CN112350780B

CN112350780B - Coherent optical communication system and method for high-speed PON

Info

Publication number: CN112350780B
Application number: CN202011239894.1A
Authority: CN
Inventors: 林如俭; 姚毅; 杨健; 陈海辉
Original assignee: Luster Tianbo Optoelectronic Technology Co ltd
Current assignee: Luster Tianbo Optoelectronic Technology Co ltd
Priority date: 2020-11-09
Filing date: 2020-11-09
Publication date: 2022-09-30
Anticipated expiration: 2040-11-09
Also published as: CN112350780A

Abstract

A coherent optical communication system and method for high-speed PON, the system includes the first optical transmitter, the first wavelength division multiplexer WDM, the second wavelength division multiplexer WDM, the first optical receiver, the second optical transmitter and the second optical receiver, use the auxiliary light wave to inject the SOA backward in the first optical transmitter and the second optical transmitter, promote the linearization of the SOA optical gain; the method comprises the steps that a local laser and an optical phase-locked loop are locked by jointly applying optical polarization tracking and optical injection at a first optical receiver of a downlink ONU (optical network unit), so that optical phase noise is minimized, and homodyne detection is realized; the second optical receiver of the OLT in the uplink direction adopts an optical polarization tracking technology to jointly use an optical mixing module and a square sum arithmetic unit, and abandons complex frequency estimation and phase estimation algorithms in a traditional digital signal processor, so that the internal difference detection with frequency offset and optical phase noise nonlinear cancellation functions is realized, and the hardware of a coherent receiver and a signal processing method are simplified.

Description

Coherent optical communication system and method for high-speed PON

Technical Field

The present application relates to the field of optical communication technologies, and in particular, to a coherent optical communication system for a high-speed passive optical network PON, and a communication method for the system.

Background

The existing communication capacity is vacated, so that the direct detection of optical wave envelope (electric field amplitude) is not enough to solve the information bearing problem, but because the hardware and software of the optical coherent detection technology are complex and expensive, the optical coherent communication system is only applied to ultra-long land and sea cable main lines, and cannot be used commercially in an optical access network.

The optical coherent detection has the advantages that the output voltage of the optical receiver is proportional to the multiplication of the optical electric field of the received signal and the optical electric field of the local oscillator, and even if the received signal light is very weak, the local oscillator light can generate considerable receiver output as long as the local oscillator light is strong enough, so that the receiver sensitivity is high, and the system loss tolerance is large. However, the complexity of optical coherence detection comes from the independent operation of a transmitting end laser and a receiving end Local (LO) laser, and even if the same-brand laser and the same current and temperature control are adopted, the LO optical wave is different from the signal optical wave in frequency and phase, and the polarization state fluctuation of the signal optical wave in a long-distance external line is added, and the signal optical wave electric field needs to be multiplied by the LO optical wave electric field, so that a polarization diversity optical receiver formed by a polarization mixer and four balanced optical detector pairs is required to be adopted. And then, the output of the receiver is subjected to complex digital signal processing after analog-to-digital (A/D) conversion, carrier frequency offset estimation and compensation are firstly carried out, then phase estimation and suppression are carried out, dispersion compensation, channel equalization, data demodulation and the like are also carried out, the system complexity is high, and the power consumption is high, so that the traditional coherent communication technology called interpolation detection is not suitable for a high-speed PON with particularly strict requirements on cost.

Disclosure of Invention

The application provides a coherent optical communication system and a coherent optical communication method for a high-speed PON, which aim to solve the problems that the existing coherent optical communication hardware system is complex in structure, high in cost, large in power consumption, complex in communication method and not suitable for being used in the high-speed PON.

The technical scheme adopted by the application is as follows:

a coherent optical communication system for a high-speed PON, the coherent optical communication system comprising:

a first optical transmitter, a first wavelength division multiplexer WDM108, a second wavelength division multiplexer WDM109, a first optical receiver, a second optical transmitter, and a second optical receiver;

the output end of the first optical transmitter is connected to a first semiconductor optical amplifier SOA105 for emitting an amplified signal optical wave in the downlink of the optical line terminal OLT;

the output end of the first semiconductor optical amplifier SOA105 is respectively connected with a first SOA gain clamping device and a first wavelength division multiplexer WDM 108;

the first wavelength division multiplexer WDM108 is connected with the second wavelength division multiplexer WDM109 through a feeder optical fiber, and the first wavelength division multiplexer WDM108 is connected with the second optical receiver;

the second wavelength division multiplexer WDM109 is connected to the first optical receiver;

the first optical receiver comprises a first external feedback polarization tracking module, an optical injection locking device and a homodyne detection device which are sequentially connected, and is used for receiving the signal optical wave from the first optical transmitter and completing homodyne detection in an Optical Network Unit (ONU);

the first external feedback polarization tracking module is connected with the second wavelength division multiplexer WDM109 and is used for adjusting the polarization direction of the received signal light wave to enable the polarization direction to be stabilized in the slow axis direction of the output polarization-maintaining optical fiber;

the output end of the second optical transmitter is connected to a second semiconductor optical amplifier SOA204, and is configured to emit an amplified signal optical wave in an uplink of an optical network unit ONU;

the output end of the second semiconductor optical amplifier SOA204 is respectively connected with a second SOA gain clamping device and the second wavelength division multiplexer WDM 109;

the second optical receiver comprises a second external feedback polarization tracking module and an internal difference detection device which are sequentially connected, and is used for receiving the signal optical wave from the second optical transmitter and completing internal difference detection at an optical line terminal OLT;

the input end of the second external feedback polarization tracking module is connected with the first wavelength division multiplexer WDM108 and is used for controlling the polarization direction of the received uplink signal light wave to be consistent with the uplink local oscillator light wave.

Preferably, the first optical transmitter comprises a first laser 101, a first lithium niobate MZM optical modulator 104, and a first drive amplifier 103;

the first laser 101 is connected with an optical signal input end of a first lithium niobate MZM optical modulator 104, and is used for emitting an intensity continuous optical carrier, and converting the intensity continuous optical carrier into an intensity modulated signal optical wave through the first lithium niobate MZM optical modulator 104;

the first drive amplifier 103 is connected to an electrical signal input end of a first lithium niobate MZM optical modulator 104, and is configured to perform signal amplification on a line-coded data electrical signal through the first drive amplifier 103, and then modulate the intensity of an input optical carrier in the first lithium niobate MZM optical modulator 104 to form a signal optical wave;

the output end of the first lithium niobate MZM optical modulator 104 is connected to the first semiconductor optical amplifier SOA105, and is configured to amplify the average intensity of the modulated signal light waves to a level required for sending to an optical fiber line;

the output end of the first semiconductor optical amplifier SOA105 is connected to the intermediate port p2 of the first optical circulator 106, and the end port p3 of the first optical circulator 106 is connected to the first wavelength division multiplexer WDM 108.

Preferably, the first SOA gain clamping device comprises a second laser 102 and an optical splitter 107 and the first optical circulator 106;

the second laser 102 is connected to the input end of the optical splitter 107, and is configured to emit an auxiliary light wave;

the first output port of the optical splitter 107 is connected to the head port p1 of the first optical circulator 106, and is used for injecting the auxiliary optical wave back into the first semiconductor optical amplifier SOA105, so as to implement gain clamping and gain linearization.

Preferably, the light injection locking device comprises a first polarization maintaining splitter 111, an optical band pass filter OBPF112, a second polarization maintaining splitter 113, a polarization maintaining optical circulator 114, a local laser ILL115 and a third polarization maintaining splitter 116;

the input end of the first polarization maintaining splitter 111 is connected to the first external feedback polarization tracking module;

two output ends of the first polarization splitter 111 are respectively connected to the homodyne detection device and the optical band pass filter OBPF112, and are configured to split the received signal optical wave to form a signal optical branch and an injection optical branch, where the signal optical branch directly generates a signal optical electric field and adds the signal optical electric field to the homodyne detection device, and the injection optical branch outputs a master control optical wave;

the output end of the optical bandpass filter OBPF112 is connected to the second polarization maintaining optical splitter 113, and is configured to extract a pure main control carrier spectral line from the main control optical wave;

the first output port of the second polarization maintaining optical splitter 113 is connected to the head port p1 of the polarization maintaining optical circulator 114, the middle port p2 of the polarization maintaining optical circulator 114 is connected to the local laser ILL115, and the tail port p3 of the polarization maintaining optical circulator 114 is connected to the input end of the third polarization maintaining optical splitter 116, so as to lock the operating wavelength of the local laser ILL115 to be consistent with the main control carrier spectrum line.

Preferably, an optical phase-locked loop module is further disposed in the optical injection locking device, and the optical phase-locked loop module includes a local laser ILL115, the third polarization-maintaining optical splitter 116, an optical phase shifter 117, an optical detector 118, and a PIC loop filter 119;

a first output port of the third polarization maintaining optical splitter 116 is connected to an input end of the optical phase shifter 117;

the output end of the optical phase shifter 117 is connected to the first input port of the optical detector 118, the second output port of the second polarization maintaining optical splitter 113 is connected to the second input port of the optical detector 118, and the optical detector 118 is configured to multiply the two optical waves from the optical phase shifter 117 and the second polarization maintaining optical splitter 113, respectively, to generate baseband optical phase noise;

the output end of the optical detector 118 is electrically connected to the PIC loop filter 119, the PIC loop filter 119 is electrically connected to the electrical driving end of the local laser ILL115, and the PIC loop filter 119 is configured to feed back the optical phase noise current generated by the optical detector 118 to the local laser ILL115, so that the phase difference between the injected and locked local optical wave and the received signal optical wave is minimized.

Preferably, the homodyne detection device comprises a polarization maintaining coupler 120, a balanced photodetector pair 121, a low noise transimpedance amplifier TIA122, a band-pass filter BPF123, a main amplifier 124 and a digital signal processor 125;

the signal terminal of the polarization maintaining coupler 120 is connected to one of the output terminals of the first polarization maintaining splitter 111, and the local oscillator terminal of the polarization maintaining coupler 120 is connected to the second output terminal of the third polarization maintaining splitter 116;

two output ends of the polarization maintaining coupler 120 are respectively connected with the balanced optical detector pair 121;

the output end of the balanced optical detector pair 121 is electrically connected with the low-noise transimpedance amplifier TIA 122;

the low-noise transimpedance amplifier TIA122, the band-pass filter BPF123, the main amplifier 124 and the digital signal processor 125 are electrically connected in sequence;

the digital signal processor 125 includes an a/D conversion unit, an equalization unit, a clock recovery unit, and a decision regeneration unit, and is configured to perform a/D conversion, equalization, clock recovery, and decision regeneration on a data signal.

Preferably, the first external feedback polarization tracking module comprises a first external feedback polarization tracker PT110 and a first detection and logarithm amplifier 126;

the first external feedback polarization tracker PT110 is fiber-connected between the second wavelength division multiplexer WDM109 and the first polarization-preserving splitter 111 input;

the first external feedback polarization tracker PT110 is electrically connected with the first detection and logarithm amplifier 126;

the first detection and logarithm amplifier 126 is electrically connected to the signal output end of the low-noise transimpedance amplifier TIA122, and is configured to convert a signal output by the low-noise transimpedance amplifier TIA122 into a feedback control voltage of the first external feedback polarization tracker PT 110.

Preferably, the second optical transmitter comprises a third laser 201, a second lithium niobate MZM optical modulator 203, and a second drive amplifier 202;

the third laser 201 is connected to the optical signal input end of the second lithium niobate MZM optical modulator 203, and is configured to send an intensity continuous optical carrier in the uplink of the optical network unit ONU, and convert the intensity continuous optical carrier into an intensity modulated signal optical wave through the second lithium niobate MZM optical modulator 104;

the second drive amplifier 202 is connected to an electrical signal input end of the second lithium niobate MZM optical modulator 203, and is configured to perform signal amplification on the line-coded data electrical signal through the second drive amplifier 202, and then modulate the intensity of the input optical carrier in the second lithium niobate MZM optical modulator 203 to form a signal optical wave;

the output end of the second lithium niobate MZM optical modulator 203 is connected to the second semiconductor optical amplifier SOA204, and is configured to amplify the average intensity of the signal light wave to a level required for sending the signal light wave to an optical fiber line;

the output end of the second semiconductor optical amplifier SOA204 is connected to the intermediate port p2 of the second optical circulator 205, and the end port p3 of the second optical circulator 205 is connected to the second wavelength division multiplexer WDM 109.

Preferably, the second SOA gain clamping device comprises the third polarization maintaining optical splitter 116 and the second optical circulator 205;

the third output port of the third polarization maintaining optical splitter 116 is connected to the head port p1 of the second optical circulator 205, and is configured to transmit the auxiliary optical wave split by the third polarization maintaining optical splitter 116 to the intermediate port p2 through the head port p1 of the second optical circulator 205 in a unidirectional manner, and inject the auxiliary optical wave into the second semiconductor optical amplifier SOA204 from the intermediate port p2 in a reverse direction, so as to implement gain clamping and gain linearization of the SOA.

Preferably, the internal difference detection device comprises a 90 ° optical mixer 207, a first balanced optical detection pair 208, a second balanced optical detection pair 209, a first low-noise preamplifier TIA210, a second low-noise preamplifier TIA211, a first band-pass filter 212, a second band-pass filter 213, and a digital signal processing device 214;

the signal end of the 90 ° optical hybrid 207 is connected to the second external feedback polarization tracking module, and the local oscillation end of the 90 ° optical hybrid 207 is connected to the second output port of the optical splitter 107;

the output end of the 90 ° optical mixer 207 is respectively connected to the first balanced optical detection pair 208 and the second balanced optical detection pair 209;

the output end of the first balanced optical detection pair 208 is connected with the first low-noise preamplifier TIA 210;

the output end of the second balanced optical detection pair 209 is connected with the second low-noise preamplifier TIA 211;

the output end of the first low noise preamplifier TIA210 is electrically connected with a first band-pass filter 212;

the output end of the second low-noise preamplifier TIA211 is electrically connected with a second band-pass filter 213;

the output ends of the first band-pass filter 212 and the second band-pass filter 213 are both connected with a digital signal processing device 214;

the digital signal processing apparatus 214 includes an a/D conversion unit, a square sum and root operator, an equalization unit, a clock recovery unit, and a decision regeneration unit, and is configured to perform a/D conversion, square sum and root operation for eliminating frequency offset and optical phase noise, equalization, clock recovery, and decision regeneration on a data signal.

Preferably, the second external feedback polarization tracking module comprises a second external feedback polarization tracker PT206 and a second detection and logarithm amplifier 215;

the second external feedback polarization tracker PT206 is fiber-connected between the signal end of the 90 ° optical hybrid 207 and the first wavelength division multiplexer WDM 108;

the second external feedback polarization tracker PT206 is electrically connected with the second detection and logarithm amplifier 215;

the second detection and logarithm amplifier 215 is electrically connected to the signal output end of the second low-noise preamplifier TIA211, and is configured to convert the signal output by the low-noise transimpedance amplifier TIA211 into a feedback control voltage of the second external feedback polarization tracker PT 206.

The communication method of the coherent optical communication system for the high-speed PON comprises the following steps:

in a first optical transmitter of a downlink of an optical line terminal OLT, a first laser 101 is adopted to emit a downlink optical carrier;

the first drive amplifier 103 amplifies the data electrical signal coded by the line and then adds the amplified data electrical signal to an electrode of a first lithium niobate MZM optical modulator 104;

the first lithium niobate MZM optical modulator 104 modulates the intensity of the downlink optical carrier with the data electrical signal to form a downlink signal optical wave;

the downlink signal light wave is amplified by a first semiconductor optical amplifier SOA105 and then is sent to an optical line;

the auxiliary light wave is reversely injected into the first semiconductor optical amplifier SOA105, so that the optical gain characteristic of the first semiconductor optical amplifier SOA105 is clamped, and the linearization of the optical gain characteristic is realized;

the amplified downlink signal light wave is transmitted to a first optical receiver of an optical network unit ONU through an optical line;

in a first optical receiver of an optical network unit ONU, the polarization direction of a received signal optical wave is stabilized in the direction of a slow axis of an output polarization-maintaining optical fiber through a first external feedback polarization tracking module;

the local optical wave frequency after injection locking is consistent with the received signal optical wave frequency through an optical injection locking device;

minimizing the phase difference between the injected and locked local optical wave and the received signal optical wave through an optical phase-locked loop module;

performing homodyne detection by using the local optical wave after injection locking as downlink local oscillator light and the received signal optical wave as downlink signal light to generate a downlink baseband data waveform;

performing a/D conversion, equalization, clock recovery and decision regeneration on the downlink baseband data waveform in the digital signal processor 125, and recovering the data signal transmitted by the first optical transmitter;

in the uplink second optical transmitter of the optical network unit ONU, the third laser 201 is used to emit an uplink optical carrier;

the second driving amplifier 202 amplifies the data electrical signal coded by the line, and then adds the amplified data electrical signal to an electrode of a second lithium niobate MZM optical modulator 203;

the second lithium niobate MZM optical modulator 203 modulates the intensity of the uplink optical carrier with the data electrical signal to form an uplink signal optical wave;

the uplink signal light wave is amplified by a second semiconductor optical amplifier SOA204 and then is sent to an optical line;

the auxiliary light wave is reversely injected into the second semiconductor optical amplifier SOA204, so that the optical gain characteristic of the second semiconductor optical amplifier SOA204 is clamped, and the linearization of the optical gain characteristic is realized;

the uplink signal optical wave is transmitted to a second optical receiver of an optical line terminal OLT through an optical line;

in a second optical receiver of the optical line terminal OLT, firstly, a second external feedback polarization tracking module is used for controlling the polarization direction of received uplink signal optical waves to be consistent with uplink local oscillator optical waves;

performing inner difference detection on the uplink signal light wave and the uplink local oscillator light wave to form an uplink baseband data waveform;

the uplink baseband data waveform is transmitted to the digital signal processing device 214 for a/D conversion, square sum and root operation for eliminating frequency offset and optical phase noise, equalization, clock recovery, and decision regeneration, and the data signal transmitted in the ONU uplink second optical transmitter is restored.

The technical scheme of the application has the following beneficial effects:

the invention replaces the light polarization diversity receiving technology by the light polarization tracking technology, so that the hardware of the receiver is greatly simplified; for a downlink structure homodyne detection device based on optical injection locking and an optical phase-locked loop, the frequency drift output by an optical receiver is eliminated, the optical phase noise is greatly reduced, and perfect homodyne detection is realized; and for the uplink, an inner difference detection device based on frequency offset and phase noise nonlinear offset operation is constructed, a frequency offset estimation module and a phase estimation module of the traditional digital signal processing software are eliminated, the cost and the power consumption of a coherent detection system are reduced, and the real-time performance of the coherent detection system is improved, so that the coherent detection can be used for receiving ends of an OLT and an ONU of a high-speed optical access network.

Drawings

In order to more clearly explain the technical solution of the present application, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious to those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.

Fig. 1 is a schematic structural diagram of a coherent optical communication system for a high-speed PON according to the present invention;

FIG. 2 is a power spectrum diagram of a raw NRZ code signal of a driving electrical signal of a lithium niobate MZM optical modulator;

FIG. 3 is a schematic diagram of signal light wave spectrum after line coding and light modulation;

FIG. 4 is an experimental demonstration of the SOA reverse injection gain clamp effect;

fig. 5(a) is a schematic diagram illustrating the effect of auxiliary light reverse injection on reducing the gain saturation characteristics of the SOA;

FIG. 5(b) is a schematic diagram illustrating the effect of assisting backward light injection to linearize the static gain;

FIG. 6 is a graph of optical phase error factor versus nominal detuning;

FIG. 7 is a schematic structural diagram of the optical phase-locked loop module;

FIG. 8 is a circuit diagram of a proportional-integral control PIC low-pass filter;

FIG. 9 is a graph of the transfer function of a proportional-integral control PIC;

fig. 10 is a schematic structural diagram of a 2x8 polarization diversity coherent detection module;

fig. 11 is a schematic structural diagram of a 2x4 non-polarization diversity optical coherent detection module;

illustration of the drawings:

wherein 101-a first laser, 102-a second laser, 103-a first driver amplifier, 104-a first lithium niobate MZM optical modulator, 105-a first semiconductor optical amplifier SOA, 106-a first optical circulator, 107-an optical splitter, 108-a first wavelength division multiplexer WDM, 109-a second wavelength division multiplexer WDM, 110-a first external feedback polarization tracker PT, 111-a first polarization maintaining splitter, 112-an optical bandpass filter OBPF, 113-a second polarization maintaining optical splitter, 114-a polarization maintaining optical circulator, 115-a local laser ILL, 116-a third polarization maintaining optical splitter, 117-an optical phase shifter, 118-an optical detector, 119-a PIC loop filter, 120-a polarization maintaining coupler, 121-a balanced optical detector pair, 122-a low noise transimpedance amplifier, 123-band pass filter BPF, 124-main amplifier, 125-digital signal processor, 126-first detection and logarithm amplifier, 201-third laser, 202-second driver amplifier, 203-second lithium niobate MZM optical modulator, 204-second semiconductor optical amplifier SOA, 205-second optical circulator, 206-second external feedback polarization tracker PT, 207-90 ° optical hybrid, 208-first balanced optical detection pair, 209-second balanced optical detection pair, 210-first low noise preamplifier TIA, 211-second low noise preamplifier, 212-first band pass filter, 213-second band pass filter, 214-digital signal processing device, 215-second detection and logarithm amplifier.

Detailed Description

The embodiments will now be described in detail, including the necessary mathematical demonstration of the principles of operation, examples of which are illustrated in the accompanying drawings. When the following description refers to the accompanying drawings, like numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following examples do not represent all embodiments consistent with the present application. But merely as examples of systems and methods consistent with certain aspects of the application, as detailed in the claims.

The invention adopts five key technologies: 1) the gain clamp semiconductor optical amplifier is used for realizing the transmission optical amplification of an O wave band; 2) the traditional optical wave polarization diversity reception is replaced by the optical wave polarization tracking reception, the structure of a coherent receiver is simplified, and the receiving sensitivity is improved; 3) the wavelength of a local oscillator laser of a downlink optical receiver is coincided with the wavelength of a far-end optical transmitter by using an optical injection locking technology, so that Homodyne Detection (Homodyne Detection) of optical signals is realized, and digital signal processing required by the traditional inner difference Detection is simplified; 4) minimizing the phase noise of the homodyne detection light by using an optical phase-locked loop; 5) the nonlinear cancellation of frequency offset and optical phase noise in the inner difference detection output is realized in the uplink optical receiver by using the square sum operation.

Referring to fig. 1, a schematic diagram of a coherent optical communication system for a high-speed PON is shown.

The application provides a coherent optical communication system for a high-speed PON, the coherent optical communication system includes:

According to the optical transmission method and device, the gain clamped semiconductor optical amplifier SOA is adopted at the optical transmission end, and the optical transmission power of an O waveband is improved. The first optical receiver of the ONU in the downlink direction jointly uses the external feedback polarization tracking, the optical injection locking, the local laser and the optical phase-locked loop to ensure that the frequency of the local LO optical wave is completely the same as that of the signal optical wave, and the optical phase noise is minimized, thereby realizing the homodyne detection. The second optical receiver of the uplink OLT jointly uses the 2x4 optical mixer and the square sum arithmetic unit output by the receiver, and the complex frequency estimation and phase estimation algorithm in the traditional digital signal processor is abandoned, so that the interpolation detection with the frequency offset and optical phase noise nonlinear cancellation function is realized. Especially, the external feedback polarization tracking technology is adopted at the optical receiving end to replace the polarization diversity receiving technology, and the traditional pi/22 x8 optical mixer/four balanced optical detector pair is abandoned, so that the hardware of the coherent receiver is simplified, and the sensitivity of the optical receiver is improved. The method and the device greatly reduce the software and hardware cost and the power consumption of the coherent optical communication system, improve the real-time performance of operation and ensure the feasibility of the application of the coherent optical communication system in a high-speed optical access network.

The first optical transmitter comprises a first laser 101, a first lithium niobate MZM optical modulator 104, and a first drive amplifier 103;

the first drive amplifier 103 is connected to an electrical signal input end of a first lithium niobate MZM optical modulator 104, and is configured to perform signal amplification on a line-coded data electrical signal through the first drive amplifier 103, and then modulate the intensity of an input optical carrier in the first lithium niobate MZM optical modulator 104 to form a signal optical wave; the data electrical signal is a high-speed data signal with a speed of 25 or 50Gb/s, the original waveform of a binary NRZ code is shown in FIG. 2, and the binary NRZ code is converted into a binary 64B66B code or converted into a quaternary PAM-4 code after being subjected to power spectrum conversion by a line encoder. The spectrum of the signal light wave formed by the 64B66B code double-sideband intensity modulation is shown in fig. 3, where fc is the emission frequency of the first laser 101, the dc component after the power spectrum conversion is zero, the low-frequency component is reduced, the spectrum of the formed signal light wave contains a pure light carrier spectral line, and the light injection locking depends on the spectral line of the main control light;

the output end of the first lithium niobate MZM optical modulator 104 is connected to the first semiconductor optical amplifier SOA105, and is configured to amplify the average intensity of the modulated signal light waves to a level required for sending to an optical fiber line through the first semiconductor optical amplifier SOA 105;

The first SOA gain clamping device comprises a second laser 102, an optical splitter 107 and the first optical circulator 106;

the second laser 102 is connected to the input end of the optical splitter 107, and is used for emitting an auxiliary light wave;

the first output port of the optical splitter 107 is connected to the head port p1 of the first optical circulator 106, and is used for injecting the auxiliary optical wave back into the first semiconductor optical amplifier SOA105, so as to implement gain clamping and gain linearization. As shown in fig. 4, 5(a) and 5(b), the auxiliary light is injected reversely through the optical circulator, so that the gain of the SOA can be clamped at a low value, nearly linear optical amplification can be realized, and the optical pulse eye diagram can be opened. The nonlinearity of the SOA is overcome, and the four-level PAM-4 data format can be used at 50 Gb/s.

The injection locking is realized by connecting the master control light wave with the controlled laser through the polarization-maintaining circulator. The wavelength of the output light wave of the controlled laser after locking depends on the wavelength of the master control light wave, but the phase of the output light wave has a deviation (residual phase error) with the master control light wave, and the power of the output light wave is determined by the controlled laser, and can be much higher than that of the master control light wave. As shown in fig. 1, in the ONU, the optical receiver uses the received optical wave to realize injection locking of the local laser wavelength.

The light injection locking device comprises a first polarization maintaining light splitter 111, an optical band pass filter OBPF112, a second polarization maintaining light splitter 113, a polarization maintaining light circulator 114, a local laser ILL115 and a third polarization maintaining light splitter 116;

the two output ends of the first polarization splitter 111 are respectively connected to the homodyne detection device and the optical band-pass filter OBPF112, and are configured to split the received signal light wave to form a signal light branch and an injection light branch, where the signal light branch directly generates a signal light electric field to be applied to the homodyne detection device, and the injection light branch outputs a master control light wave;

a first output port of the second polarization-maintaining optical splitter 113 is connected to a head port p1 of the polarization-maintaining optical circulator 114, a middle port p2 of the polarization-maintaining optical circulator 114 is connected to the local laser ILL115, and a tail port p3 of the polarization-maintaining optical circulator 114 is connected to an input end of a third polarization-maintaining optical splitter 116, so as to lock an operating wavelength of the local laser ILL115 to be consistent with the main control carrier spectrum line.

The injected light branch outputs main control light, and first extracts an optical carrier line through the narrow-band optical filter OBPF112, and then is connected to the port 1 of the polarization-maintaining optical circulator 114 through the second polarization-maintaining optical splitter 113, and is injected into the active region of the local laser ILL 115. The local laser ILL115, originally tuned near fc, is locked by the master control optics, producing a laser quasi-located at fc, which is input to port 2 of the polarization maintaining optical circulator 114. After injection locking, the optical wave frequency of the local laser ILL115 is consistent with the signal optical wave frequency, which creates a condition for homodyne detection of a receiver, and the line width of the local laser ILL115 reaches 100kHz and is consistent with the line width of the OLT sending laser 1, so that only a wide-line-width laser is needed to serve as the local laser, which is beneficial to reducing the cost and power consumption of the PON system. The output power of the local laser ILL115 is determined by itself and may reach the order of +10 dBm. Therefore, the third polarization maintaining optical splitter (116) can output the second optical wave for optical gain clamping of the SOA at the transmitting end of the ONU. However, the phase of the local laser output optical wave is offset from the master optical wave, which forms optical phase noise in the coherent receiver output signal.

Injection locking of lasers has two basic relationships: injection ratio versus locking frequency (wavelength) range; the relationship between the remaining phase error and the locking frequency (wavelength) range is expressed as follows:

wherein, I ₁ Is the injected light intensity of the master control laser, I _o Is the transmitted light intensity, Δ f, of the slave laser _L Is the injection locking frequency range of the slave laser;

the residual phase shift of the transmitting light wave of the slave laser during injection locking is shown, and delta f is the initial frequency difference of the transmitting light waves of the master laser and the slave laser; alpha is alpha _L Is the linewidth enhancement factor of the slave laser. For semiconductor lasers, α _L Phase error factor of 5

Close to 2 deltaf/deltaf off-resonance from nominal _L In a linear relationship as shown in fig. 6. For a noisy master laser, there is an optimum injection ratio value, which can be found experimentally. The injection ratio adopted by the invention is about-47 dB, and when a common DFB laser is used as a controlled laser, the delta f _L Up to 600 MHz. If the original frequency deviation of the master and slave lasers is 150MHz, then the nominal detuning is 2 Δ f/Δ f _L 0.5, the optical phase error factor due to residual phase difference

Corresponding to the remainderPhase difference

In order to reduce the residual phase error of the controlled laser, an optical phase-locked loop module is further disposed in the optical injection locking device, and the optical phase-locked loop module includes a local laser ILL115, the third polarization-maintaining optical splitter 116, an optical phase shifter 117, an optical detector 118, and a PIC loop filter 119;

the output end of the optical detector 118 is electrically connected to the PIC loop filter 119, the PIC loop filter 119 is electrically connected to the electrical driving end of the local laser ILL115, and the PIC loop filter 119 is configured to feed back an optical phase noise current generated by the optical detector 118 to the local laser ILL115, so that a phase difference between the injected and locked local optical wave and the received signal optical wave is minimized.

The model of the optical phase-locked loop is shown in fig. 7, where the master light is coupled to the controlled LO light through a polarization-maintaining optical circulator, and a residual phase error occurs in the LO light relative to the master light. The optical detector generates a phase error signal, and the phase of the local laser is controlled through the feedback of the loop filter.

As shown in fig. 8, the Proportional Integral Controller (PIC) circuit of the PIC loop filter 119 is an inverting type active loop filter with a low frequency operational amplifier as a core, and its transfer function is shown in fig. 9.

The optical phase shifter 117 is a 90 ° voltage-adjustable electro-optical phase shifter. The cascade of the optical phase shifter 117 and the optical detector 118 is used as a phase error detector of the phase locked loop, and they must ensure that the output of the optical detector is a sine function of the phase error, which is a requirement for stable convergence of the optical phase locked loop. The signal is transmitted through the PIC loop filter 119 and then applied to the local laser ILL115, and the phase of the local laser approaches the phase of the injected laser by using the conversion from the current frequency modulation effect in the laser cavity to the phase increment, thereby minimizing the residual phase error.

The homodyne detection device comprises a polarization maintaining coupler 120, a balanced optical detector pair 121, a low noise transimpedance amplifier TIA122, a band-pass filter BPF123, a main amplifier 124 and a digital signal processor 125;

the signal terminal of the polarization maintaining coupler 120 is connected to one of the output terminals of the first polarization maintaining splitter 111, and the local oscillator terminal of the polarization maintaining coupler 120 is connected to the second output port of the third polarization maintaining splitter 116;

the core principle of the homodyne detection device is the multiplication of the signal light and the local oscillator light with the same wavelength, which is realized by the balanced optical detector pair 121 in the photoelectric conversion process after the signal light and the local oscillator light are mixed by the polarization-maintaining coupler 120, and the following proves that:

the transmission matrix of the polarization-maintaining 2x 23 decibel coupler is

Signal light wave electric field e _s (t) and LO optical wave electric field e _L (t) have the same polarization direction, and are applied to the two inputs of the polarization maintaining 2 × 23 db coupler, respectively, two combinations of two electric fields are obtained at its two outputs as follows:

applying the two electric field combinations to the PD of a balanced photodetector pair ₁ And PD ₂ A photocurrent i is generated ₁ (t)、i ₂ (t)

And the difference between the two:

wherein R is the responsivity of the optical detector, and K is the conversion coefficient from the electric field to the power in the optical fiber;

the signal optical electric field of the receiving end comes from the output of the optical modulator of the transmitting end, and has a form after considering the power loss coefficient L of the link

Wherein E _c For transmitting the output optical electric field amplitude, omega, of the laser _c For transmitting the angular frequency, theta, of the optical carrier of the laser _c Is the random phase of the transmit laser; b _k Is the power of the k-th symbol of the electrical signal pulse sequence, h _s (T) is the waveform of a single pulse, and T is the symbol period. V _π Is the half-wave voltage of the MZM intensity modulator;

through the injection locking process, the output electric field of the local oscillator laser at the receiving end is

Wherein E _L Is the amplitude of the local oscillator photoelectric field, theta _L Is a local oscillator light random phaseA bit;

substituting (6) and (7) into (5) to obtain the following relation:

the output voltage of the optical receiver is

Where the received signal light power P has been substituted _s ＝KLE _c ² /2 local oscillator optical power P _L ＝KE _L ² 2; the first term is low frequency interference caused by optical phase noise, and the second term is a data signal polluted by the optical phase noise;

by passing through the optical phase-locked loop, theta has been made _c (t)-θ _L (t) minimized, then sin [ theta ] _c (t)-θ _L (t)]≈0,cos[θ _c (t)-θ _L (t)]1, the final output voltage of the optical receiver is

It fully reproduces the originally transmitted data waveform;

the digital signal processor 125 includes an a/D conversion unit, an equalization unit, a clock recovery unit, and a decision regeneration unit, and is configured to perform a/D conversion, equalization, clock recovery, and decision regeneration processing on the data signal of equation (10), and restore the transmission data of the OLT.

The first external feedback polarization tracking module comprises a first external feedback polarization tracker PT110 and a first detection and logarithm amplifier 126;

the first detection and logarithm amplifier 126 is electrically connected to the signal output end of the low-noise transimpedance amplifier TIA122, and is configured to convert the signal output by the low-noise transimpedance amplifier TIA122 into a feedback control voltage of the first external feedback polarization tracker PT110, where a voltage level of a direct current output by the first detection and logarithm amplifier 126 represents a matching degree between a polarization direction of an output signal optical wave of the first external feedback polarization tracker PT and a slow axis direction of an input end of the first polarization splitter 111.

The second optical transmitter comprises a third laser 201, a second lithium niobate MZM optical modulator 203, and a second drive amplifier 202;

the second drive amplifier 202 is connected to an electrical signal input end of the second lithium niobate MZM optical modulator 203, and is configured to perform signal amplification on a line-coded data electrical signal through the second drive amplifier 202, and then modulate the intensity of an input optical carrier in the second lithium niobate MZM optical modulator 203 to form an uplink signal optical wave;

the output end of the second lithium niobate MZM optical modulator 203 is connected to the second semiconductor optical amplifier SOA204, and is used for amplifying the average intensity of the signal light wave to a level required for sending the signal light wave to an optical fiber line;

The second SOA gain clamping device includes the third polarization maintaining optical splitter 116 and the second optical circulator 205;

the third output port of the third polarization maintaining optical splitter 116 is connected to the head port p1 of the second optical circulator 205, and is configured to transmit the auxiliary optical wave split by the third polarization maintaining optical splitter 116 to the intermediate port p2 through the head port p1 of the second optical circulator 205 in a unidirectional manner, and inject the auxiliary optical wave into the second semiconductor optical amplifier SOA204 in a reverse direction from the intermediate port p2 of the second optical circulator 205, so as to implement gain clamping.

The second SOA gain clamping device overcomes the problems of reduction of extinction ratio of digital optical signals and crosstalk between wavelength channels caused by the existing SOA nonlinear optical gain used for a coherent optical communication system, and particularly, when the transmission data rate is as high as 50Gb/s, the gain of the SOA can be clamped at a low value by adopting auxiliary optical reverse injection, so that almost linear optical amplification is realized, an optical pulse eye diagram is opened, and crosstalk between the wavelength channels is eliminated, and the device is suitable for being used in a high-speed PON.

Because the optimal local oscillator optical power required by a coherent receiver in the OLT and the ONU is usually in the order of +5dBm, and the output optical power of the local oscillator laser can reach +10dBm, the residual optical power of the split light is used as the auxiliary light of the SOA, which is feasible.

As shown in fig. 10, the existing coherent optical receiver generally employs four pairs of balanced optical detector modules of polarization diversity, and such a receiving module is expensive, and the insertion loss of the 2x 890 ° hybrid therein is large, so that the responsivity of the synthesized optical detector is generally only 0.05 to 0.08A/W, and the improvement of the receiver sensitivity brought by coherent detection (by the multiplication of local oscillator light and signal light) is almost consumed by the insertion loss of the optical hybrid.

As shown in fig. 11, the present application does not adopt a polarization diversity four-pair balanced optical detector module, the internal difference detection apparatus includes a 90 ° optical hybrid 207, a first balanced optical detection pair 208, a second balanced optical detection pair 209, a first low-noise preamplifier TIA210, a second low-noise preamplifier TIA211, a first band-pass filter 212, a second band-pass filter 213, and a digital signal processing apparatus 214, the present application adopts an external feedback optical polarization tracking technique to solve the polarization matching problem between the local oscillation light and the signal light, and then adopts a polarization-preserving 90 ° optical hybrid 207 to cascade two balanced optical detector pairs;

the output end of the first low-noise preamplifier TIA210 is electrically connected with a first band-pass filter 212;

The traditional digital signal processing algorithm of the coherent optical receiver relies on frequency offset estimation and phase noise estimation to overcome frequency offset and phase noise, and complex processing steps are needed. Considering that information is only carried by the intensity of signal light in the intensity modulation optical fiber transmission system and is reflected in the amplitude of the output voltage of the receiver, the frequency offset and the phase noise factor in the output voltage of the receiver can be completely eliminated by a simple square sum operation, and the signal is not damaged. The idea of this innovation proves as follows:

according to the transmission matrix of the 2X 490 degree mixer in FIG. 11, the output electric fields of its four ports are

Wherein e _s (t) is the input signal optical electric field, e _L And (t) inputting a local oscillator optical electric field.

The current of the photodetector is i ₁ (t)、i ₃ (t)、i ₂ (t)、i ₄ (t) is given by the following equation:

the sending end of the ONU is used for carrying out light intensity modulation on the lithium niobate MZM by using baseband data. The output modulated optical wave electric field is also shown as formula (6).

The expression of the local oscillator optical electric field of the OLT is as follows:

wherein E _L The amplitude of the electric field of the local oscillator light wave and the angular frequency of the local oscillator light wave are omega _c + Δ ω, Δ ω ═ 2 pi Δ f, Δ f is the frequency difference between the free-running lasers in the ONU and OLT, θ _L And (t) is the random phase of the local oscillator light wave.

Substitution of (6) and (13) into (12) to obtain

The output voltages of the first path and the second path of the optical receiver are respectively

For frequency offset delta omega and phase noise factor theta in output voltage of receiver _L (t)-θ _c (t) can be completely eliminated by a simple sum of squares operation without signal impairment:

after the operation result is subjected to blocking and frequency multiplication component filtering, the square root is calculated, and the final synthesized output of the first path and the second path of the optical receiver is obtained

It restores the transmission signal of the ONU. The above operation of square sum and root finding is performed in the digital signal processing means 214 after sampling and digitizing the output voltage of the optical receiver, which replaces the frequency offset and phase noise estimation in the conventional algorithm.

The second external feedback polarization tracking module comprises a second external feedback polarization tracker PT206 and a second detection and logarithm amplifier 215;

the second detection and logarithm amplifier 215 is electrically connected to the signal output end of the second low-noise preamplifier TIA211, and the signal voltage output by the second low-noise preamplifier TIA211 is acquired by the second detection and logarithm amplifier 215, detected and amplified to form a direct-current control voltage, and fed back to the second external feedback polarization tracker PT206, so that the polarization state control of the signal light wave is realized.

The method adopts an external feedback polarization tracking technology to solve the problem of polarization matching of signal light and local oscillator light, amplifies the variation trend of the output voltage of a receiver by a detection and logarithm amplifier (126 in ONU and 215 in OLT), converts the variation trend into the driving voltages of three or four piezoelectric polarization controllers, and adds the driving voltages to a piezoelectric adjustment polarization tracker (110 in ONU and 206 in OLT), so that the polarization state of the signal light wave is adaptively adjusted to achieve the optimal state of matching with the local oscillator light wave. The subsequent coherent optical receiver does not need a traditional polarization diversity coherent detection module with four balanced optical detector pairs after the polarization state matching problem is solved, hardware is simplified, and a communication method is simplified. And the insertion loss of the 90-degree optical mixer is reduced, while the insertion loss of the piezoelectric adjusting external feedback polarization tracker is only 0.5dB, which is beneficial to the application of the optical coherent detection technology in a high-speed optical access network communication system. The transition response time of the piezoelectric adjusting external feedback polarization tracker is in millisecond level, the time constant of the polarization state of the optical wave in the line optical fiber changing along with the environmental temperature and the mechanical stress is also in millisecond level, and the optical fiber online polarization controller of the external feedback tracking has the capability of maintaining the polarization state matching of the signal optical wave and the local oscillation optical wave, so that the optical fiber online polarization controller is suitable for high-speed data transmission.

The communication method for the coherent optical communication system of the high-speed PON comprises the following steps:

the first lithium niobate MZM optical modulator 104 modulates the intensity of the downlink optical carrier according to the data electrical signal to form a downlink signal optical wave;

in a first optical receiver of an optical network unit ONU, the polarization direction of a received signal light wave is stabilized in the direction of a slow axis of an output polarization-maintaining optical fiber through a first external feedback polarization tracking module;

taking the local optical wave after injection locking as downlink local oscillator light, taking the received signal optical wave as downlink signal light, and performing homodyne detection on the downlink signal light and the downlink local oscillator light to generate a downlink baseband data waveform;

the second driving amplifier 202 amplifies the data electrical signal coded by the line and adds the amplified data electrical signal to an electrode of a second lithium niobate MZM optical modulator 203;

the second lithium niobate MZM optical modulator 203 modulates the intensity of the uplink optical carrier according to the data electrical signal to form an uplink signal optical wave;

performing inner difference detection on the uplink signal light wave and the uplink local oscillation light wave to form an uplink baseband data waveform;

As shown in fig. 1, in this embodiment, a communication method of a coherent optical communication system for a high-speed PON according to the present application specifically includes the following steps:

in a downlink first optical transmitter of an optical line terminal OLT, a first laser (101) is adopted to transmit an optical carrier, a data electrical signal is subjected to power spectrum conversion through line coding and then amplified by a first driving amplifier (103), and then the amplified data electrical signal is added to a first lithium niobate MZM optical modulator (104) to carry out intensity modulation on the optical carrier;

the first lithium niobate MZM optical modulator (104) outputs a modulated signal light wave which is amplified by a first semiconductor optical amplifier SOA (105); in order to clamp the optical gain of the first semiconductor optical amplifier SOA105 and realize the linearization of optical gain characteristics, a first optical circulator (106) is inserted between the output end of the SOA (105) and a first wavelength division multiplexer WDM (108), the p2 port of the first optical circulator (106) is connected with the output end of the SOA (105), an auxiliary optical wave sent by a second laser (102) is split by an optical splitter (107) and then is transmitted to the intermediate port p2 of the first optical circulator (106) in a unidirectional mode through the head port p1 of the first optical circulator (106), and the optical gain of the SOA (105) is clamped by injecting the first semiconductor optical amplifier SOA (105) in a reverse direction; the amplified signal optical wave is transmitted to a first optical receiver of an optical network unit ONU from an intermediate port p2 of a first optical circulator (106) through an end port p3, a first wavelength division multiplexer WDM (108), an optical line and a second wavelength division multiplexer WDM (109) in sequence;

in a first optical receiver of an optical network unit ONU, the polarization direction of a received signal light wave is firstly stabilized in the slow axis direction of an input optical fiber of a first polarization maintaining splitter (111) through the control of a first external feedback polarization tracker PT (110), and then is divided into two signal light waves through the first polarization maintaining splitter (111), wherein one signal light wave is directly applied to the signal end of a polarization maintaining coupler (120), and the other signal light wave is used for injection locking of a local laser. The signal is added to a head end port p1 of a polarization-maintaining optical circulator (114) through an optical band pass filter OBPF (112) and a second polarization-maintaining optical splitter (113). Injecting a signal lightwave unidirectionally transmitted from the head port p1 to the intermediate port p2 as a master light into the local laser ILL (115);

the continuous light wave emitted by the local laser ILL (115) is transmitted to the end port p3 in a single direction through the middle port p2 of the polarization-maintaining optical circulator (114), and is split through the third polarization-maintaining optical splitter (116): one beam is added to a local oscillation end of the polarization-maintaining coupler (120) and is used as local oscillation light; the other beam is phase shifted by an optical phase shifter (117) and multiplied by the master light from the second polarization maintaining optical splitter (113) as the optical phase reference in an optical detector (118) to generate baseband optical phase noise, and then fed back to the local laser ILL (115) through a PIC loop filter (119) to realize the closed loop control of the optical phase locked loop and minimize the residual phase error of the injection locking process;

the signal light and the local oscillator light are mixed through the polarization-maintaining coupler (120), then multiplied in a balanced light detector pair (121) to generate a baseband photocurrent, then form a voltage through a low-noise trans-impedance amplifier TIA (122), further filtered through a band-pass filter BPF (123), amplified by a main amplifier (124), output a baseband data waveform, transmitted to a digital signal processor (125), subjected to analog-to-digital conversion, equalization, clock recovery and decision regeneration, and restored with a data signal sent by a downlink first optical transmitter of the OLT;

the signal voltage formed in the low-noise transimpedance amplifier TIA (122) is further output to a first detection and logarithm amplifier (126), is detected and amplified into a direct-current control voltage and is fed back to the first external feedback polarization tracker PT (110);

in an uplink second optical transmitter of an optical network unit, ONU, an optical carrier is transmitted using a third laser (201). The data electric signal after line coding is amplified through a second driving amplifier (202), and then is added to a second lithium niobate MZM optical modulator (203) to carry out intensity modulation on an optical carrier; the modulated signal light wave output by the second lithium niobate MZM optical modulator (203) is amplified by a second semiconductor optical amplifier SOA (204), and then is transmitted to a second optical receiver of the optical line terminal OLT through a middle port p2, an end port p3, a second wavelength division multiplexer WDM (109), an optical fiber line and a first wavelength division multiplexer WDM (108) of a second optical circulator (205) in sequence;

the third light wave output after being split by the third polarization maintaining optical splitter (116) is transmitted to a head port p1 of the second optical circulator (205), then is transmitted to an intermediate port p2 through a head port p1 in a single direction, and then is injected into the second semiconductor optical amplifier SOA (204) from the intermediate port p2 in a reverse direction, so that gain clamping is realized;

in a second optical receiver of the optical line terminal OLT, an uplink signal optical wave reaching the OLT firstly passes through a second external feedback polarization tracker PT (206), and the polarization direction of the signal optical wave is controlled to be matched with a local oscillation optical wave from the optical splitter (107);

the signal light and the local oscillator light are mixed in a 90-degree light mixer (207), then photocurrents are generated in a first balanced light detection pair (208) and a second balanced light detection pair (209), and then converted into sine signal voltage and cosine signal voltage through a first low-noise preamplifier TIA (210) and a second low-noise preamplifier TIA (211) respectively, and then sine output voltage and cosine output voltage are generated through a first band-pass filter (212) and a second band-pass filter (213); the two output voltages are both transmitted to a digital signal processing device (214). Performing analog-to-digital conversion on the two paths of output voltages respectively in the digital signal processing device (214), then performing square sum and root operation for eliminating frequency offset and optical phase noise, and then performing equalization, clock recovery and decision regeneration processing to restore a data signal transmitted in an uplink second optical transmitter of the ONU;

and the output signal of the second low-noise preamplifier TIA (211) is coupled to a second detection and logarithm amplifier (215), a direct current control voltage is formed by detection and amplification and is added to a second external feedback polarization tracker PT (206), so that the polarization state of the received signal light wave is adaptively adjusted, and the optimal state matched with the local oscillation light wave is achieved.

The detailed description provided above is only a few examples under the general concept of the present application, and does not constitute a limitation to the scope of the present application. Any other embodiments extended according to the scheme of the present application without inventive efforts will be within the scope of protection of the present application for a person skilled in the art.

Claims

1. A coherent optical communication system for a high-speed PON, the coherent optical communication system comprising:

a first optical transmitter, a first wavelength division multiplexer, WDM, (108), a second wavelength division multiplexer, WDM, (109), a first optical receiver, a second optical transmitter and a second optical receiver;

the output end of the first optical transmitter is connected to a first semiconductor optical amplifier SOA (105) for emitting amplified signal light waves in the downlink of an optical line termination OLT;

the output end of the first semiconductor optical amplifier SOA (105) is respectively connected with a first SOA gain clamping device and a first wavelength division multiplexer WDM (108);

the first wavelength division multiplexer WDM (108) is connected with the second wavelength division multiplexer WDM (109) through a feeder optical fiber, and the first wavelength division multiplexer WDM (108) is connected with the second optical receiver;

said second wavelength division multiplexer WDM (109) connected to said first optical receiver;

the first optical receiver comprises a first external feedback polarization tracking module, an optical injection locking device and a homodyne detection device which are sequentially connected;

the optical injection locking device is used for splitting the received signal light wave to form a signal light branch and an injection light branch, outputting a main control light wave, extracting a pure main control carrier spectral line from the main control light wave, locking the wavelength to be consistent with the main control carrier spectral line, generating baseband optical phase noise, and minimizing the phase difference between the injected and locked local light wave and the received signal light wave;

the homodyne detection device is used for carrying out A/D conversion, equalization, clock recovery and judgment regeneration processing on the data signal;

the first external feedback polarization tracking module is used for converting an output signal into a feedback control voltage;

the first optical receiver receives the signal light wave from the first optical transmitter through the first external feedback polarization tracking module, the optical injection locking device and the homodyne detection device which are connected in sequence, and homodyne detection is completed in an Optical Network Unit (ONU);

the first external feedback polarization tracking module is connected with the second wavelength division multiplexer WDM (109) and is used for adjusting the polarization direction of the received signal light wave to enable the polarization direction to be stabilized in the slow axis direction of the output polarization-maintaining optical fiber;

the output end of the second optical transmitter is connected with a second semiconductor optical amplifier SOA (204) and is used for emitting amplified signal light waves in an uplink of the optical network unit ONU;

the output end of the second semiconductor optical amplifier SOA (204) is respectively connected with a second SOA gain clamping device and the second wavelength division multiplexer WDM (109);

the second optical receiver comprises a second external feedback polarization tracking module and an internal difference detection device which are connected in sequence;

the inner difference detection device is used for carrying out A/D conversion, square sum and root operation for eliminating frequency deviation and optical phase noise, equalization, clock recovery and judgment regeneration processing on data signals;

the second external feedback polarization tracking module is used for converting the output signal into a feedback control voltage;

the second optical receiver receives the signal optical wave from the second optical transmitter through the second external feedback polarization tracking module and the internal difference detection device which are connected in sequence, and internal difference detection is completed at an optical line terminal OLT;

the input end of the second external feedback polarization tracking module is connected with the first wavelength division multiplexer WDM (108) and used for controlling the polarization direction of the received uplink signal light wave to be consistent with the uplink local oscillator light wave.

2. The coherent optical communication system for a high-speed PON according to claim 1,

the first optical transmitter comprises a first laser (101), a first lithium niobate MZM optical modulator (104), and a first drive amplifier (103);

the first laser (101) is connected with an optical signal input end of a first lithium niobate MZM optical modulator (104), and is used for emitting continuous optical carriers with intensity, and the continuous optical carriers are converted into intensity modulated signal optical waves through the first lithium niobate MZM optical modulator (104);

the first drive amplifier (103) is connected with an electric signal input end of a first lithium niobate MZM optical modulator (104) and is used for amplifying a data electric signal subjected to line coding through the first drive amplifier (103), and then modulating the intensity of an input optical carrier wave in the first lithium niobate MZM optical modulator (104) to form a signal optical wave;

the output end of the first lithium niobate MZM optical modulator (104) is connected with the first semiconductor optical amplifier SOA (105) and is used for amplifying the average intensity of the signal light wave to a required level for transmitting to an optical fiber line through the first semiconductor optical amplifier SOA (105);

the output end of the first semiconductor optical amplifier SOA (105) is connected with an intermediate port p2 of a first optical circulator (106), and an end port p3 of the first optical circulator (106) is connected with the first wavelength division multiplexer WDM (108).

3. The coherent optical communication system for a high-speed PON according to claim 2,

the first SOA gain clamping device comprises a second laser (102) and an optical splitter (107) and the first optical circulator (106);

the second laser (102) is connected with the input end of the optical splitter (107) and is used for emitting auxiliary light waves;

the first output port of the optical splitter (107) is connected with the head port p1 of the first optical circulator (106) and is used for injecting the auxiliary optical wave into the first semiconductor optical amplifier SOA (105) in the reverse direction, so that gain clamping and gain linearization are realized.

4. A coherent optical communication system for high speed PONs in accordance with claim 1, 2 or 3,

the light injection locking device comprises a first polarization-maintaining optical splitter (111), an Optical Band Pass Filter (OBPF) (112), a second polarization-maintaining optical splitter (113), a polarization-maintaining optical circulator (114), a local laser ILL (115) and a third polarization-maintaining optical splitter (116);

the input end of the first polarization-preserving light splitter (111) is connected with the first external feedback polarization tracking module;

two output ends of the first polarization splitter (111) are respectively connected with the homodyne detection device and an optical band-pass filter (OBPF) (112) and are used for splitting received signal light waves to form a signal light branch and an injection light branch, the signal light branch directly generates a signal light electric field to be added to the homodyne detection device, and the injection light branch outputs a master control light wave;

the output end of the optical band-pass filter OBPF (112) is connected with the second polarization maintaining optical splitter (113) and is used for extracting a pure main control carrier spectral line from the main control optical wave;

the first output port of the second polarization-maintaining optical splitter (113) is connected with the head port p1 of the polarization-maintaining optical circulator (114), the middle port p2 of the polarization-maintaining optical circulator (114) is connected with the local laser ILL (115), and the tail port p3 of the polarization-maintaining optical circulator (114) is connected with the input end of the third polarization-maintaining optical splitter (116), so that the operating wavelength of the local laser ILL (115) is locked to be consistent with the main control carrier spectral line.

5. The coherent optical communication system for a high-speed PON according to claim 4,

an optical phase-locked loop module is also arranged in the optical injection locking device, and comprises a local laser ILL (115), the third polarization-maintaining optical splitter (116), an optical phase shifter (117), an optical detector (118) and a PIC loop filter (119);

a first output port of the third polarization-maintaining optical splitter (116) is connected with an input end of the optical phase shifter (117);

the output end of the optical phase shifter (117) is connected with a first input port of the optical detector (118), a second output port of the second polarization-maintaining optical splitter (113) is connected with a second input port of the optical detector (118), and the optical detector (118) is used for multiplying two light waves from the optical phase shifter (117) and the second polarization-maintaining optical splitter (113) respectively to generate baseband optical phase noise;

the output end of the optical detector (118) is electrically connected with the PIC loop filter (119), the PIC loop filter (119) is electrically connected with the electric driving end of the local laser ILL (115), and the PIC loop filter (119) is used for feeding back optical phase noise current generated by the optical detector (118) to the local laser ILL (115), so that the phase difference between the injection-locked local optical wave and the received signal optical wave is minimized.

6. The coherent optical communication system for a high-speed PON according to claim 5,

the homodyne detection device comprises a polarization maintaining coupler (120), a balanced optical detector pair (121), a low noise transimpedance amplifier (TIA) (122), a band-pass filter (BPF) (123), a main amplifier (124) and a digital signal processor (125);

the signal end of the polarization-maintaining coupler (120) is connected with one output end of the first polarization-maintaining optical splitter (111), and the local oscillator end of the polarization-maintaining coupler (120) is connected with the second output port of the third polarization-maintaining optical splitter (116);

two output ends of the polarization maintaining coupler (120) are respectively connected with the balanced light detector pair (121);

the output end of the balanced optical detector pair (121) is electrically connected with the TIA (122);

the low-noise transimpedance amplifier TIA (122), the band-pass filter BPF (123), the main amplifier (124) and the digital signal processor (125) are electrically connected in sequence;

the digital signal processor (125) comprises an A/D conversion unit, an equalization unit, a clock recovery unit and a decision regeneration unit, and is used for carrying out A/D conversion, equalization, clock recovery and decision regeneration processing on the data signal.

7. The coherent optical communication system for a high-speed PON of claim 6,

the first external feedback polarization tracking module comprises a first external feedback polarization tracker PT (110) and a first detection and logarithm amplifier (126);

said first external feedback polarization tracker PT (110) is fiber connected between said second wavelength division multiplexer WDM (109) and said first polarization maintaining splitter (111) input;

the first external feedback polarization tracker PT (110) is electrically connected with the first detection and logarithm amplifier (126);

the first detection and logarithm amplifier (126) is electrically connected with a signal output end of the low-noise transimpedance amplifier TIA (122) and used for converting a signal output by the low-noise transimpedance amplifier TIA (122) into a feedback control voltage of the first external feedback polarization tracker PT (110).

8. A coherent optical communication system for high-speed PONs according to claim 1 or 6,

the second optical transmitter comprises a third laser (201), a second lithium niobate MZM optical modulator (203), and a second drive amplifier (202);

the third laser (201) is connected to an optical signal input end of the second lithium niobate MZM optical modulator (203), and is configured to emit an intensity continuous optical carrier in an uplink of an optical network unit ONU, and convert the intensity continuous optical carrier into an intensity modulated signal optical wave through the second lithium niobate MZM optical modulator (104);

the second drive amplifier (202) is connected with an electrical signal input end of the second lithium niobate MZM optical modulator (203), and is used for amplifying the data electrical signal subjected to line coding through the second drive amplifier (202), and then modulating the intensity of an input optical carrier in the second lithium niobate MZM optical modulator (203) to form a signal optical wave;

the output end of the second lithium niobate MZM optical modulator (203) is connected with the second semiconductor optical amplifier SOA (204) and is used for amplifying the average intensity of the signal light wave to a level required for sending the signal light wave to an optical fiber line;

the output end of the second semiconductor optical amplifier SOA (204) is connected with an intermediate port p2 of a second optical circulator (205), and an end port p3 of the second optical circulator (205) is connected with the second wavelength division multiplexer WDM (109).

9. The coherent optical communication system for a high-speed PON according to claim 8,

the second SOA gain clamping device comprises a third polarization maintaining optical splitter (116) and the second optical circulator (205);

and the third output port of the third polarization maintaining optical splitter (116) is connected with the head port p1 of the second optical circulator (205) and is used for unidirectionally transmitting the auxiliary optical wave split by the third polarization maintaining optical splitter (116) to the intermediate port p2 through the head port p1 of the second optical circulator (205) and reversely injecting the auxiliary optical wave into the second semiconductor optical amplifier SOA (204) from the intermediate port p2, so that gain clamping and gain linearization of the SOA are realized.

10. A coherent optical communication system for a high speed PON according to claim 3,

the internal difference detection device comprises a 90-degree optical mixer (207), a first balanced optical detection pair (208), a second balanced optical detection pair (209), a first low-noise preamplifier TIA (210), a second low-noise preamplifier TIA (211), a first band-pass filter (212), a second band-pass filter (213) and a digital signal processing device (214);

the signal end of the 90-degree optical mixer (207) is connected with the second external feedback polarization tracking module, and the local oscillation end of the 90-degree optical mixer (207) is connected with the second output port of the optical splitter (107);

the output end of the 90-degree optical mixer (207) is respectively connected with the first balanced optical detection pair (208) and the second balanced optical detection pair (209);

the output end of the first balanced optical detection pair (208) is connected with the first low-noise preamplifier TIA (210);

the output end of the second balanced optical detection pair (209) is connected with the second low-noise preamplifier TIA (211);

the output end of the first low-noise preamplifier TIA (210) is electrically connected with a first band-pass filter (212);

the output end of the TIA (211) is electrically connected with a second band-pass filter (213);

the output ends of the first band-pass filter (212) and the second band-pass filter (213) are connected with a digital signal processing device (214);

the digital signal processing device (214) comprises an A/D conversion unit, a square sum and root operation unit, an equalization unit, a clock recovery unit and a judgment regeneration unit, and is used for carrying out A/D conversion, square sum and root operation for eliminating frequency offset and optical phase noise, equalization, clock recovery and judgment regeneration on data signals.

11. The coherent optical communication system for a high-speed PON according to claim 10,

the second external feedback polarization tracking module comprises a second external feedback polarization tracker PT (206) and a second detection and logarithm amplifier (215);

the second external feedback polarization tracker PT (206) is fiber-connected between the signal end of the 90 ° optical hybrid (207) and the first wavelength division multiplexer WDM (108);

the second external feedback polarization tracker PT (206) is electrically connected with the second detection and logarithmic amplifier (215);

the second detection and logarithm amplifier (215) is electrically connected with the signal output end of the second low-noise preamplifier TIA (211) and is used for converting the signal output by the low-noise transimpedance amplifier TIA (211) into the feedback control voltage of the second external feedback polarization tracker PT (206).

12. A coherent optical communication method for a high-speed PON, applied to the coherent optical communication system for a high-speed PON according to claim 1, 2, 3, 5, 6, 7, 9, 10 or 11, comprising the steps of:

in a first optical transmitter of a downlink of an Optical Line Terminal (OLT), a first laser (101) is adopted to emit a downlink optical carrier;

the first drive amplifier (103) amplifies the line-coded data electric signal and then adds the amplified signal to the electrode of the first lithium niobate MZM optical modulator (104);

the first lithium niobate MZM optical modulator (104) modulates the intensity of the downlink optical carrier wave by the data electrical signal to form a downlink signal optical wave;

the downlink signal light wave is amplified by a first semiconductor optical amplifier SOA (105) and then is sent to an optical line;

the optical gain characteristic of the first semiconductor optical amplifier SOA (105) is clamped by reversely injecting an auxiliary light wave into the first semiconductor optical amplifier SOA (105), so that the linearization of the optical gain characteristic is realized;

performing A/D conversion, equalization, clock recovery and decision regeneration on the downlink baseband data waveform in a digital signal processor (125), and restoring the data signal transmitted by the first optical transmitter;

in an uplink second optical transmitter of an optical network unit, ONU, emitting an uplink optical carrier using a third laser (201);

the second drive amplifier (202) amplifies the data electric signal after line coding and then adds the amplified data electric signal to an electrode of a second lithium niobate MZM optical modulator (203);

the second lithium niobate MZM optical modulator (203) modulates the intensity of the uplink optical carrier wave by the data electrical signal to form an uplink signal optical wave;

the uplink signal light wave is amplified by a second semiconductor optical amplifier SOA (204) and then is sent to an optical line;

the auxiliary light wave is injected into the second semiconductor optical amplifier SOA (204) in a reverse direction, so that the optical gain characteristic of the second semiconductor optical amplifier SOA (204) is clamped, and the linearization of the optical gain characteristic is realized;

in a second optical receiver of the optical line terminal OLT, firstly, a polarization direction of a received uplink signal optical wave is controlled to be consistent with an uplink local oscillator optical wave through a second external feedback polarization tracking module;

and transmitting the uplink baseband data waveform to a digital signal processing device (214) for A/D conversion, eliminating the square sum of frequency offset and optical phase noise and root operation, equalization, clock recovery and decision regeneration, and restoring the data signal transmitted in the second optical transmitter of the ONU uplink.