CN108051400A - A kind of scanning laser interference-type optical fiber sound wave lock phase detection system and method - Google Patents

Abstract

The invention provides a scanning laser interference optical fiber acoustic wave phase-locked detection system and method, belonging to the technical field of trace gas detection. The system includes a photoacoustic excitation light source, an optical modulator, a photoacoustic cell, a fiber optic microphone, a broadband scanning laser source, a fiber optic 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 spectrum sampling control on 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 lock-in 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 photoacoustic spectrum trace gas detection, and provides a highly competitive technical solution for photoacoustic spectrum trace gas detection.

Description

A scanning laser interference type optical fiber 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

Photoacoustic spectroscopy trace gas detection technology has shown a wide range of applications in the fields of environmental pollution gas monitoring, dissolved gas analysis in transformer oil, and flammable and explosive gas monitoring in coal mines due to its significant advantages such as high sensitivity and small sampling volume. prospect.

In photoacoustic spectroscopy, gas molecules in the gas cell absorb light energy and undergo non-radiative transitions to generate heat changes, which cause gas vibrations to generate sound waves. For extremely low concentration trace gas detection, the photoacoustic signal is usually only on the order of micropascals. According to the principle of photoacoustic spectroscopy, the sensitivity of photoacoustic measurements is proportional to the sensitivity of the acoustic wave detector. In order to improve the detection limit of the system, the literature Wang Q, Wang J, Li L, et al.An all-optical photoacoustic spectrometer fortrace gas detection[J].Sensors and Actuators B:Chemical,2011,153(1):214-218 design A miniaturized microphone based on 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 make the cavity length change periodically. This scheme adopts the intensity demodulation method. When the central wavelength of the detection laser is locked at the Q point and the acoustic 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 acoustic wave. After the light detector 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 area, the laser wavelength must be dynamically adjusted accordingly, which increases the complexity of the system. In addition, this intensity demodulation method generally has problems such as increased measurement errors caused by light source power fluctuations and optical path loss. Literature 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 adjustment method demodulates the Fabry-Perot cavity length. This phase demodulation algorithm based on spectral measurement is not affected by light source power fluctuations and optical path loss, and can perform high-precision and High stability measurement. However, the current wavelength interrogator cannot be matched with the lock-in amplifier, and cannot be applied to 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 fiber optic Fabry-Perot microphone has important application value in the detection of trace gases in photoacoustic spectroscopy.

Contents of the invention

The object of the present invention is to propose a scanning laser interference optical fiber acoustic wave phase-locked detection system and method for photoacoustic spectroscopy, aiming to solve the photoacoustic problems existing in the all-optical photoacoustic spectrometer based on fiber optic Fabry-Perot microphones. Problems such as poor signal demodulation stability and low precision further improve the detection sensitivity of trace gas detection and expand a larger space for the application of photoacoustic spectroscopy in trace gas detection.

The principle of the present invention is as follows: the Fabry-Perot cavity length absolute measurement technology based on the fiber-optic 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 excitation of gas molecules to be measured in the photoacoustic cell acts on the fiber optic microphone to cause periodic changes in the length of the Fabry-Perot cavity, and the frequency and phase of the optical frequency domain spectrum generated by scanning laser interference change accordingly. The acoustic signal can be recovered by fast cavity length demodulation of the laser interference spectrum. The phase-locked loop generates a high-frequency trigger signal to perform synchronous sampling control on the high-speed wavelength query module, so that the frequency of the Fabry-Perot cavity length 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 phase-locked amplification function and improve the signal-to-noise ratio of photoacoustic signal detection.

Technical scheme of the present invention:

A scanning laser interference optical fiber acoustic wave phase-locked detection system, including a photoacoustic excitation light source 1, an optical modulator 2, a photoacoustic cell 3, a fiber optic microphone 4, a broadband scanning laser light source 5, a fiber optic 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, used to detect the acoustic wave signal generated by the absorption of gas molecules 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 square wave signal generator 9 and digital signal processor 10 respectively; Described digital signal processor 10 provides feedback signal for phase-locked loop 8; Described digital signal processor 10 controls square wave signal generator The square wave signals generated by 9 are 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 optic microphone 4 after the optical fiber circulator 6; the optical fiber The reflected light of the microphone 4 is incident to the high-speed wavelength query module 7 through the optical fiber circulator 6; after the described 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 optical fiber acoustic phase-locked detection method, which combines the Fabry-Perot cavity length high-speed synchronous demodulation technology based on optical fiber scanning laser interferometer with phase-locked amplification technology to achieve high sensitivity to weak photoacoustic signals with 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 it is incident into the photoacoustic cell 3; the gas molecules in the photoacoustic cell 3 undergo a non-radiative transition after absorbing light energy, and the heat energy generated by the transition makes The gas moves periodically and forms sound waves; then the sound waves act on the fiber optic microphone 4, causing the Fabry-Perot cavity length to change periodically; Phase locking, producing the same-frequency signal and 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, and the frequency-multiplied signal provides the main clock for the square wave signal generator 9, and the digital signal processor 10 controls the TTL trigger signal produced by the square wave signal generator 9 to perform synchronous wavelength scanning control and synchronous spectrum sampling control on the broadband scanning laser source 5 and the high-speed wavelength query module 7 respectively; the broadband wavelength scanning laser emitted by the broadband scanning laser source 5 passes through the optical fiber After the circulator 6 is incident on the fiber optic microphone 4; the interference light reflected from the fiber optic microphone 4 is incident on the high-speed wavelength query module 7 through the fiber optic circulator 6, and the high-speed wavelength query module 7 collects the spectral signal of the incident light; the digital signal processor 10 passes 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 conversion, and then the fast phase demodulation method is adopted to realize the dynamic measurement of the Fabry-Perot cavity length; Furthermore, the digital signal processor 10 performs cross-correlation calculations with 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 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 fiber optic microphone 4 is a diaphragm microphone based on a fiber optic Fabry-Perot interferometer structure, and has a relatively high responsivity to low-frequency sound waves.

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

The high-speed wavelength query module 7 is a module with high-precision optical wavelength and fast calibration functions. It works in an 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.

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 1528nm-1563nm.

Beneficial effects of the present invention: The Fabry-Perot cavity length dynamic measurement technology based on the fiber scanning laser interferometer adopts the phase demodulation method different from the intensity demodulation method, 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 precision and limit sensitivity of photoacoustic spectroscopy trace gas detection can be greatly improved. The invention provides a very competitive technical solution for the ultra-low concentration trace gas detection by photoacoustic spectroscopy.

Description of drawings

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

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

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

Fig. 4 is the amplitude of the photoacoustic signal 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 fiber optic microphone;

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

8 phase-locked loop; 9 square wave signal generator; 10 digital signal processor; 11 computer.

Detailed ways

The specific embodiments of the present invention will be described in detail below in conjunction with the technical solutions and accompanying drawings.

The schematic diagram of the system structure of the present invention is shown in Figure 1, mainly including a photoacoustic excitation light source 1, an optical modulator 2, a photoacoustic cell 3, a fiber optic microphone 4, a broadband scanning laser light source 5, a fiber optic 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.

After the photoacoustic excitation light source 1 is modulated by 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 heat energy generated by the non-radiative transition causes the gas to move periodically and form sound waves; the sound pressure acts on the diaphragm surface of the fiber optic microphone 4, causing periodic changes in the length of the Fabry-Perot cavity; frequency signal and 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, and the frequency multiplied 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 spectrum sampling control on the broadband scanning laser source 5 and the high-speed wavelength query module 7 respectively; the broadband scanning laser emitted by the broadband scanning laser source 5 is incident on the optical fiber circulator 6 The fiber optic microphone 4; the interference light reflected from the fiber optic microphone 4 enters the high-speed wavelength query module 7 through the fiber optic circulator 6, and the high-speed wavelength query module 7 collects the Fabry-Perot interference spectrum; the digital signal processor 10 passes through 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 adopted to realize the dynamic absolute measurement of the Fabry-Perot cavity length; the digital signal The processor 10 performs a cross-correlation operation on the measured value of the Fabry-Perot cavity length and a reference signal of the same frequency to realize the phase-locked amplification function and improve the signal-to-noise ratio of photoacoustic signal detection. The computer 11 sets the working parameters of the digital signal processor 10, collects the photoacoustic signal output by the digital signal processor 10 to measure the amplitude, performs further signal processing and displays it.

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

The fiber optic microphone 4 is a diaphragm microphone based on a fiber optic 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 a Fabry-Perot interference spectrum diagram synchronously measured by the high-speed wavelength query module. The static length of the Fabry-Perot cavity is calculated to be about 600 μm by the high-speed phase demodulation method.

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

Fig. 4 is the amplitude of the photoacoustic signal 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 amplitude of the photoacoustic signal with a 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 modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (9)

1. a kind of scanning laser interference-type optical fiber sound wave locks phase detection system, which is characterized in that including photo-acoustic excitation light source (1), Optical modulator (2), photoacoustic cell (3), fiber microphone (4), broadband scanning laser light source (5), optical fiber circulator (6), fast wave Long enquiry module (7), phaselocked loop (8), square wave signal generator (9), digital signal processor (10) and computer (11)；

Optical modulator (2) is set between the photo-acoustic excitation light source (1) and photoacoustic cell (3), and photo-acoustic excitation light source (1) is sent Photoacoustic cell (3) is incided into after the optically modulated device of exciting light (2) modulation；The fiber microphone (4) is mounted on photoacoustic cell (3) On, for detecting the acoustic signals that the interior gas molecules sorb of photoacoustic cell (3) generates；The output square wave of the optical modulator (2) Signal transmission to phaselocked loop (8), the output signal of the phaselocked loop (8) is separately input to square wave signal generator (9) sum number Word signal processor (10)；The digital signal processor (10) provides feedback signal for phaselocked loop (8)；The number letter The square-wave signal that number processor (10) control square wave signal generator (9) generates is transmitted separately to broadband scanning laser light source (5) With high speed wavelength enquiry module (7)；The broadband scanning laser of broadband scanning laser light source (5) transmitting is through optical fiber circulator (6) fiber microphone (4) is incided into after；The reflected light of the fiber microphone (4) incides into height through optical fiber circulator (6) again Fast wavelength enquiry module (7)；The digital signal processor (10) reads the spectroscopic data of high speed wavelength enquiry module (7) Afterwards, lock phase enlarging function is realized；The computer (11) is connected with digital signal processor (10), for setting digital signal The running parameter of processor (10) is simultaneously acquired the photoacoustic signal measurement amplitude of digital signal processor (10) output, handles And display.

2. a kind of scanning laser interference-type optical fiber sound wave locks phase detection method, which is characterized in that will be done based on optical fiber scanning laser The long high speed combined synchronous demodulation technique of Fabry-Perot-type cavity of interferometer is combined with phase lock amplifying technology, is realized to faint photoacoustic signal It is highly sensitive to be detected with high stability；

It is as follows：

After optical modulator (2) carries out intensity modulated to the exciting light for coming from photo-acoustic excitation light source (1) first, photoacoustic cell is incided into (3) in；Radiationless transition occurs after gas molecules sorb luminous energy in photoacoustic cell (3), the thermal energy that transition generates makes gas Cycle movement simultaneously forms sound wave；Then sound wave effect makes Fabry-Perot-type cavity length that week occur therewith in fiber microphone (4) Phase property changes；Meanwhile the square-wave signal that phaselocked loop (8) exports optical modulator (2) carries out PGC demodulation, generate homogenous frequency signal with Frequency-doubled signal, wherein homogenous frequency signal are input to reference signal of the digital signal processor (10) as lock-in amplifier, frequency multiplication letter Master clock number then is provided for square wave signal generator (9), digital signal processor (10) control square wave signal generator (9) generates TTL trigger signals length scanning control is synchronized to broadband scanning laser light source (5) and high speed wavelength enquiry module (7) respectively System and synchronous spectrum controlling of sampling；The broad band wavelength scanning laser of broadband scanning laser light source (5) transmitting is through optical fiber circulator (6) After incide into fiber microphone (4)；The interference light reflected from fiber microphone (4) incides at a high speed again through optical fiber circulator (6) Wavelength enquiry module (7), high speed wavelength enquiry module (7) gather the spectral signal of incident light；Digital signal processor (10) is logical After crossing the spectroscopic data that high-speed communication interface reads high speed wavelength enquiry module (7), spectrum is filtered and spectral domain-frequency domain Using fast phase demodulation method after the pretreatments such as conversion, the dynamic measurement of Fabry-Perot-type cavity length is realized；And then digital signal The reference signal of the long measured value of Fabry-Perot-type cavity and same frequency is carried out computing cross-correlation by processor (10), realizes that lock is mutually put Big function improves the signal-to-noise ratio of photoacoustic signal detection；Computer (11) sets the running parameter of digital signal processor (10), most The photoacoustic signal measured value that computer (11) exports digital signal processor (10) afterwards is acquired, handles and shows.

A kind of 3. scanning laser interference-type optical fiber sound wave lock phase detection method according to claim 2, which is characterized in that institute The photo-acoustic excitation light source (1) stated is the narrow linewidth laser for gas detection；The optical modulator (2) is optics copped wave Device；The photoacoustic cell (3) is non-resonance photoacoustic cell or single order longitudinal resonance photoacoustic cell；The fiber microphone (4) is Diaphragm type microphone based on optical fibre Fabry-perot interferometer structure has compared with high-responsivity low-frequency sound wave signal；It is described Square wave signal generator (9) generate duty cycle be 50% TTL signal, reference frequency output 10Hz-200Hz.

4. a kind of scanning laser interference-type optical fiber sound wave lock phase detection system according to Claims 2 or 3, feature exist In the broadband scanning laser light source (5) is a kind of scanning laser light source, and spectrum width is more than 20nm.

5. a kind of scanning laser interference-type optical fiber sound wave lock phase detection system according to Claims 2 or 3, feature exist In the high speed wavelength enquiry module (7) is a kind of module with optical wavelength high-precision and Fast Calibration, is worked in External trigger synchronized sampling pattern, sample frequency are M/N times of light modulation frequency, and wherein M and N are integers, and M/N is more than 2.

A kind of 6. scanning laser interference-type optical fiber sound wave lock phase detection system according to claim 4, which is characterized in that institute The high speed wavelength enquiry module (7) stated is a kind of module with optical wavelength high-precision and Fast Calibration, works in outside Synchronized sampling pattern is triggered, sample frequency is M/N times of light modulation frequency, and wherein M and N are integers, and M/N is more than 2；Described The sampling rate of high speed wavelength enquiry module (7) is 200Hz, spectral measurement ranges 1528nm-1563nm.

7. a kind of scanning laser interference-type optical fiber sound wave lock phase detection system according to claim 2,3 or 6, feature exist In the wavelength scanning range of the broadband scanning laser light source (5) is 1528-1563nm, sweep speed 200Hz.

A kind of 8. scanning laser interference-type optical fiber sound wave lock phase detection system according to claim 4, which is characterized in that institute The wavelength scanning range for the broadband scanning laser light source (5) stated is 1528-1563nm, sweep speed 200Hz.

A kind of 9. scanning laser interference-type optical fiber sound wave lock phase detection system according to claim 5, which is characterized in that institute The wavelength scanning range for the broadband scanning laser light source (5) stated is 1528-1563nm, sweep speed 200Hz.