CN111007526B - System and method for suppressing optical noise of continuous wave all-fiber coherent Doppler laser speed measurement radar - Google Patents

Abstract

The invention belongs to the technical field of laser speed measuring radar, and in particular relates to an optical noise suppression system of continuous wave all-fiber coherent Doppler laser speed measuring radar, which adopts 1/4 wave plate and optical fiber polarization beam splitter to realize the rotation and polarization state of echo signal light Screening; the APC end face coating of the second port of the optical fiber circulator or polarization beam splitter reduces end face reflection, thereby reducing interference; using a balance detector to reduce the relative intensity noise of local oscillator light, the present invention also provides a suppression method based on the above system, In this way, the additional noise caused by the light leakage caused by the fiber end face and the circulator port in the continuous wave all-fiber laser coherent Doppler speed measuring radar is reduced, thereby improving the signal-to-noise ratio and ensuring the detection accuracy of the system.

Description

Optical Noise Suppression System of CW All-Fiber Coherent Doppler LiDAR and method

technical field

The invention belongs to the technical field of laser speed measuring radar, and in particular relates to a system and method for suppressing optical noise of continuous wave all-fiber coherent Doppler laser speed measuring radar.

Background technique

In recent years, laser speed radar technology has made great progress. Specific application fields include laser wind measurement, laser liquid speed measurement, and laser radar for unmanned driving. To achieve the above functions, the widely used technical basis includes Doppler velocity measurement technology, optical coherent measurement technology and all-fiber optical path structure. Among them, the Doppler speed measurement technology is based on the principle that the relative motion of the light source and the target produces a laser frequency shift, and the relative motion speed of the target and the light source is obtained by measuring the movement of the laser frequency; optical coherence measurement is a method to know the specific Doppler The frequency shift method uses the local oscillator light and the signal light carrying the Doppler frequency shift information for coherent detection, and obtains the Doppler frequency shift by measuring the difference frequency information. This method can use a strong local oscillator Light can achieve the amplification effect on weak signals, especially when the balance detector is used, it can reduce the relative intensity noise of local oscillator light, so it is especially suitable for the detection of weak signal light; finally, due to the remarkable advantages of stable structure, the all-fiber structure It has also been widely used in such devices.

As a typical example of weak signal light detection, laser wind radar, including continuous light and pulse light laser wind radar, realizes wind detection by detecting the Doppler information carried in the backscattering of detection light by aerosol particles in the atmosphere. Measurement of field size and direction. Due to the extremely low backscattering coefficient caused by atmospheric aerosol particles, the ability of the system to extract Doppler information of weak signal light is particularly critical. The key to solving the above problems is to improve the signal-to-noise ratio of the signal. At present, generally proceed from two aspects, one is to try to increase the signal optical power, and the other is to reduce the noise in the measurement process. In addition to the relative intensity noise of local oscillator light mentioned above, coherent Doppler laser speed measurement radar, especially the continuous wave all-fiber laser coherent Doppler speed measurement radar also faces an important noise source, such as the reflected light from the end face of the optical fiber in the system. And the coherent noise introduced by part of the optical path leakage light.

In the prior art, fiber laser radar consists of a fiber-coupled MOPA-SL light source, a circulator, an inline polarization controller, and a photodetector. The PC end of port 2 of the circulator is coated with an anti-reflection coating (<0.25%) to obtain local oscillator light with a suitable power. In this solution, the local oscillator light has been mixed with the signal light in advance and returned from the same optical path, so it is difficult to adopt The balanced detection scheme reduces the optical intensity noise caused by the strong local oscillator light, which affects the further improvement of the system signal-to-noise ratio.

Contents of the invention

In view of the above deficiencies, the present invention provides a system and method for suppressing optical noise of continuous wave all-fiber coherent Doppler laser speed measuring radar. In the Doppler speed radar, the additional noise caused by the leaked light at the end face of the optical fiber and the port of the circulator can improve the signal-to-noise ratio and ensure the detection accuracy of the system.

For realizing above technical purpose, a technical scheme of the present invention is:

All-fiber continuous coherent Doppler laser radar optical noise suppression system, including narrow linewidth laser, first coupler, polarization beam splitter, second coupler, collimator, 1/4 wave plate, beam expander system , the detection target and the signal processing unit, the narrow linewidth laser is connected to the input end of the first coupler, the first output end of the first coupler is connected to the first port of the polarization beam splitter through an amplifier, The collimator, the 1/4 wave plate and the beam expander system are sequentially arranged between the polarizing beam splitter and the detection target, and the APC end face of the second port of the polarizing beam splitter is coated with Anti-reflection film, the third port of the polarization beam splitter is connected to the second coupler, the fast axis of the 1/4 wave plate is set at an angle of 45 degrees to the polarization direction, and the first coupler’s The second output end is also connected to the second coupler, and the second coupler is connected to the signal processing unit through a balance detector.

Further, the narrow linewidth laser is one of a distributed feedback laser, a distributed Bragg reflection laser, and an external cavity semiconductor laser.

Further, the amplifier is an erbium-doped fiber amplifier, the first coupler is a high-resolution fiber coupler, and the second coupler is a 3dB fiber coupler.

Based on the above-mentioned system, the present invention correspondingly provides a method for suppressing optical noise of an all-fiber continuous coherent Doppler laser speed measuring radar, and the specific steps include:

S1, the narrow-linewidth continuous laser emitted by the narrow-linewidth laser is divided into local oscillator light and seed light by the first coupler,

S2, the seed light in step S1 is amplified by the amplifier, and injected as the probe light into the first port of the polarizing beam splitter, the APC end face of the second port of the polarizing beam splitter is coated with an anti-reflection film,

S3, through the 1/4 wave plate with a 45-degree angle between the fast axis and the polarization direction, it is converted into circularly polarized light, and then returns to the target after being shot through the beam expander system.

S4, the backscattered light carrying the detection target information is converted into linearly polarized light through the 1/4 wave plate again, and the polarization direction of the linearly polarized light will be perpendicular to the polarization direction of the detection light,

S5, the linearly polarized light in step S4 is injected from the second port of the polarization beam splitter, and emitted from the third port of the polarization beam splitter to obtain signal light,

S6, the signal light and local oscillator light in step S5 are mixed and sent to a balanced detector for detection, and the difference frequency signal output by the balanced detector is sent to a signal processing unit for processing.

For realizing above technical purpose, another technical scheme of the present invention is:

All-fiber continuous coherent Doppler laser radar optical noise suppression system, including narrow linewidth laser, first coupler, fiber circulator, polarization beam splitter, second coupler, collimator, 1/4 wave plate , a beam expander system, a detection target and a signal processing unit, the narrow linewidth laser is connected to the input end of the first coupler, the first output end of the first coupler is connected to the first output end of the fiber circulator through an amplifier One port is connected, the collimator, the 1/4 wave plate and the beam expander system are sequentially arranged between the optical fiber circulator and the detection target, and the APC end face of the second port of the optical fiber circulator Coated with an anti-reflection film, the third port of the optical fiber circulator is connected to the second coupler through the polarization beam splitter, and the fast axis of the 1/4 wave plate is set at an angle of 45 degrees to the polarization direction ,

The second output end of the first coupler is also connected to the second coupler, and the second coupler is connected to the signal processing unit through a balance detector.

Further, the narrow linewidth laser is one of a distributed feedback laser, a distributed Bragg reflection laser, and an external cavity semiconductor laser.

Further, the amplifier is an erbium-doped fiber amplifier, the first coupler is a high-resolution fiber coupler, and the second coupler is a 3dB fiber coupler.

Based on the above-mentioned system, the present invention correspondingly provides a method for suppressing optical noise of an all-fiber continuous coherent Doppler laser speed measuring radar, and the specific steps include:

S1, the narrow-linewidth continuous laser emitted by the narrow-linewidth laser is divided into local oscillator light and seed light by the first coupler,

S2, the seed light in step S1 is amplified by the amplifier, injected as the probe light into the first port of the optical fiber circulator, the APC end face of the second port of the optical fiber circulator is coated with an anti-reflection film,

S3, through the 1/4 wave plate with the fast axis and the polarization direction at an angle of 45 degrees, it is converted into circularly polarized light, and then returns to the target after being shot through the beam expander system 7,

S4, the backscattered light carrying the detection target information is converted into linearly polarized light through the 1/4 wave plate again, and the polarization direction of the linearly polarized light will be perpendicular to the polarization direction of the detection light,

S5, the linearly polarized light in step S4 is injected from the second port of the optical fiber circulator, and emitted from the third port of the optical fiber circulator to obtain signal light,

S6, the signal light in step S5 is mixed with the local oscillator light by the polarization beam splitter and sent to the balance detector for detection, and the difference frequency signal output by the balance detector is sent to the signal processing unit for processing.

The present invention has the following beneficial effects:

1. Use a 1/4 wave plate to adjust the polarization state of the echo signal light and the detection light to an angle of 90 degrees, and use a fiber optic polarization beam splitter to separate the signal light from the interference light, thereby reducing the influence of interference light;

2. The coating on the end face of the second port of the circulator or fiber polarization beam splitter reduces end face reflection, thereby reducing interference;

3. A balanced detector is used to reduce the relative intensity noise of local oscillator light.

Description of drawings

Fig. 1 is the structural schematic diagram of the suppression system of the optical noise of the continuous wave all-fiber laser coherent Doppler velocity measuring radar in the prior art;

Fig. 2 is a schematic structural diagram of the suppression system of the optical noise of the continuous wave all-fiber laser coherent Doppler speed measuring radar of embodiment 1;

Fig. 3 is the noise measurement spectrogram of embodiment 1;

4 is a schematic structural diagram of the suppression system of the optical noise of the continuous wave all-fiber laser coherent Doppler speed measuring radar of embodiment 2;

Fig. 5 is the noise measurement spectrogram of embodiment 2;

Fig. 6 is the structural representation of comparative example;

Fig. 7 is a noise measurement spectrogram of a comparative example.

Detailed ways

The purpose of the present invention is to reduce the additional noise caused by the light leakage from the fiber end face and the circulator port in the continuous wave all-fiber laser coherent Doppler speed measuring radar by using the polarization characteristics of light, the end face coating technology and the balance detection technology, thereby improving the signal-to-noise ratio and ensuring System detection accuracy. Based on the above theories and methods, two implementing devices can be provided. The two technical solutions of the present invention will be described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

Embodiment 1 of the present invention will be described in detail below in conjunction with accompanying drawing 2, but the claims of the present invention are not limited in any way.

The optical noise suppression system of the continuous wave all-fiber laser coherent Doppler speed measuring radar provided in this embodiment includes a narrow linewidth laser 1-1, a first coupler 1-2, a polarization beam splitter 1-4, a second Coupler 1-9, collimator 1-5, 1/4 wave plate 1-6, beam expander system 1-7, detection target 1-8 and signal processing unit 1-11.

Narrow linewidth laser 1-1 is used to emit narrow linewidth continuous laser, narrow linewidth laser 1-1 is distributed feedback laser, distributed Bragg reflection laser, external cavity semiconductor laser, narrow linewidth laser 1-1 is connected to the first coupling The input end of the first coupler 1-2 is a high-resolution fiber coupler for splitting the passed output light into two paths. The polarization beam splitter 1-4 has three ports: the first port, the second port and the third port. The optical signal enters from the first port and outputs from the second port. The second port will also return from the detection target 1-8. The backscattered light signal is output from the third port. The first output end of the first coupler 1-2 is connected to the first port of the polarization beam splitter 1-4 through the amplifier 1-3, and the amplifier 1-3 is an erbium-doped fiber amplifier 1-3, which amplifies the incoming optical signal . A collimator 1-5, a 1/4 wave plate 1-6 and a beam expander system 1-7 are sequentially arranged between the polarizing beam splitter 1-4 and the detection target 1-8, and the second part of the polarizing beam splitter 1-4 The APC end face of the port is coated with an anti-reflection film, so that the reflected light intensity of the end face is reduced as much as possible, and its influence is further reduced. The third port of the polarization beam splitter 1-4 is connected to the second coupler 1-9, and the second coupler 1-9 is a 3dB fiber coupler. The fast axis of the 1/4 wave plate 1-6 is set at an angle of 45 degrees to the polarization direction, and the 1/4 wave plate 1-6 converts circularly polarized light and linearly polarized light. The second output end of the first coupler 1-2 and the third port of the polarization beam splitter 1-4 are all connected to the second coupler 1-9, and the second coupler 1-9 is connected through a balanced detector 1-10 Signal processing units 1-11.

Based on the above system, the method for suppressing the optical noise of the continuous wave all-fiber laser coherent Doppler speed measuring radar provided in this embodiment, the specific steps are as follows:

S1, the narrow linewidth laser 1-1 emits narrow linewidth continuous laser, the narrow linewidth continuous laser is linearly polarized light output through the optical fiber, and the output light is divided into local oscillator light and seed light by the first coupler 1-2. Among them, the weaker one is used as the local oscillator light, and the other is the seed light;

S2, the seed light is injected into the first port of the optical fiber polarization beam splitter 1-4 as probe light after being amplified by the fiber amplifier 1-3, and the APC end face of the second port of the polarization beam splitter 1-4 is coated with an anti-reflection film;

S3, the probe light passes through the polarization beam splitter 1-4 and the optical collimator 1-5 in turn, and then reaches the 1/4 wave plate 1-6 whose fast axis and polarization direction form an angle of 45 degrees to become circularly polarized light , and finally shoot to the target space through the beam expander system 1-7;

S4, the detection target 1-8 in space will cause the detection light to produce weak backscattering carrying the velocity signal, and the backscattered light carrying the information of the detection target 1-8 returns to the optics through the beam expander system 1-7 along the original path system, the 1/4 wave plate 1-6 converts circularly polarized light into linearly polarized light again, and the polarization direction of the linearly polarized light obtained at this time will be perpendicular to the original linearly polarized light’s polarization direction,

S5, the linearly polarized light in step S4 is injected from the second port of the polarization beam splitter 1-4, and is emitted from the third port of the polarization beam splitter 1-4 to obtain signal light;

S5, the signal light and the local oscillator light are mixed by the second coupler 1-9 and sent to the balanced detector 1-10 for detection, and the difference frequency signal output by the balanced detector 1-10 is followed by the signal processing unit 1-11 Processing such as analog-to-digital conversion, fast Fourier transform, spectrum accumulation, and peak-finding calculation.

As shown in Figure 3, the bandwidth of the spectrum analyzer is set to 1KHz, and the average number of times is 30. After being processed by the system provided in this embodiment, the noise floor is about -80dBm, and the fluctuation range is less than 3dB. The system can be stably in the state of high signal-to-noise ratio, so as to stably measure the weak return optical signal.

Embodiment 2 of the present invention will be described in detail below with reference to FIG. 4 , but the claims of the present invention are not limited in any way.

The continuous wave all-fiber laser coherent Doppler velocity radar optical noise suppression system provided in this embodiment includes a narrow linewidth laser 2-1, a first coupler 2-2, a fiber circulator 2-4, and a polarization beam splitter 2-9, a second coupler 2-10, a collimator 2-5, a 1/4 wave plate 2-6, a beam expander system 2-7, a detection target 2-8 and a signal processing unit 2-12.

Narrow linewidth laser 2-1 is used to emit narrow linewidth continuous laser, narrow linewidth laser 2-1 is distributed feedback laser, distributed Bragg reflection laser, external cavity semiconductor laser, narrow linewidth laser 2-1 is connected to the first coupling The first coupler 2-2 is a high-resolution fiber coupler for splitting the passed output light into two paths. The optical fiber circulator 2-4 has three ports: the first port, the second port and the third port. The optical signal enters from the first port and outputs from the second port. The backscattered light signal is output from the third port. The first output end of the first coupler 2-2 is connected to the first port of the optical fiber circulator 2-4 through the amplifier 2-3, and the amplifier 2-3 is an erbium-doped optical fiber amplifier 2-3, which amplifies the incoming optical signal . A collimator 2-5, a 1/4 wave plate 2-6 and a beam expander system 2-7 are sequentially arranged between the fiber optic circulator 2-4 and the detection target 2-8, and the second port of the fiber optic circulator 2-4 The end face of APC is coated with anti-reflection film, so that the intensity of reflected light on the end face can be reduced as much as possible, and its influence can be further reduced. The third port of the fiber optic circulator 2-4 is connected to the second coupler 2-10 through the polarization beam splitter 2-9, and the second coupler 2-10 is a 3dB fiber optic coupler. The fast axis of the 1/4 wave plate 2-6 is set at an angle of 45 degrees to the polarization direction, and the 1/4 wave plate 2-6 converts circularly polarized light and linearly polarized light. Both the second output end of the first coupler 2-2 and the polarization beam splitter 2-9 are connected to the second coupler 2-10, and the second coupler 2-10 is connected to the signal processing unit 2 through a balanced detector 2-11 -12.

Based on the above system, the method for suppressing the optical noise of the continuous wave all-fiber laser coherent Doppler speed measuring radar provided in this embodiment, the specific steps are as follows:

S1, the narrow linewidth laser 2-1 emits narrow linewidth continuous laser, the narrow linewidth continuous laser is linearly polarized light output through the optical fiber, and the output light is divided into local oscillator light and seed light by the first coupler 2-2, wherein , the weaker one is used as the local oscillator light, and the other is the seed light;

S2, the seed light is injected into the first port of the optical fiber circulator 2-4 as the detection light after being amplified by the optical fiber amplifier 2-3, and the APC end face of the second port of the optical fiber circulator 2-4 is coated with an anti-reflection film;

S3, the probe light passes through the fiber circulator 2-4 and the collimator 2-5 in sequence, and then reaches the 1/4 wave plate 2-6 whose fast axis and polarization direction form an angle of 45 degrees to become circularly polarized light, and finally Shoot to the target space through the beam expander system 2-7;

S4, the detection target 2-8 in space will cause the detection light to produce weak backscattering carrying the velocity signal, and the backscattered light carrying the information of the detection target 2-8 returns to the optics through the beam expander system 2-7 along the original path system, 1/4 wave plate 2-6 converts circularly polarized light into linearly polarized light again, and the polarization direction of the linearly polarized light obtained at this time will be perpendicular to the original linearly polarized light’s polarization direction,

S5, the linearly polarized light in step S4 is injected from the second port of the optical fiber circulator 2-4, and is emitted from the third port of the optical circulator to obtain signal light;

S5, the signal light passes through the polarization beam splitter 2-9 and the local oscillator light is mixed through the second coupler 2-10 and sent to the balance detector 2-11 for detection, and the difference frequency signal output by the balance detector 2-11 is determined by the signal The processing unit 2-12 performs subsequent processing such as analog-to-digital conversion, fast Fourier transform, spectrum accumulation, and peak-finding calculation.

As shown in Figure 5, the bandwidth of the spectrum analyzer is set to 1KHz, and the average number of times is 30. After being processed by the system provided in this embodiment, the noise floor is about -80dBm, and the fluctuation range is less than 3dB. The system can be stably in the state of high signal-to-noise ratio, so as to stably measure the weak return optical signal.

The comparative example will be described below in conjunction with accompanying drawing 6 .

Compared with Example 2, in the system provided in the comparative example, the narrow linewidth laser 3-1 is connected to the input end of the first coupler 3-2, and the first output end of the first coupler 3-2 passes through the amplifier 3- 3 is connected to the first port of the optical fiber circulator 3-4, only the collimator 3-5 and the beam expander system 3-6 are set between the optical fiber circulator 3-4 and the detection target 3-7, and the optical fiber circulator 3-4 The third port of the first coupler 3-8 is directly connected with the second coupler 3-8, and the second output of the first coupler 3-2 is also connected with the second coupler 3-8, and the second coupler 3-8 passes through the balance detector 3 -9 is connected to the signal processing unit 3-10. The second port of the optical fiber circulator 3-4 is not provided with an anti-reflection coating, and the front end of the beam expander system 3-6 is not provided with a 1/4 wave plate. At the same time, the optical fiber circulator 3-4 and the second coupler 3- There is no polarizing beam splitter between 8.

As shown in Figure 7, the bandwidth of the spectrum analyzer is set to 1KHz, and the average number of times is 30. After being processed by the system provided in this comparative example, the fluctuation range of the noise floor is about 20dB. The system noise fluctuates in a large range, and can cover the weak return optical signal at some times, resulting in the inability to perform measurement stably.

In summary, the present invention has the following beneficial effects:

1. Use a 1/4 wave plate to adjust the polarization state of the echo signal light and the detection light to an angle of 90 degrees, and use a polarization beam splitter to separate the signal light from the interference light, thereby reducing the influence of interference light;

2. The coating on the APC end face of the second port of the circulator or polarization beam splitter reduces end face reflection, thereby reducing interference;

3. A balanced detector is used to reduce the relative intensity noise of local oscillator light.

Using the technical solution of the present invention can reduce the additional noise caused by the leaked light of the fiber end face and the circulator port in the continuous wave all-fiber laser coherent Doppler speed measuring radar; at the same time, compared with the existing reference solution, the relative intensity noise introduced by the local oscillator light can be reduced , thereby improving the system signal-to-noise ratio.

The above is only a preferred embodiment of the present invention, and is not a limitation of the present invention, and the present invention is not limited to the above examples. Those skilled in the art, within the scope of the present invention, the changes, modifications, and additions made Or replace, also should belong to the protection scope of the present invention.

Claims (8)

1. A system for suppressing optical noise of an all-fiber continuous coherent Doppler laser speed measuring radar is characterized in that it comprises a narrow linewidth laser, a first coupler, a polarization beam splitter, a second coupler, a collimator, 1 /4 wave plate, beam expander system, detection target and signal processing unit, the narrow linewidth laser is connected to the input end of the first coupler, and the first output end of the first coupler is connected to the polarization by an amplifier The first port of the beam splitter is connected, and the collimator, the 1/4 wave plate and the beam expander system are sequentially arranged between the polarizing beam splitter and the detection target, and the polarizing beam splitter The APC end face of the second port is coated with an anti-reflection film, the third port of the polarization beam splitter is connected to the second coupler, and the fast axis of the 1/4 wave plate is set at an angle of 45 degrees to the polarization direction , the second output end of the first coupler is also connected to the second coupler, and the second coupler is connected to the signal processing unit through a balance detector. 2. The suppression system of a kind of all-fiber continuous coherent Doppler laser speed measuring radar optical noise according to claim 1, is characterized in that, described narrow-linewidth laser is distributed feedback laser, distributed Bragg reflection laser, external cavity A type of semiconductor laser. 3. The suppression system of a kind of all-fiber continuous coherent Doppler laser speed measuring radar optical noise according to claim 1, is characterized in that, described amplifier is the erbium-doped optical fiber amplifier, and described first coupler is high resolution ratio A fiber optic coupler, the second coupler is a 3dB fiber optic coupler. 4. A suppression method based on the suppression system of the optical noise of the all-fiber continuous coherent Doppler laser speed measuring radar according to any one of claims 1-3, wherein the specific steps include: S1, the narrow-linewidth continuous laser emitted by the narrow-linewidth laser is divided into local oscillator light and seed light by the first coupler, S2, the seed light in step S1 is amplified by the amplifier, and injected as the probe light into the first port of the polarizing beam splitter, the APC end face of the second port of the polarizing beam splitter is coated with an anti-reflection film, S3, through the 1/4 wave plate with the fast axis and the polarization direction at an angle of 45 degrees, it is converted into circularly polarized light, and then returns to the target after being shot through the beam expander system 7, S4, the backscattered light carrying the detection target information is converted into linearly polarized light through the 1/4 wave plate again, and the polarization direction of the linearly polarized light will be perpendicular to the polarization direction of the detection light, S5, the linearly polarized light in step S4 is injected from the second port of the polarization beam splitter, and emitted from the third port of the polarization beam splitter to obtain signal light, S6, the signal light and local oscillator light in step S5 are mixed and sent to a balanced detector for detection, and the difference frequency signal output by the balanced detector is sent to a signal processing unit for processing. 5. A system for suppressing optical noise of an all-fiber continuous coherent Doppler laser speed measuring radar is characterized in that it comprises a narrow linewidth laser, a first coupler, an optical fiber circulator, a polarization beam splitter, a second coupler, a quasi Straightener, 1/4 wave plate, beam expander system, detection target and signal processing unit, the narrow linewidth laser is connected to the input end of the first coupler, and the first output end of the first coupler is passed through the amplifier Connected to the first port of the optical fiber circulator, the collimator, the 1/4 wave plate and the beam expander system are sequentially arranged between the optical fiber circulator and the detection target, and the optical fiber circulator The APC end face of the second port of the device is coated with an anti-reflection film, the third port of the optical fiber circulator is connected to the second coupler through the polarization beam splitter, and the fast axis of the 1/4 wave plate is connected to the second coupler. The polarization direction is set at an included angle of 45 degrees, the second output end of the first coupler is also connected to the second coupler, and the second coupler is connected to the signal processing unit through a balanced detector. 6. The suppression system of optical noise of a kind of all-fiber continuous coherent Doppler laser velocity measuring radar according to claim 5, is characterized in that, described narrow-linewidth laser is distributed feedback laser, distributed Bragg reflection laser, external cavity A type of semiconductor laser. 7. The suppression system of optical noise of a kind of all-fiber continuous coherent Doppler laser speed measuring radar according to claim 5, is characterized in that, described amplifier is the erbium-doped fiber amplifier, and described first coupler is high resolution ratio A fiber optic coupler, the second coupler is a 3dB fiber optic coupler. 8. A suppression method based on the suppression system of the optical noise of the all-fiber continuous coherent Doppler laser speed measuring radar according to any one of claims 5-7, wherein the specific steps include: S1, the narrow-linewidth continuous laser emitted by the narrow-linewidth laser is divided into local oscillator light and seed light by the first coupler, S2, the seed light in step S1 is amplified by the amplifier, injected as the probe light into the first port of the optical fiber circulator, the APC end face of the second port of the optical fiber circulator is coated with an anti-reflection film, S3, through the 1/4 wave plate with the fast axis and the polarization direction at an angle of 45 degrees, it is converted into circularly polarized light, and then returns to the target after being shot through the beam expander system 7, S4, the backscattered light carrying the detection target information is converted into linearly polarized light through the 1/4 wave plate again, and the polarization direction of the linearly polarized light will be perpendicular to the polarization direction of the detection light, S5, the linearly polarized light in step S4 is injected from the second port of the optical fiber circulator, and emitted from the third port of the optical fiber circulator to obtain signal light, S6, the signal light in step S5 is mixed with the local oscillator light by the polarization beam splitter and sent to the balanced detector for detection, and the difference frequency signal output by the balanced detector is sent to the signal processing unit for processing.

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