CN117792513A - Self-coherent detection system and detection method supporting polarization multiplexing - Google Patents
Self-coherent detection system and detection method supporting polarization multiplexing Download PDFInfo
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Abstract
The invention discloses a self-coherent detection system supporting polarization multiplexing and a detection method, wherein the self-coherent detection system supporting polarization multiplexing comprises: the device comprises a dual-polarization IQ modulation module, a polarization module, a local oscillator module, an optical coupling module, a photoelectric detection module, an analog-to-digital conversion module and a digital processing module, wherein the dual-polarization IQ modulation module, the optical coupling module, the photoelectric detection module, the analog-to-digital conversion module and the digital processing module are sequentially connected. The self-coherent detection system supporting polarization multiplexing uses a polarization multiplexing mode, can realize the detection of polarization multiplexing signals without adding an additional photoelectric device, effectively improves the utilization rate of spectrum and light source power, and has the advantages of low equipment cost, small device bandwidth requirement, low calculation complexity and the like.
Description
Technical Field
The invention relates to the field of optical communication, in particular to an autocorrelation detection system supporting polarization multiplexing, namely a detection method.
Background
In recent years, new applications based on the internet are endless, and the number of users in the global network is rapidly increasing. High-speed, low-delay and ultra-large scale access 5G technology; the demands of technologies such as cloud computing for remote data storage and 4K/8K high-definition video streaming media on network bandwidth are increasing. How to realize high-speed, long-distance and high-quality data transmission at low cost becomes a key problem to be solved in all communication between access network and data center. Compared with a coherent detection system, the self-coherent detection system needs less hardware and is expected to be widely applied to communication among data centers. In the self-coherent detection system, the existing schemes mostly have the problems of high carrier signal power ratio, large operation amount, high device complexity and the like, and an additional photoelectric device is often required to be added to realize the detection of the polarization multiplexing signal, so that the practicability of the self-coherent detection system in a medium-short distance interconnection system is limited.
Disclosure of Invention
In view of the above-mentioned drawbacks of the prior art, the present invention aims to solve the technical problems of high carrier signal power ratio, large operation amount, high device complexity, and the like in the existing self-coherent detection system, and the problems of limited practicality in a medium-short distance interconnection system due to the fact that an additional photoelectric device is often added to realize the detection of polarization multiplexing signals. Therefore, the invention provides the self-coherent detection system and the detection method supporting polarization multiplexing, the self-coherent detection system can realize the detection of the polarization multiplexing signal without adding an additional photoelectric device, can effectively improve the utilization rate of spectrum and light source power, and has the advantages of low equipment cost, small device bandwidth requirement, low calculation complexity and the like.
In order to achieve the above purpose, the invention provides an auto-coherent detection system supporting polarization multiplexing, which comprises a dual-polarization IQ modulation module, a polarization module, a local oscillation module, an optical coupling module, a photoelectric detection module, an analog-to-digital conversion module and a digital processing module, wherein the dual-polarization IQ modulation module, the optical coupling module, the photoelectric detection module, the analog-to-digital conversion module and the digital processing module are sequentially connected; wherein,
the dual-polarization IQ modulation module is used for dividing a light source signal into two paths of mutually orthogonal polarized light waves and respectively modulating data to be transmitted onto the two paths of mutually orthogonal polarized light waves;
the polarization module is connected with the optical coupling module and is used for adjusting the polarization state of the light source signal to obtain an optical transmission signal and transmitting the optical transmission signal to the optical coupling module;
the local oscillation module is used for outputting local oscillation signals;
the optical coupling module is used for receiving the optical transmission signal and the local oscillation signal, wherein the frequency value of the local oscillation signal is different from the frequency value of the optical transmission signal by a preset offset frequency value, and a coupling signal is obtained according to the optical transmission signal and the local oscillation signal;
the photoelectric detection module is connected with the optical coupling module and used for converting the coupling signal into an analog signal;
the analog-to-digital conversion module is connected with the photoelectric detection module and is used for sampling the analog signal according to a preset sampling frequency value and converting the analog signal into a digital signal, wherein the preset sampling frequency value is three times of the symbol rate value of the optical transmission signal; and the digital processing module is connected with the analog-to-digital conversion module and is used for processing the digital signals.
Further, the dual-polarization state IQ modulation module is configured as a dual-polarization state IQ modulator, and is configured to divide the light source signal into two paths of polarized light waves orthogonal to each other, perform IQ modulation on one path of polarized light wave to obtain a complex signal, perform intensity modulation on the other path of polarized light wave to obtain an intensity signal, and couple the two paths of polarized light waves through the polarization multiplexer to obtain an optical transmission signal.
Further, the light source signal is a combined light signal obtained by combining the light transmission signal and the local oscillation signal, and the polarization module is used for adjusting the polarization state of the light source signal so as to obtain the light transmission signal and the local oscillation signal, and respectively transmitting the light transmission signal and the local oscillation signal to the optical coupling module.
Further, the preset sampling frequency value is set to be greater than or equal to twice the symbol rate value of the optical transmission signal.
Further, the light source signal is a combined light signal obtained by combining the light transmission signal and the local oscillation signal, and the polarization module divides the light source signal into the light transmission signal and the local oscillation signal and respectively transmits the light transmission signal and the local oscillation signal to the optical coupling module.
Further, the preset sampling frequency value sets an arbitrary value.
Further, the device also comprises an adjustable optical attenuation module for adjusting the amplitudes of the optical transmission signal and the local oscillation signal, and the adjustable optical attenuation module is connected with the output end of the polarization module.
Further, the preset offset frequency values are set to one or more of 1/2,3/4 and 1 times the symbol rate of the optical transmission signal, respectively.
Further, the photodetection module is configured as a single photodetector, and a single photodetector is used as the receiving end.
In a preferred embodiment of the present invention, the present invention provides a method for using a self-coherent detection system supporting polarization multiplexing as described in any one of the above, comprising the steps of:
respectively setting 3 preset offset frequency values and sampling frequency values;
transmitting the optical transmission signal and the local oscillation signal to an optical coupling module respectively;
the optical coupling module couples the optical transmission signal and the local oscillation signal to obtain a coupling signal, and transmits the coupling signal to the photoelectric detection module;
the photoelectric detection module performs photoelectric conversion on the coupling signal to obtain an analog signal, and transmits the analog signal to the analog-to-digital conversion module;
the analog-to-digital conversion module samples the analog signal with a preset sampling frequency value and converts the analog signal into a digital signal to calculate an analog signal E of every three samples s Phase differences respectively existing between the two;
three sampled analog signals E s The method comprises the steps of respectively obtaining complex signals and intensity signals of two mutually orthogonal polarization states of an optical transmission signal through calculation, wherein the emphasis is that the signals of the two polarization states are complex signals, and real signals are all the same;
and the digital processing module processes the digital signals converted from the sampled analog signals to complete demodulation of the optical transmission signals.
Further, an analog signal E is calculated for every third sample s The phase differences respectively exist between the two phases specifically comprise the following steps:
let it be assumed that the optical transmission signal O t Symbol rate R of (2) symbol By changing the frequency value offset by the frequency offset unit, the local oscillation signal O is enabled lo And optical transmission signal O t A certain frequency offset exists between the two;
the preset frequency offset is R symbol /2，3R symbol R/4 and R symbol ；
Analog signal E output to photodetector by preset sampling frequency value s Sampling, and setting a preset sampling frequency value as an optical transmission signal O t The symbol rate is three times 3R symbol The sampling period T is 1/3R symbol ；
Three analog signals E sampled in succession s Is expressed by the following formula:
wherein R represents the responsivity of the photodetector,power of complex signal for IQ modulation on X-polarization state, +.>Power P of intensity signal transmitted in Y polarization state LO Representing local oscillator signal O lo Beta represents the optical transmission signal O t And θ represents phase noise, 2pinΔfT represents due to local oscillator signal O lo And optical transmission signal O t Phase shift due to frequency shift existing between them;
analog signal E of three consecutive samples s Phase difference between
Further, three sampled analog signals E s Respectively obtaining complex signals and intensity signals of two mutually orthogonal polarization states of an optical transmission signal through calculation, specifically three sampled analog signals, and firstly obtaining an in-phase component and a quadrature component of an IQ modulation on an X polarization state through calculation; subtracting the calculated intensity information on the X polarization state from the obtained sampled analog signal to obtain intensity information on the Y polarization state subjected to intensity modulation, wherein the method specifically comprises the following steps of:
representing three consecutive sampled analog signals as in-phase component I of the same symbol on a complex signal IQ modulated in the X-polarization state k And quadrature component Q k And intensity information on the Y polarization state
In the above formula, γ is a normalized coefficient.
Technical effects
According to the self-coherent detection system and the detection method supporting polarization multiplexing, the mutual independence of light in an orthogonal polarization state in beat frequency is utilized, polarization demultiplexing can be directly supported, single PD-based polarization multiplexing signal detection can be realized without adding an additional photoelectric device, and the utilization rate of spectrum and light source power is effectively improved. The requirement on the bandwidth of the device can be reduced by changing the preset frequency offset value, beat interference can be eliminated through simple calculation, and the optical transmission signal is recovered, so that the self-coherent detection system and the detection method supporting polarization multiplexing have the advantages of low equipment cost, small device bandwidth requirement and low calculation complexity.
The conception, specific structure, and technical effects of the present invention will be further described with reference to the accompanying drawings to fully understand the objects, features, and effects of the present invention.
Drawings
FIG. 1 is a schematic diagram of an autocorrelation detection system supporting polarization multiplexing in accordance with a preferred embodiment of the present invention;
fig. 2 is a schematic diagram of a frequency offset manner between an optical transmission signal and a local oscillation signal received by an optical coupling module supporting a polarization multiplexing self-coherent detection method according to another embodiment of the present invention;
fig. 3 is a schematic diagram of a sampling period of an analog-to-digital conversion module of an auto-coherent detection method supporting polarization multiplexing according to another embodiment of the present invention;
fig. 4 is a schematic diagram of an autocorrelation detection method supporting polarization multiplexing according to another embodiment of the present invention;
FIG. 5 is a schematic diagram of subsequent digital signal processing in accordance with another embodiment of the invention;
FIG. 6 is a 16QAM constellation modulated on the X polarization state at 10GHz of polarization under experiment in accordance with another embodiment of the present invention; the method comprises the steps of carrying out a first treatment on the surface of the
FIG. 7 is a PAM4 signal modulated on Y polarization at 10GHz of polarization in an experiment in accordance with another embodiment of the present invention;
FIG. 8 is a 16QAM constellation modulated on the X polarization state at 15GHz of polarization under experiment according to another embodiment of the present invention;
FIG. 9 is a PAM4 signal modulated on Y polarization at 15GHz for polarization in an experiment in accordance with another embodiment of the present invention;
FIG. 10 is a 16QAM constellation modulated on X polarization at 20GHz of polarization under experiment in accordance with another embodiment of the present invention;
fig. 11 is a PAM4 signal modulated on the Y polarization state at 20GHz in the polarization under experiment according to another embodiment of the present invention.
Detailed Description
In order to make the technical problems, technical schemes and beneficial effects to be solved more clear, the invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular internal procedures, techniques, etc. in order to provide a thorough understanding of embodiments of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known systems, devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.
As shown in fig. 1, a preferred embodiment of the present invention provides an auto-coherent detection system supporting polarization multiplexing, which comprises a dual-polarization IQ modulation module 110, an optical coupling module 120, a photoelectric detection module 130, an analog-to-digital conversion module 140, and a digital processing module 150, which are sequentially connected, wherein the dual-polarization IQ modulation module 110 is used for transmitting a light source signal O s Dividing into two paths of mutually orthogonal polarized light waves, respectively modulating data to be transmitted into the two paths of mutually orthogonal polarized light waves, and combining signals in two polarization states through a polarization combiner to obtain an optical transmission signal O t Entering an optical fiber for transmission; the optical coupling module 120 is configured to receive an optical transmission signal O carrying information t And local oscillation signal O lo Local oscillation signal O of the same source as the optical transmission signal lo Frequency of (2)The frequency value is different from the frequency value of the optical transmission signal by a preset offset frequency value, and the preset offset frequency value is set to be any value; the local oscillation signals of the same source only change the phase difference among three sampling signals through different preset offset frequency values; whether the preset offset frequency value is set to three different values, three samples can be obtained, and the preset offset frequency value is set to any value, and in this embodiment, the preset offset frequency value is 1/2,3/4 and 1 times of the symbol rate of the optical transmission signal respectively.
And according to the optical transmission signal O t And local oscillation signal O lo Obtaining a coupling signal O m The method comprises the steps of carrying out a first treatment on the surface of the The photo-detection module 130 is used for coupling the signal O m Converted into analog signal E s ；
The analog-to-digital conversion module 140 is configured to convert the analog signal E according to a preset sampling frequency value s Sampling and applying analog signal E s Converted into digital signal E d The preset sampling frequency value is set to be in accordance with the sampling theorem, namely, the value is more than or equal to 2 times of the symbol rate value; the digital processing module 150 is used for processing the digital signal E d . Due to local oscillator signal O lo And optical transmission signal O t The frequency values of (2) differ by a preset offset frequency, and the preset offset frequency value is the optical transmission signal O t The symbol rate of (2), 3/4 and 1 times of the symbol rate of (C), when the optical transmission signal O is passed through the preset sampling frequency value t Three times the symbol rate value of (2) by local oscillator signal O lo And optical transmission signal O t Analog signal E converted from combined optical signal s When sampling is performed, the analog signal E of every three samples can be made s Phase differences of 60 degrees, 90 degrees and 120 degrees exist respectively, and the three sampled analog signals can respectively obtain optical transmission signals O through calculation t The complex and intensity signals on two mutually orthogonal polarization states can be used to sample the analog signal E by the digital processing module 150 s Converted digital signal E d The processing is performed to complete demodulation of the optical transmission signal. The self-coherent detection system supporting polarization multiplexing in this embodiment can directly support polarization demultiplexing by utilizing the mutual independence of light in orthogonal polarization state in beat frequency, and has no need ofThe polarization multiplexing signal detection based on single PD can be realized by adding an additional photoelectric device, and the utilization rate of spectrum and light source power is effectively improved. The requirement on the bandwidth of the device can be reduced by changing the preset frequency offset value, beat interference can be eliminated through simple calculation, and the optical transmission signal is recovered, so that the polarization multiplexing self-coherent detection system has the advantages of low equipment cost, small device bandwidth requirement and low calculation complexity.
Wherein the photo-detection module 130 is a single photo-detector. The photodetector may be a PIN photodiode, which is not limited solely by this embodiment.
The optical coupling module 120 may be an optical coupler, which is not limited solely by the present embodiment.
Local oscillator signal O received by optical coupling module 120 lo Can be transmitted to the self-coherent detection device through the optical signal transmitting device, and the embodiment is used for local oscillation signal O lo The source of (c) is not particularly limited.
In the current self-coherent detection solution, a single photoelectric detector is often adopted at the receiving end, and beat interference generated by square rate detection of the photoelectric detector becomes a main factor affecting signal transmission performance. Aiming at beat interference, several existing processing methods are used at present: a guard interval method, which results in a half reduction in spectral efficiency; the iteration elimination algorithm is adopted, and the iteration times required by the method are often higher; the clamerer-kroneger (KK) receiver, while simple, has high requirements on the carrier signal power ratio and the sampling rate. Therefore, there is an urgent need for an auto-coherent detection method with low equipment cost, small device bandwidth requirement, and low computational complexity. Meanwhile, the scheme often needs to add an extra photoelectric device to realize the detection of the polarization multiplexing signal, so that the practicability of the polarization multiplexing signal in a medium-short distance interconnection system is limited. Therefore, the self-coherent detection system supporting polarization multiplexing of the embodiment can realize detection of polarization multiplexing signals by adopting a single photoelectric detector so as to eliminate beat interference as much as possible, and therefore, the self-coherent detection system has low equipment cost.
The working principle of the self-coherent detection system supporting polarization multiplexing in the embodiment of the invention is as follows:
as shown in fig. 2, assume that an optical transmission signal O t Is R symbol Local oscillation signal O can be caused by changing the frequency value offset by the frequency offset unit lo And optical transmission signal O t With a certain frequency offset, R symbol /2，3R symbol R/4 and R symbol . And can output an analog signal E to the photodetector through a preset sampling frequency value as shown in FIG. 3 s Sampling, and setting a preset sampling frequency value as an optical transmission signal O t Three times the symbol rate value (3R symbol ) Then the sampling period T is 1/3R symbol 。
Three analog signals E sampled successively s The expression can be represented by the following formula:
wherein R represents the responsivity of the photodetector,power of complex signal for IQ modulation on X-polarization state, +.>Power P of intensity signal transmitted in Y polarization state LO Representing local oscillator signal O lo Beta represents the optical transmission signal O t θ represents phase noise, 2pi n ΔfT represents a phase due to a local oscillation signal O lo With optical transmission signalsNumber O t Phase shift due to frequency shift existing between them. From the above formula, three consecutive sampled analog signals E s Phase difference between(if the frequency offset value is R symbol 2, then α=pi/3; if the frequency offset value is 3R symbol 4, then α=pi/2; if the frequency offset value is R symbol α=2pi/3), then this three analog signals E sampled in succession s Can be expressed as:
representing three successively sampled analog signals as in-phase component I of the same symbol on an IQ-modulated complex signal in the X polarization state by simple calculation k And quadrature component Q k And intensity information on the Y polarization state
In the above formula, γ is a normalized coefficient.
So when the local oscillation signal O lo And optical transmission signal O t When the analog signal converted by the combined optical signal composed of the local oscillator signal and the optical transmission signal is sampled by three times the value of the preset sampling frequency which is the value of the symbol rate of the optical transmission signal, the phase difference of 60 degrees, 90 degrees and 120 degrees exists between every three sampled analog signals respectively, and the same phase component I of the same symbol in the complex signal subjected to IQ modulation on X polarization state can be obtained by simple calculation of the three sampled analog signals k And quadrature component Q k And intensity information on the Y polarization stateDemodulation of the optical transmission signal may be accomplished by processing the digital signal converted from the sampled analog signal by the digital processing module 150. The self-coherent detection system supporting polarization multiplexing can directly support polarization demultiplexing by utilizing the mutual independence of light in orthogonal polarization states in beat frequency, can realize single PD-based polarization multiplexing signal detection without adding an extra photoelectric device, and effectively improves the utilization rate of spectrum and light source power. The requirement on the bandwidth of the device can be reduced by changing the preset frequency offset value, beat interference can be eliminated through simple calculation, and the optical transmission signal is recovered, so that the self-coherent detection system supporting polarization multiplexing in the embodiment of the invention has the advantages of low equipment cost, small device bandwidth requirement and low calculation complexity.
Note that, the local oscillation signal O lo Frequency value F of (2) lo And optical transmission signal O t Frequency value F of (2) s The difference of the preset offset frequency value can be + -R symbol /2,±3R symbol /4,±R symbol 。
As shown in FIG. 4, in a further preferred embodiment, based on the embodiment of FIG. 1 described above, the self-coherentThe detection system further includes a polarization module 410 coupled to the optical coupling module 120, the polarization module 410 for receiving the light source signal O s Light source signal O s Is adjusted to obtain the optical transmission signal O t And transmits the optical signal O t To the optical coupling module 120.
In one embodiment, the self-coherent detection system may be provided with a polarization module 410, and the received light source signal O may be transmitted through the polarization module 410 s Is adjusted to obtain an optical transmission signal O t So that the light transmits the signal O t Polarization state and local oscillation signal O of IQ modulation lo Is consistent in polarization state and is capable of coupling with local oscillator signal O through optical coupling module 120 lo (local oscillation signal O) lo Frequency value of (2) and optical transmission signal O t The frequency values of (2) differ by a preset offset frequency, and the preset offset frequency value is the optical transmission signal O t Symbol rate 1/2,3/4 and 1 times) of the symbol rate of the signal to obtain a coupled signal O m And will couple signal O m To the photodetection module 130, the photodetection module 130 transmits the coupling signal O m Photoelectric conversion is carried out to obtain an analog signal E s And convert the analog signal E s Is transmitted to the analog-to-digital conversion module 140, and the analog-to-digital conversion module 140 uses a predetermined sampling frequency value (the predetermined sampling frequency value is the optical transmission signal O t Three times the symbol rate of (a) analog signal E s Sampling and applying analog signal E s Converted into digital signal E d Every third sampled analog signal E s With 60, 90 and 120 degrees of phase difference between them, respectively, and three sampled analog signals E s The optical transmission signals O can be obtained by simple calculation t The complex and intensity signals on two mutually orthogonal polarization states can be used to sample the analog signal E by the digital processing module 150 s Converted digital signal E d The processing is performed to complete demodulation of the optical transmission signal. The self-coherent detection system supporting polarization multiplexing can directly support polarization demultiplexing by utilizing the mutual independence of light in orthogonal polarization state in beat frequency without adding additional photoelectric devicesThe polarization multiplexing signal detection based on single PD can be realized, and the utilization rate of spectrum and light source power is effectively improved. The requirement on the bandwidth of the device can be reduced by changing the preset frequency offset value, beat interference can be eliminated through simple calculation, and the optical transmission signal is recovered, so that the self-coherent detection system supporting polarization multiplexing has the advantages of low equipment cost, small requirement on the bandwidth of the device and low calculation complexity.
It should be noted that, the polarization module 410 is a polarization controller.
The system was verified using optical communication simulation software, wherein the light source used in the simulation was a continuous fiber laser (CW-laser) with a center wavelength of 1550.00nm and a linewidth of 0.1MHz, and the other light source used was a continuous fiber laser with center wavelengths of 1549.92nm, 1549.88nm and 1549.84nm (preset frequency deviations of 10GHz,15GHz and 20GHz, respectively, in the signal path) and a linewidth of 0.1MHz.
At the transmitting end, the light source is divided into a signal path and a local oscillation path through the optical divider, and the signal path is divided into two paths of X polarization state and Y polarization state which are mutually orthogonal through the polarization beam splitter. The signal in the X polarization state is processed by a pseudo-random sequence generator to generate a binary sequence, and after bit mapping, a Quadrature Amplitude Modulation (QAM) sequence generator generates a 16QAM modulation signal with the speed of 80Gbit/s, and the baud rate of 20Gbaud/s. The 16QAM signal drives an IQ modulator in the X polarization state in the dual-polarization IQ modulator to generate a quadrature modulated optical wave signal, and the characteristic of carrier suppression of the IQ modulator is utilized to output a radio frequency signal without carrying a carrier. The signal in the Y polarization state is routed to a pseudo random sequence generator to generate a binary sequence, and after bit mapping, a Pulse Amplitude Modulation (PAM) sequence generator generates a PAM4 modulation signal with the speed of 40Gbit/s, and the baud rate is 20Gbaud/s. The PAM4 signal drives an IQ modulator in the Y polarization state to produce an intensity modulated lightwave signal. The frequency value of the local oscillator signal and the frequency value of the optical transmission signal differ by a preset frequency value of 10GHz,15GHz and 20GHz respectively.
At the receiving end, the signal firstly adjusts the received light power through a tunable optical attenuator (VOA) and adjusts the polarization state of the signal through a Polarization Controller (PC) so that the polarization state of the X polarization state subjected to IQ modulation is consistent with the polarization state of local oscillation light, and then the signal is input into a coupling ratio of 50:50 is mixed with local oscillator light. And then a PIN photoelectric detector with the bandwidth of 40GHz and the sensitivity of 1A/W is adopted to carry out photoelectric conversion on the mixed optical signals.
The signal output by the PIN detector is received by a digital storage oscilloscope, namely an analog-to-digital converter (ADC), and the sampling rate of the oscilloscope is set to be 3 times of the signal symbol rate and 60 GSamp/s. In the middle-short distance optical fiber transmission, polarization mode dispersion and other polarization mode related crosstalk can be ignored, so that only signal components with the same polarization can beat frequency at a receiving end, and received photocurrent signals are superposition of photocurrents obtained by beat frequency of each of the X polarization state and the Y polarization state. Since the photocurrent signal contains three consecutive sampled signals that can be converted into an in-phase component I and a quadrature component Q of the complex signal in the X-polarization state, the three sampled signals in the serial signal data need to be extracted in a specific order. The signal receiving sequence is shown in fig. 3, and when sampling is performed at a sampling rate of three times the symbol rate, the in-phase component and the quadrature component of each symbol in the X polarization state and the intensity information in the Y polarization state can be obtained by simple operation of three continuous sampled analog signalsThus, the complex signal in the X-polarization state and the intensity signal in the Y-polarization state can be obtained by sequentially three samples.
The subsequent digital signal processing in this scheme is shown in fig. 5, where the signal output by the oscilloscope is resampled to three times of symbol rate, then symbol timing and normalization processing are performed on the signal, and then equalization is performed. And finally, carrying out simple calculation in the mode to respectively obtain a complex signal in the X polarization state and an intensity signal in the Y polarization state, and finishing the recovery of the polarization multiplexing signal.
FIGS. 6-11 are simulation results, in which the optical signal-to-noise ratio is set to 35dB, the carrier signal power ratio is set to 0dB, and the received optical power is set to 2dBm, wherein FIGS. 6-7 are dual-polarization simulation result constellations in which the optical signal-to-noise local oscillation light and the signal light have a frequency offset of 10 GHz; FIGS. 8-9 are diagrams of double polarization simulation result constellations in which 15GHz frequency offset exists between local oscillation light and signal light; FIGS. 10-11 are diagrams of dual polarization simulation result constellations in which the local oscillation light and the signal light have 20GHz frequency offset. It can be seen that for different frequency offsets, the self-coherent detection method can well eliminate the influence of beat interference, and can demodulate different modulation signals in the X polarization state and the Y polarization state under the condition of different frequency offsets.
The foregoing describes in detail preferred embodiments of the present invention. It should be understood that numerous modifications and variations can be made in accordance with the concepts of the invention by one of ordinary skill in the art without undue burden. Therefore, all technical solutions which can be obtained by logic analysis, reasoning or limited experiments based on the prior art by the person skilled in the art according to the inventive concept shall be within the scope of protection defined by the claims.
Claims (10)
1. The self-coherent detection system supporting polarization multiplexing is characterized by comprising a dual-polarization IQ modulation module, a polarization module, a local oscillator module, an optical coupling module, a photoelectric detection module, an analog-to-digital conversion module and a digital processing module, wherein the dual-polarization IQ modulation module, the optical coupling module, the photoelectric detection module, the analog-to-digital conversion module and the digital processing module are sequentially connected; wherein,
the dual-polarization-state IQ modulation module is used for dividing a light source signal into two paths of mutually orthogonal polarized light waves and modulating data to be transmitted onto the two paths of mutually orthogonal polarized light waves respectively;
the polarization module is connected with the optical coupling module and is used for adjusting the polarization state of the light source signal to obtain an optical transmission signal and transmitting the optical transmission signal to the optical coupling module;
the local oscillation module is used for outputting local oscillation signals;
the optical coupling module is used for receiving the optical transmission signal and the local oscillation signal, the frequency value of the local oscillation signal and the frequency value of the optical transmission signal differ by a preset offset frequency value, and a coupling signal is obtained according to the optical transmission signal and the local oscillation signal;
the photoelectric detection module is used for converting the coupling signal into an analog signal;
the analog-to-digital conversion module is used for sampling the analog signal according to a preset sampling frequency value and converting the analog signal into a digital signal;
the digital processing module is connected with the analog-to-digital conversion module and is used for processing the digital signals.
2. The system of claim 1, wherein the dual-polarization IQ modulator is configured as a dual-polarization IQ modulator for dividing the light source signal into two mutually orthogonal polarized light waves, IQ modulating one of the polarized light waves to obtain a complex signal, intensity modulating the other polarized light wave to obtain an intensity signal, and coupling the two polarized light waves through a polarization multiplexer to obtain an optical transmission signal.
3. The system of claim 1, wherein the optical source signal is a combined optical signal obtained by combining the optical transmission signal and the local oscillator signal, and the polarization module is configured to adjust a polarization state of the optical source signal to obtain the optical transmission signal and the local oscillator signal, and transmit the optical transmission signal and the local oscillator signal to the optical coupling module, respectively.
4. The polarization-multiplexed enabled self-coherent detection system of claim 2, wherein said predetermined sample frequency value is set to be equal to or greater than twice the symbol rate value of said optical transmission signal.
5. The polarization multiplexing supported self-coherent detection system of claim 1, further comprising a tunable optical attenuation module for adjusting the amplitudes of the optical transmission signal and the local oscillator signal, the tunable optical attenuation module being coupled to an output of the polarization module.
6. The polarization-multiplexed enabled self-coherent detection system of claim 1, wherein said predetermined offset frequency value is set to an arbitrary value.
7. The polarization-multiplexed enabled self-coherent detection system of claim 1, wherein the photo-detection module is configured as a single photo-detector, using a single photo-detector as the receiving end.
8. A method of using a polarization multiplexing-supported self-coherent detection system according to any one of claims 1 to 7, comprising the steps of:
respectively setting 3 preset offset frequency values and sampling frequency values;
transmitting the optical transmission signal and the local oscillation signal to an optical coupling module respectively;
the optical coupling module couples the optical transmission signal and the local oscillation signal to obtain a coupling signal, and transmits the coupling signal to the photoelectric detection module;
the photoelectric detection module performs photoelectric conversion on the coupling signal to obtain an analog signal, and transmits the analog signal to the analog-to-digital conversion module;
the analog-to-digital conversion module samples the analog signal at a preset sampling frequency value and converts the analog signal into a digital signal to calculate an analog signal E of every three samples s Phase differences respectively existing between the two;
three sampled analog signals E s Respectively obtaining complex signals and intensity signals of two mutually orthogonal polarization states of the optical transmission signal through calculation;
and the digital processing module processes the digital signals converted from the sampled analog signals to complete demodulation of the optical transmission signals.
9. A method of supporting polarization-multiplexed self-coherent detection according to claim 8, wherein the analog signal E is calculated every three samples s The phase differences respectively exist between the two phases specifically comprise the following steps:
let it be assumed that the optical transmission signal O t Symbol rate R of (2) symbol By changing the frequency value offset by the frequency offset unit, the local oscillation signal O is enabled lo And optical transmission signal O t A certain frequency offset exists between the two;
the preset frequency offset is R symbol /2，3R symbol R/4 and R symbol ；
Analog signal E output to photodetector by preset sampling frequency value s Sampling, and setting a preset sampling frequency value as an optical transmission signal O t The symbol rate is three times 3R symbol The sampling period T is 1/3R symbol ；
Three analog signals E sampled in succession s Is expressed by the following formula:
wherein R represents the responsivity of the photodetector,for power of complex signal IQ modulated on X polarization state, +.>For transmission in the Y polarization statePower of intensity signal, P LO Representing local oscillator signal O lo Beta represents the optical transmission signal O t θ represents phase noise, 2pi n ΔfT represents a phase due to a local oscillation signal O lo And optical transmission signal O t Phase shift due to frequency shift existing between them;
analog signal E of three consecutive samples s Phase difference between
10. A method of supporting polarization-multiplexed self-coherent detection according to claim 9, wherein three sampled analog signals E s The complex signals and the intensity signals on two mutually orthogonal polarization states of the optical transmission signals are respectively obtained through calculation, namely three sampled analog signals, and an in-phase component and a quadrature component on an X polarization state of IQ modulation are obtained through calculation; subtracting the calculated intensity information on the X polarization state from the obtained sampled analog signal to obtain intensity information on the Y polarization state subjected to intensity modulation, wherein the method specifically comprises the following steps of:
representing three consecutive sampled analog signals as in-phase component I of the same symbol in an IQ-modulated complex signal in the X polarization state k And quadrature component Q k And intensity information on the Y polarization state
In the above formula, γ is a normalized coefficient.
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