Optical receiving device

Abstract

To demodulate information on the basis of the values of two Stokes parameters without adjusting the direction of an optical receiving device to match the direction of the signal light.SOLUTION: An optical receiving device includes branching means for branching a signal light obtained by polarization multiplexing modulated light of a first circular polarization and continuous light of a second circular polarization orthogonal to the first circular polarization into at least a first signal light and a second signal light, and demodulation means for demodulating the modulated light on the basis of the value of a Stokes parameter S1 of the first signal light and the value of a Stokes parameter S2 of the second signal light.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an optical receiving device. [Background technology]

[0002] Non-Patent Document 1 discloses an optical communication system using an optical receiving device employing a Stokes vector detection method. In this optical communication system, an optical transmitting device generates signal light by combining continuous light (unmodulated light) of one of two linearly polarized waves that are orthogonal to each other, that is, X-polarized wave, with modulated light of the other, Y-polarized wave. The optical transmitting device then transmits the generated signal light to an optical receiving device via an optical fiber. The optical receiving device calculates the Stokes parameters S2 and S3 of the received signal light and demodulates the information transmitted by the optical transmitting device from the values ​​of S2 and S3.

[0003] Non-Patent Document 2 discloses a free-space optical communication system that transmits signal light through space without using optical fibers. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Qian Hu,et al.,"Advanced modulation formats for high performance short reach optical interconnects", OPTICS EXPRESS,Vol.23,No.3,pp.3245-3259,2015 [Non-patent document 2] K. Matsuda, et al., "Demonstration of a Real-Time 14Tb / s Multi-Aperture Transmit Single-Aperture Receive FSO System With Class 1 Eye-Safe Transmit Intensity", in Journal of Lightwave Technology, vol. 40, no. 5, pp. 1494-1501, March 1, 2022 Summary of the Invention [Problem to be solved by the invention]

[0005] 7(A) is an explanatory diagram of an optical receiving device disclosed in Non-Patent Document 1. Signal light is split into three by a coupler 1 into first signal light, second signal light, and third signal light. The first signal light, second signal light, and third signal light are output to a first path, a second path, and a third path, respectively. The first signal light output to the first path is directly input to a detector 5. The second signal light output to the second path is input to the detector 5 via a half-wave plate (HWP) 2. The third signal light output to the third path is input to the detector 5 via a quarter-wave plate (QWP) 3.

[0006] 7(B) is a diagram showing the configuration of the detector 5. A polarizing beam splitter (PBS) 51 splits the input light into a first light having a first linear polarization and a second light having a second linear polarization that is orthogonal to the first linear polarization. The first light is input to a photodiode (PD) 52, and the second light is input to a PD 53. The PD 52 and PD 53 perform photoelectric conversion on the input light and output an electrical signal. A subtraction unit 54 outputs the difference between the electrical signal from the PD 52 and the electrical signal from the PD 53. The PD 52, PD 53, and subtraction unit 54 form a so-called balanced PD.

[0007] In the following description, the electric field direction (direction of the polarization plane) of the first linearly polarized wave output by the PBS 51 of the detector 5 is referred to as the "first direction of the optical receiving device." The slow axis of the HWP2 of the optical receiving device is positioned so that it forms an angle of 22.5 degrees with respect to the first direction of the optical receiving device. When linearly polarized light having an angle X with respect to the slow axis of the HWP2 is input, the HWP2 outputs linearly polarized light with its polarization plane rotated by an angle 2X. The slow axis of the QWP3 of the optical receiving device is positioned so that it forms an angle of 45 degrees with respect to the first direction of the optical receiving device. When linearly polarized light having an angle of 45 degrees with respect to the slow axis of the QWP3 is input to the QWP3, the QWP3 converts the input linearly polarized light into a circularly polarized light and outputs it. Note that the slow axis angles of the HWP2 and QWP3 may be an integer multiple of 90 added to the above values. That is, the slow axis of HWP2 can be 22.5 degrees, 112.5 degrees, 202.5 degrees, or 292.5 degrees relative to the first direction, and similarly, the slow axis of QWP3 can be 45 degrees, 135 degrees, 225 degrees, or 315 degrees relative to the first direction.

[0008] As shown in Figure 7(A), the output of the detector 5 on the first path indicates the Stokes parameter S1. By setting the slow axis of the HWP2 as described above, the output of the detector 5 on the second path indicates the Stokes parameter S2. By setting the slow axis of the QWP3 as described above, the output of the detector 5 on the third path indicates the Stokes parameter S3.

[0009] For example, the polarization direction of the continuous light included in the signal light is made to coincide with an integer multiple of 90 degrees with respect to the first direction of the optical receiving device. In other words, the angle between the polarization direction (electric field direction) of the continuous light and the first direction of the optical receiving device is set to 0, 90, 180, or 270 degrees. Hereinafter, this state will be expressed as "the direction of the signal light and the direction of the optical receiving device coincide." In the following, the explanation will basically be given assuming that the polarization direction of the continuous light included in the signal light coincides with the first direction of the optical receiving device. In this case, if the electric field component of the continuous light is Ex and the electric field component of the modulated light is Ey, the Stokes parameters S1, S2, and S3 will have values ​​expressed by the following equations, respectively. S1=|Ex| 2 -|Ey| 2 (1) S2 = 2Re(Ex × Ey * ) (2) S3 = 2Im(Ex × Ey * ) (3) In addition, "Re(A)" in equation (2) is a function that extracts the real part of the complex number A, and "Im(A)" in equation (3) is a function that extracts the imaginary part of the complex number A. * is the complex conjugate of Ey.

[0010] The Stokes parameters S2 and S3 are the beat components of the modulated light and continuous light, respectively, and the optical receiving device can demodulate the information transmitted by the optical transmitting device by calculating the value of S2 + jS3. If the direction of the signal light and the direction of the optical receiving device are aligned, the Stokes parameter S1 is not necessary for demodulating the information, so the first pass in Figure 7 can be omitted. On the other hand, if it is difficult to align the direction of the signal light and the direction of the optical receiving device, the Stokes parameter S1 is required to correct the value, and the first pass in Figure 7 cannot be omitted.

[0011] For example, in a free-space optical communication system, it may be difficult to align the direction of the signal light with the direction of the optical receiving device. In such cases, the optical receiving device requires three paths that output three Stokes parameters S1, S2, and S3.

[0012] The present disclosure provides a technique that enables information to be demodulated using the values ​​of two Stokes parameters without adjusting the direction of an optical receiving device to match the direction of the signal light. [Means for solving the problem]

[0013] According to one aspect of the present disclosure, an optical receiving device includes a branching means for branching a signal light obtained by polarization multiplexing modulated light of a first circular polarization and continuous light of a second circular polarization orthogonal to the first circular polarization into at least a first signal light and a second signal light, and a demodulating means for demodulating the modulated light based on a value of a Stokes parameter S1 of the first signal light and a value of a Stokes parameter S2 of the second signal light. [Effects of the Invention]

[0014] According to the present disclosure, information can be demodulated using the values ​​of two Stokes parameters without adjusting the direction of the optical receiving device to the direction of the signal light. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a block diagram of an optical transmitting device, according to some embodiments. [Figure 2] FIG. 1 is a diagram illustrating the configuration of an optical receiving device according to an embodiment. [Figure 3] FIG. 1 is a diagram illustrating the configuration of an optical receiving device according to an embodiment. [Figure 4] FIG. 1 is a diagram illustrating the configuration of an optical receiving device according to an embodiment. [Figure 5] FIG. 1 is a diagram illustrating the configuration of an optical transmitting device according to an embodiment. [Figure 6] FIG. 1 is a diagram illustrating the configuration of an optical receiving device according to an embodiment. [Figure 7] FIG. 1 is an explanatory diagram of the background art. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more features among the multiple features described in the embodiments may be arbitrarily combined. Furthermore, the same reference numerals are used for the same or similar components, and redundant explanations will be omitted.

[0017] First Embodiment FIG. 1 is a configuration diagram of an optical transmission device according to this embodiment. Light source 11 generates linearly polarized continuous light. The continuous light generated by light source 11 is separated by polarization separator 12 into a first continuous light with X polarization and a second continuous light with Y polarization, which is orthogonal to the X polarization. Polarization separator 12 may be configured to separate the input continuous light so that the amplitudes of the first and second continuous light are equal. The first continuous light output by polarization separator 12 is input to polarization multiplexer 14. Meanwhile, the second continuous light output by polarization separator 12 is input to modulator 13. Modulator 13 modulates the second continuous light with data to be transmitted and outputs the modulated light to polarization multiplexer 14. Polarization multiplexer 14 outputs a multiplexed signal light obtained by polarization multiplexing the first continuous light with X polarization and the modulated light with Y polarization to converter 15.

[0018] Converter 15 converts the X-polarized and Y-polarized light into circularly polarized light that are orthogonal to each other. As an example, converter 15 converts the X-polarized light into right-handed circularly polarized light and the Y-polarized light into left-handed circularly polarized light. Converter 15 may be, for example, a QWP. Converter 15 transmits signal light that is polarization-multiplexed with continuous light and modulated light that are orthogonal to each other, to an optical receiving device, for example, via space.

[0019] FIG. 2 is a configuration diagram of an optical receiving device according to this embodiment. The optical receiving device includes a coupler 21 and a demodulator 200. The coupler 21 is a branching unit that branches the signal light received from the optical transmitter into at least two signal lights, a first signal light and a second signal light, and outputs them to the demodulator 200. In the demodulator 200, the first signal light is directly input to the detector 5. Meanwhile, the second signal light is input to the detector 5 via the HWP2. The detector 5 is the same as that shown in FIG. 7(B). In the optical receiving device of FIG. 2, the first direction is also defined based on the direction of polarization output by the PBS 51 of the detector 5. The slow axis of the HWP2 is set to 22.5 degrees with respect to the first direction. In other words, the optical receiving device of FIG. 2 has the first and second paths shown in FIG. 7, but does not have a third path. The optical receiving device demodulates the information transmitted by the optical transmitting device based on the value of the Stokes parameter S1 output by the detector 5 on the first path and the value of the Stokes parameter S2 output by the detector 5 on the second path.

[0020] The reason why the information transmitted by the optical transmitting device can be demodulated based on the outputs of the first path detector 5 and the second path detector 5 will be explained below. If the electric field component of the continuous light is Ex and the electric field component of the modulated light is Ey, the continuous light and the modulated light are circularly polarized waves with different rotation directions, and therefore the signal light can be expressed by the following Jones vector:

[0021]

number

[0022] 2 is obtained by replacing Ex in equation (1) with (1 / √2)(Ex+Ey) and Ey with j(1 / √2)(Ex-Ey). Therefore, the output A of the detector 5 in the first path of the optical receiving device in Fig. 2 is given by the following equation (4).

[0023]

number

[0024] Similarly, the output B of the detector 5 on the second path of the optical receiving device in Figure 2 is obtained by changing Ex in equation (2) to (1 / √2)(Ex+Ey) and Ey to j(1 / √2)(Ex-Ey). Therefore, the output B of the detector 5 on the second path is given by the following equation (5).

[0025]

number

[0026] As shown in equations (5) and (6), the output A of the detector 5 on the first path corresponds to the value of equation (2), and the output B of the detector 5 on the second path corresponds to the value of equation (3). Therefore, the optical receiving device 2 can demodulate information using the values ​​of the two Stokes parameters.

[0027] As described above, in this embodiment, information can be demodulated using the values ​​of the two Stokes parameters without adjusting the direction of the optical receiving device relative to the signal light.

[0028] Second Embodiment Next, the second embodiment will be described, focusing on the differences from the first embodiment. The configuration of the optical transmitting device of this embodiment is the same as that of the first embodiment. Figure 3 is a configuration diagram of the optical receiving device according to this embodiment.

[0029] The optical receiving device according to this embodiment includes a QWP 22, a coupler 21, and a demodulation unit 201. The demodulation unit 201 includes a second path for detecting the value of the Stokes parameter S2 and a third path for detecting the value of the Stokes parameter S3, as shown in FIG. 7A. The slow axis directions of the HWB 2 and QWP 3 provided in the second and third paths are as described with reference to FIG. 7A. In the optical receiving device according to this embodiment, the first direction is also defined based on the direction of polarization output by the PBS 51 of the detector 5. The first direction of the optical receiving device may also be referred to as the first direction of the demodulation unit 201.

[0030] The QWP 22 converts the received circularly polarized signal light into linearly polarized light. The linearly polarized signal light includes linearly polarized continuous light and modulated light, which are orthogonal to each other. The QWP 22 is configured so that its slow axis is an integer multiple of 90 degrees with respect to the first direction. In other words, when circularly polarized light is input, the QWP 22 is configured to output either first or second linearly polarized light to the coupler 21 depending on the circularly polarized light's rotation direction. Therefore, the polarization direction of the continuous light input to the coupler 21 is an integer multiple of 90 degrees with respect to the first direction. Therefore, as described in FIG. 7(A), the detector 5 of the second path outputs a value corresponding to Equation (2), and the detector 5 of the third path outputs a value corresponding to Equation (3). Therefore, the optical receiving device 2 can demodulate information using the values ​​of the two Stokes parameters without adjusting the direction of the optical receiving device with respect to the signal light.

[0031] Third Embodiment Next, the third embodiment will be described, focusing on the differences from the first and second embodiments. The configuration of the optical transmitting device of this embodiment is the same as that of the first embodiment. Figure 4 is a configuration diagram of the optical receiving device according to this embodiment.

[0032] The optical receiving device according to this embodiment includes a QWP 22, a coupler 21, and a demodulator 202. The demodulator 202 includes a first path for detecting the value of the Stokes parameter S1 and a third path for detecting the value of the Stokes parameter S3, as shown in FIG. 7A. In the optical receiving device according to this embodiment, a first direction is defined based on the direction of polarization output by the PBS 51 of the detector 5 for the third path. The first direction of the optical receiving device may also be referred to as the first direction of the demodulator 202. In addition, in this embodiment, the first path is rotated by 45 degrees with respect to the third path. That is, the polarization plane of the third linearly polarized wave output by the PBS 51 of the detector 5 for the first path and the fourth linearly polarized wave orthogonal to the third linearly polarized wave are at an angle of 45 degrees with respect to the polarization planes of the first and second linearly polarized waves output by the PBS 51 of the detector 5 for the third path.

[0033] The QWP 22 is the same as in the second embodiment and converts the circularly polarized wave into the first linear polarization or the second linear polarization. The third path is the same as in the second embodiment, and the detector 5 of the third path outputs a value corresponding to equation (3). On the other hand, in this embodiment, the detector 51 of the first path receives linearly polarized signal light whose polarization is rotated by 45 degrees relative to the two linearly polarized waves output by the PBS 51. This is the same as the signal light input to the detector 5 of the second path in the second embodiment. Therefore, the detector 5 of the first path outputs a value corresponding to equation (2). Therefore, the optical receiving device 2 can demodulate information using the values ​​of the two Stokes parameters without adjusting the direction of the optical receiving device relative to the signal light.

[0034] Note that the detector 5 of the first path in this embodiment is rotated by 45 degrees relative to the detector of the third path. Therefore, when considering the demodulation unit 202 as a whole, the first path can be considered to detect the value of the Stokes parameter S2 rather than the value of the Stokes parameter S1. In other words, the first path in this embodiment can be considered to be the detector 5 of the second path in the second embodiment rotated by 45 degrees, instead of omitting the HWP2 provided in the second path in the second embodiment. However, when looking at the first path alone, since the HWP2 is not provided, in this embodiment, the first path is considered to detect the value of the Stokes parameter S1. Note that whether the first path detects the Stokes parameter S1 or the Stokes parameter S2, the fact remains that information is demodulated using only two Stokes parameters (S1 or S2 and S3).

[0035] <Fourth embodiment> Next, the fourth embodiment will be described, focusing on the differences from the second and third embodiments. Fig. 5 is a configuration diagram of an optical transmitting device according to this embodiment. The following description will focus on the differences from the configuration of the optical transmitting device shown in Fig. 1. In this embodiment, the first continuous light output by the polarization separator 12 is also modulated by the modulator 13. Therefore, the optical transmitting device transmits signal light obtained by polarization multiplexing modulated light of mutually orthogonal circularly polarized waves to the optical receiving device.

[0036] 6 is a configuration diagram of an optical receiving device according to this embodiment. The optical receiving device 2 of this embodiment has a light source 24, a QWP 22, a PBS 25, two couplers 21, and two demodulators 201. In this embodiment, the first directions of the two demodulators 201 are aligned. However, it is sufficient if the angle between the first direction of one demodulator 201 and the first direction of the other demodulator 201 is an integer multiple of 90 degrees.

[0037] The QWP 22 is the same as in the second embodiment. That is, the slow axis of the QWP 22 is arranged so as to be an integer multiple of 90 degrees with respect to the first direction of the optical receiving device. Therefore, the QWP 22 outputs polarization-multiplexed light obtained by multiplexing modulated light of the first linear polarization and modulated light of the second linear polarization to the PBS 25. One of the modulated light of the first linear polarization and the modulated light of the second linear polarization is modulated light of the X polarization in the optical transmitting device, and the other is modulated light of the Y polarization in the optical transmitting device. The light source 24 generates local light, which is unmodulated continuous light, and outputs it to the PBS 25. The light source 24 and the PBS 25 are arranged so that the PBS 25 generates signal light obtained by polarization-multiplexing the modulated light of the first linear polarization and the local light of the second linear polarization and outputs it to one coupler 21, and generates signal light obtained by polarization-multiplexing the modulated light of the second linear polarization and the local light of the first linear polarization and outputs it to the other coupler 21. Therefore, as explained in the second embodiment, the two demodulation units 201 can demodulate the modulated light of X polarization and the modulated light of Y polarization in the optical transmission device, respectively.

[0038] The configuration of the third embodiment can also be applied to this embodiment. In this case, the demodulation unit 201 in Fig. 6 becomes the demodulation unit 202. Also, one of the two demodulation units can be the demodulation unit 201 of the second embodiment, and the other can be the demodulation unit 202 of the third embodiment.

[0039] As described above, in this embodiment, information can be demodulated using the values ​​of two Stokes parameters without adjusting the direction of the optical receiving device relative to the signal light. Furthermore, in this embodiment, the transmission capacity of the optical communication system can be increased compared to the optical communication systems of the first to third embodiments.

[0040] With the above configuration, it is possible to demodulate information using the values ​​of the two Stokes parameters without adjusting the direction of the optical receiving device to match the direction of the signal light. This makes it possible to contribute to Goal 9 of the United Nations' Sustainable Development Goals (SDGs), which is to "Build resilient infrastructure, promote sustainable industrialization, and foster innovation."

[0041] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention. [Explanation of symbols]

[0042] 21: coupler, 2: half-wave plate, 5: detector, 200: demodulator

Claims

1. a branching means for branching a signal light obtained by polarization multiplexing modulated light of a first circular polarization and continuous light of a second circular polarization orthogonal to the first circular polarization into at least a first signal light and a second signal light; The Stokes parameter S of the first signal light １ and the value of the Stokes parameter S of the second signal light ２ a demodulation means for demodulating the modulated light based on the value of An optical receiving device comprising:

2. The demodulation means a first detecting means to which the first signal light is input; a second detecting means to which the second signal light is input via a half-wave plate; Equipped with The first detection means and the second detection means each include: a splitter for splitting input light into a first light of a first linear polarization and a second light of a second linear polarization orthogonal to the first linear polarization; an output means for outputting a difference between a signal obtained by photoelectrically converting the first light and a signal obtained by photoelectrically converting the second light; Equipped with 2. The optical receiving device according to claim 1, wherein an angle between the slow axis of the half-wave plate and the polarization plane of the first linearly polarized light is 22.5 degrees, 112.5 degrees, 202.5 degrees, or 292.5 degrees.

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