JP2019028082A - Optical receiver and coherent light receiving method - Google Patents

Abstract

To provide an optical receiver for configuring a coherent light communication system at lower cost.SOLUTION: An optical receiver includes: a light source for generating local light; adjustment means for adjusting at least one amplitude of the local light and signal light so as to coincide the amplitude of the local light with the amplitude of the signal light to be received; output means for outputting multiplexed light including the local light and the signal light whose polarization planes intersect with each other at right angles based on the local light and the signal light after adjustment by the adjustment means; and measurement means for measuring stokes parameters Sand Sof the multiplexed light.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical receiver and a coherent optical reception method of a coherent optical communication system.

  A coherent optical communication system is used to cope with an increase in communication capacity. An optical receiver of a coherent optical communication system requires a complicated optical circuit and signal processing. For example, in a system connecting relatively short distances, the cost of the optical receiver is relatively high. Therefore, there is a demand for an inexpensive optical receiver that can be used in a coherent optical communication system. Non-Patent Document 1 discloses a homodyne optical receiver including a 120 ° optical hybrid, three photodiodes (PD), and three analog-digital converters (ADC).

K. Y. Cho et al., “Long-reach coherent WDM PON Employing self-polarization-stabilization technique”, IEEE JLT, vol. 29,456-462, 2011

  However, it is required to configure the coherent optical communication system at a lower cost.

  The present invention provides an inexpensive optical receiver and coherent optical receiving method.

According to an aspect of the present invention, the optical receiver adjusts at least one amplitude of the local light and the signal light so that the light source that generates the local light matches the amplitude of the received signal light. And output means for outputting the combined light including the local light and the signal light whose polarization planes are orthogonal to each other based on the local light and the signal light after adjustment by the adjusting means, and a Stokes parameter S of the combined light measuring means for measuring the ₂ and S _3, characterized in that it comprises.

  According to the present invention, a coherent optical communication system can be configured at a lower cost.

The block diagram of the optical receiver by one Embodiment. Explanatory drawing of Stokes parameter measurement. The figure which shows the substitution form of the optical receiver of FIG. The figure which shows the substitution form of the optical receiver of FIG. The figure which shows the substitution form of the optical receiver of FIG.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. In addition, the following embodiment is an illustration and does not limit this invention to the content of embodiment. In the following drawings, components that are not necessary for the description of the embodiments are omitted from the drawings.

  FIG. 1 is a configuration diagram of an optical receiver according to the present embodiment. The optical receiver receives and demodulates the modulated signal light. Note that the polarization of the signal light is Y polarization. The light source 1 generates and emits local light for coherent reception. Note that the local light is X-polarized light orthogonal to Y-polarized light. The optical variable attenuator (optical variable ATT) 21 adjusts the amplitude of the local light, and outputs the local light after the amplitude adjustment to the 2 × 2 optical coupler 3. Similarly, the optical variable ATT 22 adjusts the amplitude of the signal light, and outputs the signal light after the amplitude adjustment to the 2 × 2 optical coupler 3. The optical variable ATTs 21 and 22 adjust the amplitudes of the local light and the signal light so that the adjusted local light and the signal light have the same amplitude.

  The 2 × 2 optical coupler 3 combines the input local light and signal light, and outputs the combined light to the 45-degree polarizer 51 and the quarter wavelength plate 4, respectively. Therefore, the amplitude of the local light and the signal light included in the combined light input to the 45-degree polarizer 51 and the combined light input to the quarter wavelength plate 4 are the same, and the local light and the signal light The planes of polarization are orthogonal. In the present embodiment, the quarter-wave plate 4 delays Y-polarized light by ¼ wavelength with respect to X-polarized light and outputs the delayed light to the 45-degree polarizer 52. The quarter-wave plate 4 may delay the X-polarized light by a quarter wavelength with respect to the Y-polarized light.

  The 45 degree polarizers 51 and 52 pass only the components of the polarization plane having an angle of 45 degrees with respect to the polarization planes of the X polarization and the Y polarization, respectively. Therefore, the photodiode (PD) 61 that is the photoelectric conversion unit includes a local light component and a signal light component having a polarization plane having an angle of 45 degrees with respect to each of the polarization planes of the X polarization and the Y polarization. Wave light is received, and the beat signal of these two components is output as an electrical signal. Similarly, the PD 62 also receives the combined light including the local light component and the signal light component of the polarization plane having an angle of 45 degrees with respect to the polarization planes of the X polarization and the Y polarization, respectively. The beat signal is output as an electrical signal. However, the signal light component input to the PD 62 is delayed by ¼ wavelength by the ¼ wavelength plate 4. The electrical signals output from the PD 61 and the PD 62 are converted into digital signals by analog / digital converters (ADC) 71 and 72, respectively, and these digital signals are output to the processing unit 8.

Next, the Stokes parameters of the combined light output from the 2 × 2 optical coupler 3 will be described. When the X-polarized local light is represented by a complex number Ex and the Y-polarized signal light is represented by a complex number Ey, the Stokes parameters S ₀ , S ₁ , S ₂ and S _{3 of} the combined light output from the 2 × 2 optical coupler 3 are used. Are represented by the following equations, respectively.
S ₀ = | Ex | ² + | Ey | ² (1)
S ₁ = | Ex | ² − | Ey | ² (2)
S ₂ = 2Re [Ex ^* Ey] (3)
S ₃ = 2Im [Ex ^* Ey] (4)
In Expressions (3) and (4), Ex ^* is a conjugate complex number of Ex, and Re and Im mean that a real part and an imaginary part are extracted, respectively. As is apparent from the equations (3) and (4), S ₂ + jS ₃ corresponds to a signal obtained by coherent detection of the signal light Ey, and the signal light can be demodulated by S ₂ + jS ₃ .

The Stokes parameters S ₀ , S ₁ , S ₂ and S ₃ have the following relationship.
S ₀ ² = S ₁ ² + S ₂ ² + S ₃ ² (5)

Next, measurement of Stokes parameters will be described. The optical signal to be measured is branched into four and input to the circuits shown in FIGS. It is assumed that the four optical signals after branching have the same amplitude. In the circuit of FIG. 2A, the PD outputs a current I ₀ corresponding to the total amount of the branched light. In the circuit of FIG. 2B, only the light component of the reference polarization plane of the branched light is extracted by the 0 degree polarizer, and the PD outputs a current I ₁ corresponding to this light component. In the circuit of Fig. 2 (C), extraction of light components of the polarization plane having an angle of 45 degrees with respect to the reference plane of polarization from the branched light, PD outputs a current I ₂ corresponding to the optical component. In the circuit of FIG. 2D, the phase of the light component of the polarization plane at an angle of 90 degrees with respect to the reference polarization plane is delayed by ¼ wavelength, and then the angle of 45 degrees with respect to the reference polarization plane is set. The light component having the polarization plane is extracted from the branched light, and the PD outputs a current I ₃ corresponding to the light component. As is well known, the Stokes parameter is obtained from the currents I ₀ , I ₁ , I ₂ , and I ₃ by the following equation.
S ₀ = I ₀ (6)
S ₁ = 2 × I ₁ −I ₀ (7)
S ₂ = 2 × I ₂ −I ₀ (8)
S ₃ = 2 × I ₃ −I ₀ (9)

Here, the 45-degree polarizer 51 and the PD 61 in FIG. 1 correspond to the circuit in FIG. 2C, and the quarter-wave plate 4, the 45-degree polarizer 52 and the PD 62 in FIG. Corresponds to the circuit. That is, the PD 61 in FIG. 1 outputs the current I ₂ for the combined light, and the PD 62 outputs the current I ₃ for the combined light.

Therefore, when the current output from the PD 61 is expressed as I ₂ and the current output from the PD 62 is expressed as I ₃ , the Stokes parameters S ₂ and S ₃ of the combined light can be obtained by the following equations.
S ₂ = 2 × I ₂ −I ₀ (10)
S ₃ = 2 × I ₃ −I ₀ (11)
Substituting equation (6) into equations (10) and (11) yields the following equation:
S ₂ = 2 × I ₂ −S ₀ (12)
S ₃ = 2 × I ₃ −S ₀ (13)

Further, as described above, in the present embodiment, the optical variable ATTs 21 and 22 adjust the amplitudes of the local light and the signal light so that the amplitudes of the adjusted local light and the signal light are the same. That is, | Ex | = | Ey |, and S ₁ = 0 from Equation (2). Substituting S ₁ = 0 into equation (5) yields:
S ₀ ² = S ₂ ² + S ₃ ² (14)

By solving the simultaneous equations of Expressions (12) to (14), S ₂ and S ₃ can be obtained from the current I ₂ output from the PD 61 and the current I ₃ output from the PD 62, and thereby the signal light Demodulation can be performed. The processing unit 8 in FIG. 1 solves the simultaneous equations of Expressions (12) to (14) from digital signals indicating the currents I ₂ and I ₃ output from the ADC 71 and the ADC 72, thereby obtaining S ₂ and S ₃ . Find and demodulate.

  As described above, in this embodiment, the number of PDs and ADCs is two, and the number of PDs and ADCs can be reduced one by one from the configuration described in Non-Patent Document 1. In the present embodiment, the optical variable ATTs 21 and 22 are provided corresponding to the signal light and the local light, respectively. However, for example, since the amplitude of the signal light is usually smaller than the amplitude of the local light, the optical variable is possible. A configuration in which only the ATT 21 is provided may be used. Further, the attenuation amount in the optical variable ATTs 21 and 22 may be set manually before starting the service. For example, the amplitude of the optical signal output from the optical variable ATTs 21 and 22 is monitored and becomes the target amplitude. A configuration in which the attenuation amount is automatically adjusted may be employed.

  Further, the 2 × 2 optical coupler 3 of FIG. 1 can be replaced with a polarizing beam splitter 31 and a 1 × 2 optical coupler 32 as shown in FIG. The polarization beam splitter 31 combines the X-polarized local light and the Y-polarized signal light and outputs them to the 1 × 2 optical coupler 32. The 1 × 2 optical coupler 32 branches the combined light, respectively. 1 is output to the 45-degree polarizer 51 of FIG.

  In the configuration of FIG. 1, the signal light has a polarization orthogonal to the local light. This can be achieved, for example, by using a polarization maintaining fiber in the optical transmission line. Even when the polarization maintaining fiber cannot be used, for example, the polarization plane of the signal light can be orthogonal to the local light by using the polarization controller. Furthermore, even when the polarization maintaining fiber cannot be used, the polarization plane of the signal light can be made orthogonal to the local light by adopting the configuration shown in FIG.

  In FIG. 4, the light source 1 outputs X-polarized local light to the 1 × 2 optical coupler 91, and the 1 × 2 optical coupler 91 supplies the X-polarized local light to the optical variable ATT 21 and the polarization beam splitter 92, respectively. Output. The polarization beam splitter 92 outputs X-polarized local light to the optical transmission line 100. The optical transmitter connected to the other end of the optical transmission line 100 reflects and amplifies the received local light using, for example, a reflective reaction body optical amplifier (RSOA), and modulates the reflected light. And transmit to the optical receiver. At this time, the optical transmitter rotates the polarization plane of the reflected light by 90 degrees by reciprocating the Faraday rotator.

  Since the optical fiber constituting the optical transmission line 100 is not a polarization maintaining type, the plane of polarization of the X-polarized local light transmitted from the optical receiver to the optical transmission line 100 can vary in the optical transmission line 100. However, since the optical signal reciprocates in the optical transmission line, the fluctuation of the polarization plane is canceled out. Therefore, only the 90-degree rotation of the polarization plane by the optical transmitter remains, and thus the optical receiver receives Y-polarized signal light. Then, the polarization beam splitter 92 of the optical receiver outputs Y-polarized signal light to the optical variable ATT 22. With this configuration, the polarization planes of the local light and the signal light can be made orthogonal without using a polarization maintaining optical fiber. In this case, the optical receiver is a homodyne optical receiver. However, the present invention can be applied to both a heterodyne optical receiver and a homodyne optical receiver in a configuration in which signal light and local light are separately generated, such as when a polarization maintaining optical fiber is used.

  FIG. 5 is a diagram showing a modification of the optical receiver of FIG. In the configuration of FIG. 5, the local light and the signal light have the same Z polarization, and the polarization plane of the Z polarization is an angle of 45 degrees with the polarization plane of each of the X polarization and the Y polarization. The polarization beam splitter 33 linearly advances the X polarization component and reflects and outputs the Y polarization component. That is, the polarization beam splitter 33 outputs the X polarization component of the Z-polarized local light output from the optical variable ATT 21 from the port # 4 and outputs the Y polarization component from the port # 3. Further, the polarization beam splitter 33 outputs the Y-polarized component of the Z-polarized signal light output from the optical variable ATT 22 and outputs the X-polarized component from the port # 3. Therefore, the combined light of the X-polarized signal light and the Y-polarized local light is output from the port # 3 of the polarization beam splitter 33. On the other hand, from the port # 4 of the polarization beam splitter 33, the combined light of the Y-polarized signal light and the X-polarized local light is output. That is, the polarization beam splitter 33 outputs combined light of signal light and local light that are orthogonal to each other and have the same amplitude. The combined light output from the port # 3 of the polarization beam splitter 33 passes through the 45-degree polarizer 51 and is then photoelectrically converted by the PD 61. Note that the 45 degree polarizer 51 allows the Z polarization component to pass through. On the other hand, the combined light output from the port # 4 of the polarization beam splitter 33 passes through the quarter-wave plate 4 and the 45-degree polarizer 52 and is then photoelectrically converted by the PD 62. Note that the 45-degree polarizer 52 passes the Z-polarized component.

  In the configuration of FIG. 5, the local light and the signal light have the same Z polarization, but the local light may be a Z polarization and the signal light may be a Z ′ polarization orthogonal to the Z polarization. Also in this case, the polarization beam splitter 33 outputs the combined light of the signal light and the local light that are orthogonal to each other and have the same amplitude from the port # 3 and the port 4. However, in this case, the polarization planes of the polarization components that the 45-degree polarizer 51 and the 45-degree polarizer pass through are orthogonal to each other. In FIG. 5, the combined light output from the port # 3 of the polarization beam splitter 33 is output to the 45-degree polarizer 51, and the combined light output from the port # 4 is output to the quarter wavelength plate 4. . However, the combined light output from the port # 3 is branched into two and output to the 45 degree polarizer 51 and the quarter wavelength plate 4, or the combined light output from the port # 4 is branched into two and 45 degrees. The light can also be output to the polarizer 51 and the quarter-wave plate 4.

  1: Light source, 21, 22: Optical variable attenuator, 3: 2 × 2 optical coupler, 4: 1/4 wavelength plate, 51, 52: 45 degree polarizer, 61, 62: Photodiode, 71, 72: Analog・ Digital converter, 8: Processing unit

Claims (9)

A light source that generates local light;
An adjusting means for adjusting the amplitude of at least one of the local light and the signal light so that the amplitude of the local light and the amplitude of the received signal light coincide with each other;
Based on the local light and the signal light after adjustment by the adjustment means, output means for outputting the combined light including the local light and the signal light whose polarization planes are orthogonal to each other;
Measuring means for measuring the Stokes parameters S ₂ and S ₃ of the combined light;
An optical receiver comprising: The measuring means includes
A first polarization means for passing a component of a polarization plane at an angle of 45 degrees with each of the polarization planes of the local light and the signal light among the combined light;
First conversion means for converting a component of the combined light that has passed through the first polarization means into an electrical signal;
Delay means for delaying the local light or the signal light included in the combined light by a quarter wavelength;
A second polarization means for passing a component of a polarization plane at an angle of 45 degrees with each of the polarization planes of the local light and the signal light among the combined light that has passed through the delay means;
Second conversion means for converting the component of the combined light that has passed through the second polarization means into an electrical signal;
The optical receiver according to claim 1, further comprising: The measuring means includes
Digital conversion means for converting the output of each of the first conversion means and the second conversion means into a digital signal;
The optical receiver of claim 2 sipping and measuring the Stokes parameters S ₂ and S ₃ of the multiplexed light based on an output of the digital conversion means. 4. The optical receiver according to claim 1, wherein the measuring unit does not measure the Stokes parameters S ₀ and S ₁ of the combined light. 5. The local light generated by the light source and the polarization of the received signal light are orthogonal to each other,
5. The light according to claim 1, wherein the output unit generates the combined light by combining the local light and the signal light that have been adjusted by the adjusting unit. Receiving machine. The local light generated by the light source and the polarization of the received signal light are orthogonal to each other,
The output means separates the local light after the adjustment by the adjustment means into a first component of the first polarization and a second component of the second polarization orthogonal to the first polarization, and the adjustment means The adjusted signal light is separated into the third component of the first polarization and the fourth component of the second polarization, and the second component is combined by combining the first component and the fourth component. And combining the third component, or combining the first component and the fourth component, and combining the second component and the third component, thereby combining the combined light. Generate
5. The light according to claim 1, wherein polarizations of the local light and the received signal light differ from each of the first polarization and the second polarization by 45 degrees. Receiving machine. The local light generated by the light source and the polarization of the received signal light are the same,
The output means separates the local light after the adjustment by the adjustment means into a first component of the first polarization and a second component of the second polarization orthogonal to the first polarization, and the adjustment means The adjusted signal light is separated into the third component of the first polarization and the fourth component of the second polarization, and the second component is combined by combining the first component and the fourth component. And combining the third component, or combining the first component and the fourth component, and combining the second component and the third component, thereby combining the combined light. Generate
The polarization of the local light and the received signal light is different from the first polarization and the second polarization by 45 degrees, respectively. The optical receiver according to item.   The received signal light transmits the local light generated by the light source to an optical transmitter via an optical transmission path, and the optical transmitter modulates the local light and rotates the polarization plane by 90 degrees. The optical receiver according to claim 1, which is transmitted to the optical receiver by the optical transmitter via the optical transmission path after being performed. Generating local light,
Adjusting the amplitude of at least one of the local light and the signal light so as to match the amplitude of the local light and the amplitude of the received signal light;
Based on the local light and the signal light after amplitude adjustment, outputting the combined light including the local light and the signal light whose polarization planes are orthogonal to each other;
Measuring the Stokes parameters S ₂ and S ₃ of the combined light;
A coherent light receiving method.

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