JP5792086B2 - Coherent light receiving apparatus and coherent light receiving method - Google Patents

Description

  The present invention relates to an optical receiving apparatus and an optical receiving method in coherent optical communication. The present invention relates to a wavelength division multiplexing coherent transmission system having good reception characteristics.

  In recent years, research and development of a digital coherent fiber transmission system in which a digital signal processing technique is applied to a coherent transmission method has progressed, and a part of the system has been introduced. The current optical communication system is premised on the wavelength multiplexing transmission system, and the coherent transmission system is also premised on the wavelength multiplexing transmission. However, in selecting the desired signal light, it is larger than the conventional direct detection system. There are advantages.

  Originally, the coherent transmission method combines local light emission at a frequency very close to the carrier frequency of a desired optical signal from among a large number of optical signals received simultaneously at the receiver, and receives the electrical signal by a photodiode. In contrast to the direct detection method, optical components for wavelength selection for extracting only the desired optical signal are not required in principle. [Non-Patent Document 1].

  Therefore, for example, if a digital coherent method is introduced into a ROADM (Reconfigurable Optical Drop Drop Multiplexer) system, a WADM (Wavelength Selective Switch) or an AWG (Wavelength Selective Wave) or AWG (Wavelength Selective Wave) or AWG (Wavelength Selective Wave) or AWG (Wavelength Selective Wave) It is considered that a device such as a variable filter becomes unnecessary, and cost reduction is realized. In the case of the direct detection method, if the performance of such wavelength selection components is insufficient, noise due to crosstalk due to non-selected wavelengths (channels) may occur. There is no. However, it is assumed that the receiver band is sufficiently smaller than the channel spacing (typically 50 GHz).

  As described above, in the coherent transmission method, it is possible in principle to receive light of all channels at once without using an optical filter or the like.

S. Ryu, “Coherent Lightwave Communication Systems”, Arttech House, 1995, pp. 1-4. S. Ryu, “Coherent Lightwave Communication Systems”, Arttech House, 1995, pp. 117-122. M.M. Seimetz, “High-Order Modulation for Optical Fiber Transmission”, Springer, 2009, pp. 79-84. Optical Internetworking Forum, Implementation Agreement for Integrated Dual Polarization Intradyne Coherent Receivers, IA # OIF-DPC-RX-01.0. Watanabe, et al., “Ultra-low power consumption PLC optical VOA switch”, IEICE Technical Report, OPE 2007-14 (2007-05).

  As described above, in the coherent transmission system, in general, crosstalk due to a non-selected wavelength, that is, a different wavelength has not been originally considered. However, such recognition is not based on the premise of using an optical amplifier. In an existing optical fiber transmission system, an optical signal passes through the optical amplifier, so that ASE (Amplified Spontaneous Emission) generated from the optical amplifier is reduced. It will mix. For this reason, ASE is included in all received optical signals regardless of selection or non-selection, and the beat frequencies of the non-selection optical signal and the ASE signal included therein fall within the band of the receiver. , It becomes noise (beat noise). Therefore, even in the case of coherent reception, in reality, the effect of the non-selected optical signal becomes a problem, and the noise increases as the power of the non-selected optical signal and the number of channels increase. Occurs. Further, if there is intensity fluctuation in the non-selected optical signal, intensity noise due to the intensity fluctuation may also occur.

  On the other hand, if a coherent optical receiver using a balanced photodiode originally proposed to cancel intensity noise due to local light emission is used, not only the intensity noise of the unselected optical signal but also the unselected optical signal and the unselected optical signal are included in it. Although it is possible to cancel the influence of beat noise caused by ASE, an actual receiver has imperfections defined by CMRR (Common Mode Rejection Ratio) [Non-Patent Document 2] It is considered difficult to completely cancel the influence of noise and beat noise.

The above will be described by taking the case of a reception system of the polarization multiplexing QPSK modulation method as a specific example. Hereinafter, the display of the electric field or the like is referred to [Non-Patent Document 3]. FIG. 1 shows a model diagram of the receiving system. Let E _s (t) be the electric field of the selected optical signal, and let E _i (t) be the signal optical field of the other channel that is not selected. These optical amplifiers, for example, _{EDFA (Erbium Doped Fiber Amplifier) n} s as a final noise to pass _(t), ASE field _n i (t) is added. Here, n _s (t) is the ASE electric field at the selected optical signal frequency, and n _i (t) is the ASE electric field at the non-selected channel frequency. Each signal is divided into an X polarization and a Y polarization by a PBS (Polarization Beam Splitter) and is incident on a 2 × 4 90 degree hybrid. Since the following description is not directly related to polarization, attention is paid only to X polarization. The relationship between input and output to the 90-degree hybrid is expressed by equations (1) to (4).

式（５）の第２項は、選択光信号に付随するＡＳＥ成分であり、第３項は、非選択チャネル信号および非選択チャネル周波数におけるＡＳＥ成分の和である。 Here, E _s (t) and E _lo (t) are a signal electric field and a local light emission electric field, respectively. E _out1 (t) and E _out3 (t) are I-phase component electric fields, and E _out2 (t) and E _out4 (t) are Q-phase component electric fields. Each coefficient is ideally ½, but actually deviates slightly from ½, so strictly speaking, the coefficients a _{s1 to} a _s4 , a _l1 to, as in (1) to (4). a ₁₄ is used. Hereinafter, E _s (t) is replaced as shown in Equation (5) because it is assumed that multi-wavelength optical signals are collectively received.

The second term of equation (5) is the ASE component associated with the selected optical signal, and the third term is the sum of the unselected channel signal and the ASE component at the unselected channel frequency.

The output from the 90-degree hybrid is received by each photodiode. At this time, the current flowing through the photodiode is given by (6) to (9). Here, R _{1 to} R ₄ are the sensitivity of each photodiode. Also _{i sh1, i sh2, i sh3} , i sh4 is the shot noise current.

Next, CMRR is defined as (10) to (13) with respect to the signal light and the local light for the I phase and the Q phase, respectively. Here, the absolute values of these values correspond to the usual definition of CMRR [Non-Patent Document 4], but the essence is not changed.

ここで、

は、それぞれ選択光信号とＡＳＥとのビート成分（雑音）、非選択光信号とＡＳＥとのビート成分（雑音）、選択光信号と局発光とのビート成分（信号成分）、局発光とＡＳＥとのビート成分（雑音）である。 As described above, the output of the balanced photodiode is expressed as (14) and (15) with respect to the I phase and the Q phase. However, the phase fluctuation of the laser beam and shot noise were ignored.

here,

Are the beat component (noise) of the selected light signal and ASE, the beat component (noise) of the unselected light signal and ASE, the beat component (signal component) of the selected light signal and local light, and the local light and ASE, respectively. Beat component (noise).

On the other hand, in the case of an ideal receiving system, each CMRR is 0 and is expressed as (16) and (17). The first term is the beat signal between the selection signal light and the local light, and the second term is the beat noise between the local light and the ASE light.

  When CMRR is 0, the signal-to-noise ratio depends only on the second term, but when CMRR cannot be ignored due to imperfections due to manufacturing errors of a coherent optical receiver using a balanced photodiode, various noise currents are generated. Present and the signal-to-noise ratio will be degraded. However, in actual manufacture of a coherent optical receiver, it is unavoidable that an error occurs, and CMRR has some value, so it is difficult to completely cancel various noise currents.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a coherent optical receiver using a balanced photodiode having a small CMRR.

In order to solve the problem, it is only necessary to set CMRR _SI = 0 and CMRR _SQ = 0 using some correction means from the equations (10) and (12) and the equations (14) and (15). I understand. That is, in principle, R ₁ a _s1 ² −R ₂ a _s3 ² = 0 and R ₃ a _s2 ² −R ₄ a _s4 ² = 0 may be satisfied. R _{1 to} R ₄ are photodiode sensitivities, which are difficult to control. With regard to a _{s1 to} a _s4 , a mechanism for adjusting the light input to the balanced photodiode is added, so that a _s1 ~ _As4 can be changed and each CMRR becomes controllable.

  Therefore, the present invention provides a coherent light receiving apparatus that demodulates only signal light of one wavelength to be received using the frequency selection characteristic of coherent detection without removing signal light other than the reception target from the WDM signal by an optical filter. In order to suppress the influence of signal light interference other than the reception target, the common mode rejection ratio (CMRR) relating to the signal light to be received is adjusted.

Specifically, in the coherent optical receiver according to the present invention, a multi-wavelength signal in which a plurality of signal lights are wavelength-multiplexed and local light for demodulating the signal light are input, and the signal light and the local light are transmitted. An optical circuit that distributes and outputs the mixed light for each phase difference between the signal light and the local light, and receives the mixed light from the optical circuit, and the mixed light having the phase difference opposite in phase. By controlling the power of the mixed light for each phase difference and the balanced photodiode that outputs an electrical signal corresponding to the difference, the CMRR determined from the characteristics of the optical circuit and the balanced photodiode is zero. as approaching, and a power control means for changing the power of the mixed light for each of the phase difference, the power control unit, the multi-wavelength signal or the local light has been input to the optical circuit Monitoring the output current of the balanced photodiode in a negative state, controlling the power of the mixed light for each phase difference so that the output current is minimized, and the power control means includes the balanced photodiode monitoring the signal quality of the optical signal demodulated by using the output current of, so that the signal quality is optimized, that controls the power of the mixed light for each of the phase difference.

  In the coherent optical receiver according to the present invention, the power control means is an optical attenuator provided in an optical path between the optical circuit and the balanced photodiode, for attenuating the power of the mixed light for each phase difference. There may be.

  In the coherent light receiving device according to the present invention, the power control means may be a movable mechanism that controls the position of the balanced photodiode for each phase difference of the mixed light.

  In the coherent optical receiver according to the present invention, the power control means may be an optical attenuation circuit that is provided in the optical circuit and attenuates the power of the mixed light for each phase difference.

Specifically, in the coherent light receiving method according to the present invention, the signal light is transmitted from an optical circuit into which a multi-wavelength signal in which a plurality of signal lights are wavelength-multiplexed and a local light for demodulating the signal light is input. The mixed light with the local light is output for each phase difference between the signal light and the local light, and the mixed light from the optical circuit is received using a balanced photodiode to receive the multi-wavelength signal. A coherent light receiving method for receiving, wherein by controlling the power of the mixed light for each phase difference, the CMRR determined from characteristics of the optical circuit and the balanced photodiode approaches zero. the power of the mixed light and a power control procedure for changing for each of said phase difference, have a, a receiving step of receiving said multiwavelength signal sequentially, said power control step, the multi-wavelength signal Monitors the output current of the balanced photodiode in a state where the local light is not input to the optical circuit, and controls the power of the mixed light for each phase difference so that the output current is minimized. The signal quality of the optical signal demodulated using the output current of the balanced photodiode is monitored, and the power of the mixed light is controlled for each phase difference so that the signal quality is optimized .

  The above inventions can be combined as much as possible.

  In a coherent optical receiver using a balanced photodiode, CMRR can be made sufficiently small, so that good reception characteristics can be obtained when receiving wavelength multiplexed signals all at once.

It is explanatory drawing of the model of the coherent optical receiving system of a polarization multiplexing QPSK modulation system using a balance type photodiode. It is explanatory drawing of a structure of the coherent optical receiver using the conventional balance type | mold photodiode. It is explanatory drawing of the 90 degree hybrid which provided the variable optical attenuator. It is a figure explaining controlling the electric current to the thin film heater of the variable optical attenuator on PLC so that the signal voltage in a front end may become the minimum. It is a figure explaining the structure for adjusting CMRR single-piece | unit using local light. It is explanatory drawing of the coherent optical receiver which provided the variable optical attenuator between 90 degree | times hybrid and a photodiode. It is explanatory drawing of the coherent optical receiver provided with the mechanism which can change the position of a photodiode.

  Hereinafter, embodiments of the present invention will be described in detail. A significant difference from a coherent optical receiver using a conventional balanced photodiode is that a mechanism capable of controlling light input to the balanced photodiode is provided. A schematic diagram of a conventional coherent optical receiver is shown in FIG. The coherent light receiving device includes a local laser 11, a 90-degree hybrid 12 as an optical circuit, a balanced photodiode 13, and a signal processing unit 14. The 90-degree hybrid 12 receives a multi-wavelength signal in which a plurality of signal lights are wavelength-multiplexed and a local light from the local laser 11 and mixes the multi-wavelength signal and the local light so that the signal light and the local light are mixed. A mixed light having a phase difference of 0 °, 90 °, 180 °, and 270 ° is generated. The balanced photodiode 13 receives the mixed light having a phase difference of 0 ° and the mixed light having a phase difference of 180 °, and outputs an electric signal corresponding to the difference between the mixed light and the mixed light having a phase difference of 90 °. Receives mixed light of 270 ° and outputs an electrical signal corresponding to the difference between them. The signal processing unit 14 demodulates the signal light of each channel using the output signal from the balanced photodiode 13 and reproduces the transmission data. Between 90-degree hybrid 12 and balanced photodiode 13, a mechanism for adjusting the delay between mixed lights having different phase differences may be provided.

The coherent optical receiver according to the present embodiment includes power control means in order to reduce CMRR. The coherent light receiving method according to the present embodiment includes a power control procedure in which the power control means independently controls the power of mixed light having a phase difference between the signal light from the 90-degree hybrid 12 and the local light, and a multi-wavelength signal. Receiving procedures for receiving. Thereby, a _{s1 to} a _s4 can be changed, and each CMRR can be controlled. Ideally, each CMRR should be set to 0, but in reality, it should be as close to 0 as possible in some way.

In addition, since fluctuation of local light emission may be a problem, it is desirable that CMRR _LI and CMRR _LQ be as small as possible. However, if CMRR _SI and CMRR _SQ are small, CMRR _LI and CMRR _LQ are also considered to be small. Here, the actual target values of CMRR _SI and CMRR _SQ may be primarily the recommended values described in [Non-Patent Document 4]. However, local light emission and signal light can be obtained by the following simple consideration. A target value or a guideline of necessary conditions can also be obtained from the power ratio.

First, Equation (14) is considered by dividing it into a component (18) depending on local light and an independent component (19). The same applies to (15).

As described above, originally, as can be seen from (16) and (17), only the beat noise between the local light and the ASE is noise, which limits the reception sensitivity and transmission quality. On the other hand, as described above, when the balanced reception is incomplete, various noises are manifested. However, from the equation (19), | E _i (t) | ^{2 and the} like are almost DC components. The sum of the beat noise of each optical signal and ASE is considered to be dominant. If the sum of the beat noises is about the same as the beat noise of local light and ASE, which is a limiting factor of the original transmission quality, it is considered that the transmission quality is significantly deteriorated from the original value and the transmission penalty becomes remarkable. From the equations (18) and (19), the condition that the noise current caused by the beat between the local light and the ASE and the noise current caused by the beat between each optical signal and the ASE are approximately equal to the equation (20) It is expressed as Further, since the equation (21) also holds, the condition becomes simple as the equation (22).

Equation (22) shows the condition that the transmission penalty becomes significant, but if the left side is larger than the right side, the transmission penalty becomes smaller.
Local light emission power> | CMRR _SI | × Sum of powers of all received optical signals (23)
It can be said that this condition is a necessary condition that the transmission penalty does not appear remarkably. Here, the ASE is assumed to be the same level in all channels.

In addition, there is no difference between the I phase and the Q phase, and the polarization multiplexed signal may be an equation as it is, and using a general CMRR expression,
Local light emission power> CMRR × sum of the power of all received optical signals (24)
Can be expressed as

Alternatively, the local emission power (dBm)> CMRR (dB) + the sum of the power of all received optical signals (dBm) in decibel display.
(25)
But you can.

Alternatively, the above is modified so that the ratio of the excitation light and the total received optical signal power (dB)> CMRR (dB) (26)
(The left side is 10 × log ₁₀ [excitation light power ÷ total optical signal power]).
In this manner, a target value of CMRR or a necessary condition to be satisfied is obtained.

It should be noted here that since the characteristics of optical components and the sensitivity of the photodiode are generally wavelength-dependent, the CMRR value is considered to be wavelength-dependent, and the CMRR value is determined using a certain wavelength (channel). Even if it is minimized, it is not always the optimum condition when receiving signals of different wavelengths. For this reason, when the selected wavelength changes due to an optical path change or the like, it is necessary to change the conditions so that noise does not increase. Further, in deriving (14) and (15), ignoring the wavelength dependence of the characteristics of the optical component and the photodiode described above, which is the coefficient a _{s1 to} a _s4 in (1) to (4)? Although the wavelength is assumed to be constant, it is considered that the wavelength actually changes slightly. Therefore, even if the CMRR is optimized at a certain single wavelength, there is a possibility that the multi-wavelength collective reception may deviate from the optimum condition.

  Therefore, in the present invention, the CMRR is roughly controlled by signal light or local light of a certain wavelength before operation, and the CMRR is minimized as much as possible. Then, after the operation, the information on the state of the electrically converted selection signal is always fed back to the physical adjustment mechanism for controlling the CMRR. Thus, the present invention does not optimize the CMRR in advance and use it as a fixed value, but may deteriorate from the initial value CMRR, but the state of the electrically converted signal is improved. The CMRR is changed at any time. As a result, even if the selected wavelength is changed or the number of wavelengths to be collectively received is changed due to the dynamic change of the optical path, the transmission quality can be kept as high as possible by the ratio between the local light and the ASE. it can.

(Embodiment 1)
One embodiment of the present invention is shown in FIG. As power control means, a variable optical attenuator (VOA) is formed in the planar lightwave circuit itself forming the 90-degree hybrid 12. The variable optical attenuator 23 is, for example, a thin film heater 232 that independently attenuates the power of light separated into each phase component. The variable light attenuator 23 on the planar lightwave circuit forms a 3 dB coupler 231 that splits the mixed light separated into each phase component into two, forms a thin film heater 232 on each branch path, and passes these branch paths. This can be realized by forming a coupler 234 to be coupled. The amount of attenuation can be realized by controlling the current flowing through the thin film heater 232 [Non-Patent Document 5].

  Regarding the actual attenuation setting, as can be seen from the equations (16) and (17), when CMRR is 0, no current flows unless local light is incident. And the attenuation amount may be adjusted so that the signal voltage at the front end is minimized.

  FIG. 4 shows an explanatory diagram thereof. The signal processing unit 14 calculates a signal voltage value of each output signal from the balanced photodiode 13 and outputs the signal voltage value to the CMRR control unit 27. The CMRR control unit 27 controls the current flowing through each thin film heater 232 so that the signal voltage value from the signal processing unit 14 is minimized. This makes it possible to minimize the CMRR as much as possible before operation.

  FIG. 5 includes a 2 × 2 optical switch 28 in front of the 90-degree hybrid 12 and switches the local light to be incident on the signal light port in the absence of signal light. It is the structure for using for. This is also for controlling the CMRR before operation. Also in this case, the CMRR control unit 27 controls the current flowing through the thin film heater 232 so that the signal voltage value from the signal processing unit 14 is minimized.

  Next, after the multi-wavelength collective reception actually starts, the CMRR control unit 27 monitors the demodulated signal quality, for example, the Q value, and controls the CMRR value so that the signal quality is optimized. Specifically, for example, in the case of FIG. 3, the CMRR control unit 27 slightly gives the attenuation amount of the VOA 23 on one side of the balanced photodiode 13 and slightly changes the balance of the balanced photodiode 13. For example, the amount of attenuation is changed by changing the current flowing through the thin film heater 232 that attenuates one of the mixed lights having a phase difference that is in reverse phase. The signal processing unit 14 calculates the demodulated signal quality and outputs it to the CMRR control unit 27. The CMRR control unit 27 further attenuates the VOA 23 in the same direction when the signal quality from the signal processing unit 14 slightly increases, while the balance in the opposite direction when the transmission quality from the signal processing unit 14 decreases. What is necessary is just to operate VOA23 so that it may change.

(Embodiment 2)
FIG. 6 shows an example of a coherent light receiving apparatus according to this embodiment. In the coherent optical receiver according to this embodiment, a VOA 21 is provided as a power control unit between the 90-degree hybrid 12 and the balanced photodiode 13. The VOA 21 attenuates the power of the mixed light for each phase component.

  The attenuation setting is the same as that in the first embodiment. The difference from the first embodiment is that the CMRR control unit 27 controls the attenuation amount of the variable light source attenuator 21 for each mixed light having a different phase difference between the signal light and the local light.

(Embodiment 3)
FIG. 7 shows an example of a coherent light receiving apparatus according to this embodiment. The coherent optical receiver according to this embodiment includes power control means for controlling the input power to the two balanced photodiodes 13 by changing the position of the balanced photodiodes 13. For example, the position of the balanced photodiode 13 is moved using the piezo actuator 22. For example, the movement is performed horizontally in a direction substantially perpendicular to the optical axis of the output light from the 90-degree hybrid 12.

  The attenuation setting is the same as that in the first embodiment. The difference from the first embodiment is that the CMRR control unit 27 controls the moving distance of the balanced photodiode 13 for each mixed light in which the phase difference between the signal light and the local light is different.

  As described above, the present invention is applied to a wavelength division multiplexing optical transmission system, and is particularly useful for a wavelength division multiplexing optical network having an optical add / drop function.

11: Local emission laser 12: 90 degree hybrid 13: Balanced photodiode 14: Signal processing unit 21, 23: VOA
231, 234: Coupler 232: Thin film heater 27: CMRR control unit 28: 2 × 2 switch

Claims (5)

A multi-wavelength signal in which a plurality of signal lights are wavelength-multiplexed and a local light for demodulating the signal light are input, and a mixed light of the signal light and the local light is mixed for each phase difference between the signal light and the local light. An optical circuit that distributes and outputs to
A balanced photodiode that receives the mixed light from the optical circuit and outputs an electrical signal corresponding to a difference between the mixed lights, the phase difference of which is in reverse phase;
CMRR (Common Mode Rejection Ratio) that is determined from the characteristics of the pre-Symbol light circuit and the balanced photodiode so approaches zero, and a power control means for changing the power of the mixed light for each of the phase difference ,
The power control means monitors the output current of the balanced photodiode in a state where the multi-wavelength signal or the local light is not input to the optical circuit, so that the mixed light is minimized. For each phase difference,
The power control means monitors the signal quality of the optical signal demodulated using the output current of the balanced photodiode and controls the power of the mixed light for each phase difference so that the signal quality is optimized. To
Coherent optical receiver.   The power control means is an optical attenuator that is provided in an optical path between the optical circuit and the balanced photodiode and attenuates the power of the mixed light for each phase difference. The coherent optical receiver described.   The coherent light receiving apparatus according to claim 1, wherein the power control unit is a movable mechanism that controls the position of the balanced photodiode for each phase difference of the mixed light.   The coherent optical receiver according to claim 1, wherein the power control unit is an optical attenuation circuit that is provided in the optical circuit and attenuates the power of the mixed light for each phase difference. A mixed light of the signal light and the local light is converted into the signal light and the local light from an optical circuit to which a multi-wavelength signal in which a plurality of signal lights are wavelength-multiplexed and local light for demodulating the signal light is input. A coherent light receiving method for receiving the multi-wavelength signal by receiving the mixed light from the optical circuit using a balanced photodiode.
As CMRR determined from the characteristics of the pre-Symbol light circuit and the balanced photodiode approaches zero, the power control procedure for changing the power of the mixed light for each of the phase difference,
A receiving procedure for receiving the multi-wavelength signal;
Have a in order,
The power control procedure is:
The output current of the balanced photodiode in a state where the multi-wavelength signal or the local light is not input to the optical circuit is monitored, and the power of the mixed light is set to the phase difference so that the output current is minimized. Control every
Monitoring the signal quality of the optical signal demodulated using the output current of the balanced photodiode, and controlling the power of the mixed light for each phase difference so that the signal quality is optimal;
Coherent light reception method.

Images