Optical receiving device and optical transmitting device

Abstract

This reduces the cost of optical receiving equipment that receives signal light transmitted via multimode fiber. [Solution] An optical receiving device for receiving signal light via a multimode fiber, wherein the multimode fiber carries the signal light in each of a plurality of carrier modes, including a first mode to the Nth mode, where N is an integer of 2 or more. The optical receiving device comprises: a generation means for generating local light, which includes N consecutive lights from the first mode to the Nth mode, each of which has a different frequency; an optical processing means for generating one or more output lights including the signal light and the local light; and a detection means for photoelectrically converting the one or more output lights.

Description

[Technical Field]

[0001] This disclosure relates, for example, to optical receivers and optical transmitters that may be used in free space optics (FSO) communication systems. [Background technology]

[0002] Non-patent documents 1 and 2 disclose optical receiving devices for FSO communication systems, respectively. According to non-patent documents 1 and 2, multimode fiber (MMF) is used to transport signal light collected by an optical antenna. Since the cross-sectional area of ​​the core of an MMF is larger than that of a single-mode fiber (SMF), using MMF allows more of the signal light collected by the optical antenna to be injected into the optical fiber compared to using SMF. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] N. Fontaine,et al,"Digital turbulence compensation of free space optical link with multimode optical amplifier", in 45th European Conference on Optical Communication (ECOC), 2019 [Non-Patent Document 2] F. Wang,et al,"Free-Space Optical Communication Based on Mode Diversity Reception Using a Nonmode Selective Photonic Lantern and Equal Gain Combining", in IEEE Photonics Journal,vol.15,no.1,pp.1-7,February 2023 [Overview of the project] [Problems that the invention aims to solve]

[0004] To receive a signal light coherently, it is necessary to interfere the signal light and local light of the same propagation mode. For this reason, Non-Patent Documents 1 and 2 describe a method in which the signal light carried by MMF is separated by a mode separator, and a number of coherent receivers equal to the number of separated modes are used. Specifically, if the number of separated modes is N (where N is an integer greater than or equal to 2), each of the N coherent receivers performs photoelectric conversion on the mixed light of the signal light and local light to output an electrical signal that represents the beat component of the mixed light. The signal light is then demodulated by MIMO (Multi Input Multi Output) processing of the N electrical signals output by the N coherent receivers. Note that if two orthogonal polarizations are considered, 2 × N coherent receivers are required for N modes, and MIMO processing is performed on 2 × N electrical signals.

[0005] The configurations described in Non-Patent Documents 1 and 2 use a number of coherent receivers corresponding to the number of modes used. Coherent receivers have photoelectric converters such as analog-to-digital converters (ADCs) and photodiodes (PDs). Therefore, the configurations described in Non-Patent Documents 1 and 2 require a number of ADCs and PDs corresponding to the number of modes, which increases the cost of the optical receiver.

[0006] This disclosure provides a technology to reduce the cost of optical receiving equipment that receives signal light transported by MMF. [Means for solving the problem]

[0007] According to one aspect of the present disclosure, there is provided an optical receiver that receives signal light via a multimode fiber, wherein the multimode fiber carries the signal light in each of a plurality of propagation modes including a first mode to an N-th mode, N is an integer of 2 or greater, and the optical receiver comprises: generation means for generating local light that is local light including N pieces of continuous light from the first mode to the N-th mode, wherein frequencies of the N pieces of continuous light are different from each other; optical processing means for generating one or more output lights including the signal light and the local light; and detection means for performing photoelectric conversion on the one or more output lights. Effects of the Invention

[0008] According to the present disclosure, the cost of an optical receiver that receives signal light carried by an MMF can be reduced. Brief Description of the Drawings

[0009] [Figure 1] Figure showing a configuration example of an optical transmitter. [Figure 2] Figure showing a configuration example of an optical receiver. [Figure 3] Figure showing a configuration example of a local light generation unit. [Figure 4] Figure showing frequencies of each mode of signal light and frequencies of each mode of local light. [Figure 5] Figure showing a configuration example of an optical processing unit [Figure 6] Figure showing an example of frequency components of a detection signal. [Figure 7] Figure showing a configuration example of a demodulation unit. [Figure 8] Figure showing another configuration example of an optical receiver. [Figure 9] Figure showing another configuration example of an optical processing unit. [Figure 10] Explanatory diagram of filter processing in an optical transmitter. Mode for Carrying Out the Invention

[0010] The embodiments will be described in detail below with reference to the attached drawings. Note that the following embodiments do not limit the invention as defined in the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more of the features described in the embodiments may be combined arbitrarily. Furthermore, identical or similar configurations will be given the same reference numeral, and redundant descriptions will be omitted.

[0011] <First Embodiment> Figure 1 shows an example of the configuration of an optical transmission device according to this embodiment. The light source 10 generates continuous light as carrier light. In this embodiment, the frequency of the continuous light (carrier frequency) is f S The electrical signal carrying the information (data) is filtered by filter 13. Figure 10(A) shows the frequency waveform of the filtered electrical signal. As shown in Figure 10(A), the center frequency of the filtered electrical signal is f C The bandwidth is B. The center frequency is f. C This value is 0 if the electrical signal is a baseband signal, and greater than 0 if the electrical signal is an intermediate frequency band (IF) signal or a radio frequency band (RF) signal. The filtered electrical signal is input to the modulator 11. The modulator 11 generates signal light (or modulated light) by modulating the carrier light with the electrical signal. The frequency waveform of the signal light is the center frequency f shown in Figure 10(A). C carrier frequency f S This corresponds to the above. In this embodiment, the bandwidth of the signal light is limited to B by limiting the bandwidth of the electrical signal with filter 13. However, it is also possible to have a configuration in which the bandwidth of the signal light is limited to B by filtering the signal light with an optical filter. In this case, filter 13 can be omitted. The transmitting unit 12 performs amplification processing of the signal light and outputs the processed signal light to an optical antenna (not shown). The optical antenna transmits the signal light into space as an optical beam.

[0012] FIG. 2 shows a configuration example of an optical receiver according to the present embodiment. The optical receiver is connected to an unillustrated optical antenna that condenses a light beam transmitted into space by an optical transmitter. The light receiving unit 20 causes the light beam condensed by the optical antenna to enter the MMF as signal light. The light receiving unit 20 can perform amplification processing, filter processing, and the like on the signal light incident on the MMF. The light receiving unit 20 outputs the processed signal light to the optical processing unit 22 via the MMF. In the following description, it is assumed that signal light carried by N propagation modes among the plurality of propagation modes in the MMF is used for demodulation. Note that N is an integer of 2 or more. The number of possible plurality of propagation modes in the MMF may be N or more. In the following description, when distinguishing N propagation modes, they are referred to as a first mode to an N-th mode. FIG. 4(A) shows signal light of the first mode to the N-th mode input to the optical processing unit 22. As described above, the center frequency of the signal light in each mode is the carrier frequency f S .

[0013] A local light generator 21 generates local light for coherent reception. The local light generator 21 outputs the generated local light to the optical processing unit 22 via the MMF. FIG. 3 shows a configuration example of the local light generator 21.

[0014] A light source 211 generates an optical comb. That is, the light source 211 generates an optical signal obtained by frequency-multiplexing continuous light arranged at equal intervals on the frequency axis. In this example, the number of continuous light components included in the optical comb is N, but the number of continuous light components included in the optical comb may be greater than N. In the following description, the frequencies of N continuous light components included in the optical comb are from f1 to f N , and the frequency f n (where n is an integer from 1 to N) is referred to as the n-th continuous light. Note that the larger the value of n, the higher the frequency. That is, the frequency f1 is the lowest, and the frequency f N is the highest.

[0015] The frequency separation unit 212 performs frequency separation of the optical comb and outputs the first to the nth continuous light to the mode conversion multiplexer 213. The propagation modes of the first to the nth continuous light are the same and will be referred to as the "light source mode" below. The light source mode may be the same as one of the first to the nth modes, or it may be different from each of the first to the nth modes. The mode conversion multiplexer 213 has at least N ports, including the first to the nth ports. The nth continuous light from the frequency separation unit 212 is input to the nth port. The nth port of the mode conversion multiplexer 213 is configured to convert the light source mode to the nth mode. Note that if the light source mode is the mth mode (where m is one from 1 to N), no mode conversion is performed at the mth port. The mode conversion multiplexer 213 generates local light by frequency multiplexing the N continuous light after mode conversion. Figure 4(B) is an explanatory diagram of local light. The local light includes the first continuous light of the first mode to the nth continuous light of the nth mode. As described above, the frequency of the nth continuous light of the nth mode is f n That is the case.

[0016] Returning to Figure 2, the optical processing unit 22 mixes the signal light from the light receiving unit 20 with the local light from the local light generation unit 21 and outputs the first output light and the second output light. Figure 5 shows an example of the configuration of the optical processing unit 22. The optical processing unit 22 is equipped with a beam splitter (BS) 221. The BS 221 splits the signal light and the local light, respectively, and outputs the first output light and the second output light, which include the signal light and the local light. Due to the characteristics of the BS 221, the phase of the local light included in the first output light and the phase of the local light included in the second output light differ by π (180 degrees).

[0017] Returning to Figure 2, the detection unit 23 is a balanced detector and includes a first PD that photoelectrically converts the first output light to output a first electrical signal, a second PD that photoelectrically converts the second output light to output a second electrical signal, and a circuit that outputs an electrical signal corresponding to the difference between the first electrical signal and the second electrical signal as a detection signal. As described above, the phase of the local light contained in the first output light and the phase of the local light contained in the second output light differ by π, so the second electrical signal corresponds to the inverted amplitude of the first electrical signal.

[0018] Figure 6 shows the frequency components of the first electrical signal output by the first PD of the detection unit 23 when it performs photoelectric conversion of the first output light. The first electrical signal includes the nth beat signal, which is the beat between the nth continuous light of the nth mode contained in the local light and the signal light of the nth mode. In other words, the first electrical signal includes a total of N beat signals, from the first beat signal to the nth beat signal. Reference numeral 5-1 in Figure 6 represents the first beat signal, and its center frequency is the frequency f1 of the first continuous light and the carrier frequency f of the signal light. s This represents the frequency difference. Reference numeral 5-2 is the second beat signal, and reference numeral 5-N is the nth beat signal. Note that in Figure 6, the carrier frequency f s at frequency f N It is considered to be higher than that. However, the carrier frequency f s The frequency f can be lower than f1. Also, the carrier frequency f s It is higher than frequency f1, but frequency f N A lower value is also acceptable.

[0019] In this example, the frequencies f1 to f of the continuous light are set such that the N beat signals do not overlap on the frequency axis. N The bandwidth B of the signal light is set accordingly. As shown in Figure 6, if S is the frequency difference between two adjacent continuous lights on the frequency axis, the condition for N beat signals not to overlap on the frequency axis is S ≥ B.

[0020] Furthermore, in this example, the frequencies f1 to f are not all baseband signals, so that the first to the Nth beat signals are not all baseband signals. N and f S The value of is set so that none of the first beat signal to the Nth beat signal contain a component with frequency 0, with frequencies f1 to f N and f S The value is set.

[0021] Since the second electrical signal is the inverted amplitude of the first electrical signal, the beat signal included in the second electrical signal is as shown in Figure 6. Furthermore, since the detection signal is the difference between the first electrical signal and the second electrical signal (which is the inverted amplitude of the first electrical signal), the beat signal included in the detection signal is as shown in Figure 6.

[0022] Returning to Figure 2, the detection signal is output to the demodulation unit 24. Figure 7 shows an example of the configuration of the demodulation unit 24. The ADC 241 performs analog-to-digital conversion of the detection signal, which is an analog signal, and outputs a digital detection signal. Subsequent processing is performed in the digital domain, but the same signal names as in the analog domain are used. The separation unit 242 performs frequency separation of the digital detection signal and outputs N beat signals. The determination unit 243 determines the information (data) transmitted by the optical transmitter based on each of the N beat signals. As an example, the determination unit 243 has a frequency conversion unit to make the frequency bands of each of the N beat signals the same, and determines the information (data) transmitted by the optical transmitter by performing maximum ratio synthesis of the N beat signals after frequency conversion.

[0023] Note that Figure 5 shows only the components necessary for understanding the embodiment, and the optical processing unit 22 may have optical components other than those shown in Figure 5. For example, the optical processing unit 22 may have a lens that converts the input signal light or local light into parallel light before it is incident on the BS221. The optical processing unit 22 may also have a lens for causing the first output light or second output light to be incident on the light-receiving surfaces of the first PD or second PD of the detection unit 23.

[0024] The optical processing unit 22 illustrated in Figure 5 spatially realizes a 180-degree hybrid, but the configuration of the optical processing unit 22 is not limited to that shown in Figure 5. Furthermore, in this embodiment, the optical processing unit 22 generates a first output light and a second output light in order to use a balanced detection unit 23, but it is also possible to have a configuration in which only one output light including the signal light and local light is generated, and the detection signal is generated by photoelectric conversion of this.

[0025] Next, we will explain the reason for making the frequencies of the first continuous light of the first mode, the Nth continuous light of the Nth mode, and the Nth continuous light of the Nth mode, which are included in the local light, different. For example, the frequencies of the first continuous light and the Nth continuous light are the same f L In this case, the frequency bandwidth of the beat signals for each of the first to Nth continuous light sources and the signal light will be the same. That is, the frequency bandwidth of the first to Nth beat signals will be the same. Therefore, the detected signal will be the sum of the first to Nth beat signals, all of which have the same frequency bandwidth. If the phases of the first to Nth beat signals are not in phase, adding them together will not necessarily increase the amplitude, and depending on the phase relationship, the amplitude may decrease, which could affect demodulation.

[0026] On the other hand, in this embodiment, since the frequencies of the first continuous light of the first mode to the Nth continuous light of the Nth mode are different, the detection signal is obtained by frequency multiplexing the first beat signal to the Nth beat signal. Therefore, for example, the first beat signal to the Nth beat signal can be individually extracted by frequency separation of the detection signal, and the determination unit 243 can accurately determine the data transmitted by the optical transmitter by combining the first beat signal to the Nth beat signal by the maximum ratio, etc.

[0027] In this embodiment, regardless of the number of modes, the number of PDs required for coherent reception is 2, and the number of ADCs is 1. Furthermore, if the balanced detection unit 23 is not used, the number of required PDs becomes 1. Therefore, the cost of the optical receiving device that receives the signal light transported by the MMF can be reduced.

[0028] In this embodiment, the detection signal is converted to a digital signal by the ADC 241, and therefore the separation unit 242 performs frequency separation in the digital domain. However, it is also possible to configure the separation unit 242 to perform frequency separation in the analog domain. In this case, an ADC is provided that converts the analog nth beat signal output by the separation unit 242 into a digital signal. Therefore, when a separation unit 242 that performs frequency separation in the analog domain is provided, the number of ADCs required for coherent reception is the same as the number of modes, N, but the number of PDs required for coherent reception is 2 or 1, and the cost of the optical receiving device that receives the signal light carried by the MMF can be reduced.

[0029] <Transformed form> In the optical receiver shown in Figure 2, the frequencies f1 to f are set such that not all of the first to Nth beat signals are baseband signals. N and f S The value was to be set. Below, we will explain the case where one of the first to Nth beat signals is the baseband signal. That is, the frequencies f1~f N One of them is frequency f S Let's explain the case where it is the same as [this].

[0030] Figure 8 shows an example of the configuration of the optical receiver in this modified form. The optical processing unit 22 receives the signal light and local light described above. The optical processing unit 22 mixes the signal light from the light receiving unit 20 and the local light from the local light generation unit 21 and outputs the first to fourth output lights. The first and second output lights correspond to the I (in-phase) component in quadrature modulation, and the third and fourth output lights correspond to the Q (quadrature phase) component in quadrature modulation. In other words, the optical processing unit 22 supports 90-degree hybrid modulation.

[0031] Figure 9 shows an example configuration of the optical processing unit 22. The local light is converted to circular polarization by the quarter-wave plate 222. That is, if two orthogonal linear polarizations are X-polarization and Y-polarization, the quarter-wave plate 222 sets the phase difference between the X-polarization and Y-polarization components of the local light to π / 2 (90 degrees). The polarization plane of the signal light is set to an angle of 45 degrees with respect to the polarization planes of the X-polarization and Y-polarization, respectively. BS221 splits the signal light and the local light from the quarter-wave plate 222 into two branches each, and outputs a first mixed light and a second mixed light obtained by mixing the signal light and the local light. Polarization beam splitters (PBS) 233 and PBS234 perform polarization separation of the first mixed light and the second mixed light, respectively, and output the first and second output lights with X polarization and the third and fourth output lights with Y polarization.

[0032] As described above, the phase of the local light contained in the third and fourth output lights, which are Y-polarized, is shifted by π / 2 relative to the phase of the local light contained in the first and second output lights, which are X-polarized. Therefore, if the first and second output lights are the I component in quadrature modulation, then the third and fourth output lights become the Q component in quadrature modulation. Furthermore, due to the characteristics of the BS221, the first and second output lights are out of phase with respect to each other, and the third and fourth output lights are out of phase with respect to each other.

[0033] Returning to Figure 8, the detection unit 231 is a balanced detector and includes a first PD that photoelectrically converts the first output light to output a first electrical signal, a second PD that photoelectrically converts the second output light to output a second electrical signal, and a circuit that outputs an electrical signal corresponding to the difference between the first electrical signal and the second electrical signal as the first detection signal. The first detection signal corresponds to the I component. The detection unit 232 is a balanced detector and includes a first PD that photoelectrically converts the third output light to output a third electrical signal, a second PD that photoelectrically converts the fourth output light to output a fourth electrical signal, and a circuit that outputs an electrical signal corresponding to the difference between the third electrical signal and the fourth electrical signal as the second detection signal. The second detection signal corresponds to the Q component. The demodulation unit 24 determines the information transmitted by the optical transmitter based on the N beat signals contained in the first detection signal and the second detection signal, respectively.

[0034] As shown in Figure 8, regardless of the number of modes, the number of PDs required for coherent reception is 4. Furthermore, if the balanced detection units 231 and 232 are not used, the number of required PDs becomes 2. In this case, the optical processing unit 22 outputs the first output light and the third output light. Note that if the demodulation unit 24 converts the first detection signal and the second detection signal into digital signals, the number of required ADCs becomes 2.

[0035] In the configurations shown in Figures 2 and 8, only single polarization is used, but the configuration of this embodiment can also be applied to polarization multiplexing communication that multiplexes two orthogonal polarizations. In this case, the optical processing unit 22 first performs polarization separation of the signal light, and then applies the processing shown in Figures 5 and 9 to each of the two signal lights generated by polarization separation. Therefore, the number of output lights output by the optical processing unit is twice that of the case when only single polarization is used. Similarly, the number of PDs and ADCs required for the optical receiver is also twice that of the case when only single polarization is used.

[0036] Furthermore, in this embodiment, an optical comb is used in the generation of local light, and therefore, when N is 3 or greater, the frequency difference between each pair of adjacent continuous light on the frequency axis among the first to the Nth continuous light is always the same. However, the frequency difference between pairs of adjacent continuous light on the frequency axis may be different for each pair. In this case, it is sufficient that the minimum value of the frequency difference between each pair is greater than or equal to B, which is the bandwidth of the signal light. Also, it is sufficient that the local light is generated by frequency multiplexing multiple continuous light of different frequencies, and that each propagation mode of the multiple continuous light is different, and the configuration of the local light generation unit 21 is not limited to that shown in Figure 3. Similarly, it is sufficient that output light including local light and signal light can be generated, and the configuration of the optical processing unit 22 is not limited to that shown in Figures 5 and 9.

[0037] <Second Embodiment> Next, the second embodiment will be described, focusing on the differences from the first embodiment. In the first embodiment, the filter 13 allowed the portion of the frequency components of the electrical signal with bandwidth B to pass through, while blocking (or attenuating) the remaining portion. In this embodiment, the filter 13 filters the electrical signal so that the frequency components of the filtered electrical signal follow a waveform that follows a sinc function as shown in Figure 10(B) (hereinafter referred to as the sinc waveform).

[0038] Unlike the first embodiment, the electrical signal filtered according to this embodiment is not bandwidth-limited to a predetermined value, but rather has a center frequency f c The amplitude is maximum at the center frequency f c As you move away from the point, the amplitude gradually decreases while repeatedly increasing and decreasing. In Figure 10(B), the frequency f U1 The center frequency is f s This is the frequency at which the amplitude first reaches its minimum value, for example, 0, on the higher frequency side. D1 The center frequency is f S This is the frequency at which the amplitude first reaches its minimum value, for example, 0, on the lower frequency side. As shown in Figure 10(B), the center frequency f s and frequency f D1 and f U1 Let E be the frequency difference between them.

[0039] In the frequency waveform shown in Figure 10(B), the frequency at which the amplitude is minimum, for example, 0, is frequency f. U1 and f D1 Others also exist. If we denote the frequency at which the amplitude is smallest, for example, 0, as the null frequency, then the center frequency f c The frequency difference between this frequency and the null frequency is E × K (where K is an integer greater than or equal to 1).

[0040] Note that in Figure 10(B), the amplitude is shown as a negative value according to the sinc function, but the actual frequency waveform is obtained by inverting the negative value portion of Figure 10(B) with respect to the frequency axis.

[0041] In this embodiment, continuous light is modulated with an electrical signal with the frequency waveform shown in Figure 10(B), so the frequency waveform of the signal light is also as shown in Figure 10(B). However, the center frequency f in Figure 10(B) C The carrier frequency is f S The frequency waveforms of the N beat signals included in the detection signal are as shown in Figure 10(B). However, the center frequency f in Figure 10(B) is... C The carrier frequency f S This becomes the frequency difference with continuous light.

[0042] As is clear from the waveform in Figure 10(B), in this embodiment, the frequency bands of the N beat signals included in the detection signal partially overlap with each other. However, in this embodiment, the frequency difference S of two adjacent pairs of continuous light on the frequency axis among the first to the Nth continuous light is equal to the center frequency f c The frequency is set to be an integer multiple of E × K, which is the frequency difference between the null frequency and the beat signal. By setting the frequencies of the first to the Nth continuous beat signals in this way, interference between the N beat signals can be suppressed even if the frequency bands of the N beat signals included in the detection signal overlap, similar to orthogonal frequency division multiplexing (OFDM). The number of PDs and ADCs required for demodulation is the same as in the first embodiment.

[0043] Furthermore, the optical receiving devices described in each of the above embodiments can be used not only in FSO communication systems but also in optical communication systems that transmit and receive signal light via MMF.

[0044] This configuration reduces the cost of the optical receiving device that receives the signal light transmitted by the MMF. Therefore, it becomes possible to contribute to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs): "Build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0045] The invention is not limited to the embodiments described above, and various modifications and changes are possible within the scope of the gist of the invention. [Explanation of Symbols]

[0046] 21: Local light generation unit, 22: Light processing unit, 23: Detection unit

Claims

1. An optical receiving device that receives signal light via a multimode fiber, The multimode fiber carries the signal light in each of the multiple carrier modes, including the first to the nth mode. N is an integer greater than or equal to 2, The aforementioned optical receiving device is A generating means for generating local light comprising N consecutive light beams from the first mode to the Nth mode, wherein the frequencies of the N consecutive light beams are each different; A photoprocessing means that generates one or more output lights including the signal light and the local light, A detection means for photoelectrically converting one or more output lights, An optical receiving device equipped with the following features.

2. The optical receiving device according to claim 1, wherein the minimum value of the frequency difference between two adjacent continuous light beams on the frequency axis among the N continuous light beams contained in the local light beam is greater than or equal to the bandwidth of the signal light beam.

3. The optical receiving device according to claim 2, wherein N is 3 or more, and the frequency difference between each pair of adjacent continuous light rays on the frequency axis among the N continuous light rays included in the local light is equal.

4. The frequency waveform of the signal light is sin wave-shaped. The optical receiving device according to claim 1, wherein the frequency difference between each pair of adjacent continuous light rays on the frequency axis among the N continuous light rays included in the local light is an integer multiple of the difference between the frequency at which the amplitude is maximum and the frequency at which the amplitude is minimum in the frequency waveform of the signal light.

5. The detection means outputs one or more detection signals by photoelectric conversion of one or more output lights. The optical receiving device according to any one of claims 1 to 4, further comprising demodulation means for determining information carried by the signal light based on one or more detection signals.

6. The one or more output lights include a first output light and a second output light that includes the local light included in the first output light and which has a phase difference of π from the local light. The optical receiving device according to claim 5, wherein the detection means outputs a single detection signal based on the difference between an electrical signal obtained by photoelectric conversion of the first output light and an electrical signal obtained by photoelectric conversion of the second output light.

7. The one or more output lights include a first output light and a second output light that includes the local light included in the first output light and having a phase difference of π / 2 from the local light, The optical receiving device according to claim 5, wherein the detection means outputs one detection signal based on an electrical signal obtained by photoelectric conversion of the first output light, and another detection signal based on an electrical signal obtained by photoelectric conversion of the second output light.

8. The demodulation means determines the information to be carried by the signal light based on the first to Nth beat signals included in the one or more detection signals, The nth beat signal is a signal generated by the beat between the nth mode continuous light and the nth mode signal light. The optical receiving device according to claim 5, wherein n is an integer from 1 to N.

9. The optical receiving device according to claim 8, wherein the demodulation means includes a frequency conversion means for making the frequency bands of the first beat signal to the N beat signal the same.

10. The optical receiving device according to claim 5, wherein the demodulation means further comprises an analog-to-digital conversion means for converting one or more detection signals into digital signals.

11. The generation means includes a mode conversion multiplexing means that generates the local light by converting continuous light of a predetermined propagation mode at the nth frequency into continuous light of the nth mode and wavelength multiplexing, The optical receiving device according to any one of claims 1 to 4, wherein n is an integer from 1 to N.

12. The generating means is The optical receiving device according to claim 11, comprising a light source that generates an optical comb containing continuous light of a predetermined propagation mode from a first frequency to the Nth frequency.

13. An optical transmitting device that transmits signal light to an optical receiving device that receives signal light via a multimode fiber, A light source that generates continuous light, A modulation means that generates the signal light by modulating the continuous light with an electrical signal that carries information, Equipped with, An optical transmitting device in which the frequency waveform of the signal light has a sin-wave shape.

14. The system further includes a filter that filters the electrical signal so that the frequency waveform of the electrical signal takes on the sin-wave shape. The optical transmitting device according to claim 13, wherein the modulation means modulates the continuous light with the electrical signal after filtering by the filter.

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