A temperature measurement method and device based on single-cavity double optical comb

Abstract

The application provides a temperature measurement method and device based on single-cavity double optical comb, and belongs to the technical field of laser absorption spectrum. A single-cavity double optical comb generation module generates a probe optical comb and a local oscillator optical comb with slightly different repetition frequencies; a temperature control module makes the single-cavity double optical comb generation module work in a constant temperature environment; the probe optical comb and the local oscillator optical comb are respectively amplified and spectrally broadened; the two optical combs are combined through a 2*1 optical fiber coupler, then enter an optical band-pass filter for filtering; the filtered double optical combs pass through a gas medium to be measured, and an interference signal of asynchronous sampling between the double optical combs is obtained through a photodetector; a signal acquisition and processing module is used to acquire the interference signal filtered through an electrical low-pass filter, so that a gas absorption spectrum is obtained; finally, through frequency domain difference and time domain separation, a multi-spectrum fitting method is used to solve the gas temperature. The application combines the advantages of the single-cavity double optical comb, such as high spectral resolution, high coherence and wide spectrum, and the advantages of the multi-spectrum fitting algorithm, such as high robustness, strong anti-noise ability and strong immunity to baseline, so that the gas absorption spectrum measurement and temperature measurement are realized, and the application has a wide application prospect.

Description

TECHNICAL FIELD

[0001] The application provides a single-cavity double-optical comb based temperature measurement method and device, and belongs to the technical field of laser absorption spectroscopy. BACKGROUND

[0002] Gas temperature measurement plays an important role in combustion diagnosis, chemical reaction monitoring and other fields. Typical temperature measurement methods are invasive or non-invasive. The invasive method (such as thermocouple and gas sampling) has the advantage of simple measurement process and is widely used, but the invasive process will interfere with the gas flow field and cannot obtain the correct gas parameters. The non-invasive method uses a non-contact sensor to measure the gas parameters, which has the advantages of high sensitivity, fast response, and simultaneous measurement of multiple parameters. The non-invasive method is usually realized by laser spectroscopy technology, such as coherent anti-Stokes Raman scattering spectroscopy, planar laser-induced fluorescence spectroscopy and tunable diode laser absorption spectroscopy (TDLAS).

[0003] TDLAS is widely used in industrial and aerospace combustion chamber and other harsh environments due to its high adaptability, fast response, high precision and multi-parameter detection. In 2020, Xu et al. published a paper in the 2020 IEEE International Instrumentation and Measurement Technology Conference (2020IEEE International Instrumentation and Measurement Technology Conference) on page 105, "A Compact Noise-Immune TDLAS Temperature Sensor using Intensity Modulation", which proposed a method of using Mach-Zehnder fiber interferometer for light intensity modulation to achieve noise immunity. Even in the case of strong background light interference, this method has high noise suppression ability, providing a new means for measuring the exhaust plume of the aircraft engine and other harsh environments with strong thermal radiation interference. In 2022, Wang et al. published a paper in the journal Spectro chimica Acta Part A: Molecular and Biomolecular Spectroscopy (Spectro chimica Acta Part A: Molecular and Biomolecular Spectroscopy) Vol. 265, page 120333, "Multi-species hydrocarbon measurement using TDLAS with a wide scanning range DFG laser", which designed a TDLAS system based on the difference frequency generation method, and simultaneously measured the direct absorption spectrum covering different groups (such as alkanes, alkenes and aromatics), verifying the ability of TDLAS in multi-component measurement. However, TDLAS technology often needs the help of two or more distributed feedback (DFB) lasers to obtain the temperature and concentration parameters of the gas, and two or more independent spectral lines need to be scanned; to obtain a wide spectral coverage, a complex optical adjustment system and hardware system are generally required.

[0004] Compared with TDLAS technology, dual-comb spectroscopy (DCS) uses two optical frequency comb lasers for coherent measurement to obtain the absorption spectrum information of the gas. The spectral width of DCS technology is usually tens of times larger than the spectral width of the DFB laser used in TDLAS technology, and it also provides high spectral resolution and high measurement accuracy as a reference for HITRAN database calibration.

[0005] Optical frequency comb laser outputs a sequence of femtosecond pulses with equal time interval in time domain, and its Fourier transform corresponds to a series of equally spaced frequency comb teeth in frequency domain. By locking the carrier envelope frequency and repetition frequency of the optical frequency comb to an atomic clock, high resolution and high stability of spectral characteristics can be obtained. In 2004, Keilmann et al. of Max-Planck Institute in Germany first reported a dual-comb multi-heterodyne spectroscopy scheme in the research paper Time-domain mid-infrared frequency-comb spectrometer published in Optics Letters. In frequency domain, each frequency comb tooth generates a microwave beat frequency, which reduces the detection signal from the optical frequency band to the radio frequency microwave band, greatly reducing the detection difficulty. In time domain, the local light pulse moves with equal time step and is superimposed with the probe light pulse to achieve the effect of delay scanning. The absorption spectrum and phase spectrum can be obtained simultaneously, the detection signal frequency is low and there is no mechanical scanning, and the measurement speed is fast and the signal-to-noise ratio is high.In 2008, I. Coddington et al. published a paper in Physical Review Letters, Vol. 100, p. 013902, entitled Coherent Multiheterodyne Spectroscopy Using Stabilized Optical Frequency Combs, in which they measured the absorption and phase spectra of HCN molecules at 155,000 frequency points with a 100-MHz frequency interval between wavelengths of 1495-1620 nm; in 2011, E. Baumann et al. published a paper in Physical Review A, Vol. 84, p. 062513, entitled Spectroscopy of the methane v3 band with an accurate midinfrared coherent dual-comb spectrometer, in which they improved the experimental scheme and measured the spectral resolution of CH4 to be less than 10 kHz; in 2012, Zolot et al. widened the near-infrared spectral width to 43 THz, achieving ultra-wide absorption spectrum measurement covering absorption lines of multiple molecules, as reported in a paper published in Optics Letters, Vol. 4, pp. 638-640, entitled Direct-comb molecular spectroscopy with accurate, resolved comb teeth over 43 THz. Zuo et al. of East China Normal University used optical-optical modulation frequency comb technology to achieve parallel detection of multiple gas molecules (including C2H2, CH4, H2CO, H2S, COS and H2O), as reported in a paper published in Photonics Research, Vol. 9, p. 1358, entitled Broadband mid-infrared molecular spectroscopy based on passive coherent optical-optical modulated frequency combs. In addition to measuring ultra-wide range absorption spectra and achieving parallel measurement of multiple molecules, DCS technology can also simultaneously obtain parameters such as temperature and concentration of multiple gas components.In 2016, P.J. Schroeder et al. of the University of Colorado at Boulder published a paper entitled "Dual frequency comb laser absorption spectroscopy in a 16 MW gas turbine exhaust" in the 36th volume of the Proceedings of the Combustion Institute, pages 4565-4573, which first applied dual-comb absorption spectroscopy technology to a commercial environment and simultaneously monitored the temperature and concentration of H2O and CO2 in a stationary gas turbine. However, the optical comb system composed of multiple cavities involves locking between each optical comb to generate stable coherent absorption spectroscopy; the generation of two frequency combs requires a complex frequency feedback control system, which makes it difficult to miniaturize and reduce the cost of the dual-comb laser absorption spectroscopy technology, limiting its practical application.

[0006] In contrast, single-cavity dual-comb laser absorption spectroscopy technology only needs to use a single-cavity dual-comb laser as a light source without complex frequency feedback control circuits. The single-cavity dual-comb laser used simultaneously generates two pairs of pulses with very small repetition rate differences in the same mode-locked laser; the two pulses come from the same cavity and experience the same structural and environmental conditions, having very good relative stability and mutual coherence.

[0007] In 2016, Zheng et al. measured the absorption spectrum of acetylene gas using a wavelength multiplexed single-cavity dual-comb laser, and obtained the absorption spectrum data with a precision of picometer. The corresponding research results have been published in the journal Optics Express, Vol. 24, pp. 21833, in the paper Picometer-resolution dual-comb spectroscopy with a free-running fiber laser. In the paper Dual-comb spectroscopy of methane based on a free-running Erbium-doped fiber laser published in the journal Optics Express, Vol. 27, pp. 11406, the research group used the soliton self-frequency shift phenomenon to convert the laser wavelength generated by the wavelength multiplexed single-cavity dual-comb laser from 1550 nm to 1650 nm, and measured the absorption spectrum of methane. After 1.96 s of coherent averaging, the residual error of the obtained absorption spectrum curve compared with the HITRAN database was only about 0.006. In the same year, Takuro Ideguchi et al. of the University of Tokyo published the paper Kerr-lens mode-locked bidirectional dual-comb ring in the journal Optica, Vol. 3, pp. 748, in which they used a mode-locked bidirectional dual-comb to measure the absorption spectrum of a Nd:YVO4 crystal and verified that the laser has high coherence and stability. In 2017, S.M. Link et al. of the Swiss Federal Institute of Technology in Zurich published the paper Dual-comb spectroscopy of water vapor with a free-running semiconductor disk laser in the journal Science, Vol. 356, pp. 1164-1168, in which they used a polarization multiplexed dual-comb laser as the light source and successfully measured the absorption spectrum data of water vapor. The experimental device is very simple.In 2021, Xu et al. published a paper entitled “Dual-comb Spectroscopy for Laminar Premixed Flames with a Free-running Fiber Laser” in the journal Combustion Science and Technology, vol. 194, pp. 2523-2538, which used adaptive single-cavity dual-comb absorption spectroscopy to measure CH4 / O2 / air premixed flames and obtained absorption spectra in the range of 6503 cm. -1 ~ 6535 cm -1 The authors achieved an equivalent noise absorption coefficient of 9.5 x 10 -5 cm -1 -1 / Hz1 / 2. In addition to obtaining amplitude spectral data, single-cavity dual-comb spectroscopy has the ability to obtain phase spectral data. In 2019, G. Philippe et al. of Laval University in Canada published a paper entitled “Precision spectroscopy of H 13 CN using a free-running all-fiber dual electro-optic frequency comb system” in the journal Optics letters, vol. 43, p. 1407, which used a free-running all-fiber dual electro-optic frequency comb system to achieve high-precision spectroscopy of H 13The absorption spectrum data of methane in R and Q bands were measured and the amplitude and phase spectrum data were obtained simultaneously in (Dual-comb spectroscopy of methane using a free-running, all-fiber dual electro-optic frequency comb system). Single-cavity dual-comb lasers are widely used in the fields of spectral analysis, distance measurement, 3D terrain mapping, and flow rate measurement, etc. In 2021, Nürnberg et al. demonstrated direct dual-comb ranging from free-running diode-pumped semiconductor and solid-state laser oscillators in (Dual-comb ranging with frequency combs from single cavity free-running laser oscillators) published in Optics express, vol. 29, p. 24910, achieving a measurement resolution of 0.55 pm. With further optimization, the resolution of this technique will be far below 1 pm and the range will be far higher than 1 km. P. Trocha et al. used microcavity dual-comb technology for ranging in (Ultrafast optical ranging using microresonator soliton frequency combs) published in Science, vol. 359, pp. 887-891, which reduced the Allan deviation to 12 nm at an average time of 13 us, allowing ultrafast ranging at a 100 MHz acquisition rate and sampling a projectile moving at 150 m / s. This technology combines an integrated soliton dual-comb ranging system with a chip-level nanophotonic phased array, enabling a small ultrafast ranging system. In 2021, Zhou et al. designed a 3D sensor in (Dual-comb spectroscopy resolved three-degree-of-freedom sensing) published in Photonics research, vol. 9, p. 243, which combined dual-comb technology to measure the distance, pitch, and yaw angles of a moving object.

[0008] Although single cavity dual comb laser has been applied in the field of spectral analysis, distance measurement, 3D terrain mapping, etc., the requirements of high spectral accuracy, small baseline deviation and narrow comb spectral measurement range make it difficult to apply single cavity dual comb spectral technology to gas temperature measurement. For single cavity dual comb laser with obvious phase drift, it is a severe challenge to obtain high spectral accuracy and extract baseline, but the common multi-spectral fitting algorithm for solving gas temperature is sensitive to baseline deviation. Since the comb spectral measurement range is determined by the repetition frequency and frequency difference of two asynchronous pulses, these two factors are negatively correlated. In other words, the comb spectral measurement range will be limited by the spectral resolution and measurement speed. When measuring gas parameters, in order to obtain high spectral resolution and fast response, it is necessary to reduce the comb spectral measurement range, which will affect the accuracy of gas parameter measurement. In addition, the above-mentioned single cavity dual comb laser is realized by material saturable absorber or single heavy nonlinear polarization rotation mode locking effect or the mixed mode locking of the two. Among them, the material saturable absorber mode locking is not easily affected by the environmental temperature and vibration, but it is limited by the relaxation time. In other words, the saturable absorption effect of the saturable absorber is not available within the bleaching recovery time, at which time the noise light will pass through without loss, which makes the pulse generated by the saturable absorber mode locking have larger amplitude noise. The single cavity dual comb laser based on single heavy nonlinear polarization rotation mode locking technology does not rely on the saturable absorption effect of the material itself, has no relaxation time limit, has the advantages of wavelength adjustable, large modulation depth, short response time, etc., and can generate high stability pulses. However, its working state will be affected by environmental temperature, external vibration, polarization decay and other factors, which will change the mode locking state or even lose lock, making it difficult to lock and difficult to achieve self-starting.

[0009] Based on the above background, the present application proposes a single-cavity double optical comb based temperature measurement method and device, which can realize fast and accurate measurement of the temperature of the gas to be measured. Instead of using material saturable absorber mode locking technology, double nonlinear polarization rotation mode locking technology is used, which can significantly improve the signal-to-noise ratio of the pulse repetition frequency comb teeth and the stability of the pulse power, thereby suppressing the amplitude noise of the local light comb and the probe light comb, and improving the temperature measurement accuracy. Compared with single nonlinear polarization rotation mode locking technology, the use of double nonlinear polarization rotation mode locking technology causes the noise light of the pulse wings to be absorbed twice, so that the pulse experiences twice narrowing, further improving the signal-to-noise ratio and stability of the pulse, and making it easier to achieve mode locking self-starting. Using a temperature controller to ensure that the single-cavity double optical comb laser works in a constant temperature condition can solve the problem of the laser losing lock due to temperature changes and polarization decay during operation. The local light comb and the probe light comb perform asynchronous optical sampling to obtain interference graph data, and then the absorption spectrum information of the gas can be extracted. Through frequency domain difference and time domain separation, the measured absorption spectrum data is fitted using a multi-spectrum fitting algorithm, and the composition and temperature of the gas can be extracted. The method and device effectively suppress the influence of baseline error on the multi-spectrum fitting accuracy, improve the temperature solving accuracy, and have strong baseline immunity, noise immunity and robustness. Even when the measurement spectral range is narrow and the available spectral lines are few, the method and device can still achieve high-precision temperature measurement. SUMMARY

[0010] The present application proposes a single-cavity double optical comb based temperature measurement method and device, which includes a temperature control module, a single-cavity double optical comb generation module, an optical power amplification module, a 2*1 optical fiber coupler, an optical bandpass filter, a gas medium to be measured, a photodetector, an electrical low-pass filter, and a signal acquisition and processing module. The temperature control module can ensure that the single-cavity double optical comb generation module works in a constant temperature environment. The single-cavity double optical comb generation module emits a probe light comb and a local light comb with slightly different repetition frequencies. The probe light comb is amplified in pulse power and spectrally broadened by a first optical power amplification module, and the local light comb is amplified in pulse power and spectrally broadened by a second optical power amplification module. Then the probe light comb and the local light comb are combined by a 2*1 optical fiber coupler, filtered by an optical bandpass filter, pass through the gas medium to be measured, and the interference signal of the probe light comb asynchronously sampled by the local light comb is detected by a photodetector; then the signal acquisition and processing module acquires and processes the interference signal to obtain the measured absorption spectrum information; finally, the multi-spectrum fitting algorithm is used to fit the measured absorption spectrum information, and the measured value of the temperature of the gas medium to be measured is extracted.

[0011] The repetition frequencies of the probe light comb and the local light comb generated by the single-cavity double optical comb generation module are represented as f r1 and f r2 , respectively. The repetition frequency difference between the probe light comb and the local light comb is df = f r1 -f r2 ; the optical power and spectral width of the probe optical comb and the local optical comb can be amplified and broadened by the first optical power amplification module and the second optical power amplification module, so as to ensure that the probe optical comb and the local optical comb have sufficient optical power in the absorption spectrum region of the gas to be measured. The optical field E p1 of the probe optical comb and the optical field E q2 of the local optical comb can be represented as the superposition of a plurality of optical longitudinal modes:

[0012]

[0013] wherein f p1 and f q2 represent the p1th longitudinal mode frequency of the probe optical comb and the q2th longitudinal mode frequency of the local optical comb respectively, and satisfy the relationship f p1 = f c + p1fr1 + f o1 , f q2 = f c + q2f r2 + f o2 ; A p1 and A q2 represent the amplitudes of the longitudinal modes f p1 and f q2 , and represent the initial phases of the longitudinal modes f p1 and f q2 , f c represents the carrier frequency, and f o1 and f o2 represent the carrier envelope phase shift frequencies of the longitudinal modes f p1 and f q2 . The interference signal obtained after the asynchronous optical adoption of the probe optical comb on the local optical comb is actually the coherent superposition between the optical fields of the probe optical comb and the local optical comb:

[0014]

[0015] The probe optical comb and the local optical comb need to pass through an optical fiber band-pass filter with a spectral bandwidth Δλ satisfying formula (3) and an electrical low-pass filter with a bandwidth BW < f r1 / 2 at the same time, wherein the spectral range of the optical band-pass filter needs to be selected according to the absorption spectrum range of the gas medium to be measured.

[0016]

[0017] where λ and c represent the center wavelength of the optical band-pass filter and the speed of light, respectively. Since the photoelectric detector used in the present application extracts the interference signal through the optical band-pass filter and the electrical low-pass filter in an AC coupling manner, the voltage signal V detected by the photoelectric detector actually represents the AC part in equation (2):

[0018]

[0019] Under the action of the optical band-pass filter and the electrical low-pass filter, the adjacent comb teeth of the probe light comb and the local light comb are beat to the radio frequency domain, satisfying the relationship p = p1 = q2. Thus, equation (4) can be rewritten as equation (5).

[0020]

[0021] In equation (5), Δf o = f o1 -f o2 , Finally, under the action of the gas medium to be measured, the light intensity of the probe light comb and the local light comb is attenuated, so that the voltage signal satisfies the relationship of equation (6).

[0022]

[0023] Taking the fast Fourier transform of equation (6), the transmission spectrum information, that is, I = |A| 2 e -α(f) , can be extracted. The measured absorption spectrum can be expressed as α m (f) = -ln(I), that is:

[0024] α m (f) = α(f) + β(f) (7)

[0025] where β(f) and α(f) represent the baseline and the actual absorption spectrum, respectively. In order to obtain the gas parameters, it is necessary to fit the measured absorption spectrum with the simulated absorption spectrum from the HITRAN database, so the baseline information needs to be estimated in order to fit the accurate absorption spectrum. In order to eliminate the absorption information that may be included in the estimated baseline, a difference and time domain separation strategy is adopted. The specific details are as follows: the simulated absorption spectrum can be calculated by the HITRAN database and expressed as α H (f). At the same time, by performing cubic spline interpolation on the reference points on α H (f), a simulated reference curve expressed as α Hr (f) can be obtained. Here, the reference points are defined as the minimum values between two absorption peaks. Similarly, by performing cubic spline interpolation on the reference points on α m (f), a measured reference curve on the measured absorption spectrum expressed as αmr (f). α mr (f) simultaneously contains the absorption information α r (f) and the baseline information β r (f), which is expressed as:

[0026] α mr (f) = α r (f) + β r (f) (8) Therefore, the difference between the simulated reference curve and the measured reference curve can be written as:

[0027] β'(f) = α mr (f) - α Hr (f) = α r (f) - α Hr (f) + β r (f) (9)

[0028] In equation (9), β'(f) represents the estimated baseline. Only when the simulated absorption spectrum α H (f) is consistent with the measured absorption spectrum α(f), the estimated baseline β'(f) can represent the true baseline. Therefore, by the difference operation described in equation (9), the part of the absorption information contained in the estimated baseline can be eliminated. Then, by using the time-domain separation strategy, the baseline signal and the absorption signal are separated in the time domain, so as to further reduce the influence of the baseline estimation error. According to equation (8) and equation (9), the measured absorption signal A m (t) and the estimated baseline signal B(t) in the time domain are derived as:

[0029]

[0030]

[0031] By inverse Fourier transform of the simulated absorption spectrum α H (f), the simulated absorption signal A H (t) in the time domain can also be obtained.

[0032]

[0033] Finally, by solving the nonlinear optimization problem in equation (13) using the Nelder-Mead nonlinear optimization algorithm, the parameter information of the gas can be obtained.

[0034]

[0035] In equation (13), T, x, v0, dv represent the temperature, the concentration, the initial frequency of the fitted spectrum and the resolution of the fitted spectrum, respectively. A suitable time window [t s , T eThus, the fitting accuracy is improved.

[0036] The present application has the following advantages: the present application uses a single-cavity dual-comb laser as a probe light source, and has a simple structure, and can realize miniaturization of application of dual-comb absorption spectroscopy. The local light comb and the probe light comb are emitted from the same laser, and have very high relative stability and mutual coherence, so that coherent measurement of gas absorption spectroscopy is realized. The present application uses multiple mode locking technology, and improves the light power stability of the pulse, so that the amplitude noise of the probe light source can be suppressed, and the temperature measurement accuracy is improved. The multi-spectrum fitting algorithm used in the present application uses a differential and time domain separation strategy, effectively suppresses the influence of baseline estimation error on multi-spectrum fitting, and improves the gas parameter solving accuracy and algorithm robustness. The present application can measure wide spectrum information of gas, obtain gas component information and temperature information, and has the ability of simultaneously measuring multiple parameters. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 Fig. 1 is a typical structural diagram of a single-cavity dual-comb based temperature measurement method and device,

[0038] 102 single-cavity dual-comb generation module, 103 first optical power amplification module, 104 second optical power amplification module, 105 2*1 optical fiber coupler, 106 optical bandpass filter, 107 to-be-measured gas medium, 108 photodetector, 109 electrical low-pass filter, 1010 signal acquisition and processing module.

[0039] Figure 2 Fig. 2 is a structural diagram of the single-cavity dual-comb generation module in the embodiment

[0040] Figure 3 Fig. 3 is an output spectrum diagram of the single-cavity dual-comb generation module in the embodiment

[0041] Figure 4 Fig. 4 is an output spectrum diagram of the single-cavity dual-comb generation module in the embodiment

[0042] Figure 5 Fig. 5 is an absorption spectrum fitting result of acetylene measured in the embodiment DETAILED DESCRIPTION

[0043] The technical solutions in the embodiments of the present application will be described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments.

[0044] The light source structure of the single-cavity dual-comb generation module used in the present embodiment is as shown in Figure 2The light source is composed of a semiconductor laser, a multifunctional device, an online polarizer with a polarization maintaining fiber, a polarization maintaining fiber, a polarization rotator, a dispersion compensation fiber, and a doped fiber, and each device is connected by a common single-mode fiber. The order of device connection can be changed. The multifunctional device is composed of a coupler, an isolator, and a wavelength division multiplexer, which can be replaced by a single functional device with equivalent functions. The polarization rotator is composed of a half-wave plate (H), a quarter-wave plate (Q), and a polarizer (P). The polarization rotator can control the polarization state inside the laser and introduce polarization-dependent loss to realize nonlinear polarization rotation mode locking. Compared with the material saturable absorber mode locking technology, the pulse output by the nonlinear polarization rotation mode locking technology has higher optical power stability. The online polarizer can also realize nonlinear polarization rotation function, and the combination of the polarization rotator and the online polarizer can realize double mode locking effect, which can further improve the output pulse power stability, thereby suppressing the amplitude noise of the local light comb and the probe light comb, and improving the gas temperature measurement accuracy. The combination of the polarization rotator, the online polarizer, and the polarization maintaining fiber can realize birefringent filtering effect, so as to generate two optical pulses with different center wavelengths.

[0045] In this example, the spectrum of the two optical pulses with different repetition frequencies output by the single-cavity double-comb generation module is shown in Figure 3 The center wavelengths of the two pulses output by the seed source of the single-cavity double-comb generation module are 1531.6 nm and 1550.6 nm, respectively, one of which is used as the local light comb, and the other is used as the probe light comb. Due to the chromatic dispersion inside the laser, the two optical pulses with different center wavelengths have different repetition frequencies. The repetition frequency of the local light comb is 34.329851 MHz, and the repetition frequency of the probe light comb is 34.330791 MHz, and the repetition frequency difference between the two is 940 Hz, as shown in Figure 4 Due to the multiple mode locking effect formed by the combination of the online polarizer and the polarization rotator, the output pulse power stability is improved, so that the signal-to-noise ratio of the repetition frequency comb teeth of the local light comb and the probe light comb both exceeds 90 dB.

[0046] After passing through the optical power amplification module, the output power and spectral width of the local light comb and the probe light comb are significantly improved, thereby realizing coherent measurement of gas. Taking acetylene as the gas molecule to be measured, the near-infrared spectral range 6491.6-6497.42 cm -1The absorption spectrum data in the range is used to obtain the temperature and concentration information of the gas. According to the repetition frequency and the repetition frequency difference of the local light comb and the probe light comb, the non-aliasing spectral bandwidth of 4.97 nm can be calculated. Therefore, an optical filter with a 3dB bandwidth of 1.31 nm is needed to filter out the spectral region of interest, and an electrical low-pass filter with a bandwidth of 14 MHz is used to low-pass filter the collected interference data, so as to prevent the occurrence of spectral aliasing. The sampling frequency of the data acquisition and processing module is set to 100 MHz, which is about 3 times the repetition frequency of the probe light comb, satisfying the Nyquist sampling law. The collected interference data is processed, and combined with the multi-spectral fitting algorithm, the parameter information of the gas can be calculated.

[0047] In this embodiment, the multi-spectral fitting algorithm used includes the following four steps:

[0048] First step: extract the absorption spectrum data. The frequency jitter in the interference graph is corrected by using the phase correction algorithm, and then the transmission spectrum information is obtained by using the fast Fourier transform. The absorption spectrum data containing the baseline is obtained by taking the negative logarithm of the transmission spectrum.

[0049] Second step: estimate the baseline information. First, four free parameters (temperature, concentration, initial frequency of the fitting spectrum and resolution of the fitting spectrum) are substituted into the HITRAN database to obtain a simulated absorption spectrum. Second, the simulated and measured reference curves can be obtained by respectively performing cubic spline interpolation on the reference points on the simulated and measured absorption spectra, wherein the minimum value between two simulated absorption peaks is selected as the reference point. Finally, the estimated baseline is represented by the difference between the simulated reference curve and the measured reference curve.

[0050] Third step: calculate the time domain signal. The zero padding operation is performed on the frequency domain signal, which is equivalent to interpolating in the time domain signal to improve the time resolution of the time domain signal. In the case where there are few spectral lines available for fitting, zero padding the absorption spectrum signal in the frequency domain is very important. Because the narrower the fitting spectrum range, the coarser the time resolution, and the more difficult it is to select a suitable time window. The time domain signal is obtained by zero padding the measured absorption spectrum, the simulated absorption spectrum and the estimated baseline, and inverse Fourier transform.

[0051] Fourth step: calculate the gas parameters. The Nelder-Mead nonlinear optimization algorithm is used to calculate the initial optimal values of the temperature, concentration, initial frequency of the fitting spectrum and resolution of the fitting spectrum. In order to further improve the calculation accuracy, the initial optimal values of the initial frequency of the fitting spectrum and the resolution of the fitting spectrum are set as known parameters and substituted into step two to reduce the number of free parameters. By repeating steps two to five, the final optimized values of the temperature and concentration can be obtained.

[0052] Using the multi-spectral fitting algorithm, the temperature and concentration of the gas can be obtained asFigure 5 Fitting results of the acetylene absorption spectrum.

[0053] The above description of the application and its embodiments is not limited to this, the drawings shown is only one of the embodiments of the application. Without departing from the purpose of the invention, not creative design of similar structures or embodiments of the technical solution, all belong to the scope of the invention.

Claims

1. A single-cavity dual-comb based temperature measurement device, employing a temperature control module to regulate the operating temperature of a single-cavity dual-comb generation module; the single-cavity dual-comb generation module emits a probe comb and a local comb with slightly different repetition rates; wherein, The probe light comb is amplified in pulse power and spectrum width by a first optical power amplifier module, and the local light comb is amplified in pulse power and spectrum width by a second optical power amplifier module; then the probe light comb and the local light comb are combined by a 2*1 optical fiber coupler, filtered by an optical band-pass filter, pass through the gas medium to be measured, and the interference signal of the asynchronous sampling of the probe light comb to the local light comb is obtained by a photodetector; the interference signal is low-pass filtered by an electrical low-pass filter, and the measurement absorption spectrum information is obtained by a signal acquisition and processing module; finally, the multi-spectrum fitting algorithm is used to fit the measurement absorption spectrum information, and the measurement value of the temperature of the gas medium to be measured is obtained; the single-cavity double-comb generation module is realized by double nonlinear polarization rotation mode-locked; wherein the double nonlinear polarization rotation mode-locked refers to that two nonlinear polarization rotation mode-locked devices are used to generate ultrashort pulses in the same single-cavity double-comb generation module.

2. The single-cavity dual-comb based temperature measurement device of claim 1, wherein, The optical field E of the probe comb p1 The optical field E of the local comb q2 The optical field E can be represented as a superposition of several optical longitudinal modes: where f p1 and f q2 represent the p1th longitudinal mode frequency of the probe comb and the q2th longitudinal mode frequency of the local comb respectively; f p1 = f c + p1f r1 + f o1 , f q2 = f c + q2f r2 + f o2 ; f r1 and f r2 represent the repetition frequencies of the probe comb and the local comb respectively; A p1 and A q2 represent the amplitudes of the longitudinal modes f p1 and f q2 , and represent the initial phases of the longitudinal modes f p1 and f q2 , f c represents the carrier frequency, f o1 and f o2 represent the carrier envelope phase offset frequencies of the longitudinal modes f p1 and f q2 ; the interference signal obtained after the probe comb asynchronously optically samples the local comb is the coherent superposition between the probe comb and the local comb optical fields: The optical band-pass filter and the electrical low-pass filter with a bandwidth of 1 / 2 satisfy formula (3), and the spectral range of the optical band-pass filter needs to be determined according to the absorption spectral range of the gas medium to be measured r1 The optical band-pass filter and the electrical low-pass filter with a bandwidth of 1 / 2 satisfy formula (3), and the spectral range of the optical band-pass filter needs to be determined according to the absorption spectral range of the gas medium to be measured Wherein, λ and c represent the center wavelength of the optical band-pass filter and the speed of light respectively; the repetition frequency difference between the probe light comb and the local light comb is df = f r1 -f r2 ; the photoelectric detector extracts the interference signal passing through the optical band-pass filter and the electrical low-pass filter in an alternating coupling manner, and the detected voltage signal V can be represented as: Under the action of the optical band-pass filter and the electrical low-pass filter, the adjacent comb teeth of the probe light comb and the local light comb are beat to the radio frequency domain, and the relationship p=p1=q2 is satisfied; therefore, formula (4) can be rewritten as formula (5); Wherein, Δf o = f o1 -f o2 , Finally, under the action of the gas medium to be measured, the light intensity of the probe light comb and the local light comb decays due to gas absorption, so that the voltage signal satisfies the relationship of formula (6); The Fourier transform of equation (6) extracts the transmission spectral information, i.e. The measured absorption spectrum can be expressed as α m (f) = -ln(I), i.e. α m (f) = a(f) + β(f) (7) where β(f) and a(f) represent the baseline and actual absorption spectra, respectively; to obtain the gas parameters, the measured absorption spectrum needs to be fitted with the absorption spectrum simulated by the HITRAN database, and the baseline information needs to be estimated during the fitting; To eliminate the influence of baseline estimation error, a difference and time domain separation strategy was adopted; the simulation absorption spectrum was calculated by HITRAN database, denoted as α H (f); the minimum value between two absorption peaks was taken as the reference point, and the reference points on α H (f) were interpolated by cubic spline to obtain the simulation reference curve denoted as α Hr (f); the minimum value between two absorption peaks was taken as the reference point, and the reference points on α m (f) were interpolated by cubic spline to obtain the measurement reference curve on the measured absorption spectrum, denoted as α mr (f); α mr (f) contains absorption information α r (f) and baseline information β r (f), denoted as: α mr (f) = a r (f) + β r (f) (8) Therefore, the difference between the simulation reference curve and the measurement reference curve can be written as: β'(f) = a mr (f) - a Hr (f) = a r (f) - a Hr (f) + β r (f) (9) where β'(f) represents the estimated baseline; only when the simulated absorption spectrum α H (f) is consistent with the measured absorption spectrum α(f) can the estimated baseline β'(f) represent the true baseline; therefore, the partial absorption information contained in the estimated baseline can be eliminated by the difference operation described in equation (9); using the time-domain separation strategy, the baseline signal and the absorption signal are separated in the time domain, reducing the influence of the baseline error; according to equations (8) and (9), the measured absorption signal A m (t) and the estimated baseline signal B(t) in the time domain are: By performing an inverse Fourier transform on the simulated absorption spectrum α H (f) By performing an inverse Fourier transform, the simulated absorption signal A in the time domain can also be obtained H (t); Finally, the nonlinear optimization algorithm is used to solve formula (13), and the parameter information of the gas can be obtained. where T, X, v0, dv represent temperature, concentration, initial frequency of the fitted spectrum and resolution of the fitted spectrum, respectively; finally a suitable time window [t s ,t e ] needs to be selected in the time domain to improve the fitting accuracy.

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