CN106911395A - A kind of biorthogonal palarization multiplexing intensity modulated system and its Deplexing method - Google Patents

Abstract

The invention discloses a dual orthogonal polarization multiplexing intensity modulation system and a demultiplexing method thereof. The system is as follows: two paths of orthogonal polarization multiplexing intensity modulation optical signals are jointly connected to an optical coupler, and the optical coupling A device, a wavelength division multiplexer, an optical amplifier, an optical fiber, a wavelength division multiplexer, a Stokes analyzer, and a digital signal processing unit are connected in sequence. The method includes: at the transmitting end, generating dual orthogonal polarization state multiplexing signals, and multiplexing the signals together at different polarization angles through a polarization controller and an optical coupler; at the receiving end, using Stokes analysis The instrument is used as a receiving method to obtain electrical signals I ₀ , I _0deg , I _45deg and I _RC ; real-time DSP signal processing is used to realize the steps of demodulation and recovery of multiplexed signals. While ensuring the flexibility of the transmitting end and the receiving end in the optical network, the invention simplifies the structure of the receiving end, doubles the communication capacity and reduces the cost.

Description

A dual-orthogonal polarization multiplexing intensity modulation system and its demultiplexing method

technical field

The invention relates to the fields of short-distance optical interconnection, data center, access network, etc., in particular to a bi-orthogonal polarization multiplexing intensity modulation/direct detection system and a demultiplexing method based on a Stokes analyzer.

Background technique

Since Dr. Kao published the article "Optical Frequency Dielectric Fiber Surface Waveguide" in 1966, optical fiber communication has experienced nearly half a century of development. With the joint efforts of several generations of researchers, optical fiber communication has been widely used in transoceanic communication, backbone network, intercity network and other fields. The rapid development of coherent optical communication and digital signal processing (DSP) technology has increased the capacity-distance product of optical communication to the Exabit/s·km level.

However, the high cost of coherent optical communication is not suitable for short-distance optical communication systems that require strict cost control. At the same time, with the rise of short-distance optical interconnection, cloud services (cloud storage, cloud computing, and cloud query, etc.) Modulation/direct detection (IM/DD) systems have attracted more attention. Compared with the coherent detection method, the IM/DD system can greatly simplify the structures of the sending end and the receiving end and reduce costs. However, the detection sensitivity and transmission rate are reduced.

In order to increase the transmission rate of the IM/DD system and reduce the cost, the main technical means include: 1. Using multiple wavelength multiplexing transmission to improve the communication capacity of the entire system; 2. Using pulse amplitude modulation (PAM), discrete multi-tone modulation ( Efficient modulation formats such as DMT) and carrierless amplitude-phase modulation (CAP) to improve system spectrum utilization; 3. Adopt advanced coding methods (duobinary coding, etc.) to reduce the system capacity while ensuring the same system capacity required device bandwidth. The above technologies can greatly reduce the cost of the system and increase the capacity of the system, but it is still difficult to meet the increasing demand for communication capacity. Therefore, in order to further improve the communication capacity of the IM/DD system, the polarization multiplexing technology is introduced into the system, which can achieve the purpose of doubling the spectrum efficiency while changing the system structure to a minimum.

As early as 1990, A.D. Kersey and others realized the transmission and demultiplexing of polarization multiplexing signals in optical fibers. However, due to the random changes in the polarization state during optical fiber transmission, it is difficult for the receiving end to demultiplex them. This technology has not been widely used. With the breakthrough of coherent reception and DSP technology, it is of practical significance to use the method to realize polarization demultiplexing. In this way, the demultiplexing flexibility of the polarization multiplexing system is greatly improved. However, unlike the coherent reception method, the direct detection system cannot obtain the phase information of the optical signal. Therefore, it is difficult for the PDM-IM/DD system to realize polarization demultiplexing by using the DSP method.

In 2014, K.Kikuchi from the University of Tokyo proposed a scheme based on Stokes vectors to realize polarization tracking and demultiplexing. This scheme is simple and can quickly track the change of polarization state, but the receiving end needs four photodetectors (PD) and related polarization devices to obtain the four parameters of Stokes, which increases the system cost to a certain extent. At the same time, if the initial reference vector is not properly selected, it will lead to the inability to correctly distinguish the polarization state of the demultiplexed signal. In the same year, J.Estarán from the Technical University of Denmark used a Stokes analyzer to realize the transmission and demultiplexing of four different polarization state signals. Due to the same receiving method and polarization tracking method as K.Kikuchi, there is also the problem of selecting the initial reference vector. At the same time, the scheme uses four different wavelengths to eliminate crosstalk between different polarization states. Therefore, from this perspective, this solution does not improve the spectral efficiency of the system. On the other hand, W. Shieh's research group at the University of Melbourne in Australia used a 90° optical mixer and a balanced detector to obtain Stokes parameters and successfully demodulated a single-polarization 16-QAM signal in 2014. Because the system adopts a method similar to coherent reception (but does not require a local oscillator), it has high sensitivity and is effective for signals such as QAM. However, the polarization tracking and demultiplexing methods are complicated. Using this structure, the D.V.Plant research group of McGill University in Canada used 56GBaud PAM-4 signals to realize a PDM-IM/DD system with 224Gbit/s transmission over 10km single-mode fiber in 2014. Also, the demultiplexing method of this system is complicated.

Therefore, the research on the PDM-IM/DD transmission system with high spectrum efficiency, low cost and low method complexity has theoretical guidance and practical application significance.

Contents of the invention

The technical problem to be solved by the present invention is to provide a dual orthogonal polarization multiplexing intensity modulation system and its demultiplexing method, without increasing the complexity of the transmitter, receiver and method, only one Stokes The analyzer demultiplexes the 2×PDM-IM/DD system, and at the same time, can quickly track the change of the polarization state.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A double orthogonal polarization multiplexing intensity modulation system, two orthogonal polarization multiplexing intensity modulation optical signals are connected to an optical coupler, the optical coupler, wavelength division multiplexer, optical amplifier, optical fiber, wave decomposition The multiplexer, the Stokes analyzer, and the digital signal processing unit are connected in sequence.

Further, there are 2N paths of orthogonal polarization multiplexing intensity modulated optical signals, there are N corresponding optical couplers, and there are N Stokes analyzers, where N is a positive integer.

A method for demultiplexing a bi-orthogonal polarization multiplexing intensity modulation system, comprising the following steps:

Step 1: At the transmitting end, dual orthogonal polarization multiplexing signals are generated, and the signals are multiplexed together at different polarization angles through a polarization controller and an optical coupler;

Step 2: At the receiver end, use a Stokes analyzer as a receiving method to obtain electrical signals I ₀ , I _0deg , I _45deg and I _RC ;

Step 3: Use real-time DSP signal processing to realize the demodulation and recovery of the multiplexed signal; the signal demodulation includes: first resampling the received signal, correcting the time delay and DC compensation; then calculating the Stokes parameters S ₀ , S ₁ , S ₂ and S ₃ and implement polarization tracking; finally use the tracked reference vector to implement demultiplexing and demodulation to obtain the final output signal.

Further, the step 1 is specifically:

Step 1.1: The first signal of the two orthogonal polarization multiplexed intensity modulated optical signals passes through the first phase shifter so that the phases of the two signals have a fixed phase difference of π/2, and the second signal passes through the first half of the glass so that the phase of the two signals The polarization states are in an orthogonal relationship;

Step 1.2: The signals output from the first phase shifter and the first half-wave plate pass through the first polarization beam combiner to form the first orthogonal polarization multiplexing signal; similarly, the other two signals are combined with the first signal and the second The signals are formed in the same way as the orthogonal polarization multiplexing signal, but the phase difference between the two signals is fixed to -π/2 through the second phase shifter;

Step 1.3: Use the polarization controller to adjust the polarization states of the two orthogonal polarization multiplexing signals to a fixed 45° state, so that the polarization states of the multiplexed signals output through the coupler are 0°, 90°, -45° and 45° respectively. °.

Further, the specific acceptance method of the Stokes analyzer in step 2 is:

The input signal is divided into four channels by a 1×4 coupler, and the first optical signal directly passes through the first photodetector to realize the acquisition of the first electrical signal I ₀ ;

The second optical signal passes through a 0° polarizer/analyzer, and then passes through the second photodetector to obtain the second electrical signal I _0deg ;

The third optical signal passes through a 45° polarizer/analyzer, and then obtains the third electrical signal I _45deg through the third photodetector;

The last optical signal first passes through a quarter slide, then passes through a 45° polarizer/analyzer, and finally obtains a fourth electrical signal I _RC through the fourth photodetector.

Further, the step 3 is specifically:

Define S _t1 , S _t2 and S _t3 as the Stokes parameters of the training sequence, v ₁ , v ₂ and v ₃ are the initial reference vectors pointing to 0°, 45° and right-handed polarization directions respectively, for 2×PDM- OOK system, polarization state tracking and demultiplexing method are described as:

1) When S _ti (n)>U _thi , the update method of v _i (n+1) is:

v _i (n+1)＝{v _i (n)+u·[s(n)-v _i (n)]}/||v _i (n)+u·[s(n)-v _i ( n)]||

When S _ti (n)<-U _thi , the update method of v _i (n+1) is:

v _i (n+1)＝{v _i (n)+u·[-s(n)-v _i (n)]}/||v _i (n)+u·[-s(n)-v _i (n)]||

Where i=1, 2 or 3, s(n)=[S ₁ (n), S ₂ (n), S ₃ (n)], u is the update step size, U _thi is the corresponding S _ti decision threshold; v After the _i (n+1) update converges, the three Stokes vectors obtained after polarization tracking are v _t1 , v _t2 and v _t3 ;

2) According to the distribution of S ₀ , the signal is divided into five parts;

A. When S ₀ <S _th0 , it is determined that the status of the signal at the sending end is low;

B. When S ₀ >S _th3 , it is determined that the state of the signal at the sending end is high;

C. When S _th0 <S ₀ <S _th1 , only one of the four polarization states at the transmitting end carries a high-level signal, calculate v _t1 s and v _t2 s, and determine the signal at the transmitting end according to its probability distribution;

If abs(v _t1 s)>abs(v _t2 s), when v _t1 s<S _th4 , it is determined that only the 90° polarization state of the signal at the sending end carries a high level; when v _t1 s>S _th4 When , it is determined that only the 0° polarization state of the signal at the sending end carries a high level;

If abs(v _t1 s)<abs(v _t2 s), when v _t2 s<S _th5 , it is determined that only the -45° polarization state of the sender signal carries a high level; when v _t2 s>S _{When th5} , it is determined that only the 45° polarization state of the sending end signal carries a high level;

D. When S _th1 <S ₀ <S _th2 , two of the four polarization states at the transmitting end carry high-level signals and two carry low-level signals; calculate (v _t1 s+v _t2 s), ( v _t1 ·sv _t2 ·s) and v _t3 ·s, according to their probability distribution, determine the signal at the sending end;

When (v _t1 s+v _t2 s)<S _th6 , it is determined that the 90° polarization state and the -45° polarization state in the signal at the sending end carry a high level;

When (v _t1 s+v _t2 s)>S _th7 , it is determined that the 0° polarization state and the 45° polarization state in the signal at the sending end carry a high level;

When (v _t1 sv _t2 s)<S _th8 , it is determined that the 90° polarization state and the 45° polarization state in the signal at the sending end carry a high level;

When (v _t1 sv _t2 s)>S _th9 , it is determined that the 0° polarization state and the -45° polarization state in the signal at the sending end carry a high level;

When v _t3 s<S _th10 , it is determined that the 0° polarization state and the 90° polarization state in the signal at the sending end carry a high level;

When v _t3 s>S _th10 , it is determined that the 45° polarization state and -45° polarization state in the signal at the sending end carry a high level;

E. When S _th2 <S ₀ <S _th3 , only one of the four polarization states at the sending end carries a low-level signal; similarly, calculate v _t1 s and v _t2 s, and determine the signal at the sending end according to its probability distribution ; where S _thj (j=0-10) is the decision threshold corresponding to the probability distribution.

Compared with prior art, the beneficial effect of the present invention is:

1) Simultaneous demultiplexing of two orthogonal polarization multiplexing systems can be realized by using one Stokes analyzer.

2) The random perturbation of the polarization state can be quickly tracked by correlation methods.

3) Both the sending end and the receiving end devices are very mature, and it can be realized only by making a small amount of improvement to the existing commercial system. By using three orthogonal vectors in the Stokes space, the method realizes the demultiplexing of two orthogonal polarization multiplexing systems by using only one Stokes analyzer.

4) The division method can be combined with other multiplexing technologies such as wavelength division multiplexing (WDM) to realize the construction of a low-cost, large-capacity, and dynamic adaptive next-generation short-distance optical transmission network.

5) The present invention is suitable for short-distance optical interconnection, data center and access network signal transmission, and has the advantages of high spectrum efficiency, low cost and low method complexity.

Description of drawings

FIG. 1 is a Stokes-space-based dual-orthogonal polarization multiplexing intensity modulation/direct detection transmission scheme in the present invention.

Fig. 2 is a structural diagram of generating a biorthogonal polarization multiplexing intensity modulation signal in the present invention.

FIG. 3 shows the time-domain state and synthesized polarization state of the 2×PDM-OOK signal in the present invention.

Fig. 4 is a structural diagram of a Stokes analyzer in the present invention.

Fig. 5 is a schematic diagram of a digital signal processing method in the present invention.

FIG. 6 is the probability distribution of the total power S ₀ at the receiving end in the present invention.

FIG. 7 shows the probability distribution of v _t1 ·S when only one polarization state is loaded with a high-level signal in the present invention.

FIG. 8 shows the probability distribution of v _t2 ·S when only one polarization state is loaded with a high-level signal in the present invention.

FIG. 9 shows the probability distribution of (v _t1 ·S+v _t2 ·S) when two polarization states are loaded with low-level signals and two polarization states are loaded with high-level signals in the present invention.

FIG. 10 shows the probability distribution of (v _t1 ·Sv _t2 ·S) when two polarization states are loaded with low-level signals and two polarization states are loaded with high-level signals in the present invention.

FIG. 11 shows the probability distribution of v _t3 ·S when two polarization states are loaded with low-level signals and two polarization states are loaded with high-level signals in the present invention.

FIG. 12 is a graph showing the relationship between the bit error rate (Log(BER)) and the received power (ReceivedPower per Pol.) of the system after back-to-back transmission in the present invention.

FIG. 13 is a graph showing the relationship between the bit error rate (Log (BER)) and the received power (Received Power per Pol.) of the system after 2 kilometers of transmission in the present invention.

FIG. 14 is a graph showing the relationship between the bit error rate (Log (BER)) and the sampling point (Sampling Number) of the system after adding a scrambler at the receiving end in the present invention.

detailed description

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figure 1, the method of the present invention consists of two or 2N orthogonal polarization multiplexed intensity modulated optical signals 101 ₁ to 1012 _N , one or N optical couplers 102 ₁ to 102 _N , and one wavelength division multiplexer 103 , an optical amplifier 104 , a section of optical fiber 105 , a wave division multiplexer 106 , one or N Stokes analyzers 107 ₁ to 107 _N and a digital signal processing unit 108 . The specific logical relationship is as follows: After multiple channels of PDM-IM optical signals 101 ₁ to 101 _2N with different wavelengths are transmitted to the central office, N channels of 2×PDM-IM optical signals are synthesized by N couplers 102 ₁ to 102 _N and passed through a wave The division multiplexer 103 synthesizes a wavelength division multiplexed 2×PDM-IM optical signal; the multiplexed optical signal is amplified by an amplifier 104 for power compensation, and then enters a section of optical fiber 105 for transmission; at the receiving end, the multiplexed The signal is first separated into N channels of independent bi-orthogonal polarization multiplexing signals through a wave division multiplexer 106, and then each channel of 2×PDM-IM signals is received by a corresponding Stokes analyzer 107 _N to obtain four signals (I ₀ , I _0deg , I _45deg and I _RC ). The four signals are correlated to obtain three Stokes parameters S ₁ , S ₂ and S ₃ . Finally, it is input to the digital signal processing unit 108, and the signal is demodulated by a corresponding method in the digital signal processing unit.

FIG. 2 is a structural diagram of 2×PDM-IM/DD signal generation in the present invention. Among the two intensity modulation signals, the signal 1 passes through the phase shifter 1 so that the phases of the two signals differ by π/2, and the signal 2 passes through the half glass 1 so that the polarization states of the two signals are in an orthogonal relationship. Then, the first orthogonal polarization multiplexing signal is formed through the polarization beam combiner 1 . Similarly, the other two signals form an orthogonal polarization multiplexing signal in the same way. However, the phase difference between the two signals is fixed to -π/2 by the phase shifter 2 . Finally, use the polarization controller to adjust the polarization states of the two orthogonal polarization multiplexing signals to a fixed 45° state, so that the polarization states of the multiplexed signals output through the coupler are 0°, 90°, -45° and 45° respectively .

Fig. 3 is a schematic diagram of time-domain state and synthesized polarization state of 2×PDM-OOK signal in the present invention. According to the total power received, the signal is divided into 5 parts. It can be seen from Figure 3 that when the total power is 0 or 4, the combined polarization state is not clear; when the total power is 1, the final polarization state is consistent with the polarization state of the high-level signal; when the total power is 2 , according to the three vector directions (0°, 45° and right-handed polarization direction) in the Stokes space as the orthogonal basis, the synthesized polarization states can be divided into six situations as shown in the figure. It is worth noting that for two quadrature signals with the same power and a fixed phase difference of π/2 or -π/2, the final synthesized polarization state is right-handed polarization state (RC) or left-handed polarization state (LC). Similarly, when the total power is 3, there are four synthesized polarization states. According to this relationship, polarization state tracking and demultiplexing can be realized very simply.

Fig. 4 is a basic structure diagram of the Stokes analyzer used in the present invention, its structure is relatively simple, and it is easy to realize the preparation of the device by using the existing photon integration technology. The input signal is divided into four channels by a 1×4 coupler. The first optical signal directly passes through the photodetector 1 (PD1) to realize the acquisition of the first electrical signal I ₀ ; the second optical signal is in After passing through a 0° polarizer/analyzer, the second electrical signal I _0deg is obtained through PD2; the third optical signal is obtained through PD3 after passing through a 45° polarizer/analyzer. Signal I _45deg ; the last optical signal passes through a quarter slide first, then passes through a 45° polarizer/analyzer, and finally obtains the fourth electrical signal I _RC through PD4.

Fig. 5 is a schematic diagram of the signal processing method of the present invention. After the Stokes analyzer obtains four electrical signal outputs, the real-time oscilloscope with a sampling rate of 80GS/s realizes sampling of the signals and stores relevant data. In order to obtain better sampling points, it is necessary to resample the signal before processing the signal. Since it is necessary to ensure the bit synchronization of the four signals, it is necessary to perform time lag correction before performing the correlation operation. On the other hand, the received signal has been processed by DC blocking, and the DC component needs to be preserved when calculating the Stokes parameter. Therefore, a DC compensation process is required here. Further, the Stokes parameters S ₁ , S ₂ and S ₃ are obtained by formula (1).

After obtaining the Stokes parameters, the present invention implements polarization tracking using a method similar to that proposed by K. Kikuchi et al. At the same time, in order to avoid the problems caused by the improper setting of the initial reference vector, the present invention uses the training sequence as a reference to realize the correct tracking of the polarization state. Define S _t1 , S _t2 and S _t3 as the Stokes parameters of the training sequence, v ₁ , v ₂ and v ₃ are the initial reference vectors pointing to 0°, 45° and RC polarization direction respectively, then the specific polarization tracking method can be The description is as follows: when S _ti (n)>U _thi , v _i (n+1) is updated according to formula (2); when S _ti (n)<U _thi , v _i (n+1) is updated according to formula ( 3) Mode update.

v _i (n+1)＝{v _i (n)+u·[s(n)-v _i (n)]}/||v _i (n)+u·[s(n)-v _i ( n)]|| (2)

v _i (n+1)＝{v _i (n)+u·[-s(n)-v _i (n)]}/||v _i (n)+u·[-s(n)-v _i (n)]|| (3)

Where i=1, 2 or 3, s(n)=[S ₁ (n), S ₂ (n), S ₃ (n)], u is the update step size, and U _thi is the decision threshold corresponding to S _ti . Finally, three Stokes vectors obtained after polarization tracking are v _t1 , v _t2 and v _t3 .

After polarization tracking, polarization demultiplexing and decision processing are performed on the signal. Combining Fig. 3 with Fig. 6-11, wherein Fig. 6-11 is a probability distribution diagram of the relevant Stokes vector product in the present invention. Figure 6 shows the probability distribution of the total power S ₀ at the receiving end; Figures 7 and 8 respectively show the probabilities of v _t1 s and v _t2 s when only one polarization state is loaded with a high-level signal and the other polarization states are at a low level distribution; Figures 9, 10 and 11 show that when two polarization states are loaded with low-level signals and two polarization states are loaded with high-level signals, (v _t1 s+v _t2 s), (v _t1 sv _t2 s) and the probability distribution of v _t3 ·s.

The method of this part is described as follows: 1) According to the distribution of _S0 , the signal is divided into five parts.

When S ₀ <S _th0 , it can be determined that the signal at the sending end is in the first state in Figure 3, that is, the signals at the sending end are all at low level.

When S ₀ >S _th3 , it can be determined that the signals at the sending end are in the 16th state in FIG. 3 , that is, the signals at the sending end are all at high level.

When S _th0 <S ₀ <S _th1 , only one polarization state is loaded with a high-level signal. Calculate v _t1 ·s and v _t2 ·s and obtain their probability distributions, as shown in Figures 7 and 8. If abs(v _t1 s)>abs(v _t2 s), when v _t1 s<S _th4 , it can be determined that the signal at the sending end is in the fourth state in Figure 3, that is, the signal at the sending end has only 90° polarization state carries a high level; when v _t1 ·s>S _th4 , it can be determined that the signal at the sending end is in the fifth state in Figure 3, that is, only the 0° polarization state of the signal at the sending end carries a high level. If abs(v _t1 s)<abs(v _t2 s), when v _t2 s<S _th5 , it can be determined that the signal at the sending end is in the second state in Figure 3, that is, the signal at the sending end is only -45° The polarization state carries a high level; when v _t2 s>S _th5 , it can be determined that the signal at the sending end is in the third state in Figure 3, that is, only the 45° polarization state of the signal at the sending end carries a high level.

When S _th1 <S ₀ <S _th2 , there are two polarization states loaded with high-level signals, calculate (v _t1 s+v _t2 s), (v _t1 sv _t2 s) and v _t3 s And get its probability distribution as shown in Figures 9, 10 and 11. When (v _t1 s+v _t2 s)<S _th6 , it can be determined that the signal at the sending end is in the seventh state in Figure 3, and when (v _t1 s+v _t2 s)>S _th7 , it can be Determine that the signal at the sending end is the tenth state in Figure 3; when (v _t1 sv _t2 s)<S _th8 , it can be determined that the signal at the sending end is in the eighth state in Figure 3, when (v _t1 sv sv When _t2 ·s)>S _th9 , it can be determined that the signal at the sending end is in the ninth state in Figure 3; when v _t3 ·s<S _th10 , it can be determined that the signal at the sending end is in the eleventh state in Figure 3, when When v _t3 ·s>S _th10 , it can be determined that the signal at the sending end is in the sixth state in Figure 3.

When S _th2 <S ₀ <S _th3 , only one polarization state is loaded with a low level, and the 12th to 15th states in Figure 3 can be obtained by calculating v _t1 ·s and v _t2 ·s. So far, all the possibilities in Figure 3 have been determined, that is, the signal has been demultiplexed and demodulated. A key point here is that in the Stokes space, the vector directions of 0°, 45° and RC polarization states are orthogonal to each other, and the 0° and 90° polarization states point in opposite directions, and -45° Same with 45° and RC and LC polarization states.

Figures 12-14 are the bit error rate curves of the system in the present invention for back-to-back transmission, 2-kilometer transmission, and adding a scrambler at the receiving end. Fig. 12 is a comparison chart of bit error rates of a single polarization state, a PDM system and a 2×PDM system (the system proposed by the present invention) in the case of back-to-back transmission. The figure uses a forward error correction (FEC) threshold of 7% as a comparison, indicating that any bit error rate below this threshold can be restored to an error-free signal.

It can be seen from FIG. 12 that the performance of the system proposed by the present invention is about 1.5 dB worse than that of a single polarization state or an orthogonal polarization state multiplexing system under back-to-back transmission. At the same time, there is basically no difference in the bit error rate of each polarization state. Figure 13 shows the performance comparison of the 2×PDM system in the case of back-to-back and 2km transmission. It can be seen from the figure that the performance of 2km transmission is only about 0.5dB worse than that of back-to-back transmission.

On the other hand, in order to verify the performance of the polarization tracking method in the present invention, when the power at the fixed receiving end is -11.5dBm (average to the power of each polarization state), a polarization scrambler is used to change the input to the Stokes analysis The polarization state of the instrument. Set the scrambling rate of the scrambler to 708.75deg/s. When sampling data, it is fixed every 2 minutes. The finally calculated bit error rate is shown in FIG. 14 , and it can be seen that the polarization tracking method used in the present invention can quickly and accurately realize polarization state tracking.

It can be observed from the above experimental results that the present invention realizes the polarization demultiplexing and signal demodulation of the 2×PDM-IM/DD system based on the Stokes space. This scheme can realize simultaneous demultiplexing of two orthogonal polarization multiplexing systems by using a Stokes analyzer. At the same time, only a small amount of improvement needs to be made to the sending end and the receiving end of the existing transmission system. Due to the advantages of simple digital signal processing method and good compatibility, the invention is suitable for short-distance optical interconnection, data center and access network signal transmission.

Claims (6)

1. a kind of biorthogonal palarization multiplexing intensity modulated system, it is characterised in that two-way cross-polarization multiplexing intensity-modulated light letter Number (101) are commonly connected to a photo-coupler (102), the photo-coupler (102), wavelength division multiplexer (103), image intensifer (104), optical fiber (105), Wave decomposing multiplexer (106), Stokes analyzer (107), digital signal processing unit (108) according to It is secondary to be connected.

2. a kind of biorthogonal palarization multiplexing intensity modulated system as claimed in claim 1, it is characterised in that the cross-polarization Multiplexing intensitymodulated optical signals (101) is 2N roads, and corresponding photo-coupler (102) is N number of, and Stokes analyzer (107) is N Individual, N is positive integer.

3. a kind of Deplexing method of biorthogonal palarization multiplexing intensity modulated system, it is characterised in that comprise the following steps：

Step 1：In transmitting terminal, produce biorthogonal polarization state multiplexed signals, by Polarization Controller and photo-coupler, by signal with Different polarization angles are multiplexed together；

Step 2：In receiver end, using Stokes analyzer as the mode of reception, electric signal I is obtained₀、I_0deg、I_45degAnd I_RC；

Step 3：The demodulation for realizing multiplexed signals using real-time DSP signal transactings recovers；The demodulation of signal includes：Dock first The collection of letters number is done resampling, time lag and is corrected and DC compensation；Stokes parameter S is calculated again₀、S₁、S₂And S₃And realize polarization Follow the trail of；Demultiplexing and demodulation finally are realized using the reference vector after following the trail of, final output signal is obtained.

4. a kind of Deplexing method of biorthogonal palarization multiplexing intensity modulated system as claimed in claim 3, it is characterised in that The step 1 is specially：

Step 1.1：The first signal causes that two-way is believed by the first phase shifter in two-way cross-polarization multiplexing intensitymodulated optical signals Number phase fixed phase difference pi/2, secondary signal causes the polarization state orthogonal relationship of two paths of signals by the first half slides；

Step 1.2：The signal exported from the first phase shifter and the first half-wave plate is constituting first just by the first polarization beam combiner Hand over polarisation-multiplexed signal；Equally, two other signal is using orthogonal partially with the first signal and secondary signal identical method composition Shake multiplexed signals, but causes that the phase difference of two paths of signals is fixed to-pi/2 by the second phase shifter；

Step 1.3：Using Polarization Controller by the regulation of the polarization state of two cross-polarization multiplexed signals to fixing 45 ° of states so that The multiplexed signals polarization state for eventually passing coupler output is respectively 0 °, 90 °, -45 ° and 45 °.

5. a kind of Deplexing method of biorthogonal palarization multiplexing intensity modulated system as claimed in claim 3, it is characterised in that The specific receiving method of Stokes analyzer is in step 2：

Luminous power is divided into four tunnels by input signal by the coupler of 1 × 4, and first via optical signal directly passes through the first photoelectricity Detector realizes first via electric signal I₀Acquisition；

Second road optical signal obtains the second tunnel telecommunications by the 0 ° of polarizer/analyzer, then by the second photodetector Number I_0deg；

3rd road optical signal obtains the 3rd tunnel telecommunications by the 45 ° of polarizer/analyzers, then by the 3rd photodetector Number I_45deg；

Last optical signal all the way first passes through an a quarter slide, then by the 45 ° of polarizer/analyzers, finally by the 4th Photodetector obtains the 4th road electric signal I_RC。

6. as claimed in claim 3 a kind of Deplexing method of biorthogonal palarization multiplexing intensity modulated system, it is characterised in that institute Step 3 is stated to be specially：

Define S_t1、S_t2And S_t3It is the stokes parameter of training sequence, v₁、v₂And v₃To be respectively directed to 0 °, 45 ° and the right side The initial reference vector in rotatory polarization direction, for the system of the PDM-OOK synthesis of 2 × PDM-OOK systems, i.e., two, polarization state is chased after Track and Deplexing method are described as：

1) S is worked as_ti(n)>U_thiWhen, v_i(n+1) update mode is：

v_i(n+1)={ v_i(n)+u[s(n)-v_i(n)]}/||v_i(n)+u[s(n)-v_i(n)]||

Work as S_ti(n)<-U_thiWhen, v_i(n+1) update mode is：

v_i(n+1)={ v_i(n)+u[-s(n)-v_i(n)]}/||v_i(n)+u[-s(n)-v_i(n)]||

Wherein i=1,2 or 3, s (n)=[S₁(n),S₂(n),S₃(n)], u is renewal step-length, U_thiIt is correspondence S_tiDecision threshold；v_i (n+1) update after convergence, obtain three Stokes vectors after polarization is followed the trail of and be respectively v_t1、v_t2And v_t3；

2) according to S₀Distribution, by signal point quinquepartite；

A, work as S₀<S_th0When, the state for judging to send end signal is all low level；

B, work as S₀>S_th3When, the state for judging to send end signal is all high level；

C, work as S_th0<S₀<S_th1When, four polarization state only one of which of transmitting terminal carry high level signal, calculate v_t1S and v_t2S, according to its probability distribution, judges to send end signal；

If abs (v_t1·s)>abs(v_t2S), v is worked as_t1·s<S_th4When, judge that sending end signal only has 90 ° of polarization states to carry High level；Work as v_t1·s>S_th4When, judge that sending only 0 ° polarization state of end signal carries high level；

If abs (v_t1·s)<abs(v_t2S), v is worked as_t2·s<S_th5When, judge that sending end signal only has -45 ° of polarization states to carry High level；Work as v_t2·s>S_th5When, judge that sending only 45 ° polarization states of end signal carries high level；

D, work as S_th1<S₀<S_th2When, four polarization states of transmitting terminal have two to carry high level signal and two carry low level and believe Number；Calculate (v_t1·s+v_t2·s)、(v_t1·s-v_t2And v s)_t3S, according to its probability distribution, judges to send end signal；

As (v_t1·s+v_t2·s)<S_th6When, judge to send 90 ° of polarization states and -45 ° of polarization states carrying high level in end signal；

As (v_t1·s+v_t2·s)>S_th7When, judge to send 0 ° of polarization state and 45 ° of polarization states carrying high level in end signal；

As (v_t1·s-v_t2·s)<S_th8When, judge to send 90 ° of polarization states and 45 ° of polarization states carrying high level in end signal；

As (v_t1·s-v_t2·s)>S_th9When, judge to send 0 ° of polarization state and -45 ° of polarization states carrying high level in end signal；

Work as v_t3·s<S_th10When, judge to send 0 ° of polarization state and 90 ° of polarization states carrying high level in end signal；

Work as v_t3·s>S_th10When, judge to send 45 ° of polarization states and -45 ° of polarization states carrying high level in end signal；

E, work as S_th2<S₀<S_th3When, four polarization state only one of which of transmitting terminal carry low level signal；Equally, v is calculated_t1· S and v_t2S, according to its probability distribution, judges to send end signal；Wherein S_thj(j=0-10) be correspondence probability distribution judgement threshold Value.