Optical communication system with control of output level

The present invention relates to an optical communication system, and particularly to an optical communication system with an optical output level control function for controlling the optical output level of an access section in the optical transmission, and to a two-way optical communication system for carrying out 1×L two-way communications between one optical transmitting/receiving unit (master station) and L optical transmitting/receiving units (slave stations). Here, an optical signal transmitted from the master station to a slave station is called a downlink optical signal, and an optical signal transmitted from a slave station to the master station is called an uplink optical signal.

Figs. 1 to 5 each show an example of a conventional optical communication system with an optical transmitter. Fig. 1 shows a configuration of an optical communication system including an optical transmitter with an optical splitter for splitting an optical signal to optical signals with equal optical output levels, and optical receivers connected to the optical transmitter via optical fibers.

Fig. 2 shows a configuration of an optical communication system including an optical transmitter with an optical splitter for optically splitting an optical signal to signals with equal optical output levels, optical couplers connected to the optical transmitter via optical fibers, and optical receivers connected to the optical couplers via optical fibers. Fig. 3 shows a calculation example of a minimum optical reception level in Fig. 1, and Fig. 4 shows a calculation example of a minimum optical reception level in Fig. 2.

In the present specification, although optical signal is split by an optical transmitter, by a remote variable splitter or by an optical coupler, optical signal passing through the splitting do not always have the same optical levels at the optical receivers. In the present specification, the minimum level among the levels received by the optical receivers is called a minimum optical reception level.

The optical communication system of Fig. 1 includes an optical transmitter 201 with an optical splitter 2012 for splitting an optical signal fed from an optical fiber 2011 to N optical signals with the same optical output level, and N optical receivers 401, 402, ..., and 40N connected to the optical transmitter 201 via N optical fibers 301, 302, ..., and 30N.

Fig. 3 shows a calculation example of obtaining the minimum optical reception level Pin_min (dBm) in the optical communication system as shown in Fig. 1. Here, the optical receiver connected to the optical transmitter 201 via an ith (i = 1 to N) optical fiber is denoted as an optical receiver (i). Assume that the optical output level of the optical fiber 2011 is Pout (dBm), the transmission loss of the ith optical fiber is L(i) (dB), and the optical reception level of the optical receiver (i) is Pin(i) (dBm). All the coupling ratios of the optical output levels of the optical splitter 2012 are 1/N. This is because the optical splitter 2012 splits the input optical signal to the same output levels, and the number of the optical receivers connected to the optical splitter 2012, that is, the splitting number of the optical splitter 2012 is N.

The coupling ratio gives the optical loss of the optical splitter 2012 of 10log₁₀N (dB) per branch. Thus, the relationship between the optical output level Pout (dBm) and the optical reception level Pin(i) (dBm) at the optical receiver (i) is given by the following equation.<maths id="math0001" num="(1)"><math display="block"><mrow><mtext mathvariant="italic">Pout</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext mathvariant="italic">N-L</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>)=</mtext><mtext mathvariant="italic">Pin</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>) (</mtext><mtext mathvariant="italic">i</mtext><mtext>=1 to </mtext><mtext mathvariant="italic">N</mtext><mtext>)</mtext></mrow></math><img file="EP1306990A2_D0001.tif" /></maths>

Since the left-hand terms Pout and 10log₁₀N of equation (1) are constant for all the optical receivers, the optical reception level becomes minimum when L(i) is maximum. By denoting the minimum optical reception level as Pin_min (dBm), and the transmission loss when the optical reception level of the optical receiver is minimum as Lmax (dB), the following equation holds.<maths id="math0002" num="(2)"><math display="block"><mrow><mtext mathvariant="italic">Pout</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext mathvariant="italic">N</mtext><mtext>-</mtext><mtext mathvariant="italic">L</mtext><mtext>max=</mtext><mtext mathvariant="italic">Pin</mtext><mtext>_</mtext><mtext mathvariant="italic">min</mtext></mrow></math><img file="EP1306990A2_D0002.tif" /></maths>

Next, the configuration of Fig. 2 will be described. The optical communication system of Fig. 2 comprises: an optical transmitter 501 including an optical splitter 5012 for splitting the optical signal fed through an optical fiber 5011 to optical signals with the same optical output level; N optical couplers 701, 702,..., and 70N connected to the optical transmitter 501 via N optical fibers 601, 602, ..., and 60N; m₁ optical receivers 911, 912, ..., and 91m₁ connected to the first optical coupler 701 via m₁ optical fibers 811, 812, ..., and 81m₁; m₂ optical receivers 921, 922, ..., and 92m₂ connected to the second optical coupler 702 via m₂ optical fibers 821, 822, ..., and 82m₂; and m_N optical receivers 9N1, 9N2, ..., and 9Nm_N connected to Nth optical coupler 70N via m_N optical fibers 8N1, 8N2, ..., and 8Nm_N.

Fig. 4 shows a calculation example of obtaining the minimum optical reception level Pin_min (dBm) in the optical communication system as shown in Fig. 2. Here, the optical receiver connected to the optical transmitter 501 via an ith optical fiber and jth optical fiber (j = 1 to m_i) is denoted as an optical receiver (i,j) (i = 1 to N,j = 1 to m_i).

Assume that the optical output level of the optical fiber 5011 is Pout (dBm), the total transmission loss of the ith and jth optical fibers is L(i,j) (dB), and the optical reception level of the optical receiver (i,j) is Pin(i,j) (dBm). All the coupling ratios of the optical output levels of the optical splitter 5012 are 1/N.

This is because the optical splitter 5012 splits the input optical signal to the same output levels, and the number of the optical couplers connected to the optical splitter 5012, that is, the splitting number of the optical splitter 5012 is N. Thus, the coupling ratio gives the optical loss of the optical splitter 5012 of 10log₁₀N (dB) per branch. In addition, since the optical couplers 701, 702, ..., and 70N each have the splitting number m_i, and the optical couplers each split the optical input into the same output levels, the optical loss of the optical couplers 701, 702, ..., and 70N is given by 10log₁₀m_i (dB) per branch.

Accordingly, the relationship between the optical output level Pout (dBm) and the optical reception level Pin(i,j) (dBm) at the optical receiver (i,j) is given by the following equation.<maths id="math0003" num="(3)"><math display="block"><mrow><mtext mathvariant="italic">Pout</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext mathvariant="italic">N</mtext><msub><mrow><mtext>-{10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">m</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msub><mtext>+</mtext><mtext mathvariant="italic">L</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>,</mtext><mtext mathvariant="italic">j</mtext><mtext>)}=</mtext><mtext mathvariant="italic">Pin</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>,</mtext><mtext mathvariant="italic">j</mtext><mtext>) (</mtext><mtext mathvariant="italic">i</mtext><mtext>=1 to </mtext><mtext mathvariant="italic">N</mtext><mtext>, </mtext><mtext mathvariant="italic">j</mtext><mtext>=1 to </mtext><msub><mrow><mtext mathvariant="italic">m</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msub><mtext>)</mtext></mrow></math><img file="EP1306990A2_D0003.tif" /></maths>

Since the left-hand terms Pout and 10log₁₀N of equation (3) are constant for all the optical receivers, the optical reception level becomes minimum when 10log₁₀m_i + L(i,j) is maximum. By denoting (i,j) when the optical reception level of the optical receiver is minimum as (u, v), the following equation holds.<maths id="math0004" num="(4)"><math display="block"><mrow><mtext mathvariant="italic">Pout</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext mathvariant="italic">N</mtext><msub><mrow><mtext>- {10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">m</mtext></mrow><mrow><mtext mathvariant="italic">u</mtext></mrow></msub><mtext> +</mtext><mtext mathvariant="italic">L</mtext><mtext>(</mtext><mtext mathvariant="italic">u</mtext><mtext>,</mtext><mtext mathvariant="italic">v</mtext><mtext>)} =</mtext><mtext mathvariant="italic">Pin_min</mtext></mrow></math><img file="EP1306990A2_D0004.tif" /></maths>

As described above, the optical reception levels of the optical receivers of the conventional optical communication system depend on the transmission loss L(i), splitting number and transmission loss {10log₁₀m_i + L(i,j)}.

The optical communication systems as shown in Figs. 1 and 2, however, have a problem of high cost. This is because in the optical communication systems as shown in Figs. 1 and 2, it is necessary for the optical receivers with smaller optical reception levels to be more sensitive than the optical receivers with greater optical reception levels to receive the lower intensity light.

Thus, the optical communication systems as shown in Figs. 1 and 2 have the problem of increasing the cost because of the highly sensitive optical receivers.

Furthermore, the optical communication systems as shown in Figs. 1 and 2 have another problem of having low output efficiency of the optical signals as the optical communication systems in their entirety. This is because receiving the light with the intensity more than necessary, the optical receivers with higher optical reception levels in the optical communication systems as shown in Figs. 1 and 2 waste the light as the total optical communication systems.

Fig. 5A shows a conventional two-way optical communication system including one optical transmitting/receiving unit 1 and L optical transmitting/receiving units 2-1 to 2-L, which are connected in a 1×L fashion via an optical fiber 3-0, 1×L optical coupler 4, and L optical fibers 3-1 to 3-L. The optical coupler 4 splits a downlink optical signal with a wavelength λd transmitted from the optical transmitting/receiving unit 1 to L parts, and transmits them to the optical transmitting/receiving units 2-1 to 2-L. Reversely, the optical coupler 4 combines uplink optical signals with a wavelength λu transmitted from the optical transmitting/receiving units 2-1 to 2-L, and transmits the combined signal to the optical transmitting/receiving unit 1.

The optical coupler 4 has equal 1/L coupling ratios so that the downlink optical signal through the optical fiber 3-0 is split to the optical fibers 3-1 to 3-L at the same level, and the uplink optical signals via the optical fibers 3-1 to 3-L are combined to the optical fiber 3-0 at the level with the same coupling loss subtracted.

However, the optical fibers 3-1 to 3-L connecting the optical coupler 4 with L optical transmitting/receiving units 2-1 to 2-L as shown in Fig. 5A have different transmission losses in accordance with the transmission distances, though the optical coupler 4 has the equal coupling ratios. Accordingly, although the downlink optical signals have the same level at the input terminals of the optical fibers 3-1 to 3-L as illustrated in Fig. 5B, the optical transmitting/receiving units 2-1 to 2-L have different optical reception levels because of the variations in the transmission loss. On the other hand, as for the uplink optical signals, even if they have variations in the optical levels at the output terminals of the optical fibers 3-1 to 3-L, they are combined as they are and received by the optical transmitting/receiving unit 1. Specifically, as illustrated in Fig. 5C, the optical reception levels of the signals sent from the optical transmitting/receiving units 2-1 to 2-L differ at the optical transmitting/receiving unit 1.

Therefore the optical transmitting/receiving unit 1 and optical transmitting/receiving units 2-1 to 2-L each require a highly sensitive, wide dynamic range photo-detection circuit that can cope with the variations in the optical reception levels and the minimum optical reception level, thereby increasing the system cost.

As a technique to circumvent such problems, Japanese Patent Application Laid-open No. 4-269023 (1992) discloses a method in which slave stations transmit constant level signals to enable a master station to adjust optical attenuation and to transmit different level signals to the slave stations. In addition, Japanese Patent Application Laid-open No. 11-136192 (1999) discloses a method in which a master station adjusts the transmission level for each slave station so that the reception levels of the individual slave stations with different transmission losses become equal.

However, configuring a 1×L optical coupler 4 by combining Mach-Zehnder interferometer type 1×2 optical couplers, for example, presents a problem of the wavelength dependence of the individual 1×2 optical couplers. Specifically, the equal coupling ratio for the downlink optical signals with the wavelength λd is not always equal for the uplink optical signals with the wavelength λu. Furthermore, when the downlink optical signals and the uplink optical signals are wavelength division multiplexed optical signals including multiple wavelengths, the coupling ratios are never equal for the wavelength division multiplexed optical signals because the coupling ratios vary in accordance with the individual wavelengths.

The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide an optical communication system with an optical output level control function capable of increasing the minimum optical reception level by controlling the optical output levels of its optical transmitter to promote economy of its optical receivers, and by extension of the entire optical communication system, or to provide an optical communication system with an optical output level control function capable of increasing the number of connectable optical receivers.

Another object of the present invention to provide a two-way optical communication system capable of reducing the dynamic range and sensitivity of photo-detectors in the two-way optical communication system that includes optical couplers for coupling different wavelengths optical signals for its downlink and uplink, or wavelength division multiplexed optical signals, by carrying out the control in such a manner that makes the difference between the maximum optical reception level and minimum optical reception level of each optical transmitting/receiving unit less than a specified value, or that makes its minimum optical reception level more than a specified value.

According to a first aspect of the present invention, there is provided an optical communication system comprising: an optical transmitter including an optical variable splitting controller for splitting received light into N parts, and supplying them to N optical fibers, where N is an integer greater than one; and N optical receivers for receiving the output light of the optical transmitter via the N optical fibers, wherein the optical variable splitting controller regulates its output levels to be delivered to the optical receivers independently by controlling its optical coupling ratios.

With the foregoing configuration in accordance with the present invention, the optical variable splitting controller controls its output levels in such a manner that it does not supply the optical signals with levels more than necessary to the optical receivers connected via the optical fibers with a smaller transmission loss. At the same time, the optical variable splitting controller increases the optical output levels for the optical receivers connected via the optical fibers with a larger transmission loss by exploiting the extra power obtained by reducing the power for the optical receivers connected via the optical fibers with the smaller transmission loss, thereby increasing the optical reception levels of the optical receivers. Thus, the minimum optical reception level of the optical receivers can be increased.

In the first aspect of the present invention, the optical variable splitting controller may regulate its optical output levels by controlling its optical coupling ratios such that optical losses from an input of the optical variable splitting controller to inputs of the N optical receivers become equal for all the N optical receivers.

According to a second aspect of the present invention, there is provided an optical communication system comprising: an optical transmitter including an optical variable splitting controller for splitting received light into N parts, and supplying them to N optical fibers, where N is an integer greater than one; N optical couplers for receiving outputs of the optical transmitter via the N optical fibers, each of the N optical couplers delivering its received signal to m_i optical fibers, where suffix i varies from one to N; and m_i optical receivers for receiving outputs of one of the N optical couplers via the m_i optical fibers, wherein the optical variable splitting controller regulates its output levels to be delivered to the optical receivers independently by controlling its optical coupling ratios.

With the foregoing configuration in accordance with the present invention, the optical variable splitting controller controls its output levels in such a manner that it does not supply the optical signals with levels more than necessary to the optical receivers connected via the optical fibers with a smaller transmission loss. At the same time, the optical variable splitting controller increases the optical output levels for the optical receivers connected via the optical fibers with a larger transmission loss by exploiting the extra power obtained by reducing the power for the optical receivers connected via the optical fibers with the smaller transmission loss, thereby increasing the optical reception levels of the optical receivers. Thus, the minimum optical reception level of the optical receivers can be increased.

In the second aspect of the present invention, as for N optical receivers, each of which has a maximum optical loss from an input of the one of the optical couplers to inputs of the m_i optical receivers connected to the one of the optical couplers, the optical variable splitting controller may regulate its optical output levels by controlling its optical coupling ratios such that optical losses from an input of the optical variable splitting controller to inputs of the N optical receivers become equal for all of the N optical receivers.

According to a third aspect of the present invention, there is provided an optical communication system comprising: an optical transmitter including an optical splitter for splitting received light into N parts with equal output levels, and supplying them to N optical fibers, where N is an integer greater than one; N remote variable splitters for receiving outputs of the optical transmitter via the N optical fibers, each of the N remote variable splitters including an optical variable splitting controller that splits a received optical signal to m_i parts and supplies them to m_i optical fibers, where suffix i varies from one to N; and m_i optical receivers for receiving outputs of one of the N remote variable splitters via the m_i optical fibers, wherein the optical variable splitting controller regulates its output levels to be delivered to the optical receivers independently by controlling its optical coupling ratios.

With the foregoing configuration in accordance with the present invention, the optical variable splitting controller controls its output levels in such a manner that it does not supply the optical signals with levels more than necessary to the optical receivers connected via the optical fibers with a smaller transmission loss. At the same time, the optical variable splitting controller increases the optical output levels for the optical receivers connected via the optical fibers with a larger transmission loss by exploiting the extra power obtained by reducing the power for the optical receivers connected via the optical fibers with the smaller transmission loss, thereby increasing the optical reception levels of the optical receivers. Thus, the minimum optical reception level of the optical receivers can be increased.

In the third aspect of the present invention, the optical variable splitting controller may regulate its optical output levels by controlling its optical coupling ratios such that optical losses from an input of one of the remote variable splitters to inputs of the m_i optical receivers become equal for all m_i optical receivers.

According to a fourth aspect of the present invention, there is provided a two-way optical communication system comprising: a first optical transmitting/receiving unit; L second optical transmitting/receiving units, where L is an integer greater than one; and a 1×L optical coupler connected to the first optical transmitting/receiving unit and to the L second optical transmitting/receiving units via optical fibers, the 1×L optical coupler splitting a downlink optical signal with a wavelength λd transmitted from the first optical transmitting/receiving unit into L parts and transmitting them to the second optical transmitting/receiving units, and combining uplink optical signals with a wavelength λu transmitted from the second optical transmitting/receiving units and transmitting the combined signal to the first optical transmitting/receiving unit, wherein the optical coupler includes means for controlling coupling ratios for the second optical transmitting/receiving units such that a difference between a minimum optical reception level and a maximum optical reception level of the downlink optical signals arriving at the second optical transmitting/receiving units is less than a specified value, and that a difference between a minimum optical reception level and a maximum optical reception level of the uplink optical signals arriving at the first optical transmitting/receiving unit from the second optical transmitting/receiving units is less than a specified value.

According to a fifth aspect of the present invention, there is provided a two-way optical communication system comprising: a first optical transmitting/receiving unit; L second optical transmitting/receiving units, where L is an integer greater than one; and a 1×L optical coupler connected to the first optical transmitting/receiving unit and to the L second optical transmitting/receiving units via optical fibers, the 1×L optical coupler splitting a downlink optical signal with a wavelength λd transmitted from the first optical transmitting/receiving unit into L parts and transmitting them to the second optical transmitting/receiving units, and combining uplink optical signals with a wavelength λu transmitted from the second optical transmitting/receiving units and transmitting the combined signal to the first optical transmitting/receiving unit, wherein the optical coupler includes means for controlling coupling ratios for the second optical transmitting/receiving units such that a minimum optical reception level of the downlink optical signals arriving at the second optical transmitting/receiving units is greater than a specified value, and that a minimum optical reception level of the uplink optical signals arriving at the first optical transmitting/receiving unit from the second optical transmitting/receiving units is greater than a specified value.

In the fourth aspect of the present invention, the optical coupler may determine its coupling ratios such that the difference between the minimum optical reception level and the maximum optical reception level becomes less than the specified value in accordance with a premeasured optical transmission level of the first optical transmitting/receiving unit and premeasured optical reception levels of the second optical transmitting/receiving units.

In the fifth aspect of the present invention, the optical coupler may determine its coupling ratios such that the minimum optical reception level of the downlink optical signals and the minimum optical reception level of the uplink optical signals each become greater than the specified value in accordance with a premeasured optical transmission level of the first optical transmitting/receiving unit, premeasured optical reception levels of the second optical transmitting/receiving units, and premeasured optical transmission levels of the second optical transmitting/receiving units.

In the fourth or fifth aspect of the present invention, optical coupler may determine its coupling ratios such that the difference between the minimum optical reception level and the maximum optical reception level becomes less than the specified value in accordance with premeasured optical transmission levels of the uplink optical signals of the second optical transmitting/receiving units, and optical reception levels of the uplink optical signals measured at the optical coupler.

In the fourth or fifth aspect of the present invention, optical coupler may determine its coupling ratios such that the minimum optical reception level becomes greater than the specified value in accordance with premeasured optical transmission levels of the uplink optical signals of the second optical transmitting/receiving units, and optical reception levels of the uplink optical signals measured at the optical coupler.

In the fourth or fifth aspect of the present invention, the downlink optical signals may be a wavelength division multiplexed optical signal that multiplexes at least two wavelengths, wherein the second optical transmitting/receiving units each receive optical signals with one or more wavelengths from the wavelength division multiplexed optical signals, and the optical coupler may determine the coupling ratios considering wavelength dependence of the coupling ratios.

In the fourth or fifth aspect of the present invention, the second optical transmitting/receiving units may each transmit an optical signal with one or more wavelengths as the uplink optical signal, and the optical coupler may determine the coupling ratios considering wavelength dependence of the coupling ratios.

According to the present invention, since the optical communication system with an optical output level control function can be implemented, the minimum optical reception level of the optical receivers can be established at a higher value in the optical communication system. In addition, implementing the present invention enables increasing the number of the connectable optical receivers, thereby being able to effectively utilizing the light that is wasted in the conventional systems. Thus, it can encourage economy of the optical communication system in its entirety.

Furthermore, according to the present invention, setting the coupling ratios in accordance with the wavelengths used by optical variable couplers can make the difference between the maximum optical reception level and the minimum optical reception level of the individual optical transmitting/receiving units less than the specified value, or make the minimum optical reception level more than the specified value. As a result, it can reduce the dynamic range of the individual optical transmitting/receiving units and the sensitivity of their photo-detectors.

The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of exemple embodiments thereof taken in conjunction with the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a block diagram showing a configuration of a conventional optical communication system;</li><li>Fig. 2 is a block diagram showing a configuration of another conventional optical communication system;</li><li>Fig. 3 is a block diagram showing a calculation example of the minimum optical reception level in the optical communication system as shown in Fig. 1;</li><li>Fig. 4 is a block diagram showing a calculation example of the minimum optical reception level in the optical communication system as shown in Fig. 2;</li><li>Fig. 5A is a block diagram showing a configuration of a conventional two-way optical communication system;</li><li>Figs. 5B and 5C are diagrams illustrating optical losses in the conventional two-way optical communication system as shown in Fig. 5A.</li><li>Fig. 6 is a block diagram showing the entire configuration of an embodiment of the optical communication system in accordance with the present invention;</li><li>Fig. 7 is a diagram illustrating an optical receiver distribution model in the optical communication system as shown in Fig. 6;</li><li>Fig. 8 is a block diagram showing the entire configuration of an embodiment of the optical communication system in accordance with the present invention;</li><li>Fig. 9 is a diagram illustrating an optical receiver distribution model in the optical communication system as shown in Fig. 8;</li><li>Fig. 10 is a block diagram showing the entire configuration of an embodiment of the optical communication system in accordance with the present invention;</li><li>Figs. 11A and 11B are diagrams showing a configuration of an optical variable splitting controller in the optical communication system in accordance with the present invention;</li><li>Fig. 12 is a block diagram showing a configuration of an embodiment of a two-way optical communication system in accordance with the present invention;</li><li>Fig. 13 is a diagram illustrating a control example of a 1×2 optical variable coupler; and</li><li>Fig. 14 is a diagram illustrating another control example of the 1×2 optical variable coupler.</li></ul>

The preferred embodiments of the optical communication system in accordance with the invention will now be described with reference to Figs. 6 to 14.

FIRST EMBODIMENT

As shown in Fig. 6, the first embodiment of the optical communication system in accordance with the present invention comprises: an optical transmitter 11 including an optical variable splitting controller 112 for splitting the optical signal passing through an optical fiber 111 with regulating the optical output levels by controlling the optical coupling ratios; and N optical receivers 31, 32, ..., and 3N connected to the transmitter 11 via N optical fibers 21, 22, ..., and 2N.

In the present embodiment 1, receiving the optical signal from the optical fiber 111, the optical transmitter 11 controls the optical output levels (optical levels to be supplied to the individual optical fibers 21 to 2N) independently of all the optical signals to be sent to the individual optical receivers 31 to 3N via the individual optical fibers 21 to 2N. Thus, in the present embodiment 1, the output levels of the optical signals can differ from each other for respective optical receivers, that is, for the individual optical fibers 21 to 2N connected to the optical receivers 31 to 3N.

It is preferable that the optical transmitter 11 control the optical output levels such that the optical losses from the input of the optical variable splitting controller to the inputs of the N optical receivers become equal for all the N optical receivers. To explain this, let us compare the minimum optical reception level of the conventional example as shown in Fig. 1 with that of the present embodiment. In the following description, the optical receiver connected to the optical transmitter via the ith (i = 1 to N) optical fiber is denoted as an optical receiver (i).

Assume that the optical output level of the preceding stage of the optical variable splitting controller is Pout (dBm), the optical loss from the output of the optical variable splitting controller associated with the optical receiver (i) to the optical receiver is L(i) (dB), and the optical reception level of the optical receiver (i) is Pin(i) (dBm). In addition, assume that the coupling ratios of the optical output levels of the optical variable splitting controller associated with the optical receivers (i) are given by the following equation.<maths id="math0005" num=""><img file="EP1306990A2_D0005.tif" /></maths>

Then, the optical losses of the optical variable splitting controller are given by 10log₁₀(1/K(i)) (dB). As for the optical reception levels Pin(i) (dBm), since the optical losses from the input of the optical variable splitting controller to the inputs of the individual optical receivers are controlled in such a manner that they are equal for all the N optical receivers, the optical reception levels Pin(i) are denoted as a constant value Pin_con (dBm).

Thus, the relationships between the optical output levels Pout (dBm) and the optical reception levels Pin_con (dBm) of the optical receivers (i) are given by the following equation.<maths id="math0006" num="(6)"><math display="block"><mrow><mtext mathvariant="italic">Pout</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>(1/</mtext><mtext mathvariant="italic">K</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>))-</mtext><mtext mathvariant="italic">L</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>)=</mtext><mtext mathvariant="italic">Pin</mtext><mtext>_</mtext><mtext mathvariant="italic">con</mtext><mtext> (</mtext><mtext mathvariant="italic">i</mtext><mtext>=1 to </mtext><mtext mathvariant="italic">N</mtext><mtext>)</mtext></mrow></math><img file="EP1306990A2_D0006.tif" /></maths>

Since the optical losses from the input of the optical variable splitting controller to the inputs of the optical receivers are controlled such that they are equal for all the N optical receivers, the following equations hold in equation (6).<maths id="math0007" num=""><img file="EP1306990A2_D0007.tif" /></maths> That is,<maths id="math0008" num=""><img file="EP1306990A2_D0008.tif" /></maths> Summing up equations (8), the following equation is obtained.<maths id="math0009" num=""><img file="EP1306990A2_D0009.tif" /></maths> In equation (9), the following equation holds.<maths id="math0010" num=""><img file="EP1306990A2_D0010.tif" /></maths> Consequently,<maths id="math0011" num="(11)"><math display="block"><mrow><msup><mrow><mtext>10</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msup><mrow><mtext>​</mtext></mrow><mrow><mtext mathvariant="italic">A</mtext></mrow></msup><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>/10</mtext></mrow></msup></mrow></math><img file="EP1306990A2_D0011.tif" /></maths> is given by the following equation.<maths id="math0012" num=""><img file="EP1306990A2_D0012.tif" /></maths> From equations (8) and (12), the coupling ratios K(i) are given by the following equation.<maths id="math0013" num=""><img file="EP1306990A2_D0013.tif" /></maths> Substituting equation (13) into equation (6), the following equation is obtained.<maths id="math0014" num=""><img file="EP1306990A2_D0014.tif" /></maths>

Next, to compare the value of the Pin_con with the Pin_min, equation (14) - equation (2) is calculated as follows.<maths id="math0015" num=""><img file="EP1306990A2_D0015.tif" /></maths> The left-hand side of equation (15) is given by<maths id="math0016" num=""><img file="EP1306990A2_D0016.tif" /></maths> in which since<maths id="math0017" num=""><img file="EP1306990A2_D0017.tif" /></maths> holds, the following expression is given.<maths id="math0018" num=""><img file="EP1306990A2_D0018.tif" /></maths> Accordingly,<maths id="math0019" num=""><img file="EP1306990A2_D0019.tif" /></maths>

From equation (19), the right-hand side of equation (15) is given by the following expression.<maths id="math0020" num="(20)"><math display="block"><mrow><mtext mathvariant="italic">Pin_con-Pin_min</mtext><mtext> &gt;0</mtext></mrow></math><img file="EP1306990A2_D0020.tif" /></maths>

From the foregoing description, it is known that the optical reception level Pin_con of the present embodiment of the optical communication system is higher than the minimum optical reception level Pin_min of the conventional system. Consequently, it is shown that the optical communication system of the present embodiment is superior to the conventional optical communication system.

The advantage of the present embodiment will be described in detail with reference to Fig. 7. Fig. 7 shows an example of an optical receiver distribution model in the optical communication system as shown in Fig. 6.

The optical receiver distribution model of Fig. 7 shows the transmission losses 1, 2, ..., and 8 (dB), and the number of the optical receivers with the individual transmission losses. First, the minimum optical reception level in the optical receiver distribution model as shown in Fig. 7 is calculated for the conventional optical communication system as shown in Fig. 1.

The sum total of the optical receivers is 8 × 32 = 256. Since the splitting number of the optical splitter equals the sum total of the optical receivers, the splitting number N is 256. Hence, the optical loss of the optical splitter is 10log₁₀256 (dB) per branch. In addition, in the optical receiver distribution model of Fig. 7, Lmax is 8 (dB). Accordingly, the minimum optical reception level Pin_min (dBm) is obtained from equation (2) as follows.<maths id="math0021" num="(21)"><math display="block"><mrow><mtext mathvariant="italic">Pout-</mtext><msub><mrow><mtext>10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>256-8</mtext><mtext mathvariant="italic">=Pin_min</mtext></mrow></math><img file="EP1306990A2_D0021.tif" /></maths>

Next, the minimum optical reception level is calculated for the optical communication system of Fig. 6, when the control of the present embodiment is applied to the optical receiver distribution model as shown in Fig. 7.

Let us assume that the transmission losses of the optical receivers (i) (i = (1 + 32(n-1)) to 32n, n = 1 to 8) are L(i) = n (dB). Then the following term of equation (14)<maths id="math0022" num=""><img file="EP1306990A2_D0022.tif" /></maths> is given by the following expression.<maths id="math0023" num=""><img file="EP1306990A2_D0023.tif" /></maths> Accordingly equation (14) is expressed as<maths id="math0024" num="(24)"><math display="block"><mrow><mtext mathvariant="italic">Pout</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>826=</mtext><mtext mathvariant="italic">Pin</mtext><mtext>_</mtext><mtext mathvariant="italic">con</mtext></mrow></math><img file="EP1306990A2_D0024.tif" /></maths> Subsequently, calculate equation (24) - equation (21) as follows to compare Pin_con with Pin_min.<maths id="math0025" num="(25)"><math display="block"><mrow><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext>826 + 10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>256 + 8 =</mtext><mtext mathvariant="italic">Pin_con-Pin_min</mtext></mrow></math><img file="EP1306990A2_D0025.tif" /></maths> The left-hand side of equation (25) is given by the following expression.<maths id="math0026" num="(26)"><math display="block"><mrow><mtext mathvariant="italic">left-hand side</mtext><msub><mrow><mtext> =-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext>826 + 10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>256 + 8 ≈ 2.9</mtext></mrow></math><img file="EP1306990A2_D0026.tif" /></maths> Accordingly, the right-hand side of equation (25) is given by the following expression.<maths id="math0027" num="(27)"><math display="block"><mrow><mtext mathvariant="italic">Pin_con-Pin_min≈</mtext><mtext>2.9</mtext></mrow></math><img file="EP1306990A2_D0027.tif" /></maths>

Consequently, the minimum optical reception level can be set higher than that of the conventional optical communication system by about 2.9 (dB).

SECOND EMBODIMENT

As shown in Fig. 8, the second embodiment of the optical communication system in accordance with the present invention comprises: an optical transmitter 41 including an optical variable splitting controller 412 for splitting the optical signal fed from an optical fiber 411 with regulating the optical output levels by controlling the optical coupling ratios; N optical couplers 61, 62, ..., and 6N connected to the optical transmitter 41 via N optical fibers 51, 52, ..., and 5N; and optical receivers 181, 182, ..., and 18m₁, 281, 282, ..., and 28m₂, and N81, N82, ..., and N8m_N connected to the individual optical couplers 61, 62, ..., and 6N via optical fiber 171, 172, ..., and 17m₁, 271, 272, ..., and 27m₂, and N71, N72, ..., and N7m_N.

Here, receiving the optical signal passing through the optical fiber 411, the optical transmitter 41 controls the optical output levels (optical levels to be supplied to the individual optical fibers 51 to 5N) independently of all the optical signals to be sent to the individual optical couplers 61 to 6N via the individual optical fibers 51 to 5N. Thus, the output levels of the optical signals can differ from each other for respective optical couplers, that is, for the individual optical fibers 51 to 5N connected to the optical couplers 61 to 6N.

Next, the minimum optical reception level of the present embodiment of the optical communication system will be compared with that of the conventional example as shown in Fig. 2. In the following description, the optical coupler connected to the ith optical fiber is denoted as C(i), and the optical receiver whose optical loss from the optical coupler C(i) to the optical receiver itself is maximum is denoted as optical receiver (i)max.

Assume that the optical output level of the preceding stage of the optical variable splitting controller is Pout (dBm), the optical loss from the output of the optical variable splitting controller associated with the optical receiver (i)max to the optical receiver is L(i) (dB), and the optical reception level of the optical receiver (i)max is Pin(i)max (dBm). In addition, assume that the coupling ratios of the optical output levels of the optical variable splitting controller associated with the optical receivers (i)max are given by the following equation.<maths id="math0028" num=""><img file="EP1306990A2_D0028.tif" /></maths>

Then, the optical losses of the optical variable splitting controller are given by 10log₁₀(1/K(i)max) (dB). In addition, the optical loss of the optical coupler C(i) is calculated as 10log₁₀m_i (dB) per branch because the splitting number of the optical coupler C(i) is m_i, and the optical coupler splits the input light into parts with the same output levels.

In addition, since the optical reception levels Pin(i)max (dBm) are controlled such that the optical losses from the input of the optical variable splitting controller to the inputs of the optical receivers are equal for all the optical receivers (i)max, the values are denoted as a constant value Pin_con_n (dBm).

Thus, the relationships between the optical output levels Pout (dBm) and the optical reception levels Pin_con_n (dBm) of the optical receivers (i)max are given by the following equation.<maths id="math0029" num="(29)"><math display="block"><mrow><mtext mathvariant="italic">Pout</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>(1/</mtext><mtext mathvariant="italic">K</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>)</mtext><mtext mathvariant="italic">max</mtext><msub><mrow><mtext>)-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext mathvariant="italic">m</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msub><mtext>-</mtext><mtext mathvariant="italic">L</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>)</mtext><mtext mathvariant="italic">max</mtext><mtext>=</mtext><mtext mathvariant="italic">Pin</mtext><mtext>_</mtext><mtext mathvariant="italic">con</mtext><mtext>_</mtext><mtext mathvariant="italic">n</mtext><mtext> (</mtext><mtext mathvariant="italic">i</mtext><mtext>=1 to </mtext><mtext mathvariant="italic">N</mtext><mtext>)</mtext></mrow></math><img file="EP1306990A2_D0029.tif" /></maths>

It will be shown in the following that the optical reception level Pin_con_n (dBm) of the present embodiment given by equation (29) takes a greater value than the conventional minimum optical reception level Pin_min (dBm) given by equation (4).

Consider the N optical receivers whose optical losses from the optical couplers to the inputs of the optical receivers are maximum. Since the optical losses from the input of the optical variable splitting controller to the inputs of the optical receivers are controlled such that they are equal for all the N optical receivers, the following equations hold in equation (29).<maths id="math0030" num=""><img file="EP1306990A2_D0030.tif" /></maths> That is,<maths id="math0031" num=""><img file="EP1306990A2_D0031.tif" /></maths> Summing up equations (31), the following equation is obtained.<maths id="math0032" num=""><img file="EP1306990A2_D0032.tif" /></maths> In equation (32), the following equation holds.<maths id="math0033" num=""><img file="EP1306990A2_D0033.tif" /></maths> Consequently, 10^-A/10 is given by the following equation.<maths id="math0034" num=""><img file="EP1306990A2_D0034.tif" /></maths> From equations (31) and (34), the coupling ratios K(i)max are given by the following equation.<maths id="math0035" num=""><img file="EP1306990A2_D0035.tif" /></maths> Substituting equation (35) into equation (29), the following equation is obtained.<maths id="math0036" num=""><img file="EP1306990A2_D0036.tif" /></maths>

Next, equation (36) - equation (4) is calculated as follows to compare the Pin_con_n with the Pin_min.<maths id="math0037" num=""><img file="EP1306990A2_D0037.tif" /></maths> The left-hand side of equation (37) is given by<maths id="math0038" num=""><img file="EP1306990A2_D0038.tif" /></maths> in which since<maths id="math0039" num=""><img file="EP1306990A2_D0039.tif" /></maths> holds, the following expression is given.<maths id="math0040" num=""><img file="EP1306990A2_D0040.tif" /></maths> Accordingly,<maths id="math0041" num=""><img file="EP1306990A2_D0041.tif" /></maths> From equation (41), the right-hand side of equation (37) is given by the following expression.<maths id="math0042" num="(42)"><math display="block"><mrow><mtext mathvariant="italic">Pin_con_n-Pin_min</mtext><mtext> &gt; 0</mtext></mrow></math><img file="EP1306990A2_D0042.tif" /></maths>

From the foregoing description, it is shown that the optical reception levels Pin_con_n of the present embodiment of the optical communication system is higher than the minimum optical reception level Pin_min of the conventional system. Consequently, it is proved that the optical communication system of the present embodiment is superior to the conventional optical communication system.

Fig. 9 shows an example of the optical receiver distribution model in the present embodiment of the optical communication system as shown in Fig. 8.

The optical receiver distribution model of Fig. 9 shows the transmission losses 1, 2, ..., and 8 (dB), and the number of the optical receivers with the individual transmission losses. Assume that each optical coupler has a splitting number 16, and is connected to 16 optical receivers with transmission losses of 2(n-1) to 2n (dB), where n = 1 to 4, as illustrated in the shaded portion of Fig. 9.

First, the minimum optical reception level in the optical receiver distribution model as shown in Fig. 9 is calculated for the conventional optical communication system as shown in Fig. 2. The sum total of the optical receivers is 8 × 32 = 256. Since the splitting number of each optical splitter equals the quotient obtained by dividing the sum total of the optical receivers by the splitting number of the optical couplers, the splitting number N is 256/16 = 16. Hence, the optical loss of the optical splitter is 10log₁₀16 (dB) per branch.

Likewise, the optical loss of each optical coupler is 10log₁₀16 (dB) per branch. In addition, in the optical receiver distribution model of Fig. 9, the maximum loss L(u, v) is 8 (dB). Accordingly, the minimum optical reception level Pin_min (dBm) is obtained as follows from equation (4).<maths id="math0043" num="(43)"><math display="block"><mrow><mtext mathvariant="italic">Pout</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext>16-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>16-8=</mtext><mtext mathvariant="italic">Pin</mtext><mtext>_</mtext><mtext mathvariant="italic">min</mtext></mrow></math><img file="EP1306990A2_D0043.tif" /></maths>

Next, the minimum optical reception level in the second embodiment of the optical communication system of Fig. 8 is calculated when applying the control of the present embodiment to the optical receiver distribution model as shown in Fig. 9.

The transmission losses of the optical receivers (i)max whose optical losses from the optical couplers to the optical receivers themselves are maximum are 2, 4, 6, and 8 (dB). Then the following term of equation (36)<maths id="math0044" num=""><img file="EP1306990A2_D0044.tif" /></maths> is given by the following expression.<maths id="math0045" num=""><img file="EP1306990A2_D0045.tif" /></maths> Accordingly, equation (36) is expressed as<maths id="math0046" num="(46)"><math display="block"><mrow><mtext mathvariant="italic">Pout</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>921=</mtext><mtext mathvariant="italic">Pin</mtext><mtext>_</mtext><mtext mathvariant="italic">con</mtext><mtext>_</mtext><mtext mathvariant="italic">n</mtext></mrow></math><img file="EP1306990A2_D0046.tif" /></maths> Subsequently, equation (46) - equation (43) is calculated as follows to compare Pin_con_n with Pin_min.<maths id="math0047" num="(47)"><math display="block"><mrow><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext>921 +10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext>16 + 10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>16 + 8 </mtext><mtext mathvariant="italic">= Pin_con_n-Pin_min</mtext></mrow></math><img file="EP1306990A2_D0047.tif" /></maths> The left-hand side of equation (47) is given by the following expression.<maths id="math0048" num="(48)"><math display="block"><mrow><mtext mathvariant="italic">left-hand side =</mtext><msub><mrow><mtext> -10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext>921+10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><msub><mrow><mtext>16+10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>16 + 8 ≈ 2.4</mtext></mrow></math><img file="EP1306990A2_D0048.tif" /></maths> Accordingly, the right-hand side of equation (47) is given by the following expression.<maths id="math0049" num="(49)"><math display="block"><mrow><mtext mathvariant="italic">Pin_con_n-Pin_min ≈</mtext><mtext> 2.4</mtext></mrow></math><img file="EP1306990A2_D0049.tif" /></maths> Consequently, the control of the present embodiment can set the minimum optical reception level at a value higher than that of the conventional optical communication system by about 2.4 (dB).

THIRD EMBODIMENT

As shown in Fig. 10, the third embodiment of the optical communication system in accordance with the present invention comprises: an optical transmitter 101 including an optical splitter 1012 for splitting the optical signal fed from an optical fiber 1011 to optical signals with the same optical output levels; N remote variable splitters 121, 122, ..., and 12N including optical variable splitting controllers 1211, 1221, ..., and 12N1 that are connected to the optical transmitter 101 via N optical fibers 111, 112, ..., and 11N, and splitting the optical signals from the optical fibers 111, 112, ..., and 11N to optical signals with regulating their output levels for the optical receivers by controlling the optical coupling ratios; and m₁ optical receivers 141, 142, ..., and 14m₁, m₂ optical receivers 241, 242, ..., and 24m₂, and m_N optical receivers N41, N42, ..., and N4m_N that are connected to the individual remote variable splitters 121, 122, ..., and 12N via m₁ optical fibers 131, 132, ..., and 13m₁, m₂ optical fibers 231, 232, ..., and 23m₂, and m_N optical fibers N31, N32, ..., and N3m_N.

In the present embodiment, the remote variable splitters 121 to 12N independently control the optical output levels of the optical signals which are produced by splitting the optical signal received from the optical fiber 111, and are sent to the individual optical receivers 141, 142, ..., and 14m₁, the optical output levels of the optical signals which are produced by splitting the optical signal received from the optical fiber 112, and are sent to the individual optical receivers 241, 242, ..., and 24m₂, and the optical output levels of the optical signals which are produced by splitting the optical signal received from the optical fiber 11N, and are sent to the individual optical receivers N41, N42, ...and N4m_N.

It is preferable that the remote variable splitters control the optical output levels in such manner that the optical losses from the input of the optical variable splitting controllers to the inputs of the m; optical receivers become equal for all the m; optical receivers. To explain this, let us compare the minimum optical reception level of the conventional example as shown in Fig. 2 with that of the present embodiment of the optical communication system. In the following description, the optical receiver connected to the optical transmitter via the ith and jth (j = 1 to m_i) optical fibers is denoted as an optical receiver (i,j) (i = 1 to N, j = 1 to m_i).

Assume that the optical output level of the preceding stage of each optical variable splitting controller is Pout (dBm), the optical loss from the output of the optical variable splitting controller to the optical receiver (i,j) associated with the optical variable splitting controller is L(i,j) (dB), and the optical reception level of the optical receiver (i,j) is Pin(i,j) (dBm). In addition, assume that the coupling ratios of the optical output levels of the optical variable splitting controller associated with the optical receivers (i,j) are given by the following equation.<maths id="math0050" num=""><img file="EP1306990A2_D0050.tif" /></maths> Then, the optical losses of the optical variable splitting controllers are given by 10log₁₀(1/K(i,j)) (dB). In addition, since the optical splitter has the splitting number N, and carries out the optical splitting such that the output signals have the same output levels, the optical splitter has an optical loss of 10log₁₀N (dBm) per branch.

Furthermore, since the optical reception levels Pin(i,j) are controlled such that the optical losses from the input of the optical variable splitting controller to the inputs of the optical receivers are equal for all the m_i optical receivers, the value is denoted as a constant value Pin_con_m(i) (dBm). Thus, the relationships between the optical output levels Pout (dBm) and the optical reception levels Pin_con_m(i) (dBm) are given by the following equation.<maths id="math0051" num="(51)"><math display="block"><mrow><mtext mathvariant="italic">Pout-</mtext><msub><mrow><mtext>10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext mathvariant="italic">N</mtext><msub><mrow><mtext>-10log</mtext></mrow><mrow><mtext>10</mtext></mrow></msub><mtext>(1/</mtext><mtext mathvariant="italic">K</mtext><mtext>(</mtext><mtext mathvariant="italic">i,j</mtext><mtext>))-</mtext><mtext mathvariant="italic">L</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>,</mtext><mtext mathvariant="italic">j</mtext><mtext>)=</mtext><mtext mathvariant="italic">Pin_con_m</mtext><mtext>(</mtext><mtext mathvariant="italic">i</mtext><mtext>) (</mtext><mtext mathvariant="italic">i</mtext><mtext>=1 to </mtext><mtext mathvariant="italic">N</mtext><mtext>,</mtext><mtext mathvariant="italic">j</mtext><mtext>=1 to </mtext><msub><mrow><mtext mathvariant="italic">m</mtext></mrow><mrow><mtext mathvariant="italic">i</mtext></mrow></msub><mtext>)</mtext></mrow></math><img file="EP1306990A2_D0051.tif" /></maths>

Since the optical losses from the inputs of the optical variable splitting controllers to the inputs of the optical receivers are controlled such that they are equal for all the m; optical receivers, the following equations hold in equation (51).<maths id="math0052" num=""><img file="EP1306990A2_D0052.tif" /></maths> That is,<maths id="math0053" num=""><img file="EP1306990A2_D0053.tif" /></maths> Summing up equations (53), the following equation is obtained.<maths id="math0054" num=""><img file="EP1306990A2_D0054.tif" /></maths> In equation (54), the following equation holds.<maths id="math0055" num=""><img file="EP1306990A2_D0055.tif" /></maths> Consequently, 10^-A/10 is given as<maths id="math0056" num=""><img file="EP1306990A2_D0056.tif" /></maths> From equations (53) and (56), the coupling ratio K(i,j) is given by the following equation.<maths id="math0057" num=""><img file="EP1306990A2_D0057.tif" /></maths> Substituting equation (57) into equation (51), the following equation is obtained.<maths id="math0058" num=""><img file="EP1306990A2_D0058.tif" /></maths>

Next, equation (58) - equation (4) is calculated as follows with changing i to u in equation (58) in order to compare the value Pin_con_m(i) (dBm) with Pin_min (dBm) under the same conditions.<maths id="math0059" num=""><img file="EP1306990A2_D0059.tif" /></maths> The left-hand side of equation (59) is given by<maths id="math0060" num=""><img file="EP1306990A2_D0060.tif" /></maths> in which since<maths id="math0061" num=""><img file="EP1306990A2_D0061.tif" /></maths> holds, the following expression is given.<maths id="math0062" num=""><img file="EP1306990A2_D0062.tif" /></maths> Accordingly,<maths id="math0063" num=""><img file="EP1306990A2_D0063.tif" /></maths> From equation (63), the right-hand side of equation (59) is given by the following expression.<maths id="math0064" num="(64)"><math display="block"><mrow><mtext mathvariant="italic">Pin_con_m</mtext><mtext>(</mtext><mtext mathvariant="italic">u</mtext><mtext>)</mtext><mtext mathvariant="italic">-Pin_min</mtext><mtext> &gt; 0</mtext></mrow></math><img file="EP1306990A2_D0064.tif" /></maths> From the foregoing description, it is seen that the optical reception levels Pin_con_m of the present embodiment of the optical communication system is higher than the minimum optical reception level Pin_min of the conventional system. Consequently, it is proved that the optical communication system of the present embodiment is superior to the conventional optical communication system.

Figs. 11A and 11B show a configuration of the optical variable splitting controller of the first to third embodiments of the optical communication system in accordance with the present invention, in which the optical variable splitting controller 2001 is configured by combining variable splitters 2002 in multiple stages.

The variable splitter 2002 has a thin film heater 20023 on the surface of an upper cladding of an optical waveguide 20022. It controls the optical output level by altering the refractive index or phase of the optical waveguide 20022 by the heat from the thin film heater 20023 utilizing the temperature dependence of the refractive index of the optical waveguide, that is, the thermooptic effect.

More specifically, assume that temperature change in the optical waveguide 20022 is ΔT, the refractive index of the optical waveguide 20022 before the temperature changes is n0, the refractive index of the optical waveguide 20022 after the temperature changes is n1, and the refractive index temperature coefficient of the optical waveguide 20022 is α. Then the following relationship holds.<maths id="math0065" num="(65)"><math display="block"><mrow><mtext mathvariant="italic">n</mtext><mtext>1</mtext><mtext mathvariant="italic">=n</mtext><mtext>0</mtext><mtext mathvariant="italic">+αΔT</mtext></mrow></math><img file="EP1306990A2_D0065.tif" /></maths> In addition, assume that the length of the thin film heater 20023 is d, the phase variation due to the variations of the refractive index of the optical waveguides 20021 and 20022 are φ, and the wavelength of the light traveling through the optical waveguides 20021 and 20022 is λ. Then the following equation holds.<maths id="math0066" num="(66)"><math display="block"><mrow><mtext>φ=-(2π/λ)×(</mtext><mtext mathvariant="italic">n</mtext><mtext>1-</mtext><mtext mathvariant="italic">n</mtext><mtext>0)×</mtext><mtext mathvariant="italic">d</mtext></mrow></math><img file="EP1306990A2_D0066.tif" /></maths> From equation (65), the following equation is obtained.<maths id="math0067" num="(67)"><math display="block"><mrow><mtext>φ=-(2π/λ)×αΔ</mtext><mtext mathvariant="italic">T</mtext><mtext>×</mtext><mtext mathvariant="italic">d</mtext></mrow></math><img file="EP1306990A2_D0067.tif" /></maths> Assume that the optical intensity of the optical waveguide 20021 at its input A0 is |A0|², the optical intensity of the optical waveguide 20022 at its input B0 is |B0|² = 0, the optical output level of the optical waveguide 20021 at its output A1 is |A1|², and the optical output level of the optical waveguide 20022 at its output B1 is |B1|². Then the following equations hold using equation (67).<maths id="math0068" num=""><img file="EP1306990A2_D0068.tif" /></maths><maths id="math0069" num=""><img file="EP1306990A2_D0069.tif" /></maths> Equations (68) and (69) show that the control of the temperature of the thin film heater 20023 makes it possible to freely control the optical output levels |A1|² and |B1|² (coupling ratio) of the optical waveguides 20021 and 20022.

FOURTH EMBODIMENT

Fig. 12 shows an embodiment of a two-way optical communication system in accordance with the present invention. A single optical transmitting/receiving unit 1 is connected with L optical transmitting/receiving units 2-1 to 2-L in a one-to-L manner via an optical fiber 3-0, a 1×L optifcal varibale coupling unit 10, and optical fibers 3-1 to 3-L. The optifcal varibale coupling unit 10 splits a downlink optical signal with a wavelength λd transmitted from the optical transmitting/receiving unit 1 to L parts, and sends them to the optical transmitting/receiving units 2-1 to 2-L. On the other hand, it combines uplink optical signals with a wavelength λu transmitted from the optical transmitting/receiving units 2-1 to 2-L, and transmits the combined signal to the optical transmitting/receiving unit 1.

The optifcal varibale coupling unit 10 of the present embodiment has a configuration in which a plurality of 1×2 optical variable couplers 11 are connected successively to implement the 1×L coupling. A coupling ratio controller 12 controls the coupling ratios of the individual 1×2 optical variable couplers 11 considering the transmission losses of the optical fibers 3-1 to 3-L and the wavelength, thereby carrying out variable control of the 1×L coupling ratios in their entirety. As the 1×2 optical variable couplers 11, Mach-Zehnder interferometer type couplers are used whose coupling ratios are varied by temperature control or by electric field control, for example. To control the coupling ratios by temperature, the 1×2 optical variable couplers can be configured as shown in Fig. 11B.

Since the coupling ratio has wavelength dependence, it must be controlled in accordance with the wavelength used. A control example will be described below.

The coupling ratio controller 12 of the optifcal varibale coupling unit 10 stores a premeasured optical transmission level Pout(1) of the optical transmitting/receiving unit 1 and premeasured individual optical reception levels Pin(21) to Pin(2L) of the optical transmitting/receiving units 2-1 to 2-L.

The individual total optical losses between the optical transmitting/receiving unit 1 and the optical transmitting/receiving units 2-1 to 2-L are obtained as the difference between the premeasured Pout(1) and Pin(21), ..., and the difference between the Pout(1) and Pin(2L), respectively. Here, the individual optical losses depend on the transmission loss of the optical fiber 3-0, the coupling ratios (coupling losses) of the optifcal varibale coupling unit 10 (individual 1×2 optical variable couplers 11 on the passage) and the transmission losses of the individual optical fibers 3-1 to 3-L, and the coupling losses depend on the wavelength. Accordingly, the coupling ratios that make the difference between the maximum and minimum values of the individual total optical losses less than a specified value at the individual wavelengths can be obtained by carrying out calculation with varying the coupling ratios at the wavelengths λd and λu.

The coupling ratio controller 12 controls the individual 1×2 optical variable couplers 11 to set their coupling ratios at such values that can make the following differences less than the specified values. The differences are the difference between the minimum optical reception level and maximum optical reception level of the downlink optical signals with the wavelength λd received by the optical transmitting/receiving units 2-1 to 2-L, and the difference between the minimum optical reception level and maximum optical reception level of the uplink optical signals with the wavelength λu sent from the optical transmitting/receiving units 2-1 to 2-L to the optical transmitting/receiving unit 1.

Fig. 13 illustrates a first control example of the 1×2 optical variable couplers 11. Assume that a first total optical loss between the transmitter and a receiver at wavelength λd is L1(λd), and a second total optical loss is L2(λd), that a first total optical loss between the transmitter and the receiver at the wavelength λu is L1(λu) and a second total optical loss is L2(λu), and that L1(λd) &gt; L1(λu) &gt; L2(λd) &gt; L2(λu). The coupling ratios are controlled such that the difference between the maximum value (L1(λd)) and the minimum value (L2(λu)) becomes less than a specified value.

When a wavelength division multiplexed optical signal is used as the downlink optical signals or uplink optical signals, the coupling ratio is set that makes the difference between the maximum value and the minimum value of the total optical loss between the transmitter and receiver at each wavelength less than the specified value.

FIFTH EMBODIMENT

The coupling ratio controller 12 of the optifcal varibale coupling unit 10 stores a premeasured optical transmission level Pout(1) of the optical transmitting/receiving unit 1 and premeasured individual optical reception levels Pin(21) to Pin(2L) of the optical transmitting/receiving units 2-1 to 2-L, and premeasured individual optical transmission levels Pout(21) to Pout(2L) of the optical transmitting/receiving units 2-1 to 2-L.

The individual total optical losses between the optical transmitting/receiving unit 1 and the optical transmitting/receiving units 2-1 to 2-L are each obtained as the difference between the premeasured Pout(1) and Pin(21), ..., and the difference between the Pout(1) and Pin(2L). Here, as for the individual optical transmission levels Pout(1), and Pout(21) to Pout(2L), the optical reception levels of the downlink optical signals at the optical transmitting/receiving units 2-1 to 2-L and the optical reception levels of the uplink optical signals at the optical transmitting/receiving unit 1 depend on the transmission loss of the optical fiber 3-0, the coupling ratios (coupling losses) of the optifcal varibale coupling unit 10 and the transmission losses of the individual optical fibers 3-1 to 3-L, and the coupling losses depend on the wavelength. Accordingly, the coupling ratios that make the minimum values of the individual received levels greater than a specified value can be obtained by carrying out calculation with varying the coupling ratios at the wavelengths λd and λu.

The coupling ratio controller 12 can place the minimum optical reception levels of the optical transmitting/receiving unit 1 and optical transmitting/receiving units 2-1 to 2-L at a value greater than the specified value by setting the coupling ratios of the individual 1×2 optical variable couplers 11 by controlling them.

Fig. 14 illustrates a second control example of the 1×2 optical variable couplers 11. Assume that a first total optical loss between the transmitter and a receiver at wavelength λd is L1(λd), and a second total optical loss is L2(λd), that a first total optical loss between the transmitter and the receiver at the wavelength λu is L1(λu) and a second total optical loss is L2(λu), and that L1(λd) &gt; L1(λu) &gt; L2(λd) &gt; L2(λu). The coupling ratios are controlled such that the maximum value (L1(λd)) is less than the specified value, that is, the minimum values of the individual optical reception levels become greater than the specified value.

When a wavelength division multiplexed optical signal is used as the downlink optical signals or uplink optical signals, the coupling ratios are calculated such that the minimum reception levels become greater than a specified value for the respective wavelengths.

SIXTH EMBODIMENT

The coupling ratio controller 12 of the optifcal varibale coupling unit 10 stores premeasured individual optical transmission levels Pout(21) to Pout(2L) of the optical transmitting/receiving units 2-1 to 2-L, and measures and stores the individual optical reception levels Pin(101) to Pin(10L) of the uplink optical signals at the optifcal varibale coupling unit 10. Incidentally, the means for measuring the individual optical reception levels Pin(101) to Pin(10L) of the incoming uplink optical signals from the optical fibers 3-1 to 3-L to the optifcal varibale coupling unit 10 is omitted from Fig. 12.

The individual optical losses between the optifcal varibale coupling unit 10 and the optical transmitting/receiving units 2-1 to 2-L are each obtained as the difference between the Pout(21) and Pin(101), ..., and the difference between the Pout(2L) and Pin(10L). Here, the individual total optical losses between the optical transmitting/receiving unit 1 and the optical transmitting/receiving units 2-1 to 2-L depend on the transmission loss of the optical fiber 3-0, coupling ratios (coupling losses) at the optifcal varibale coupling unit 10 and the transmission losses (measured values) of the individual optical fibers 3-1 to 3-L, and the coupling losses depend on the wavelength. Accordingly, the coupling ratios that make the difference between the maximum and minimum values of the individual total optical losses less than a specified value at the individual wavelengths can be obtained by carrying out calculation with varying the coupling ratios at the wavelengths λd and λu. The coupling ratio controller 12 controls the individual 1×2 optical variable couplers 11 to set their coupling ratios at such values that can make the following differences less than the specified values. The differences are the difference between the minimum optical reception level and maximum optical reception level of the downlink optical signals with the wavelength λd received by the optical transmitting/receiving units 2-1 to 2-L, and the difference between the minimum optical reception level and maximum optical reception level of the uplink optical signals with the wavelength λu sent from the optical transmitting/receiving units 2-1 to 2-L to the optical transmitting/receiving unit 1.

When a wavelength division multiplexed optical signal is used as the downlink optical signals or uplink optical signals, the coupling ratio is set that makes the difference between the maximum value and the minimum value of the total optical loss between the transmitter and receiver at each wavelength less than the specified value.

In addition, the optical reception levels of the uplink optical signals at the optical transmitting/receiving unit 1 depend on the individual optical transmission levels Pout(21) to Pout(2L), the coupling ratios (coupling losses) of the optifcal varibale coupling unit 10 and the transmission losses of the individual optical fibers 3-1 to 3-L, and the coupling losses depend on the wavelength. Accordingly, the coupling ratios that make the minimum values of the individual received levels greater than the specified value can be obtained by carrying out calculation with varying the coupling ratios at the wavelengths λd and λu.

The coupling ratio controller 12 can place the minimum optical reception levels of the optical transmitting/receiving unit 1 and optical transmitting/receiving units 2-1 to 2-L at a value greater than the specified value by setting the coupling ratios of the individual 1×2 optical variable couplers 11 by controlling them.

When a wavelength division multiplexed optical signal is used as the downlink optical signals or uplink optical signals, the coupling ratios are calculated such that the minimum reception levels become greater than a specified value for the respective wavelengths.

An optical communication system can increase a minimum optical reception level or the number of connectable optical receivers by controlling optical output levels of its optical transmitter. The optical communication system includes the optical transmitter having an optical variable splitting controller for splitting received light into N parts and supplies them to N optical fibers, and N optical receivers for receiving the signals sent via the N optical fibers. The optical variable splitting controller regulates its optical output levels to be supplied to the N optical fibers by controlling the optical coupling ratios. For example, the optical variable splitting controller may regulate its optical output levels by controlling its optical coupling ratios such that optical losses from its input to inputs of the N optical receivers become equal for all the N optical receivers.

The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect, and it is the intention, therefore, in the apparent claims to cover all such changes and modifications as fall within the true spirit of the invention.