefore, in FIG. 23, a switch 104 is controlled by monitor signal processing circuit 70 to switch between automatic level control provided by automatic level control circuit 66 and automatic gain control provided by an automatic gain control circuit 60₃. More specifically, for example, as illustrated in FIG. 4(A), monitor signal processing circuit 70 causes switch 104 to select automatic level control circuit 66 prior to, and subsequent to, a variation in the number of channels. While the number of channels is being varied, monitor signal processing circuit 70 causes switch 104 to select automatic gain control circuit 60₃.

0111FIG. 23 also illustrates a laser diode (LD) 105 which is controlled by monitor signal processing circuit 70 to transmit information to downstream optical components, such as downstream optical repeaters. For example, as described in more detail further below, laser diode (LD) 105 can be used by monitor signal processing circuit 70 to transmit information to downstream optical components.

0112FIG. 24 is a more detailed diagram of the optical amplifying apparatus in FIG. 23. Referring now to FIG. 24, the operation is as follows: <ol id="ol0001" compact="compact"><li>(1) Normally (that is, when the number of channels are not being varied), switch 104 selects automatic level control circuit 66 so that the power level of light output from optical attenuator 64 is monitored and maintained at a constant level.</li><li>(2) When monitor signal processing circuit 70 receives a signal warning of a change in the number of channels, a gain monitoring signal 107 of automatic gain control circuit 60₃ is read so that an average gain (attenuation) with respect to a time constant on the order of 10 -100 ms is determined.</li><li>(3) A reference voltage V_AGC corresponding to the average gain determined in (2) is output from monitor signal processing circuit 70 to automatic gain control circuit 60₃.</li><li>(4) Switch 104 then selects automatic gain control circuit 60₃.</li><li>(5) Monitor signal processing circuit 70 receives information indicating the new number of channels to be included in the wavelength-multiplexed optical signal.</li><li>(6) Monitors signal processing circuit 70 provides to automatic level control circuit 66 a reference voltage V_ALC corresponding to the new number of channels.</li><li>(7) Monitor signal processing circuit 70 receives a signal indicating that the variation in the number of channels is complete. Alternatively, a predetermined period of time lapses from the receipt of the signal warning of the change in the number of channels.</li><li>(8) Switch 104 selects automatic level control circuit 66.</li></ol>

0113The relationship between an attenuation provided by optical attenuator 64 and a driving current of control element 86 provided by transistor 80 may depend on a parameter such as an operating temperature, but is generally a one-to-one relationship. Therefore, (2), above, may be replaced by a process whereby the driving current is monitored (with respect to the time constant on the order of 10 - 100 ms) so as to determine an average gain (attenuation) based on the monitored driving current. The driving current may be controlled so that its average level is maintained constant.

0114FIG. 25 is a diagram illustrating a fiber optical communication system employing an optical amplifying apparatus according to embodiments of the present invention. Referring now to FIG. 25, a transmitter (Tx) 108 transmits an SV light beam to a receiver (Rx) 110, where an SV light beam is light that is wavelength-multiplexed with a main signal. The main signal is used to transmit information downstream. An optical amplifier (O-AMP) 112 amplifies the SV light beam. Main signal control 114 and monitor signal processing 116 are performed.

0115FIG. 26 is a more detailed diagram illustrating an optical amplifying apparatus which includes optical amplifier 112, main signal control 114 and monitor signal processing 116 of FIG. 25. The optical amplifying apparatus in FIG. 26 is similar to the optical amplifying apparatus in FIG. 3, but includes laser diode (LD) 105 for sending an SV light beam downstream.

0116More specifically, monitor signal processing circuit 70 inserts, in the SV light beam, information indicating when the attenuation, or light transmissivity, of the optical attenuator 64 will be held constant, or "frozen". The SV light beam, carrying that information, is transmitted by laser diode (LD) 105 to the transmission line.

0117FIG. 27 is a diagram illustrating a transmission line employing a plurality of optical amplifying apparatuses, according to embodiments of the present invention. Referring now to FIG. 27, a wavelength-multiplexed optical communication system includes transmitters Tx 120, wavelength-multiplexed optical fiber amplifiers/repeaters OAMPs 122 and receivers Rx 124. When a variation in the number of channels is processed, all the OAMPs 122 in the upstream (or downstream) line in the system are set into a constant optical gain control.

0118A wavelength-multiplexed optical postamplifier (not illustrated) that may be provided in each transmitter Tx 120 and a wavelength-multiplexed optical preamplifier (not illustrated) that may be provided in each receiver Rx 124 are also set into a constant gain control. When all OAMPS 122 are in a constant gain control state, the power of an optical signal fed to a light receiving element in receivers Rx 124 may vary.

0119In a transmission line having optical amplifying apparatuses as illustrated in FIG. 25-27, it is possible to determine whether or not all the optical fiber amplifiers in the path managed by a receiving end (Rx) on the transmission line have their attenuation fixed and their optical gain maintained at a constant level. Once it is determined that all the optical fiber amplifiers have their optical gain maintained at a constant level, information indicating the same is sent to the transmitting end (Tx) via the backward path, whereupon a variation in the number of channels can be started.

0120The following is an example of the operation flow in a transmission line having optical amplifying apparatuses as illustrated in FIGS. 25-27, for processing a variation in the number of channels. <ol id="ol0002" compact="compact"><li>(1) A signal warning of a variation in the number of channels is issued from the upstream SV transmitting end (SVTx).</li><li>(2) Monitor signal processing circuit 70 of each OAMP receives the signal warning of the variation in the number of channels.</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 carrying that information on the monitor signal (an identification number for identifying the individual OAMPs is also inserted on the monitor signal).</li><li>(5) The upstream SV receiving end (SVRx) acknowledges that all of the upstream OAMPS are in the constant optical gain state.</li><li>(6) The downstream SV transmitting end (SVTx) announces that all the upstream OAMPs are in the constant optical gain state.</li><li>(7) The downstream SV receiving end (SVRx) acknowledges that all the upstream OAMPS are in the constant optical gain state.</li><li>(8) The upstream transmitting end (Tx) actually varies the number of channels.</li><li>(9) The upstream SV transmitting end (SVTx) issues information indicating that the variation in the number of channels is completed.</li><li>(10) The monitor signal processing circuit 70 in each DAMP receives the information indicating that the variation in the number of channels is completed.</li><li>(11) Each OAMP cancels the freezing operation for freezing the operation of the associated optical attenuator and proceeds to the constant optical output control.</li><li>(12) Each OAMP sends downstream information indicating that a shift to the constant optical output control is completed, in the form of the monitor 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 have processed the variation in the number of channels.</li><li>(14) The information indicating that all of the OAMPs have processed the variation in the number of channels is sent to the transmitting end.</li></ol>

0121FIG. 28 is a timing diagram illustrating the above-described operation flow.

0122Therefore, in the processing of the variation in the number of channels, a wavelength-multiplexed optical fiber amplifier is temporarily stopped from performing an automatic level control function and, instead, is made to perform a constant gain control function, or to cause the optical amplifying apparatus, as a whole, to perform a constant gain function.

0123However, in an optical communication system, it is usually necessary to maintain the power of an optical signal supplied to a light receiving element at a constant level. Although a variation in the input power due to polarization variation occurs under conventional circumstances, the control for maintaining the optical gain of the optical fiber amplifier at a constant level causes the power of the optical signal supplied to the light receiving element to vary.

0124This problem can be overcome by demultiplexing the optical signal into individual channels, and controlling the power level of the individual demultiplexed channels.

0125More specifically, FIG. 29 is a diagram illustrating a portion of an optical communication system, according to an embodiment of the present invention. Referring now to FIG. 29, a demultiplexer (DEMUX) 125 demultiplexes a wavelength-multiplexed optical signal into individual channels to be received by individual receivers 126. An optical preamplifier 127 and an automatic level control unit 128 is provided for each channel, so that the associated receiver 126 receives an optical signal at a constant power level.

0126According to the above embodiments of the present invention, an optical attenuator or an optical amplifier can be controlled to provide a constant gain while the number of channels in a wavelength-multiplexed optical signal are being varied. In this case, the gain G can be in the range (0 ≤ G ≤ 1). Thus, an optical attenuator can be controlled to provide a constant gain by maintaining a constant ratio between the input and the output of the optical attenuator.

0127According to the above embodiments of the present invention, a rare-earth doped optical fiber used in an optical amplifier, where the dopant is erbium (Er). However, the present invention is not intended to be limited to an erbium (Er) doped optical fiber. Instead, other rare-earth-doped optical fibers, such as a neodymium(Nd)-doped optical fiber or a praseodymium(Pd)-doped optical fiber, may also be used, depending on the wavelength involved. Further, for example, the various photodiodes disclosed herein can be replaced by phototransistors.

0128According to the above embodiments of the present invention, specific embodiments of automatic gain control circuits and automatic level control circuits are disclosed. However, the present invention is not intended to be limited to any specific circuit configuration for these circuits, or for other circuits disclosed herein. Instead, many different circuit configuration can be used.

0129Moreover, according to the above embodiments of the present invention, an optical attenuation is used to provide a variable attenuation. There are many different types of known optical attenuators, and the embodiments of the present invention are not intended to be limited to any specific type of optical attenuator.

0130Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles of the invention, the scope of which is defined in the claims and their equivalents.