CN110855367A

CN110855367A - Optical balance detection method and system capable of eliminating beat frequency interference between signals of PDM SSB-OOFDM signals

Info

Publication number: CN110855367A
Application number: CN201911052192.XA
Authority: CN
Inventors: 马健新; 胡志奇
Original assignee: Beijing University of Posts and Telecommunications
Current assignee: Beijing University of Posts and Telecommunications
Priority date: 2019-10-31
Filing date: 2019-10-31
Publication date: 2020-02-28
Anticipated expiration: 2039-10-31
Also published as: CN110855367B

Abstract

The invention provides an optical balance detection method and an optical balance detection system capable of eliminating beat frequency interference among signals of PDM SSB-OOFDM signals. Two SSB-OOFDM signals in the PDM SSB-OOFDM signals are orthogonal in polarization, the frequency spectrums of the two OOFDM signals are overlapped, and optical carriers in the X and Y polarization directions are located on two sides of the OOFDM signals; the receiving end uses the power divider to divide the optical signal into four equal parts, then the four optical band-pass filters filter the optical carriers with different polarization directions, and inject the optical carriers into the two optical balanced detectors to perform photoelectric conversion, the differential output current of the two optical balanced detectors only contains the transmitted OFDM radio frequency signal component and the direct current component, and the SSBI is eliminated. By adopting the method and the system, the receiving without SSBI can be realized, the bandwidth does not need to be protected theoretically, and the frequency spectrum utilization efficiency of the PDM SSB-OOFDM system is further improved; meanwhile, because the polarization demultiplexing is not carried out by using a polarization beam splitter, the problems of polarization mode dispersion and complex MIMO processing are avoided.

Description

Optical balance detection method and system capable of eliminating beat frequency interference between signals of PDM SSB-OOFDM signals

Technical Field

The invention relates to the field of optical communication, in particular to a problem of eliminating beat frequency interference (SSBI) among signals in a PDM SSB-OOFDM signal detection process with a smaller wavelength guard interval and a problem of improving spectral efficiency in a PDM SSB-OOFDM system.

Background

Currently, an Optical Orthogonal frequency division Multiplexing (Optical Orthogonal frequency division Multiplexing O-OFDM) technology based on high-order QAM modulation receives wide attention due to its high spectrum utilization rate and high tolerance to Optical fiber dispersion. Although a coherent light OFDM (CO-OFDM) system can reach the spectrum utilization rate (Spectral Efficiency SE) of 14bit/s/Hz, the CO-OFDM system is sensitive to frequency offset and phase noise and requires extremely narrow line widths of lasers at a transmitting end and a receiving end, which makes the design of the transmitting and receiving systems of the CO-OFDM very complicated.

Because the Direct detection OFDM (Direct detection DD-OFDM) signal contains the optical carrier component, the receiving end of the Direct detection OFDM signal can realize heterodyne beat frequency detection between the OOFDM signal and the optical carrier only by one square optical detector (PD), so that the receiving end of the Direct detection OFDM signal is greatly simplified compared with coherent detection; in addition, because the OOFDM signal and the optical carrier come from the same light source, the frequency offset and the phase drift are completely synchronous, and can be completely offset in an heterodyne beat frequency optical path, the requirement on the line width of the laser is low.

In the DD-OOFDM system, compared with a Double-Sideband (Double-Sideband) modulation technique, a Single-Sideband Optical OFDM (Optical Single-Sideband Optical OFDM SSB-OOFDM) modulation technique is more preferable because of higher spectrum utilization efficiency of an Optical signal and capability of overcoming an amplitude fading effect caused by chromatic dispersion of an Optical fiber.

In general, an O-OFDM signal transmitted in a DD-OFDM system is restored to the electrical domain by a square-rate photodetector at a receiving end, and a signal-to-signal beat interference (SSBI) is generated. Currently, several methods for reducing signal impairment by SSBI have been reported in the literature. A Guard Band (GB) large enough is set between the optical carrier and the optical OFDM signal, so that the spectrum of the SSBI in the photocurrent does not overlap with the spectrum of the radio frequency OFDM signal. To ensure the elimination of the effect of SSBI, the bandwidth of GB must not be smaller than the bandwidth of the optical OFDM signal, which reduces the spectral efficiency of the DD-OFDM system by more than half of that of the CO-OFDM system. Reducing GB in SSB-OOFDM may improve the SE of the system, but as GB decreases, the impact of SSBI on system performance increases. Although some schemes for reducing the impact of SSBI on system performance have been proposed, there are also deficiencies in system complexity and power cost. The spectrum efficiency of the polarization multiplexing technology SSB-OOFDM system is improved, but the influence of SSBI still exists, and meanwhile, polarization mode dispersion of the optical fiber link can cause the polarization directions of the two SSB-OOFDM signals which are polarization multiplexed together to randomly rotate, thereby causing the difficulty of polarization demultiplexing of the two SSB-OOFDM signals at the receiving end. Several methods for reducing the impact of SSBI on system performance are reported in the literature, but none address the impact of polarization mode dispersion, even if SSBI is eliminated, because of the coupling between polarization modes. The conventional method for solving the polarization direction rotation caused by polarization mode dispersion is to use a polarization stabilizer (polarization stabilizer) to extract polarization rotation information and then perform polarization direction rotation correction, but the polarization stabilizer has a relatively high cost and makes the receiving end more complicated. In addition, the two mixed signals can be separated by performing the Output signal processing through a multiple Input multiple Output (Multi Input Multi Output) technology at the receiving end, but a quite complicated digital signal processing process is required, and the implementation difficulty of the system is increased.

Disclosure of Invention

In order to solve the problems, the invention provides a simple novel receiving method and a system based on optical balance detection, which realize the correct receiving of signals by reasonably designing the spectrum structure of PDM SSB-OOFDM signals, thereby not only eliminating SSBI, but also reducing the limit of frequency band protection bandwidth, improving the spectrum utilization rate of the PDM SSB-OFDM system, and simultaneously avoiding the influence caused by polarization mode dispersion.

The invention provides a PDM SSB-OOFDM signal receiving method and system based on an optical balance detector, which can eliminate beat frequency interference among signals, and can eliminate SSBI generated by photoelectric conversion of the PDM SSB-OOFDM signal by reasonably designing the spectrum of the PDM SSB-OOFDM signal and utilizing the differential cancellation function of the optical balance detector, further reduce the protection bandwidth between an optical carrier and the OOFDM signal, relax the limit of a PDM SSB-OOFDM system on GB, and improve the spectral efficiency of the system, wherein the system comprises:

as a preferred method, the spectral structure of the PDM SSB-OOFDM signal generated by the transmitting end is designed reasonably, so that the frequency spectrums of the OOFDM signal sidebands in the two SSB-OOFDM signals in the X and Y polarization directions are completely overlapped to improve the spectral efficiency, and the frequency of one optical carrier is higher than that of the OOFDM signal, and the frequency of the other optical carrier is lower than that of the OOFDM signal, that is, the optical carriers are respectively placed on both sides of the OOFDM signal.

As a preferred method, at the receiving end, the received PDMSSB-OOFDM signal is divided into four paths of equal power by a polarization insensitive optical power splitter, and different optical carriers are filtered by four different polarization insensitive optical band pass filters: the first path filters the optical carrier wave in the Y polarization direction, and the OOFDM signal in the Y polarization direction, the optical carrier wave in the X polarization direction and the OOFDM signal are left; the second path and the third path are the same, and simultaneously optical carriers in the X and Y polarization directions are filtered, and only OOFDM signals in the X and Y polarization directions are left; and the fourth path filters the optical carrier wave in the X polarization direction, and the OOFDM signal in the X polarization direction, the optical carrier wave in the Y polarization direction and the OOFDM signal are remained.

As a preferred method, the four optical bandpass filters may adopt a filter with a steep falling edge, so that the protection bandwidth may be sufficiently reduced, and simultaneously, the damage of the optical filter to the optical carrier and the optical OFDM signal may be avoided to the greatest extent.

As a preferred method, the first and second filtered optical signals are injected into a first optical balanced detector, the differential current output by the optical balanced detector only contains the OFDM signal carried by the X-polarized light wave and a dc component, and the SSBI in two polarization directions are eliminated; similarly, after filtering, the third and fourth optical signals are photoelectrically converted by the second optical balance detector, the output differential photocurrent only contains the Y polarization direction OFDM signal and one dc component, and the SSBI components in the two polarization directions are eliminated. The method can eliminate the influence of beat frequency interference between signals on OFDM signals, so that a guard band between an optical carrier and OOFDM signals can be reduced or canceled, the spectrum utilization rate of a system is improved, and the selection of a guard interval (GB) between the optical carrier and the OOFDM signals needs to be balanced by considering the spectral efficiency of the system and the requirement of the steepness of edge roll-off of a filter in the filtering process. The polarization direction rotation of the optical signal shows randomness at different moments due to the polarization film dispersion of the optical fiber, but at a certain moment, the polarization direction rotations of the optical signals in all polarization directions are synchronous, that is, it can be considered that two paths of SSB-OOFDM signals in the PDM SSB-OOFDM signals can maintain the orthogonality of polarization at any moment or in a small time period, but the polarization direction rotates randomly relative to the initial moment; meanwhile, the polarization beam splitter is not adopted at the receiving end for polarization demultiplexing, and the optical power splitter and the optical bandpass filter are polarization insensitive devices, so that the influence of polarization mode dispersion is avoided.

Preferably, the two photodetectors in the light balance detector have the same performance parameters and sufficient response bandwidth, and the output is in a differential form, that is, the current I (t) of each light balance detection output is the photocurrent I (t) output by the two photodetectors in the light balance detector₁(t) and I₂(t) the subtracted difference I (t) I₁(t)-I₂(t)。

As a preferred method, the present invention provides an optical balanced detection transceiver system capable of eliminating beat interference between PDM SSB-OOFDM signals, comprising:

the optical power divider is used for equally dividing the received PDM SSB-OOFDM signals into four parts which are the same and insensitive to polarization mode dispersion;

the four optical bandpass filter devices are used for realizing optical carrier suppression in different polarization directions and are insensitive to polarization mode dispersion;

and the two optical balance detection circuits are used for carrying out optical balance detection on the four paths of optical signals output by the optical filter, are insensitive to polarization mode dispersion, and output differential photocurrents respectively comprise RF-OFDM signals modulated onto optical waves by the transmitting end.

After the technical scheme provided by the invention is adopted, SSBI generated by the PDM SSB-OOFDM signal in the photoelectric conversion process is eliminated, the output photocurrent only comprises OFDM signal components and direct current components, a required protection bandwidth GB is not required to be reserved for avoiding the frequency spectrum overlapping of the SSBI and the OFDM signal, the limit of the PDM SSB-OOFDM system on the GB is relaxed, a precondition is provided for reducing the protection bandwidth GB between an optical carrier and the OOFDM signal, and the frequency spectrum efficiency of the system can be improved. Meanwhile, the instantaneous synchronism of the polarization direction of the optical signal caused by the polarization film dispersion of the optical fiber adopts an optical power divider and an optical bandpass filter which are insensitive to polarization, so that the influence of polarization mode dispersion is effectively avoided; compared with other PDMSSB-OOFDM receivers which utilize a polarization beam splitter to carry out polarization demultiplexing, the proposed PDM SSB-OOFDM system receiver is quite simple because complex MIMO algorithm and digital signal processing process are needed to solve the problem caused by polarization mode dispersion.

Drawings

Fig. 1 is a schematic block diagram of PDM SSB-OOFDM signal transceiving based on optical balance detection according to the present invention;

FIG. 2 is a schematic block diagram of a transmitting end of the present invention;

FIG. 3 is a spectrum diagram of a launch end injection fiber of the present invention;

FIG. 4 and FIG. 5 are graphs of the spectrum of RF-OFDM signals in X and Y polarization directions after polarization demultiplexing at the receiving end according to the present invention;

fig. 6 and 7 are constellation diagrams of 16-QAM symbols transmitted in X and Y polarization directions after polarization demultiplexing at the receiving end according to the present invention, respectively;

fig. 8 relationship between Error Vector Magnitude (EVM) and guard interval.

FIG. 9 is a constellation diagram of an EVM curve and a demodulated 16-QAM signal after the PDM SSB-OOFDM signal is transmitted through optical fibers at different distances

Detailed Description

To make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Fig. 1 is a block diagram and a spectrum diagram of a receiving system according to the present embodiment.

The invention adopts a novel receiving method for PDM SSB-OOFDM signals based on light balance detection to eliminate the influence of SSBI on signal demodulation, improve the spectrum utilization efficiency of a system and simultaneously avoid the influence of polarization mode dispersion when the PDM SSB-OOFDM signals are transmitted in optical fibers. In order to achieve the above effect, the following steps are required:

firstly, PDM SSB-OOFDM signals are generated at a transmitting end in the following specific process2, respectively. To improve the spectral efficiency, the frequency f of the optical carriers in the X and Y polarization directions of the PDM SSB-OOFDM signal overlap spectrally, and to meet this_CX、f_CYFrequency f of radio frequency carrier of radio frequency OFDM signal_RFX、f_RFYThe conditions are required to be satisfied: f. of_CX+f_RFX＝f_CY-f_RFY. Firstly, the continuous laser outputs 193.100THz of central carrier frequency and 100MHz of line width, MZM biased at the minimum transmission point is modulated by a radio frequency local oscillator with the frequency of 9GHz, and two first-order optical sidebands with the frequency of 193.091THz and 193.109THz are output. An optical carrier E in the X and Y polarization directions is passed through an optical wavelength Interleaver (IL)_CX(t) and E_CY(t) are separated and their spectra are shown in the insets (a) and (b) of FIG. 2, respectively.

For generating a baseband OFDM signal S in two polarization directions_X(t)、S_Y(t), a transmitting terminal maps two independent binary sequences with the bit rate of 56Gb/s into two 16QAM signal sequences, and then the two 16QAM signal sequences are respectively subjected to IFFT conversion through 256 points, wherein 128 subcarriers carrying information bear 16QAM signals, and the rest subcarriers are subjected to zero padding to realize oversampling of OFDM symbols; generation of an analog baseband OFDM signal S by digital-to-analog conversion_X(t)、S_Y(t) of (d). Since this example is intended to demonstrate that the system and method of the present invention can eliminate the effect of SSBI on signal demodulation and verify the insensitivity of the system to polarization mode dispersion, the OFDM signal generated adds a cyclic prefix of 32 points in view of fiber transmission. The two baseband OFDM signals S_X(t)、 S_Y(t) applying the signal to a radio frequency signal with a frequency of 9GHz by quadrature modulation in the electrical domain to generate a frequency band OFDM signal V_RFX(t)、V_RFY(t)。

Radio frequency OFDM signal V_RFX(t)、V_RFY(t) modulating optical carrier E modulated by MZM to frequencies 193.091THz and 193.109THz in DSB form_CX(t) and E_CY(t), the bandwidth of the sideband OOFDM signal is 14GHz, and the protection bandwidth between the OOFDM signal and the optical carrier is 2 GHz; adjusted by a polarization controller to make the two polarizations orthogonal, polarized by PBC and then passed through a central frequency of 193.100An optical bandpass filter with THz and a bandwidth of 20GHz filters out an external unwanted optical variable band, a schematic diagram of a spectrogram is shown in fig. 1(c), and an optical power spectrum obtained by simulation superimposes signals in two polarization directions together, as shown in fig. 3. The resulting PDM SSB-OOFDM signal may be represented as

The guard interval between the optical carrier and the OOFDM signal may be adjusted by changing the frequency of the radio frequency local oscillator signal, while the bandwidth of the filter may also be adaptively changed.

At the receiving end, the optical receiving method and system proposed by the patent perform photoelectric conversion on the PDM SSB-OOFDM signal. The PDM SSB-OOFDM signals transmitted by the optical fiber are equally divided into four paths of same PDM SSB-OOFDM signals by the optical power divider, and the first path filters optical carrier waves E in the Y polarization direction_CY(t), leaving OOFDM signal E in the Y polarization direction_SY(t) and optical carrier and OOFDM signal E in the X polarization direction_CX(t)+E_SX(t) that isThe second path is the same as the third path, and simultaneously filters the optical carriers E in the X and Y polarization directions_CX(t) and E_CY(t), leaving only the OOFDM signals E in the X and Y polarization directions_SX(t) and E_SY(t) that is

Fourth path of optical carrier E for filtering out X polarization direction_CX(t), leaving OOFDM signal E in the X polarization direction_SX(t) optical carrier and OOFDM signal E in both Y and Y polarization directions_CY(t)+E_SY(t) that isThe spectral diagrams of the four optical signals after being filtered by the optical filters are shown in fig. 1(b) - (e), wherein the dotted lines represent the optical carriers filtered by the filters. The four optical signals are then injected into two optical balanced detectors, respectively. Each light balance probeThe detector comprises two Photodetectors (PDs) with the same performance parameters, and the two photodetectors are connected in a differential mode to realize subtraction operation of two paths of photocurrents. The first and second paths of optical signals are injected into the first optical balance detector, wherein the output photocurrents of the two photodetectors are

Output photocurrent as

The photocurrent only contains the RF-OFDM signal and dc components carried by the X-polarization direction light waves, while the SSBI term is eliminated. Two photoelectric detectors output photocurrent I₁(t)、I₂(t) and differential output photocurrent I_XThe schematic diagram of the radio frequency power spectrum of (t) is shown in fig. 1(f), (g) and (j), respectively, and the radio frequency power spectrum of the output photocurrent of the first photo-balance detector obtained by simulation is shown in fig. 4.

Similarly, the third and fourth optical signals are injected into the second optical balanced detector, and the output photocurrent of the two photodetectors realizes the output photocurrent of the optical current by the electric domain subtraction through the differential circuit

The photocurrent only contains the RF-OFDM signal and dc components carried by the Y polarization direction optical waves, while the SSBI term is eliminated. Two photoelectric detectors output photocurrent I₃(t)、I₄(t) and differential output photocurrent I_YThe schematic diagram of the radio frequency power spectrum of (t) is shown in fig. 1(h), (i) and (k), respectively, and the radio frequency power spectrum of the output photocurrent of the second photo-balance detector obtained by simulation is shown in fig. 5.

By adopting the transceiving system and the transceiving method provided by the invention, SSBI is eliminated for photoelectric conversion of PDM SSB-OOFDM signals, optical domain demultiplexing of RF-OFDM signals carried in two polarization directions is realized, and the necessity of setting GB for avoiding spectrum overlapping of the SSBI and the RF-OFDM signals in photocurrent is theoretically eliminated. Even if the bandwidth 14GHz of the PDM SSB-OOFDM signal is greater than the guard bandwidth GB of 2GHz in this example, the demodulation of the OFDM signal is not seriously affected. 16QAM constellations obtained by demodulating the baseband OFDM signals obtained by coherently demodulating the RF OFDM signals carried in the X and Y polarization directions by an OFDM signal receiver are shown in fig. 6 and 7, showing that the performance is good. Adjusting the radio frequency local oscillator frequency to change between 7GHz and 25GHz, changing the corresponding guard interval GB between 0GHz and 18GHz, calculating an Error Vector Magnitude (EVM) under different GB conditions by a simulation system, and giving a change relationship of the calculated EVM with respect to GB in fig. 8. It can be seen that the EVM of the system remains below 16.3% at all times when the guard bandwidth is greater than 2 GHz.

In addition, the polarization film dispersion of the optical fiber shows randomness to the polarization direction of the optical signal at different moments, but at a certain moment, the polarization direction rotations of the optical signals in all polarization directions are synchronous, and it can be considered that the two SSB-OOFDM signals in the PDMSSB-OOFDM signal can maintain polarization orthogonality at any moment or in a small time period; meanwhile, the polarization beam splitter is not adopted at the receiving end for polarization demultiplexing, and the optical power splitter and the optical bandpass filter are polarization insensitive devices, so that the influence of polarization mode dispersion is effectively avoided, and the problem caused by the polarization mode dispersion is not required to be processed by a complex MIMO technology. Fig. 9 shows EVM curves of PDM SSB-OOFDM signals transmitted through optical fibers at different distances and constellations of demodulated 16-QAM signals.

The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the appended claims.

Claims

1. An optical balance detection method capable of eliminating beat frequency interference among signals of a PDM SSB-OOFDM signal is characterized in that through properly designing a PDM SSB-OOFDM signal structure, different optical carrier components are filtered by an optical filter at a receiving end, photoelectric conversion is carried out by two optical balance detectors, beat frequency interference among signals can be eliminated, radio frequency OFDM signals carried in two polarization directions are output, the limitation of a protection bandwidth GB between an optical carrier and the OOFDM signal in a PDM SSB-OOFDM system is further relaxed, the spectrum efficiency of the system is improved, and meanwhile the influence of polarization mode dispersion can be avoided, the method comprises the following steps:

the PDM SSB-OOFDM signal generated by the transmitting end is formed by polarization multiplexing of two SSB-OOFDM signals, wherein the two OOFDM signals are orthogonal in polarization and completely overlapped in frequency spectrum, two optical carriers are respectively parallel to the respective OOFDM signal in polarization, the frequency of one optical carrier is higher than that of the OOFDM signal, and the frequency of the other carrier is lower than that of the OFDM signal;

at a receiving end, a power divider insensitive to polarization divides received PDM SSB-OOFDM signals into four paths of equal power, and different optical carriers are filtered by four paths of different polarization insensitive optical band pass filters: the first path filters the optical carrier wave in the Y polarization direction, and the OOFDM signal in the Y polarization direction, the optical carrier wave in the X polarization direction and the OOFDM signal are left; the second path and the third path are the same, optical carriers in the X and Y polarization directions are filtered, and only OOFDM signals in the X and Y polarization directions are left; the fourth path filters the optical carrier wave in the X polarization direction, and an OOFDM signal in the X polarization direction and an optical carrier wave and an OOFDM signal in the Y polarization direction are left;

injecting the filtered first and second paths of optical signals into a first optical balance detector, wherein differential current output by the optical balance detector only comprises an OFDM signal and a direct current component carried by an X-polarized light wave, and beat frequency interference among the signals is eliminated; similarly, after filtering, the third and fourth optical signals are photoelectrically converted by the second optical balance detector, and the output differential photocurrent only contains the Y polarization direction OFDM signal and a dc component.

2. The method of claim 1, wherein the transmitting end:

the PDM SSB-OOFDM signal is formed by polarization multiplexing of two SSB-OOFDM signals carrying different information, each SSB-OOFDM signal comprises an OOFDM signal carrying the information and an optical carrier not carrying the information, a guard interval GB is arranged between the optical carrier and the OOFDM signal in a frequency domain, and the GB is more than or equal to 0; in the PDM SSB-OOFDM signal, two OOFDM signals have orthogonal polarizations and completely coincident frequency spectrums, two optical carriers are respectively parallel to the respective OOFDM signals in polarization, wherein the frequency of one optical carrier is higher than that of the OOFDM signal, the frequency of the other optical carrier is lower than that of the OOFDM signal, and a guard interval GB is provided between the optical carrier and the OOFDM signal.

3. The method of claim 1, wherein the optical power splitter:

the received PDM SSB-OOFDM signals are divided into four paths by the optical power splitter in an equal-power mode, optical signals output by the four output ports are completely the same, and the optical power splitter has no selectivity on wavelength and polarization.

4. The method of claim 1, wherein the four optical bandpass filters:

the four paths of PDM SSB-OOFDM signals pass through four polarization-insensitive optical band-pass filters to respectively filter different optical carriers: the first optical bandpass filter filters the optical carrier wave in the Y polarization direction, and an OOFDM signal in the Y polarization direction, an optical carrier wave in the X polarization direction and an OOFDM signal are reserved; the second optical band-pass filter and the third optical band-pass filter are the same and simultaneously filter optical carriers in the X polarization direction and the Y polarization direction, and only OOFDM signals in the X polarization direction and the Y polarization direction are left; the fourth optical bandpass filter filters out the optical carrier in the X polarization direction, leaving the OOFDM signal in the X polarization direction and the optical carrier and the OOFDM signal in the Y polarization direction.

5. The method of claim 1, wherein the two light balance detectors:

the four paths of filtered optical signals are subjected to photoelectric conversion by two optical balance detectors, and the two photoelectric detectors in each optical balance detector have the same performance parameters and sufficient response bandwidth; the filtered first path of optical signal and the filtered second path of optical signal are injected into a first optical balance detector, the first path of optical signal and the second path of optical signal are converted into optical current by the balance optical detector, the optical current and the optical current are subtracted by a differential circuit and then output, and the output optical current differential current only comprises an OFDM signal and a direct current component which are carried by X-polarized light waves; and injecting the third optical signal and the fourth optical signal into a second optical balance detector, converting the third optical signal and the fourth optical signal into photocurrent by the balance optical detector, subtracting the photocurrent by a differential circuit, and outputting the photocurrent differential current, wherein the photocurrent differential current only comprises an OFDM signal and a direct current component which are carried by the Y-polarized light wave.

6. A balanced photoelectric detection receiving system capable of eliminating beat frequency interference among polarization division multiplexing SSB-OOFDM signals can eliminate beat frequency interference among signals generated by photoelectric conversion of PDM SSB-OOFDM signals, effectively reduces a protection bandwidth between an optical carrier and the OOFDM signals, improves the spectrum efficiency of the PDM SSB-OOFDM system, can realize demultiplexing of the polarization division multiplexing SSB-OOFDM signals, and avoids the influence of polarization mode dispersion, and specifically comprises:

the optical power divider is used for dividing the received PDM SSB-OOFDM signals into four paths of same optical signals in an equal power mode, and is insensitive to polarization mode dispersion and wavelength;

the four optical band-pass filters are respectively used for filtering optical carriers in different polarization directions and are insensitive to polarization mode dispersion, wherein the first optical band-pass filter is used for filtering optical carriers in the Y polarization direction in the first path of optical signals, the second optical band-pass filter and the third optical band-pass filter are the same and are respectively used for filtering optical carriers in the X polarization direction and the Y polarization direction in the second path of optical signals and the third path of optical signals, and the fourth optical band-pass filter is used for filtering optical carriers in the X polarization direction in the fourth path of optical signals;

the two balanced photoelectric detectors are used for performing photoelectric detection on optical signals output by the optical bandpass filter and are insensitive to polarization mode dispersion, each balanced photoelectric detector comprises two photoelectric detectors with the same bandwidth, response sensitivity and other parameters and a differential circuit, and the output of the two photoelectric detectors realizes subtraction of analog electric signals through the differential circuit and outputs differential light current.