CN108683453B - Linear direct detection method and system based on synthetic single sideband signal - Google Patents

Linear direct detection method and system based on synthetic single sideband signal Download PDF

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CN108683453B
CN108683453B CN201810346891.4A CN201810346891A CN108683453B CN 108683453 B CN108683453 B CN 108683453B CN 201810346891 A CN201810346891 A CN 201810346891A CN 108683453 B CN108683453 B CN 108683453B
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CN108683453A (en
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崔晟
周可籍
刘德明
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Huazhong University of Science and Technology
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
    • H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
    • H04B10/079—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using measurements of the data signal
    • H04B10/0795—Performance monitoring; Measurement of transmission parameters
    • H04B10/07953—Monitoring or measuring OSNR, BER or Q
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
    • H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
    • H04B10/079—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using measurements of the data signal
    • H04B10/0795—Performance monitoring; Measurement of transmission parameters
    • H04B10/07955—Monitoring or measuring power
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/616—Details of the electronic signal processing in coherent optical receivers
    • H04B10/6165—Estimation of the phase of the received optical signal, phase error estimation or phase error correction

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Abstract

The invention discloses an optical signal linear direct detection method and system based on a synthetic single sideband signal. The system comprises a local oscillator laser, a coupler, an optical detector, a radio frequency amplifier, an electrical filter, an analog-to-digital converter (ADC) and a Digital Signal Processor (DSP). The frequency of the local oscillator laser is arranged at one side edge of an incident light signal frequency spectrum, and the local oscillator laser and the incident light signal are combined through the coupler to form a single-sideband signal input light detector. Analog signals output by the light detector are amplified and filtered and then converted into digital signals by the ADC. The DSP takes the input digital signal as a real part and the Hilbert transform thereof as an imaginary part to be added to obtain a complex signal representing the synthesized single-sideband optical signal optical field, and then recovers the incident optical signal optical field through a frequency offset compensation algorithm. The invention can realize the digital coherent detection of the incident light signal light field by using a single optical detector and a single ADC, and has the advantages of simple structure, low cost, low power consumption and the like.

Description

Linear direct detection method and system based on synthetic single sideband signal
Technical Field
The invention belongs to the fields of optical fiber communication, free space optical communication, optical signal detection and digital signal processing, and particularly relates to a linear direct detection method and system based on a synthetic single-sideband signal.
Background
Currently, optical communication systems mainly employ intensity modulation and direct detection techniques. The optical receiver based on the direct detection technology adopts a single photodiode to convert an optical signal into an electrical signal, has a simple structure and low cost, but the output current of the photodiode is in direct proportion to the square of the mode field (namely, optical power) of an input optical signal, so that only the intensity information of signal light can be detected. Since intensity modulation does not fully utilize all modulatable dimensions of the optical field, transmission efficiency is low, and it is difficult to meet the increasing demand for high-speed large-capacity communication systems. Coherent detection can detect the information of each dimension of intensity, frequency, phase and polarization of signal light, and has higher transmission efficiency and sensitivity. The optical field in the coherent detection can be linearized and restored into an electric signal, so the method is a linearized detection technology. The digital coherent detection can restore a signal light field in a digital domain, and further can realize carrier synchronization through an algorithm without a complex optical phase-locked loop and a high-performance laser, so that a coherent light receiver based on a digital coherent detection technology is mostly adopted in the conventional high-speed large-capacity long-distance optical communication system. However, at present, such a coherent optical receiver generally includes 2 optical mixers, 4 optical balance detectors (each optical balance detector includes 2 photodiodes, which require 8 photodiodes in total) and 4 Analog-to-Digital converters (ADCs), and is complex in structure, high in cost and power consumption, and not suitable for data centers, metropolitan area networks, optical access networks and other occasions which are sensitive to cost, and satellites, space stations and other occasions which have strict requirements on the volume power consumption of communication terminals, so that the coverage range of a high-speed large-capacity optical network is limited. Recent foreign researchers have proposed some new types of so-called "digital linearization" techniques. The technology adopts a single photodiode to detect optical signals, eliminates nonlinear crosstalk introduced by a photodiode square rate response function through Digital Signal Processing (DSP) operation, and realizes linear Digital coherent detection of a Signal optical field. The scheme can greatly reduce the system cost and power consumption, and is a green low-cost digital coherent detection method.
The currently proposed "digital linearization" techniques mainly include five types, namely single-stage and two-stage linearization filtering techniques, iterative linearization filtering techniques, signal-to-signal beat interference cancellation techniques, and minimum phase signal-based linearization techniques. The first four techniques calculate the non-linear terms due to the individual photodiode squaring rate response functions as a distortion and subtract from the output signal. The linearization technology based on the minimum phase signal adopts direct current light and signal light to synthesize an optical minimum phase signal, then a single photodiode is used for detecting the intensity of the optical minimum phase signal, and finally a signal light field is reconstructed in a DSP based on the Hilbert transform relation existing between the amplitude and the phase of the minimum phase signal, so that the frequency spectrum efficiency and the output signal quality are higher than those of the former four technologies. However, the existing linearization technology based on minimum phase signals has some disadvantages, for example, the DSP has a spectrum broadening effect in the logarithm operation when reconstructing the light field information, which causes that the ADC is not enough to recover the full field information under the conventional 2 times sampling rate (at least 3 times sampling rate is needed), and introducing an iterative algorithm to reduce the sampling rate to the conventional 2 times brings extra huge calculation amount, which puts a very high requirement on the rate of DSP data processing.
Disclosure of Invention
Aiming at the defects or the improvement requirements of the prior art, the invention provides a linear direct detection method and a system based on a synthetic single-sideband signal, thereby solving the technical problems of high requirements on ADC sampling multiplying power and extremely high requirements on data processing rate of DSP in the current linearization technology based on minimum phase signals.
To achieve the above object, according to one aspect of the present invention, there is provided a linear direct detection system based on a synthesized single sideband signal, comprising: the system comprises a local oscillator laser, a coupler, an optical detector, a radio frequency amplifier, an electric filter, an analog-to-digital converter and a digital signal processor which are connected in sequence;
the coupler is provided with 2 input ports and 2 output ports, the optical detector comprises a common optical detector PD with 1 input port and 1 output port, or an optical balance detector BPD with 2 input ports and 1 output port, the coupler arbitrarily selects 1 port from the 2 output ports to be connected with 1 input port of the common optical detector PD, or the coupler connects 2 output ports with 2 input ports of the optical balance detector BPD;
the local oscillator laser is used for providing a local oscillator optical signal which carries out beat frequency with an incident optical signal, wherein the frequency of the local oscillator optical signal is located in a preset range on the left side or the right side of the frequency spectrum of the incident optical signal;
the coupler is used for combining the incident light signal and the local oscillator optical signal to obtain two paths of optical signals meeting the single-sideband condition;
the optical detector is used for converting the combined incident optical signal and the local oscillator optical signal into a path of radio frequency electric signal;
the radio frequency amplifier is used for amplifying the radio frequency electric signal from the optical detector so as to meet the requirement of the sampling level of the analog-to-digital converter;
the electric filter is used for filtering noise in the amplified radio frequency electric signal;
the analog-to-digital converter is used for converting the radio-frequency electric signal after noise filtering into a digital signal;
and the digital signal processor is used for processing the digital signal input by the analog-to-digital converter so as to recover a signal light field in a digital domain and realize digital coherent detection.
The local oscillator optical signal can be generated at the transmitting end, is transmitted to the receiving end after being multiplexed with the incident optical signal, can also be generated at the receiving end, and is input to the optical detector after being multiplexed with the incident optical signal transmitted to the receiving end through the coupler.
Preferably, if the photodetector is a normal photodetector PD, the radio frequency electrical signal is:
Figure BDA0001632166580000031
the digital signals are:
Figure BDA0001632166580000032
wherein, IPD(t) represents the radio frequency electrical signal, RPDRepresents the responsivity, P, of the ordinary photodetector PDSRepresenting the optical power, P, of said incident optical signalLORepresenting the optical power, ω, of said local oscillator optical signalIFRepresenting the difference, phi, between the angular frequency of the incident optical signal and the angular frequency of the local oscillator optical signalSRepresenting phase information carried by said incident optical signal, Idig-PD(nT) represents the digital signal, T represents the sampling interval time of the digital signal processor, and n represents the sampling point serial number.
Preferably, the digital signal processor is used for Iim-PD(nT)＝HT[Idig-PD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-PD(nT) of
Figure BDA0001632166580000041
Restoring the optical field of the signal in the digital domain to realize numbersWord coherent detection, in whichim-PD(nT) corresponds to ES-PD(nT) sampling values of magnitude of imaginary part of the light field information, HT representing Hilbert transform,
Figure BDA0001632166580000042
is a signal Idig-PDMean value of (nT).
Preferably, if the light detector is a light balance detector BPD, the radio frequency electrical signal is:
Figure BDA0001632166580000043
the digital signals are:
Figure BDA0001632166580000044
wherein, IBPD(t) represents the radio frequency electrical signal, RBPDRepresenting the responsivity, P, of the light balance detector BPDSRepresenting the optical power, P, of said incident optical signalLORepresenting the optical power, ω, of said local oscillator optical signalIFRepresenting the difference, phi, between the angular frequency of the incident optical signal and the angular frequency of the local oscillator optical signalSRepresenting phase information carried by said incident optical signal, Idig-BPD(nT) represents the digital signal, T represents the sampling interval time of the digital signal processor, and n represents the sampling point serial number.
Preferably, if the light detector is the light balance detector BPD, the digital signal processor is used for Iim-BPD(nT)＝HT[Idig-BPD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-BPD(nT) from ES-BPD(nT)＝Idig-BPD(nT)+j[Iim-BPD(nT)]Recovering the optical field of the signal in the digital domain to achieve digital coherent detection, wherein Iim-BPD(nT) corresponds to ES-BPD(nT) sampled values of magnitude of imaginary part of light field information, HT, represents Hilbert transform.
The amplification factor of the radio frequency amplifier is matched with the rated input voltage dynamic range of the analog-to-digital converter.
According to another aspect of the present invention, there is provided a linear direct detection method based on a synthesized single sideband signal, comprising:
combining an incident optical signal and a local oscillator optical signal to obtain two paths of optical signals meeting a single-sideband condition, and converting the combined incident optical signal and local oscillator optical signal into a path of radio frequency electrical signal, wherein the frequency of the local oscillator optical signal is positioned at the left side or right side edge of the frequency spectrum of the incident optical signal;
amplifying the radio frequency electric signal to meet the requirement of analog-to-digital conversion sampling level, and filtering noise in the amplified radio frequency electric signal;
and converting the radio-frequency electric signal after noise filtering into a digital signal, and processing the digital signal to recover a signal optical field in a digital domain to realize digital coherent detection.
The local oscillator optical signal can be flexibly configured at the transmitting end or the receiving end according to actual requirements. When the local oscillator optical signal is generated at the transmitting end, the incident optical signal and the local oscillator optical signal are combined and then transmitted to the receiving end through the optical fiber or the space. When the local oscillation optical signal is generated at the receiving end, the local oscillation optical signal and the incident optical signal transmitted to the receiving end are subjected to wave combination through the coupler and then input to the optical detector.
Preferably, the combining the incident optical signal and the local oscillator optical signal to obtain two optical signals, and converting the two optical signals into one radio frequency electrical signal includes:
and after the incident light signal and the local oscillator optical signal are combined into two paths of incident light signals through a coupler, converting any one path of optical signal in the two paths of incident light signals into one path of radio frequency electrical signal through a common optical detector PD, or converting the two paths of optical signals into one path of radio frequency electrical signal through an optical balance detector BPD.
Preferably, if any one of the two paths of incident light signals is converted into one path of radio frequency electrical signal by the ordinary photodetector PD, the radio frequency electrical signal is:
Figure BDA0001632166580000051
the digital signals are:
Figure BDA0001632166580000052
wherein, IPD(t) represents the radio frequency electrical signal, RPDRepresents the responsivity, P, of the ordinary photodetector PDSRepresenting the optical power, P, of said incident optical signalLORepresenting the optical power, ω, of said local oscillator optical signalIFRepresenting the difference, phi, between the angular frequency of the incident optical signal and the angular frequency of the local oscillator optical signalSRepresenting phase information carried by said incident optical signal, Idig-PD(nT) represents the digital signal, T represents the sampling interval time when the digital signal is processed, and n represents the sampling point number.
Preferably, said processing said digital signal to recover a signal light field in the digital domain, implementing digital coherent detection comprises:
from Iim-PD(nT)＝HT[Idig-PD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-PD(nT) of
Figure BDA0001632166580000061
Recovering the optical field of the signal in the digital domain to achieve digital coherent detection, wherein Iim-PD(nT) corresponds to ES-PD(nT) sampling values of magnitude of imaginary part of the light field information, HT representing Hilbert transform,
Figure BDA0001632166580000062
is a signal Idig-PDMean value of (nT).
Preferably, if the two paths of incident light signals are converted into one path of radio frequency electrical signal by the optical balance detector BPD, the radio frequency electrical signal is:
Figure BDA0001632166580000063
the digital signals are:
Figure BDA0001632166580000064
wherein, IBPD(t) represents the radio frequency electrical signal, RBPDRepresenting the responsivity, P, of the light balance detector BPDSRepresenting the optical power, P, of said incident optical signalLORepresenting the optical power, ω, of said local oscillator optical signalIFRepresenting the difference, phi, between the angular frequency of the incident optical signal and the angular frequency of the local oscillator optical signalSRepresenting phase information carried by said incident optical signal, Idig-BPD(nT) represents the digital signal, T represents the sampling interval time when the digital signal is processed, and n represents the sampling point number.
Preferably, said processing said digital signal to recover a signal light field in the digital domain, implementing digital coherent detection comprises:
from Iim-BPD(nT)＝HT[Idig-BPD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-BPD(nT) from ES-BPD(nT)＝Idig-BPD(nT)+j[Iim-BPD(nT)]Recovering the optical field of the signal in the digital domain to achieve digital coherent detection, wherein Iim-BPD(nT) corresponds to ES-BPD(nT) sampled values of magnitude of imaginary part of light field information, HT, represents Hilbert transform.
In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:
(1) the digital coherent detection system only uses one local oscillator laser, one coupler, one optical detector (PD or BPD), one radio frequency amplifier, one electric filter, one analog-to-digital converter (ADC) and one Digital Signal Processor (DSP). Compared with the prior digital coherent optical receiver, the required optical detector, ADC and DSP are greatly reduced, and an optical mixer is not needed, so that the system is simple in structure, good in reliability, low in cost and low in power consumption.
(2) Compared with the existing linear direct detection system based on the minimum phase signal, the linear direct detection system formed by the ordinary light detector PD greatly reduces the complexity and the operation amount of a demodulation algorithm by a linear direct detection method based on a synthetic single-sideband signal, so that the calculation rate requirement of a DSP (digital signal processor) can be greatly reduced, meanwhile, the demodulation algorithm avoids an algorithm which can cause spectrum broadening by logarithmic operation and the like, reduces the high requirement on ADC (analog to digital converter) sampling bandwidth, is very suitable for occasions which are sensitive to cost, such as a data center, a metropolitan area network, an optical access network and the like, and occasions which have strict requirements on the volume power consumption of a communication terminal, such as a satellite, a space station and the like, and can greatly improve the coverage range of a high-speed high-capacity optical network.
(3) Compared with the current linear direct detection system based on the minimum phase signal, the linear direct detection system formed by the optical balance detector BPD has the advantages of (2) and greatly reduces the requirement on the local oscillator optical power. Meanwhile, the invention eliminates Relative Intensity Noise (RIN) brought by signal light and local oscillator light through a balanced detection structure, improves the signal to noise ratio of the output radio frequency electric signal, avoids the problem of power loss of wave combination light by performing photoelectric conversion on the combined two paths of optical signals, is very suitable for the application of occasions requiring ultrahigh sensitivity and ultrahigh energy efficiency ratio, is also suitable for the occasions requiring strict requirements on the volume power consumption of communication terminals such as satellites and space stations, and can greatly improve the communication distance and improve the communication quality.
Drawings
Fig. 1 is a schematic structural diagram of a linear direct detection system based on a synthesized single sideband signal according to an embodiment of the present invention;
fig. 2 is a graph showing the variation of the signal quality (represented by the error vector magnitude EVM) output by the phase diversity coherent optical receiver based on the present invention and the current minimum phase signal and the conventional optical mixer based in a typical application environment, as a function of the input signal OSNR;
FIG. 3 shows the output signal quality (represented by error vector magnitude EVM) of the conventional optical mixer-based phase diversity coherent optical receiver and the linear direct detection system based on the present invention and the minimum phase signal based on the present invention under the condition of a fixed signal input power (-40dBm) according to the ratio of the added DC optical power to the added signal optical power (represented by PLO/PsRepresentation);
the same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
the system comprises a 1-local oscillator laser, a 2-coupler, a 3-optical detector, a 4-radio frequency amplifier, a 5-electric filter, a 6-analog-to-digital converter (ADC) and a 7-Digital Signal Processor (DSP).
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The invention provides a linear direct detection method and system based on a synthetic single-sideband signal, and solves the technical problems of high requirements on ADC (analog to digital converter) sampling multiplying power and extremely high requirements on data processing rate of a DSP (digital signal processor) in the existing linearization technology based on a minimum phase signal.
Fig. 1 is a schematic structural diagram of a linear direct detection system based on a synthesized single sideband signal according to an embodiment of the present invention, where the system shown in fig. 1 includes: the system comprises a local oscillator laser 1, a coupler 2, an optical detector 3, a radio frequency amplifier 4, an electric filter 5, an analog-to-digital converter 6 and a digital signal processor 7 which are connected in sequence;
the Photodetector may use a normal Photodetector (PD), and has 1 input port and 1 output port; a Balanced Photo-detector (BPD) with 2 input ports and 1 output port may also be used.
In the embodiment of the present invention, the frequency of the local oscillator optical signal is located in a preset range on the left or right of the frequency spectrum of the incident optical signal, where the preset range may be determined according to an actual use condition.
In the embodiment of the invention, the local oscillator optical signal can be generated at the transmitting end and is transmitted to the receiving end after being combined with the signal light; or generated at the receiving end, and is input to the optical detector after being combined with the incident signal light transmitted to the receiving end through the coupler.
In an embodiment of the invention, an incident optical signal
Figure BDA0001632166580000091
And local oscillator optical signal
Figure BDA0001632166580000092
The two paths of optical signals meeting the single-sideband condition are obtained by wave combination of the coupler and then input into 1 or 2 input ports (determined according to the difference of the number of the input ports of PD and BPD) of the optical detector, wherein P iss、PLOOptical power, omega, of the incident optical signal and the local oscillator optical signal, respectivelyS、ωLOAngular frequencies, phi, corresponding to the incident optical signal and the local oscillator optical signal, respectivelyS(t) is the phase information carried by the incident optical signal.
The optical detector is used for converting 1 path or 2 paths of optical signals (which are judged according to the difference of the number of input ports of PD and BPD) into 1 path of radio frequency electric signals;
in the embodiment of the present invention, when a general photodetector PD is used, the radio frequency electrical signal (without considering the loss of the power of the multiplexed light) is:
Figure BDA0001632166580000093
when using the optical balanced detector BPD, the rf signal is:
Figure BDA0001632166580000094
wherein, IPD(t) or IBPD(t) represents a radio frequency electric signal, RPDRepresenting the responsivity, R, of a conventional photodetector PDBPDRepresenting the responsivity, P, of the light balance detector BPDSRepresenting the optical power of the incident optical signal, PLORepresenting the optical power, omega, of the local oscillator optical signalIFRepresenting the difference, phi, between the angular frequency of the incident optical signal and the angular frequency of the local oscillator optical signalSRepresenting the phase information carried by the incident optical signal.
The radio frequency amplifier is used for amplifying the radio frequency electric signal from the optical detector so as to meet the requirement of sampling level of the analog-to-digital converter;
the amplification factor of the radio frequency amplifier is matched with the rated input voltage dynamic range of the analog-to-digital converter.
The electric filter is used for filtering noise in the amplified radio frequency electric signal;
the analog-to-digital converter is used for converting the radio-frequency electric signal after the noise is filtered into a digital signal;
in the embodiment of the present invention, when a general photodetector PD is used, the digital signals are:
Figure BDA0001632166580000101
when using the light balance detector BPD, the digital signal is:
Figure BDA0001632166580000102
wherein, Idig-PD(nT) or Idig-BPD(nT) represents a digital signal, T represents a sampling interval time of the digital signal processor, and n represents a sampling point number.
And the digital signal processor is used for processing the digital signal input by the analog-to-digital converter so as to recover a signal light field in a digital domain and realize digital coherent detection.
In the embodiment of the invention, when a common light detector PD is used, a digital signal processor is used for processing signals from Iim-PD(nT)＝HT[Idig-PD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-PD(nT) of
Figure BDA0001632166580000103
Recovering the optical field of the signal in the digital domain to achieve digital coherent detection, wherein Iim-PD(nT) corresponds to ES-PD(nT) sampling values of magnitude of imaginary part of the light field information, HT representing Hilbert transform,
Figure BDA0001632166580000104
is a signal Idig-PDMean value of (nT).
Digital signal processor for use by I when using a light balance detector BPDim-BPD(nT)＝HT[Idig-BPD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-BPD(nT)From ES-BPD(nT)＝Idig-BPD(nT)+j[Iim-BPD(nT)]Recovering the optical field of the signal in the digital domain to achieve digital coherent detection, wherein Iim-BPD(nT) corresponds to ES-BPD(nT) sampled values of magnitude of imaginary part of light field information, HT, represents Hilbert transform.
The invention also provides a linear direct detection method based on the synthetic single sideband signal, which comprises the following steps:
combining an incident optical signal and a local oscillator optical signal to obtain two optical signals meeting a single-sideband condition, and converting the combined incident optical signal and local oscillator optical signal into a radio frequency electrical signal, wherein the frequency of the local oscillator optical signal is within a preset range on the left side or the right side of the frequency spectrum of the incident optical signal, and the preset range can be determined according to the actual use condition;
amplifying the radio-frequency electric signal to meet the requirement of analog-to-digital conversion sampling level, and filtering noise in the amplified radio-frequency electric signal;
and converting the radio-frequency electric signal after noise filtering into a digital signal, and processing the digital signal to recover a signal optical field in a digital domain to realize digital coherent detection.
Fig. 2 is a graph showing the variation of the signal quality (represented by the error vector magnitude EVM) output by the present invention and the current minimum-phase signal-based linear direct detection system and the conventional optical mixer-based phase diversity coherent optical receiving system according to the OSNR of the input optical signal in a typical application environment. Wherein the detected optical signal is a Quadrature Phase Shift Keying (QPSK) signal at 20Gbps (baud rate of 10 GBaud/s). BER 1e when EVM 32.5%-3. As can be seen from fig. 2, when EVM is 32.5%, BER is 1e-3When the linear direct detection system based on the invention adopts a common light detector PD, the sensitivity is equivalent to that of the current linear direct detection system based on the minimum phase signal; when the linear direct detection system based on the invention adopts the optical balance detector BPD, the sensitivity is improved by 1.1dB (the required OSNR is reduced by 1.1dB) compared with the current linear direct detection system based on the minimum phase signal, and the sensitivity is highest compared with the current traditional optical mixer-based linear direct detection system based on the optical mixerCompared with a phase diversity coherent optical receiver, the sensitivity is kept consistent.
FIG. 3 shows the output signal quality (represented by error vector magnitude EVM) of the linear direct detection system based on the present invention and based on the minimum phase signal and the conventional phase diversity coherent optical receiving system based on the optical mixer as a function of the ratio of the added local oscillator light to the signal light power (represented by P)LO/PsRepresentation) of the change in the profile. It can be seen from fig. 3 that the EVM is 32.5%, i.e. BER is 1e under the same operating conditions-3The DC optical power P required by the technology of the invention when adopting a common photodetector PDLO＝30PsCompared with the minimum phase signal linearization based technique (P)LO＝8Ps) Although the local oscillator optical power requirement is improved, the total system cost is greatly reduced due to the reduction of the sampling bandwidth requirement and the algorithm complexity. In practical application, because the power of the incident optical signal is weak, the system cost is greatly reduced by improving the local oscillator optical power on the premise of ensuring equivalent receiving performance, and the system has great practicability and economic value. Meanwhile, the technology of the invention adopts the direct current light power P required by the light balance detector BPDLO＝2PsCompared with the minimum phase signal linearization based technique (P)LO＝8Ps) The optical frequency mixer reduces 4 times, compared with the common optical detector (PD), the system cost is slightly improved, but the receiving performance is greatly improved and the local oscillator optical power requirement is greatly reduced, so that the comprehensive performance of the optical frequency mixer-based phase diversity coherent optical receiving system is consistent with that of the traditional optical frequency mixer-based phase diversity coherent optical receiving system.
In a word, when the common light detector PD is adopted, the method has the advantages of lower system cost, lower algorithm complexity and better stability compared with a linear direct detection system based on a minimum phase signal; compared with a linear direct detection system based on a minimum phase signal, the linear direct detection system based on the optical balance detector has higher sensitivity, lower local oscillator optical power requirement, lower algorithm complexity and better system stability when the optical balance detector BPD is adopted. Meanwhile, compared with the traditional phase diversity coherent light receiving system based on the optical mixer, the same receiving performance and system energy efficiency ratio are obtained under the condition of greatly reducing the system cost.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (7)

1. A linear direct detection system based on a synthesized single sideband signal, comprising: the system comprises a local oscillator laser, a coupler, an optical detector, a radio frequency amplifier, an electric filter, an analog-to-digital converter and a digital signal processor which are connected in sequence;
the coupler is provided with 2 input ports and 2 output ports, the optical detector comprises a common optical detector PD with 1 input port and 1 output port, or an optical balance detector BPD with 2 input ports and 1 output port, the coupler arbitrarily selects 1 port from the 2 output ports to be connected with 1 input port of the common optical detector PD, or the coupler connects 2 output ports with 2 input ports of the optical balance detector BPD;
the local oscillator laser is used for providing a local oscillator optical signal which carries out beat frequency with an incident optical signal, wherein the frequency of the local oscillator optical signal is located in a preset range on the left side or the right side of the frequency spectrum of the incident optical signal;
the coupler is used for combining the incident light signal and the local oscillator optical signal to obtain two paths of optical signals meeting the single-sideband condition;
the optical detector is used for converting the combined incident optical signal and the local oscillator optical signal into a path of radio frequency electric signal;
the radio frequency amplifier is used for amplifying the radio frequency electric signal from the optical detector so as to meet the requirement of the sampling level of the analog-to-digital converter;
the electric filter is used for filtering noise in the amplified radio frequency electric signal;
the analog-to-digital converter is used for converting the radio-frequency electric signal after noise filtering into a digital signal;
the digital signal processor is used for processing the signals by Iim-PD(nT)＝HT[Idig-PD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-PD(nT) of
Figure FDA0002575639950000011
Recovering the optical field of the signal in the digital domain to achieve digital coherent detection, wherein Iim-PD(nT) corresponds to ES-PD(nT) sampling values of magnitude of imaginary part of the light field information, HT representing Hilbert transform,
Figure FDA0002575639950000021
is a signal Idig-PD(nT);
or the digital signal processor, for processing the signals from Iim-BPD(nT)＝HT[Idig-BPD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-BPD(nT) from ES-BPD(nT)＝Idig-BPD(nT)+j[Iim-BPD(nT)]Recovering the optical field of the signal in the digital domain to achieve digital coherent detection, wherein Iim-BPD(nT) corresponds to ES-BPD(nT) sampled values of magnitude of imaginary part of light field information, HT, represents Hilbert transform.
2. The system of claim 1, wherein if the photo detector is a normal photo detector PD, the rf signal is:
Figure FDA0002575639950000022
the digital signals are:
Figure FDA0002575639950000023
wherein, IPD(t) represents the radio frequency electrical signal, RPDRepresents the responsivity, P, of the ordinary photodetector PDSRepresenting the optical power, P, of said incident optical signalLORepresenting the optical power, ω, of said local oscillator optical signalIFRepresenting the difference, phi, between the angular frequency of the incident optical signal and the angular frequency of the local oscillator optical signalSRepresenting phase information carried by said incident optical signal, Idig-PD(nT) represents the digital signal, T represents the sampling interval time of the digital signal processor, and n represents the sampling point serial number.
3. The system of claim 1, wherein if the light detector is a balanced light detector (BPD), the radio frequency electrical signal is:
Figure FDA0002575639950000024
the digital signals are:
Figure FDA0002575639950000025
wherein, IBPD(t) represents the radio frequency electrical signal, RBPDRepresenting the responsivity, P, of the light balance detector BPDSRepresenting the optical power, P, of said incident optical signalLORepresenting the optical power, ω, of said local oscillator optical signalIFRepresenting the difference, phi, between the angular frequency of the incident optical signal and the angular frequency of the local oscillator optical signalSRepresenting phase information carried by said incident optical signal, Idig-BPD(nT) represents the digital signal, T represents the sampling interval time of the digital signal processor, and n represents the sampling point serial number.
4. A linear direct detection method based on a synthesized single sideband signal is characterized by comprising the following steps:
combining an incident optical signal and a local oscillator optical signal to obtain two paths of optical signals meeting a single-sideband condition, and converting the combined incident optical signal and local oscillator optical signal into a path of radio frequency electrical signal, wherein the frequency of the local oscillator optical signal is located in a preset range on the left side or the right side of the frequency spectrum of the incident optical signal;
amplifying the radio frequency electric signal to meet the requirement of analog-to-digital conversion sampling level, and filtering noise in the amplified radio frequency electric signal;
converting the radio frequency electric signal after noise filtering into a digital signal, and processing the digital signal to recover a signal light field in a digital domain to realize digital coherent detection;
the processing the digital signal comprises:
from Iim-PD(nT)＝HT[Idig-PD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-PD(nT) of
Figure FDA0002575639950000031
Recovering the optical field of the signal in the digital domain to achieve digital coherent detection, wherein Iim-PD(nT) corresponds to ES-PD(nT) sampling values of magnitude of imaginary part of the light field information, HT representing Hilbert transform,
Figure FDA0002575639950000032
is a signal Idig-PD(nT);
or from Iim-BPD(nT)＝HT[Idig-BPD(nT)]Hilbert conversion is carried out on the digital signal to obtain a new digital signal Iim-BPD(nT) from ES-BPD(nT)＝Idig-BPD(nT)+j[Iim-BPD(nT)]Recovering the optical field of the signal in the digital domain to achieve digital coherent detection, wherein Iim-BPD(nT) corresponds to ES-BPD(nT) sampled values of magnitude of imaginary part of light field information, HT, represents Hilbert transform.
5. The method according to claim 4, wherein the combining the incident optical signal and the local oscillator optical signal to obtain two optical signals, and converting the two optical signals into one radio frequency electrical signal comprises:
and after the incident light signal and the local oscillator optical signal are combined into two paths of incident light signals through a coupler, converting any one path of optical signal in the two paths of incident light signals into one path of radio frequency electrical signal through a common optical detector PD, or converting the two paths of optical signals into one path of radio frequency electrical signal through an optical balance detector BPD.
6. The method according to claim 5, wherein if any one of the two incident optical signals is converted into a radio frequency electrical signal by the normal photo detector PD, the radio frequency electrical signal is:
Figure FDA0002575639950000041
the digital signals are:
Figure FDA0002575639950000042
wherein, IPD(t) represents the radio frequency electrical signal, RPDRepresents the responsivity, P, of the ordinary photodetector PDSRepresenting the optical power, P, of said incident optical signalLORepresenting the optical power, ω, of said local oscillator optical signalIFRepresenting the difference, phi, between the angular frequency of the incident optical signal and the angular frequency of the local oscillator optical signalSRepresenting phase information carried by said incident optical signal, Idig-PD(nT) represents the digital signal, T represents the sampling interval time when the digital signal is processed, and n represents the sampling point number.
7. The method according to claim 5, wherein if the two incident optical signals are converted into one rf electrical signal by the optical balanced detector BPD, the rf electrical signal is:
Figure FDA0002575639950000043
the digital signals are:
Figure FDA0002575639950000044
wherein, IBPD(t) represents the radio frequency electrical signal, RBPDRepresenting the responsivity, P, of the light balance detector BPDSRepresenting the optical power, P, of said incident optical signalLORepresenting the optical power, ω, of said local oscillator optical signalIFRepresenting the angular frequency of the incident optical signal and the local oscillator lightAngular frequency difference of signal, phiSRepresenting phase information carried by said incident optical signal, Idig-BPD(nT) represents the digital signal, T represents the sampling interval time when the digital signal is processed, and n represents the sampling point number.
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