Line width detection system of narrow line width laser based on spectrum reconstruction

Abstract

The invention relates to a narrow linewidth laser linewidth detection system based on spectrum reconstruction. The narrow linewidth laser linewidth detection system comprises a beam expanding collimation unit, an N-step etalon frequency discrimination unit, a beam splitting focusing unit, N point detectors and a multi-channel acquisition and data processing unit. The beam expanding and collimating unit is used for carrying out light beam collimation on incident measured laser, the N-step etalon frequency discrimination unit comprises N F-P cavities which are spliced into a multi-step etalon with a circular clear aperture, and the F-P cavities are used for respectively gating different spectral ranges of the measured laser and outputting parallel light with different energies. The beam splitting and focusing unit focuses the parallel light output by the N F-P cavities to the light sensing surfaces of the N point detectors respectively, and electric signals are formed through photoelectric conversion. And the multi-channel acquisition and data processing unit obtains a power measured value IM of each point detector, and calculates a power theoretical calculation value IjM of the N point detectors at the same time, thereby obtaining a line width value sigma of the measured single-frequency narrow-line-width laser. According to the invention, the problem that the 3dB bandwidth is difficult to directly read due to insufficient distinguishing capability of the existing frequency discriminator is solved.

Description

Technical Field

[0001] The invention belongs to the field of laser hyperspectral detection and relates to a system that adopts a multi-step etalon to perform high-spectral resolution detection, which can achieve kHz-level spectrum detection accuracy. Background Art

[0002] Narrow linewidth lasers have important application prospects in the fields of gravitational wave detection, fiber optic sensing, coherent lidar, coherent communications, coherent synthesis, optical frequency standards, and precision measurement due to their ultra-long coherence length and other characteristics. The linewidth control and frequency stabilization of narrow linewidth lasers have an important impact on high-precision measurements, and accurate measurement of linewidth is a prerequisite. Interferometers can theoretically measure laser linewidths of arbitrary precision, but when the linewidth of narrow linewidth lasers reaches the kHz level, linewidth measurement using 3dB bandwidth as a direct criterion requires extremely high precision or a kilometer-long cavity length. Such a test system is difficult to implement or is extremely expensive.

[0003] At present, there are two commonly used linewidth detection methods: heterodyne method and delayed heterodyne method. The measurement equipment of the heterodyne method can only detect wavelengths within a specific range. It requires a reference light source. For kHz-level linewidth measurement, the wavelength difference between the reference light source and the wavelength of the laser to be measured needs to be controlled within 100 Hz. The development of the reference light source is difficult and expensive, and it must be customized according to the wavelength to be measured. The delayed heterodyne method is insensitive to the central wavelength. The resolution of narrow linewidth is proportional to the length of the delayed optical fiber. The linewidth resolution of tens of kHz requires the optical fiber extension line to be hundreds of kilometers, making the system large and complex. The low-precision wedge-shaped discriminator combined with computational spectrum reconstruction can realize the detection of the central wavelength and linewidth of narrow linewidth lasers, breaking through the problem that the existing discriminator is difficult to directly measure the spectrum. However, the high-sensitivity linear array detector is expensive and adjacent pixels are prone to crosstalk, which increases the error of spectral solution. Summary of the invention

[0004] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art and provide a linewidth detection system for kHz-level narrow linewidth lasers, using a low-cost, low-fineness multi-step standard tool in combination with a corresponding number of point detectors, and according to the characteristics that the extremely narrow linewidth laser will present different energy distributions on the point detector according to the wavelength size after being collimated through the multi-step calibration tool, the central wavelength and linewidth of the laser are calculated by using spectral reconstruction, thereby solving the problem that the existing discriminator has insufficient resolution and is difficult to directly read out the 3dB bandwidth.

[0005] The technical solution of the present invention is: a line width detection system of a narrow line width laser based on spectral reconstruction, comprising a beam expansion and collimation unit, an N-step etalon frequency discrimination unit, a beam splitting and focusing unit, N point detectors, a multi-channel acquisition and data processing unit, N is a positive integer not less than 2, wherein:

[0006] Beam expansion and collimation unit: used to collimate the incident single-frequency narrow-linewidth laser to be measured, adjust the beam into parallel light, and make the spot of the parallel beam just completely cover the aperture of the N-step etalon frequency discrimination unit;

[0007] N-step etalon frequency discrimination unit: It includes N FP cavities with different cavity lengths, which are spliced ​​into a multi-step etalon with a circular aperture. Each FP cavity selects different spectrum ranges of the single-frequency narrow-linewidth laser to be measured. After the single-frequency narrow-linewidth laser to be measured undergoes multi-beam interference between the two ends of the FP cavity, the FP cavities with different cavity lengths output parallel light of different energies to the beam splitting and focusing unit.

[0008] Beam splitting and focusing unit: focuses the parallel light output by N FP cavities onto the photosensitive surfaces of N point detectors respectively;

[0009] N point detectors: Each detector pixel corresponds to the FP cavity one by one, each performs photoelectric conversion, and inputs the electrical signal to the multi-channel acquisition and data processing unit;

[0010] Multi-channel acquisition and data processing unit: obtain the power measured value I of each point detector M , M represents the Mth point detector, M = 1, 2, ..., N; at the same time, the power theoretical value I of N point detectors is calculated jM , according to the measured power value I M And the power theoretical calculation value I jM , the linewidth value σ of the single-frequency narrow-linewidth laser under test is calculated.

[0011] Preferably, the value of N is 3, 4 or 5.

[0012] Furthermore, the multi-channel acquisition and data processing unit calculates the line width value σ of the single-frequency narrow line width laser being measured, specifically:

[0013] Get the theoretical power value I of the Mth point detector pixel jM ,

[0014]

[0015] Among them, I jM (ν i ) are the different frequency components of the single-frequency narrow-linewidth laser being measured (ν0+ν i ) After passing through the two plates at both ends of the FP cavity, the detection power of the Mth detector changes with the laser incident frequency; v i Represents the frequency difference between the frequency contained in the single-frequency narrow-linewidth laser being measured and the center frequency of the laser, v iThe value range of is set to [-FSR / 2+(i-1)×FSR / K,-FSR / 2+i×FSR / K], i=1,2,…K, K is a positive integer, and K>>1, v0=c / n / λ0 is the center frequency of the laser, c is the speed of light in vacuum, FSR is the free spectral range of the FP cavity; v F-P , d, σ is the Gaussian line shape characteristic of the single-frequency narrow line width laser being measured, v F-P represents the frequency difference between the center frequency of the FP cavity and the center frequency of the laser, σ is the spectrum line width of the laser, d is the normalized power amplitude, and Parameter F = 4R / (1-R) 2 , where R is the reflectivity of the plates at both ends of the FP cavity;

[0016] The measured power value I for each pixel M And the power theoretical calculation value I jM Perform normalization processing to obtain the normalized power measured value I of each pixel M1 And the power theoretical calculation value I jM1 ,

[0017]

[0018] The nonlinear regression method is used to solve N simultaneous equations {I M1 =I jM1}, and obtain the optimal solution for the line width value σ.

[0019] Preferably, the laser frequency range of the single-frequency narrow-linewidth laser to be measured is 1 kHz to 500 kHz.

[0020] Preferably, the beam expansion and collimation unit comprises an aperture, a collimating optical path, an interference filter and a beam expansion optical component, wherein the aperture is located at the front end of the collimating optical path for shielding stray light; the collimating optical path is used to perform beam collimation processing on the input single-frequency narrow-linewidth laser to be measured, and send the parallel light beam into the interference filter; the interference filter performs narrow-band filtering on the ambient stray light, and sends the parallel light beam into the beam expansion optical component after filtering out the stray light outside the band; the beam expansion optical component performs beam expansion and shaping on the parallel light beam, and makes the light spot of the parallel light beam just completely cover the light aperture of the N-step standard frequency discrimination unit.

[0021] Preferably, the FP cavity is vacuum packaged, and the cavity material is selected to have an expansion coefficient better than 10 -7 / K microcrystalline glass; the cavity consists of two opposite plates, and the inner side of the plate is coated with a reflective film of the laser center wavelength.

[0022] Preferably, the reflectivity of the reflective film ranges from 70% to 80%.

[0023] Furthermore, the cavity length h of the Mth step etalon of the N-step etalon frequency discrimination unit is M Satisfy h M =[q+(M-1) / (2(N-1))]×(λ0 / 2n), where n represents the refractive index of the medium in the FP cavity, for vacuum medium n=1, M is a positive integer and M=1, 2,…N, the value of q is the quotient of c / λ0 / FSR rounded off, c is the speed of light in vacuum, λ0 represents the central wavelength of the single-frequency narrow-linewidth laser being measured, and FSR is the free spectral range of the FP cavity.

[0024] Preferably, the beam splitting and focusing unit is N cylindrical lenses.

[0025] Preferably, the point detector is a photomultiplier tube PMT or an avalanche diode APD.

[0026] The advantages of the present invention compared with the prior art are:

[0027] (1) The system of the present invention targets single-frequency narrow-linewidth lasers at the kHz level, uses a low-cost, low-fineness, multi-step standard discriminator to modulate the energy of the input laser, and then calculates the line width by a computational spectrum reconstruction method. The measurement accuracy of the line width is no longer proportional to the fineness of the discriminator, which reduces the index requirements of the discriminator for line width detection. In addition, the transmittance function response of different frequencies can be obtained by using several discrete low-cost point detectors, which can reduce system complexity and cost.

[0028] (2) The low-fineness multi-step etalon discriminator of the system of the present invention has high transmittance and low energy loss, and can be used for line width detection of laser echoes reflected by the target; it does not require a high-sensitivity array detector, and can measure the center wavelength and line width of the laser only by using a few discrete high-sensitivity point detectors;

[0029] (3) The method of calculating spectrum reconstruction by the system of the present invention is based on the reflectivity, aperture D, and spacing h of the multi-beam interferometer. M The distribution relationship between the incident laser spectrum and the output laser power is established by using parameters such as the free spectral range (FSR) and other parameters. Combined with the output results of the measured multi-point detector, the complete spectral shape of the incident laser beam can be obtained at the same time, that is, the central wavelength and line width can be obtained, and the measured wavelength range is adjustable. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a block diagram of the composition principle of the system of the present invention;

[0031] Figure 2 Schematic diagram of the splicing configuration of the N-step etalon under the conditions of N=3, 4, 5 of the present invention;

[0032] Figure 3Schematic diagram of three transmittance functions of the three-step etalon of the present invention relative to the laser spectrum;

[0033] Figure 4 It is a schematic diagram of the power response of the laser spectrum of the present invention for different spectra after being modulated by a high-fineness 3-step etalon (fineness 3140) and a low-fineness 3-step etalon (fineness 8.76). DETAILED DESCRIPTION

[0034] like Figure 1 As shown, the system of the present invention mainly includes: a beam expansion and collimation unit 1, an N-step etalon frequency discrimination unit 2, a beam splitting and focusing unit 3, N point detectors 4, and a multi-channel acquisition and data processing unit 5, where the value of N is 3, 4 or 5.

[0035] The beam expansion and collimation unit 1 is used to collimate the incident single-frequency narrow-linewidth laser beam, adjust the beam into parallel light, and make the beam spot completely cover the aperture of the N-step etalon frequency discrimination unit 2 through beam expansion and shaping. The laser frequency range of the single-frequency narrow-linewidth laser that can be processed by the system of the present invention is 1kHz to 500kHz.

[0036] The beam expansion and collimation unit 1 comprises an aperture, a collimation optical path, an interference filter and a beam expansion optical component, wherein the aperture is located at the front end of the collimation optical path for shielding stray light; the collimation optical path is used to collimate the input single-frequency narrow-linewidth laser beam, send the parallel light beam into the interference filter, perform narrow-band filtering on the ambient stray light, filter out the out-of-band stray light of the interference filter, and then send the parallel light beam into the beam expansion optical component, which performs beam expansion and shaping so that the light spot of the output parallel light beam just covers the clear aperture of the N-step standard frequency discrimination unit 2.

[0037] The N-step etalon frequency discrimination unit 2 is composed of N Fabry-Perot cavities (FP cavities) with different cavity lengths, which respectively select different spectrum ranges of single-frequency narrow-linewidth lasers. N FP cavities are spliced ​​into a multi-step etalon with a circular aperture. The FP cavity is vacuum packaged, and the cavity material is selected with an expansion coefficient better than 10 -7 / K microcrystalline glass. The cavity is composed of two opposite plates, and the inner side of the plate is coated with a reflective film of the laser center wavelength (the reflectivity R ranges from 70% to 80%). After the single-frequency narrow-linewidth laser undergoes multi-beam interference between the two ends of the cavity, parallel light of different energies is output correspondingly after the FP cavity with different cavity lengths. The energy output by N FP cavities is input to the beam splitting and focusing unit 3.

[0038] The cavity length h of the Mth step etalon of the N-step etalon frequency discrimination unit 2 M Satisfy h M=[q+(M-1) / (2(N-1))]×(λ0 / 2n), where n represents the refractive index of the medium in the cavity of the etalon, for vacuum medium n=1, M is a positive integer and M=1,2,…N. q represents a positive integer, and its value is the integer of the quotient of c / λ0 / FSR, where c is the speed of light in vacuum, λ0 represents the central wavelength of the laser, and FSR is the free spectral range of the frequency discrimination unit 2 of the N-step etalon.

[0039] The beam splitting and focusing unit 3 focuses the energy output by the N FP cavities onto the photosensitive surfaces of the N point detectors 4. The detector pixels of the point detectors 4 correspond to the FP cavities one by one. The beam splitting and focusing unit 3 is composed of N cylindrical lenses.

[0040] The N point detectors 4 each perform photoelectric conversion and input the electrical signal to the multi-channel acquisition and data processing unit 5. The point detector 4 uses a high-sensitivity point detector (each point detector has only one pixel), and can use a corresponding photomultiplier tube PMT or avalanche diode APD.

[0041] The multi-channel acquisition and data processing unit 5 obtains the power measured value I of each point detector 4 M , M represents the Mth point detector, M = 1, 2, ..., N. According to the linear distribution model of the spectrum of the single-frequency narrow-linewidth laser and the device function of the N-step standard frequency discrimination unit 2, the power theoretical calculation value I of the N point detectors can be calculated. jM ; According to the measured power value I M And the power theoretical calculation value I jM , the line width σ of the laser is calculated.

[0042] The multi-channel acquisition and data processing unit 5 calculates the line width value σ, which is specifically:

[0043] Get the theoretical power value I of the Mth point detector pixel jM ,

[0044]

[0045] Among them, I jM (ν i ) Different frequencies of incident laser (ν0+ν i ) component passes through the plate, the change in the detection power of the Mth detector with the laser incident frequency. i Represents the frequency difference between the frequency contained in the laser and the center frequency of the laser, the frequency difference v iThe value range of is set to [-FSR / 2+(i-1)×FSR / K,-FSR / 2+i×FSR / K], i=1,2,…K, K is a positive integer, and K>>1, v0=c / n / λ0 is the center frequency of the laser, c is the speed of light in vacuum, and FSR is the free spectral range of the FP cavity in the N-step etalon frequency discrimination unit 2. F-P , d, σ are the characteristic quantities of the Gaussian line shape of the input laser to be measured; v F-P represents the frequency difference between the center frequency of the FP cavity and the center frequency of the laser, σ is the spectrum line width of the laser, d is the normalized power amplitude, and Parameter F = 4R / (1-R) 2 , where R is the reflectivity of the plates at both ends of the FP cavity in the N-step etalon frequency discrimination unit 2.

[0046] The measured energy value I of each pixel M and energy theoretical calculation value I jM , perform normalization processing on the energy measurement value of the first point detector pixel. The first detector refers to the point detector corresponding to the cavity with a cavity length of h1. h1 = q × (λ0 / 2n) represents the cavity length of the first step etalon, where q is the integer of the quotient of c / λ0 / FSR. Thus, the energy measurement value I of each pixel after normalization processing is obtained. M1 and energy theoretical calculation value I jM1 , specifically:

[0047]

[0048] The measured energy value I after normalization of N pixels one by one M1 and energy theoretical calculation value I jM1 By combining them, we get:

[0049] {I M1 =I jM1}, M = 1, 2, ... N;

[0050] The nonlinear regression method is used to solve N simultaneous equations to obtain the line width values ​​σ and v i The optimal solution of .

[0051] Example

[0052] The following is an example of a wavelength to be measured with a central wavelength λ=1060nm and a line width ≤100kHz, but the present invention is not limited to solving the line width detection of a single-frequency narrow-line width laser with a central wavelength of 1060nm. The present invention is effective for line width detection of wavelengths from ultraviolet to near-infrared.

[0053] The laser frequency value of the single-frequency narrow-linewidth laser is 45kHz.

[0054] The bandwidth of the narrowband filter in the beam expansion and collimation unit 1 is ±0.3 nm.

[0055] The N-step etalon frequency discrimination unit 2 is composed of N FP cavities with different cavity lengths. The N FP cavities of the N-step etalon frequency discrimination unit are spliced ​​into a multi-step etalon with a circular clear aperture, such as Figure 2 As shown, the schematic diagram of the splicing configuration of N-step etalon with N=3, 4, 5, the full aperture diameter is 30mm. The frequency difference of the center frequency of the FP cavity relative to 1060nm is 0.2MHz, and the free spectral range FSR=150MHz.

[0056] Each FP cavity is composed of two oppositely placed plates, with a vacuum in the middle of the cavity. The opposite inner sides of the two plates are coated with a high-reflection film with a laser central wavelength λ=1060nm, and the reflectivity of the high-reflection film is R=0.7.

[0057] The cavity length h of the Mth step etalon of the N-step etalon frequency discrimination unit 2 M The value is around 1m and satisfies h M =[q+(M-1) / (2(N-1))]×(λ0 / 2n), where q is the integer of c / λ0 / FSR. The cavity material should have an expansion coefficient better than 10 -7 / K microcrystalline glass, free spectral range FSR = 150MHz, the vacuum chamber of the FP cavity adopts active temperature control, and the temperature is stable at 20℃±0.5℃.

[0058] like Figure 3 As shown, the estimated value of the central wavelength is λ=1060nm, the line width is through a multi-step standard with a flat plate reflectivity R=0.7 (fineness F=8.76), with the number of steps N=3, and the transmittance functions of the three standard instruments with different cavity lengths and the spectrum of the narrow linewidth Gaussian laser (linewidth 45Hz) are shown in the figure. The first standard instrument has a higher transmittance, and the third standard instrument has a lower transmittance.

[0059] Theoretical calculation value of the power of the Mth point detector pixel I jM , I jM (ν i ) Different frequencies of incident laser ν i After the component passes through the plate, the detection power of the Mth detector changes with the laser incident frequency. i Represents the frequency difference between the frequency contained in the laser and the center frequency of the laser, the frequency difference v i The value range of is set to [-FSR / 2+(i-1)×FSR / K,-FSR / 2+i×FSR / K], i=1,2,…K, and K is 4000.

[0060] like Figure 4As shown, the frequency difference between the center frequency of the 3-step etalon and the center frequency of the laser is 0.2MHz, the center wavelength of the laser is λ=1060nm, and its spectrum (line width 45kHz) passes through the 3-step etalon with a plate reflectivity R=0.999 (fineness F=3140) and the peak power (max(I jM ( i ))=0.065) The peak power (max(I jM ( i ))=0.409) is more than 60 times lower. A low-fineness 3-step etalon is used in conjunction with 3 point detectors. Different energy distributions are presented on the point detectors according to the wavelength. The central wavelength of the laser to be measured is calculated to be 1060nm, the line width is 45.2kHz, and the error of the line width is about 0.44%.

[0061] The contents not described in detail in the specification of the present invention belong to the common knowledge of those skilled in the art.

Claims

1. A linewidth detection system for narrow linewidth laser based on spectral reconstruction, characterized in that: It comprises a beam expansion and collimation unit (1), an N-step etalon frequency discrimination unit (2), a beam splitting and focusing unit (3), N point detectors (4), and a multi-channel acquisition and data processing unit (5), wherein N is a positive integer not less than 2, wherein: The beam expansion and collimation unit (1) is used to collimate the incident single-frequency narrow-linewidth laser to be measured, adjust the beam into parallel light, and make the light spot of the parallel beam just completely cover the light aperture of the N-step standard instrument frequency discrimination unit (2); The N-step etalon frequency discrimination unit (2) comprises N FP cavities of different cavity lengths, the N FP cavities are spliced ​​into a multi-step etalon with a circular aperture, each FP cavity respectively selects a different spectrum range of the single-frequency narrow-linewidth laser to be measured; after the single-frequency narrow-linewidth laser to be measured generates multi-beam interference between the two ends of the FP cavity, the FP cavities with different cavity lengths output parallel light of different energies to the beam splitting and focusing unit (3); A beam splitting and focusing unit (3) is used to focus the parallel light outputted by the N FP cavities onto the photosensitive surfaces of the N point detectors (4); N point detectors (4): each detector pixel corresponds to the FP cavity one by one, each performs photoelectric conversion, and inputs the electrical signal to the multi-channel acquisition and data processing unit (5); Multi-channel acquisition and data processing unit (5): obtains the power measured value I of each point detector (4) M , M represents the Mth point detector, M = 1, 2, ..., N; at the same time, the power theoretical value I of N point detectors is calculated jM , according to the measured power value I M And the power theoretical calculation value I jM , the linewidth value σ of the single-frequency narrow-linewidth laser under test is calculated.

2. The linewidth detection system of narrow linewidth laser based on spectrum reconstruction according to claim 1, characterized in that: The value of N is 3, 4 or 5.

3. A linewidth detection system for narrow linewidth laser based on spectrum reconstruction according to claim 1 or 2, characterized in that: The multi-channel acquisition and data processing unit (5) calculates the line width value σ of the single-frequency narrow line width laser being measured, which is specifically: Get the theoretical power value I of the Mth point detector pixel jM , Among them, I jM (ν i ) are the different frequency components of the single-frequency narrow-linewidth laser being measured (ν0+ν i ) After passing through the two plates at both ends of the FP cavity, the detection power of the Mth detector changes with the laser incident frequency; v i Represents the frequency difference between the frequency contained in the single-frequency narrow-linewidth laser being measured and the center frequency of the laser, v i The value range of is set to [-FSR / 2+(i-1)×FSR / K,-FSR / 2+i×FSR / K], i=1,2,…K, K is a positive integer, and K>>1, v0=c / n / λ0 is the center frequency of the laser, c is the speed of light in vacuum, FSR is the free spectral range of the FP cavity; v F-P , d, σ is the Gaussian line shape characteristic of the single-frequency narrow line width laser being measured, v F-P represents the frequency difference between the center frequency of the FP cavity and the center frequency of the laser, σ is the spectrum line width of the laser, d is the normalized power amplitude, and Parameter F = 4R / (1-R) 2 , where R is the reflectivity of the plates at both ends of the FP cavity; The measured power value I for each pixel M And the power theoretical calculation value I jM Perform normalization processing to obtain the normalized power measured value I of each pixel M1 And the power theoretical calculation value I jM1 , The nonlinear regression method is used to solve N simultaneous equations {I M1 =I jM1 }, and obtain the optimal solution for the line width value σ.

4. A linewidth detection system for narrow linewidth laser based on spectrum reconstruction according to claim 1 or 2, characterized in that: The laser frequency range of the single-frequency narrow-linewidth laser to be measured is 1kHz to 500kHz.

5. The linewidth detection system of narrow linewidth laser based on spectrum reconstruction according to claim 1 or 2, characterized in that: The beam expansion and collimation unit (1) comprises an aperture, a collimation optical path, an interference filter and a beam expansion optical component, wherein the aperture is located at the front end of the collimation optical path and is used to block stray light; the collimation optical path is used to collimate the input single-frequency narrow-linewidth laser to be measured and send the parallel light beam into the interference filter; The interference filter performs narrow-band filtering on the ambient stray light, and after filtering out the out-of-band stray light, sends the parallel light beam to the beam expansion optical component; the beam expansion optical component performs beam expansion and shaping on the parallel light beam, and makes the light spot of the parallel light beam just completely cover the light aperture of the N-step standard frequency discrimination unit (2).

6. The linewidth detection system of narrow linewidth laser based on spectrum reconstruction according to claim 1 or 2, characterized in that: The FP cavity is vacuum packaged, and the cavity material is selected with an expansion coefficient better than 10 -7 / K microcrystalline glass; the cavity consists of two opposite plates, and the inner side of the plate is coated with a reflective film of the laser center wavelength.

7. The linewidth detection system of narrow linewidth laser based on spectrum reconstruction according to claim 6, characterized in that: The reflectivity of the reflective film ranges from 70% to 80%.

8. The linewidth detection system of narrow linewidth laser based on spectrum reconstruction according to claim 1 or 2, characterized in that: The cavity length h of the Mth step etalon of the N-step etalon frequency discrimination unit (2) is M Satisfy h M =[q+(M-1) / (2(N-1))]×(λ0 / 2n), where n represents the refractive index of the medium in the FP cavity, for vacuum medium n=1, M is a positive integer and M=1, 2,…N, the value of q is the quotient of c / λ0 / FSR rounded off, c is the speed of light in vacuum, λ0 represents the central wavelength of the single-frequency narrow-linewidth laser being measured, and FSR is the free spectral range of the FP cavity.

9. The linewidth detection system of narrow linewidth laser based on spectrum reconstruction according to claim 1 or 2, characterized in that: The beam splitting and focusing unit (3) is N cylindrical lenses.

10. The linewidth detection system of narrow linewidth laser based on spectrum reconstruction according to claim 1 or 2, characterized in that: The point detector (4) adopts a photomultiplier tube PMT or an avalanche diode APD.

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