CN108051400B - A scanning laser interference type fiber optic acoustic wave phase-locked detection system and method - Google Patents

Abstract

The invention provides a scanning laser interference type optical fiber acoustic wave phase locking detection system and method, belonging to the technical field of trace gas detection. The system includes a photoacoustic excitation light source, a light modulator, a photoacoustic cell, an optical fiber microphone, a broadband scanning laser light source, an optical fiber circulator, a high-speed wavelength query module, a phase-locked loop, a square wave signal generator, a digital signal processor and a computer. The invention performs synchronous wavelength scanning control and synchronous spectral sampling control for the broadband scanning laser light source and the high-speed wavelength query module, and combines the Fabry-Perot cavity length high-speed synchronous demodulation technology based on the fiber scanning laser interferometer with the phase-lock amplification technology , to achieve high sensitivity and high stability detection of weak photoacoustic signals. The invention can greatly improve the precision and limit sensitivity of the photoacoustic spectrum trace gas detection, and provides a highly competitive technical solution for the photoacoustic spectrum trace gas detection.

Description

A scanning laser interference type fiber optic acoustic wave phase-locked detection system and method

technical field

The invention belongs to the technical field of trace gas detection, and relates to a scanning laser interference type optical fiber acoustic wave phase-locked detection system and method.

Background technique

Due to its significant advantages such as high sensitivity and small sampling volume, photoacoustic spectroscopy trace gas detection technology has been widely used in the fields of environmental pollution gas monitoring, analysis of dissolved gas in transformer oil, and monitoring of flammable and explosive gases in coal mines. prospect.

In the photoacoustic spectroscopy measurement, the gas molecules in the gas chamber absorb light energy and undergo non-radiative transitions to generate heat changes, which cause the gas to vibrate and generate sound waves. For the detection of very low concentrations of trace gases, the photoacoustic signal is usually only in the order of micropascals. According to the principle of photoacoustic spectroscopy, the sensitivity of photoacoustic measurement is proportional to the sensitivity of the acoustic wave detector. In order to improve the detection limit of the system, Wang Q, Wang J, Li L, et al. An all-optical photoacoustic spectrometer for trace gas detection [J]. Sensors and Actuators B: Chemical, 2011, 153(1): 214-218 designed A miniaturized microphone based on a fiber-optic Fabry-Perot interferometer was developed and applied to the measurement of trace acetylene gas by photoacoustic spectroscopy. The air gap between the internal reflection surface of the acoustic wave sensitive diaphragm and the end face of the optical fiber constitutes a Fabry-Perot cavity, and the acoustic wave acts on the surface of the diaphragm to produce periodic changes in the cavity length. The scheme adopts the intensity demodulation method. When the central wavelength of the detection laser is locked at the Q point and the sound wave intensity is small, the Fabry-Perot interferometer works in the linear region, that is, the output light intensity changes periodically with the action of the sound wave. After the photodetector converts the light intensity signal into an electrical signal, it is input to the lock-in amplifier to improve the signal-to-noise ratio of photoacoustic signal detection. However, environmental factors such as temperature will cause the drift of the cavity length. In order to ensure that the demodulation system is always in the linear working region, the laser wavelength must be dynamically adjusted accordingly, which increases the complexity of the system. In addition, this intensity demodulation method generally suffers from the problem of increased measurement error caused by the fluctuation of light source power and optical path loss. Document Zhang Y, Shibru H, Cooper K L, et al. Miniature fiber-optic multicavity Fabry–Perotinterferometric biosensor[J]. Optics letters, 2005, 30(9): 1021-1023 Using a scanning laser interferometric solution based on a wavelength interrogator The modulation method demodulates the Fabry-Perot cavity length. This phase demodulation algorithm based on spectral measurement is not affected by the fluctuation of light source power and optical path loss, and can perform high-precision and optical fiber Fabry-Perot sensor modulation. High stability measurement. However, the current wavelength interrogator cannot be matched with the lock-in amplifier, and cannot be used in the detection of weak signals in photoacoustic spectroscopy. Therefore, designing a photoacoustic phase-locked detection system with high signal-to-noise ratio and high stability based on optical fiber Fabry-Perot microphone has important application value in the detection of trace gases in photoacoustic spectroscopy.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose a scanning laser interference type fiber optic acoustic wave phase-locked detection system and method for photoacoustic spectroscopy, which aims to solve the problem of photoacoustic existing in all-optical photoacoustic spectrometers based on fiber Fabry-Perot microphones. The problems of poor signal demodulation stability and low precision further improve the detection sensitivity of trace gas detection and expand more space for the application of photoacoustic spectroscopy in trace gas detection.

The principle of the invention is as follows: the absolute measurement technology of the Fabry-Perot cavity length based on the fiber scanning laser interferometer is combined with the lock-in amplification technology to realize high sensitivity, high stability and large dynamic range detection of weak photoacoustic signals. The acoustic wave generated by the excited gas molecules in the photoacoustic cell acts on the optical fiber microphone to periodically change the length of the Fabry-Perot cavity, and the frequency and phase of the optical frequency domain spectrum generated by the scanning laser interference change accordingly. The acoustic signal can be recovered by the fast cavity-length demodulation of the laser interference spectrum. The phase-locked loop generates a high-frequency trigger signal to synchronously sample and control the high-speed wavelength query module, so that the frequency of the Fabry-Perot cavity demodulation signal is exactly the same as the optical modulation frequency; the digital signal processor converts the Fabry-Perot The measured value of the cavity length is cross-correlated with the reference signal of the same frequency to realize the lock-in amplification function and improve the signal-to-noise ratio of photoacoustic signal detection.

Technical scheme of the present invention:

A scanning laser interference type optical fiber acoustic phase-locked detection system, comprising a photoacoustic excitation light source 1, an optical modulator 2, a photoacoustic cell 3, a fiber microphone 4, a broadband scanning laser light source 5, a fiber circulator 6, and a high-speed wavelength query module 7 , phase-locked loop 8, square wave signal generator 9, digital signal processor 10 and computer 11;

A light modulator 2 is arranged between the photoacoustic excitation light source 1 and the photoacoustic cell 3, and the excitation light emitted by the photoacoustic excitation light source 1 is modulated by the light modulator 2 and then enters the photoacoustic cell 3; the optical fiber microphone 4 Installed on the photoacoustic cell 3, it is used to detect the acoustic wave signal generated by gas molecule absorption in the photoacoustic cell 3; the output square wave signal of the optical modulator 2 is transmitted to the phase-locked loop 8, and the phase-locked loop 8 The output signal is input to the square wave signal generator 9 and the digital signal processor 10 respectively; the digital signal processor 10 provides a feedback signal for the phase-locked loop 8; the digital signal processor 10 controls the square wave signal generator 9. The square wave signal generated is transmitted to the broadband scanning laser light source 5 and the high-speed wavelength query module 7 respectively; the broadband scanning laser light emitted by the broadband scanning laser light source 5 is incident on the fiber microphone 4 through the fiber circulator 6; the fiber optic The reflected light of the microphone 4 is then incident on the high-speed wavelength query module 7 through the optical fiber circulator 6; after the digital signal processor 10 reads the spectral data of the high-speed wavelength query module 7, the lock-in amplification function is realized; the computer 11 It is connected with the digital signal processor 10, and is used for setting the working parameters of the digital signal processor 10 and collecting, processing and displaying the measured amplitude of the photoacoustic signal output by the digital signal processor 10.

A scanning laser interference type fiber acoustic phase-locked detection method, which combines the Fabry-Perot cavity length high-speed synchronous demodulation technology based on the fiber scanning laser interferometer and the lock-in amplification technology to achieve high sensitivity to weak photoacoustic signals and high stability detection;

Specific steps are as follows:

First, the light modulator 2 modulates the intensity of the excitation light from the photoacoustic excitation light source 1, and then enters the photoacoustic cell 3; the gas molecules in the photoacoustic cell 3 absorb the light energy and undergo a non-radiative transition, and the thermal energy generated by the transition occurs. The gas moves periodically and forms sound waves; then the sound waves act on the optical fiber microphone 4, so that the length of the Fabry-Perot cavity changes periodically; Phase locking, generating the same frequency signal and frequency multiplication signal, wherein the same frequency signal is input to the digital signal processor 10 as the reference signal of the lock-in amplifier, and the frequency multiplication signal provides the main clock for the square wave signal generator 9, and the digital signal processor 10. Control the TTL trigger signal generated by the square wave signal generator 9 to carry out synchronous wavelength scanning control and synchronous spectrum sampling control to the broadband scanning laser light source 5 and the high-speed wavelength query module 7 respectively; The circulator 6 is incident on the optical fiber microphone 4; the interference light reflected from the optical fiber microphone 4 is then incident on the high-speed wavelength query module 7 through the optical fiber circulator 6, and the high-speed wavelength query module 7 collects the spectral signal of the incident light; the digital signal processor 10 passes through After the high-speed communication interface reads the spectral data of the high-speed wavelength query module 7, the spectrum is preprocessed by filtering and spectral domain-frequency domain transformation, and then the fast phase demodulation method is used to realize the dynamic measurement of the Fabry-Perot cavity length; Furthermore, the digital signal processor 10 performs a cross-correlation operation between the measured value of the Fabry-Perot cavity length and the reference signal of the same frequency, so as to realize the lock-in amplification function and improve the signal-to-noise ratio of the photoacoustic signal detection; the computer 11 sets the digital signal The working parameters of the processor 10, and finally the computer 11 collects, processes and displays the measured value of the photoacoustic signal output by the digital signal processor 10.

The photoacoustic excitation light source 1 is a narrow linewidth laser for gas detection.

The optical modulator 2 is an optical chopper.

The photoacoustic cell 3 is a non-resonant photoacoustic cell or a first-order longitudinal resonant photoacoustic cell.

The optical fiber microphone 4 is a diaphragm microphone based on the structure of the optical fiber Fabry-Perot interferometer, and has high responsivity to low-frequency acoustic signals.

The broadband scanning laser light source 5 is a scanning laser light source with a spectral width greater than 20 nm.

The high-speed wavelength query module 7 is a module with optical wavelength high precision and fast calibration function, works in an external trigger synchronous sampling mode, and the sampling frequency is M/N times the optical modulation frequency, wherein M and N are integers, And M/N is greater than 2.

The square wave signal generator 9 generates a TTL signal with a duty cycle of 50% and an output frequency range of 10Hz-200Hz.

The wavelength scanning range of the broadband scanning laser light source 5 is 1528-1563 nm, and the scanning speed is 200 Hz.

The sampling rate of the high-speed wavelength query module 7 is 200 Hz, and the spectral measurement range is 1528 nm-1563 nm.

Beneficial effects of the invention: The Fabry-Perot cavity length dynamic measurement technology based on fiber scanning laser interferometer adopts a phase demodulation method which is different from intensity demodulation, and can detect low-frequency photoacoustic signals with high precision and high stability . Through the synchronous control of high-speed spectral sampling, combined with lock-in amplification technology, the accuracy and limit sensitivity of photoacoustic spectroscopy trace gas detection can be greatly improved. The invention provides a very competitive technical solution for the detection of ultra-low concentration trace gas by photoacoustic spectroscopy.

Description of drawings

FIG. 1 is a schematic diagram of the system structure of the present invention.

Fig. 2 is the Fabry-Perot interference spectrogram measured synchronously by the high-speed wavelength query module.

Figure 3 is a photoacoustic signal synchronously measured by a scanning laser interferometer.

Figure 4 is the photoacoustic signal amplitude output by the lock-in amplifier module in the signal processor.

In the figure: 1 photoacoustic excitation light source; 2 optical modulator; 3 photoacoustic cell; 4 optical fiber microphone;

5 broadband scanning laser light source; 6 optical fiber circulator; 7 high-speed wavelength query module;

8 phase-locked loops; 9 square wave signal generators; 10 digital signal processors; 11 computers.

Detailed ways

The specific embodiments of the present invention are described in detail below with reference to the technical solutions and the accompanying drawings.

The schematic diagram of the system structure of the present invention is shown in FIG. 1, which mainly includes a photoacoustic excitation light source 1, an optical modulator 2, a photoacoustic cell 3, an optical fiber microphone 4, a broadband scanning laser light source 5, an optical fiber circulator 6, and a high-speed wavelength query module 7. , a phase-locked loop 8 , a square wave signal generator 9 , a digital signal processor 10 and a computer 11 .

After the photoacoustic excitation light source 1 is modulated by the light intensity of the light modulator 2, it is incident into the photoacoustic cell 3; after the gas molecules in the photoacoustic cell 3 absorb the light energy, the thermal energy generated by the non-radiative transition can cause the gas to move periodically. and form sound waves; the sound pressure acts on the diaphragm surface of the optical fiber microphone 4, causing the Fabry-Perot cavity length to change periodically; the phase-locked loop 8 locks the phase of the square wave signal output by the optical modulator 2 to generate the same frequency signal and frequency multiplication signal, wherein the same frequency signal is input to the digital signal processor 10 as the reference signal of the lock-in amplifier, the frequency multiplication signal provides the main clock for the square wave signal generator 9, and the digital signal processor 10 controls the square wave signal The TTL trigger signal generated by the generator 9 performs synchronous wavelength scanning control and synchronous spectral sampling control on the broadband scanning laser light source 5 and the high-speed wavelength query module 7 respectively; The optical fiber microphone 4; the interference light reflected from the optical fiber microphone 4 is incident on the high-speed wavelength query module 7 through the optical fiber circulator 6, and the high-speed wavelength query module 7 collects the Fabry-Perot interference spectrum; the digital signal processor 10 passes the high-speed communication interface After reading the spectral data of the high-speed wavelength query module 7, the spectrum is preprocessed by filtering and spectral domain-frequency domain transformation, and then the fast phase demodulation method is used to realize the dynamic absolute measurement of the Fabry-Perot cavity length; digital signal The processor 10 performs a cross-correlation operation between the measured value of the Fabry-Perot cavity length and the reference signal of the same frequency, so as to realize the lock-in amplification function and improve the signal-to-noise ratio of the photoacoustic signal detection. The computer 11 sets the working parameters of the digital signal processor 10, collects the measured amplitude of the photoacoustic signal output by the digital signal processor 10, and performs further signal processing and display.

Among them, the photoacoustic excitation light source 1 is a narrow linewidth laser used for gas detection. The light modulator 2 is an optical chopper. The photoacoustic cell 3 is a non-resonant photoacoustic cell or a first-order longitudinal resonant photoacoustic cell. The square wave signal generator 9 generates a TTL signal with a duty cycle of 50% and an output frequency range of 10Hz-200Hz.

The optical fiber microphone 4 is a diaphragm microphone based on the optical fiber Fabry-Perot interferometer structure, and has high responsivity to low-frequency acoustic signals. The wavelength scanning range of the broadband scanning laser light source 5 is 1528-1563 nm, and the maximum scanning speed is 200 Hz. The high-speed wavelength query module 7 is a near-infrared high-speed wavelength query module with a maximum sampling rate of 200Hz and a spectral measurement range of 1528nm-1563nm.

Fig. 2 is the Fabry-Perot interference spectrogram measured synchronously by the high-speed wavelength query module. The static length of the Fabry-Perot cavity is calculated to be about 600μm by the high-speed phase demodulation method.

Figure 3 is a photoacoustic signal synchronously measured by a scanning laser interferometer. The chopping frequency is set to 20Hz, and the wavelength scan and query frequency is set to 160Hz. The low-concentration acetylene gas molecules in the photoacoustic cell absorb the intensity-modulated excitation light to generate a photoacoustic signal, and the effect of sound pressure changes the cavity length. The scanning laser interferometer demodulates the cavity length value and performs bandpass filtering.

Figure 4 is the photoacoustic signal amplitude output by the lock-in amplifier module in the signal processor. After the cavity length demodulated by the scanning laser interferometer is phase-locked and amplified, the photoacoustic signal amplitude with high signal-to-noise ratio is obtained.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (9)

1. A scanning laser interference type optical fiber acoustic phase-locked detection system, characterized in that it comprises a photoacoustic excitation light source (1), an optical modulator (2), a photoacoustic cell (3), an optical fiber microphone (4), a broadband scanning a laser light source (5), an optical fiber circulator (6), a high-speed wavelength query module (7), a phase-locked loop (8), a square wave signal generator (9), a digital signal processor (10) and a computer (11); A light modulator (2) is arranged between the photoacoustic excitation light source (1) and the photoacoustic cell (3), and the excitation light emitted by the photoacoustic excitation light source (1) is modulated by the light modulator (2) and then incident on the light. Acoustic cell (3); the optical fiber microphone (4) is installed on the photoacoustic cell (3), and is used for detecting the acoustic wave signal generated by the absorption of gas molecules in the photoacoustic cell (3); the optical modulator (2) The output square wave signal of ) is transmitted to the phase-locked loop (8), and the output signal of the phase-locked loop (8) is respectively input to the square-wave signal generator (9) and the digital signal processor (10); the The digital signal processor (10) provides a feedback signal for the phase-locked loop (8); the digital signal processor (10) controls the square wave signal generated by the square wave signal generator (9) to be respectively transmitted to the broadband scanning laser light source ( 5) and a high-speed wavelength query module (7); the broadband scanning laser emitted by the broadband scanning laser light source (5) is incident on the fiber microphone (4) after passing through the fiber circulator (6); the fiber microphone (4) The reflected light is then incident on the high-speed wavelength inquiry module (7) through the optical fiber circulator (6); after the digital signal processor (10) reads the spectral data of the high-speed wavelength inquiry module (7), the lock-in amplification function is realized The computer (11) is connected with the digital signal processor (10), for setting the working parameters of the digital signal processor (10) and collecting the measured amplitude of the photoacoustic signal output by the digital signal processor (10) , processing and display. 2. A scanning laser interference type fiber acoustic phase-locked detection method, characterized in that the Fabry-Perot cavity length high-speed synchronous demodulation technology based on the fiber scanning laser interferometer is combined with the lock-in amplification technology to realize the detection of weak signals. High sensitivity and high stability detection of photoacoustic signals; Specific steps are as follows: First, the light modulator (2) modulates the intensity of the excitation light from the photoacoustic excitation light source (1), and then enters the photoacoustic cell (3); the gas molecules in the photoacoustic cell (3) absorb the light energy and generate no Radiation transition, the heat generated by the transition can make the gas move periodically and form sound waves; then the sound waves act on the optical fiber microphone (4), so that the length of the Fabry-Perot cavity changes periodically; at the same time, the phase-locked loop (8 ) phase-locks the square wave signal output by the optical modulator (2) to generate the same frequency signal and the frequency multiplied signal, wherein the same frequency signal is input to the digital signal processor (10) as the reference signal of the lock-in amplifier, the frequency multiplied signal Then the square wave signal generator (9) is provided with a master clock, and the digital signal processor (10) controls the TTL trigger signal generated by the square wave signal generator (9) to respectively scan the broadband scanning laser light source (5) and the high-speed wavelength query module ( 7) Perform synchronous wavelength scanning control and synchronous spectral sampling control; the broadband wavelength scanning laser emitted by the broadband scanning laser light source (5) is incident on the fiber microphone (4) after passing through the fiber circulator (6); The interference light is then incident on the high-speed wavelength query module (7) through the optical fiber circulator (6), and the high-speed wavelength query module (7) collects the spectral signal of the incident light; the digital signal processor (10) reads the high-speed wavelength query through the high-speed communication interface. After the spectral data of module (7), the spectrum is filtered and preprocessed by spectral domain-frequency domain transformation, and then the fast phase demodulation method is used to realize the dynamic measurement of the Fabry-Perot cavity length; furthermore, the digital signal processor ( 10) The cross-correlation operation is performed between the measured value of the Fabry-Perot cavity length and the reference signal of the same frequency to realize the lock-in amplification function and improve the signal-to-noise ratio of photoacoustic signal detection; the computer (11) is equipped with a digital signal processor ( 10), and finally the computer (11) collects, processes and displays the measured value of the photoacoustic signal output by the digital signal processor (10). 3. A scanning laser interference type fiber acoustic phase-locked detection method according to claim 2, wherein the photoacoustic excitation light source (1) is a narrow linewidth laser for gas detection; the The optical modulator (2) is an optical chopper; the photoacoustic cell (3) is a non-resonant photoacoustic cell or a first-order longitudinal resonant photoacoustic cell; the optical fiber microphone (4) is based on a fiber Fabry- The diaphragm microphone of the Perot interferometer structure has high responsivity to low-frequency sound wave signals; the square wave signal generator (9) generates a TTL signal with a duty cycle of 50% and an output frequency range of 10Hz-200Hz. 4 . The scanning laser interference type optical fiber acoustic phase locking detection method according to claim 2 or 3 , wherein the broadband scanning laser light source ( 5 ) is a scanning laser light source with a spectral width greater than 20 nm. 5 . 5. A scanning laser interference type optical fiber acoustic phase-locked detection method according to claim 2 or 3, characterized in that the high-speed wavelength query module (7) is a high-precision optical wavelength and fast calibration function The module works in the external trigger synchronous sampling mode, and the sampling frequency is M/N times the optical modulation frequency, where M and N are integers, and M/N is greater than 2. 6 . A scanning laser interference type optical fiber acoustic phase-locked detection method according to claim 4 , wherein the high-speed wavelength query module (7) is a module with high-precision optical wavelength and fast calibration functions. 7 . , work in the external trigger synchronous sampling mode, the sampling frequency is M/N times the optical modulation frequency, where M and N are integers, and M/N is greater than 2; the sampling rate of the high-speed wavelength query module (7) is 200Hz , the spectral measurement range is 1528nm-1563nm. 7. A scanning laser interference type optical fiber acoustic phase-locked detection method according to claim 2, 3 or 6, characterized in that, the wavelength scanning range of the broadband scanning laser light source (5) is 1528-1563 nm, and the scanning range is 1528-1563 nm. The speed is 200Hz. 8 . The scanning laser interference type fiber acoustic phase locking detection method according to claim 4 , wherein the wavelength scanning range of the broadband scanning laser light source ( 5 ) is 1528-1563 nm, and the scanning speed is 200 Hz. 9 . 9 . The scanning laser interference type fiber acoustic phase locking detection method according to claim 5 , wherein the wavelength scanning range of the broadband scanning laser light source ( 5 ) is 1528-1563 nm, and the scanning speed is 200 Hz. 10 .

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