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 that receives signal light modulated from pulsed light via an MMF includes: a mode processing means that separates the signal light into multiple modes of light and generates received light by multiplexing the pulse positions of each of the multiple modes of light at different time positions; a generation means that generates local light including continuous light of the multiple modes; an optical processing means that generates output light including the received light and the local light; and a detection means that converts the output light into photoelectric light.

Description

Technical Field

[0001] The present disclosure relates to an optical receiving device and an optical transmitting device that can be used, for example, in a free space optics (FSO) communication system.

Background Art

[0002] Non-Patent Documents 1 and 2 each disclose an optical receiving device for an FSO communication system. According to Non-Patent Documents 1 and Non-Patent Document 2, a multimode fiber (MMF) is used for the conveyance of signal light collected by an optical antenna. Since the cross-sectional area of the core of the MMF is larger than the cross-sectional area of the core of a single-mode fiber (SMF), by using the MMF, more of the signal light collected by the optical antenna can be made to enter the optical fiber as compared with using the SMF.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Non-Patent Document 2

[0004] To receive signal light coherently, it is necessary to interfere signal light and local light of the same mode. For this reason, Non-Patent Documents 1 and 2 describe separating the modes of signal light carried by MMF using a mode separator and using a number of coherent receivers equal to the number of modes. Specifically, if the number of 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 and outputs an electrical signal that represents the beat component of the mixed light. Then, the signal light is 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 the number of modes N, and MIMO processing is performed on 2 × N electrical signals.

[0005] In the configurations described in Non-Patent Documents 1 and 2, a coherent receiver is used in proportion to the number of modes utilized. The coherent receiver has photoelectric converters such as analog-to-digital converters (ADCs) and photodiodes (PDs). Therefore, in the configurations described in Non-Patent Documents 1 and 2, a number of ADCs or PDs corresponding to the number of modes is required, 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 carried by MMF. [Means for solving the problem]

[0007] According to one aspect of the present disclosure, an optical receiving device that receives signal light modulated from pulsed light via a multimode fiber includes: a mode processing means that separates the signal light into multiple modes of light and generates received light by multiplexing the pulse positions of each of the multiple modes of light at different time positions; a generation means that generates local light including continuous light of the multiple modes; an optical processing means that generates output light including the received light and the local light; and a detection means that performs photoelectric conversion of the output light. [Effects of the Invention]

[0008] According to this disclosure, the cost of an optical receiving device that receives signal light transported by MMF can be reduced. [Brief explanation of the drawing]

[0009] [Figure 1] A diagram showing an example configuration of an optical transmission device. [Figure 2] Diagram illustrating the carrier light. [Figure 3] A diagram showing an example configuration of an optical receiving device. [Figure 4] A diagram showing an example configuration of the mode processing unit. [Figure 5] A diagram showing an example of the configuration of a local photogenerator. [Figure 6] A diagram showing an example of the configuration of the optical processing unit. [Figure 7] A diagram showing another example configuration of an optical receiving device. [Figure 8] A diagram showing another example configuration of the optical processing unit. [Figure 9] A diagram showing yet another example configuration of an optical receiver. [Figure 10] A diagram showing yet another example configuration of the optical processing unit. [Figure 11] A diagram showing another example configuration of an optical transmission device. [Modes for carrying out the invention]

[0010] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are essential for the invention. Two or more of the features described in the embodiments may be arbitrarily combined. Also, the same or similar configurations are denoted by the same reference numerals, and duplicate descriptions are omitted.

[0011] <First Embodiment> FIG. 1 is a diagram showing a configuration example of an optical transmission device according to this embodiment. The light source 10 generates pulsed light as shown in FIG. 2 as the carrier light. In FIG. 2, the shaded squares indicate the pulses included in the pulsed light. According to FIG. 2, the pulse period in the pulsed light is T, the duration of one pulse is C, and the "idle time" between two temporally continuous pulses is V.

[0012] An electrical signal for carrying information is input to the modulator 11 in synchronization with the period during which the pulses of the carrier light are input. The modulator 11 generates signal light (or modulated light) by modulating the pulses of the carrier light with the electrical signal. The transmission unit 12 performs amplification processing and the like on the signal light, and outputs the processed signal light to an optical antenna (not shown). The optical antenna transmits the signal light as an optical beam into space.

[0013] FIG. 3 shows a configuration example of an optical reception device according to this embodiment. The light receiving unit 20 makes the light beam collected by the optical antenna 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 mode processing unit 21 via the MMF.

[0014] The mode processing unit 21 processes the signal light from the light receiving unit 20 to generate received light. The light receiving unit 20 outputs the received light to the optical processing unit 22 via the MMF. FIG. 4 shows a configuration example of the mode processing unit 21.

[0015] The mode separation unit 210 separates the signal light for each mode. For example, assuming that the number of propagation modes in the MMF is N (N is an integer of 2 or more), the mode separation unit 210 separates the signal light into each of the N modes and outputs N signal lights. Hereinafter, as a specific numerical example, N = 3 is used, and the three modes are respectively denoted as the first mode, the second mode, and the third mode. As described above, since the pulse period of the signal light is T, the pulse periods of the signal lights of the first mode, the second mode, and the third mode output by the mode separation unit 210 are also T.

[0016] The delay unit 212 gives a delay to the signal light of the second mode, and the delay unit 213 gives a delay to the signal light of the third mode. The mode multiplexing unit 211 generates received light by mode multiplexing the signal light of the first mode from the mode separation unit 210, the signal light of the second mode from the delay unit 212, and the signal light of the third mode from the delay unit 213.

[0017] In the present embodiment, the delay unit 212 and the delay unit 213 give delays so that the pulses of each mode do not overlap in time in the received light generated by the mode multiplexing unit 211. As an example, the delay unit 212 gives a delay of T / 3 to the received light of the second mode, and the delay unit 213 gives a delay of (2×T) / 3 to the received light of the third mode. By giving such a delay, the received light has the pulses of each mode not overlapping in time and is cyclically arranged on the time axis. In this case, the empty period between two consecutive pulses in the received light is (T / 3) - C. Therefore, T and C are set so as to satisfy the condition of (T / 3) - C ≧ 0. In other words, T ≧ 3C.

[0018] More generally speaking, assuming that the number of modes in the MMF is N from the first mode to the Nth mode, the mode processing unit 21 can be configured to give a delay of {(n - 1)×T} / N to the signal light of the nth mode (n is an integer from 1 to N) after the mode separation of the signal light. In this case, T and C are set so as to satisfy T ≧ N×C.

[0019] Furthermore, it is sufficient that the received light is generated such that the pulses of each mode do not overlap in time, and the configuration of the mode processing unit 21 is not limited to that shown in Figure 4.

[0020] Returning to Figure 3, the local light generation unit 23 generates local light for coherent reception. The local light generation unit 23 outputs the generated local light to the optical processing unit 22 via the MMF. Note that the optical receiver shown in Figure 3 performs heterodyne detection, so the frequency of the local light and the frequency of the carrier light are different.

[0021] Figure 5 shows an example configuration of the local light generation unit 23. The light source 231 generates a single-mode continuous light. The coupler 232 branches the continuous light generated by the light source 231 into at least N parts. Here, N is the number of mode separations in the mode processing unit 21, which is 3 in this example. The mode conversion multiplexer 233 has at least N ports. The N ports correspond one-to-one with the first to the Nth modes. In this example, N=3, so the mode conversion multiplexer 233 has a first port associated with the first mode, a second port associated with the second mode, and a third port associated with the third mode.

[0022] The mode conversion multiplexer 233 converts the light input to a port into the mode corresponding to that port. If the assigned mode is the same as the mode of the continuous light from the light source 231, no mode conversion is performed. The mode conversion multiplexer 233 generates local light by mode multiplexing the converted continuous light. The local light contains the continuous light of N modes included in the received light.

[0023] Returning to Figure 3, the optical processing unit 22 mixes the received light from the mode processing unit 22 and the local light from the local light generation unit 23 to output the first output light and the second output light. Figure 6 shows an example of the configuration of the optical processing unit 22 in this embodiment. The optical processing unit 22 in this embodiment is equipped with a beam splitter (BS) 221. The BS221 splits the received light and the local light into two branches each, and outputs the first output light and the second output light, which are a mixture of the received light and the local light. Due to the characteristics of the BS221, 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).

[0024] Returning to Figure 3, the detection unit 24 is a balanced detector and includes a first photovoltaic diode (PD) that converts the first output light into an electrical signal, a second photovoltaic diode (PD) that converts the second output light into an electrical signal, and a circuit that outputs an electrical signal corresponding to the difference between the output of the first PD and the output of the second PD as a detection signal. The demodulation unit 25 has an ADC for converting the detection signal into a digital signal and demodulates and determines the information transmitted by the optical transmitter based on the digital signal output by the ADC.

[0025] Note that Figure 6 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 6. For example, the optical processing unit 22 may have a lens that converts the input received 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 surface of the first PD or second PD of the detection unit 24.

[0026] The optical processing unit 22 illustrated in Figure 6 spatially realizes a 180-degree hybrid, but the configuration of the optical processing unit 22 is not limited to that shown in Figure 6. 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 24, but it is also possible to generate only one output light by mixing the received light and local light, and generate a detection signal by photoelectric conversion of this.

[0027] As described above, according to this embodiment, the optical transmitting device generates signal light by modulating each pulse of pulsed light and transmits it to the optical receiving device. The mode processing unit 21 of the optical receiving device receives the signal light via MMF. The optical receiving device then separates the signal light into modes and generates received light by applying different delays to each mode. The local light generation unit 23 of the optical receiving device generates local light containing multiple modes included in the received light. The optical processing unit 22 and detection unit 24 of the optical receiving device then coherently receive the received light using the local light.

[0028] Since the local light contains the continuous light of each of the multiple modes included in the received light, a beat component with respect to the continuous light is obtained for each pulse included in the received light. For example, when N=3, the detected signal will be a signal in which the beat component of the pulse of the first mode and the continuous light of the first mode (hereinafter referred to as the first beat component) continues for a period of C, followed by the signal corresponding to the beat component of the pulse of the second mode and the continuous light of the second mode (hereinafter referred to as the second beat component) continues for a period of C, followed by the signal corresponding to the beat component of the pulse of the third mode and the continuous light of the third mode (hereinafter referred to as the third beat component) continues for a period of C, and this process is repeated.

[0029] Next, the reason for providing the mode processing unit 21 will be explained. If the mode processing unit 21 is not provided and the signal light is input to the optical processing unit 22, the detected signal will be a signal in which the signal corresponding to the component obtained by adding the first beat component, the second beat component, and the third beat component continues for a period C, and this is repeated with a period T. Here, if the phases of each beat component are not in phase, the amplitude will decrease when the first beat component, the second beat component, and the third beat component are added together, which may affect demodulation. For this reason, in this embodiment, the mode processing unit 21 is provided, thereby separating the beat components of different modes in time and extracting them individually. As a result, the demodulation unit 25 can perform demodulation with high accuracy by combining the maximum ratio of the consecutive first beat component to the third beat component, etc.

[0030] 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 24 is not used, the required number of PDs becomes 1. Therefore, the cost of the optical receiving device that receives the signal light transported by the MMF can be reduced.

[0031] <In the case of homodyne detection> Next, we will explain the case of performing homodyne detection.

[0032] Figure 7 shows an example configuration of an optical receiver when performing homodyne detection. Note that, for homodyne detection, the wavelength of the local light and the carrier light are the same. Similar to heterodyne detection, the received light and the local light are input to the optical processing unit 26.

[0033] The optical processing unit 26 mixes the received light from the mode processing unit 22 with the local light from the local light generation unit 23 and outputs the first to fourth output lights. The first and second output lights correspond to the I (in-phase) component in quadrature modulation, while the third and fourth output lights correspond to the Q (quadrature phase) component in quadrature modulation. In other words, the optical processing unit 26 supports 90-degree hybrid modulation.

[0034] Figure 8 shows an example configuration of the optical processing unit 26. The local light is converted to circular polarization by the quarter-wave plate 262. That is, if two orthogonal linear polarizations are X-polarization and Y-polarization, the quarter-wave plate 262 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 received light is set to an angle of 45 degrees with respect to the polarization planes of the X-polarization and Y-polarization, respectively. BS261 splits the received light and the local light from the quarter-wave plate 262 into two branches each, and outputs a first mixed light and a second mixed light obtained by mixing the received light and the local light. Polarization beam splitters (PBS) 263 and PBS264 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.

[0035] 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 BS261, 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.

[0036] Returning to Figure 7, the detection unit 241 is a balanced detector and includes two photodiodes (PDs) and a circuit that outputs an electrical signal corresponding to the difference between the outputs of the two PDs as a first detection signal. The first detection signal corresponds to the I component. The detection unit 242 is also a balanced detector and includes two photodiodes (PDs) and a circuit that outputs an electrical signal corresponding to the difference between the outputs of the two PDs as a second detection signal. The second detection signal corresponds to the Q component. The demodulation unit 25 has two ADCs that convert the first detection signal and the second detection signal, respectively, into digital signals, and determines the information transmitted by the optical transmitter based on the digital signals output by the two ADCs.

[0037] As shown in Figure 7, regardless of the number of modes, the number of PDs required for coherent reception is 4, and the number of ADCs is 2. Note that if the balanced detection units 241 and 242 are not used, the number of required PDs is 2. In this case, the optical processing unit 26 outputs the first output light and the third output light.

[0038] Figure 9 shows another configuration example of an optical receiver performing homodyne detection. The difference from Figure 7 is that polarization multiplexing is used for communication with the optical transmitter. The optical processing unit 27 receives the received light and local light as in the configuration of Figure 7.

[0039] The optical processing unit 27 mixes the received light from the mode processing unit 22 with the local light from the local light generation unit 23 and outputs the first to eighth output lights. In Figure 9, for the sake of simplification of the diagram, only the "L" portion of the L output light is shown. The first to fourth output lights correspond to the processing results of one polarization, and the fifth to eighth output lights correspond to the processing results of the other polarization. Furthermore, the first and second output lights correspond to the I component in quadrature modulation, and the third and fourth output lights correspond to the Q component in quadrature modulation. Similarly, the fifth and sixth output lights correspond to the I component in quadrature modulation, and the seventh and eighth output lights correspond to the Q component in quadrature modulation.

[0040] Figure 10 shows an example configuration of the optical processing unit 27. The local light is converted to circular polarization by the quarter-wave plate 272. BS273 splits the circularly polarized local light into two and outputs them to BS274 and BS275. PBS271 polarizes the received light and outputs it to BS274 and BS275. The processing in BS274, PBS276, and PBS277 is the same as the processing in BS261, PBS263, and PBS264 in Figure 8. Also, the processing in BS275, PBS278, and PBS279 is the same as the processing in BS261, PBS263, and PBS264 in Figure 8.

[0041] The detection units 241 and 242 in Figure 9 are the same as those in Figure 7 and output the first detection signal and the second detection signal to the demodulation unit 25. Note that in Figure 9, for the sake of simplicity, only the "M" portion of the Mth detection signal is shown. The detection units 243 and 244 in Figure 9 are the same as those in Figure 7 and output the third detection signal and the fourth detection signal, corresponding to the first detection signal and the second detection signal in Figure 7, to the demodulation unit 25.

[0042] As shown in Figure 9, regardless of the number of modes, the number of PDs required for coherent reception is 8, and the number of ADCs is 4. Note that if the balanced detection units 241-244 are not used, the required number of PDs is 4. In this case, the optical processing unit 27 outputs the first, third, fifth, and seventh output light.

[0043] Although not shown in the diagram, polarization multiplexing can also be performed with heterodyne detection. In this case, regardless of the number of modes, the number of PDs required for coherent reception is 4, and the number of ADCs is 2. If a balanced detection unit is not used, the number of required PDs is 2.

[0044] <Second Embodiment> Next, the second embodiment will be described, focusing on the differences from the first embodiment. In the first embodiment, the signal light included pulses with a period C that repeated with a period T. The optical receiver then separated the signal light into multiple modes of light and generated the received light by multiplexing the pulses of each mode of light at different time positions so that they did not overlap on the time axis. In this embodiment, the received light after mode multiplexing in the optical receiver is made to allow for partial overlap of pulses on the time axis.

[0045] Figure 11 shows an example of the configuration of an optical transmission device according to this embodiment. The difference from the first embodiment is that the electrical signal is filtered by a Nyquist filter 13, and the carrier light is modulated by the filtered electrical signal. In the following description, it is assumed that the Nyquist filter 13 is configured to filter based on the Nyquist interval S. In this case, for example, the period T of the carrier light pulse is set to be an integer multiple of S or greater. As an example, if the number of modes is N, the pulse period T is set to N × S × P or greater, where P is an integer of 1 or greater.

[0046] The mode processing unit 21 of the optical receiver applies a delay of (n-1) × S × P to the light of the nth mode (where n is an integer from 1 to N). By applying such a delay, the pulses of each mode in the received light overlap in time, but interference between pulses is suppressed because each pulse is arranged at a Nyquist interval.

[0047] As described above, according to this embodiment, signal light is generated by modulating pulses of pulsed light with an electrical signal filtered by the Nyquist filter 13. Furthermore, the mode processing unit 21 multiplexes the time positions of the pulses of each of the N modes of light by an integer multiple of the Nyquist interval of the Nyquist filter 13. This configuration allows for a higher pulse density than in the first embodiment, and therefore, an increase in the amount of information transmitted.

[0048] Furthermore, the optical transmitting and 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.

[0049] 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."

[0050] 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]

[0051] 21: Mode processing unit, 23: Local light generation unit, 22: Light processing unit, 24: Detection unit

Claims

1. An optical receiving device that receives signal light modulated from pulsed light via a multimode fiber, Mode processing means for separating the signal light into multiple modes of light, and generating received light by multiplexing the pulses of each of the multiple modes of light at different time positions, A generation means for generating local light including the aforementioned multiple modes of continuous light, A photoprocessing means that generates output light including the received light and the local light, A detection means for converting the output light into photoelectric light, An optical receiving device equipped with the following features.

2. The optical receiving device according to claim 1, wherein the mode processing means multiplexes the pulses of the multiple modes of light at different time positions so that the pulses of each of the multiple modes of light do not overlap on the time axis.

3. The aforementioned multiple modes are the first to the Nth mode (where N is an integer greater than or equal to 2), The period of the pulse of the signal light is T. The optical receiving device according to claim 2, wherein the mode processing means includes a delay means that delays the light of the nth mode (where n is an integer from 1 to N) by {(n-1) × T} / N}.

4. The duration of the pulse of the signal light is C. The optical receiving device according to claim 3, wherein T is N × C or greater.

5. The aforementioned signal light is generated by modulating the pulsed light with an electrical signal filtered by a Nyquist filter. The optical receiving device according to claim 1, wherein the mode processing means multiplexes the time positions of the pulses of each of the multiple modes of light by an integer multiple of the Nyquist interval of the Nyquist filter.

6. The aforementioned multiple modes are the first to the Nth mode (where N is an integer greater than or equal to 2), The aforementioned Nyquist interval is S, The mode processing means includes a delay means that delays the light of the nth mode (where n is an integer from 1 to N) by (n-1) × S × P (where P is an integer of 1 or more), The optical receiving device according to claim 5, wherein the period of the pulse of the signal light is N × S × P or greater.

7. The wavelength of the pulsed light and the wavelength of the local light are different, The detection means converts the output light into photoelectric form and outputs a detection signal. The optical receiving device according to any one of claims 1 to 6, further comprising demodulation means for determining information carried by the signal light based on the detection signal.

8. The wavelength of the pulsed light and the wavelength of the local light are the same. The optical processing means generates first output light and second output light, which include the received light and the local light, wherein 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 / 2. The detection means outputs a first detection signal obtained by photoelectric conversion of the first output light and a second detection signal obtained by photoelectric conversion of the second output light. The optical receiving device according to any one of claims 1 to 6, further comprising demodulation means for determining information to be carried by the signal light based on the first detection signal and the second detection signal.

9. The wavelength of the pulsed light and the wavelength of the local light are the same. The optical processing means outputs first output light and second output light, which include a first polarization component of the received light and the first polarization component of the local light, wherein 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 / 2; and third output light and fourth output light, which include a second polarization component orthogonal to the first polarization component of the received light and the second polarization component of the local light, wherein the phase of the local light included in the third output light and the phase of the local light included in the fourth output light differ by π / 2. The detection means outputs a first detection signal obtained by photoelectric conversion of the first output light, a second detection signal obtained by photoelectric conversion of the second output light, a third detection signal obtained by photoelectric conversion of the third output light, and a fourth detection signal obtained by photoelectric conversion of the fourth output light. The optical receiving device according to any one of claims 1 to 6, further comprising demodulation means for determining information to be carried by the signal light based on the first to fourth detection signals.

10. 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 pulsed light, A modulation means that generates the signal light by modulating the pulse light with an electrical signal that carries information, An optical transmission device equipped with the following features.

11. The optical receiving device separates the signal light into N modes (where N is an integer of 2 or more), The duration of the pulse of the pulsed light is C. The optical transmitting device according to claim 10, wherein the period of the pulse of the pulsed light is N × C or greater.

12. With even more Nyquist filters, The optical transmitting device according to claim 11, wherein the electrical signal is the signal after filtering by the Nyquist filter.

13. The optical receiving device separates the signal light into N modes (where N is an integer of 2 or more), The Nyquist interval of the aforementioned Nyquist filter is S. The optical transmitting device according to claim 12, wherein the period of the pulse of the signal light is N × S × P (where P is an integer of 1 or more).

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