Method for suppressing OA-ICOS cavity mode noise using active high-frequency vibration

Abstract

The present invention discloses a method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration, which belongs to the field of off-axis integrating cavity output spectrum technology, including obtaining off-axis integrating cavity parameters; calculating the initial cavity film density, generating an excitation force on the rear high-reflective mirror of the off-axis integrating cavity to make it vibrate along the cavity axis in a simple harmonic manner, adjusting the cavity length change at high frequency, calculating the light spot offset angle, the offset angle of each round trip cycle, analyzing the center angle, number and re-incident conditions between the newly added light spots, and when the re-incident state is met, or when the infinite off-axis state is met, the cavity mode density increases. The present invention provides a method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration, proposes a new vibration cavity (VC) scheme to suppress the cavity mode noise of the OA-ICOS sensor system, and theoretically studies the effect of vibration on cavity mode density for the first time. After analysis, this method reduces the cavity mode noise of the designed OA-ICOS sensor system by more than two times.

Description

Technical Field

[0001] The present invention belongs to the technical field of off-axis integral cavity output spectrum, and in particular relates to a method for suppressing OA-ICOS cavity mode noise by utilizing active high-frequency vibration. Background Art

[0002] Since the Cavity Ring-Down Absorption Spectroscopy (CRDS) technique was proposed, it has been widely used in trace gas detection, free radical molecular spectroscopy, high-sensitivity molecular spectroscopy research and other fields. Cavity-enhanced absorption spectroscopy technology uses a high-precision optical resonant cavity to increase the effective optical path of the interaction between light and matter. This technology has a high detection sensitivity of up to 10 -9 cm -1 , slightly lower than the cavity ring-down absorption spectrum (10 -10 cm -1 ). However, the experimental setup is simpler than that of cavity ring-down absorption spectroscopy. Integrated cavity output spectroscopy (ICOS) has higher output power and larger free spectral range (FSR), and its optical path length enhancement factor is in This represents cavity finesse, but requires rigorous optical alignment. Compared to ICOS, off-axis integrated cavity output spectroscopy (OA-ICOS) employs a method where the laser is incident off the cavity optical axis. This method excites more high-order transverse modes within the cavity, reduces the cavity's free spectral range (FSR), and effectively mitigates the inherent interference effects of the Fabry-Perot cavity. The off-axis integrated cavity is less sensitive to external perturbations, significantly improving its anti-interference capabilities and environmental adaptability. Therefore, OA-ICOS is well-suited for trace gas detection in a variety of environments.

[0003] However, even when the incident light is sufficiently off-axis, some residual cavity modes cannot be eliminated, becoming the dominant noise in the system. In the field of direct absorption OA-ICOS, various methods for improving the signal-to-noise ratio (SNR) have been demonstrated. One approach is to add a third cavity mirror with an entrance aperture in front of the first cavity mirror, further improving the cavity output and SNR. The beam reflected by the first mirror is reinjected into the cavity to boost power. However, while this approach increases the intracavity coupled light intensity, it also exacerbates spot overlap on the mirror surface, resulting in limited suppression of intracavity optical noise. To suppress cavity mode noise while maintaining the same mechanical structure, a method of perturbing the laser with radio frequency (RF) noise has been developed. Perturbing the laser drive with white noise is relatively simple to implement, but it inevitably introduces frequency and amplitude modulation, which in turn leads to additional signal noise, spectral line distortion, and reduced absorption. Furthermore, researchers have further improved the detection limit by combining wavelength modulation (WM). In 2019, Wang et al. proposed a new method of injecting radio frequency (RF) white noise perturbations into the laser drive current. In this scheme, radio frequency white noise is used to suppress the noise level caused by residual cavity mode fluctuations, and the detection limit is 1.2×10 -9 This method also suffers from the above-mentioned problems caused by the RF white noise perturbation of the laser. Zheng et al. developed a new multi-input multi-output (DIDO) laser-cavity coupling scheme for modal noise suppression. Through experimental research on methane measurement based on OA-ICOS, this method improved the signal-to-noise ratio by 2.5 times and the detection limit by 2.2 times. However, the total energy of the incident beam of the laser beam splitting scheme is reduced and the incident structure is more complex. Summary of the Invention

[0004] The purpose of the present invention is to suppress cavity mode noise while keeping the absorption spectrum unchanged, and a method of suppressing OA-ICOS cavity mode noise by using active high-frequency vibration is proposed. Active vibration is introduced into the OA-ICOS sensor system, and a new vibration method for suppressing cavity mode noise is proposed.

[0005] The technical solution adopted by the present invention to achieve the above-mentioned object is: a method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration is proposed, comprising the following steps, which are performed in sequence:

[0006] Step 1: Obtain off-axis integrating cavity parameters;

[0007] The off-axis integrating cavity parameters include the reflectivity R of the high-reflection mirror, the curvature radius r of the high-reflection mirror, the diameter D of the high-reflection mirror and the cavity length d, and the cavity length d is equal to the distance between the front and rear high-reflection mirrors located on the cavity axis of the off-axis integrating cavity;

[0008] Step 2: According to the cavity length d and the high reflective mirror curvature radius r in the obtained off-axis integral cavity parameters, the one-way spot deviation angle θ is obtained by the formula cos(θ)=1-d / r; according to the re-incident condition formula 2mθ=2Nπ, the number of spots that meet the re-incident condition m is calculated, where N is an integer; according to the formula Get the free spectral range (FSR), where c is the speed of light. The cavity mode density is inversely proportional to the free spectral range (FSR). The smaller the free spectral range (FSR), the greater the cavity mode density.

[0009] Step 3: Generate an excitation force on the rear high-reflection mirror of the off-axis integrating cavity to vibrate along the cavity axis in a simple harmonic manner, so that the rear high-reflection mirror of the off-axis integrating cavity vibrates in a simple harmonic manner according to the frequency f and the amplitude A, thereby causing the cavity length d to change periodically according to the frequency f;

[0010] Step 4: Calculation of light spot offset angle;

[0011] According to the formula Where φ is the angle between the light beam and the cavity axis, and the time τ for the light to travel back and forth once in the off-axis integrating cavity is obtained. Then, according to the formula The cavity length displacement from the i-1th round trip to the i-th round trip time τ to form the kth light spot is obtained For the kth i-1 The time it takes for a round trip to return, k i-1 The i-1th round trip of the kth light spot added between two adjacent light spots on the high-reflectivity mirror surface; then the cumulative displacement is obtained Simple harmonic vibration will cause the cavity length to increase changes; according to the formula The spot angle formed by the cavity length corresponding to the i-th round trip in the process of forming the k-th spot cycle is obtained

[0012] Step 5: Obtain the offset angle of each round trip cycle;

[0013] According to the formula The spot angle Δθ between the kth spot and the k-1th spot is obtained k , and the spot angles Δθ1, Δθ2, Δθ3, ..., Δθ are obtained in sequence when k = 1, 2, 3, ..., K. K ;

[0014] Step 6: Analyze the center angle Δθ between the newly added light spots k and quantity K;

[0015] When the angles between the newly added K light spots satisfy Δθ1+Δθ2+Δθ3+···Δθ K =Δθ minWhen , the number of spots on the off-axis integrating cavity high reflective mirror increases from m to Km, and the length of the re-incident optical path becomes 2Kmd; the free spectrum range is FSR VR It is the free spectral region of the off-axis integrating cavity of the resonator type;

[0016] Step 7: Pass The free spectral range is reduced to K times of the original value, and the cavity film density is increased by K times. At this point, the use of active high-frequency vibration to suppress OA-ICOS cavity mode noise is completed.

[0017] Furthermore, the method of suppressing OA-ICOS cavity mode noise by active high-frequency vibration defines a critical value When the light spots overlap, the angle Δθ between the two adjacent light spots min Less than or equal to the critical value When Δθ is equal to Δθ, the overlapping light spots will produce early re-incident or quasi-re-incident; min Greater than the critical value When the light is reflected by the light beam, interference fringes will appear between the overlapping light spots.

[0018] Furthermore, the method of suppressing OA-ICOS cavity mode noise by active high-frequency vibration is as follows: when the angles between the newly added K light spots meet When the center of the newly added light spot does not coincide with the center of the original light spot, the re-incident condition cannot be met; or the light spot overlap occurs when the light intensity of the light spot has decayed to a sufficiently weak level; these two situations correspond to the infinite off-axis mode.

[0019] Furthermore, in the method of suppressing OA-ICOS cavity mode noise by active high-frequency vibration, the cavity mode noise in the integration time T is expressed as:

[0020] It can be seen that within the integration time T, K=TN will appear T Transmission peaks; N T is the number of laser and cavity mode couplings per second, and the modulation frequency is f o , the number of modules is n, and we get N T =2f o n; ξ is the relative discreteness of a single transmission peak, ranging from 0.15 to 0.5, and σ represents the cavity mode noise within the integration time T.

[0021] Through the above-mentioned design scheme, the present invention can bring the following beneficial effects: The present invention provides a method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration, proposes a new vibration cavity (VC) scheme to suppress the cavity mode noise of the OA-ICOS sensor system, and theoretically studies the effect of vibration on the cavity mode density for the first time. After analysis, the method reduces the cavity mode noise by more than two times. The direct absorption of standard methane (CH4) was measured by the designed VC-OA-ICOS sensor system, and the cavity mode noise suppression capability of the method was verified from two indicators: signal-to-noise ratio and minimum detection limit. In order to reduce the interference of burr noise generated near the absorption peak, the collected spectral signal is filtered. The signal-to-noise ratio of the filtered VC-OA-ICOS sensor system is 71, and the minimum detection limit is 0.88ppmv. Compared with the non-vibration OA-ICOS method, the signal-to-noise ratio of the designed VC-OA-ICOS sensor system is improved by 7.78 times, and the minimum detection limit is improved by 3.4 times. The present invention provides a new idea for suppressing cavity mode noise of OA-ICOS sensor systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of this application. The exemplary embodiments of the present invention and their descriptions are used to understand the present invention and do not constitute improper limitations of the present invention. In the drawings:

[0023] Figure 1 This is a flow chart of the method proposed in the present invention for suppressing OA-ICOS cavity mode noise using active high-frequency vibration.

[0024] Figure 2 Schematic diagram of light spot re-incident and overlap, in which (a) represents the schematic diagram of re-incident distribution; (b) represents the schematic diagram of quasi-re-incident distribution; and (c) represents the schematic diagram of overlap distribution.

[0025] Figure 3 This is a simulation diagram of the mode density of a conventional off-axis integrated cavity.

[0026] Figure 4 This is a simulation diagram of the off-axis integrated cavity mode density of the vibration cavity type.

[0027] Figure 5 is the cavity mode noise diagram.

[0028] Figure 6 This is a structural diagram of the VC-OA-ICOS resonant cavity type off-axis integrating cavity sensor system.

[0029] Figure 7 The signal-to-noise ratio and vibration glitch interval diagram at each vibration frequency.

[0030] Figure 8The experimental data and fitting curves of different concentration levels of standard CH4 gas and direct absorption peaks are shown.

[0031] Figure 9 This is the detection result of 1500ppmv standard CH4 gas.

[0032] Figure 10 This is the Allan deviation diagram of 1500ppmv standard CH4 gas. DETAILED DESCRIPTION

[0033] To more clearly illustrate the present invention, the present invention is further described below with reference to preferred embodiments and the accompanying drawings. Those skilled in the art should understand that the following detailed description is illustrative rather than restrictive and should not be used to limit the scope of protection of the present invention. Unless otherwise defined, technical or scientific terms used herein should have the same meaning as those having ordinary skill in the art to which the present invention belongs.

[0034] like Figure 1 As shown, the method proposed by the present invention for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration includes the following steps, which are performed in sequence:

[0035] Step 1: Obtain off-axis integrating cavity parameters;

[0036] The off-axis integrating cavity parameters include the reflectivity R of the high-reflection mirror, the curvature radius r of the high-reflection mirror, the diameter D of the high-reflection mirror and the cavity length d;

[0037] Step 2: Calculate the initial cavity mode density;

[0038] According to the cavity length d and the high reflective mirror curvature radius r in the obtained off-axis integral cavity parameters, the one-way spot deviation angle θ is obtained by the formula cos(θ)=1-d / r; according to the re-incident condition formula 2mθ=2Nπ, the number of spots m that meet the re-incident condition is calculated, where N is an integer; according to the formula Where c is the speed of light, and the free spectral range (FSR) is obtained. The cavity mode density is inversely proportional to the free spectral range (FSR). The smaller the free spectral range (FSR), the greater the cavity mode density.

[0039] Step 3: The vibration module works;

[0040] The full-range speaker is driven and operated by the host computer software, so that the full-range speaker fixed to the high-reflection mirror support frame behind the off-axis integrating cavity vibrates according to the frequency f preset by the host computer;

[0041] Step 4: High frequency adjustment of cavity length change;

[0042] The full-range speaker drives the rear high-reflection mirror support frame to vibrate at frequency f, causing the rear high-reflection mirror of the off-axis integrating cavity to perform simple harmonic vibration at frequency f and amplitude A, thereby causing the cavity length d to change periodically at frequency f;

[0043] Step 5: Calculate the light spot offset angle;

[0044] According to the formula φ is the angle between the light beam and the cavity axis, and the time τ for the light to travel back and forth once in the off-axis integrating cavity is obtained. Then, according to the formula The cavity length displacement from the i-1th round trip to the i-th round trip time τ to form the kth light spot is obtained Then the cumulative displacement is obtained According to the formula The spot angle formed by the cavity length corresponding to the i-th round trip in the process of forming the k-th spot cycle is obtained

[0045] Step 6: offset angle of each round trip cycle;

[0046] According to the formula The spot angle Δθ between the kth spot and the k-1th spot is obtained k , and the spot angles Δθ1, Δθ2, Δθ3, …, Δθ are obtained in sequence when k = 1, 2, 3, …, K. K ;

[0047] Step 7: Analyze the center angle Δθ between the newly added light spots k and quantity K;

[0048] Step 8: If the re-incident condition is met, then the re-incident state is reached; if the re-incident condition is not met, then the infinite off-axis state is reached; when the re-incident state or the infinite off-axis state is met, the cavity mode density increases;

[0049] Step 9: At this point, the use of active high-frequency vibration to suppress OA-ICOS cavity mode noise is completed.

[0050] The main contributions of this invention are as follows: 1) A VC-OA-ICOS method is proposed, which actively introduces beneficial controllable vibrations on the high-reflectivity mirror behind the off-axis integrating cavity; 2) A theoretical explanation and detailed derivation of the OA-ICOS sensor system's vibration cavity method are presented, providing theoretical support for further research on vibration technology in OA-ICOS sensor systems; and 3) A short-cavity, long-cavity off-axis integrating cavity system is designed, which has the advantages of small size, simple structure, and low cost.

[0051] 1. Analysis of the working principle of the present invention

[0052] 1.1 Principle of Off-Axis Integrating Cavity Technology

[0053] The present invention only considers Lambert-Beer absorption and ignores the loss caused by scattering. For a beam of light with an intensity of I0(ν) incident on an off-axis integrating cavity, the total light intensity passing through the off-axis integrating cavity is:

[0054]

[0055] Among them I in is the initial light intensity, I is the light intensity passing through the cavity when there is absorption, α(ν) is the absorption coefficient, ν is the light frequency, d is the distance between the two high-reflection mirrors (i.e., cavity length), R is the reflectivity of the high-reflection mirror (assuming that the reflectivity of the two high-reflection mirrors is the same), then I o =I in (ν)(1-R) / 1+R is the light intensity transmitted through the cavity when there is no absorption in the cavity. When the reflectivity of the high-reflectivity mirror is very high and the single absorption is very small, that is, R→1, exp[-α(ν)d]→1, the simplified absorption coefficient α(ν) obtained from formula (1) is:

[0056]

[0057] In formula (2), d and R are constants. Fluctuations in the transmittance I / I0 will result in fluctuations in the absorption coefficient α(ν).

[0058] When the off-axis beam is incident on the off-axis integrating cavity, it is reflected multiple times between the two highly reflective mirrors, forming an elliptical pattern on the mirror surface. After m rounds of reflection, it coincides with the incident beam, and the re-incidence condition is met.

[0059] 2mθ=2Nπ (3)

[0060] Where m is the number of times the light beam bounces back and forth, 2θ is the angle between adjacent reflected light spots (one round trip) on the high-reflectivity mirror, and N is an integer. θ is determined by the following formula:

[0061] cos(θ)=1-d / r (4)

[0062] Where d is the cavity length, i.e., the distance between the two highly reflective mirrors in formula (1), and r is the radius of curvature of the highly reflective mirror. When the re-incidence condition is met, the optical path reaches 2md, and the free spectral range (FSR) becomes:

[0063]

[0064] This is 1 / m times smaller than the coaxial state, where c is the speed of light. Therefore, the off-axis integrating cavity can excite a large number of high-order transverse modes by injecting laser light off the cavity axis, thereby increasing the cavity mode density.

[0065] 1.2 Re-incidence and interference fringes

[0066] For an off-axis integrating cavity with set parameters, adjust the incident position and angle of the incident light beam to make the light spot present an ideal circular distribution. According to formula (4), 2θ is completely determined by the off-axis integrating cavity parameters (d and r). When the light beam reflects back and forth and just meets the conditions of formula (3), the light beam reaches the re-incident condition after m round trips. At this time, m light spots are formed on the high-reflection mirror, and the corresponding free spectral range is FSR = c / 2md, as shown in Figure 2 As shown in (a).

[0067] When the light beam has not reached the re-incident condition 2mθ≠2Nπ after m times of back and forth reflection, countless light spots will theoretically be formed on the high-reflection mirror. The corresponding free spectral range FSR is infinitely small, and the transmission spectrum tends to be continuous. This state is the infinite off-axis state. However, the off-axis integrating cavity is always limited by the diameter of the high-reflection mirror (usually only 1 foot), and the spot diameter (usually in the order of mm) cannot be infinitely small. Therefore, the above two situations cannot be achieved in practice. In practice, whether in the re-incident state or the infinite off-axis state, after the light beam travels back and forth multiple times in the off-axis integrating cavity, the light spots on the high-reflection mirror will always overlap to varying degrees.

[0068] Here, the present invention defines a critical value When the light spots overlap, the angle Δθ between the two adjacent light spots min Less than or equal to the critical value When the overlapping light spots are overlapped, they will produce early re-incident or quasi-re-incident, such as Figure 2 (b) As shown. Depending on the overlap state, the early re-incidence will lead to a variety of re-incidence or quasi-re-incidence with different optical path lengths in the off-axis integrating cavity. This will cause a variety of resonant states with different optical path lengths in the off-axis integrating cavity. This will produce free spectral ranges (FSRs) of different widths. When the angle Δθ between two adjacent light spots is min Greater than the critical value When , interference fringes will appear between overlapping light spots (etalon effect), such as Figure 2 (c) Depending on the overlap state, interference fringes with various optical path lengths can be generated. Therefore, in practice, multiple re-incident and quasi-re-incident modes, as well as multiple interference fringes, exist within the off-axis integrating cavity. This is a superposition process of the interactions of all these modes.

[0069] 1.3 Principle of increasing cavity mode density using the cavity method

[0070] In practical applications, even if the light spot on the high-reflectivity mirror has the ideal perfect circular distribution, a large number of cavity mode structures will still exist. To further increase the cavity mode density and more effectively suppress cavity mode noise, the present invention incorporates a frequency-adjustable full-range speaker on the rear high-reflectivity mirror support frame of the off-axis integrating cavity. Vibration significantly increases the cavity mode density, as detailed below:

[0071] Z=A sin(2πft) (6)

[0072] After adding the full-range speaker, the rear high reflector will vibrate along the cavity axis in a simple harmonic manner. Where Z is the simple harmonic displacement of the rear high reflector, A is the simple harmonic amplitude of the rear high reflector, and f is the simple harmonic frequency of the rear high reflector.

[0073]

[0074] In formula (7), d is the cavity length, τ is the time it takes for light to travel back and forth once in the off-axis integrating cavity, c is the speed of light, and φ is the angle between the light beam and the cavity axis.

[0075]

[0076] In formula (8), for Time has come The cavity length displacement corresponding to the moment, For the kth i-1 The time it takes for a round trip to return, k i-1 is the i-1th round trip of the kth spot added between two adjacent spots on the mirror. Simple harmonic vibration will cause the cavity length to produce changes.

[0077]

[0078] From formula (4), we can see that in formula (9), The angle of the light spot formed by the cavity length corresponding to the i-th round trip in the k-th light spot cycle. Taking the inverse cosine of formula (9) yields:

[0079]

[0080] It can be concluded from formula (10) that the introduction of simple harmonic vibration will lead to The change of cavity length will make θ become

[0081]

[0082] In formula (11), Δθ k The angle between the kth spot and the previous adjacent spot is Δθ. The light beam starts from the k-1th spot and reaches the kth spot after m round trips. The total displacement formed during the round trip cycle is Δθ. k .

[0083] Δθ1+Δθ2+Δθ3+···Δθ K =Δθ min (12)

[0084] When the angles between the newly added K light spots satisfy formula (12), it is equivalent to inserting K light spots between any two conventional light spots. The number of light spots of the off-axis integrating cavity high reflectivity mirror increases from m to Km, and the re-incident optical path length becomes 2Kmd. The free spectral range is as follows:

[0085]

[0086] Among them, FSR VR It is the free spectral region of the off-axis integrating cavity of the resonant cavity type.

[0087] It can be concluded from formula (13) that the free spectral range (FSR) is reduced to K times of the original value, which theoretically increases the transmission spectral density by K times.

[0088]

[0089] When the angles between the newly added K spots satisfy formula (14), the centers of the newly added spots do not overlap with the original spots, failing to meet the re-incident condition. Alternatively, spot overlap occurs only after the intensity of the spots has decayed sufficiently. These two situations can theoretically be considered infinite off-axis modes. In theory, in this mode, the FSR is infinitely small, the cavity mode density is sufficiently high, and the transmission spectrum tends to be continuous.

[0090] 1.4 Effect of cavity mode density on cavity mode noise

[0091] Cavity mode noise can be described as the relative dispersion of the current integral value within the integration time T. The cavity mode noise within the integration time T can be expressed as:

[0092]

[0093] From formula (15), we can see that within the integration time T, K=TN T Transmission peak. N T is the number of laser and cavity mode couplings per second, and the modulation frequency is f o , the number of modules is n, and we can get N T =2f o n. ξ is the relative dispersion of a single transmission peak, ranging from 0.15 to 0.5. As can be seen from the formula, increasing the number of modes, n, can reduce cavity mode noise and improve detection limits and sensitivity.

[0094] 2. Simulation and calculation

[0095] The higher the cavity mode density, the stronger its ability to smooth the cavity mode structure. The present invention simulates the cavity mode density of the off-axis integrating cavity and finds that the cavity mode density in the effective vibration state is increased by 5 times compared with the non-vibration state. Figure 3 and Figure 4According to the simulation results and formula (15), the cavity mode noise level of the system is reduced by about 2.3 times. Figure 3 shows the conventional off-axis integrated cavity mode density simulation diagram, Figure 3 The middle line Ⅰ represents the CH4 absorption line Line II represents the DFB laser line width (Δf laser =3MHz), line III represents the conventional off-axis integrated cavity mode density. Figure 4 The simulation diagram of the off-axis integrated cavity mode density of the resonant cavity is shown: Figure 4 The middle line Ⅰ represents the CH4 absorption line Line II represents the DFB laser line width (Δf laser =3MHz), and line IV represents the off-axis integrated cavity mode density of the cavity type.

[0096] After calculation Figure 5 The average optical noise values ​​for the two signals, the mid-V and VI lines, were 0.128V and 0.048V, respectively. The addition of vibration reduced the optical noise by an average of approximately 2.6 times. The measured optical noise reduction factor is close to the simulated reduction factor, with the error sources being interference noise and glitch noise. This also indicates that the primary noise in the system is cavity mode noise.

[0097] 3. VC-OA-ICOS sensor system structure and design

[0098] The structure of the VC-OA-ICOS sensor system is as follows Figure 6 As shown, it consists of a gas distribution system, an optical part, an electrical system and a vibration module.

[0099] The electrical system includes a computer, a DAQ acquisition card, a full-range speaker driver, a current driver (LDC202C, Thorlabs, USA), and a laser temperature control (TED200C, Thorlabs, USA). The DFB laser temperature is set to 10°C. A signal generator generates a sawtooth waveform (10 Hz) to drive the input current for wavelength sweeping. The DFB laser stands for distributed feedback laser, OA-ICOS stands for off-axis integrated cavity output spectrum, and the DAQ acquisition card stands for data acquisition card.

[0100] In the optical part, the light source used is a DFB diode laser with a central wavelength of 1.6537μm. A collimator (50-1550A-APC, Shorlabs, USA) is used to collimate the beam to obtain a Gaussian beam with a waist diameter of 1.2mm. The collimator is mounted on a five-dimensional optical adjustment frame and couples the output beam into the off-axis integrating cavity. The optical adjustment frame is used to adjust the angle of the incident beam to achieve good coupling between the laser and the off-axis integrating cavity. The cavity is 104.45mm long and consists of two high-reflection mirrors (99% @ 1450-1670nm) in front and back, with a curvature radius of 20cm. The transmitted output beam of the off-axis integrating cavity is collected by a converging lens (focal length 6cm) and focused on an InGaAs amplified detector.

[0101] In the gas distribution system, nitrogen (N2) was used as the balance gas and mixed with methane gas with a volume concentration of 10,000 ppmv. Methane samples at different concentration levels were obtained using a gas mixing system (4000 series, American Environmental Company).

[0102] In the vibration module, a full-range speaker is mounted on the upper portion of the rear high-reflection mirror support frame. A MATLAB program controls the full-range speaker to vibrate the rear high-reflection mirror. By adjusting the full-range speaker's output frequency, the program increases the cavity mode density, thereby reducing cavity mode noise and interference noise.

[0103] 4. Results and Discussion

[0104] 4.1SNR Analysis and Frequency Optimization

[0105] In order to obtain the optimal vibration frequency of the off-axis integrating cavity system, the present invention applies vibration frequencies ranging from 0 to 2000 Hz to the system, and fits the vibration frequency and SNR curve to obtain the optimal vibration frequency. Figure 7 The SNR curve shows several irregular peaks between 0 and 1200 Hz, indicating that different vibration frequencies have different effects on the suppression of cavity mode noise, with a clear downward trend after 1200 Hz. This is because the mechanical structure of the cavity loses its response to the vibration frequency after the vibration frequency increases to a certain level. At the same time, when selecting the signal-to-noise ratio (SNR), it is also important to consider whether the interval of the burr noise is appropriate, such as Figure 7 Curve VIII. Because the glitch interval decreases with increasing vibration frequency, the SNR decreases, gradually losing its effectiveness. When the vibration is below 500Hz, the glitch interval is relatively large. Based on the SNR curve, 250Hz was ultimately selected as the optimal vibration frequency for this system (SNR of 23.2 and glitch interval of 8ms), and the glitch interval corresponding to this vibration frequency is also suitable. However, when glitch noise occurs at the absorption peak, it interferes with the acquired data, requiring filtering to minimize its impact on the absorption peak.

[0106] 4.2 System Calibration

[0107] To calibrate the system, a gas distribution system was used to mix 15,000 ppm methane standard gas and high-purity nitrogen into seven groups of methane mixed gases with different concentrations ranging from 1,000 ppmv to 1,600 ppmv. The direct absorption amplitudes at these seven different concentration levels were measured. The average value was taken and plotted as a function of the methane concentration, as shown in Figure 1. Figure 8 shown. Figure 8 The experimental data and fitting curves of different concentration levels of standard CH4 gas and direct absorption peak are shown. Figure 8 Where IX is the experimental data of standard CH4 gas with different concentration levels and direct absorption peak and the fitting curve without vibration state, X is the experimental data of standard CH4 gas with different concentration levels and direct absorption peak and the fitting curve with 250Hz vibration state, XI is the experimental data of standard CH4 gas with different concentration levels and direct absorption peak and the fitting curve with 250Hz vibration filtering state; the concentration C and direct absorption peak A are obtained. da The two linear relationships between them are fitted using a quadratic polynomial. The linearity of the vibration-free absorption is 0.99449, as shown in Figure 8 In the middle IX curve, the linearity of 250Hz vibration absorption is 0.99934. Figure 8 In the X curve, the linearity of 250Hz vibration plus filtering absorption is 0.99953, such as Figure 8 The XI curve in the middle shows that 250Hz vibration has an excellent effect on improving the system's concentration response performance.

[0108]

[0109] Among them C Non-vibration is the vibration-free concentration C Vibration is the vibration concentration C Vibration-filter is the concentration of vibration filtering.

[0110] 4.3 Detection limit is

[0111] The present invention conducts a long-term measurement of 10 minutes on a standard methane gas with a concentration of 1500 ppmv to evaluate the stability of the sensor system, and uses Allan deviation to characterize its detection limit performance. Figure 9 The gas concentration of 1500 ppmv methane measured by the system at R-OA-ICOS, VC-OA-ICOS (interval sampling) and VCF-OA-ICOS with a sampling time of 1 second. Figure 9Figure 1 shows the results of a 1500ppmv standard CH4 gas detection. Figures XII and XIII show the 1500ppmv standard CH4 gas detection under vibration-free conditions, XIII under 250Hz vibration conditions, and XIV under 250Hz vibration filtering conditions. Figures XII, XIII, and XIV represent the average concentration data under the three modes, respectively. The relative errors are less than 1.13%, 0.6%, and 0.33%, respectively, demonstrating that the sensor systems using VC-OA-ICOS (interval sampling) and VCF-OA-ICOS have good measurement accuracy. The concentration data fluctuations during the measurement process were less than ±40ppmv, ±15ppmv, and ±10ppmv, respectively, demonstrating the good stability of the VRF-OA-ICOS sensor system.

[0112] Figure 10 Figure 1 is an Allan deviation plot calculated from concentration data. XV is the Allan deviation of a 1500 ppmv standard CH4 gas with no vibration, XVI is the Allan deviation of a 1500 ppmv standard CH4 gas with 250 Hz vibration, and XVII is the Allan deviation of a 1500 ppmv standard CH4 gas with 250 Hz vibration filtered. With an integration time of 2 seconds, the R-OA-ICOS sensor system can achieve a detection limit of 41.67 ppmv, and a minimum detection limit of 3.03 ppmv is achieved with an integration time of 119 seconds. Figure 10 The XV curve in the middle. The VC-OA-ICOS sensor system can reach a detection limit of 11.15ppmv at 2s, and a minimum detection limit of 1.36ppmv at an integration time of 79s. Figure 10 Middle XVI curve.

[0113] The VCF-OA-ICOS sensor system can reach a detection limit of 8.8 ppmv at 2 s, and a minimum detection limit of 0.88 ppmv at an integration time of 86 s. Figure 10 Curve XVII in the figure demonstrates that under the same conditions, the VCF-OA-ICOS sensor system can improve the detection limit by 3.4 times compared to the R-OA-ICOS sensor system.

[0114] 5. Conclusion

[0115] In order to reduce the cavity mode noise of the OA-ICOS sensor system and improve the measurement accuracy of the system, the present invention proposes a cavity filtering method applied to the OA-ICOS sensor, and for the first time proposes a theoretical explanation and detailed derivation process of the cavity method. The sensor system based on this method has the advantages of low cavity mode noise, simple structure, low cost and easy implementation. After actual verification, it can be concluded that the performance of the sensor system using the cavity method is significantly improved. At a vibration frequency of 250Hz, the signal-to-noise ratio of the VC-OA-ICOS and filtering-based VC-OA-ICOS sensor systems increased by 7.78 times and 23.4 times respectively. The filtering-based VC-OA-ICOS sensor system achieved a minimum detection limit performance of 0.88ppmv at an integration time of 86s. This shows that the system can increase the detection limit by 3.4 times under vibration conditions.

Claims

1. A method for suppressing OA-ICOS cavity mode noise using active high-frequency vibration, characterized in that: The method includes the following steps, which are performed in sequence: Step 1: Obtain off-axis integrating cavity parameters; The off-axis integrating cavity parameters include the reflectivity R of the high-reflection mirror, the curvature radius r of the high-reflection mirror, the diameter D of the high-reflection mirror and the cavity length d, and the cavity length d is equal to the distance between the front and rear high-reflection mirrors located on the cavity axis of the off-axis integrating cavity; Step 2: According to the cavity length d and the high reflective mirror curvature radius r in the obtained off-axis integral cavity parameters, the one-way spot deviation angle θ is obtained by the formula cos(θ)=1-d / r; according to the re-incident condition formula 2mθ=2Nπ, the number of spots that meet the re-incident condition m is calculated, where N is an integer; according to the formula Get the free spectral range (FSR), where c is the speed of light. The cavity mode density is inversely proportional to the free spectral range (FSR). The smaller the free spectral range (FSR), the greater the cavity mode density. Step 3: Generate an excitation force on the rear high-reflection mirror of the off-axis integrating cavity to vibrate along the cavity axis in a simple harmonic manner, so that the rear high-reflection mirror of the off-axis integrating cavity vibrates in a simple harmonic manner according to the frequency f and the amplitude A, thereby causing the cavity length d to change periodically according to the frequency f; Step 4: Calculation of the light spot offset angle; According to the formula Where φ is the angle between the light beam and the cavity axis, and the time τ for the light to travel back and forth once in the off-axis integrating cavity is obtained. Then, according to the formula The cavity length displacement from the i-1th round trip to the i-th round trip time τ to form the kth light spot is obtained For the kth i-1 The time it takes for a round trip to return, k i-1 The i-1th round trip of the kth light spot added between two adjacent light spots on the high-reflectivity mirror surface; then the cumulative displacement is obtained Simple harmonic vibration will cause the cavity length to increase changes; according to the formula The light spot angle formed by the cavity length corresponding to the i-th round trip in the process of forming the k-th light spot cycle is obtained Step 5: Obtain the offset angle of each round trip cycle; According to the formula The spot angle Δθ between the kth spot and the k-1th spot is obtained k , and the spot angles Δθ1, Δθ2, Δθ3, ..., Δθ are obtained in sequence when k = 1, 2, 3, ..., K. K ; Step 6: Analyze the center angle Δθ between the newly added light spots k and quantity K; When the angles between the newly added K light spots satisfy Δθ1+Δθ2+Δθ3+···Δθ K =Δθ min When , the number of spots on the off-axis integrating cavity high reflective mirror increases from m to Km, and the length of the re-incident optical path becomes 2Kmd; the free spectrum range is FSR VR It is the free spectral region of the off-axis integrating cavity of the resonator type; Step 7: Pass The free spectral range is reduced to K times of the original value, and the cavity film density is increased by K times. At this point, the use of active high-frequency vibration to suppress OA-ICOS cavity mode noise is completed.

2. The method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration according to claim 1, characterized in that: Defining critical values When the light spots overlap, the angle Δθ between the two adjacent light spots min Less than or equal to the critical value When Δθ is equal to Δθ, the overlapping light spots will produce early re-incident or quasi-re-incident; min Greater than the critical value When the light is reflected by the light beam, interference fringes will appear between the overlapping light spots.

3. The method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration according to claim 1, characterized in that: When the angles between the newly added K light spots meet When the center of the newly added light spot does not coincide with the center of the original light spot, the re-incident condition cannot be met; or the light spot overlap occurs when the light intensity of the light spot has decayed to a sufficiently weak level; these two situations correspond to the infinite off-axis mode.

4. The method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration according to claim 1, characterized in that: The cavity mode noise in the integration time T is expressed as: It can be seen that within the integration time T, K=TN will appear T Transmission peaks; N T is the number of laser and cavity mode couplings per second, and the modulation frequency is f o , the number of modules is n, and we get N T =2f o n; ξ is the relative discreteness of a single transmission peak, ranging from 0.15 to 0.5, and σ represents the cavity mode noise within the integration time T.

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