Calibration-free measuring device and method based on wavelength locked cavity ring-down spectroscopy

Abstract

The invention discloses a calibration-free measurement device and method based on wavelength locked cavity ring-down spectroscopy, and the method comprises the steps: S1, carrying out the frequency modulation of laser wavelengths through an electro-optical modulator driven by a radio frequency signal, and obtaining laser of three wavelengths; s2, when the lasers with the three wavelengths enter the optical resonant cavity, turn-off control is carried out on the lasers with the three wavelengths, and a ring-down curve of e index attenuation is obtained; s3, carrying out ring-down signal fitting processing on the ring-down curves of the three wavelengths to obtain ring-down time data at the three wavelengths; and S4, converting the ring-down time data at the three wavelengths into absorption coefficient data at the three wavelengths, and comparing the absorption coefficient data with spectral parameters of absorption lines in a database to obtain the concentration of the gas to be measured so as to realize calibration-free gas measurement. According to the invention, the gas measurement rate is greatly improved, and the complexity and measurement cost of the system are reduced.

Description

Technical Field

[0001] The present invention relates to the field of optical sensing technology, and in particular to a calibration-free measurement device and method based on wavelength-locked cavity ring-down spectroscopy. Background Art

[0002] With the development of sensing technology, absorption spectroscopy technology based on the Lambert-Beer law has been widely used in the fields of atmospheric greenhouse gas monitoring, human exhaled gas monitoring, and industrial process emission gas monitoring.

[0003] As an extension of absorption spectroscopy technology, cavity ring-down spectroscopy technology can achieve extremely high measurement sensitivity by using highly reflective lenses at both ends of the absorption cavity to increase the distance of interaction between laser and matter. It has now been widely used in the field of gas monitoring.

[0004] In the process of realizing gas sensing by cavity ring-down spectroscopy technology, the traditional method is usually to change the wavelength of the incident laser to obtain a complete absorption spectrum, that is, wavelength scanning cavity ring-down spectroscopy. The gas concentration accuracy and sensitivity obtained by this method are high enough, but because the coupling of each wavelength of laser and the optical cavity requires a certain amount of time, the biggest drawback of wavelength scanning cavity ring-down spectroscopy is that the rate of obtaining gas concentration is slow, only in the MHz order.

[0005] Another commonly used method is wavelength-locked cavity ring-down, the principle of which is to lock the wavelength of the laser at the molecular absorption line to obtain the ring-down time at the molecular absorption line.

[0006] Since wavelength-locked cavity ring-down spectroscopy cannot obtain the ring-down time of the position without gas absorption, the general solution is to use another laser that does not absorb the gas to be measured to obtain the cavity ring-down time of the position without gas absorption. Although this method can effectively improve the acquisition rate of gas concentration, the measurement system requires the use of two lasers with different wavelengths, and it cannot be guaranteed that the light intensity of the two wavelengths after the laser beam is combined is similar, which increases the complexity of the measurement system. Summary of the invention

[0007] The purpose of the present invention is to propose a calibration-free measurement device and method based on wavelength-locked cavity ring-down spectroscopy to avoid the use of additional lasers, effectively reduce the complexity of the system, and provide a new idea for calibration-free gas concentration measurement.

[0008] To this end, the present invention provides, on one hand, a calibration-free measurement method based on wavelength-locked cavity ring-down spectroscopy, comprising: S1, using an electro-optical modulator driven by a radio frequency signal to frequency modulate the laser wavelength, thereby obtaining lasers of three wavelengths; S2, when the lasers of the three wavelengths enter the optical resonant cavity, the lasers of the three wavelengths are shut off to obtain a ring-down curve with e exponential decay; S3, performing ring-down signal fitting processing on the ring-down curves of the three wavelengths to obtain ring-down time data at the three wavelengths; S4, converting the ring-down time data at the three wavelengths into absorption coefficient data at the three wavelengths, and obtaining the concentration of the gas to be measured by comparing with the spectral parameters of the absorption line in the database, thereby realizing calibration-free gas measurement.

[0009] According to another aspect of the present invention, a calibration-free measurement device based on wavelength-locked cavity ring-down spectroscopy is provided, comprising: a laser, an electro-optic modulator and a modulation device thereof, an acousto-optic modulator and a driver thereof, an optical resonant cavity, a photoelectric detection device, and a ring-down signal acquisition and processing device, wherein the electro-optic modulator and the modulation device thereof comprise an electro-optic modulator and a radio frequency signal generator, and are used to frequency modulate the laser wavelength, thereby obtaining lasers of three wavelengths, the photoelectric detection device comprises a collimator, a photodetector, and a signal comparator, the acousto-optic modulator and the driver thereof comprise an acousto-optic modulator and an acousto-optic modulator driver, and are used to shut off and control the lasers of three wavelengths entering the optical resonant cavity according to the output signal of the signal comparator, thereby obtaining a ring-down curve of e exponential decay, and the ring-down signal acquisition and processing device is used to perform ring-down signal fitting processing on the ring-down curves of the three wavelengths, thereby obtaining ring-down time data at the three wavelengths, and simultaneously converting the ring-down time data at the three wavelengths into absorption coefficient data at the three wavelengths, and obtaining the concentration of the gas to be measured by comparing with the spectral parameters of the absorption line in the database.

[0010] Compared with the prior art, the present invention has the following advantages:

[0011] 1. Compared with the traditional frequency scanning cavity ring-down technology, the present invention can significantly improve the gas measurement rate by locking the laser wavelength at a desired position.

[0012] 2. Compared with the commonly used wavelength-locked cavity ring-down spectroscopy technology, the present invention can measure the gas concentration with only one laser, eliminating the need for another laser for measuring the cavity ring-down time, thereby reducing the complexity of the system and the measurement cost.

[0013] 3. The present invention does not require calibration of gas concentration and can achieve calibration-free gas measurement in the implementation of wavelength-locked cavity ring-down spectroscopy technology.

[0014] In addition to the above-described purposes, features and advantages, the present invention has other purposes, features and advantages. The present invention will be further described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0016] Figure 1 It is a structural schematic diagram of a calibration-free measurement device based on wavelength-locked cavity ring-down spectroscopy provided according to an embodiment of the present invention;

[0017] Figure 2 It is a flow chart of a calibration-free measurement method for wavelength-locked cavity ring-down spectroscopy provided in accordance with an embodiment of the present invention;

[0018] Figure 3 is a schematic diagram of the positions of gas absorption spectral lines and absorption coefficients corresponding to light of three wavelengths, and three ring-down time curves provided in an embodiment of the present invention;

[0019] Figure 4 It is a laser frequency domain diagram in which the laser beam emitted by the laser is converted into a carrier and two sidebands with a total of three wavelengths after passing through an electro-optic modulator driven by a radio frequency signal.

[0020] Description of Reference Numerals

[0021] 1. Laser; 21. Electro-optic modulator; 22. RF signal generator; 31. Acousto-optic modulator; 32. Acousto-optic modulator driver; 4. Optical resonator; 41. Front-end high-reflection mirror; 42. Rear-end high-reflection mirror; 51. Collimator; 52. Photodetector; 53. Signal comparator; 61. Signal acquisition card; 62. Computer. DETAILED DESCRIPTION

[0022] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0023] The calibration-free measurement device based on wavelength-locked cavity ring-down spectroscopy of the present invention comprises: a laser 1, an electro-optic modulator and a modulation device thereof, an acousto-optic modulator and a driver thereof, an optical resonant cavity 4, a photoelectric detection device, and a ring-down signal acquisition and processing device.

[0024] The laser 1 emits a laser beam, which is converted into a laser beam with three wavelengths, namely a carrier wave and two sidebands, after passing through the electro-optic modulator 21 driven by the radio frequency signal; the laser beam with three wavelengths passes through the acousto-optic modulator and enters the optical resonant cavity 4.

[0025] The electro-optic modulator and the modulation device thereof include: an electro-optic modulator 21 and a radio frequency signal generator 22 .

[0026] The AOM and its driver include: a polarization-maintaining AOM 31 and an AOM driver 32 .

[0027] The optical resonant cavity 4 comprises: a cavity, a front-end high-reflection mirror 41 and a rear-end high-reflection mirror 42 .

[0028] The transmitted light after passing through the optical resonant cavity 4 enters the photoelectric detection device.

[0029] The photoelectric detection device includes: a collimator 51 , a photoelectric detector 52 and a signal comparator 53 .

[0030] The collimator 51 collects the transmission signal of the optical resonant cavity 4 into the optical fiber, the photodetector 52 converts the optical signal into an electrical signal, the default output signal of the signal comparator 53 is a high-level signal, and a trigger threshold of the electrical signal is set. When an electrical signal exceeding the trigger threshold is detected, the output signal of the signal comparator 53 changes from a high-level signal to a low-level signal, and the duration is 2ms.

[0031] The output signal of the signal comparator 53 is connected to the modulation port of the AOM driver 32. When the output signal of the signal comparator, i.e., the input signal of the modulation port of the AOM driver, is at a high level, the AOM is in a light-transmitting state; when the output signal of the signal comparator 53, i.e., the input signal of the modulation port of the AOM driver, is at a low level, the AOM is in a light-blocking state; therefore, when the longitudinal mode in the cavity accumulates to a sufficiently strong level (e.g., Figure 4 As shown in the figure, the light path is cut off, so that an exponential decay ring-down curve can be obtained; for different wavelengths, the gas absorption is different, so different ring-down time data can be obtained.

[0032] The ring-down curves at the three wavelengths obtained can be used to obtain data of three ring-down times through a ring-down signal acquisition and processing device, and data of three absorption coefficients can be obtained through conversion.

[0033] Through the line intensity and broadening data of the gas to be measured at the measurement wavelength in the database, the parameters of the unit concentration absorption coefficient at three wavelengths can be obtained. By processing it with the absorption coefficient data measured at the three wavelengths, the concentration data of the gas to be measured can be obtained, thereby realizing calibration-free gas measurement.

[0034] The laser is a distributed feedback diode laser.

[0035] The electro-optic modulator and the modulation device thereof include: an electro-optic modulator and a radio frequency signal generator, which are used to perform frequency modulation on the laser so as to generate two sidebands, namely lasers of other two frequencies.

[0036] The acousto-optic modulator and its driver include: a polarization-maintaining acousto-optic modulator and an acousto-optic modulator driver, which are used to turn off the laser and obtain an e-exponential decay ring-down curve.

[0037] The photoelectric detection device includes: a collimator, a photodetector and a signal comparator; the collimator is used to collect spatial light and convert it into optical fiber light; the photodetector is used to convert the ring-down optical signal into an electrical signal to facilitate subsequent signal processing and fitting; the signal comparator is used to send a trigger signal to the AOM driver, thereby enabling the on-off light of the polarization-maintaining AOM to be controlled.

[0038] The ring-down signal acquisition and processing device includes: a signal acquisition card and a computer; the signal acquisition card is used for fitting the acquired ring-down curve, performing concentration calculation and other operations; the computer is used for observing and displaying the acquired ring-down time data and concentration data.

[0039] The front-end high-reflection mirror and the rear-end high-reflection mirror in the ring-down cavity are high-reflection mirrors with a reflectivity greater than 99.995%.

[0040] The calibration-free measurement method based on wavelength-locked cavity ring-down spectroscopy of the present invention comprises the following steps S1-S4.

[0041] S1. The laser emits a laser beam, which is converted into a laser with three wavelengths, one carrier and two sidebands, after passing through an electro-optic modulator driven by a radio frequency signal. The frequency domain diagram of the three wavelengths of laser is shown in the figure. Figure 4 As shown in the figure, they are the carrier frequency ω and a pair of sidebands ω±Ω. After passing through the acousto-optic modulator, the three wavelengths of laser light enter the optical resonant cavity.

[0042] The main frequency of the laser light source is ω. When the laser passes through the electro-optic modulator, the phase of the laser will change, thus affecting the change of the laser frequency, because the electro-optic modulator receives high-frequency modulation with a frequency of Ω (above 500MHz). Therefore, after the laser passes through the electro-optic modulator, a pair of symmetrical sidebands will be generated, with frequencies of ω-Ω and ω+Ω respectively. The specific electric field formula is as follows:

[0043]

[0044] Where E0 is the electric field amplitude of the laser light source, M is the modulation index, and Ein is the expression of the laser electric field after modulation.

[0045] S2. The transmitted light after passing through the optical resonant cavity is received by the photoelectric detection device and converted into an electrical signal, and the output signal of the signal comparator is connected to the input port of the AOM driver as a trigger signal, thereby shutting down the AOM.

[0046] S3. The ring-down curve received by the photoelectric detector is processed by the ring-down signal acquisition and processing device for data acquisition and ring-down signal fitting to obtain the ring-down time data at three wavelengths.

[0047] S4. The obtained ring-down time data at three wavelengths can be converted to obtain the absorption coefficient data at three wavelengths. By performing specific processing with the line intensity and broadening data obtained in the database, the concentration of the gas to be measured can be obtained, thereby realizing calibration-free gas measurement.

[0048] In step S3, the ring-down signal fitting process is as follows:

[0049] The ring-down curve is a descending e-exponential curve. By fitting the e-exponential curve, we can get the fitting function:

[0050] y=a×e -x / b +c

[0051] The horizontal axis is the number of acquisition points, and the vertical axis is the acquired voltage value. If the acquisition rate is z, the unit is MHz, then the ring-down time value is b / z, the unit is 10 -6 s, then the ring-down times at the three wavelengths λ1, λ2 and λ3 are:

[0052] τ1=b1 / z, τ2=b2 / z, τ3=b3 / z

[0053] Through the above fitting function and the formula of the ring-down time, the ring-down time data at three wavelengths can be obtained.

[0054] Step S4 includes the following steps:

[0055] S41, converting the obtained ring-down time data at the three wavelengths to obtain the absorption coefficients A1, A2 and A3 at the three wavelengths, the conversion formula is:

[0056]

[0057] Where C is the speed of light; τ1, τ2 and τ3 are the ring-down times at three wavelengths λ1, λ2 and λ3 respectively;

[0058] τ0 is the ring-down time data when there is no gas absorption, also known as the cavity ring-down time.

[0059] S42. In the HITRAN database, the line intensity S and air broadening γ of the gas molecules to be measured at a specific wavelength λ under the conditions of a certain measurement temperature T and measurement pressure P are obtained. air and the self-broadening data γ self , by processing the simulated data in the database, it can be obtained that the absorption coefficient at wavelength λ can be expressed as:

[0060] A(λ)=Sφ(λ)×P

[0061] Where: φ(λ) is the area normalized linear function, which is widened by air γ air and the self-broadening data γ self Etc. calculated.

[0062] Taking Lorentz linearity as an example, the expression is as follows:

[0063]

[0064] Where ν is the wave number, which is expressed as ν = 1 / λ, ν0 is the central wave number, Δν is the half-height full width of the Lorentzian line type associated with the broadening data, and the expression of Δν is as follows:

[0065]

[0066] Where P is the gas pressure (unit: atm), γ B-A Refers to the pressure broadening coefficient of the measured gas B to other gases (including B itself), X A is the molecular number concentration of gas A. When the gas other than the gas to be measured is air, the above formula can be changed to the following formula:

[0067]

[0068] T is the temperature of the gas to be measured (unit: K), n self is the temperature-dependent self-broadening exponent, n air is the temperature-dependent air broadening exponent, γ self Refers to the self-broadening coefficient at 296K (unit: cm -1 ﹒ atm -1 ),γ air Refers to the air broadening coefficient at 296K (unit: cm -1 ﹒ atm -1 ), X B is the molecular number concentration of gas B, that is, the concentration of the interfering gas, and in this formula is the molecular number concentration of air.

[0069] x is the concentration of the gas.

[0070] S43, adjust and lock the laser wavelength at λ2, then the wavelengths of the two sidebands generated by the frequency modulated laser are λ1 and λ3 respectively, and the absorption coefficients at the three frequencies are:

[0071]

[0072]

[0073] Since the interval between τ1 and τ3 is larger, these two ring-down time data are selected to calculate the concentration, and the expression of the measured gas concentration is obtained as follows:

[0074]

[0075] Thus, a calibration-free method for a wavelength-locked cavity ring-down measurement system can be realized.

[0076] The above description is only an embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A calibration-free measurement method based on wavelength-locked cavity ring-down spectroscopy, characterized in that: include: S1, using an electro-optic modulator driven by a radio frequency signal to frequency modulate the laser wavelength, thereby obtaining three wavelengths of laser light; S2. When the lasers of the three wavelengths enter the optical resonant cavity, the lasers of the three wavelengths are shut off to obtain a ring-down curve of exponential decay of e; S3, performing ring-down signal fitting processing on the ring-down curves of the three wavelengths to obtain the ring-down time data at the three wavelengths; S4. Convert the ring-down time data at three wavelengths into absorption coefficient data at three wavelengths, and compare them with the spectral parameters of the absorption lines in the database to obtain the concentration of the gas to be measured, thereby realizing calibration-free gas measurement.

2. The calibration-free measurement method based on wavelength-locked cavity ring-down spectroscopy according to claim 1 is characterized in that: The shutoff control of the three wavelength lasers includes the following steps: The acousto-optic modulator is used to control the laser light entering the optical resonant cavity. At the same time, the signal comparator is used to trigger the control signal according to the strength of the electrical signal detected by the photodetector, and the control signal is used to control the acousto-optic modulator. The default output signal of the signal comparator is a high-level signal. When the strength of the electrical signal detected by the photodetector exceeds the trigger threshold, a low-level signal is triggered and lasts for a set time. When the modulation port of the acousto-optic modulator is at a high level, the acousto-optic modulator is in a light-transmitting state. When the modulation port is at a low level, the acousto-optic modulator is in a light-blocking state, and then the transmitted light that conforms to the ring-down curve of the e-exponential decay is obtained through the optical resonant cavity.

3. The calibration-free measurement method based on wavelength-locked cavity ring-down spectroscopy according to claim 1 is characterized in that: The ring-down signal is fitted by the ring-down signal fitting formula: The ring-down curve is a descending e-exponential curve. By fitting the e-exponential curve, we can get the fitting function: y=a×e -x / b +c The horizontal axis x is the number of acquisition points, and the vertical axis y is the acquired voltage value. If the acquisition rate is z, the unit is MHz, then the ring-down time value is b / z, the unit is 10 -6 s, then the ring-down times at the three wavelengths λ1, λ2 and λ3 are: τ1=b1 / z, τ2=b2 / z, τ3=b3 / z By using the above fitting function and the formula of the ring-down time, the ring-down time data at three wavelengths can be obtained.

4. The calibration-free measurement method based on wavelength-locked cavity ring-down spectroscopy according to claim 1 is characterized in that: Converting the ring-down time data at three wavelengths into absorption coefficient data at three wavelengths includes the following steps: S41, converting the obtained ring-down time data at the three wavelengths to obtain the absorption coefficients A1, A2 and A3 at the three wavelengths, the conversion formula is: Where C is the speed of light; τ1, τ2 and τ3 are the ring-down times at three wavelengths λ1, λ2 and λ3 respectively; τ0 is the ring-down time data when there is no gas absorption, also known as the cavity ring-down time.

5. The calibration-free measurement method based on wavelength-locked cavity ring-down spectroscopy according to claim 1, characterized in that: The concentration of the gas to be measured is obtained by comparing the spectral parameters of the absorption line with those in the database. The process includes the following steps: S42. In the HITRAN database, the line intensity S and air broadening γ of the gas molecules to be measured at a specific wavelength λ under the conditions of a certain measurement temperature T and measurement pressure P are obtained. air and the self-broadening data γ self , by processing the simulated data in the database, the absorption coefficient at wavelength λ is obtained: A(λ)=Sφ(λ)×P Where: φ(λ) is the area normalized linear function; x is the concentration of the gas, S43, adjust and lock the laser wavelength at λ2, then the wavelengths of the two sidebands generated by the frequency modulated laser are λ1 and λ3 respectively, and the absorption coefficients at the three frequencies are: Since the interval between τ1 and τ3 is larger, these two ring-down time data are selected to calculate the concentration, and the expression of the measured gas concentration is obtained as follows: Thus, a calibration-free method for a wavelength-locked cavity ring-down measurement system can be realized.

6. A calibration-free measurement device based on wavelength-locked cavity ring-down spectroscopy, characterized in that: include: Lasers, electro-optic modulators and modulation devices, acousto-optic modulators and drivers, optical resonators, photoelectric detection devices, and ring-down signal acquisition and processing devices, The electro-optic modulator and the modulation device thereof include an electro-optic modulator and a radio frequency signal generator, which are used to frequency modulate the laser wavelength to obtain three wavelengths of laser light. The photoelectric detection device includes a collimator, a photodetector and a signal comparator. The acousto-optic modulator and its driver include an acousto-optic modulator and an acousto-optic modulator driver, which are used to control the shutoff of three wavelengths of laser light entering the optical resonant cavity according to the output signal of the signal comparator to obtain an e-exponential decay ring-down curve. The ring-down signal acquisition and processing device is used to perform ring-down signal fitting processing on the ring-down curves of the three wavelengths to obtain the ring-down time data at the three wavelengths, and at the same time convert the ring-down time data at the three wavelengths into the absorption coefficient data at the three wavelengths, and compare them with the spectral parameters of the absorption line in the database to obtain the concentration of the gas to be measured.

7. The calibration-free measurement device based on wavelength-locked cavity ring-down spectroscopy according to claim 6, characterized in that: The laser is a distributed feedback diode laser.

8. The calibration-free measurement device based on wavelength-locked cavity ring-down spectroscopy according to claim 6, characterized in that: The ring-down signal acquisition and processing device comprises a signal acquisition card and a computer, wherein the signal acquisition card is used to calculate the ring-down time data, the absorption coefficient data, and the concentration of the gas to be measured from the collected ring-down curve; The computer is used to observe and display the obtained measurement parameters.

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