Cavity ring-down spectral measurement system and method based on narrow-linewidth single-frequency laser sideband scanning

Abstract

The invention discloses a cavity ring-down spectral measurement system and method based on narrow linewidth single-frequency laser sideband scanning. The system comprises a computer, a radio frequency signal generator, a radio frequency signal amplification device, a laser, an electro-optical modulator, an optical switch, a resonant cavity, a threshold comparison circuit and the like. A computer controls a radio frequency signal generator to generate sinusoidal modulation signals with different frequencies and inputs the sinusoidal modulation signals into an electro-optical modulator, and light output by a laser passes through the electro-optical modulator, an optical switch, a lens and a reflector and then enters a ring-down cavity. The detector receives transmission light of the ring-down cavity and divides the transmission light into two paths for output, the first path is input to the threshold comparison circuit to generate a pulse signal to drive the optical switch, and the second path is collected by the collection card and then recorded and processed by the computer. The cavity ring-down spectrum measurement system and method have the advantages that the influence of laser phase noise on ring-down time stability can be avoided, the wavelength of a scanning spectrum point by point can be obtained by using the radio frequency signal frequency, integration is easy, and the like.

Description

Technical Field

[0001] The present invention relates to an optical sensing technology, in particular to a cavity ring-down spectroscopy measurement system and method based on sideband scanning of a narrow linewidth single-frequency laser. Background Art

[0002] Laser Absorption Spectroscopy is a technology for analysis and detection using the principle of the interaction between laser and matter. It is based on the resonance absorption of specific energy photons by specific molecules and has the detection characteristics of non-contact, high precision, and path integration. Laser Absorption Spectroscopy has the advantages of high spectral resolution, high sensitivity, and easy system integration. With the development of low-cost and small-size lasers, Laser Absorption Spectroscopy has been gradually widely applied in different fields such as greenhouse gas detection, engine combustion flow field diagnosis, deep-sea dissolved gas measurement, and coal mine gas leakage.

[0003] Cavity ring-down spectroscopy has the characteristics of not being affected by the intensity fluctuation of the laser, having a long absorption length, and being very sensitive, and is often used to detect the absolute optical extinction of samples, including light scattering and absorption. Cavity ring-down spectroscopy technology can achieve a light path of kilometer level in a cavity with a length less than 1 meter by using a high-finesse optical resonator, enhancing the interaction length between light and the gas to be measured by thousands of times. At the same time, cavity ring-down spectroscopy technology measures the time required for the target signal to decay rather than the amplitude, immune to the influence of laser intensity noise, and can achieve a measurement sensitivity of 10 -9 to 10 -12 cm -1 level, corresponding to a gas detection limit in the order of ppm (Parts Per Million) to ppt (Parts Per Trillion).

[0004] The most widely used in cavity ring-down spectroscopy technology is continuous-wave cavity ring-down spectroscopy. Continuous-wave cavity ring-down spectroscopy can achieve the coupling of a single laser frequency and the cavity mode, and its detection sensitivity is mainly affected by factors such as system mechanical vibration, measurement chamber temperature and pressure stability, laser-cavity coupling efficiency, and circuit noise of the detection and acquisition system.

[0005] In current experimental research and commercial instruments, distributed feedback semiconductor lasers are mostly used in the measurement process of continuous-wave cavity ring-down spectroscopy. By utilizing the wide tuning characteristics of distributed feedback semiconductor lasers, the spectral signals of the absorption characteristics to be measured are measured. However, since the linewidth of distributed feedback semiconductor lasers (in the order of MHz) is much larger than the cavity mode linewidth of high-finesse optical resonators (in the order of kHz), the coupling efficiency between distributed feedback semiconductor lasers and high-finesse optical resonators is very low, and the intensity of the output cavity mode signal is unstable. At the same time, the phase noise output by distributed feedback semiconductor lasers will also cause the excitation of transverse modes in the resonator, making the measured ring-down signal contain residual beat frequency terms generated by the interference of different transverse mode excitations, reducing the stability of the measured ring-down time. In addition, in a continuous-wave cavity ring-down spectroscopy system based on semiconductor lasers, in order to determine the laser frequency corresponding to each spectral point composed of ring-down times, additional wavelength measurement equipment is required. Currently, commercial wavelength detection equipment and frequency calibration equipment such as optical frequency combs have problems such as large volume and high cost, and cannot be adapted to the application of integrated commercial products. Summary of the Invention

[0006] The present invention aims to avoid the deficiencies existing in the above-mentioned prior art, and provides a cavity ring-down spectroscopy measurement system and method based on sideband scanning of a narrow-linewidth single-frequency laser, so as to improve the stability of ring-down time measurement and the occurrence rate of ring-down events. At the same time, the accuracy of the measured spectral frequency axis is determined by a radio frequency signal source and an electro-optic modulator, and a complex frequency calibration device is not required to achieve a frequency axis with an accuracy better than the MHz level.

[0007] The present invention adopts the following technical solutions to solve the technical problems.

[0008] The present invention discloses a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser, including a computer 1, a radio frequency signal generator 2, a radio frequency signal amplification device 3, a laser 4, an electro-optic modulator 5, an optical switch 6, a collimating lens 7, a reflecting mirror, a piezoelectric ceramic 10, a resonator 11, a detector 12, a threshold comparison circuit 13 and a data acquisition card 14; the reflecting mirror includes a first reflecting mirror 8 and a second reflecting mirror 9;

[0009] The computer 1 controls the radio frequency signal generator 2 to output modulation signals of different frequencies, and the radio frequency signal amplification device 3 receives the modulation signals and amplifies the signals; the amplified modulation signals are input into the electro-optic modulator 5; the output signal of the laser 4 is input into the electro-optic modulator 5 for modulation to generate sidebands; the output signal of the electro-optic modulator 5 is input and sequentially passes through the optical switch 6, the collimating lens 7, the first reflecting mirror 8 and the second reflecting mirror 9 and then enters the resonator 11; the output signal of the resonator 11 enters the detector 12 and is divided into two paths. The first path enters the threshold comparison circuit 13 to generate a pulse signal and input it to the optical switch 6, and the second path is collected by the data acquisition card 14 and then input into the computer 1.

[0010] The structural characteristics of a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser in the present invention also lie in that:

[0011] Further, the laser 4 is a narrow-linewidth single-frequency laser.

[0012] Further, the detector 12 includes an amplifier circuit, which receives the output signal of the detector 12 for amplification and then outputs in two paths.

[0013] Further, the output linewidth of the laser 4 is less than 1 kHz.

[0014] Further, the optical switch 6 is an acousto-optic modulator.

[0015] Further, the modulation bandwidth of the acousto-optic modulator is greater than 1 GHz.

[0016] Further, two highly reflective mirrors are arranged in the resonator 11.

[0017] The present invention also discloses a measurement method of a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser, including the following steps:

[0018] Step 1: The computer 1 controls the modulation signal output by the radio frequency signal generator 2 to the electro-optic modulator 5, and the output signal of the laser 4 is input to the electro-optic modulator 5 for modulation to generate sidebands and output sideband light; the wavelength of the sideband light is controlled by the modulation signal;

[0019] Step 2: The sideband light enters the resonator 11 after passing through the optical switch 6, the collimating lens 7, the first reflector 8 and the second reflector 9;

[0020] Step 3: The output signal of the resonator 11 is divided into two paths after entering the detector 12: the first path enters the threshold comparison circuit 13 to generate a pulse signal and input it to the optical switch 6 to control the optical switch 6 to turn off the laser to generate a ring-down signal; the second path is collected by the data acquisition card 14 and input to the computer 1, and the computer 1 performs data processing to obtain the ring-down time result and save it;

[0021] Step 4: The computer 1 controls the radio frequency signal generator 2 to adjust the sideband wavelength, repeats the above steps 2 to 3 and records the ring-down time at different wavelengths, and finally obtains the entire spectral data covering the absorption spectral lines of the target component.

[0022] Compared with the prior art, the beneficial effects of the present invention are reflected in that:

[0023] The present invention discloses a cavity ring-down spectroscopy measurement system and method based on sideband scanning of a narrow-linewidth single-frequency laser. The system includes a computer, a radio frequency signal generator, a radio frequency signal amplification device, a laser, an electro-optic modulator, an optical switch, a resonant cavity, a threshold comparison circuit, etc. The computer controls the radio frequency signal generator to generate sine modulation signals with different frequencies and inputs them into the electro-optic modulator. The light output by the laser enters the ring-down cavity after passing through the electro-optic modulator, the optical switch, a lens, and a mirror. The detector receives the transmitted light of the ring-down cavity and outputs it in two paths: the first path is input to the threshold comparison circuit to generate a pulse signal to drive the optical switch, and the second path is collected by the acquisition card and recorded and processed by the computer.

[0024] The present invention develops a laser based on sideband scanning of a narrow-linewidth single-frequency laser. Compared with the traditional cavity ring-down spectroscopy system, it avoids the influence of the laser phase noise on the stability of the ring-down time. At the same time, the wavelength of each point of the scanned spectrum is obtained by using the radio frequency signal frequency, without expensive and complex frequency measurement and calibration equipment, and has the potential for miniaturization and commercialization.

[0025] The cavity ring-down spectroscopy measurement system and method based on sideband scanning of a narrow-linewidth single-frequency laser of the present invention have the advantages of avoiding the influence of the laser phase noise on the stability of the ring-down time, being able to obtain the wavelength of each point of the scanned spectrum by using the radio frequency signal frequency, and being easy to integrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser of the present invention.

[0027] Figure 2 It is a schematic diagram of the carrier wave and sidebands generated by the laser of a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser of the present invention through an electro-optic modulator.

[0028] Figure 3 It is a comparison diagram of the ring-down curves collected by using a traditional semiconductor laser (upper figure) and the narrow-linewidth laser of the present invention (lower figure).

[0029] Figure 4 It is a spectral signal diagram of a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser of the present invention.

[0030] The following further illustrates the present invention through specific embodiments and in conjunction with the drawings. SPECIFIC EMBODIMENTS

[0031] See Figures 1 to 4, the present invention discloses a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser, which includes a computer 1, a radio frequency signal generator 2, a radio frequency signal amplification device 3, a laser 4, an electro-optic modulator 5, an optical switch 6, a collimating lens 7, a reflecting mirror, a piezoelectric ceramic 10, a resonant cavity 11, a detector 12, a threshold comparison circuit 13, and a data acquisition card 14; the reflecting mirror includes a first reflecting mirror 8 and a second reflecting mirror 9;

[0032] The computer 1 controls the radio frequency signal generator 2 to output modulation signals of different frequencies, and the radio frequency signal amplification device 3 receives the modulation signals and amplifies the signals; the amplified modulation signals are input into the electro-optic modulator 5; the output signal of the laser 4 is input into the electro-optic modulator 5 and modulated to generate sidebands; the output signal of the electro-optic modulator 5 is input and sequentially passes through the optical switch 6, the collimating lens 7, the first reflecting mirror 8, and the second reflecting mirror 9 and then enters the resonant cavity 11; the output signal of the resonant cavity 11 enters the detector 12 and is divided into two paths. The first path enters the threshold comparison circuit 13 to generate a pulse signal and inputs it to the optical switch 6, and the second path is collected by the data acquisition card 14 and then input into the computer 1.

[0033] As Figure 1 is a schematic diagram of a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser according to the present invention. The computer controls the radio frequency signal generator to output modulation signals of different frequencies, and the modulation signals are amplified and then input into the electro-optic modulator after signal amplification. The output of the laser generates sidebands through the electro-optic modulator, and the frequency difference between the sidebands and the carrier is the modulation frequency. After the light beam passes through the optical switch and mode matching, it is coupled into the resonant cavity. Figure 2 is a schematic diagram of the carrier and sidebands generated by the laser through the electro-optic modulator, where the frequency difference v between the sidebands and the central carrier m is the frequency of the modulation signal applied by the radio frequency signal amplification device to the electro-optic modulator, and the central carrier frequency is v c .

[0034] The resonant cavity includes two highly reflective mirrors and an intermediate connection structure, and one of the highly reflective mirrors is connected to the piezoelectric ceramic to realize the adjustment of the resonant frequency of the cavity mode. The transmitted light passing through the resonant cavity is received by the detector, and after being amplified by the amplifier circuit of the detector, it is divided into two paths. The first path enters the threshold comparison circuit to generate a pulse signal to control the optical switch to cut off the incident light entering the resonant cavity to generate a ring-down signal, and the second path is digitally processed by the high-speed data acquisition card and then input into the computer, and the data is processed by the computer.

[0035] A cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser of the present invention uses a fiber laser with a linewidth smaller than the cavity mode linewidth to measure the ring-down signal, improving the cavity mode coupling efficiency and measurement sensitivity. At the same time, the sideband light generated by the light source through an electro-optic modulator is used to measure the spectrum of the target absorption characteristics. At this time, the corresponding frequency interval of each spectral point is determined by the frequency of the radio frequency signal, and no additional wavelength measurement equipment is required.

[0036] In specific implementation, the laser 4 is a narrow-linewidth single-frequency laser.

[0037] In specific implementation, the detector 12 includes an amplification circuit. The amplification circuit receives the output signal of the detector 12 for amplification and then outputs in two paths.

[0038] In specific implementation, the output linewidth of the laser 4 is less than 1 kHz.

[0039] In specific implementation, the optical switch 6 is an acousto-optic modulator.

[0040] In specific implementation, the modulation bandwidth of the acousto-optic modulator is greater than 1 GHz.

[0041] The modulation bandwidth of the acousto-optic modulator is greater than 1 GHz to ensure that the generated sideband frequencies can cover the entire absorption spectral frequency range.

[0042] In specific implementation, two highly reflective mirrors are arranged in the resonator 11.

[0043] The reflectivity of the highly reflective mirror in the resonator is usually >99.995%, ensuring sufficient absorption optical path.

[0044] The present invention also discloses a measurement method of a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow-linewidth single-frequency laser, including the following steps:

[0045] Step 1: The computer 1 controls the modulation signal output by the radio frequency signal generator 2 to the electro-optic modulator 5. The output signal of the laser 4 is input to the electro-optic modulator 5 for modulation to generate sidebands and output sideband light; the wavelength of the sideband light is controlled by the modulation signal;

[0046] The laser outputs a narrow-linewidth single-frequency light source to generate sidebands through the electro-optic modulator. The frequency difference v between the sidebands and the laser carrier m is determined by the modulation frequency of the electro-optic modulator, and the modulation frequency is determined by the modulation signal output by the frequency signal generator 2.

[0047] Step 2: The sideband light enters the resonator 11 after passing through the optical switch 6, the collimating lens 7, the first reflector 8 and the second reflector 9;

[0048] The sideband light enters the resonant cavity after passing through an acousto-optic modulator and mode matching, and the optical signal passing through the transparent resonant cavity is received and amplified by a detector.

[0049] Step 3: The output signal of the resonant cavity 11 is divided into two paths after entering the detector 12: The first path enters the threshold comparison circuit 13 to generate a pulse signal, which is input to the optical switch 6 to control the optical switch 6 to turn off the laser to generate a ring-down signal; the second path is collected by the data acquisition card 14 and then input to the computer 1, and the computer 1 processes the data to obtain the ring-down time result and save it.

[0050] Step 4: The computer 1 controls the radio frequency signal generator 2 to adjust the sideband wavelength, repeats the above steps 2 to 3, and records the ring-down times at different wavelengths, and finally obtains the entire spectral data covering the absorption lines of the target component.

[0051] By changing the computer to change the frequency of the modulation signal output by the radio frequency signal generator, that is, changing the sideband wavelength, repeating the above steps and recording the ring-down time at this wavelength, and finally obtaining the entire spectral data covering the absorption lines of the target component.

[0052] Under resonance conditions, the laser radiation accumulates in the resonant cavity. When the incident light is quickly turned off by the optical switch, at this time, a part of the laser leaks out during each reflection. Define 1 / e of the light intensity decay time as the ring-down time τ(v), as shown in the following formula (1).

[0053]

[0054] In formula (1), L is the length of the resonant cavity (unit: cm), R is the reflectivity of the high-reflection mirror in the resonant cavity, c is the speed of light, t is the time required for light to travel back and forth in the resonant cavity (unit: s); τ(v) is the ring-down time of light in the resonant cavity (unit: μs), v is the laser output frequency when the optical switch is quickly turned off (unit: Hz), I0(v) is the initial incident laser light intensity (unit: W / m 2 ), I(v) is the transmitted light intensity after the light source is turned off (unit: W / m 2 ).

[0055] In the case where there is no absorption medium in the resonant cavity, the attenuation of light in the resonant cavity depends on the physical properties of the resonant cavity. The physical properties of the resonant cavity include the length L of the resonant cavity and the reflectivity R of the high-reflection mirror in the resonant cavity, as shown in the following formula (2).

[0056]

[0057] In formula (2), L is the length of the resonant cavity (unit: cm), R is the reflectivity of the high-reflection mirror in the resonant cavity, and c is the speed of light.

[0058] When the value of the reflectivity R approaches 1 infinitely (R > 0.999) and there is no other absorption medium in the resonant cavity, that is, in a vacuum state, the main source of light loss at this time is mirror reflection. Then, the decay time τ0(v) of light in the resonant cavity can be approximated by formula (3).

[0059]

[0060] In formula (3), L is the length of the resonant cavity (unit: cm), R is the reflectivity of the highly reflective mirror in the resonant cavity, c is the speed of light, and τ0(v) is the decay time of light in the resonant cavity in a vacuum state (unit: μs).

[0061] When additional losses occur due to the presence of an absorption medium in the resonant cavity, the decay time τ(v) will decrease, and then there is the following formula (4).

[0062]

[0063] In formula (4), L is the length of the resonant cavity (unit: cm), R is the reflectivity of the highly reflective mirror in the resonant cavity, c is the speed of light, α(v) is the absorption coefficient of the absorption medium for light (unit: cm -1 ), and l is the effective optical path length increased due to resonance (unit: cm). The absorption coefficient α(v) of the absorption medium for light can be calculated by measuring the decay time τ0 of light in the resonant cavity in a vacuum state and the decay time τ reduced after passing through the absorption medium. The calculation formula is shown in the following formula (5).

[0064]

[0065] In formula (5), α(v) is the absorption coefficient of the absorption medium (cm -1 ), c is the speed of light, τ0 is the cavity decay time of light in the resonant cavity in a vacuum state, and τ is the decay time of light when there is an absorption medium in the resonant cavity.

[0066] The concentration X of a given absorption medium can be calculated by the following formula (6):

[0067]

[0068] In formula (6), α(v) is the absorption coefficient of the absorption medium, P is the total pressure of the gas serving as the absorption medium (unit: atm); the gas serving as the absorption medium includes J components. X j is the concentration (ppm) of the j-th component among the J components; S i,j is the line strength (unit: cm -2 atm -1 ) of the absorption transition i of the j-th component, and φ i，jThe absorption line shape of the absorption transition i of the j-th component is (ν) (unit: cm). Here we adopt the Voigt line shape function:

[0069]

[0070] In formula (7), a and w are dimensionless parameters, y is the integration variable,

[0071] Δν G refers to the Gaussian broadening of the entire absorption spectrum (unit: cm -1 ), Δν L refers to the Lorentz broadening of the entire absorption spectrum (unit: cm -1 ). v is the laser output frequency when the optical switch is quickly turned off (unit: Hz), and v0 is the center frequency of the absorption transition (unit: Hz).

[0072] By adjusting the laser wavelength over the entire absorption characteristics of the target molecule to obtain the entire absorption spectrum profile, the concentration X of the absorption medium can be obtained through the integral ∫α(v)dv and the known absorption characteristic spectral parameters. At the same time, in the measured spectral results, in addition to the absorption signal of the gas, there is also cavity loss information superimposed. Since the reflectivity R of the high-reflection mirror changes according to the wavelength, the loss of the cavity also changes with frequency. To sum up, the function expression to be fitted is transformed into the following formula (8):

[0073]

[0074] In formula (8), A i,j = PX j S i,j represents the integrated absorbance of the spectral signal, N j is the total number of molecular absorption transitions, α′(v) is the absorption coefficient obtained from actual measurement (unit: cm -1 ); v is the laser output frequency when the optical switch is quickly turned off (unit: Hz), v0 is the center frequency of the absorption transition (unit: Hz); A i,j = PX j S i,j represents the integrated absorbance of the spectral signal (unit: cm -2 ), P is the total pressure of the gas serving as the absorption medium (unit: atm); the gas serving as the absorption medium includes J components; X j is the concentration (ppm) of the j-th component in the J components; S i,j is the spectral line intensity of the i-th absorption transition of the j-th component (unit: cm -2 atm -1 ) of the j-th component, φ i，j(ν) is the absorption spectral line shape of the i-th absorption transition of the j-th component (unit: cm). Δν G refers to the Gaussian broadening of the entire absorption spectrum (unit: cm -1 ), Δν L refers to the Lorentz broadening of the entire absorption spectrum (unit: cm -1 ); k (unit: (hz*cm) -1 ) and b (unit: cm -1 ) represent the slope and intercept of the relationship between the loss of the cavity and the frequency respectively, which are known parameters of the resonant cavity; the concentration X i,j of the j-th component among the J components of the absorption medium can be calculated from the A j value obtained by fitting according to formula (8) j (X i,j = A i,j / PS

[0075] The present invention discloses a cavity ring-down spectroscopy measurement system based on sideband scanning of a narrow linewidth single-frequency laser. The computer controls the radio frequency signal generator to generate sinusoidal modulation signals of different frequencies and acts on the electro-optic modulator. The light output by the laser passes through the electro-optic modulator and the optical switch device, and after mode matching by the lens and the mirror, it enters the ring-down cavity. The transmitted light passing through the ring-down cavity is received by the detector and converted into an electrical signal. One path of the amplified signal by the amplifier circuit is input to the threshold comparison circuit to generate a pulse signal to drive the optical switch, and the other path is collected by the acquisition card and recorded and processed by the computer. The present invention avoids the fluctuation of the ring-down signal caused by the phase noise of the laser in the traditional cavity ring-down system, improves the stability of the ring-down time measurement and the occurrence rate of the ring-down event. At the same time, the accurate wavelength data of the scanned spectrum is provided by the wavelength of the single-frequency laser and the radio frequency frequency, and there is no need to use an additional wavelength meter.

[0076] Figure 3 The figure shows the ring-down curves collected by using the traditional semiconductor laser (upper figure) and the narrow linewidth laser of the present invention (lower figure). Figure 3 In the upper figure, the linewidth of the light source used in the traditional cavity ring-down spectroscopy system is much larger than the cavity mode width, and the signal amplitude passing through the resonant cavity is in an unstable state. Figure 3 In the lower figure, the linewidth of the light source used in the present system is smaller than the cavity mode width, so the ring-down signal amplitude is relatively stable.

[0077] Figure 4 It is a schematic diagram of the spectral signal. By changing the frequency of the generated radio frequency signal, the target absorption line is step-scanned, and the frequency interval between each spectral point is determined by the radio frequency signal frequency difference.

[0078] ​It is obvious to those skilled in the art that the present invention is not limited to the details of the above-described exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, in any aspect, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

[0079] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment only contains an independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A cavity ring-down spectroscopy measurement system based on narrow linewidth single-frequency laser sideband scanning, characterized in that: The invention comprises a computer (1), a radio frequency signal generator (2), a radio frequency signal amplifying device (3), a laser (4), an electro-optic modulator (5), an optical switch (6), a collimating lens (7), a reflector, a piezoelectric ceramic (10), a resonant cavity (11), a detector (12), a threshold comparison circuit (13) and a data acquisition card (14); the reflector comprises a first reflector (8) and a second reflector (9); The computer (1) controls the radio frequency signal generator (2) to output modulation signals of different frequencies, and the radio frequency signal amplifying device (3) receives the modulation signals and amplifies the signals; the amplified modulation signals are input into the electro-optic modulator (5); the output signal of the laser (4) is input into the electro-optic modulator (5) to generate sidebands after modulation; the output signal of the electro-optic modulator (5) is input into the resonant cavity (11) after passing through the optical switch (6), the collimating lens (7), the first reflector (8) and the second reflector (9) in sequence; the output signal of the resonant cavity (11) enters the detector (12) and is divided into two paths, the first path enters the threshold comparison circuit (13) to generate a pulse signal and inputs the optical switch (6), and the second path is collected by the data acquisition card (14) and input into the computer (1).

2. The cavity ring-down spectroscopy measurement system based on narrow linewidth single-frequency laser sideband scanning according to claim 1 is characterized in that: The laser (4) is a narrow-linewidth single-frequency laser.

3. The cavity ring-down spectroscopy measurement system based on narrow linewidth single-frequency laser sideband scanning according to claim 1 is characterized in that: The detector (12) comprises an amplifier circuit, which receives an output signal of the detector (12), amplifies the signal, and then outputs the signal in two paths.

4. The cavity ring-down spectroscopy measurement system based on narrow linewidth single-frequency laser sideband scanning according to claim 1 is characterized in that: The output line width of the laser (4) is less than 1 kHz.

5. The cavity ring-down spectroscopy measurement system based on narrow linewidth single-frequency laser sideband scanning according to claim 1 is characterized in that: The optical switch (6) is an acousto-optic modulator or a semiconductor optical amplifier.

6. The cavity ring-down spectroscopy measurement system based on narrow linewidth single-frequency laser sideband scanning according to claim 5 is characterized in that: The modulation bandwidth of the acousto-optic modulator is greater than 1 GHz.

7. The cavity ring-down spectroscopy measurement system based on narrow linewidth single-frequency laser sideband scanning according to claim 1 is characterized in that: Two high-reflection mirrors are arranged in the resonant cavity (11).

8. A measurement method of a cavity ring-down spectroscopy measurement system based on narrow linewidth single-frequency laser sideband scanning according to claim 1, characterized in that: The steps include: Step 1: The computer (1) controls the radio frequency signal generator (2) to output a modulation signal to the electro-optical modulator (5), and the output signal of the laser (4) is input to the electro-optical modulator (5) to generate a sideband after modulation, and output the sideband light; Step 2: The sideband light passes through the optical switch (6), the collimating lens (7), the first reflector (8) and the second reflector (9) and then enters the resonant cavity (11); Step 3: The output signal of the resonant cavity (11) enters the detector (12) and is divided into two paths: the first path enters the threshold comparison circuit (13) to generate a pulse signal which is input into the optical switch (6), and controls the optical switch (6) to turn off the laser to generate a ring-down signal; the second path is collected by the data acquisition card (14) and input into the computer (1), and the computer (1) performs data processing to obtain the ring-down time result and saves it; Step 4: The computer (1) controls the radio frequency signal generator (2) to adjust the sideband wavelength, repeats the above steps 2 to 3 and records the decay time at different wavelengths, and finally obtains the entire spectrum data covering the absorption spectrum line of the target component.

9. The measurement method of the cavity ring-down spectroscopy measurement system based on narrow linewidth single-frequency laser sideband scanning according to claim 8 is characterized in that: In the step 1, the wavelength of the sideband light is controlled by a modulation signal.

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