Method for restraining OA-ICOS cavity mode noise through active high-frequency vibration

Abstract

The invention discloses a method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration, which belongs to the technical field of off-axis integral cavity output spectrum, and comprises the following steps: acquiring off-axis integral cavity parameters; the method comprises the following steps: calculating initial cavity membrane density, generating excitation force on a rear high-reflectivity mirror of an off-axis integral cavity to enable the rear high-reflectivity mirror to vibrate along a cavity axis in a simple harmonic manner, adjusting cavity length change at high frequency, calculating a light spot deviation angle, calculating a back-and-forth circulation deviation angle, analyzing a central included angle between newly added light spots, the number and a re-incidence condition, and when a re-incidence state is met, calculating the initial cavity membrane density; or when an infinite off-axis state is met, the cavity mold density is increased. The invention provides a method for inhibiting the OA-ICOS cavity mode noise by utilizing active high-frequency vibration, a new vibration cavity (VC) scheme is provided to inhibit the cavity mode noise of an OA-ICOS sensor system, and the influence of vibration on the cavity mode density is researched theoretically for the first time. By analyzing the method, the cavity mode noise of the designed OA-ICOS sensor system is reduced by more than two times.

Description

Technical Field

[0001] The 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 It indicates the fineness of the cavity, but requires strict optical alignment. Compared with ICOS, the off-axis integrated cavity output spectrum (OA-ICOS) uses the method of laser incidence away from the cavity optical axis. This method can excite more high-order transverse modes in the cavity, reduce the free spectral range (FSR) of the cavity, and effectively weaken the interference effect of the Fabry-Perot cavity itself. The off-axis integrated cavity is not particularly sensitive to external disturbances. Its anti-interference ability and environmental adaptability are significantly improved. Therefore, OA-ICOS is very suitable for the detection of trace gases in a variety of environments.

[0003] However, even when the incident light is sufficiently off-axis, there are still some residual cavity modes that cannot be eliminated, which become the main noise of the system. In the field of direct absorption OA-ICOS, a variety of methods to improve the signal-to-noise ratio (SNR) have also been verified. One method is to further improve the cavity output and signal-to-noise ratio by adding a third cavity mirror with an entrance hole in front of the first cavity mirror. The beam reflected by the first reflector is re-injected into the cavity to enhance the power, but this scheme increases the intensity of coupled light in the cavity while also aggravating the spot overlap on the mirror, which has a limited effect on suppressing the intracavity optical noise. In order to suppress cavity mode noise without changing the mechanical structure, a method of radio frequency noise perturbation of the laser was developed. It is relatively simple to implement the laser drive by perturbing it with white noise, but it will inevitably introduce frequency and amplitude modulation, which in turn will lead to additional signal noise, spectral line deformation and absorption reduction. In addition, relevant scholars have further improved the detection limit by combining wavelength modulation technology (WM). In 2019, Wang et al. proposed a new method of injecting radio frequency (RF) white noise perturbation into the laser drive current. In this scheme, RF white noise is used to suppress the noise level caused by residual cavity mode fluctuations, and the detection limit is 1.2×10 -9 It has been improved by 6 times. This method also has the problems caused by the above-mentioned RF white noise perturbation laser. Zheng et al. developed a new multiple-input multiple-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 complicated. 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 purpose is: a method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration is proposed, comprising the following steps, and the following steps 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 radius of curvature 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-reflection 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; according to the formula Get the free spectral range FSR, 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: generating an exciting force on the rear high-reflection mirror of the off-axis integrating cavity to make it vibrate along the cavity axis in a simple harmonic manner, so that the rear high-reflection mirror of the off-axis integrating cavity performs simple harmonic vibration 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 of the kth light spot is obtained. 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 light spot added between two adjacent light spots on the high-reflection 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

[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 obtain the spot angle Δθ when k=1,2,3,......,K in turn 1 , Δθ 2 , Δθ 3 ,…,Δθ 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 Δθ 1 +Δθ 2 +Δθ3 +···Δθ K =Δθ min When , the number of spots on the high-reflection mirror of the off-axis integrating cavity increases from m to Km, and the length of the re-incident optical path becomes 2Kmd; the free spectral 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] Further, 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 two adjacent light spots min Less than or equal to the critical value When , the overlapping light spots will produce early re-incident or quasi-re-incident; Δθ min Greater than critical value When the light is overlapping, interference fringes will appear between the overlapping light spots.

[0018] Furthermore, in the method of suppressing OA-ICOS cavity mode noise by active high-frequency vibration, when the angle between the newly added K light spots satisfies When the center of the newly added light spot does not overlap with the center of the original light spot, the re-incident condition cannot be met; or the light spot overlap occurs only 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 lasers coupled to cavity modes per second, with a modulation frequency of 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 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 cavity mode density for the first time. After analysis, the method reduces the cavity mode noise by more than two times. The designed VC-OA-ICOS sensor system is used to measure the standard methane (CH 4 ) direct absorption was measured, and the cavity mode noise suppression capability of the method was verified from the two indicators of 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 was 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 increased by 7.78 times, and the minimum detection limit is increased by 3.4 times. The present invention provides a new idea for the cavity mode noise suppression of the OA-ICOS sensor system. 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 the present 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 by the present invention for suppressing OA-ICOS cavity mode noise by 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 schematic diagram of the structure 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 8 For standard CH4 Experimental data and fitting curves of different gas concentration levels and direct absorption peaks.

[0031] Fig. 9 1500ppmv standard CH 4 Gas detection results graph.

[0032] Fig.10 1500ppmv standard CH 4 Gas Allan deviation diagram. DETAILED DESCRIPTION

[0033] In order to explain the present invention more clearly, the present invention is further described below in conjunction with preferred embodiments and accompanying drawings. It should be understood by those skilled in the art that the content described below is illustrative rather than restrictive, and should not be used to limit the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be the common meanings understood by people with ordinary skills in the field to which the present invention belongs.

[0034] like Figure 1 As shown, the method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration proposed by the present invention includes the following steps, and the following steps 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 radius of curvature 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-reflection mirror curvature radius r in the 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; 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 a frequency f, causing the rear high-reflection mirror of the off-axis integrating cavity to perform simple harmonic vibration at a frequency f and an amplitude A, thereby causing the cavity length d to change periodically at a frequency f;

[0043] Step 5: Calculation of 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 of the kth light spot is obtained. Then the cumulative displacement is obtained 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

[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 obtain the spot angle Δθ when k=1,2,3,…,K in turn 1 , Δθ 2 , Δθ 3 ,…,Δθ 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 it is in the re-incident state; if the re-incident condition is not met, then it is in the infinite off-axis state; when the re-incident state is met 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 vibration on the high-reflection mirror behind the off-axis integrating cavity; 2) A theoretical explanation and detailed derivation process of the vibration cavity method of the OA-ICOS sensor system are proposed, providing theoretical support for further research on subsequent vibration technology on the OA-ICOS sensor system; 3) A short-cavity and long-cavity off-axis integrating cavity system is designed. It 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. 0 The light of (ν) is incident on the off-axis integrating cavity, and 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., the cavity length), and R is the reflectivity of the high-reflection mirror (assuming that the reflectivity of the two high-reflection mirrors is the same), then Io=Iin(ν)(1-R) / 1+R is the light intensity passing through the cavity when there is no absorption in the cavity. When the reflectivity of the high-reflection mirrors 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. Transmittance I / I 0 Fluctuations in will lead to fluctuations in the absorption coefficient α(ν).

[0058] When the light beam is incident off-axis into the off-axis integrating cavity, it is reflected multiple times between the two high-reflection mirrors and forms an elliptical pattern on the mirror surface of the high-reflection mirror. After m reflections, it coincides with the incident light beam, and the re-incident condition is met.

[0059] 2mθ=2Nπ (3) Where m is the number of times the light beam is reflected 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:

[0060] cos(θ)=1-d / r (4)

[0061] Where d is the cavity length, i.e., the distance between the two high-reflection mirrors in formula (1), and r is the radius of curvature of the high-reflection mirror. When the re-incidence condition is met, the optical path reaches 2md, and the free spectral range FSR becomes:

[0062]

[0063] 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 off the cavity axis, thus increasing the cavity mode density.

[0064] 1.2 Re-incidence and interference fringes

[0065] For the 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 perfect 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 spectrum range is FSR = c / 2md, as shown in Figure 2 (a) shown.

[0066] When the light beam still fails to reach the re-incident condition 2mθ≠2nπ after m times of back and forth reflection. At this time, countless light spots will be formed on the high-reflection mirror in theory, 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 size 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 realized in practice. In practice, whether it is in the re-incident state or the infinite off-axis state, after the light beam goes back and forth in the off-axis integrating cavity for many times, the light spot on the high-reflection mirror will always overlap to varying degrees.

[0067] Here, the present invention defines a critical value When the light spots overlap, the angle Δθ between two adjacent light spots min Less than or equal to the critical value When the overlapping light spots are overlapped, early re-incident or quasi-re-incident may occur, such as Figure 2 (b) As shown. Depending on the overlap state, early re-incidence will lead to multiple re-incidences or quasi-re-incidences with different optical paths in the off-axis integrating cavity. This will cause multiple 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 critical value When , interference fringes will appear between overlapping light spots (etalon effect), such as Figure 2 (c) As shown. Depending on the overlapping state, multiple interference fringes with different optical paths will also be generated. Therefore, in practice, there are multiple re-incident and quasi-re-incident modes and multiple interference fringes in the off-axis integrating cavity, which is a superposition process of the interaction of all these modes.

[0068] 1.3 Principle of increasing cavity mode density by the cavity vibration method

[0069] In practical applications, even if the light spot on the high-reflection mirror is the most ideal perfect circular distribution, there will still be a large number of cavity mode structures. In order to further increase the cavity mode density and more effectively suppress the cavity mode noise. The present invention adds a full-range speaker with adjustable frequency to the rear high-reflection mirror support frame of the off-axis integrating cavity. Vibration greatly increases the cavity mode density, and the specific analysis is as follows:

[0070] Z=Asin(2πft) (6)

[0071] 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 vibration displacement of the rear high reflector, A is the simple harmonic vibration amplitude of the rear high reflector, and f is the simple harmonic vibration frequency of the rear high reflector.

[0072]

[0073] 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.

[0074]

[0075] 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 increase changes.

[0076]

[0077] From formula (4), we can see that in formula (9), is the light spot angle 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:

[0078]

[0079] 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

[0080]

[0081] In formula (11), Δθ k The angle between the kth spot and the previous adjacent spot is the angle between the kth spot and the previous adjacent spot. 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 .

[0082] Δθ 1 +Δθ 2 +Δθ 3 +···Δθ K =Δθmin (12)

[0083] 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 reflector increases from m to Km, and the re-incident optical path length becomes 2Kmd. The free spectral range is as follows:

[0084]

[0085] Among them, FSR VR It is the free spectral region of the off-axis integrating cavity of the resonator type.

[0086] 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.

[0087]

[0088] When the angle between the newly added K light spots satisfies formula (14), the center of the newly added light spot does not overlap with the center of the original light spot, and the re-incident condition cannot be met. Or the light spot overlap occurs only when the light intensity of the light spot has decayed to a sufficiently low level. These two situations can be theoretically considered as infinite off-axis modes. In theory, in this mode, the FSR is infinitely small, the cavity mode density is high enough, and the transmission spectrum tends to be continuous.

[0089] 1.4 Effect of cavity mode density on cavity mode noise

[0090] Cavity mode noise can be described as the relative discreteness of the current integral value within the integration time T. Cavity mode noise within the integration time T can be expressed as:

[0091]

[0092] From formula (15), we can see that within the integration time T, K = TN T Transmission peak. T is the number of lasers coupled to cavity modes per second, with a modulation frequency of f o , the number of modules is n, and we can get N T =2f o n. ξ is the relative discreteness 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 limit and sensitivity.

[0093] 2. Simulation and calculation

[0094] 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 the cavity mode density in the effective vibration state is increased by 5 times compared with the cavity mode density in the non-vibration state. Figure 3 and Figure 4 According 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 a simulation diagram of the conventional off-axis integrated cavity mode density. Figure 3 The middle line I represents CH 4 Absorption line (Δf CH4 =3.9GHz), 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 cavity mode density of the off-axis integrated cavity is shown: Figure 4 The middle line I represents CH 4 Absorption line (Δf CH4 =3.9GHz), line II represents the DFB laser line width (Δf laser =3MHz), line IV represents the off-axis integrated cavity mode density of the cavity type.

[0095] After calculation Figure 5 The average values ​​of the optical noise of the two signals, the V line and the VI line, are 0.128V and 0.048V respectively. After adding vibration, the optical noise is reduced by about 2.6 times on average. The reduction factor of the measured optical noise is close to that of the simulation calculation, and the error sources are interference noise and burr noise. This also shows that the main noise of the system is cavity mode noise.

[0096] 3. VC-OA-ICOS sensor system structure and design

[0097] The structure of the VC-OA-ICOS sensor system is as follows: Figure 6 As shown, it includes a gas distribution system, an optical part, an electrical system and a vibration module.

[0098] 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 temperature of the DFB laser is set to 10°C. A sawtooth wave (10 Hz) input current drive is generated by a signal generator to scan the wavelength. The DFB laser is a distributed feedback laser, OA-ICOS is an off-axis integrated cavity output spectrum, and the DAQ acquisition card is a data acquisition card.

[0099] 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 outgoing beam into the off-axis integrating cavity. The angle of the incident beam is adjusted using the optical adjustment frame 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 the front and back, and the radius of curvature of the high-reflection mirror is 20cm. The transmitted output beam of the off-axis integrating cavity is collected by a converging lens (focal length of 6cm) and focused on an InGaAs amplified detector.

[0100] In the gas distribution system, nitrogen (N 2 ) was used as the balance gas and mixed with methane gas with a volume concentration of 10000 ppmv. Methane samples with different concentration levels were obtained by a gas mixing system (4000 series, American Environmental Company).

[0101] In the vibration module, the full-range speaker is fixed on the upper part of the rear high reflector support frame. The full-range speaker is controlled by the MATLAB program to achieve the vibration of the rear high reflector. The output frequency of the full-range speaker is regulated by the program to increase the cavity mode density, thereby reducing the cavity mode noise and interference noise.

[0102] 4. Results and Discussion

[0103] 4.1SNR Analysis and Frequency Optimization

[0104] 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, namely Figure 7 The SNR curve shows several irregular peaks between 0 and 1200 Hz, which indicates that different vibration frequencies have different effects on the suppression of cavity mode noise, and it shows a clear downward trend after 1200 Hz. This is because when the vibration frequency increases to a certain level, the mechanical structure of the cavity has lost its response to the vibration frequency. At the same time, when selecting the signal-to-noise ratio (SNR), it is also necessary to consider whether the interval of the burr noise is appropriate, such as Figure 7 VIII curve. As the vibration frequency increases, the burr interval decreases, the SNR shows a downward trend, and the vibration effect is gradually lost. When the vibration is less than 500Hz, the burr interval is relatively large. Through the SNR curve, 250Hz is finally selected as the optimal vibration frequency of the system (SNR is 23.2, burr interval is 8ms), and the burr interval corresponding to this vibration frequency is also suitable. However, when the burr noise appears at the absorption peak position, it will interfere with the collected data, and filtering is required to reduce the impact of the burr noise on the absorption peak interference.

[0105] 4.2 System Calibration

[0106] In order to calibrate the system, a gas distribution system was used to prepare 7 groups of methane mixed gases with different concentrations in the range of 1000ppmv to 1600ppmv using methane standard gas with a concentration of 15000ppm and high-purity nitrogen. The direct absorption amplitudes at 7 different concentration levels were measured. The average value was taken and plotted as a function of the methane concentration, as shown in Figure 2. Figure 8 shown. Figure 8 shows the standard CH 4 Experimental data and fitting curves of different gas concentration levels and direct absorption peaks, Figure 8 IX is the standard CH 4 Experimental data and fitting curves of different gas concentration levels and direct absorption peaks without vibration, X is the standard CH 4 Experimental data and fitting curves of different gas concentration levels and direct absorption peaks at 250 Hz vibration state, XI is the standard CH 4 Experimental data and fitting curves of different gas concentration levels and direct absorption peaks in the 250Hz vibration filtering state; two linear relationships between concentration C and direct absorption peak Ada were obtained, and quadratic polynomial fitting was used. The linearity of vibration-free absorption is 0.99449, such as 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 figure shows that 250Hz vibration has an excellent effect on improving the system concentration response performance.

[0107]

[0108] Where CNon-vibration is the concentration C without vibration Vibration is the vibration concentration C Vibration-filter is the concentration of vibration filter.

[0109] 4.3 Detection limit

[0110] for

[0111] The present invention conducts a long-term measurement lasting 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. Fig. 9 The gas concentration of 1500ppmv 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. Fig. 9 1500ppmv standard CH4 Gas detection result diagram, XII is 1500ppmv standard CH 4 Gas detection without vibration, XIII is 1500ppmv standard CH 4 Gas detection 250Hz vibration state, 1500ppmv standard CH 4 Gas detection XIV is in 250Hz vibration filter state; XII, XIII and XIV represent the average values ​​of concentration data in three modes. The relative errors are less than 1.13%, 0.6% and 0.33% respectively, indicating that the sensor system using VC-OA-ICOS (interval sampling) and VCF-OA-ICOS has good measurement accuracy performance. The concentration data fluctuation values ​​during the measurement process are less than ±40ppmv, ±15ppmv and ±10ppmv respectively, which indicates that the VRF-OA-ICOS sensor system has good stability performance.

[0112] Fig.10 is the Allan deviation plot calculated from the concentration data, XV is the 1500ppmv standard CH 4 Gas Allan deviation vibration-free state, XVI is 1500ppmv standard CH 4 Gas Allan deviation 250Hz vibration state, XVII is 1500ppmv standard CH 4 Gas Allan deviation 250Hz vibration filter state. When the integration time is 2s, the R-OA-ICOS sensor system can reach a detection limit of 41.67ppmv, and when the integration time is 119s, the minimum detection limit performance is 3.03ppmv. Fig.10 The XV curve in the middle. The VC-OA-ICOS sensor system can reach a detection limit of 11.15ppmv at 2s, and the minimum detection limit performance is 1.36ppmv at an integration time of 79s. Fig.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. Fig.10 Curve XVII in the figure shows that under the same conditions, the VCF-OA-ICOS sensor system can increase the detection limit by 3.4 times compared with 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, VC-OA-ICOS and based on filtering

[0116] The signal-to-noise ratio of the VC-OA-ICOS sensor system was improved by 7.78 times and 23.4 times respectively.

[0117] The VC-OA-ICOS sensor system reaches the minimum detection limit performance when the integration time is 86s.

[0118] 0.88ppmv. This indicates that the system can improve the detection limit by 3.4 times under vibration conditions.

Claims

1. A method for suppressing OA-ICOS cavity mode noise by using active high-frequency vibration, characterized in that: The method comprises 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 radius of curvature 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-reflection 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; according to the formula Get the free spectral range FSR, 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: generating an exciting force on the rear high-reflection mirror of the off-axis integrating cavity to make it vibrate along the cavity axis in a simple harmonic manner, so that the rear high-reflection mirror of the off-axis integrating cavity performs simple harmonic vibration 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 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 of the kth light spot is obtained. 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 light spot added between two adjacent light spots on the high-reflection 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 turn 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 high-reflection mirror of the off-axis integrating cavity increases from m to Km, and the length of the re-incident optical path becomes 2Kmd; the free spectral 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 active high-frequency vibration according to claim 1, characterized in that: Defining critical values When the light spots overlap, the angle Δθ between two adjacent light spots min Less than or equal to the critical value When , the overlapping light spots will produce early re-incident or quasi-re-incident; Δθ min Greater than critical value When the light is overlapping, 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 overlap with the center of the original light spot, the re-incident condition cannot be met; or the light spot overlap occurs only 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 lasers coupled to cavity modes per second, with a modulation frequency of 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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