JP6728833B2 - Digital coherent receiver, optical space communication system, and Doppler shift acquisition method thereof - Google Patents

Description

The present invention relates to an optical space communication system and a Doppler shift capturing method thereof, and more particularly, to an optical space communication system for capturing a Doppler shift of a signal light frequency in an optical space communication system connecting satellites, satellite-ground, etc. It relates to the Doppler shift acquisition method.

With the development of data processing technology in recent years, expectations are gathering for attempts to extract meaningful information from abundant ground observation data acquired from satellites and aircrafts in the sky and utilize it for disaster prevention and resource search. There is. Against this background, the amount of data generated by air vehicles tends to increase with the increasing demand for improved performance, such as the resolution of sensors for ground observation mounted on air vehicles, and we will make full use of this data. Therefore, it is important to increase the capacity of the communication system that sends the acquired data to the ground.

The methods for sending the data generated by artificial satellites or aircrafts in the sky to the ground are roughly divided into two methods. One is a method in which an artificial satellite or an aircraft directly communicates with a ground station installed on the ground. However, since low-orbit satellites and aircraft that perform ground observations are moving at high speed, the time when the ground station can be directly viewed is limited. The other is a method in which data is sent once from a low-orbit satellite or an aircraft to a geostationary orbit satellite that can always communicate from the ground station, and then sent to the ground via it. Regardless of which method is used, generally used microwave communication has a large restriction on band use, and therefore, an optical space communication system that can achieve a large capacity without any band restriction is drawing attention.

Turning to optical fiber communication systems in which optical communication technology has been active so far, digital coherent technology that combines coherent detection and digital signal processing has now achieved great success. A high reception sensitivity is achieved by the coherent detection in which the received weak optical signal is received by interfering with the local oscillator light, and the digital signal processing enables flexible equalization processing. In order to enjoy such advantages, it is possible to apply the digital coherent technology to the optical space communication system.

In an optical space communication system between artificial satellites and between artificial satellites and ground, Doppler shift of signal light occurs due to high speed movement of artificial satellites. The magnitude of Doppler shift caused by optical communication between a low-orbit satellite that moves at high speed and a geosynchronous satellite reaches several GHz, and the fluctuation speed reaches several MHz/s.

In the digital coherent technology, there are roughly three methods of compensating for the Doppler shift of signal light, that is, the relative frequency difference between the carrier frequency and the local oscillator light. The first is an optical method of adjusting the frequency of the local oscillator light according to the Doppler shift amount. Non-Patent Document 1 describes a method of compensating for Doppler shift by controlling the frequency of local oscillator light using an optical phase locked loop. The second is an electrical method in which the signal is converted into an electric signal by coherent detection and then mixed with a sine wave having a frequency matching the Doppler shift amount. The third is a digital method of compensating for a frequency difference by digital signal processing after being sampled by an analog-to-digital converter (ADC: Analog-Digital Converter). Non-Patent Document 2 describes a method of the M-th power method for compensating a frequency offset corresponding to Doppler shift. The method described in Non-Patent Document 2 squares the phase difference between two consecutive samples to the sample sequence of 1 sample/symbol that has received a QPSK (Quadrature Phase Shift Keying) signal. As a result, the effect of data modulation is removed, and the effect of noise is suppressed by averaging it to estimate the frequency difference, and the frequency offset corresponding to the Doppler shift is compensated.
The method using digital signal processing enables fast and flexible compensation.

FIG. 8 is a block diagram of a general configuration in which an optical signal is coherently received, and Doppler shift is compensated by digital signal processing to perform decoding. The received optical signal is input to the 90-degree optical hybrid together with the local oscillator light, and the combined signals are converted into electric signals by the balanced photodetector, and the I (in-phase) component and the Q (quadrature) component signals are obtained. To get The electric signal is sampled by the ADC after passing through a transimpedance amplifier, a low-pass filter, etc. to obtain a sample string. Intensity normalization, resampling, DC component removal, equalization filter processing including matched filter, etc. are performed on the sample sequence by digital signal processing, and the Doppler shift compensating unit as described in Non-Patent Document 2 is described. The Doppler shift amount is estimated and compensated by. Finally, received data is obtained by performing decoding processing such as symbol determination, differential decoding, and forward error correction.

The method using digital signal processing enables high-speed and flexible compensation, but if the Doppler shift amount is very large, the following problem arises in that no signal can be detected in the digital domain. FIG. 9 is a diagram showing the relationship between the signal light and the optical frequency of the local oscillator light when the Doppler shift is large in the general configuration. The carrier frequency of the signal light is f _s , and the optical frequency of the local oscillator light is f _LO . Assuming that a wavelength near 1550 nm is used, both f _s and f _LO are about 193 THz, but two are due to the frequency difference between the laser provided on the transmission side and the laser serving as the local oscillator light and the Doppler shift of the signal light. Do not match. FIG. 10 is a diagram showing the relationship between the frequency component of the electric signal after coherent detection of the signal light by the local oscillator light and the bandwidth f _ADC of the _ADC in the general configuration. The interference of the signal light and local oscillator light, the signal is downconverted, | becomes to have a centered frequency components _| f s _-f LO. The frequency width of the frequency component of the signal is determined by the modulation method and the like, and it is general that the _ADC bandwidth f _ADC is selected to be necessary and sufficient for receiving this. When f _s −f _LO is large, no frequency component of the signal is included in the ADC bandwidth. Therefore, in such a case, since no significant signal is input to the digital signal processing, it is impossible to estimate the Doppler shift amount, and it is impossible to even determine whether or not the signal light is input to the receiver. .. Assuming optical communication between a low earth orbit satellite and a geostationary satellite, assuming that the signal light modulation symbol rate is 2.5 GHz, the ADC bandwidth is 1.25 GHz, and the Doppler shift amount is 5 GHz, The situation arises. Even for the same Doppler shift amount, such a problem is likely to occur as the data rate of the signal light is smaller and the ADC bandwidth is smaller.

One possible method to avoid this is to sweep the optical frequency of the local oscillator light in the initial frequency acquisition stage, as described in Non-Patent Document 1. However, it takes a very long time to sweep the stable optical frequency in the range of several GHz required for Doppler shift compensation. Further, at the timing when the relative frequency between the optical frequency of the local oscillator light and the carrier frequency of the signal light during the sweep does not take a value close to the Doppler shift amount, no significant signal is input to the digital area. Therefore, especially when the fluctuation of the signal light intensity input to the receiver is large, a significant signal should not be input due to the fluctuation of the received signal light intensity due to the vibration of the artificial satellite under the specific local oscillator optical frequency. In order not to erroneously discriminate the influence of the Doppler shift, it is necessary to provide an observation time sufficient to average the fluctuations in the signal light intensity. Due to these influences, it is difficult to perform initial capture of Doppler shift and recovery from signal disconnection at high speed and stably.

Further, as another related technique, Patent Document 1 discloses a satellite receiver that performs satellite acquisition when the transmission frequency from the satellite changes due to the Doppler effect. The satellite receiver disclosed in Patent Document 1 searches for a frequency point at which a synchronization signal from the satellite can be detected while switching the transmission frequency of the local frequency generation unit in every 100 Hz step in the satellite downlink channel ±Doppler frequency range. .. Measure the amplitude level of the demodulated data to detect the peak, investigate whether the synchronization signal can be confirmed at the frequency at which this peak is obtained, consider the frequency point of the largest peak point as the lock point, and proceed to the received signal tracking operation. Transfer.

JP-A-8-148974

Haraguchi et al., "Development of Next Generation Optical Inter-Satellite Communication Equipment (2)-Communication Performance Evaluation of Optical Homodyne Receiver-", IEICE Technical Report, SANE2011-39. A. Leven et al. , "Frequency Estimation in Intradyne Reception," IEEE Photon. Technol. Lett. , Vol. 19, No. 6, 366 (2007).

In an optical space communication system that connects between satellites and between satellites and the ground, the Doppler shift that occurs due to the relative position of the transceiver station changing at high speed occurs. When a large amount of Doppler shift occurs, the sample sequence input to digital signal processing may not be a significant signal. As described in Patent Document 1 and Non-Patent Document 1, in the method of sweeping the frequency of the laser light source at the time of initial capture and continuing it until the interference component of the signal light and the local oscillator light falls within the ADC band, it is wide. The wide sweep range to support Doppler shift and the slow frequency sweep speed limit the time required for initial acquisition.

An object of the present invention is to always input a significant sample sequence to digital signal processing even for a large amount of Doppler shift, and to perform initial capture of Doppler shift compensation by digital signal processing and to recover from signal loss quickly and stably. To provide a way to do it.

An optical space communication system according to one aspect of the present invention is an AD conversion unit that AD-converts an electric signal generated by coherently detecting a received optical signal using local oscillator light, and a signal output by the AD conversion unit. And a number that covers a range of possible Doppler shifts in the frequency of the local oscillator light according to the band of the AD converter when the intensity of the signal is below a threshold value. A plurality of frequency generation units that generate a plurality of frequency components set in 1.

A Doppler shift capturing method for an optical space communication system according to another aspect of the present invention is an optical space communication in which a received optical signal is coherently detected using a local oscillator light and an electric signal generated is AD-converted by an AD converter. A Doppler shift capturing method for a system, wherein the intensity of a signal output from the AD conversion unit is detected, and when the intensity of the signal is below a threshold value, the local oscillator light is responsive to a band of the AD conversion unit. Generate a plurality of frequency components set by numbers that cover the range of possible Doppler shifts at different frequency intervals.

According to the present invention, it becomes possible to perform fast and stable initial acquisition of Doppler shift compensation by digital signal processing even for large Doppler shift amounts.

FIG. 1 is a block diagram of a digital coherent receiving unit of an optical space communication system according to the first embodiment of the present invention. FIG. 2 is a diagram showing an example of a plurality of frequency components of the signal light and the local oscillator light. FIG. 3 is a diagram showing the relationship between the frequency component of the electric signal and the ADC bandwidth f _ADC when the signal light is coherently detected by the local oscillator light having the plurality of frequency components of FIG. FIG. 4 is a block diagram showing an example of the configuration of the multiple frequency generation unit. FIG. 5 is a diagram showing an example of a time waveform of intensity modulation for converting light output from a laser light source into light having a plurality of frequency components. FIG. 6 is a graph showing the relationship between the Doppler shift amount and the intensity monitor output according to this embodiment. FIG. 7 is a flowchart showing the frequency component search operation by the bisection method of this embodiment. FIG. 8 is a block diagram of a general configuration in which an optical signal is coherently received, and Doppler shift is compensated by digital signal processing to perform decoding. FIG. 9 is a diagram showing a relationship between the signal light and the optical frequency of the local oscillator light when the Doppler shift is large in the general configuration. FIG. 10 is a diagram showing the relationship between the frequency component of the electric signal after coherent detection of the signal light by the local oscillator light and the bandwidth f _ADC of the _ADC in the general configuration.

<First Embodiment>
FIG. 1 is a block diagram of a digital coherent receiving unit of an optical space communication system according to the first embodiment of the present invention. The present embodiment is an example in which the modulation system of the received signal light is 2.5 Gb/s BPSK (Binary Phase Shift Keying).

As shown in FIG. 1, the digital coherent reception unit 1 includes a 90-degree optical hybrid 11, a laser light source 12, a multiple frequency generation unit 13, a photodetector 14, an analog-digital converter (ADC) 15, and a digital signal processing unit 16. , Intensity monitor 17 and generation frequency controller 18.

The signal light received by the digital coherent receiver 1 is input to the 90-degree optical hybrid 11 together with the local oscillator light.

The laser light source 12 produces laser light having a single optical frequency. The output of the laser light source 12 is input to the multiple frequency generation unit 13.

In the normal reception state in which the reception data is well demodulated, the multiple frequency generation unit 13 plays a role of performing nothing or shifting the laser light of a single optical frequency by a predetermined optical frequency. The output of the multiple frequency generation unit 13 is input to the 90-degree optical hybrid 11 together with the signal light received by the digital coherent reception unit of the optical space communication system.

The 90-degree optical hybrid 11 performs coherent detection. That is, the 90-degree optical hybrid 11 mixes the signal light and the local oscillator light and outputs two interference signals (beat signals) corresponding to the I (in-phase) component and the Q (quadrature) component generated by both. The I-component and Q-component signal lights coherently detected by the 90-degree optical hybrid 11 are input to the balanced photodetector 14, respectively.

The photodetector 14 converts the intensity of the I-component or Q-component beat signal, which is coherently detected and input, into an electric signal. The converted electric signals of the I component and the Q component are input to the ADC 15 after being amplified by a transimpedance amplifier or a low-pass filter (not shown) to reduce noise.

The ADC 15 has a necessary and sufficient bandwidth and a sampling rate for receiving a 2.5 Gb/s BPSK signal, for example. The bandwidth of the ADC 15 is, for example, 1.25 GHz, and the sampling rate of the ADC 15 is, for example, 5 GS/s. The ADC 15 outputs the sample sequence obtained as a result of sampling to the digital signal processing unit 16 and the intensity monitor 17.

The digital signal processing unit 16 performs equalization filter processing including intensity normalization, resampling, DC component removal, matched filter, and the like to minimize distortion and noise. After that, the digital signal processing unit 16 performs the Doppler shift compensation process, and the Doppler shift amount is estimated and compensated. For the Doppler compensation process, the frequency offset estimation/compensation using the above-described M-th power method, the decision-directed digital phase locked loop, or the like can be used. Finally, the digital signal processing unit 16 performs decoding processing such as symbol determination, differential decoding, and forward error correction on the sample sequence in which the Doppler shift is compensated, and outputs the received data.

The intensity monitor 17 generates a monitor intensity from a sample sequence that is a sampling result of the signal light of the I component and the Q component input from the two ADCs 15, and outputs it to the generation frequency control unit 18.

The generation frequency control unit 18 has a threshold value set for the monitor intensity. The generation frequency control unit 18 controls the multiple frequency generation unit 13 to start generation of multiple frequencies when the monitor intensity generated by the intensity monitor 17 is equal to or less than the threshold value. For example, such a state occurs when the Doppler shift is large at the time of capturing the initial frequency.

The multiple frequency generation unit 13 generates local oscillator light having a plurality of frequency components arranged at specific frequency intervals Δf above and below the original optical frequency f _LO of the laser light source 12 that is the local oscillator light. The frequency interval Δf is predetermined and set in the multiple frequency generation unit 13.

FIG. 2 is a diagram showing an example of a plurality of frequency components of the signal light and the local oscillator light. For example, as shown in FIG. 2, the multiple frequency generation unit 13 generates a local oscillator light having three frequency components of f _LO and f _LO ±Δf in the vicinity of the carrier frequency f _s of the signal light.

FIG. 3 is a diagram showing the relationship between the frequency component of the electric signal and the bandwidth f _ADC of the _ADC 15 when the signal light is coherently detected by the local oscillator light having the plurality of frequency components of FIG. The interference between the signal light and the frequency component of the frequency f _LO of the local oscillator light is down-converted to the center frequency f _s −f _LO . Similarly, the interference between the signal light and the frequency component of the frequency f _LO ±Δf of the local oscillator light is down-converted to the center frequency f _s −(f _LO ±Δf). As shown in FIG. 3, the signals down-converted to the center frequencies f _s −f _LO +Δf and f _s −f _LO are not within the bandwidth f _{ADC of the ADC} 15, but at the center frequency f _s −f _LO −Δf. The down-converted interference component between the signal light and the frequency component of the frequency f _LO +Δf of the local oscillator light is within the bandwidth f _{ADC of the ADC} 15. Therefore, the sample sequence sampled by the ADC 15 contains significant information about the signal and can also be detected by the intensity monitor 17.

The generation frequency control unit 18 of the present embodiment sets the frequency interval Δf of the frequency components generated by the multiple frequency generation unit 13 to about twice the ADC band. Further, the generation frequency control unit 18 sets the number of generated frequency components so that the frequency range of the plurality of frequency components generated at this frequency interval covers the range in which Doppler shift can occur. That is, the generation frequency control unit 18 sets the number of generated frequency components such that the maximum and minimum frequency difference between the plurality of frequency components generated by the multiple frequency generation unit 13 is equal to or more than the range of possible Doppler shift. Set.

By setting in this way, the frequency difference between the signal light and any frequency component of the local oscillator light falls within the range of the bandwidth f _ADC of the _ADC 15. Therefore, any interference component can be down-converted into the bandwidth f _{ADC of ADC} 15 to keep significant sample trains input to the digital signal processing.

FIG. 4 is a block diagram showing an example of the configuration of the multiple frequency generation unit 13.

The laser light source 12 outputs the generated local oscillator light of the frequency f _LO to the IQ modulator 131.

The generation frequency control unit 18 digitally generates an electric waveform for controlling the intensities of the plurality of frequency components generated by the IQ modulator 131, and outputs the electric waveform to the digital-analog converter (DAC) 132.

The DAC 132 converts the data digitally generated by the generation frequency control unit 18 into an electric waveform by digital-analog conversion and outputs the electric waveform to the IQ modulator 131.

The IQ modulator 131 performs optical modulation on the light output from the laser light source 12 according to the electrical waveform input from the digital-analog converter 132.

FIG. 5 is a diagram showing an example of a time waveform of intensity modulation for converting the light output from the laser light source 12 into light having a plurality of frequency components. The waveform shown in FIG. 5 is an example of a waveform for converting the light having the frequency f _LO output from the laser light source 12 into the light having three frequency components f _LO and f _LO ±Δf. The generation frequency control unit 18 generates digital data so that the electrical waveform as shown in FIG. 5 is output from the DAC 132 to the IQ modulator 131, and outputs the digital data to the DAC 132. The digital data of the waveform shown in FIG. 5 are functions exp(2πi×0×t), exp(2πi×Δf×t), exp(2πi×(-Δf) corresponding to the three frequency components f _LO and f _LO ±Δf. )×t) based on the generated frequency control unit 18. The generation frequency control unit 18 generates digital data having the waveform shown in FIG. 5, for example, by multiplying each of three functions by a coefficient, adding the coefficients, and performing intensity standardization. The generation frequency control unit 18 can increase the relative intensity of only a specific frequency component by giving a difference to the coefficient by which each frequency component is multiplied. For example, if exp(2πi×0×t), exp(2πi×Δf×t), and exp(2πi×(−Δf)×t) are multiplied by A, B, and C, A will be A for B and C. By increasing, the relative intensity of the frequency component of the frequency f _LO in the local oscillator light can be increased.

The output of the IQ modulator 131 is input to the optical amplifier 133, and the optical loss due to the optical modulation is compensated. When the local oscillator light input to the 90-degree optical hybrid 11 is assumed to be light in a specific polarization state, the optical amplifier 133 holds the polarization state of the input light and polarizes it orthogonally. The spontaneous emission amplified light generated in the component is removed by the polarizer.

It is assumed that the optical amplifier 133 inputs the local oscillator light having the same total light intensity to the 90-degree optical hybrid 11 regardless of the number of frequency components generated by the multiple frequency generation unit 13.

The effects of this embodiment will be described based on a specific example. Signal light is 2.5 Gb/s BPSK signal, ADC bandwidth is 1.25 GHz, sampling speed is 5 GS/s, and the light intensity of the local oscillator light input to the 90-degree optical hybrid is constant regardless of the number of frequency components. The simulation was performed under the following conditions. FIG. 6 is a graph showing the relationship between the Doppler shift amount and the intensity monitor output according to this embodiment. In the example shown in FIG. 6, the frequency interval of the generated multiple frequencies is Δf=2 GHz, and when the local oscillator light has a single frequency, f _LO and f _LO ±f are three frequency components, f _LO and f _LO In the case of five frequency components of ±Δf and f _LO ±2Δf, in the case of seven frequency components of f _LO , f _LO ±Δf, f _LO ±2Δf, and f _LO ±3Δf, Doppler shift is applied to the signal light, respectively. It is the result of the simulation about the case of giving. The carrier frequency of the signal light to be received is 193.3 THz, Optical Signal-to-Noise Ratio is -2 dB/0.1 nm, the frequency of the laser that becomes the local oscillator light is 193.3 THz, and the line width of the laser is the same as that of the transmission side. It was set to 0.

When the local oscillator light has a single frequency, the output of the intensity monitor decreases as the magnitude of the Doppler shift amount increases, and when the magnitude of the Doppler shift reaches about 5 GHz, in the range beyond that. Almost no change. This means that, as described above, no frequency component of the signal is included in the bandwidth f _ADC of the _ADC 15 in this region, and therefore, no significant signal is input to the digital signal processing.

On the other hand, when the local oscillator light has seven frequency components f _LO, f _LO ±Δf, f _LO ±2Δf, and f _LO ±3Δf, the intensity monitor output when the Doppler shift is 0 is Although it is lower than that at one frequency, the Doppler shift maintains a constant output up to about 5 GHz. That is, a significant sample sequence down-converted into the bandwidth f _{ADC of the ADC} 15 by any frequency component of the local oscillator light is detected by digital signal processing.

As described above, according to the present embodiment, by generating and using the local oscillator light having a plurality of frequency components by the multiple frequency generation unit 13, it is always significant for digital signal processing within the assumed Doppler shift range. It is possible to retain the state in which the sample string is input.

FIG. 7 is a flowchart showing the frequency component search operation by the bisection method of this embodiment. First, the generation frequency control unit 18 determines whether the output of the intensity monitor is less than or equal to the threshold value (step S1), and when detecting that the output of the intensity monitor is less than or equal to the threshold value, the generation frequency control unit 18 causes the predetermined frequency interval Δf. And the digital data for generating the number N of frequency components is generated and output to the multiple frequency generation unit 13. Based on this, the multiple frequency generation unit 13 generates the local oscillator light having N frequency components of the preset frequency interval Δf based on the digital data (step S2).

As a result, the state in which the sample sequence significant for digital signal processing is always input is held.

Next, the generation frequency control unit 18 searches for one frequency component closest to the carrier frequency of the signal light from the N frequency components by using the so-called dichotomy method. First, the generation frequency control unit 18 narrows down half of the N frequency components as frequency candidates (step S3). The number of frequency candidates does not have to be half as long as it is close to half.

The generation frequency controller 18 equalizes the relative intensities of all frequency components. After that, the generation frequency control unit 18 collectively increases the relative intensities of the frequency components that are frequency candidates (step S4). That is, the generation frequency control unit 18 generates digital data in which the coefficient corresponding to the frequency component that is a frequency candidate is increased, and outputs the digital data to the multiple frequency generation unit 13.

Then, the generation frequency control unit 18 determines whether the output of the intensity monitor 17 increases (step S5).

When the output of the intensity monitor 17 increases, the generation frequency control unit 18 maintains the current frequency candidate as it is (step S6). That is, other frequencies are rejected as not being frequency candidates.

On the contrary, if it decreases, the generation frequency control unit 18 rejects the current frequency candidate and reselects the remaining frequency components that are not candidates as frequency candidates.

It is judged whether there is one remaining frequency candidate (step S6), and if there is one, it is held as the optimum frequency component closest to the carrier frequency of the desired signal light and the search ends (step S9). If the number is not one, the process returns to step S4, and steps S4 to S8 are repeated until there is one candidate.

When the search is completed, the remaining frequency candidate is the optimum frequency component closest to the carrier frequency of the desired signal light, so the multiple frequency generation unit 13 generates a single frequency local oscillator light of this optimum frequency component, Complete the initial frequency acquisition. Then, the digital coherent reception unit 1 captures the Doppler shift with this optimum frequency component.

As described above, according to the present embodiment, since the plural frequency generation unit 13 generates and uses the local oscillator light having a plurality of frequency components, the sweep of the laser light is not required, and thus the initial acquisition is performed. The time required can be reduced.

Further, the generation frequency control unit 18 sets the frequency interval Δf of the frequency components generated by the multiple frequency generation unit 13 to about twice the bandwidth f _ADC of the _ADC 15, and the multiple frequency generation units 13 generate multiple frequencies. The number of frequency components is set so that the maximum and minimum frequency difference between the components is equal to or greater than the range of possible Doppler shifts. With this configuration, even if the Doppler shift amount of the signal light is large, the frequency difference between the signal light and any frequency component of the local oscillator light can be kept within the range of the bandwidth f _ADC of the _ADC 15. Therefore, these two interference components are down-converted into the bandwidth f _{ADC of the ADC} 15 and a significant sample sequence is input to the digital signal processing.

Further, by controlling the respective intensities of the plurality of frequency components of the local oscillator light, it is possible to search for the optimum local oscillator light frequency while always maintaining a state of being input to significant digital signal processing.

Although the present invention has been described with reference to the exemplary embodiments, the present invention is not limited to the above exemplary embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

For example, although it has been described that the light output from one laser light source 12 is converted into light having a plurality of frequency components by the multiple frequency generation unit 13, the invention is not limited to this, and a configuration including a plurality of laser light sources having different frequencies May be In this case, the generation frequency control unit outputs a signal for controlling the intensity or blocking of the light from the plurality of laser light sources to the multiple frequency generation unit, and the multiple frequency generation unit is based on the signal output from the generation frequency control unit. Then, the intensities of the coherent light beams from the plurality of laser light sources are controlled or blocked, and they are output after being combined by an optical coupler or the like.

With such a configuration, the multiple frequency generation unit can be configured by simpler elements such as an optical attenuator and an optical amplifier, instead of complicated components such as an optical modulator and a DAC.

1 Digital Coherent Receiver 11 90-degree Optical Hybrid 12 Laser Light Source 13 Multiple Frequency Generator 14 Photodetector 15 ADC
16 Digital Signal Processing Unit 17 Strength Monitor 18 Generation Frequency Control Unit 131 IQ Modulator 132 Digital-Analog Converter 133 Optical Amplifier

Claims (12)

An AD conversion unit that AD-converts the electric signal generated by coherently detecting the received optical signal using the local oscillator light;
An intensity monitor for detecting the intensity of the signal output from the AD converter,
When the intensity of the signal is below a threshold value, the frequency component of the local oscillator light is set to a number that covers the range of possible Doppler shifts at frequency intervals according to the band of the AD conversion unit. A plurality of frequency generators for generating a plurality of frequency components,
A generation frequency control unit that controls the frequency intervals, the number, and the intensities of the plurality of frequency components of the local oscillator light;
A laser light source for generating a laser light having a single frequency component,
Have
The generation frequency control unit generates a digital data corresponding to the strength of the plurality of frequency components by multiplying the functions representing the plurality of frequency components by the strength and adding the strengths, and generates one of the plurality of frequency components. Select as local oscillator light,
The multiple frequency generation unit includes a modulation unit that modulates the laser light based on the digital data to generate the local oscillator light.
Digital coherent receiver. An AD conversion unit that AD-converts the electric signal generated by coherently detecting the received optical signal using the local oscillator light;
An intensity monitor for detecting the intensity of the signal output from the AD converter,
When the intensity of the signal is below a threshold value, the frequency component of the local oscillator light is set to a number that covers the range of possible Doppler shifts at frequency intervals according to the band of the AD conversion unit. A plurality of frequency generators for generating a plurality of frequency components,
The frequency interval of the plurality of frequency components of the local oscillator light, a generation frequency control unit for controlling the number and intensity,
The generation frequency control unit,
The intensities of the plurality of frequency components are collectively changed using a bisection method, a frequency component close to the Doppler shift is searched based on the intensity of the optical signal, and one of the plurality of frequency components is locally Digital coherent receiver that selects as oscillator light . It has a laser light source that generates laser light having a single frequency component,
The generation frequency control unit generates digital data according to the intensities of the plurality of frequency components,
The multiple frequency generation unit includes a modulation unit that modulates the laser light based on the digital data to generate the local oscillator light.
The digital coherent receiver according to claim 2 . The generation frequency control unit generates the digital data by multiplying a function representing the plurality of frequency components by the intensity and adding the product.
The digital coherent receiver according to claim 3 . The generation frequency control unit,
The frequency interval is set to twice the bandwidth of the AD converter,
The digital coherent receiver according to any one of claims 2 to 4 . The generation frequency control unit,
The frequency interval is set to twice the bandwidth of the AD converter,
The digital coherent receiver according to claim 1 . The generation frequency control unit,
Narrowing half of the plurality of frequency components as frequency candidates,
Increase the intensity of the frequency candidates collectively,
If the intensity of the optical signal is increased, the remaining frequency components that have not been rejected as candidates are rejected and half of the frequency candidates are narrowed down as new frequency candidates,
When the intensity of the optical signal decreases, the frequency candidates are rejected and the remaining frequency components that have not been rejected as candidates are reselected as new frequency candidates,
The digital coherent receiver according to claim 2 . An AD conversion unit that AD-converts the electric signal generated by coherently detecting the received optical signal using the local oscillator light;
An intensity monitor for detecting the intensity of the signal output from the AD converter,
When the intensity of the signal is below a threshold value, the frequency component of the local oscillator light is set to a number that covers the range of possible Doppler shifts at frequency intervals according to the band of the AD conversion unit. A plurality of frequency generators for generating a plurality of frequency components,
A generation frequency control unit that controls the frequency intervals, the number, and the intensities of the plurality of frequency components of the local oscillator light;
A laser light source for generating a laser light having a single frequency component,
The generation frequency control unit generates a digital data corresponding to the strength of the plurality of frequency components by multiplying the functions representing the plurality of frequency components by the strength and adding the strengths, and generates one of the plurality of frequency components. Select as local oscillator light,
The multiple frequency generation unit includes a modulation unit that modulates the laser light based on the digital data to generate the local oscillator light.
Optical space communication system. An AD conversion unit that AD-converts the electric signal generated by coherently detecting the received optical signal using the local oscillator light;
An intensity monitor for detecting the intensity of the signal output from the AD converter,
When the intensity of the signal is below a threshold value, the frequency component of the local oscillator light is set to a number that covers the range of possible Doppler shifts at frequency intervals according to the band of the AD conversion unit. A plurality of frequency generators for generating a plurality of frequency components,
The frequency interval of the plurality of frequency components of the local oscillator light, a generation frequency control unit for controlling the number and intensity,
The multiple frequency generation unit,
The intensity of the plurality of frequency components is collectively changed using a dichotomy method, and a frequency component close to the Doppler shift is searched for based on the intensity of the optical signal. One of the plurality of frequency components is local. Optical space communication system to select as oscillator light . In a Doppler shift acquisition method of an optical space communication system in which a received optical signal is coherently detected by using a local oscillator light, and the generated electrical signal is AD-converted by an AD converter,
Detecting the intensity of the signal output from the AD converter,
When the intensity of the signal is below a threshold value, the local oscillator light has a plurality of frequency components set to a number that covers a range of possible Doppler shifts at frequency intervals according to the band of the AD converter. Generate,
Narrowing half of the plurality of frequency components as frequency candidates,
Increase the intensity of the frequency candidates collectively,
If the intensity of the optical signal increases, discard the remaining frequency components that are not the frequency candidates,
If the intensity of the optical signal is reduced, the frequency component that is the frequency candidate is rejected,
If the remaining frequency components one, capture the Doppler shift by the remaining frequency components
Doppler shift acquisition method for optical space communication system. In a Doppler shift acquisition method of an optical space communication system in which a received optical signal is coherently detected by using a local oscillator light, and the generated electrical signal is AD-converted by an AD converter,
Detecting the intensity of the signal output from the AD converter,
When the intensity of the signal is below a threshold value, the frequency component of the local oscillator light is set to a number that covers the range of possible Doppler shifts at frequency intervals according to the band of the AD conversion unit. Generate multiple frequency components,
Controlling the frequency spacing, the number and intensity of the plurality of frequency components of the local oscillator light;
Generates a laser beam with a single frequency component,
A function that represents the plurality of frequency components is multiplied by the strength and added to generate digital data according to the strength of the plurality of frequency components,
Selecting one of the plurality of frequency components as the local oscillator light;
Modulates the laser light based on the digital data to generate the local oscillator light,
Doppler shift capture method. In a Doppler shift acquisition method of an optical space communication system in which a received optical signal is coherently detected using a local oscillator light, and the generated electric signal is AD-converted by an AD converter,
Detecting the intensity of the signal output from the AD converter,
When the intensity of the signal is below a threshold value, the frequency component of the local oscillator light is set to a number that covers the range of possible Doppler shifts at frequency intervals according to the band of the AD conversion unit. Generate multiple frequency components ,
Controlling the frequency spacing, the number and intensity of the plurality of frequency components of the local oscillator light;
The intensities of the plurality of frequency components are collectively changed using a bisection method, a frequency component close to the Doppler shift is searched for based on the intensity of the optical signal, and one of the plurality of frequency components is set to the local region. Doppler shift acquisition method to select as oscillator light .

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