A narrow linewidth laser linewidth detection system based on spectral reconstruction

Abstract

A linewidth detection system for narrow linewidth lasers based on spectral reconstruction includes a beam expansion and collimation unit, an N-step etalon frequency discrimination unit, a beam splitting and focusing unit, N point detectors, and a multi-channel acquisition and data processing unit. The beam expansion and collimation unit collimates the incident laser to be measured. The N-step etalon frequency discrimination unit includes N F-P cavities, which are spliced ​​into a multi-step etalon with a circular aperture. Each F-P cavity selects a different spectral range of the laser to be measured and outputs parallel light of different energies. The beam splitting and focusing unit focuses the parallel light output by the N F-P cavities onto the photosensitive surfaces of the N point detectors, and forms electrical signals through photoelectric conversion. The multi-channel acquisition and data processing unit obtains the actual power measurement value I of each point detector. M , and simultaneously calculate the theoretical power value I of N point detectors jM The present invention solves the problem that the existing frequency discriminator has insufficient resolution and is unable to directly read out the 3dB bandwidth.

Description

Technical Field

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

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

[0003] Currently, two commonly used linewidth detection methods are heterodyne and delayed heterodyne. Heterodyne measurement equipment can only detect wavelengths within a specific range and requires a reference light source. For kHz-level linewidth measurements, the wavelength difference between the reference light source and the laser to be measured must be controlled within 100 Hz. Reference light sources are difficult and expensive to develop and must be customized based on the wavelength to be measured. Delayed heterodyne is insensitive to the center wavelength, and the resolution of narrow linewidths is proportional to the length of the delay fiber. Linewidth resolution of tens of kHz requires fiber extension cables on the order of hundreds of kilometers, resulting in a large and complex system. A low-fineness wedge-shaped discriminator, combined with computational spectral reconstruction, can detect the center wavelength and linewidth of narrow-linewidth lasers, overcoming the difficulty of existing discriminators in directly measuring spectra. However, high-sensitivity linear array detectors are expensive and prone to crosstalk between adjacent pixels, increasing errors in spectral resolution. Summary of the Invention

[0004] The technical problem solved by the present invention is: to overcome the shortcomings of the existing technology and provide a linewidth detection system for kHz-level narrow-linewidth lasers. The system adopts a low-cost, low-fineness multi-step etalon in combination with a corresponding number of point detectors. 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 etalon, the central wavelength and linewidth of the laser are calculated by using spectral reconstruction, which solves 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 narrow linewidth laser linewidth detection system based on spectral reconstruction, including a beam expansion and collimation unit, an N-step etalon frequency discrimination unit, a beam splitting and focusing unit, N point detectors, and a multi-channel acquisition and data processing unit, where 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 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. These N FP cavities are spliced ​​into a multi-step etalon with a circular optical aperture. Each FP cavity selects a different spectral 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.

[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, 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 theoretical calculated value of power I jM , calculate and obtain the linewidth value σ of the measured single-frequency narrow-linewidth laser.

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

[0012] Furthermore, the multi-channel acquisition and data processing unit calculates the linewidth value σ of the single-frequency narrow-linewidth 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 measured single-frequency narrow-linewidth laser and the laser center frequency, 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 linewidth 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 linewidth 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 of each pixel I M And the theoretical calculated value of power I jM Perform normalization processing to obtain the normalized power measured value I of each pixel M1 And the theoretical calculated value of power I jM1 ,

[0017]

[0018] Use nonlinear regression method 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 includes 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 and is used to block stray light; the collimating 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, filters out the out-of-band stray light, and then sends the parallel light beam into the beam expansion optical component; the beam expansion optical component expands and shapes 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 opposing plates, the inner side of which 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 inside 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 to the nearest integer, c is the speed of light in vacuum, λ0 represents the central wavelength of the single-frequency narrow-linewidth laser to be 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. It uses a low-cost, low-fineness, multi-step etalon frequency discriminator to modulate the energy of the input laser and then calculates the linewidth by a computational spectrum reconstruction method. The measurement accuracy of the linewidth is no longer proportional to the fineness of the frequency discriminator, which reduces the requirements of the frequency discriminator for linewidth detection. In addition, several discrete, low-cost point detectors can be used to obtain transmittance function responses at different frequencies, 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 to detect the line width of the laser echo reflected by the target; without the need for a high-sensitivity array detector, the central wavelength and line width of the laser can be measured by only 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 , free spectral range (FSR) and other parameters to establish the distribution relationship between the incident laser spectrum and the output laser power, and then combine it with the output results of the measured multi-point detector to obtain the complete spectral shape of the incident laser beam, that is, the central wavelength and line width, 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 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, adjusting the beam into parallel light. Through beam expansion and shaping, the beam spot is precisely aligned with the aperture of the N-step etalon frequency discrimination unit 2. The system can handle single-frequency, narrow-linewidth lasers with a frequency range of 1kHz to 500kHz.

[0036] The beam expansion and collimation unit 1 includes 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 and is used to block stray light; the collimating optical path is used to collimate the input single-frequency narrow-linewidth laser beam, send the parallel beam into the interference filter, perform narrow-band filtering on the ambient stray light, filter out the stray light out of the interference filter band, and then send the parallel beam into the beam expansion optical component, which performs beam expansion and shaping by the beam expansion optical component so that the light spot of the output parallel 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 spectral 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-encapsulated, and the cavity material is selected with an expansion coefficient better than 10 -7 / K glass-ceramic. The cavity consists of two opposing flat plates, the inner surfaces of which are coated with a reflective coating at the laser's center wavelength (reflectivity R ranges from 70% to 80%). Single-frequency, narrow-linewidth laser light undergoes multi-beam interference between the two ends of the cavity, and parallel light of varying energies is output after the FP cavities of varying lengths. The energy output from these N FP cavities is fed into 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 within the etalon cavity. For a vacuum medium, n=1, M is a positive integer, and M=1, 2, ..., N. q represents a positive integer, calculated as the quotient of c / λ0 / FSR, rounded to the nearest integer, where c is the speed of light in a vacuum, λ0 represents the central wavelength of the laser, and FSR is the free spectral range of the N-step etalon's frequency discrimination unit 2.

[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 one to one with the FP cavities. The beam splitting and focusing unit 3 is composed of N cylindrical lenses.

[0040] 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 detectors 4 use high-sensitivity point detectors (each point detector has only one pixel), which can use corresponding photomultiplier tubes PMT or avalanche diodes 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 etalon frequency discrimination unit 2, the theoretical power calculation value I of the N point detectors can be calculated. jM ; According to the measured power value I M And the theoretical calculated value of power I jM , the laser line width σ 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 the 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 linewidth 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 of each pixel I 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 quotient of c / λ0 / FSR rounded to an integer. The energy measurement value of each pixel after normalization is obtained. M1 and energy theoretical calculation value I jM1 , specifically:

[0047]

[0048] The measured energy value I after normalization of N pixels is M1 and energy theoretical calculation value I jM1 Perform the combination and 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 description takes the wavelength to be measured with a central wavelength λ=1060nm and a linewidth ≤100kHz as an example, but the present invention is not limited to solving the linewidth detection of a single-frequency narrow-linewidth laser with a central wavelength of 1060nm. The present invention is effective for linewidth 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 Figure 2 shows a schematic diagram of the spliced ​​configuration of N-step etalons with N = 3, 4, and 5, with a full aperture diameter of 30 mm. The center frequency of the FP cavity has a frequency difference of 0.2 MHz relative to 1060 nm, and a free spectral range (FSR) of 150 MHz.

[0056] Each FP cavity is composed of two oppositely placed plates forming a cavity. The middle of the cavity is vacuum. The inner surfaces of the two plates are coated with a high-reflection film with a laser center wavelength of λ = 1060nm. 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 meets h M =[q+(M-1) / (2(N-1))]×(λ0 / 2n), where q is the integer of the quotient of c / λ0 / FSR. The cavity material should have an expansion coefficient better than 10 -7 / K glass-ceramics, free spectral range FSR = 150MHz, the vacuum chamber of the FP cavity adopts active temperature control, and the temperature is stabilized at 20℃±0.5℃.

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

[0059] The theoretical calculated power value of the M-th point detector pixel is 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 (finess F=3140) and the peak power (max(I jM (υ i ))=0.065) The peak power (max(I jM (υ i )) = 0.409) is over 60 times lower. Using a low-finesse three-step etalon with three point detectors, different energy distributions are presented on the point detectors depending on the wavelength. Computational spectrum reconstruction is used to calculate the central wavelength of the measured laser to be 1060nm, with a linewidth of 45.2kHz, and a linewidth error of approximately 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 narrow linewidth laser linewidth detection system based on spectral reconstruction, characterized by: The invention 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: Beam expansion and collimation unit (1): used for collimating the incident single-frequency narrow-linewidth laser to be measured, adjusting the beam into parallel light, and making the light spot of the parallel beam just completely cover the light aperture of the N-step etalon frequency discrimination unit (2); N-step etalon frequency discrimination unit (2): comprising N FP cavities of different cavity lengths, the N FP cavities being spliced ​​into a multi-step etalon with a circular aperture, each FP cavity being respectively selected for a different spectrum range 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 of different cavity lengths output parallel light of different energies to the beam splitting and focusing unit (3); Beam splitting and focusing unit (3): focusing the parallel light outputted by N FP cavities onto the photosensitive surfaces of 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 theoretical calculated value of power I jM , calculate the linewidth value σ of the single-frequency narrow-linewidth laser under test; 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, 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 measured single-frequency narrow-linewidth laser and the laser center frequency, 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 linewidth 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 linewidth 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; h M is the cavity length of the M-th step etalon; The measured power value of each pixel I M And the theoretical calculated value of power I jM Perform normalization processing to obtain the normalized power measured value I of each pixel M1 And the theoretical calculated value of power I jM1 , Use nonlinear regression method to solve N simultaneous equations {I M1 =I jM1 }, and obtain the optimal solution for the line width value σ.

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

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

4. A narrow linewidth laser linewidth detection system based on spectral reconstruction according to claim 1 or 2, characterized in that: The beam expansion and collimation unit (1) 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 and is used to block stray light; the collimating 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 into 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).

5. The narrow linewidth laser linewidth detection system based on spectral 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 glass-ceramic; the cavity consists of two opposing plates, the inner sides of which are coated with a reflective film of the laser center wavelength.

6. The narrow linewidth laser linewidth detection system based on spectral reconstruction according to claim 5, characterized in that: The reflectivity of the reflective film ranges from 70% to 80%.

7. The linewidth detection system for 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 inside 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 to the nearest integer, c is the speed of light in vacuum, λ0 represents the central wavelength of the single-frequency narrow-linewidth laser to be measured, and FSR is the free spectral range of the FP cavity.

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

9. The narrow linewidth laser linewidth detection system based on spectral 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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