JP2006238334A - Optical communication apparatus and system in coherent optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To keep constant reception sensitivity in coherent optical communication at all the time, independently of polarization fluctuation within a signal light transmission line, by using existent cost-reduced components without using any complicated transmitter/receiver. <P>SOLUTION: A light source 11 oscillates continuous light, and a modulator 12 receives the continuous light and inserts RZ modulated signal light 101 to a polarization controller 12 by performing intensity, phase or frequency signal modulation through direct modulation or external modulation. A light demultiplexer 14 of the polarization controller 13 demultiplexes the inserted RZ modulated signal light 101 and transmits the demultiplexed light to two arms 15 and 16. At such a time, the signal light transmitted to the arm 15 is transmitted to a light multiplexer 19 as it is, and the signal light transmitted to the arm 16 is delayed by a half of modulated signal velocity by a retarder 17 and transmitted to the light multiplexer 19 as modulated signal light 102 in a polarized state made orthogonal by a polarization rotator 18. The light multiplexer 19 receives and multiplexes the modulated signal light 102 from the polarization rotator 18 and signal light from the light demultiplexer 14. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical communication apparatus or an optical communication system that does not depend on polarization fluctuations on a transmission line in coherent optical communication.

  In coherent optical communication, optical detection is performed by mixing signal light and local oscillation light. However, if the polarization states of the signal light and local oscillation light do not match, the detection efficiency deteriorates.

  For this reason, it is necessary to make the polarization states of the optical signal and the local oscillation light coincide with each other. However, it is practically difficult to maintain the polarization state in the longitudinal direction in the existing optical fiber transmission line. The polarization state cannot be kept constant. This cause is due to temperature change, vibration, etc. of the optical fiber transmission line. As a method for realizing the detection of coherent light without depending on such a change in the polarization state of the signal light, there are a diversity reception method, a polarization scrambling method, and a polarization control method.

  A receiver (polarization diversity receiver) using a conventional diversity reception system is shown in FIG. 8 (see Non-Patent Document 1). This polarization diversity receiver includes a local oscillation light source 81, an optical detector 82, a demodulation circuit 83, an adder 84, an optical polarization separation element 85, and a 90 ° polarization rotator 86. The optical polarization separation element 85 distributes the signal light transmitted from the transmission side to the orthogonal first polarization state and second polarization state. The first optical detector 82 receives one of the allocated signal lights mixed with the first polarization state of the local oscillation light from the local oscillation light source 81 via the 90 ° polarization rotator 86, and An intermediate frequency signal in the polarization state is output. The second optical detector 82 receives the other of the distributed signal lights mixed with the second polarization state of the local oscillation light from the local oscillation light source 81, and receives the intermediate frequency in the polarization state. Output a signal. The demodulating circuit 83 demodulates each of the two intermediate frequency signals, and the adder 84 adds the outputs of the demodulating circuit 83 and outputs the added signal component. Thereby, it becomes possible to obtain a signal output of a certain level or more in the receiver regardless of the polarization state of the signal light.

  Next, a coherent optical transmitter using a conventional polarization scrambling method is shown in FIG. 9 (see Non-Patent Document 2). This coherent optical transmitter includes a light source 91 and an internal or external modulator 92. In addition to this coherent optical transmitter, an oscillator 94 and a polarization modulator 93 are provided. The oscillator 94 and the polarization modulator 93 give a high-speed polarization fluctuation component exceeding the signal modulation speed to the signal light transmitted by the coherent optical transmitter. As a result, when the signal light and the local oscillation light are multiplexed on the coherent optical receiver side, components having the same polarization state are always generated, so that it is not necessary to consider the polarization state at the time of reception. Therefore, it is possible to obtain a constant reception signal that does not depend on the polarization fluctuation in the transmission path, and there is an advantage that a complicated receiver structure is not required unlike the polarization diversity receiver.

  Next, a coherent optical transmitter using a conventional polarization control system is shown in FIG. 10 (see Non-Patent Document 3). This coherent optical transmitter uses a polarization fluctuation compensation technique, and includes a light source 95 and an internal or external modulator 96. In addition to the coherent optical transmitter, polarization beam splitters 97 and 100 and optical fibers 98 and 99 are provided. The light source 95 and the modulator 96 transmit an optical signal whose modulation format is a mark in the first half of the bit interval and makes the optical signal incident on the polarization beam splitter 97. The polarization beam splitter 97 controls the polarization state at the time of incidence so that the signal light is equally divided, and separates the signal light into orthogonal polarization states. The optical fibers 98 and 99 are adjusted so that the optical path difference after separation has a delay of half the bit rate in terms of time. The polarization beam splitter 100 combines the signal light propagated through the optical fibers 98 and 99, and generates modulated signal light whose polarization states are orthogonal in the first half and the second half of the bit. As a result, when the modulated signal light is detected by the coherent optical receiver, a component in which the polarization states of the local oscillation light and the signal light coincide with each other always exists, so that a constant signal output can be obtained.

Takanori Okoshi, Shiro Ryu and Kazuro Kikuchi, "Polarization-diversity receiver for heterodyne / coherent optical fiber communications," 4th International Conference on Integrated Optics and Optical Fiber Communication '83, Tokyo, 27-30 June 1983, Paper No. 30C3-2 , Tech.Digest, pp.386-387 N. Caponio, F. Delpiano, P. Gambini, M. Puleo, V. Seano and E. Vezzoni, “Demonstration of polarisation independent coherent transmission by synchronous intra-bit polarisation spreading,” IEEE International Conference on Communications '91, 23- 26 June 1991, vol.1, pp.343-347 I.M.I.Habbab, L.j.Cimini, Jr., "Polarization-switching techniques for coherent optical communications," Journal of Lightwave Technology, vol.6, issue.10, Oct1988, pp.1537-1548

  In the conventional polarization diversity system as shown in FIG. 8, the receiver detects the optical signal by receiving the polarization diversity of the optical signal transmitted from the transmitter. Therefore, in order to receive polarization diversity of an optical signal, it is necessary to add an optical polarization separation element, an optical coupler, an optical detector, a demodulation circuit, and an adder to the receiver. Therefore, in order to realize the polarization diversity reception system, there arises a problem that the configuration of the optical receiver becomes complicated.

  Further, in the conventional polarization scrambling method as shown in FIG. 9, since it is necessary to apply polarization modulation at a speed exceeding the bit rate, an oscillator for inputting a high-speed oscillation signal to the polarization modulator is required. . Therefore, since such a high-frequency component is required, there arises a problem that the transmitter itself becomes expensive.

  Furthermore, in the conventional polarization control method as shown in FIG. 10, the coherent optical transmitter using the polarization modulation compensation technique is used when the polarization beam splitter separates the signal light into different polarization states. It is necessary to make the signal light incident at an angle of 45 ° with respect to the splitter branching direction. That is, it means that the polarization of the signal light output in a certain polarization state from the light source must be kept constant until the signal light is input to the polarization beam splitter. This is a big problem especially when performing optical wavelength division multiplexing, and when performing polarization control for each signal light collectively, it is necessary to individually maintain the polarization for each signal light, As a result, an increase in the number of parts and a cost increase due to polarization maintenance are caused. In addition, since the polarization must be kept constant, it is impossible to place such a polarization fluctuation compensator at any location in the optical transmission line where the polarization fluctuation state is not constant. Become. For this reason, there arises a problem that the polarization fluctuation compensator must always be arranged on the transmission side.

  Therefore, in order to solve the above problems, the present invention uses a complex transmitter / receiver to always maintain the reception sensitivity at the time of coherent optical communication without depending on the polarization fluctuation in the transmission path of the signal light. Instead, it is intended to be realized by using existing low-cost parts. In addition, it is possible to compensate for polarization fluctuations at any location on the optical transmission line, and to perform polarization control for a plurality of signal lights at the same time, thereby introducing a coherent optical communication system at the time of optical wavelength division multiplexing communication. The purpose is to make it easier.

  The present invention is an optical communication apparatus using a two-branch optical delay line for controlling polarization on an optical transmission line. A basic configuration of the present invention is shown in FIG. The optical communication apparatus includes a light source 11, a modulator 12, and a polarization controller 13 having an optical splitter 14, arms 15 and 16, a delay device 17, a polarization rotator 18, and an optical multiplexer 19. Yes. The optical splitter 14 and the optical multiplexer 19 are connected by an arm 15 and are connected by an arm 16 via a delay device 17 and a polarization rotator 18. The light source 11 oscillates continuous light, the modulator 12 inputs this continuous light, performs intensity, phase, or frequency signal modulation by direct modulation or external modulation, and inserts the RZ modulated signal light 101 into the polarization controller 13. To do. In this case, the signal modulation format by the modulator 12 is an RZ signal format in which the mark is not more than half the bit interval. For example, as shown in FIG. 1, the RZ-modulated signal light 101 is a signal sequence “101101” such that the left side is the first signal (1) and the next signal is one right side signal (0). It is configured. Inside the polarization controller 13, the optical splitter 14 branches the inserted RZ modulated signal light 101 and transmits it to the two arms 15 and 16. At this time, the signal light transmitted to the arm 15 is transmitted to the optical multiplexer 19 as it is, and the signal light transmitted to the arm 16 is given a delay of half the modulation signal speed, and the polarization state is orthogonalized. The modulated signal light 102 is transmitted to the optical multiplexer 19. That is, the delay unit 17 is designed to give a half delay of the modulation signal speed to the branched signal light, and the polarization rotator 18 applies to the signal light that is the transmission signal of the arm 15. It is designed to make the polarization state orthogonal. The optical multiplexer 19 receives the modulated signal light 102 from the polarization rotator 18 and the signal light from the optical splitter 14 and combines them.

  As a result, the combined modulated signal light 103 has orthogonal polarization components within one bit, and does not depend on the polarization state of the local oscillation light at the time of coherent optical detection of the transmission signal in a receiver (not shown). A constant signal output can be obtained. At this time, regardless of the polarization state of the RZ modulated signal light 101 inserted into the polarization controller 13, the modulated signal light 102 output from the polarization rotator 18 has an orthogonal component within one bit. Therefore, it is not necessary to control the polarization state of the RZ modulated signal light 101.

  In this way, by providing orthogonal polarization states within one bit of the optical transmission signal, even if polarization fluctuations occur in the signal light in the transmission line, the intra-bit polarization orthogonal state is maintained. The For this reason, when coherent light reception is performed on the receiver side, there is always a signal component in the same polarization state with respect to the local oscillation light. As a result, coherent light reception can be performed regardless of any change in the polarization state of the signal light in the transmission path, and the signal can be reproduced and output in the receiver.

  As described above, the optical communication apparatus of the present invention can obtain a constant signal output during optical heterodyne reception without controlling the polarization state of the optical signal and the local oscillation light. In addition, an optical communication system using this optical communication apparatus can perform signal detection with a simple structure without complicating the structure of the transceiver in coherent optical communication that does not depend on polarization fluctuations.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Hereinafter, the polarization controller refers to a polarization controller having the structure of the polarization controller 13 shown in FIG.

  As a first embodiment of the present invention, the polarization controller can use an optical fiber transmission line. In FIG. 1, the present embodiment uses an optical fiber transmission line for the input and output of the polarization controller, and arms 15 and 16 also use an optical fiber transmission line. In this case, for example, an optical fiber coupler or a spatial system or a waveguide type optical splitter can be used for the optical branching device 14 and the optical multiplexer 19, and an optical fiber delay device can be used for the delay device 17.

  Thus, by configuring the optical communication device using optical fibers, it becomes possible to realize polarization control using simple components, and the manufacturing cost of the polarization controller 13 itself can be kept low. It becomes possible.

  The polarization controller may be an optical waveguide type two-branch optical delay line using PLC (Planer Lightwave Circuit), for example. In FIG. 1, the present embodiment is configured by using a single optical waveguide element for the input and output of the polarization controller, the optical branching device 14, the optical multiplexer 19, the arms 15 and 16, and the delay device 17. The polarization rotator 18 is configured to dispose a wave plate or a birefringent medium, for example, designed so that the output polarization state is orthogonal to the input polarization state on the optical waveguide element.

  As described above, the optical communication device is configured by using the optical waveguide type element, so that it is more resistant to physical fluctuation of the transmission line than the case of using the optical fiber, and the stability of the two-branch optical delay line is increased. Has the advantage of greatly improving.

  Further, for example, a spatial optical device can be used as the polarization controller. In FIG. 1, for example, an optical branching unit 14 using an optical beam splitter spatially separates the input light to the polarization controller, and a spatial transmission path corresponding to the arm 15 and an arm including a delay unit 17. By adjusting the distance to the spatial transmission line corresponding to 16, an optical two-branch optical delay line can be constructed. For example, a wave plate can be used for the polarization rotator 18, and a light beam combiner can be used for the optical multiplexer 19.

  Thus, by configuring the optical communication device using a spatial system, an optically stable polarization controller can be configured, and the accuracy of the delay device 17 can be greatly improved. .

  FIG. 2 is a block diagram showing a second embodiment of the present invention. This optical communication system includes a transmitter 20 including a light source 21, an optical signal modulator 22, and a polarization controller 23, a reception including a local oscillation light source 26, an optical multiplexer 27, an optical detector 28, and a demodulation circuit 29. And an optical fiber transmission line 24 connecting the transmitter and the receiver. As in FIG. 1, the transmitter 20 uses the light source 21, the optical signal modulator 22, and the polarization controller 23 to output the modulated signal light 103 having components whose polarization states are orthogonal within the bit interval. To do. The optical fiber transmission line 24 transmits the modulated signal light 103, and the receiver 25 that receives the coherent light receives the modulated signal light 103 and outputs the signal component of the signal light transmitted from the transmitter 20. .

  FIG. 3 is a block diagram showing a third embodiment of the present invention. This optical communication system includes a plurality of transmitters 31-1 to 31 -n, a plurality of receivers 32-1 to 32 -n, optical branching couplers 33 and 34, and an optical fiber transmission line 35. The transmitters 31-1 to 31-n and the receivers 32-1 to 32-n are assumed to have the same configuration as shown in FIG. The oscillation light frequency is set for each light source 21 so that the plurality of transmitters 31-1 to 31-n transmit modulated signal light having different optical frequencies. After the transmitters 31-1 to 31 -n respectively control the polarization of the modulated signal light, the optical branching coupler 33 multiplexes these signal lights. Then, after the optical fiber transmission line 35 transmits the combined signal light, the optical branching coupler 34 branches the signal light and inserts it into each of the receivers 32-1 to 32-n.

  At this time, each of the receivers 32-1 to 32-n can receive and detect modulated signal light having different optical frequencies by changing the optical frequency of the local oscillation light. In addition, it is possible to realize a large increase in transmission capacity by transmitting multiple signals having a large number of optical frequencies as carrier waves on the same optical fiber. By using the optical branch couplers 33 and 34 as optical frequency branch couplers, it is possible to significantly reduce the propagation loss when the modulated signals are branched and coupled.

  FIG. 4 is a block diagram showing a fourth embodiment of the present invention. This optical communication system includes a plurality of transmitters 41-1 to 41-n, a plurality of receivers 42-1 to 42-n, optical branching couplers 43 and 44, a polarization controller 45, and an optical fiber transmission line 46. Consists of. Each transmitter 41-1 to n includes a light source 21 that oscillates different optical frequencies and an optical signal modulator 22 that modulates an optical signal. Each receiver 42-1 to n is shown in FIGS. It is assumed that the configuration is the same. Each of the transmitters 41-1 to 41-n transmits modulated signal light, the optical branching coupler 43 combines the signal light, and the polarization controller 45 performs polarization control. When the polarization controller 45 has the same configuration as that of the polarization controller 13 shown in FIG. 1, it is possible to collectively control the polarization of each signal light. Thereafter, the optical fiber transmission line 46 transmits the signal light, and the optical branching coupler 44 branches the signal light to each of the receivers 42-1 to 42-n. The receivers 42-1 to 42-n respectively corresponding to the optical frequencies of the modulated signal light can demodulate and output the transmission signal by receiving these signal lights coherently.

  In this way, the polarization controller 45 collectively applies polarization control to each modulation signal, thereby making it possible to solve the dependency on the polarization state without greatly changing the configuration from the conventional coherent optical communication. It becomes. Since the polarization control by the polarization controller 45 does not depend on the input polarization state, it is necessary to keep the polarization state of each signal constant when performing polarization control on a large number of modulated signal lights at once. Therefore, the configuration of the polarization controller 45 can be simplified.

  Further, according to the fourth embodiment, the polarization controllers 23 that are individually arranged in the transmitters 31-1 to 31-n in the third embodiment are shared by the plurality of transmitters 41-1 to 41-n. This is particularly effective in a network configuration in which the transmitters 41-1 to 41-n, the optical branching coupler 43, and the polarization controller 45 are centrally arranged in one place. In particular, in the form of a point-multipoint network in which the receivers 42-1 to n are distributed, the functions of the receivers 42-1 to n are reduced and the functions are concentrated on the transmitters 41-1 to n side. As a result, the management of the entire network can be simplified. Therefore, this embodiment is very effective, and it is possible to solve the dependency on the polarization without requiring a polarization diversity structure that complicates the configuration and increases the cost in the receiver.

  FIG. 5 is a block diagram showing a fifth embodiment of the present invention. This optical communication system includes transmitters 51 and 54, receivers 52 and 53, optical branching couplers 55 and 56, and an optical fiber transmission line 57. The transmitters 51 and 54 and the receivers 52 and 53 are assumed to have the same configuration as that shown in FIG. The transmitters 51 and 54 transmit modulated signal lights having different optical frequencies, and the receivers 52 and 53 perform coherent optical detection via the optical branching couplers 55 and 56 and the optical fiber transmission line 57. FIG. Same as ˜4. This optical communication system enables bidirectional optical communication by arranging the transmitter 54 and the receiver 53 in the direction opposite to the direction in which the optical signal is transmitted from the transmitter 51 to the receiver 52.

  By using optical branch couplers 55 and 56 as optical frequency branch couplers or optical circulators, it is possible to reduce optical signal loss during branch coupling. By constructing this in the same manner as FIG. 3 which is an extension of FIG. 2, bidirectional transmission can be realized using a plurality of transmitters and a plurality of receivers. At this time, as shown in FIG. 4, the polarization controller can be shared by a plurality of transmitters.

  As described above, in the second, third, and fifth embodiments, the case where the polarization controller is arranged inside the signal transmitter has been described. However, the polarization controller does not depend on the polarization state of the optical signal. In order to perform control, it may be arranged in front of the receiver.

  FIG. 6 is a block diagram showing a sixth embodiment of the present invention. This optical communication system includes a plurality of transmitters 61-1 to n, 62-1 to m, a plurality of receivers 63-1 to n, 64-1 to m, optical branch couplers 65 to 69, This is a point-multipoint network composed of wave controllers 71 and 72 and an optical fiber transmission line 73. Each of the transmitters 61-1 to n and 62-1 to m includes a light source 21 and light modulation means 22 in order to output modulated signal light having different optical frequencies. Similar to the configuration of FIG. 4, the transmitters 61-1 to 61-n transmit modulated signal lights having different optical frequencies, the optical branching coupler 65 multiplexes these optical modulated signals, and the polarization controller 71 The wave state is controlled, and the optical branching coupler 67 multiplexes the optical modulation signals and outputs them to the optical fiber transmission line 73. Thereafter, the optical fiber transmission line 73 transmits the optical signal, the optical branching couplers 68 and 69 branch the optical signal, and the receivers 63-1 to 63-n receive the branched optical signal. In this case, the receivers 63-1 to 63-n are distributed.

  Furthermore, in order to construct a bidirectional optical communication system, bidirectional multiplexing is performed by using transmitters 62-1 to n, receivers 64-1 to n, optical branching couplers 66 to 69, and polarization controller 72. Transmission can be realized. In this case, particularly in the point-multipoint topology, as shown in FIG. 6, the transmitters and receivers at one end are concentrated in one place, and the other transmitter and receiver are distributed in pairs. Be placed. Therefore, when transmitting in the direction from the transmitters 62-1 to 6-n to the receivers 64-1 to 64-n, the polarization controller 72 is disposed between the optical branching couplers 66 and 67, thereby receiving the signals. It becomes possible to reduce the dependence on the polarization state at the time. Further, by using the optical branch couplers 65 to 69 as optical frequency branch couplers or using the optical branch couplers 67 and 69 as optical circulators, branch coupling loss can be reduced.

  Further, the polarization controller used in the first to sixth embodiments has a configuration as shown in FIG. 7 in which the bit is divided into n and n polarization lines are used using n delay lines. You can also In FIG. 7, the polarization controller includes an optical branching device 75, an optical multiplexer 76, delay devices 77-1 to (n-1), and polarization rotators 78-1 to (n-1). The optical branching unit 75 inputs RZ-modulated signal light whose signal modulation format is 1 / n or less of 1 bit. Then, the optical branching device 75 branches the signal light into n and sends it to n transmission lines. After that, the delay units 77-1 to (n-1) and the polarization rotators 78-1 to (n-1) respectively perform the delay and the polarization rotation, and the optical multiplexer 76 has n patterns within one bit. A modulation signal having a polarization state of is generated. Each delay unit 77 is designed to provide a delay of 1 / n to (n-1) / n at a rate of 1 bit.

  At this time, the polarization controller has n polarization states divided into n from the polarization state of the signal light before insertion into the polarization controller to the polarization state orthogonal to the signal light before insertion. The polarization state is controlled to take each. Alternatively, the polarization controller may generate a modulation signal in which alternately orthogonal polarization states are continuous by controlling the mth polarization state to be orthogonal to the m + 1th polarization state. it can.

It is a figure explaining the basic principle of this invention. It is a figure which shows 2nd embodiment of this invention. It is a figure which shows 3rd embodiment of this invention. It is a figure which shows 4th embodiment of this invention. It is a figure which shows 5th embodiment of this invention. It is a figure which shows 6th embodiment of this invention. It is a figure which shows the application example of the polarization controller of this invention. It is a figure which shows the conventional polarization diversity reception system. It is a figure which shows the conventional polarization scrambling system. It is a figure which shows the system using the conventional polarization fluctuation compensation technique.

Explanation of symbols

11, 21, 91, 95 Light source 12, 22, 92, 96 Modulator 13, 23, 45, 71, 72 Polarization controller 14, 75 Optical splitter 15, 16 Arm 17, 77 Delay unit 18, 78, 86 Polarization rotators 19, 27, 76 Optical multiplexers 20, 31, 41, 51, 54, 61, 62 Transmitters 24, 35, 46, 57, 73 Optical fiber transmission lines 25, 32, 42, 52, 53, 63, 64 Receiver 26, 81 Local oscillation light source 28, 82 Optical detector 29, 83 Demodulation circuit
33, 34, 43, 44, 55, 56, 65 to 69 Optical branching coupler 84 Adder 85 Optical polarization separation element 93 Polarization modulator 94 Oscillator 97, 100 Polarization beam splitter 98, 99 Optical fibers 101, 102 , 103 modulated signal light

Claims (8)

In an optical communication apparatus used in an optical communication system in which a transmitter transmits signal light that has been modulated with light intensity, phase, or frequency, and a receiver receives the signal light and performs coherent optical detection.
Optical branching means for inputting modulated signal light modulated so that a half or less of the bit interval becomes a mark with respect to the oscillation light, and branching the modulated signal light;
Polarization rotating means for rotating the polarization so that one polarization state of the branched modulated signal light is orthogonal to the other polarization state;
Delay means for delaying one of the branched modulated signal lights by half a bit with respect to the other;
Optical multiplexing means for multiplexing one of the modulated signal light and the other modulated signal light that has been polarized and rotated;
A polarization control means having
An optical communication apparatus characterized in that the modulated signal light multiplexed by the optical multiplexing means of the polarization control means includes orthogonal polarization components within one bit. The optical communication device according to claim 1,
The polarization control means includes a two-branch optical delay line using an optical fiber transmission line, a two-branch optical delay line using an optical waveguide, or a two-branch optical delay line using an optical space system. Optical communication device. In an optical communication system in which a transmitter transmits signal light whose optical intensity, phase, or frequency is modulated, and a receiver receives the signal light and performs coherent optical detection,
An optical communication system, wherein the transmitter includes the polarization control unit according to claim 1. In an optical communication system in which a transmitter transmits signal light whose optical intensity, phase, or frequency is modulated, and a receiver receives the signal light and performs coherent optical detection,
A plurality of transmitters each having the polarization control means according to claim 1 and 2, wherein each of the modulated signal lights having different optical frequencies is subjected to polarization control by the polarization control means and transmitted.
Optical multiplexing means for multiplexing the transmitted signal light;
Optical branching means for branching the combined signal light;
A plurality of receivers each receiving the branched signal light;
An optical communication system comprising: In an optical communication system in which a transmitter transmits signal light whose optical intensity, phase, or frequency is modulated, and a receiver receives the signal light and performs coherent optical detection,
A plurality of transmitters each transmitting modulated signal light of different optical frequencies;
Optical multiplexing means for multiplexing the transmitted signal light;
Optical branching means for branching the combined signal light;
A plurality of receivers each receiving the branched signal light;
The optical communication device according to claim 1 or 2 provided in a transmission path between the optical multiplexing unit and the optical branching unit,
The optical communication apparatus performs polarization control collectively on a plurality of modulated signal lights combined by an optical multiplexing unit. In an optical communication system in which a transmitter transmits signal light whose optical intensity, phase, or frequency is modulated, and a receiver receives the signal light and performs coherent optical detection,
A first transmitter and a second transmitter each having the polarization control means according to claim 1 or 2,
A first receiver for receiving the signal light transmitted from the first transmitter via a transmission line;
A second receiver for receiving the transmission light transmitted from the second transmitter via a transmission line;
First optical branching and coupling means for sending signal light from the first transmitter to a transmission line and sending signal light from the second transmitter to the second receiver through the transmission line;
Second optical branching and coupling means for sending signal light from the second transmitter to the transmission line and sending signal light from the first transmitter via the transmission line to the first receiver. ,
An optical communication system characterized in that bidirectional transmission is realized through the common transmission path by using the first and second optical branching and coupling means. In an optical communication system in which a transmitter transmits signal light modulated in light intensity, phase or frequency, and a receiver receives the signal light and performs coherent optical detection.
The optical communication system includes a plurality of transmitters and a plurality of receivers arranged in a centralized manner, and a first optical branching and coupling unit for multiplexing signal lights respectively transmitted from the plurality of transmitters arranged in a centralized manner. A point-multipoint configuration comprising: a second optical branching and coupling means for branching signal light to the plurality of concentrated receivers; and a plurality of transmitters and receivers arranged in a distributed manner Configure the network,
The optical communication device according to claim 1 or 2 is provided at a stage after the first optical branching and coupling unit and at a stage before the second optical branching and coupling unit,
The optical communication apparatus performs polarization control on a plurality of modulated signal lights at once. In an optical communication apparatus used in an optical communication system in which a transmitter transmits signal light that has been modulated with light intensity, phase, or frequency, and a receiver receives the signal light and performs coherent optical detection.
Optical branching means for inputting modulated signal light modulated so that a mark is 1 / n or less of the bit interval with respect to the oscillation light, and for branching the modulated signal light into n branches;
Each of the n-1 modulated signal lights out of the n-branched modulated signal lights is changed to a predetermined polarization state so as to have n polarization states within one bit including other modulated signal lights. Polarization rotation means for rotating the polarization;
Of the n-branched modulated signal lights, n-1 modulated signal lights are delayed by a predetermined amount in time so as to have n polarization states within one bit including other modulated signal lights. Delay means,
Optical multiplexing means for multiplexing the polarization-rotated and delayed n-1 modulated signal lights and other modulated signal lights;
A polarization control means having
An optical communication apparatus characterized in that the modulated signal light combined by the optical multiplexing means of the polarization control means includes n polarization components in one bit.

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