JP4740793B2 - Polarization separator and optical receiver - Google Patents

Description

  The present invention relates to an optical receiver applied to, for example, an ultra-high-speed / long-distance transmission system, and more particularly to a polarization that enables highly accurate polarization separation of a polarization multiplexed optical signal used as a transmission signal. The present invention relates to a separation device and an optical reception device including the polarization separation device.

  Conventionally, it is known that the polarization orthogonal multiplexing method is effective when information of 40 Gbps or more is transmitted using a single wavelength. For example, if two optical signals of the same wavelength modulated by different 40 Gbps signals are multiplexed with orthogonal polarizations, an 80 Gbps signal can be transmitted at one wavelength. In general, when the polarization orthogonal multiplexed signal is separated, it can be realized by using a polarization beam combiner, a polarization beam splitter, or the like, but since the polarization state after transmission changes with time, A part for separating the polarization based on the polarization state of the received signal is required.

  In a conventional optical receiver, when transmitting a polarization multiplexed signal in which a light wave including a first signal and a light wave including a second signal are polarization multiplexed at the same wavelength, one of these two light waves is transmitted. One light wave is modulated using a third signal, and the third signal photoelectrically converted at the receiver side is used as a control signal to control the polarization state before polarization separation. (For example, Patent Document 1).

JP 2002-344426 A (FIG. 1 etc.)

  However, the conventional technique represented by Patent Document 1 has the following problems.

  First, in long-distance transmission using an ultra-high-speed signal, each frequency component on the optical spectrum is affected by polarization mode dispersion (hereinafter referred to as “PMD”) generated in an optical fiber as a transmission path. However, it is difficult to obtain a control signal for polarization separation because the orthogonality between polarization multiplexed signals is maintained, but the polarization relationship between frequencies in the same polarization component is received without being maintained. There was a problem such as.

  In addition, in the case of being affected by polarization dependent loss (hereinafter referred to as “PDL”) that occurs on a principle different from that of the PMD, reception is performed without maintaining orthogonality between polarization multiplexed signals. Therefore, there is a problem that the crosstalk light after separation increases.

  The present invention has been made in view of the above, and it is possible to efficiently obtain a control signal necessary for polarization separation even when the polarization relationship between frequencies is broken due to the influence of PMD. It is an object of the present invention to provide a polarization separation device and an optical reception device, or to provide a polarization separation device and an optical reception device for efficiently obtaining a control signal necessary for polarization separation.

  Another object of the present invention is to provide a polarization separation device and an optical reception device that can return the polarization between signals whose polarization orthogonal relationship is broken due to the influence of PDL to an orthogonal state.

  In order to solve the above-described problems and achieve the object, a polarization separation device according to the present invention includes a polarization controller that controls a polarization state of an input polarization multiplexed optical signal, and the polarization controller. A polarization separation device comprising: a polarization separation unit that separates the output polarization multiplexed optical signal into two optical signals; and one of the two optical signals separated by the polarization separation unit A narrowband optical filter that extracts the optical output included in the frequency band near the optical carrier using one of the branch outputs, and an error signal for polarization separation based on the polarization state of the extracted optical signal And a drive circuit that generates a control signal for controlling the polarization control unit using an error signal output from the orthogonal relationship monitor unit.

  According to the polarization separation device of the present invention, the optical output included in the frequency band near the optical carrier is extracted by the narrow-band optical filter using the branch output of one of the two optical signals subjected to polarization separation. In addition, since the control signal for controlling the polarization state of the polarization multiplexed optical signal is generated based on the polarization state of the extracted optical output, the polarization between the frequencies is affected by the influence of PMD. Even when the relationship is broken, the control signal necessary for polarization separation can be obtained efficiently.

  In addition, according to the polarization separation device of the present invention, while generating an error signal for polarization separation based on the polarization state of one of the two branched optical outputs of the polarization separation, Since the polarization orthogonal relationship of the polarization multiplexed optical signal input using the generated error signal is compensated, the polarization between the signals whose polarization orthogonal relationship is broken due to the influence of PDL is restored to the original orthogonality. There exists an effect that it becomes possible to return to a state.

  Preferred embodiments of a polarization beam splitter and an optical receiver according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited by each embodiment shown below.

Embodiment 1 FIG.
FIG. 1 is a diagram of a configuration of the optical receiving apparatus according to the first embodiment of the present invention. The optical receiver shown in FIG. 1 is separated by a polarization control unit 2 connected to the transmission line 1, a polarization separation unit 3 using the output of the polarization control unit 2 as an input signal, and the polarization separation unit 3. The optical receiver (RX1) 6 that receives the other optical signal, the optical receiver (RX2) 7 that receives the separated optical signal, and any output from the polarization separation unit 3 (FIG. 1). In this example, the output to the optical receiver 6 is used), the narrowband optical filter 8 using the branch output as the input signal, the orthogonal relationship monitor unit 5 using the output of the narrowband optical filter 8 as the input signal, and the orthogonal relationship And a drive circuit 4 using the output of the monitor unit 5 as an input signal.

  In the configuration of FIG. 1, each component of the polarization control unit 2, the polarization separation unit 3, the narrowband optical filter 8, the orthogonal relationship monitor unit 5, and the drive circuit 4 is a polarization input from the transmission line 1. It functions as a polarization separation device for separating the polarization of the multiplexed optical signal. Among these components, the narrow-band optical filter 8, the orthogonal relationship monitor unit 5, and the drive circuit 4 constitute a feedback loop for controlling the polarization controller 2.

  Next, the operation of the optical receiver shown in FIG. 1 will be described. The polarization controller 2 receives a polarization multiplexed optical signal input from the transmission line 1 so that each of the optical receivers 6 and 7 to receive the optical output of the polarization separator 3 has a desired polarization component. The polarization state is controlled so as to be output separately. The narrowband optical filter 8 extracts the optical output included in the frequency band near the optical carrier with respect to any one of the optical signals of the polarization separation unit 3 (output to the optical receiver 6 in the example of the figure). And output to the orthogonal relationship monitor unit 5. The orthogonal relationship monitor unit 5 verifies the optical signal component extracted by the narrowband optical filter 8, generates an error signal for controlling the polarization control unit 2, and outputs the error signal to the drive circuit 4. The drive circuit 4 generates and outputs a control signal for controlling the polarization control unit 2 using the error signal output from the orthogonal relationship monitor unit 5.

  Next, with respect to the principle of this embodiment, which is characterized in that an optical signal affected by polarization mode dispersion (PMD) is separated with high accuracy, see FIGS. 2-1 and 2-2. To explain. Here, FIG. 2-1 is a diagram showing a concept of an optical spectrum in a state where the optical signal 1 polarized in the TE mode and the optical signal 2 polarized in the TM mode are polarization multiplexed, and FIG. It is a figure which shows the concept of the optical spectrum after being influenced by.

  As shown in FIG. 2A, the optical signal input from the transmission line 1 is an optical signal 1 having an optical spectrum (K1) such that all frequency components are on the same polarization plane (TE plane). And an optical signal 2 having an optical spectrum (K2) such that all frequency components are on a polarization plane (TM plane) orthogonal to the TE plane, and a polarization orthogonal multiplexed optical signal (polarization orthogonal multiplexed light) Signal). In addition, although the polarization state after transmission cannot be uniquely determined, for convenience, the axis of each TE mode and the axis of each TM mode shown in FIGS. 2-1 and 2-2 are the same before and after transmission. Yes. Even in this assumption, generality in explaining the principle of the present embodiment is not lost.

  When affected by PMD, as shown in FIG. 2B, between the optical spectrum component (K1 ′) of the optical signal 1 and the optical spectrum component (K2 ′) of the optical signal 2, individual frequency components are individually detected. Although the orthogonal relationship is maintained, since the influence on the polarization rotation differs depending on the frequency component, each frequency component of the optical signal 1 and the optical signal 2 existing in the same optical signal exists at a position shifted from the same polarization plane. .

  Therefore, when polarization-multiplexed light having frequency components deviated from the same polarization plane as described above is subjected to polarization separation, other signal light leaks after separation, resulting in signal degradation and polarization separation. Signal degradation also occurs in the error signal for control to be performed.

  On the other hand, in the optical receiver (polarization demultiplexer) configured as shown in FIG. 1, the narrowband optical filter 8 of the optical receiver extracts the vicinity of the optical signal carrier as shown by the broken line in FIG. Since the orthogonal relationship monitor unit 5 of the optical receiver generates an error signal for polarization separation based on the polarization state of the extracted optical signal, the generated error signal includes The influence of other frequency components that may be received with the polarization axis (plane) being slightly shifted can be suppressed, and polarization separation for the polarization multiplexed optical signal can be performed with high accuracy.

  The error signal output from the orthogonal relationship monitor unit 5 to the drive circuit 4 can be generated based on the power of the optical signal, the degree of polarization (DOP), and the like. Further, a method of detecting a dither signal component from an optical signal on which a low-speed dither signal is superimposed in advance may be used.

  As described above, according to this embodiment, the optical output included in the frequency band near the optical carrier is converted into the narrow-band optical filter by using the branch output of one of the two optical signals subjected to polarization separation. And a control signal for controlling the polarization state of the polarization multiplexed optical signal is generated based on the polarization state of the extracted optical output. Even when the polarization relationship is broken, a control signal necessary for polarization separation can be obtained efficiently.

Embodiment 2. FIG.
FIG. 3 is a block diagram showing a configuration of the optical receiving apparatus according to the second embodiment of the present invention. The optical receiver shown in the figure has the PDL compensation for compensating the broken polarization orthogonal relationship based on the error signal obtained by the orthogonal relationship monitor unit 5 in the configuration of the first embodiment shown in FIG. It is characterized in that the unit 9 is provided on the input side of the polarization control unit 2. The PDL compensation unit 9 is obtained by a polarization control unit 10 to which a polarization multiplexed optical signal is input, a PDL variable control unit 11 to which an output signal of the polarization control unit 10 is input, and an orthogonal relationship monitor unit 5. And a drive circuit 12 that generates a control signal for controlling the polarization controller 10 and the PDL variable controller 11 based on the error signal. As a result of providing such a PDL compensation unit 9, it is possible to omit the narrowband optical filter 8 for extracting signal components present in frequency components in the vicinity of the optical carrier shown in FIG. In addition, about another structure, it is the same as that of the structure of FIG. 1, or those components are shown with the same code | symbol.

  Next, with respect to the principle of this embodiment, which is characterized by returning the polarization between signals whose polarization orthogonality relationship has been lost due to the influence of polarization dependent loss (PDL) to the orthogonal state, FIG. This will be described with reference to the drawings 4-3. Here, FIG. 4A is a diagram illustrating a relationship between the optical spectrums of the optical signal 1 and the optical signal 2 that are orthogonal to each other in the transmission path, and FIG. 4B is an orthogonal relationship of polarization due to the influence of PDL. FIG. 4C is a diagram illustrating the relationship between the optical spectrums of the optical signal 1 and the optical signal 2 in which the optical signals 1 and 2 are corrupted, and FIG. It is a figure which shows the relationship of a spectrum.

  For example, as shown in FIG. 4A, when a loss in the transmission path occurs (or is dominant) in the direction of the TM axis, the optical spectrums (M1, M1) of the optical signal 1 and the optical signal 2 that are orthogonal to each other In M2), the orthogonal relationship is broken as in the optical spectrum (M1 ′, M2 ′) shown in FIG.

  On the other hand, the optical receiver of this embodiment includes the PDL compensation unit 9 as described above, and the PDL compensation unit 9 has a loss in the TE-axis direction component of the optical spectrum as shown in FIG. Compensation control that gives By performing such compensation control, the polarization relationship between the optical signal 1 and the optical signal 2 can be compensated as in the optical spectrum (M1 ″, M2 ″) shown in FIG. 4-3.

  In FIG. 4A, when the transmission loss due to PDL occurs in the direction of the TE axis, the PDL compensation unit 9 performs compensation control that gives a loss only to the component in the TM axis direction orthogonal to the TE axis. However, in order to obtain a more accurate polarization orthogonal relationship, loss control not only in the TM axis direction but also in the TE axis direction may be used in combination.

  Also, unlike FIG. 4-1, when a transmission loss due to PDL occurs (or is dominant) in the direction of the TE axis, the PDL compensator 9 only has a component in the TM axis direction of the optical spectrum, or TM. Loss compensation control in both directions of the axis and the TE axis may be performed.

  Further, the error signal generated by the orthogonal relationship monitor unit 5 and output to the drive circuit 4 and the drive circuit 12 of the PDL compensation unit 9 is converted into an optical signal power, a degree of polarization, or an optical signal as in the first embodiment. It is possible to generate based on a low-speed dither signal superimposed in advance.

  As described above, according to this embodiment, an error signal for polarization separation is generated based on the polarization state of one of the two branched optical signals. Since the polarization orthogonal relationship of the polarization multiplexed optical signal input using the generated error signal is compensated, the polarization between the signals whose polarization orthogonal relationship is broken due to the influence of PDL is restored to the original orthogonality. It becomes possible to return to the state.

Embodiment 3 FIG.
FIG. 5 is a block diagram showing a configuration of the optical receiving apparatus according to the third embodiment of the present invention. The optical receiver shown in FIG. 3 is the same as that shown in FIG. 1 according to the first embodiment between the output end of the polarization separation unit 3 and the orthogonal relationship monitor unit 5 in the configuration of the second embodiment shown in FIG. The narrow band optical filter 8 shown in the configuration is arranged. Note that components that are the same as or equivalent to those of the second embodiment are denoted by the same reference numerals, and description thereof is omitted. Here, only operations different from those of the second embodiment are described.

  In FIG. 5, when the influence of polarization mode dispersion (PMD) in the transmission path cannot be ignored, as described above, the polarization state of the optical spectrum component in the optical signal varies depending on the frequency component. The error signal for performing is affected by other frequency components that may be received with the plane of polarization (axis) slightly deviated.

  On the other hand, in the optical receiver (polarization demultiplexer) configured as shown in FIG. 5, as in the first embodiment, the narrowband optical filter 8 extracts the optical signal in the vicinity of the optical carrier, and the orthogonal relationship monitor unit 5 generates an error signal for polarization separation based on the polarization state of the extracted optical signal, so that polarization separation for the polarization multiplexed optical signal can be performed with high accuracy.

  Note that the error signal generated by the orthogonal relationship monitor unit 5 and output to the drive circuit 4 and the drive circuit 12 of the PDL compensation unit 9 is the same as in the first and second embodiments, such as optical signal power, degree of polarization, or optical signal. It can be generated based on a low-speed dither signal preliminarily superimposed on the signal.

  As described above, according to this embodiment, in the configuration of the above-described second embodiment, one of the two branch outputs of the polarization-separated optical signal is used to set the frequency band near the optical carrier. Since the narrow-band optical filter for extracting the included optical output is provided, in addition to the effect of the second embodiment, the effect that the polarization separation for the polarization multiplexed optical signal can be performed with high accuracy is obtained.

  As described above, the polarization separation device and the optical reception device according to the present invention are applied to an ultrahigh-speed / long-distance transmission system or the like, and are applied to a polarization separation device and an optical reception device that use a polarization multiplexed optical signal as a transmission signal. Useful.

It is a figure which shows the structure of the optical receiver concerning Embodiment 1 of this invention. It is a figure which shows the concept of the optical spectrum of the state which carried out the polarization multiplexing of the optical signal 1 polarized in TE mode, and the optical signal 2 polarized in TM mode. It is a figure which shows the concept of the optical spectrum after receiving the influence of PMD. It is a figure which shows the structure of the optical receiver concerning Embodiment 2 of this invention. It is a figure which shows the relationship of the optical spectrum of the optical signal 1 and the optical signal 2 which are orthogonally related in a transmission line. It is a figure which shows the relationship of the optical spectrum of the optical signal 1 and the optical signal 2 which the orthogonal relationship of the polarization has broken by the influence of PDL. It is a figure which shows the relationship between the optical spectrum of the optical signal 1 and the optical signal 2 which were compensated with the loss component which a PDL compensation part generate | occur | produces. It is a figure which shows the structure of the optical receiver concerning Embodiment 3 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Transmission path 2,10 Polarization control part 3 Polarization separation part 4 Drive circuit 5 Orthogonal relationship monitor part 6,7 Optical receiver (RX1, RX2)
8 Narrowband Optical Filter 9 PDL Compensator 10 Polarization Controller 11 PDL Variable Controller 12 Drive Circuit

Claims (5)

A first polarization controller that controls the polarization state of the input polarization multiplexed optical signal, and the polarization multiplexed optical signal output from the first polarization controller is split into two optical signals. A polarization separation device comprising:
An orthogonal relationship monitoring unit that generates an error signal for polarization separation based on the polarization state of one of the branch outputs of the two optical signals separated by the polarization separation unit;
A first drive circuit that generates a control signal for controlling the first polarization controller using the error signal;
A PDL (Polarization Dependent Loss) compensation unit that is inserted on the input side of the first polarization control unit and compensates the polarization orthogonal relationship of the polarization multiplexed optical signal input using the error signal;
A polarization separation device comprising: The PDL compensation unit
A second polarization controller for controlling the polarization state of the input polarization multiplexed optical signal;
A PDL variable control unit that varies the amount of loss in the axial direction orthogonal to the polarization axis in which transmission loss due to PDL is dominant based on the output of the second polarization control unit or in both axial directions;
A second drive circuit that generates a control signal for controlling the second polarization control unit and the PDL variable control unit using an error signal output from the orthogonal relationship monitor unit;
The polarization separation device according to claim 1 , further comprising: An optical output included in a frequency band near the optical carrier is obtained using a branch output obtained by branching a part of the optical signal subjected to the polarization separation between the output end of the polarization separation unit and the orthogonal relationship monitoring unit. The polarization separation device according to claim 2 , further comprising a narrowband optical filter for extraction. When an optical signal on which a low-speed dither signal is superimposed in advance is input as the polarization multiplexed optical signal, the orthogonal relationship monitor unit includes means for detecting the low-speed dither signal. The polarization beam splitting device according to any one of claims 1 to 3 . The polarization separation device according to any one of claims 1 to 4 ,
A receiver for receiving each of the two optical signals output from the polarization beam splitter;
An optical receiver characterized by comprising:

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