Equivalent optical path calibration method for cavity attenuation phase shift spectrum

Abstract

The equivalent optical path calibration method comprises an LED light source, a resonant cavity, an optical chopper, a photomultiplier tube (PMT), an acquisition card and a computer which are arranged in sequence, and the optical chopper comprises an incident end plano-convex lens, an emergent end plano-convex lens and a chopper plate driven by a motor. The motor is connected with a motor controller, and a control signal is generated by a pulse generator; the pulse generator is also connected with the LED light source and the modulation module; a set of modulation frequency, phase difference between chopper modulation and LED modulation, direct modulation light intensity of an LED light source, transmittance of an optical chopper and total transmittance obtained along with change of the phase difference is obtained, a set of ring-down time of a signal can be inverted, fitting analysis is carried out on the ring-down time, and the ring-down time is used for calculating an equivalent optical path length L. According to the invention, the equivalent optical path length of the resonant cavity can be determined with higher resolution and higher precision, so that the use of the cavity attenuation phase shift spectrum is optimized.

Description

Technical Field

[0001] The invention relates to the technical field of optical resonant cavity detection, and in particular to an equivalent optical path calibration method for cavity attenuation phase shift spectrum. Background Art

[0002] The optical resonant cavity is used for spectral measurement of atmospheric trace gases, and a long optical path length L is required to enhance gas absorption. In phase shift spectroscopy, understanding the size of L is the basis for correctly retrieving trace gas concentrations. The optical path length L is related to the optical mirror reflectivity R. The calibration error of the lens reflectivity largely determines the final measurement error. The current calibration methods for lens reflectivity mainly include:

[0003] 1. Use cavity ring-down spectroscopy technology to calibrate the lens reflectivity by measuring the cavity ring-down time;

[0004] 2. Use phase-shift cavity ring-down spectroscopy to calibrate the reflectivity of the lens by measuring the phase shift of the modulated light source;

[0005] 3. Sampling the absorption gas with known concentration to calibrate the reflectivity of the lens;

[0006] 4. Use the difference in Rayleigh scattering of different gas molecules to calibrate the lens reflectivity.

[0007] Cavity ring-down spectroscopy was originally used to measure the reflectivity of lenses. During the process of measuring the reflectivity of lenses, a weak absorption signal of oxygen molecules in the air was measured. Since then, cavity ring-down spectroscopy has gradually been used for high-sensitivity detection of trace gases. The method of calibrating the reflectivity of lenses with cavity ring-down spectroscopy is to determine the reflectivity of the lens at the wavelength by measuring the ring-down time of the pulse signal in the cavity. The ring-down time of the cavity is determined by the reflectivity of the lens and the cavity length:

[0008] R=1-d / (c·τ0)

[0009] Where d is the cavity length, c is the speed of light, and τ0 is the local ring-down time. For actual measurements, the cavity usually refers to a cavity filled with nitrogen or zero air, without the measured gas. Therefore, in this case, the calibrated lens reflectivity includes the Ray lei gh scattering absorption of the background gas.

[0010] For phase-shift cavity ring-down spectroscopy, the phase-shift technology was first proposed by Herbe lin et al. in 1980. It is used to measure the lifetime of photons in the resonant cavity and the reflectivity of the resonant cavity. It modulates the light source used by the LED spectrum and measures the phase shift in the cavity to calibrate the reflectivity of the lens.

[0011] For the absorption spectrometry of gases with known concentrations, the purpose of calibrating the reflectivity of the lens is to quantitatively measure the concentration of the measured gas. Conversely, if the concentration of the measured gas or other gases is known in advance, the reflectivity of the lens that makes up the optical cavity can be obtained by measuring its absorption spectrum. The expression for the reflectivity of the lens can be written as: Where σ(λ) is the absorption cross section of a gas with a known concentration; n is the known number concentration of molecules, d is the cavity length, and I0 and I are the outgoing light intensities when there is an absorbing gas in the cavity and when there is no absorbing gas, respectively.

[0012] For the Rayleigh scattering difference method, for a light source with an intensity of I0(λ), the light intensity in the cavity is I in The rate of change of (λ) with time is related to the efficiency of coupling the light source I0(λ) into the resonant cavity, the light intensity loss caused by the reflection in the cavity, the Ray lei gh scattering and particle Mie scattering of the gas in the cavity, and the absorption of trace gases. It can be specifically expressed as:

[0013]

[0014] Among them, α Ray (λ) is the Rayleigh scattering coefficient, α Mie (λ) is the Mie scattering coefficient, α i (λ) is the absorption coefficient of the i-th trace gas, R(λ) is the reflectivity of the lens, d is the cavity length, c is the speed of light, k s is the light intensity coupling coefficient. The calibration of the lens reflectivity using the Ray leigh scattering difference method is relatively simple and can be applied to any ultraviolet and visible bands.

[0015] Based on the above methods, the effective optical path L(λ) or mirror reflectivity R(λ) calibration technology has certain defects:

[0016] 1. Cavity ring-down spectroscopy usually uses a laser light source, which has good collimation and a geometric loss that is significantly smaller than the incoherent light source used in the actual measurement of cavity attenuation phase-shift spectroscopy. The calibrated lens reflectivity will be correspondingly higher, so cavity ring-down technology is rarely used for the calibration of mirror reflectivity in cavity attenuation phase-shift spectroscopy measurement.

[0017] 2. The reflectivity of the lens is calibrated by the absorption spectroscopy method of a gas with a known concentration. This method requires that the gas has absorption within the calibration band and can only be used in specific circumstances.

[0018] 3. Traditional cavity ring-down phase-shift spectroscopy is easily affected by the drift of the light input intensity and the transmission optical devices. Summary of the invention

[0019] The technical problem to be solved by the present invention is to provide an equivalent optical path calibration method for cavity attenuation phase-shift spectroscopy, which can determine the optical path length with high resolution and high precision under relatively simple settings, thereby optimizing the use of cavity attenuation phase-shift spectroscopy.

[0020] In order to solve the above technical problems, the present invention provides an equivalent optical path calibration method for cavity attenuation phase shift spectrum, comprising an LED light source, a resonant cavity, an optical chopper, a photomultiplier tube PMT, an acquisition card and a computer which are arranged in sequence, the resonant cavity and the optical chopper are connected by an optical fiber, the optical chopper comprises an incident end plano-convex lens and an exit end plano-convex lens, a chopper blade driven by a motor is arranged between the incident end plano-convex lens and the exit end plano-convex lens, the motor is connected to a motor controller, and the motor controller is provided with a control signal by a pulse generator; the pulse generator is also connected to an LED power supply and a modulation module;

[0021] The pulse generator controls the LED light source to output at a modulation frequency ν; the motor controller controls the chopper to rotate so that the optical chopper also outputs at a modulation frequency ν, and adjusts the phase difference between the optical chopper and the LED light source output

[0022] Get modulation frequency ν, phase difference The direct modulated light intensity I of the LED light source, the transmittance θ(t) of the optical chopper, and the phase difference The changes are measured by a photomultiplier tube PMT to obtain a set of total transmittance η, and the signal ring-down time τ is inverted by the calculation formula of the total transmittance η, and a set of ring-down time τ is obtained, which is fitted and used to calculate the equivalent optical path length L.

[0023] Furthermore, the calculation formula of the total transmittance η is:

[0024]

[0025] In the above formula, ν is the modulation frequency, is the phase difference, I(t) is the direct modulated light intensity varying with time, and θ(t) is the transmittance of the optical chopper;

[0026] By fixing the wavelength and phase, the basic relationship between the direct modulated light intensity, the transmittance of the optical chopper and the total transmittance η under different modulation frequencies in the experiment is established, and the relationship function between the phase difference, the decay time and the total transmittance is established, which is used to calculate the corresponding decay time τ under different optical chopper modulation phases.

[0027] Furthermore, a set of total transmittances η is obtained through phase scanning, the ring-down time τ is calculated through the total transmittance η, and then the final ring-down time τ' is obtained by fitting the set of ring-down times τ, the lens reflectivity R is calculated through the final ring-down time τ', and the equivalent optical path length L is obtained based on the lens reflectivity R.

[0028] Furthermore, after determining the final ring-down time τ', the directly modulated light intensity modulation angular frequency is calculated:

[0029] tanφ c =-Ωτ', where Ω is the light intensity modulation angular frequency, φ c It is the phase shift of the resonant cavity output signal relative to the modulation signal during direct modulation;

[0030] The relationship between the lens reflectivity R and Ω is:

[0031] Where d is the cavity length of the resonant cavity, and c is the speed of light;

[0032] Therefore, the lens reflectivity R is used to calculate the equivalent optical path length L.

[0033] Furthermore, the pulse generator provides control signals to the modulation module of the LED light source and the motor of the optical chopper, generating a pulse signal with a duty cycle of 50%.

[0034] Furthermore, the light emitted from the resonant cavity is projected onto the plane of the chopper plate through the optical fiber and the plano-convex lens at the incident end. The object distance and the image distance are equal, which is used to achieve light focusing. After passing through the chopper plate, the light beam is projected onto the photomultiplier tube PMT by the plano-convex lens at the output end.

[0035] Furthermore, the diameter of the optical fiber is 400 μm and the numerical aperture N A =0.22.

[0036] Beneficial effects of the present invention:

[0037] Through the integrated calibration of the optical modulation, the optical path length can be determined with high accuracy in a relatively simple setup. The determination of the optical path length is facilitated and improved by the combined application of the single-shot measurement mode and the phase scanning mode, thereby optimizing the use of cavity attenuation phase-shift spectroscopy. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the structure of the calibration device of the present invention;

[0039] Figure 2 It is a waveform diagram of the present invention during the calibration process. DETAILED DESCRIPTION

[0040] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0041] Reference Figure 1 As shown, an embodiment of the equivalent optical path calibration method for cavity attenuation phase shift spectrum of the present invention, the calibration device specifically includes an LED light source, a resonant cavity, an optical chopper, a photomultiplier tube PMT, an acquisition card and a computer which are arranged in sequence, the resonant cavity and the optical chopper are connected by an optical fiber, the optical chopper includes an incident end plano-convex lens 11 and an exit end plano-convex lens 12, a chopper plate driven by a motor is arranged between the incident end plano-convex lens and the exit end plano-convex lens, the motor is connected to a motor controller, and a control signal is provided to the motor controller by a pulse generator; a filter 13 is arranged on the receiving end of the photomultiplier tube PMT, the LED light source includes an LED, an LED power supply and a modulation module, which cooperate with each other to emit light, the pulse generator is connected to the LED power supply and the modulation module, and the modulation module is provided with a control signal by the pulse generator; the acquisition card is used to collect the signal of the photomultiplier tube PMT and transmit it to the computer, and the computer performs subsequent calculations.

[0042] In the above structure, a collimating lens 1 is arranged between the resonant cavity and the LED light source, and the collimating lens converts the light emitted by the LED light source into a beam of parallel light and emits it to the resonant cavity. Two high-reflection mirrors 2 are arranged in the resonant cavity and fixed by an adjustment frame 3. The high-reflection lens continuously reflects the light in the resonant cavity, and only a small part of the light is emitted. A focusing lens 4 is arranged between the resonant cavity and the optical fiber, and the focusing lens focuses the light emitted from the resonant cavity onto the end of the optical fiber. The optical fiber transmits the light to the optical chopper, and the incident end plano-convex lens focuses the light emitted by the optical fiber transmission onto the chopper blade, and after being chopped by the chopper blade, the plano-convex lens at the output end focuses it onto the photomultiplier tube PMT.

[0043] In the above process, the pulse generator controls the LED light source to output at a modulation frequency ν; the motor controller controls the chopper to rotate so that the optical chopper also outputs at a modulation frequency ν. The pulse generator is also used to adjust the phase difference between the optical chopper and the LED light source output.

[0044] Specifically, in the traditional cavity attenuation phase shift spectroscopy, the light intensity is often focused on the photomultiplier tube PMT at the resonant cavity outlet. In the present application, the resonant cavity outlet and the photomultiplier tube PMT are transmitted through an optical fiber, and the optical fiber parameters are a diameter of 400 μm and a numerical aperture N A=0.22, with an optical chopper in the middle; the pulse generator gives the modulation module a control signal, and the modulation module controls the LED light source to output at a modulation frequency ν according to the control signal combined with the LED power supply. The pulse generator allows the digital selection of the modulation frequency ν up to 10kHz, and its TTL trigger output has a selective delay phase The LED light source is powered to generate a quasi-rectangular light pulse profile with a 50% duty cycle; the pulse generator gives a control signal to the motor controller, which drives the motor to rotate and drives the chopper to rotate, so that the optical chopper also outputs at a modulation frequency ν, which is the same as the light source frequency. By changing the cycle, the waveform output by the optical chopper can be adjusted to form a phase difference with the waveform output by the LED light source.

[0045] In order to collect instantaneous light changes in the system through the photomultiplier tube PMT, the sampling time must be significantly shorter than the device's ringing time τ. In the optical modulation method of the present application, by adding an optical chopper between the resonant cavity and the photomultiplier tube PMT, the time resolution of the photomultiplier tube PMT can be effectively improved.

[0046] The principle of obtaining gas concentration and calibrating effective cavity length in this application is as follows Figure 2 As shown, the LED light source is output in a modulation mode with a period of T, and the light intensity provided is I(t), as Figure 2 The waveform of a in the figure. The photons in the resonant cavity gradually accumulate and change with the change of the light input. The photons are scattered, absorbed or lost from the resonant cavity through the transmission of the reflector, which is manifested as Figure 2 The waveform of b is specifically the shape of the rise and fall, and is determined by the ring-down time τ. The transmittance θ(t) of the optical chopper between the resonant cavity and the photomultiplier tube PMT oscillates between the blocked light and the transmitted light, with the same frequency as the LED light source. Through the change of the period, it has an obvious adjustable phase relationship with the optical input signal, that is, the phase difference The light intensity reaching the photomultiplier tube PMT is the product of the unobstructed output (proportional to the number of photons in the resonator) and the transmission. The photomultiplier tube PMT of the present application does not resolve the instantaneous intensity, but integrates the arriving photons over many pulse cycles, such as Figure 2 The dashed line in the middle d is calculated by dividing the light intensity arriving at the optical chopper (I(t)) and the light intensity leaving the optical chopper (I e The average ratio of (t)) is called the total transmittance η. Since the total transmittance η is a function of the ring-down time τ, that is, a function of the equivalent optical path length L, the equivalent optical path length L can be calculated quickly and accurately.

[0047] Based on the above principles, the specific calibration method is as follows:

[0048] First obtain the modulation frequency ν and phase difference The direct modulated light intensity I(t) of the LED light source, the transmittance θ(t) of the optical chopper, and the phase difference The total transmittance η is measured by a photomultiplier tube PMT to obtain a set of total transmittance η. Specifically, the set of total transmittance η is obtained by phase scanning. The calculation formula for a single total transmittance η is:

[0049]

[0050] That is, by fixing the wavelength and phase, the basic relationship between the direct modulated light intensity, the transmittance of the optical chopper and the total transmittance η under different modulation frequencies of the experiment is established, and the relationship function between the phase difference, the decay time and the total transmittance is established, which is used to calculate the corresponding decay time τ under different optical chopper modulation phases.

[0051] The above formula is used to calculate the ring-down time τ corresponding to a single total transmittance η; thus, for a set, the ring-down time τ corresponding to each total transmittance η can be calculated. That is, the ring-down time τ of the signal is inverted by the total transmittance η calculation formula.

[0052] In this way, a set of ring-down times τ can be obtained, fitted and used to calculate the equivalent optical path length L.

[0053] Specifically, the final ring-down time τ' is obtained based on the collective fitting of the ring-down time τ, and the lens reflectivity R can be calculated through the final ring-down time τ'; after determining the final ring-down time τ', the directly modulated light intensity modulation angular frequency is calculated:

[0054] tanφ c =-Ωτ', where Ω is the light intensity modulation angular frequency, φ c It is the phase shift of the resonant cavity output signal relative to the modulation signal during direct modulation;

[0055] The relationship between the lens reflectivity R and Ω is:

[0056] Where d is the cavity length of the resonant cavity, and c is the speed of light;

[0057] Therefore, the equivalent optical path length L is calculated based on the reflectivity R of the lens.

[0058] In the above method, phase scanning can more reliably obtain each phase difference The corresponding total transmittance η can be obtained as the phase difference A series of light intensity ratios are obtained by changing the light intensity ratio, and the signal decay time τ is inverted by the light intensity ratio. The equivalent optical path length L is calculated using the decay time τ. The absolute intensity of light will not affect the detection result.

[0059] The above embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Any equivalent substitution or change made by a person skilled in the art based on the present invention is within the protection scope of the present invention.

Claims

1. A method for calibrating an equivalent optical path length for cavity attenuation phase shift spectroscopy, characterized in that: The invention comprises an LED light source, a resonant cavity, an optical chopper, a photomultiplier tube (PMT), an acquisition card and a computer which are arranged in sequence. The resonant cavity and the optical chopper are connected by an optical fiber. The optical chopper comprises an incident end plano-convex lens and an exit end plano-convex lens. A chopper blade driven by a motor is arranged between the incident end plano-convex lens and the exit end plano-convex lens. The motor is connected to a motor controller, and a pulse generator provides a control signal to the motor controller. The pulse generator is also connected to an LED power supply and a modulation module. The pulse generator controls the LED light source to output at a modulation frequency ν; the motor controller controls the chopper to rotate so that the optical chopper also outputs at a modulation frequency ν, and adjusts the phase difference between the optical chopper and the LED light source output Get modulation frequency ν, phase difference The direct modulated light intensity I of the LED light source, the transmittance θ(t) of the optical chopper, and the phase difference The changes are measured by a photomultiplier tube PMT to obtain a set of total transmittance η, and the signal ring-down time τ is inverted by the calculation formula of the total transmittance η, and a set of ring-down time τ is obtained, which is fitted and used to calculate the equivalent optical path length L.

2. The equivalent optical path calibration method for cavity attenuation phase shift spectroscopy according to claim 1, characterized in that: The calculation formula of total transmittance η is: In the above formula, ν is the modulation frequency, is the phase difference, I(t) is the direct modulated light intensity varying with time, and θ(t) is the transmittance of the optical chopper; By fixing the wavelength and phase, the basic relationship between the direct modulated light intensity, the transmittance of the optical chopper and the total transmittance η under different modulation frequencies in the experiment is established, and the relationship function between the phase difference, the decay time and the total transmittance is established, which is used to calculate the corresponding decay time τ under different optical chopper modulation phases.

3. The equivalent optical path calibration method for cavity attenuation phase shift spectroscopy according to claim 2, characterized in that: The set of total transmittance η is obtained through phase scanning, the ring-down time τ is calculated through the total transmittance η, and then the final ring-down time τ' is obtained by fitting the set of ring-down times τ. The lens reflectivity R is calculated through the final ring-down time τ', and the equivalent optical path length L is calculated based on the lens reflectivity R.

4. The equivalent optical path calibration method for cavity attenuation phase shift spectroscopy according to claim 3, characterized in that: After determining the final ring-down time τ', calculate the directly modulated light intensity modulation angular frequency: tanφ c =-Ωτ', where Ω is the light intensity modulation angular frequency, φ c It is the phase shift of the resonant cavity output signal relative to the modulation signal during direct modulation; The relationship between the lens reflectivity R and Ω is: Where d is the cavity length of the resonant cavity, and c is the speed of light; Therefore, the lens reflectivity R is used to calculate the equivalent optical path length L.

5. The equivalent optical path calibration method for cavity attenuation phase shift spectroscopy according to claim 1, characterized in that: The pulse generator provides control signals to the modulation module of the LED light source and the motor of the optical chopper, generating a pulse signal with a duty cycle of 50%.

6. The equivalent optical path calibration method for cavity attenuation phase shift spectroscopy according to claim 1, characterized in that: The light emitted from the resonant cavity is projected onto the plane of the chopper plate through the optical fiber and the plano-convex lens at the incident end. The object distance and the image distance are equal, which is used to achieve light focusing. After passing through the chopper plate, the light beam is projected onto the photomultiplier tube PMT by the plano-convex lens at the output end.

7. The equivalent optical path calibration method for cavity attenuation phase shift spectroscopy according to claim 1, characterized in that: The diameter of the optical fiber is 400 μm and the numerical aperture is N A =0.22.

Images