Pmd detector and wavelength multiple light transmission system

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a PMD detector that detects polarization mode dispersion characteristics that deteriorate the quality of a transmission signal in a wavelength multiplex optical transmission system that performs ultra-high-speed optical transmission, and a wavelength multiplex optical transmission system having a PMD compensation function. is there.

【0002】

[Conventional technology]

When light travels through substances with different refractive indexes, the propagation speed of light in the substance differs depending on the magnitude of the refractive index. In addition, the propagation of light in a material (anisotropic material) having a different refractive index depending on the polarization direction of the incident light causes a phenomenon that the delay time after passing through the material differs depending on the state of polarization in the material. .. Here, the delay time difference that occurs in the light polarized in each specific polarization direction in the material is called polarization mode dispersion (hereinafter, referred to as PMD (Polarization Mode Dispersion)).

【0003】

In general, a single-mode fiber used in an optical transmission line has a slight anisotropy locally along its longitudinal direction. Therefore, when light propagates through a fiber having such anisotropy, a delay difference depending on the polarization direction of the optical signal occurs at a location where the refractive index anisotropy exists in the propagation path. Occurs. If the delay difference due to the difference in polarization is accumulated in the optical transmission line, it causes deterioration of the signal waveform at the receiving end.

【0004】

Figure 6 shows the relationship between the PMD size of the optical transmission line and the optical transmission rate (bit rate), which causes a 1 dB penalty in optical transmission of NRZ (Non-Return to Zero) signals whose pulse width is equal to the pulse interval. is there. The relationship shown in Fig. 6 is described in the document "Fading in lightwave systems due to polarization-mode dispersion" (CD Poole, RW Tkach, AR Chraplyvy and DA Fishman, IEEE Photonics Technol. It was created based on the relational expression described in Lett. Vol.3, no.1, pp.68-70, 1991).

【0005】

That is, in the above document, the relationship between the PMD magnitude Δτ when the power penalty deteriorates by 1 dB and the pulse width T (T = 1 / B: B is the bit rate) of the NRZ signal is described by the following equation (1). It is shown as. That is, it is shown as Δτ / T ~ 0.4 ... (1). Based on this equation (1), the relationship between the PMD magnitude Δτ when the power penalty deteriorates by 1 dB and the bit rate B of the NRZ signal is obtained, and the result shown in FIG. 6 is obtained. For example, when transmitting a 40 Gbit / s NRZ signal, it can be seen that the PMD of the transmission line must be suppressed to 10 ps or less.

【0006】

On the other hand, in an actual transmission line, even with a low PMD fiber, there is an average PMD of about 0.1 ps / km. This means that a PMD of 10 ps will occur after 10000 km transmission. Furthermore, PMD in a transmission line has a characteristic that its size fluctuates with time. Therefore, after laying the optical fiber, it is necessary to perform PMD compensation according to the temporal fluctuation of PMD.

【0007】

When PMD compensation is performed according to time fluctuations, a highly accurate PMD detection method that follows the time fluctuations is required. Further, when considering implementation in a transmission system, miniaturization of the PMD detector is required.

【0008】

As a PMD detection method capable of following time fluctuations and performing high-precision measurement, for example, Jones Matrix disclosed in Japanese Patent Application Laid-Open No. 5-273082 (polarization mode dispersion determination device and method of optical device). ) The method is best known. In this Jones Matirix method, since the magnitude of PMD is calculated from the results of ellipsometry for a small number of wavelengths, high-speed and highly accurate PMD detection is possible. Furthermore, since PMD detection is performed by calculation, it is suitable for miniaturization of PMD detectors.

【0009】

Further, as means for detecting PMD based on other ellipsometry, the Poincare sphere method, the SOP method, and the FA (fixed analyzer) method are known. Since the Poancare sphere method and the SOP method calculate PMD based on the results of ellipsometry at each wavelength, they are suitable for the purpose of detecting PMD using each channel signal of wavelength division multiplexing light.

【0010】

On the other hand, the FA method is a method for PMD analysis from wavelength characteristics using a tunable light source. FIG. 7 is a diagram for explaining a PMD detection method by the FA method. This shows an example of a conventional PMD compensation method in a Wavelength Division Multiplexing (WDM) optical transmission system.

【0011】

In FIG. 7, the transmitting side includes a tunable light source 41 and a polarizer 42. The receiving side includes a polarizer 43 and an optical power meter 44. The polarizer 42 and the polarizer 43 are connected by an optical transmission line 45.

【0012】

On the transmitting side, the tunable light source 41 generates signal light of various wavelengths at a predetermined sweep cycle and outputs the signal light to the polarizer 42. The polarizer 42 imparts a known polarization state to the signal light sent from the tunable light source 41, and sends the signal light to the optical transmission line 45. On the receiving side, the signal light input from the optical transmission line 45 is input to the optical power meter 44 via the polarizer 43. In the optical power meter 44, the wavelength characteristic of the light intensity of the signal light input from the optical transmission line 45 is observed through the polarizer 43. The amount of PMD is approximately calculated by analyzing the observed wavelength characteristics. As described above, the FA method shown in FIG. 7 can detect the amount of PMD in the optical transmission line.

【0013】

[Problems to be Solved by the Invention]

However, in the conventional FA method, the PMD amount is calculated by measuring the wavelength characteristic of the delay amount of the light to be measured with a variable light source. It is difficult to detect fluctuations.

【0014】

The present invention has been made in view of the above, and is a PMD detector capable of detecting time fluctuations in the amount of PMD in an optical transmission line and capable of miniaturization, and a wavelength at which PMD compensation can be performed using the PMD detector. The purpose is to obtain a multiplex optical transmission system.

【0015】

[Means for solving problems]

In order to achieve the above object, the PMD detector according to the present invention is a part from an optical transmission path through which wavelength multiplex data signal light including any two wavelength data signal light having a known polarization relationship is transmitted. A measurement light acquisition means for extracting wavelength-multiplexed data signal light, a first wavelength selection means for extracting each of the two wavelength data signal lights from the output light of the measurement light acquisition means, and an output light of the first wavelength selection means. A first light intensity detecting means for detecting the signal light intensity of each of the two wavelengths of data signal light, a polarizer that passes a predetermined polarization component of the output light of the measurement light acquisition means, and the polarizer. The second wavelength selection means for extracting each of the two wavelength data signal lights from the output light of the above, and the signal light intensity of each of the two wavelength data signal lights received by the output light of the second wavelength selection means are detected. It is characterized by being provided with a second light intensity detecting means.

【0016】

According to the present invention, a part of the wavelength-multiplexed data signal is transmitted from the optical transmission path in which the wavelength-multiplexed data signal light including the data signal light of any two wavelengths whose polarization relationship is known by the measurement light acquisition means is transmitted. Light is taken out. When each of the data signal lights of the two wavelengths is extracted from the output light of the measurement light acquisition means by the first wavelength selection means, the extracted data signals of the two wavelengths are extracted by the first light intensity detecting means. The signal light intensity of each light is detected. On the other hand, when a predetermined polarizing component is extracted from the output light of the measurement light acquisition means by the polarizer, each of the data signal lights of the two wavelengths is released from the output light of the polarizer by the second wavelength selection means. After being extracted, the second light intensity detecting means detects the signal light intensity of each of the two wavelength data signal lights in a predetermined polarized state.

【0017】

The PMD detector according to the next invention is characterized in that, in the above invention, the first wavelength selection means and the second wavelength selection means are wavelength tunable selection means.

【0018】

According to the present invention, a tunable wavelength selection means is used as the first wavelength selection means and the second wavelength selection means.

【0019】

The PMD detector according to the next invention extracts a part of the wavelength-multiplexed data signal light from the optical transmission path through which the wavelength-multiplexed data signal light including the data signal light of any two wavelengths whose polarization relationship is known is transmitted. Each of the measurement light acquisition means, the polarizer that allows a predetermined polarization component of the output light of the measurement light acquisition means to pass through, and the wavelength-multiplexed data signal light including the two-wavelength data signal light from the output light of the polarizer. It is characterized by being provided with a light intensity detecting means for detecting the signal light intensity of the above.

【0020】

According to the present invention, a part of the wavelength-multiplexed data signal is transmitted from the optical transmission path in which the wavelength-multiplexed data signal light including the data signal light of any two wavelengths whose polarization relationship is known by the measurement light acquisition means is transmitted. When the light is extracted, the polarizer extracts a predetermined polarizing component from the output light of the measurement light acquisition means, and the light intensity detecting means includes the data signal light of the two wavelengths from the output light of the polarizer. The signal light intensity of each of the wavelength-multiplexed data signal lights is detected.

【0021】

The PMD detector according to the next invention is characterized in that a polarization beam splitter is used instead of the polarizer.

【0022】

According to the present invention, a polarization beam splitter is used instead of the polarizer. In the polarization beam splitter, the input wavelength division multiplexing data signal light is separated into polarization components having an orthogonal relationship with each other, and the light intensity is detected.

【0023】

The wavelength multiplex optical transmission system according to the next invention is a wavelength multiplex data signal light for transmitting wavelength multiplex data signal light including data signal light of arbitrary two wavelengths whose polarization relationship is known. The PMD detector according to any one of claims 1 to 4, wherein a part of the wavelength-multiplexed data signal light is taken out from the optical transmission path through which the data is transmitted, and the signal light intensity of the data signal light having at least two wavelengths is detected. And the first PMD compensating means arranged to perform the PMD compensating operation on the output light of the measuring light acquisition means in the PMD detector, and the PMD compensating operation on the wavelength multiplex data signal light transmitted through the optical transmission path. The first PMD compensating means is controlled so that the PMD analyzed from the signal light intensity detected by the PMD detector is minimized, and the PMD is minimized. It is characterized in that it is provided with a control means that gives the conditions of the first PMD compensating means to the second PMD compensating means.

【0024】

According to the present invention, the first PMD compensating means is arranged in the PMD detector so as to perform a PMD compensating operation on the output light of the measurement light acquisition means, and the second PMD compensating means is for wavelength division multiplexing data signal light on the optical transmission path. Arranged to perform PMD compensation operations. The control means controls the first PMD compensating means so that the PMD analyzed from the signal light intensity detected by the PMD detector is minimized, and the condition of the first PMD compensating means that minimizes the PMD is the second PMD compensating means. Given to.

【0025】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the PMD detector and wavelength division multiplexing optical transmission system according to the present invention will be described in detail with reference to the accompanying drawings.

【0026】

Embodiment 1. FIG. 1 is a block diagram showing a configuration of a PMD detector according to a first embodiment of the present invention. In FIG. 1, wavelength-multiplexed data signal light (λ1 ... λm ... λn ... λz) including two wavelengths (λn, λm) whose polarization relationship is known is transmitted to the optical transmission line 10. ing.

【0027】

The PMD detector shown in FIG. 1 includes an optical tap 11 that extracts a part of wavelength-multiplexed data signal light from an optical transmission path 10, an optical demultiplexer 12 that splits the output light of the optical tap 11 into two, and an optical demultiplexer. An optical demultiplexer 13 that further branches one of the 12 branched lights into two, and optical filters 14 and 15 that receive each branched light of the optical demultiplexer 13 and extract data signal light of a specific wavelength (λm, λn). , A wavelength-multiplexed data signal light having a specific polarization from the photodiodes (hereinafter referred to as "PD") 16 and 17 that electrically convert the output light of the optical filters 14 and 15 and the other branch light of the optical demultiplexer 12. An optical filter that extracts signal light of a specific wavelength (λm, λn) by receiving each branching light of the polarizer 18 to be extracted, the optical demultiplexer 19 that splits the output light of the polarizer 18 into two, and the optical demultiplexer 19. It is equipped with 20,21 and PD22,23 that electrically converts the output light of the optical filters 20,21.

【0028】

Next, the operation of the PMD detector configured in this way will be described. The optical transmission line 10 contains wavelength-multiplexed data signal light (λ1 ... λm ... λn ... λz) containing data signal light of two wavelengths (λn, λm) whose polarization relationship is known. Is being transmitted. The wavelength division multiplexing data signal light (λ1 ... λm ... λn ... λz) extracted by the optical tap 11 from the optical transmission line 10 is branched into two by the optical demultiplexer 12, the optical demultiplexer 13 and the polarizer. Entered in 18.

【0029】

The optical filter 14 that receives one of the branched lights of the optical duplexer 13 extracts data signal light having a wavelength of λm from the branched light. As a result, the intensity detection signal of the data signal light having a wavelength of λm is output from PD16. Similarly, in the optical filter 15 that receives the other branched light of the optical demultiplexer 13, the data signal light having a wavelength of λn is extracted from the branched light. As a result, the intensity detection signal of the data signal light having the wavelength λn is output from PD17.

【0030】

Further, the polarizer 18 extracts the wavelength-multiplexed data signal light having a specific polarization from the branched light input from the optical demultiplexer 12, and outputs the light to the optical demultiplexer 19. In the optical filter 20 that receives one of the branched lights of the optical duplexer 19, data signal light having a wavelength of λm is extracted from the branched light. As a result, the intensity detection signal of the data signal light having a wavelength of λm is output from PD22. Similarly, in the optical filter 21 that receives the other branched light of the optical demultiplexer 19, data signal light having a wavelength of λn is extracted from the branched light. As a result, the intensity detection signal of the data signal light having the wavelength λn is output from PD23.

【0031】

Next, in the detection processing unit (not shown) that receives the output of PD16,17,22,23, the signal light intensity of the wavelengths λm and λn detected by PD16,17 when the polarizer 18 does not intervene and the polarizer 18 intervenes. From the relationship with the signal light intensity of the wavelengths λm and λn detected by PD22 and 23, the polarization relationship at the time of output is calculated with respect to the polarization relationship at the time of input of the optical transmission line 10. The time variation of the amount of PMD in is analyzed.

【0032】

Therefore, according to the first embodiment, since the PMD can be detected by specifying the wavelength, the WDM optical transmission system can follow the time variation of the PMD amount and detect it. Further, since it is possible to detect the amount of PMD over the entire wavelength band by measuring the polarization relationship of a plurality of wavelengths, the size of the PMD detector can be reduced.

【0033】

Embodiment 2. FIG. 2 is a block diagram showing a configuration of a PMD detector according to a second embodiment of the present invention. In the second embodiment, as shown in FIG. 2, in the configuration shown in FIG. 1, variable optical filters 24 to 27 capable of varying the center wavelength are provided instead of the optical filters 14, 15, 20, 21. There is. Others are the same as the configuration shown in FIG. Here, the part related to the second embodiment will be mainly described.

【0034】

By making the optical filter variable, it is possible to detect the light intensity with and without a polarizer for data signal light of two or more wavelengths, so PMD detection with higher accuracy can be performed. It will be possible.

【0035】

Therefore, according to the second embodiment, the PMD can be detected by specifying the wavelength, so that the WDM optical transmission system can follow the time variation of the PMD amount and detect it. Further, since it is possible to detect the amount of PMD over the entire wavelength band by measuring the polarization relationship of a plurality of wavelengths, the size of the PMD detector can be reduced. Furthermore, it is possible to analyze the PMD amount from the signal light of two or more waves, and it is possible to detect the PMD amount with higher accuracy.

【0036】

Embodiment 3. FIG. 3 is a block diagram showing a configuration of a PMD detector according to a first embodiment of the present invention. In FIG. 3, wavelength multiplex data signal light (λ1 ... λm ... λn ... λz) including two wavelengths (λn, λm) whose polarization relationship is known is transmitted to the optical transmission line 10. ing.

【0037】

The PMD detector shown in FIG. 3 has an optical tap 11 that extracts a part of wavelength multiplex data signal light (λ1 ... λm ... λn ... λz) from the optical transmission path 10 and an output light of the optical tap 11. Optical spectrum that simultaneously detects the light intensity of each data signal of the frequency-multiplexed data signal light (λ1 ... λm ... λn ... λz) that receives the output light of the polarizer 18 that receives the light. It is equipped with an analyzer 28.

【0038】

Next, the operation of the PMD detector configured in this way will be described. The optical transmission line 10 contains wavelength-multiplexed data signal light (λ1 ... λm ... λn ... λz) containing data signal light of two wavelengths (λn, λm) whose polarization relationship is known. Is being transmitted.

【0039】

The wavelength division multiplexing data signal light (λ1 ... λm ... λn ... λz) extracted by the optical tap 11 from the optical transmission line 10 is input to the optical spectrum analyzer 28 via the polarizer 18. The optical spectrum analyzer 28 simultaneously detects the light intensities of the wavelength division multiplexing data signal light (λ1 ... λm ... λn ... λz).

【0040】

Then, in the detection processing unit (not shown) that receives the output of the optical spectrum analyzer 28, the polarization relationship at the time of inputting the optical transmission path 10 is determined from the relationship of each signal light intensity after the polarizer detected by the optical spectrum analyzer 28. The polarization relationship at the time of output is calculated, and the time variation of the PMD amount in the optical transmission line 10 is analyzed.

【0041】

Therefore, according to the third embodiment, the PMD can be detected by specifying the wavelength, so that the WDM optical transmission system can follow the time variation of the PMD amount and detect it. Further, since it is possible to detect the amount of PMD over the entire wavelength band by measuring the polarization relationship of a plurality of wavelengths, the PMD detector can be miniaturized. Furthermore, since it is possible to analyze the PMD amount over the entire wavelength band, it is possible to detect the PMD amount with higher accuracy.

【0042】

Embodiment 4. FIG. 4 is a block diagram showing a configuration of a PMD detector according to a fourth embodiment of the present invention. In the fourth embodiment, as shown in FIG. 4, in the configuration shown in FIG. 1, a polarization beam splitter 29 is provided in place of the polarizer 18 and the optical demultiplexer 19. Others are the same as the configuration shown in FIG. Here, the part related to the fourth embodiment will be mainly described.

【0043】

In FIG. 4, the polarization beam splitter 29 separates the wavelength-multiplexed light input from the optical demultiplexer 12 into polarization components having an orthogonal relationship with each other. Therefore, the optical filters 20, 21 and PD 22, 23 can detect the light intensity in the same manner as in the first embodiment, and can obtain the same effect as in the first embodiment.

【0044】

Therefore, according to the fourth embodiment, the PMD can be detected by specifying the wavelength, so that the WDM optical transmission system can detect the time variation of the PMD amount by using the FA method. In addition, since it is possible to detect the amount of PMD over the entire wavelength band by sweeping a variable-length light source using the FA method, the PMD detector can be miniaturized. The polarizer 18 in the third embodiment (FIG. 3) can also be replaced with a polarization beam splitter in the same manner.

【0045】

Embodiment 5. FIG. 5 is a block diagram showing a configuration of a wavelength division multiplexing optical transmission system having a PMD compensation function according to the fifth embodiment of the present invention. As shown in FIG. 5, in this wavelength multiplex optical transmission system, in the configuration shown in FIG. 1, a PMD compensating optical circuit 34 is provided between the output end of the optical tap 11 and the input end of the optical duplexer 12. A PMD-compensated optical circuit 35 is provided on the optical transmission line 10, and a control circuit 36 that electrically controls these PMD-compensated optical circuits 34 and 35 is provided. Intensity signals detected from PD16, 17, 22, and 23 are input to the control circuit 36.

【0046】

Next, the PMD compensation operation in the wavelength division multiplexing optical transmission system having the PMD compensation function configured as described above will be described. The optical transmission line 10 contains wavelength-multiplexed data signal light (λ1 ... λm ... λn ... λz) containing data signal light of two wavelengths (λn, λm) whose polarization relationship is known. Is being transmitted.

【0047】

The wavelength division multiplexing data signal light (λ1 ... λm ... λn ... λz) extracted by the optical tap 11 from the optical transmission line 10 is guided to the optical duplexer 12 via the PMD compensation optical circuit 34. The control circuit 36 analyzes the PMD amount for the data signal light of the wavelength λm and the wavelength λn based on the intensity signals from PD16, 17, 22, 23, and the PMD compensation optical circuit 34 so that the PMD amount is minimized. To control.

【0048】

Next, the control circuit 36 applies the condition of the PMD compensating optical circuit 34 that minimizes the amount of PMD obtained as a result of the control to the PMD compensating optical circuit 35. This makes it possible to compensate for PMD in the optical transmission line 10.

【0049】

As described above, according to the fifth embodiment, the PMD in the optical transmission line 10 can be compensated by combining the PMD detector, the PMD compensation optical circuit, and the control circuit. Therefore, in the WDM optical transmission system, it is possible to detect and compensate for the time variation of the PMD amount.

【0050】

In the fifth embodiment, the configuration of the PMD detector in the first embodiment is used, but it goes without saying that the configuration of the PMD detector in the second to fourth embodiments can be used in the same manner.

【0051】

[Effect of the invention]

As described above, according to the present invention, the measurement light acquisition means is transmitted from an optical transmission path in which wavelength-multiplexed data signal light including data signal light of arbitrary two wavelengths whose polarization relationship is known is transmitted. The wavelength multiplex data signal light of the part is taken out. When each of the data signal lights of the two wavelengths is extracted from the output light of the measurement light acquisition means by the first wavelength selection means, the extracted data signals of the two wavelengths are extracted by the first light intensity detecting means. The signal light intensity of each light is detected. On the other hand, when a predetermined polarizing component is extracted from the output light of the measurement light acquisition means by the polarizer, each of the data signal lights of the two wavelengths is released from the output light of the polarizer by the second wavelength selection means. After being extracted, the second light intensity detecting means detects the signal light intensity of each of the two wavelength data signal lights in a predetermined polarized state. Therefore, it is possible to analyze and detect the time variation of the PMD amount in the optical transmission line based on the signal light intensity obtained in the state without the polarizer and the state with the polarizer. This can be applied over the entire wavelength range, and it becomes possible to detect the amount of PMD that follows the time fluctuation. As a result, the PMD detector can be downsized.

【0052】

According to the following invention, in the above invention, a tunable wavelength selection means is used as the first wavelength selection means and the second wavelength selection means. Therefore, the signal light intensity with and without the polarizer can be obtained from the signal light having two or more wavelengths, so that PMD can be detected with higher accuracy.

【0053】

According to the following invention, a part of the wavelength-multiplexed data is transmitted from the optical transmission path in which the wavelength-multiplexed data signal light including the data signal light of any two wavelengths whose polarization relationship is known is transmitted by the measurement light acquisition means. When the signal light is taken out, the polarizer extracts a predetermined polarizing component from the output light of the measurement light acquisition means, and the light intensity detecting means includes the data signal light of the two wavelengths from the output light of the polarizer. The signal light intensity of each of the wavelength-multiplexed data signal lights is detected. As a result, it is possible to obtain the signal light intensity when the polarizer is present in the entire wavelength range, so that PMD detection with even higher accuracy can be performed.

【0054】

According to the following invention, a polarization beam splitter can be used instead of the polarizer. In the polarization beam splitter, the input wavelength division multiplexing data signal light is separated into polarization components having an orthogonal relationship with each other, and the light intensity is detected. Therefore, the same effect as that of the above invention can be obtained.

【0055】

According to the following invention, the first PMD compensating means is arranged so as to perform a PMD compensating operation on the output light of the measurement light acquisition means in the PMD detector, and the second PMD compensating means is the wavelength division multiplexing data signal light on the optical transmission path. Is arranged to perform PMD compensation operations. The control means controls the first PMD compensating means so that the PMD analyzed from the signal light intensity detected by the PMD detector is minimized, and the condition of the first PMD compensating means that minimizes the PMD is the second PMD compensating means. Given to. Therefore, it becomes possible to construct a wavelength division multiplexing optical transmission system capable of detecting and compensating for PMD.

[Simple explanation of drawings]

[Figure 1]

It is a block diagram which shows the structure of the PMD detector which is Embodiment 1 of this invention.

[Figure 2]

It is a block diagram which shows the structure of the PMD detector which is Embodiment 2 of this invention.

[Fig. 3]

It is a block diagram which shows the structure of the PMD detector which is Embodiment 3 of this invention.

[Fig. 4]

It is a block diagram which shows the structure of the PMD detector which is Embodiment 4 of this invention.

[Fig. 5]

It is a block diagram of the wavelength division multiplexing transmission optical system which has the PMD compensation function which is Embodiment 5 of this invention.

[Fig. 6]

It is a figure which shows the relationship between the bit rate of an optical signal and PMD which causes a 1dB power penalty.

[Fig. 7]

It is a figure for demonstrating the PMD detection method by the conventional FA method.

[Explanation of symbols]

10 optical transmission lines, 11 optical taps, 12,13,19 optical demultiplexers, 14,15,20,21 optical filters, 16,17,22,23 photodiodes (PD), 18 polarizers, 24,25, 26,27 variable optical filter, 28 optical spectrum analyzer, 29 polarized beam splitter, 34,35 PMD compensated optical circuit, 36 control circuit.