阅读说明：本技术 超高速相干光信号偏振解复用和波长变换系统及控制方法 (Ultra-high-speed coherent optical signal polarization demultiplexing and wavelength conversion system and control method ) 是由 苏玉龙 汪伟 黄新宁 谢小平 胡辉 冯欢 于 2019-09-29 设计创作，主要内容包括：本发明提供了一种超高速相干光信号偏振解复用和波长变换系统及控制方法,光滤波器与偏振控制器相连,可调谐激光器与偏振合束器相连,偏振控制器与保偏光纤耦合器相连,偏振合束器和保偏光纤耦合器相连,保偏光纤耦合器与保偏光放大器相连,保偏光放大器与保偏高非线性光纤相连,保偏高非线性光纤与波分解复用器相连,波分解复用器的各个输出端口与相干检测相连,相干检测与ADC相连,ADC与数字信号处理单元相连；本发明可同时实现偏振解复用及波长变换功能,可实现超高速率光信号的处理及交换,可实现多种调制制式兼容。(The invention provides a system and a control method for polarization demultiplexing and wavelength conversion of an ultra-high-speed coherent optical signal, wherein an optical filter is connected with a polarization controller, a tunable laser is connected with a polarization beam combiner, the polarization controller is connected with a polarization maintaining optical fiber coupler, the polarization beam combiner is connected with the polarization maintaining optical fiber coupler, the polarization maintaining optical fiber coupler is connected with a polarization maintaining optical amplifier, the polarization maintaining optical amplifier is connected with a polarization maintaining high nonlinear optical fiber, the polarization maintaining high nonlinear optical fiber is connected with a wavelength division demultiplexer, each output port of the wavelength division demultiplexer is connected with coherent detection, the coherent detection is connected with an ADC, and the ADC is connected with a digital signal processing unit; the invention can realize polarization demultiplexing and wavelength conversion functions at the same time, can realize the processing and exchange of ultrahigh-speed optical signals, and can realize the compatibility of various modulation systems.)

1. A system for polarization demultiplexing and wavelength conversion of ultra-high speed coherent optical signals is characterized in that:

the system for realizing the polarization demultiplexing and the wavelength conversion of the ultra-high-speed coherent optical signal comprises a first stage, an intermediate stage and a second stage, wherein the first stage comprises an optical filter, a polarization controller, a tunable laser A, a tunable laser B and a polarization beam combiner; the intermediate stage comprises a polarization-maintaining fiber coupler, a polarization-maintaining optical amplifier and a polarization-maintaining high nonlinear fiber, wherein the polarization controller of the first stage is connected with a port 1 of the polarization-maintaining fiber coupler, the polarization beam combiner of the first stage is connected with a port 2 of the polarization-maintaining fiber coupler, a port 3 of the polarization-maintaining fiber coupler is connected with the polarization-maintaining optical amplifier, and the polarization-maintaining optical amplifier is connected with the polarization-maintaining high nonlinear fiber; the second stage comprises a wavelength division demultiplexer, coherent detection, an analog-to-digital converter and a digital signal processing unit, wherein the polarization maintaining high nonlinear optical fiber of the middle stage is connected with the wavelength division demultiplexer, each output port of the wavelength division demultiplexer is connected with the coherent detection, the coherent detection is connected with an ADC (analog-to-digital converter), and the ADC is connected with the digital signal processing unit;

the first stage performs optical filtering and polarization state control on an ultra-high-speed coherent optical signal transmitted by an optical fiber link, continuous light with different wavelengths generated by a tunable laser A and a tunable laser B is coupled into one path through a polarization beam combiner, the polarization states of the two coupled continuous light are kept orthogonal, an intermediate polarization-maintaining optical fiber coupler couples the two paths of light into one path of signal, then the signal is subjected to optical power amplification through a polarization-maintaining optical amplifier, the amplified signal generates a double-pump orthogonal four-wave mixing effect through a polarization-maintaining high nonlinear optical fiber, polarization demultiplexing and wavelength conversion are realized simultaneously, finally a wavelength division demultiplexer of the second stage selects the signal subjected to wavelength conversion according to a wavelength position, and the signal is respectively and sequentially sent to a coherent detection unit, an ADC (analog to digital converter) and a digital signal processing unit for receiving processing, and finally transmission and exchange of the signal are completed.

2. The ultra-high speed coherent optical signal polarization demultiplexing and wavelength conversion system according to claim 1, wherein:

the ultra-high-speed coherent optical signal is a polarization multiplexing signal DP-QPSK (dual polarization-Quadrature phase Shift keying) or DP-16QAM (dual polarization-16 Quadrature amplitude modulation).

3. The ultra-high speed coherent optical signal polarization demultiplexing and wavelength conversion system according to claim 1, wherein:

the tunable laser A and the tunable laser B are wide-range (-30 nm) ITU standard tunable lasers.

4. The ultra-high speed coherent optical signal polarization demultiplexing and wavelength conversion system according to claim 1, wherein:

the polarization maintaining optical amplifier is an optical fiber amplifier or a semiconductor optical amplifier.

5. The ultra-high speed coherent optical signal polarization demultiplexing and wavelength conversion system according to claim 1, wherein:

the polarization-maintaining high nonlinear optical fiber is a third-order nonlinear polarization coefficient chi⁽³⁾Non-zero nonlinear fiber.

6. The ultra-high speed coherent optical signal polarization demultiplexing and wavelength conversion system according to claim 1, wherein:

the optical filter is any one of a fiber grating filter, a thin film dielectric filter, a fused taper fiber filter, a Fabry-Perot filter, a multilayer dielectric film filter and a Mach-Zehnder interference filter.

7. A method for implementing polarization demultiplexing and wavelength conversion system for ultra high speed coherent optical signals according to claim 1, comprising the steps of:

(1) transmitting the ultra-high-speed coherent optical signal through an optical fiber link, and then entering a narrow-band optical filter for carrying out-of-band noise filtering;

(2) the output signal of the narrow-band optical filter is connected with a port 1 of the polarization-maintaining optical fiber coupler through a polarization controller, the polarization state of the signal is changed through the polarization controller, and the signal is matched and aligned with the slow axis direction of the polarization-maintaining optical fiber coupler;

(3) the tunable laser A and the tunable laser B respectively generate a wavelength lambda_aAnd λ_bThe two continuous lights are combined into a signal by a polarization beam combiner and connected with a port 2 of the polarization-maintaining optical fiber coupler, and the lambda is at the moment_aAnd λ_bThe polarization states of the two beams of light are kept orthogonal;

(4) the polarization maintaining fiber coupler outputs the coupled light, the coupled light is amplified to a hundred mW magnitude by a polarization maintaining amplifier, and then signal light lambda in the polarization maintaining nonlinear fiber_iAnd continuous light lambda_a、λ_bGenerating orthogonal four-wave mixing effect, because the pumping light only generates four-wave mixing process with the signal light with the same polarization direction, and the four-wave mixing process generates new idler frequency light, thereby separating the signal light lambda_iTwo orthogonal polarization state information are obtained, and each polarization state information is converted to lambda on the new wavelength optical signal_a’、λ_bIn the method, the conversion efficiency depends on the phase mismatch degree in four-wave mixing, and is related to dispersion, power, nonlinear coefficient and nonlinear fiber length, the continuous light wavelength is set within plus or minus 1nm of the zero dispersion point of the fiber, and the nonlinear coefficient is 10W^-1·km^-1The length of the nonlinear optical fiber is between 300 and 600 m;

(5) subjecting the λ generated in the highly nonlinear optical fiber in step (4)_a’、λ_b' two new wavelengths are separated by a wavelength division demultiplexing unit;

(6) lambda separated in the step (5)_a’、λ_b' demodulating and receiving the optical signal; firstly, coherent detection is carried out, then high-speed ADC sampling and digital signal processing processes are carried out, original signals are recovered, and exchange and transmission of the original two polarization state signals are completed.

Technical Field

The invention relates to the field of ultra-high-speed coherent optical communication, in particular to simultaneous realization of polarization separation and wavelength conversion of a polarization multiplexing signal in optical communication.

Background

With the continuous progress of the social informatization process, the demand of the germination and development of new services such as ultra-high definition video, virtual reality, cloud computing, internet of things and the like on bandwidth is increased sharply. The service demand drives the technological progress, the capacity of a single channel of optical transmission has been increased from 10Gbit to 100Gbit, and the next generation of over 100G optical transmission system is also in breakthrough, and currently, high-order coherent modulation and polarization multiplexing are adopted for backbone network long-distance transmission to increase the channel capacity, for example, signals such as DP-QPSK, DP-16QAM and the like, and compared with OOK, the capacity can be increased by several times under the same bandwidth, so that backbone network long-distance transmission in the future will be the mainstream of ultra-high-speed coherent optical communication.

At present, the frequency spectrum utilization rate of ultra-high-speed coherent optical transmission is improved by adopting a high-order modulation and polarization multiplexing mode, the sensitivity of coherent optical communication compared with intensity communication can be improved by 10-20 dB, and therefore the transmission distance between points can be greatly improved, but with the fact that the information rate loaded on each polarization state is higher and higher, the exchange and processing of polarization multiplexing signals in a backbone network or an access network are very difficult. For example, previously, low-speed polarization multiplexing 10Gbps optical information can be processed and exchanged in an "optical-electrical-optical" manner, but when the speed reaches 100Gbps or even higher, the circuit cannot process and exchange such high-speed information, so that a new method is needed to implement the processing of ultra-high-speed polarization multiplexing signals, and particularly, a way to efficiently separate and exchange information in different polarization states of the polarization multiplexing signals is sought. At present, polarization demultiplexing and switching of ultra-high-speed coherent polarization multiplexing signals are realized by adopting series connection, namely, polarization demultiplexing is realized by adopting optical polarization tracking or digital coherent processing, and then high-speed switching is respectively carried out on each path of information, so that a large number of resources such as photoelectric devices, high-performance FPGA (field programmable gate array) and the like are needed in the process, the realization is complex, and a method for simultaneously realizing polarization demultiplexing and switching is not seen.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a system and a control method for polarization demultiplexing and wavelength conversion of an ultra-high-speed coherent optical signal. The invention overcomes the problem that the prior art can not simultaneously polarize, demultiplex and exchange the ultrahigh-speed coherent polarization multiplexing signals, innovatively provides a method for simultaneously realizing the polarization demultiplex and the wavelength conversion of the ultrahigh-speed coherent optical signals, solves the problems of processing and exchanging the ultrahigh-speed optical signals caused by the bottleneck of the optical-electrical conversion rate, and provides reliable guarantee for the transmission and the exchange of the ultrahigh-speed coherent polarization multiplexing signals in a backbone network and an access network.

The technical scheme adopted by the invention for solving the technical problems is as follows:

a system for realizing ultra-high-speed coherent optical signal polarization demultiplexing and wavelength conversion comprises a first stage, an intermediate stage and a second stage, wherein the first stage comprises an optical filter, a polarization controller, a tunable laser A, a tunable laser B and a polarization beam combiner, the optical filter is connected with the polarization controller, the tunable laser A is connected with a port 1 of the polarization beam combiner, and the tunable laser B is connected with a port 2 of the polarization beam combiner; the intermediate stage comprises a polarization-maintaining fiber coupler, a polarization-maintaining optical amplifier and a polarization-maintaining high nonlinear fiber, wherein the polarization controller of the first stage is connected with a port 1 of the polarization-maintaining fiber coupler, the polarization beam combiner of the first stage is connected with a port 2 of the polarization-maintaining fiber coupler, a port 3 of the polarization-maintaining fiber coupler is connected with the polarization-maintaining optical amplifier, and the polarization-maintaining optical amplifier is connected with the polarization-maintaining high nonlinear fiber; the second stage comprises a wavelength division demultiplexer, coherent detection, an analog-to-digital converter (ADC) and a digital signal processing unit, wherein the polarization maintaining high nonlinear optical fiber of the middle stage is connected with the wavelength division demultiplexer, each output port of the wavelength division demultiplexer is connected with the coherent detection, the coherent detection is connected with the ADC, and the ADC is connected with the digital signal processing unit;

the first stage performs optical filtering and polarization state control on an ultra-high-speed coherent optical signal transmitted by an optical fiber link, continuous light with different wavelengths generated by a tunable laser A and a tunable laser B is coupled into one path through a polarization beam combiner, the polarization states of the two coupled continuous light are kept orthogonal, an intermediate polarization-maintaining optical fiber coupler couples the two paths of light into one path of signal, then the signal is subjected to optical power amplification through a polarization-maintaining optical amplifier, the amplified signal generates a double-pump orthogonal four-wave mixing effect through a polarization-maintaining high nonlinear optical fiber, polarization demultiplexing and wavelength conversion are realized simultaneously, finally a wavelength division demultiplexer of the second stage selects the signal subjected to wavelength conversion according to a wavelength position, and the signal is respectively and sequentially sent to a coherent detection unit, an ADC (analog to digital converter) and a digital signal processing unit for receiving processing, and finally transmission and exchange of the signal are completed.

The ultra-high-speed coherent optical signal is a polarization multiplexing signal DP-QPSK (dual polarization-Quadrature phase Shift keying) or DP-16QAM (dual polarization-16 Quadrature amplitude modulation).

The tunable laser a and the tunable laser B are both wide range (-30 nm) ITU standard (international telecommunication union organization standard) tunable lasers.

The polarization maintaining optical amplifier is an optical fiber amplifier or a semiconductor optical amplifier;

the polarization-maintaining high nonlinear optical fiber is a third-order nonlinear polarization coefficient chi⁽³⁾A non-zero nonlinear fiber;

the optical filter is any one of a fiber grating filter, a thin film dielectric filter, a fused taper fiber filter, a Fabry-Perot filter, a multilayer dielectric film filter and a Mach-Zehnder interference filter.

A method for realizing a system simultaneously realizing polarization demultiplexing and wavelength conversion of an ultra-high-speed coherent optical signal comprises the following steps:

(1) transmitting the ultra-high-speed coherent optical signal through an optical fiber link, and then entering a narrow-band optical filter for carrying out-of-band noise filtering;

(2) the output signal of the narrow-band optical filter is connected with a port 1 of the polarization-maintaining optical fiber coupler through a polarization controller, the polarization state of the signal is changed through the polarization controller, and the signal is matched and aligned with the slow axis direction of the polarization-maintaining optical fiber coupler;

(3) the tunable laser A and the tunable laser B respectively generate a wavelength lambda_aAnd λ_bThe two continuous lights are combined into a signal by a polarization beam combiner and connected with a port 2 of the polarization-maintaining optical fiber coupler, and the lambda is at the moment_aAnd λ_bThe polarization states of the two beams of light are kept orthogonal;

(4) the polarization maintaining fiber coupler outputs the coupled light, the coupled light is amplified to a hundred mW magnitude by a polarization maintaining amplifier, and then signal light lambda in the polarization maintaining nonlinear fiber_iAnd continuous light lambda_a、λ_bGenerating orthogonal four-wave mixing effect, because the pumping light only generates four-wave mixing process with the signal light with the same polarization direction, and the four-wave mixing process generates new idler frequency light, thereby separating the signal light lambda_iTwo orthogonal polarization state information are obtained, and each polarization state information is converted to lambda on the new wavelength optical signal_a’、λ_bIn the method, the conversion efficiency depends on the phase mismatch degree in four-wave mixing, and is related to dispersion, power, nonlinear coefficient and nonlinear fiber length, the continuous light wavelength is set within plus or minus 1nm of the zero dispersion point of the fiber, and the nonlinear coefficient is 10W^-1·km^-1The length of the nonlinear optical fiber is between 300 and 600 m;

(5) subjecting the λ generated in the highly nonlinear optical fiber in step (4)_a’、λ_b' two new wavelengths are separated by a wavelength division demultiplexing unit;

(6) lambda separated in the step (5)_a’、λ_b' demodulating and receiving the optical signal; firstly, coherent detection is carried out, then high-speed ADC sampling and digital signal processing processes are carried out, original signals are recovered, and exchange and transmission of the original two polarization state signals are completed.

The invention has the beneficial effects that:

(1) the polarization demultiplexing and wavelength conversion functions can be realized simultaneously. Because the ultrafast nonlinear effect based on orthogonal four-wave mixing is adopted, wavelength conversion is generated while polarization demultiplexing is carried out, one set of device realizes two functions, the structure is simple, and the resource utilization rate is greatly improved. The final output wavelength can be flexibly changed by changing the wavelength of the tunable laser, and the tunable laser has a good supporting effect on a large-capacity, flexible and variable optical network processing switching node.

(2) The processing and exchange of the ultra-high speed optical signals can be realized. Because the four-wave mixing is an ultrafast nonlinear effect, the fs and even sub-fs magnitude of the response time of the four-wave mixing is realized, the whole processes of polarization demultiplexing and wavelength switching are completed in an optical domain, and an optical-electrical-optical process does not exist, so that the four-wave mixing is basically not influenced by the optical signal rate, can reach 100Gbps and even Tbps magnitude, and has good guiding significance for ultrahigh-speed optical information processing and switching of a future backbone network and an access network.

(3) And compatibility of various modulation systems can be realized. Compared with the common optical-electrical-optical processing method, the method needs to firstly carry out the processes of demodulation, photoelectric conversion, modulation and the like on optical signals to complete polarization demultiplexing and information exchange, is sensitive to the modulation system of the optical signals, adopts the nonlinear effect in the optical domain, and is transparent to parameters such as amplitude, phase and the like, so that the method can be compatible with various modulation systems such as DP-QPSK, DP-8PSK, DP-16QAM and the like.

Drawings

FIG. 1 is a schematic diagram of the principles of the present invention;

FIG. 2 is a schematic diagram of an IQ modulator according to the present invention;

fig. 3 is a schematic diagram of the principle of two-pump orthogonal four-wave mixing according to the present invention, in which fig. 3(a) is a schematic diagram of the principle of two-pump orthogonal four-wave mixing to generate idler light, and fig. 3(b) is a schematic diagram of the polarization directions of two vertical pump lights and the initial angles of two vertical signal lights.

Detailed Description

The invention is further illustrated with reference to the following figures and examples.

A system for realizing ultra-high-speed coherent optical signal polarization demultiplexing and wavelength conversion comprises a first stage, an intermediate stage and a second stage, wherein the first stage comprises an optical filter, a polarization controller, a tunable laser A, a tunable laser B and a polarization beam combiner, the optical filter is connected with the polarization controller, the tunable laser A is connected with a port 1 of the polarization beam combiner, and the tunable laser B is connected with a port 2 of the polarization beam combiner; the intermediate stage comprises a polarization-maintaining fiber coupler, a polarization-maintaining optical amplifier and a polarization-maintaining high nonlinear fiber, wherein the polarization controller of the first stage is connected with a port 1 of the polarization-maintaining fiber coupler, the polarization beam combiner of the first stage is connected with a port 2 of the polarization-maintaining fiber coupler, a port 3 of the polarization-maintaining fiber coupler is connected with the polarization-maintaining optical amplifier, and the polarization-maintaining optical amplifier is connected with the polarization-maintaining high nonlinear fiber; the second stage comprises a wavelength division demultiplexer, coherent detection, an analog-to-digital converter (ADC) and a digital signal processing unit, wherein the polarization maintaining high nonlinear optical fiber of the middle stage is connected with the wavelength division demultiplexer, each output port of the wavelength division demultiplexer is connected with the coherent detection, the coherent detection is connected with the ADC, and the ADC is connected with the digital signal processing unit;

the first stage performs optical filtering and polarization state control on an ultra-high-speed coherent optical signal transmitted by an optical fiber link, continuous light with different wavelengths generated by a tunable laser A and a tunable laser B is coupled into one path through a polarization beam combiner, the polarization states of the two coupled continuous light are kept orthogonal, an intermediate polarization-maintaining optical fiber coupler couples the two paths of light into one path of signal, then the signal is subjected to optical power amplification through a polarization-maintaining optical amplifier, the amplified signal generates a double-pump orthogonal four-wave mixing effect through a polarization-maintaining high nonlinear optical fiber, polarization demultiplexing and wavelength conversion are realized simultaneously, finally a wavelength division demultiplexer of the second stage selects the signal subjected to wavelength conversion according to a wavelength position, and the signal is respectively and sequentially sent to a coherent detection unit, an ADC (analog to digital converter) and a digital signal processing unit for receiving processing, and finally transmission and exchange of the signal are completed.

The ultra-high-speed coherent optical signal is a polarization multiplexing signal DP-QPSK (dual polarization-Quadrature phase Shift keying) or DP-16QAM (dual polarization-16 Quadrature amplitude modulation).

The tunable laser A or tunable laser B is a wide range (-30 nm) ITU standard (International telecommunication Union organization standard) tunable laser;

the polarization maintaining optical amplifier is an optical fiber amplifier or a semiconductor optical amplifier;

the polarization-maintaining high nonlinear optical fiber is a third-order nonlinear polarization coefficient chi⁽³⁾A non-zero nonlinear fiber;

the optical filter is any one of a fiber grating filter, a thin film dielectric filter, a fused taper fiber filter, a Fabry-Perot filter, a multilayer dielectric film filter and a Mach-Zehnder interference filter.

A method for realizing a system simultaneously realizing polarization demultiplexing and wavelength conversion of an ultra-high-speed coherent optical signal comprises the following steps:

(1) as shown in fig. 1, the ultra-high-speed coherent optical signal is transmitted through an optical fiber link and then enters a narrowband optical filter for out-of-band noise filtering;

(2) the output signal of the narrow-band optical filter is connected with a port 1 of the polarization-maintaining optical fiber coupler through a polarization controller, the polarization state of the signal is changed through the polarization controller, and the signal is matched and aligned with the slow axis direction of the polarization-maintaining optical fiber coupler;

(3) the tunable laser A and the tunable laser B respectively generate a wavelength lambda_aAnd λ_bThe two continuous lights are combined into a signal by a polarization beam combiner and connected with a port 2 of the polarization-maintaining optical fiber coupler, and the lambda is at the moment_aAnd λ_bThe polarization states of the two beams of light are kept orthogonal;

(4) the polarization maintaining fiber coupler outputs the coupled light, the coupled light is amplified to a hundred mW magnitude by a polarization maintaining amplifier, and then signal light lambda in the polarization maintaining nonlinear fiber_iAnd continuous light lambda_a、λ_bGenerating orthogonal four-wave mixing effect, because the pumping light only generates four-wave mixing process with the signal light with the same polarization direction, and the four-wave mixing process generates new idler frequency light, thereby separating the signal light lambda_iTwo orthogonal polarization state information are obtained, and each polarization state information is converted to lambda on the new wavelength optical signal_a’、λ_bIn the method, the conversion efficiency depends on the phase mismatch degree in four-wave mixing, and is related to dispersion, power, nonlinear coefficient and nonlinear fiber length, the continuous light wavelength is set within plus or minus 1nm of the zero dispersion point of the fiber, and the nonlinear coefficient is 10W^-1·km^-1The length of the nonlinear optical fiber is between 300 and 600m, so that the conversion efficiency is better than 50 percent;

(5) subjecting the λ generated in the highly nonlinear optical fiber in step (4)_a’、λ_b' two new wavelengths are separated by a wavelength division demultiplexing unitDischarging;

(6) lambda separated in the step (5)_a’、λ_b' demodulating and receiving the optical signal; firstly, coherent detection is carried out, then high-speed ADC sampling and digital signal processing processes are carried out, original signals are recovered, and exchange and transmission of the original two polarization state signals are completed.

The technical principle of the invention is as follows:

1. ultra-high speed coherent optical signal generation principle

Ultra-high speed coherent optical communication generation is typically implemented using quadrature modulators (IQM). The structure of the IQ modulator uses two identical Mach-Zehnder modulators (MZM) and realizes the same principle of electro-optical modulation, and also utilizes the linear electro-optical effect to realize signal modulation, because the linear electro-optical effect is obviously stronger than the high-order electro-optical effect in a crystal medium.

The MZM modulator can implement modulation in OOK, DPSK, BPSK, etc., but for the generation of ultra-high-speed high-order modulated signals (such as QPSK, 8PSK, 16QAM, etc.) in the current 100G/400G coherent transmission system, a more complex IQ modulator is required to implement, and especially, the transmission capacity can be rapidly increased by twice by using the polarization multiplexing method, so the polarization-multiplexed ultra-high-speed coherent optical signal has been the mainstream choice of the backbone network. The polarization multiplexing is usually implemented by using two IQ modulators, as shown in fig. 2, first equally dividing input light into two paths of orthogonal polarization states by a polarization beam splitter, then modulating two paths of continuous light by the two IQ modulators, and finally coupling the two paths of continuous light into one path of polarization multiplexing signal by a polarization beam combiner.

In fig. 2, a single-polarization IQ modulator is taken as an example, input light is divided into two paths I and Q, the two paths enter an MZM modulator for modulation, wherein the phase of 90 degrees is changed by the phase modulator after the Q path optical signal is modulated, and finally the two paths are synthesized and output. In an IQ modulator, U is present_I(t)、U_Q(t) and U_P(t) three voltage signals, respectively expressed as follows:

U_I(t)＝V_I+u_I(t) (1)

U_Q(t)＝V_Q+u_Q(t) (2)

U_P(t)＝V_P(3)

wherein, V_I、V_QAnd V_PI, Q and 90 degree phase shifters, respectively. u. of_I(t) and u_QAnd (t) are signal voltages loaded on the I path and the Q path respectively. Usually, MZMs in I and Q paths operate at the lowest transmission point, and one of the paths passes through a 90-degree phase shifter and then is combined, so that most high-order modulation formats, such as QPSK, 8PSK, 16QAM, and the like, can be realized. The phase difference introduced at the I and Q MZMs is expressed as follows:

therefore, when the modulator insertion loss and the initial phase shift are not considered and the 90-degree phase shifter is ensured to work in an ideal state, the output optical field of the IQ modulator is expressed as:

therefore, the IQ modulation outputs an amplitude A_IQAngle of sumAs shown in formula (6) and formula (7), respectively:

2. orthogonal four-wave mixing polarization demultiplexing principle

The resulting spectrogram from quadrature four-wave mixing is shown in fig. 3. The realization principle is as follows: firstly, the polarization multiplexing signal and two beams of pump light with orthogonal polarization states form input light, secondly, the input light enters the nonlinear optical fiber to generate obvious four-wave mixing frequency effect, the newly generated wavelengths are different combinations of the wavelengths of the input light, it can be seen that 7 idler lights with different frequencies are generated in fig. 3, and phase matching is a main factor for determining the power of the idler lights in the four-wave mixing process. The process of polarization demultiplexing is described by taking idler frequencies 1-3 as an example, and the angular frequencies of the idler frequencies 1-3 are expressed as follows:

the electric field vectors of the two pump lights P1 and P2 and the signal light are expressed as follows:

in the formula, A_p1And A_p2Representing the amplitude, omega, of the pump light 1 and 2, respectively_p1And ω_p2The angular frequencies are represented individually by the angular frequencies,

and

respectively initiating phases; a. the_s1And A_s2Representing the amplitudes of S1 and S2; omega_sAnd

indicating the angular frequency and initial phase of the signal light. The electric field vectors of idler lights 1-3 generated in the four-wave mixing process are represented as follows:

in the formula eta₁、η₂、η₃The conversion efficiency of four-wave mixing is shown, and (') the inner product of the vector field. When the polarization directions of the two vector fields coincide, the inner product is zero. By substituting formulae (9) to (11) for formulae (12) to (14):

when θ is 0 °, the subentry a of formula (15)_s2cos (90+ theta) is 0 and A_s1cos (θ) is the maximum. This indicates the electric field of the idler 1

Only the entire information of signal S1 is included. In the same way as above, the first and second,

only the entire information of signal S2 is included. When theta is equal to 90 deg.,and

the information contained is exchanged.

It can be seen from the above theoretical analysis that polarization demultiplexing of a polarization multiplexed signal can generate four by two pump lights and signal lights with orthogonal polarization statesThe wave mixing effect is realized, and the proper idler frequency needs to be selected, which is respectively 2 omega_p1-ω_sAnd 2 omega_p2-ω_sAs a result of polarization demultiplexing.