A method for fast measuring the light output frequency response of a laser

Abstract

The application provides a method for quickly measuring laser light frequency response, which extracts effective absorption information harmonic signal to obtain accurate laser light frequency response through specific environment absorption spectrum measurement, uses a method of multiplying a reference signal to transfer absorption information to a direct current part, extracts a normalized harmonic signal insensitive to a phase through low-pass filtering combined with a double-channel demodulation mode, and uses a nonlinear least square fitting to take a frequency modulation parameter of the laser as a fitting parameter to realize accurate measurement of the light frequency response. The application solves the problem of fast measurement of wavelength modulation technology laser frequency-time response, applies the laser light frequency-time response measurement method to related engineering fields, solves key technical problems for accurate measurement of complex environment component concentration, provides accurate quantitative parameters for accurate construction of an absorption model, and further expands the application range of wavelength modulation spectroscopy technology.

Description

Technical Field

[0001] This invention relates to the field of measurement and testing technology, and specifically to a method for rapidly measuring the light output frequency response of a laser. Background Technology

[0002] Tunable semiconductor absorption spectroscopy (TDLAS) utilizes a narrow-linewidth tunable semiconductor laser. By adjusting the laser's wavelength, it continuously scans the absorption line of the target gas, obtaining the absorption characteristics of the gas and thus achieving gas measurement. This technique features high sensitivity, fast response, high spectral resolution, and the ability to perform in-situ measurements of multiple components. It is widely used in fields such as spectral parameter research, atmospheric trace gas detection, industrial process control, and combustion flow field diagnostics.

[0003] TDLAS technology is divided into direct absorption spectroscopy and wavelength modulation spectroscopy based on the applied current mode. In certain measurement environments (such as weak target absorption lines, high beam transmission loss, and difficulty in selecting an absorption baseline), wavelength modulation spectroscopy is more adaptable, generally exhibiting a measurement sensitivity 2-3 orders of magnitude higher than direct absorption technology. In practical applications, the accuracy of field parameter measurements depends on the precise construction of the absorption model. When the model calculation results can be directly compared with the measured values, the model's accuracy is considered to meet the measurement requirements. The laser's output frequency-time response is a crucial input parameter in the absorption model, characterizing the laser's output characteristics; achieving accurate measurement of this response is essential for flow field parameter measurement. Traditional measurement methods utilize Fabry-Perot (FP) etalons, following the principle of multi-beam equal-inclination interference, to obtain the laser's frequency domain characteristics through fitting the frequency domain points. However, in practical applications, it is difficult to find etalons suitable for all modulation depths and output wavelengths, and there are problems such as the difficulty in etalon fabrication, the difficulty in eliminating fitting errors, and insufficient frequency domain resolution.

[0004] Therefore, a method is needed to quickly measure the frequency response of laser light output. Summary of the Invention

[0005] This invention addresses the challenge of rapidly measuring the frequency-time response of lasers in wavelength modulation technology. It provides a method for quickly measuring the output frequency response of a laser. The method involves injecting a working current and controlling the operating temperature based on the laser's operating parameters, causing the laser to pass through a standard absorption cell. The intensity signal after passing through the absorption cell is used to simulate a non-absorbed light intensity signal as background. The normalized second harmonic (NDH) after background subtraction is used as the objective function for fitting the laser, and the laser frequency-time response model is used as the fitting parameters, thus achieving accurate measurement of the output frequency response. This invention solves the problem of rapidly measuring the frequency-time response of lasers in wavelength modulation technology. Applying this laser output frequency-time response measurement method to related engineering fields solves a key technical problem for the accurate measurement of component concentrations in complex environments. Simultaneously, it provides precise quantitative parameters for the accurate construction of absorption models, further expanding the application scope of wavelength modulation spectroscopy.

[0006] This invention provides a method for rapidly measuring the output frequency response of a laser, wherein the laser is passed through an absorption cell, and the intensity I of the light after absorption is utilized. t (υ) The non-absorbed light intensity I0(υ) is obtained by simulation and the non-absorbed light intensity I0(υ) is used as the background. The normalized second harmonic after removing the background is used as the fitting objective function, and the laser frequency-time response model is used as the fitting parameter. The laser output frequency response is obtained by nonlinear least squares fitting. The method of quickly measuring the laser output frequency response is completed.

[0007] The method for rapidly measuring the output frequency response of a laser, as described in this invention, preferably includes the following steps:

[0008] S1. The output light from the laser is collimated and then passes through an absorption cell. After being absorbed by the gas medium inside the absorption cell, the light is output to the detector. The detector converts the optical signal into an electrical signal to obtain the intensity I of the absorbed light. t (υ);

[0009] S2. Obtain the intensity I of the absorbed light through nonlinear least squares fitting. t The non-absorbed light intensity I0(υ) is used as the background;

[0010] S3. Using software demodulation, the non-absorbed light intensity I0(υ) and the absorbed light intensity I t (υ) Perform the same processing to obtain the measured second harmonic signal X of channel X. 2f Background harmonic signals of the X channel The measured second harmonic signal of the Y channel Y 2f Y-channel background harmonic signal And the measured second harmonic signal S was obtained. 2f-nor-bgsub-mean ;

[0011] S4. Obtain the background second harmonic signal S from the laser frequency-time response model. 2f-nor-bgsub-sim Subtracting the background second harmonic signal S 2f-nor-bgsub-sim It is an analog signal;

[0012] S5. Change the parameters of the laser frequency-time response model to change the background second harmonic signal S. 2f-nor-bgsub-sim When the iteration condition is met, the target parameter is obtained. The target parameter υ(t) is the accurate laser output frequency response, and the rapid measurement of the laser output frequency response is completed.

[0013] The present invention provides a method for rapidly measuring the output frequency response of a laser. In a preferred embodiment, in step S1, a low-frequency sawtooth scanning is injected into the laser and superimposed with a high-frequency sinusoidal modulation, and then the output light of the laser is collimated and coupled into an absorption cell. The output light of the laser is absorbed by the target gas and then output. The gas medium includes the target gas, and the absorption cell provides a fixed temperature, pressure, component concentration, and effective optical path.

[0014] The method for rapidly measuring the light output frequency response of a laser according to the present invention, in a preferred embodiment, in step S1, the light intensity time response of the laser is:

[0015]

[0016] Where I0(t) is the initial light intensity of the laser. Let i be the average light intensity at the center frequency of the laser, and i0 be the average light intensity at the center frequency of the laser. Normalized linear intensity modulation amplitude, i2 is the amplitude of the modulated line. Normalized nonlinear intensity modulation amplitude, Let i0 be the phase difference between the linear intensity modulation and the frequency modulation. Let f be the phase difference between the nonlinear intensity modulation and frequency modulation corresponding to i2, f be the frequency of the sinusoidal modulation, and t be the time.

[0017] When a laser is modulated by a high-frequency sinusoidal current, both the light intensity and frequency are modulated. The output frequency υ0(t) of the modulated laser is:

[0018]

[0019] in, For the average emitted light frequency, a i θ i Here are the frequency description model coefficients, a is the frequency modulation amplitude, θ is the initial phase, and when i = 1, υ0(t) is a linear parameter, and when i = 2, it is a nonlinear parameter of υ0(t).

[0020] In step S2, the light intensity I after absorption t(υ) includes absorbing information and non-absorbing segment information. Nonlinear least squares fitting is a method that selects the non-absorbing segment information for piecewise fitting, using I0, i0, i2, ... The change is used to perform nonlinear least squares fitting.

[0021] The method for rapidly measuring the light output frequency response of a laser, as described in this invention, preferably includes step S2 where the absorbed light intensity I... t The light intensity I0(υ) and the non-absorbed light intensity I0(υ) follow the Beer-Lambert law;

[0022]

[0023] Where, τ υ Transmittance;

[0024] The non-absorbed light intensity I0(υ) is obtained by I(υ)=I0(υ)exp[-S(T)φ(υ-υ0,T)PχL], where υ is the laser emission frequency, υ0 is the center frequency of the absorption spectrum, S(T) is the line intensity of the absorption spectrum at temperature T, φ is the normalized absorption line shape function, P is the total pressure in the absorption cell, χ is the ratio of the number of moles of absorbing gas to the total gas, and L is the effective absorption optical path.

[0025] In a preferred embodiment of the method for rapidly measuring the output frequency response of a laser, as described in this invention, in step S3, when demodulating at a k-th harmonic of the modulation frequency using a lock-in amplifier, the k-th harmonic component is obtained, and the measured second harmonic signal S is obtained. 2f-nor-bgsub-mean .

[0026] The method for rapidly measuring the output frequency response of a laser, as described in this invention, preferably involves obtaining the measured second harmonic signal S in step S3 through low-pass filtering combined with dual-channel demodulation. 2f-nor-bgsub-mean ;

[0027]

[0028] The method for rapidly measuring the output frequency response of a laser, as described in this invention, preferably includes step S4 where the measured second harmonic signal S... 2f-nor-bgsub-mean As the fitting target, given the initial parameters of the laser's output frequency, and substituting the internal pressure, temperature, component concentration, and effective absorption path length of the absorption cell from step S1 as constants into the laser's frequency-time response model, step S3 is repeated to obtain the background second harmonic signal S. 2f-nor-bgsub-sim .

[0029] The method for rapidly measuring the output frequency response of a laser, as described in this invention, preferably includes step S5 where the background second harmonic signal S is subtracted.2f-nor-bgsub-sim Nonlinear least squares fitting is performed to seek the optimal fitting parameters.

[0030] In a preferred embodiment of the method for rapidly measuring the output frequency response of a laser, step S5 of this invention uses the following iteration condition: subtracting the background second harmonic signal S. 2f-nor-bgsub-sim and the measured second harmonic signal S 2f-nor-bgsub-mean The squared difference of the vectors is ≤10 -8 ;

[0031] When the fitting condition is met, υ(t)=υ0(t).

[0032] Based on the optical transmission law and nonlinear least squares fitting, the model parameters characterizing the laser's output frequency are used as the fitting vector. The fitting objective is to ensure that the harmonic signal passing through the standard absorption cell can be accurately described by the absorption model. The input variables in the absorption model include absorbance, line shape, and laser modulation parameters. The laser modulation parameters are divided into intensity modulation and frequency modulation. Except for the frequency parameter, all other parameters are known quantities. By changing the parameters of the frequency description model to fit it with the measured harmonic signal, the accurate descriptive parameters of the laser's output frequency can be obtained when the convergence condition is met.

[0033] The intensity change of a laser beam after passing through an absorbing medium can be expressed as:

[0034] I(υ)=I0(υ)exp[-S(T)φ(υ-υ0,T)PχL] (1)

[0035] Where υ is the laser emission frequency (unit: cm). -1 (The intensity of the light is inversely proportional to the wavelength), υ0 is the center frequency of the absorption spectral line; I0(υ) and I(υ) are the light intensities before and after absorption, respectively; S(T) is the line intensity of the absorption spectral line at temperature T (unit: cm). -2 ·atm -1 The temperature T is expressed in K; φ is the normalized absorption line shape function (unit: cm); χ is the ratio of the number of moles of absorbing gas to the total gas, i.e., the volume ratio; L is the effective absorption path length (unit: cm).

[0036] Tunable semiconductor absorption spectroscopy (TDLAS) technology is divided into direct absorption technology (DAS) and wavelength modulation spectroscopy (WMS) technology according to the form of the current applied to the semiconductor laser. When only a sawtooth wave that is repeatedly scanned is applied to the laser, it is direct absorption technology. When a sawtooth wave and a sinusoidal modulation with a frequency of f are applied, it is wavelength modulation technology.

[0037] The time response of laser intensity using wavelength modulation technology can be expressed as:

[0038]

[0039] Where I0(t) is the initial light intensity of the laser. Let i be the average light intensity at the center frequency of the laser, and i0 and i2 be the light intensity at the center frequency of the laser, respectively. Normalized linear and nonlinear intensity modulation amplitudes, These are the phase differences between the corresponding linear and nonlinear intensity modulation and frequency modulation, respectively. Only the first two intensity response terms are considered here. These parameters all depend on the modulation parameters of the laser and its frequency and intensity response characteristics.

[0040] When a laser is modulated by a high-frequency sinusoidal current, both the light intensity and frequency are modulated. The output frequency of the modulated laser can be expressed as:

[0041]

[0042] in For the average emitted light frequency, a i θ i Here, represents the frequency description model coefficients, 'a' represents the frequency modulation amplitude, and 'θ' represents the initial phase. These are linear parameters when i = 1 and nonlinear parameters when i = 2. The laser is collimated and then injected into a standard absorption cell, which provides fixed temperature, pressure, component concentration, and effective optical path.

[0043] After passing through a standard absorption cell, the laser beam is absorbed by the internal gas components. The emitted beam then strikes a detector, where it is converted into light intensity I by photoelectric conversion. t (t), the light intensity contains high-frequency absorption information, based on the obtained transmitted light intensity I. t (t) The non-absorbent light intensity I0(t) is obtained by piecewise fitting of the non-absorbent portion.

[0044] Transmitted light intensity I t The light intensities I0(t) and I0(t) without absorption are simultaneously passed through a phase-sensitive detector and a low-pass filter to obtain a second harmonic signal that has had its background removed and is normalized by peak values, which can be expressed as:

[0045]

[0046] Where X 2f , These represent the second harmonic signal and background signal of the X channel, respectively, and Y... 2f , These represent the second harmonic signal and background signal of the Y channel, respectively;

[0047] Measured second harmonic signal S 2f-nor-bgsub-mean As the fitting target, given the initial parameters of the laser's output frequency, and substituting the internal pressure, temperature, component concentration, and effective absorption optical path of the absorption cell in equation (3) as constants into the absorption model, the simulated second harmonic signal S is obtained.2f-nor-bgsub-sim By changing the absorption model parameters to fit the simulated and measured second harmonic signals, the frequency-time response parameters that meet the convergence condition can be used to describe the laser's output frequency characteristics.

[0048] This invention obtains a precise laser emission frequency response by measuring the absorption spectrum of a specific environment and extracting harmonic signals containing effective absorption information. The absorption information is transferred to the DC component by multiplying with a reference signal. A phase-insensitive normalized harmonic signal is extracted using low-pass filtering combined with dual-channel demodulation. Nonlinear least-squares fitting is then used to apply the laser's frequency modulation parameters as fitting parameters, achieving precise measurement of the emission frequency response. This invention solves the problem of rapid measurement of the frequency-time response of lasers using wavelength modulation technology. Applying this laser emission frequency-time response measurement method to related engineering fields solves key technical problems for the precise measurement of component concentrations in complex environments. Simultaneously, it provides accurate quantitative parameters for the precise construction of absorption models, further expanding the application scope of wavelength modulation spectroscopy.

[0049] The present invention has the following advantages:

[0050] (1) The present invention obtains the accurate laser output frequency response by software calculation, which solves the problems of low accuracy, low operability and difficulty in processing standard etalons in the existing traditional methods; by using the laser through a standard absorption cell combined with nonlinear least squares fitting, the measured second harmonic signal is fitted with the analog signal, and the frequency time response description parameter is used as the fitting variable, and finally the accurate laser output frequency response is obtained, which effectively solves the problem of complex laser frequency time response measurement.

[0051] (2) The present invention improves the signal-to-noise ratio of the objective function by using a dual-channel background subtraction method; the absorption signal creation process comprehensively considers the influence of all absorption lines and the selection of line type, further improving the measurement accuracy.

[0052] (3) This invention can be used for measurement of various extreme flow fields and extraction of complex absorption information, further broadening the application scope of TDLAS technology. Attached Figure Description

[0053] Figure 1 This is a flowchart of a method for rapidly measuring the output frequency response of a laser.

[0054] Figure 2 A method for rapidly measuring the frequency response of a laser; the intensity I of the laser light after absorption. t (υ) and the diagram of light intensity I0(υ) without absorption;

[0055] Figure 3A method for rapidly measuring the output frequency response of a laser is proposed, which involves experimentally normalizing the second harmonic S. 2f-nor-bgsub-mean picture;

[0056] Figure 4 An absorption spectrum of oxygen molecules near 760 nm is presented as a method for rapidly measuring the frequency response of a laser.

[0057] Figure 5 This is a schematic diagram of harmonic signal fitting results for a method used to quickly measure the output frequency response of a laser. Detailed Implementation

[0058] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0059] Example 1

[0060] like Figure 1 As shown, a method for rapidly measuring the output frequency response of a laser involves passing the laser through an absorption cell and utilizing the intensity I of the absorbed light after passing through the absorption cell. t (υ) The non-absorbed light intensity I0(υ) is obtained by simulation and the non-absorbed light intensity I0(υ) is used as the background. The normalized second harmonic after removing the background is used as the fitting objective function, and the laser frequency-time response model is used as the fitting parameter. The laser output frequency response is obtained by nonlinear least squares fitting. The method of quickly measuring the laser output frequency response is completed.

[0061] Includes the following steps:

[0062] S1. The output light from the laser is collimated and then passes through an absorption cell. After being absorbed by the gas medium inside the absorption cell, the light is output to the detector. The detector converts the optical signal into an electrical signal to obtain the intensity I of the absorbed light. t (υ);

[0063] A low-frequency sawtooth scanning is injected into the laser and superimposed with a high-frequency sinusoidal modulation. The output light of the laser is then collimated and coupled into the absorption cell. The output light of the laser is absorbed by the target gas and then output. The gas medium includes the target gas. The absorption cell provides a fixed temperature, pressure, component concentration and effective optical path.

[0064] The time response of the laser intensity is:

[0065]

[0066] Where I0(t) is the initial light intensity of the laser. Let i be the average light intensity at the center frequency of the laser, and i0 be the average light intensity at the center frequency of the laser. Normalized linear intensity modulation amplitude, i2 is the amplitude of the modulated line. Normalized nonlinear intensity modulation amplitude, Let i0 be the phase difference between the linear intensity modulation and the frequency modulation. Let f be the phase difference between the nonlinear intensity modulation and frequency modulation corresponding to i2, f be the frequency of the sinusoidal modulation, and t be the time.

[0067] When a laser is modulated by a high-frequency sinusoidal current, both the light intensity and frequency are modulated. The output frequency υ0(t) of the modulated laser is:

[0068]

[0069] in, For the average emitted light frequency, a i θ i Here are the frequency description model coefficients, a is the frequency modulation amplitude, θ is the initial phase, and when i = 1, υ0(t) is a linear parameter, and when i = 2, it is a nonlinear parameter of υ0(t).

[0070] S2. Obtain the intensity I of the absorbed light through nonlinear least squares fitting. t The non-absorbed light intensity I0(υ) is used as the background;

[0071] After absorption, the light intensity I t (υ) includes absorbed information and non-absorbing segment information. Nonlinear least squares fitting is a method that selects the non-absorbing segment information for piecewise fitting. i0, i2, The changes are subjected to nonlinear least squares fitting;

[0072] After absorption, the light intensity I t The light intensity I0(υ) and the non-absorbed light intensity I0(υ) follow the Beer-Lambert law;

[0073]

[0074] Where, τ υ Transmittance;

[0075] The non-absorbed light intensity I0(υ) is obtained from I(υ)=I0(υ)exp[-S(T)φ(υ-υ0,T)PχL], where υ is the laser emission frequency, υ0 is the center frequency of the absorption spectrum, S(T) is the line intensity of the absorption spectrum at temperature T, φ is the normalized absorption line shape function, P is the total pressure in the absorption cell, χ is the ratio of the number of moles of absorbing gas to the total gas, and L is the effective absorption optical path.

[0076] S3. Using software demodulation, the non-absorbed light intensity I0(υ) and the absorbed light intensity I t (υ) Perform the same processing to obtain the measured second harmonic signal X of channel X. 2f Background harmonic signals of the X channel The measured second harmonic signal of the Y channel Y 2f Y-channel background harmonic signal And the measured second harmonic signal S was obtained. 2f-nor-bgsub-mean ;

[0077] By using a lock-in amplifier to demodulate at a frequency k times the modulation frequency, the kth harmonic component is obtained, and the measured second harmonic signal S is also obtained. 2f-nor-bgsub-mean ;

[0078] The measured second harmonic signal S was obtained by low-pass filtering combined with dual-channel demodulation. 2f-nor-bgsub-mean ;

[0079]

[0080] S4. Obtain the background second harmonic signal S from the laser frequency-time response model. 2f-nor-bgsub-sim Subtracting the background second harmonic signal S 2f-nor-bgsub-sim It is an analog signal;

[0081] The measured second harmonic signal S 2f-nor-bgsub-mean As the fitting target, given the initial parameters of the laser's output frequency, and substituting the internal pressure, temperature, component concentration, and effective absorption path length of the absorption cell from step S1 as constants into the laser's frequency-time response model, step S3 is repeated to obtain the background second harmonic signal S. 2f-nor-bgsub-sim ;

[0082] S5. Change the parameters of the laser frequency-time response model to change the background second harmonic signal S. 2f-nor-bgsub-sim ;

[0083] For the background second harmonic signal S 2f-nor-bgsub-sim Perform nonlinear least squares fitting to find the optimal fitting parameters;

[0084] When the iteration condition is met, the target parameter is obtained. The target parameter υ(t) is the precise laser output frequency response. The iteration condition is: subtracting the background second harmonic signal S 2f-nor-bgsub-sim and the measured second harmonic signal S 2f-nor-bgsub-mean The squared difference of the vectors is ≤10 -8， The rapid measurement of the laser's output frequency response is completed.

[0085] Example 2

[0086] like Figure 1As shown, a method for rapidly measuring the output frequency response of a laser is described.

[0087] The technical solution adopted in this invention is as follows: A low-frequency sawtooth scanning superimposed with high-frequency sinusoidal modulation is injected into a laser, which is then collimated and coupled into a standard absorption cell. The working medium in the absorption cell has strong absorption in the laser's operating wavelength range. The temperature, pressure, composition, and effective optical path of the absorption cell are all known. The working medium in the absorption cell is a mixture of nitrogen and the target gas. After multiple reflections by the standard absorption cell, the light intensity signal is received by a photodetector, which converts the light intensity signal into a voltage signal. This voltage signal is then amplified to obtain the absorbed light intensity signal I. t The relationship between (υ) and the non-absorbed light intensity I0(υ) follows the Beer-Lambert law, which can be expressed as:

[0088]

[0089] Where τ υ For transmittance, α υ Let P represent absorbance, P represent the total pressure in the absorption cell, χ represent the volume fraction, L represent the effective optical path length, S(T) represent the linear intensity at temperature T, and the line shape function φ has a normalization property, meaning the integral of the line shape of a single absorption over the entire frequency domain equals 1. I0(υ) passes through I t The non-absorbing portion of (υ) is obtained by piecewise fitting.

[0090] The intensity of light after absorption can be expressed as:

[0091] I t (υ)=I0(υ)·τ(υ(t)) (6)

[0092] Since the emission frequency of the laser changes with time υ(t) as an even function of time t, the transmittance is also an even function of time t. The Fourier expansion of the absorbed light intensity can be expressed as:

[0093]

[0094] When the lock-in amplifier is used to demodulate at a frequency k times the modulation frequency, the kth harmonic component is obtained. Based on this, the measured second harmonic signal S is obtained. 2f-nor-bgsub-mean Using this as the fitting target, the pressure, temperature, component concentration, and effective absorption optical path of the standard absorption cell are then substituted into the absorption model as constants to obtain the simulated second harmonic signal S. 2f-nor-bgsub-sim Nonlinear least squares fitting is performed to minimize the sum of squared errors and find the best fitting parameters. An objective function is established so that the fitting ends when the set iteration conditions are met. The fitting parameters (the output frequency υ0(t) of the modulated laser) are obtained and used to accurately describe the output frequency time response of the laser.

[0095] The rapid measurement laser rapidly measures, such as... Figure 1 As shown, it consists of five steps:

[0096] Step 1: Obtain the light intensity I after absorption. t (υ), the laser is subjected to low-frequency sawtooth scanning and high-frequency sinusoidal modulation current, which simultaneously controls its operating temperature. The collimated beam passes through a standard absorption cell, where the gas medium absorbs the light intensity before it strikes the detector. The detector converts the optical signal into an electrical signal, obtaining I. t (υ).

[0097] Step 2: Create the non-absorbed light intensity I0(υ) and the absorbed light intensity I through nonlinear least squares fitting. t (υ) contains both absorption information and non-absorption segments. A segmented fitting method based on the non-absorption portion is used to obtain the relationship with I. t The non-absorbed light intensity I0(υ) corresponds to (υ).

[0098] In step two, when obtaining I0(υ) using nonlinear least squares fitting, the light intensity model of equation (2) is used for fitting, and through i0, i2, The best fit is obtained by changing the value of I0(υ), thus obtaining an accurate descriptive model of I0(υ).

[0099] Step 3: Obtain the measured second harmonic signal S after background subtraction. 2f-nor-bgsub-mean This is achieved using software demodulation, which is easier to implement than hardware demodulation. Once the filter parameters are fixed, the background absorbed light intensity I0(υ) and the measured absorbed light intensity I... t (υ) After the same processing, a dual-channel background harmonic signal is obtained. and Dual-channel measured harmonic signal X 2f and Y 2f S is calculated using equation (4). 2f-nor-bgsub-mean .

[0100] Step 4: Obtain the simulated second harmonic signal S after background subtraction. 2f-nor-bgsub-sim It involves multiple parameter inputs, including laser parameters, environmental parameters, and absorption spectral parameters.

[0101] The laser parameters in step four include intensity parameters and frequency parameters. The intensity parameter uses I0(υ) from step two, and the frequency model uses equation (3). The initial values ​​of the parameters are given by the laser's factory data sheet and estimated based on the current characteristics and center frequency. The environmental parameters include the temperature, pressure, composition information and effective optical path inside the standard absorption cell. The absorption spectral parameters are given by HITRAN2016. Each absorption line includes the center frequency, line intensity, air broadening factor, self-broadening factor, temperature dependence factor and pressure frequency shift factor. In order to make the simulated harmonic signal more accurate, the simulation process considers the influence of all absorption lines within the laser scanning range. In addition, the absorption information is calculated using the Voigt line shape that is closest to the actual absorption line shape.

[0102] In step four, the simulated absorbed light intensity is obtained using Beer-Lambert, and step three is repeated using the non-absorbed light intensity I0(υ) to obtain the simulated second harmonic signal S after background subtraction. 2f-nor-bgsub-sim .

[0103] Step 5: Change the simulated harmonic signal S by altering the laser output frequency model parameters from Step 4. 2f-nor-bgsub-sim When the iteration conditions are met, the target parameters are obtained, enabling rapid measurement of the laser's output frequency response.

[0104] Monitoring oxygen, an essential gas for human physiology, is crucial in various living environments. With ongoing research, the detection of trace oxygen concentrations is necessary in certain environments. Infrared absorption spectroscopy, due to its advantages of being non-contact, having a fast response, and providing high signal fidelity, has become one of the main detection methods for various applications. However, oxygen molecules are linearly symmetrical diatomic molecules, with their vibrational-rotational infrared activity concentrated around 760 nm, and their absorption line intensity is relatively weak. This highlights the advantages of wavelength modulation techniques in infrared absorption spectroscopy. Whether using harmonic fitting or ratio inversion methods, accurate acquisition of the laser's frequency-time response is essential. Therefore, this paper proposes a method for rapidly measuring the laser's output frequency response and accurately obtaining the laser's frequency response model parameters. In this embodiment, the standard absorption cell pressure is 10 atm, the effective optical path is 100 cm, the temperature is set to 296 K, the oxygen concentration (volume fraction) is 20%, and the remaining gas is high-purity nitrogen.

[0105] The intensity I of the laser beam after absorption after passing through a standard absorption cell t (υ) and the non-absorption light intensity I0(υ) obtained by piecewise fitting, as shown in... Figure 2 As shown, oxygen absorption clearly causes a dip in the original light intensity. The second harmonic S is obtained through dual-channel background subtraction and peak normalization. 2f-nor-bgsub-mean As the objective function, such as Figure 3As shown. The simulated harmonic signal is obtained using the non-absorbed light intensity I0(υ) and other input parameters. Besides the non-absorbed light intensity, the laser modulation parameters also include initial model parameters. The environmental parameters are the standard absorption cell setpoint constants, and the spectral parameters are provided by HITRAN. The absorption spectrum of oxygen molecules near 760 nm is shown below. Figure 4 As shown, to achieve the best fit, all possible absorptions are incorporated into the absorption model, and the simulated second harmonic S is obtained through the same demodulation method as the measured harmonic signal (with completely identical filtering parameters). 2f-nor-bgsub-sim With S 2f-nor-bgsub-mean To achieve the goal, by changing the parameters of the laser frequency-time response model, the result is as follows when the convergence condition is met: Figure 5 As shown, a rapid measurement of the laser's output response is achieved. The iteration condition (convergence condition) is that the squared vector difference between the simulated harmonic signal and the measured harmonic signal is ≤10. -8 (In other embodiments, the convergence condition may also be that the square of the vector difference between the simulated harmonic signal and the measured harmonic signal is ≤10.) -10 Or 10 -12 The squared vector difference between the simulated harmonic signal and the measured harmonic signal is specifically the sum of the squared differences between the corresponding points of the simulated harmonic signal and the measured harmonic signal.

[0106] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for rapidly measuring the output frequency response of a laser, characterized in that: The laser is passed through an absorption cell, and the intensity of the light after absorption is utilized. Simulation yielded light intensity without absorption and the non-absorbed light intensity As a background, the normalized second harmonic after removing the background is used as the fitting objective function, and the laser frequency-time response model is used as the fitting parameter. The laser output frequency response is obtained by nonlinear least squares fitting, and the method of rapidly measuring the laser output frequency response is completed. Includes the following steps: S1. The output light of the laser is collimated and then passes through the absorption cell. After being absorbed by the gas medium in the absorption cell, it is output to the detector. The detector converts the optical signal into an electrical signal to obtain the intensity of the absorbed light. ; S2. Obtain the intensity of the absorbed light by nonlinear least squares fitting. The corresponding non-absorbed light intensity The non-absorbed light intensity As background; S3. The non-absorbed light intensity is demodulated using software. and the light intensity after absorption The same processing was performed to obtain the measured second harmonic signal of the X channel. Background harmonic signals of the X channel Measured second harmonic signal of Y channel Y-channel background harmonic signal And the measured second harmonic signal was obtained. ; S4. Obtain the background second harmonic signal from the laser frequency-time response model. The background second harmonic signal is deducted. It is an analog signal; S5. Change the parameters of the laser frequency-time response model to change the background subtracted second harmonic signal. When the iteration condition is met, the target parameter is obtained. To accurately measure the laser's output frequency response, a rapid measurement of the laser's output frequency response is performed.

2. The method for rapidly measuring the output frequency response of a laser according to claim 1, characterized in that: In step S1, a low-frequency sawtooth scanning superimposed with a high-frequency sinusoidal modulation is injected into the laser, and then the output light of the laser is collimated and coupled into the absorption cell. The output light of the laser is absorbed by the gas medium and then output. The gas medium includes the target gas. The absorption cell provides a fixed temperature, pressure, component concentration and effective optical path.

3. The method for rapidly measuring the output frequency response of a laser according to claim 2, characterized in that: In step S1, the time response of the laser intensity is: ； in, The initial light intensity of the laser. The average light intensity at the center frequency of the laser. For being Normalized linear intensity modulation amplitude, For being Normalized nonlinear intensity modulation amplitude, for The corresponding phase difference between linear intensity modulation and frequency modulation, for The corresponding phase difference between nonlinear intensity modulation and frequency modulation, t represents the frequency of the sinusoidal modulation, and t represents time. When the laser is modulated by a high-frequency sinusoidal current, both the light intensity and frequency are modulated, resulting in a different output frequency. for: ； in, The average light output frequency, , The coefficients of the frequency-description model are used to describe the frequency. For frequency modulation amplitude, As the first prime minister, At that time The linear parameters, At that time The nonlinear parameters; In step S2, the absorbed light intensity The data includes absorbed information and non-absorbed segment information. The nonlinear least squares fitting method is a piecewise fitting method that selects the non-absorbed segment information. , , , , The change is used to perform nonlinear least squares fitting.

4. The method for rapidly measuring the output frequency response of a laser according to claim 1, characterized in that: In step S2, the absorbed light intensity and the aforementioned non-absorbed light intensity Follows the Beer-Lambert law; ； in, Transmittance; according to The non-absorbed light intensity was obtained ,in, The laser's output frequency. To the center frequency of the absorption spectral line, The intensity of the absorption spectral line at temperature T is given. For the normalized absorption line shape function, The total pressure inside the absorption tank. The ratio of the number of moles of the absorbed gas to the total number of moles of the gas. To ensure effective absorption of optical path.

5. The method for rapidly measuring the output frequency response of a laser according to claim 1, characterized in that: In step S3, a lock-in amplifier is used at the modulation frequency. During frequency up-demodulation, the following is obtained: The second harmonic component was obtained, and the measured second harmonic signal was obtained. .

6. The method for rapidly measuring the output frequency response of a laser according to claim 1, characterized in that: In step S3, the measured second harmonic signal is obtained by low-pass filtering combined with dual-channel demodulation. ; 。 7. The method for rapidly measuring the output frequency response of a laser according to claim 1, characterized in that: In step S4, the measured second harmonic signal is... As the fitting target, given the initial parameters of the laser's output frequency, and substituting the internal pressure, temperature, component concentration, and effective absorption path of the absorption cell described in step S1 as constants into the laser's frequency-time response model, step S3 is repeated to obtain the background-subtracted second harmonic signal. .

8. A method for rapidly measuring the output frequency response of a laser according to claim 1, characterized in that: In step S5, the background second harmonic signal is subtracted. Nonlinear least squares fitting is performed to seek the optimal fitting parameters.

9. The method for rapidly measuring the output frequency response of a laser according to claim 1, characterized in that: In step S5, the iteration condition is: the background second harmonic signal is subtracted. and the measured second harmonic signal Vector difference squared ≤ 10 -8 ; When the fitting conditions are met, .

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