CN108051400B - Scanning laser interference type optical fiber sound wave phase-locked detection system and method - Google Patents
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Abstract
The invention provides a scanning laser interference type optical fiber sound wave phase-locked detection system and a method, belonging to the technical field of trace gas detection. The system comprises a photoacoustic excitation light source, an optical 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 carries out synchronous wavelength scanning control and synchronous spectrum sampling control on a broadband scanning laser light source and a high-speed wavelength query module, combines a Fabry-Perot cavity length high-speed synchronous demodulation technology based on an optical fiber scanning laser interferometer with a phase-locked amplification technology, and realizes high sensitivity and high stability detection of weak photoacoustic signals. The invention can greatly improve the precision and the ultimate sensitivity of the photoacoustic spectroscopy trace gas detection, and provides a very competitive technical scheme for the photoacoustic spectroscopy trace gas detection.
Description
Technical Field
The invention belongs to the technical field of trace gas detection, and relates to a scanning laser interference type optical fiber sound wave phase-locked detection system and a scanning laser interference type optical fiber sound wave phase-locked detection method.
Background
The photoacoustic spectroscopy trace gas detection technology has the remarkable advantages of high sensitivity, small sampling volume and the like, and has wide application prospects in the fields of environmental pollution gas monitoring, analysis of dissolved gas in transformer oil, coal mine combustible and explosive gas monitoring and the like.
In photoacoustic spectrometry, gas molecules in a gas cell absorb light energy to generate nonradiative transitions to generate thermal changes, which cause gas vibration to generate sound waves. For extremely low concentration trace gas detection, the photoacoustic signal is typically only on the order of a few millipascals. 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, a miniaturized microphone based on a fiber Fabry-Perot interferometer is designed in the documents Wang Q, Wang J, Li L, et al, an all-optical Photonic spectrometer for trace gas detection [ J ]. Sensors and Actuators B, Chemical,2011,153(1):214 and 218, and is applied to photoacoustic spectroscopy trace acetylene gas measurement. An air gap between an internal reflecting surface of the acoustic wave sensitive membrane and the end face of the optical fiber forms a Fabry-Perot cavity, and the acoustic wave acts on the surface of the membrane to enable the cavity length to generate periodic change. The scheme adopts an intensity demodulation method, when the central wavelength of the detection laser is locked at a Q point and the sound wave intensity is small, the Fabry-Perot interferometer works in a linear region, namely the output light intensity is periodically changed along with the action of the sound wave. The optical detector converts the light intensity signal into an electric signal, and then the electric signal is input into the phase-locked amplifier, so that the signal-to-noise ratio of the photoacoustic signal detection is improved. However, environmental factors such as temperature can cause the drift of the cavity length, and in order to ensure that the demodulation system is always in a linear working region, the laser wavelength must be dynamically adjusted along with the cavity length, thereby increasing the complexity of the system. In addition, the intensity demodulation method generally has the problems of increased measurement errors and the like, which are easily affected by the power fluctuation of the light source and the optical path loss. In the document Zhang Y, Shibru H, Cooper K L, et al.Mini fiber-optical multi-cavity Fabry-Perot interferometric biosensor [ J ]. Optics letters,2005,30(9): 1021-. However, the existing wavelength query instrument cannot be matched with a lock-in amplifier, and cannot be applied to the detection of a photoacoustic spectrum weak signal. Therefore, the photoacoustic phase-locked detection system with high signal-to-noise ratio and high stability based on the fiber Fabry-Perot microphone has important application value in photoacoustic spectrum trace gas detection.
Disclosure of Invention
The invention aims to provide a scanning laser interference type optical fiber sound wave phase-locked detection system and method for photoacoustic spectroscopy, and aims to solve the problems of poor demodulation stability, low precision and the like of photoacoustic signals in an all-optical photoacoustic spectrometer based on an optical fiber Fabry-Perot microphone, 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 invention is as follows: the Fabry-Perot cavity length absolute measurement technology based on the optical fiber scanning laser interferometer is combined with the phase-locked amplification technology, so that the weak photoacoustic signal can be detected with high sensitivity, high stability and large dynamic range. The acoustic wave generated by exciting gas molecules to be detected in the photoacoustic cell acts on the optical fiber microphone to enable the cavity length of the Fabry-Perot to generate periodic change, the frequency and the phase of the optical frequency domain spectrum generated by scanning laser interference change along with the periodic change, and the acoustic wave signal can be recovered by demodulating the fast cavity length of the scanning laser interference spectrum. The phase-locked loop generates a high-frequency trigger signal to carry out synchronous sampling control on the high-speed wavelength query module, so that the frequency of a Fabry-Perot cavity length demodulation signal is completely the same as the light modulation frequency; and the digital signal processor performs cross-correlation operation on the Fabry-Perot cavity length measurement value and the reference signal with the same frequency, so that a phase-locked amplification function is realized, and the signal-to-noise ratio of photoacoustic signal detection is improved.
The technical scheme of the invention is as follows:
a scanning laser interference type optical fiber sound wave phase-locked detection system comprises an optical-acoustic excitation light source 1, an optical modulator 2, an optical-acoustic cell 3, an optical fiber microphone 4, a broadband scanning 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;
an optical modulator 2 is arranged between the photoacoustic excitation light source 1 and the photoacoustic cell 3, and excitation light emitted by the photoacoustic excitation light source 1 is modulated by the optical modulator 2 and then enters the photoacoustic cell 3; the optical fiber microphone 4 is arranged on the photoacoustic cell 3 and is used for detecting acoustic wave signals generated by absorption of gas molecules in the photoacoustic cell 3; the output square wave signal of the optical modulator 2 is transmitted to a phase-locked loop 8, and the output signal of the phase-locked loop 8 is respectively input to a square wave signal generator 9 and a digital signal processor 10; 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 the high-speed wavelength query module 7; the broadband scanning laser emitted by the broadband scanning laser source 5 is incident to the optical fiber microphone 4 after passing through the optical fiber circulator 6; the reflected light of the optical fiber microphone 4 is incident to the high-speed wavelength query module 7 through the optical fiber circulator 6; after the digital signal processor 10 reads the spectrum data of the high-speed wavelength query module 7, the phase-locking amplification function is realized; the computer 11 is connected to the digital signal processor 10 and configured to set a working parameter of the digital signal processor 10 and collect, process and display a measured amplitude of the photoacoustic signal output by the digital signal processor 10.
A scanning laser interference type optical fiber sound wave phase-locked detection method combines a Fabry-Perot cavity length high-speed synchronous demodulation technology based on an optical fiber scanning laser interferometer with a phase-locked amplification technology to realize high sensitivity and high stability detection of weak photoacoustic signals;
the method comprises the following specific steps:
firstly, the light modulator 2 modulates the intensity of exciting light from the photoacoustic excitation light source 1 and then emits the light into the photoacoustic cell 3; gas molecules in the photoacoustic cell 3 absorb light energy and then generate radiationless transition, and the heat energy generated by the transition enables the gas to generate periodic motion and form sound waves; then the sound wave acts on the optical fiber microphone 4, so that the cavity length of the Fabry-Perot changes periodically; meanwhile, the phase-locked loop 8 performs phase locking on the square wave signal output by the optical modulator 2 to generate a same-frequency signal and a frequency-doubled signal, wherein the same-frequency signal is input to the digital signal processor 10 to serve as a reference signal of the phase-locked amplifier, the frequency-doubled signal provides a main clock for the square wave signal generator 9, and the digital signal processor 10 controls TTL trigger signals generated by the square wave signal generator 9 to perform synchronous wavelength scanning control and synchronous spectrum sampling control on the broadband scanning laser light source 5 and the high-speed wavelength query module 7 respectively; broadband wavelength scanning laser emitted by the broadband scanning laser source 5 is incident to the optical fiber microphone 4 after passing through the optical fiber circulator 6; the interference light reflected from the optical fiber microphone 4 is incident to the high-speed wavelength query module 7 through the optical fiber circulator 6, and the high-speed wavelength query module 7 collects a spectrum signal of the incident light; after reading the spectrum data of the high-speed wavelength query module 7 through the high-speed communication interface, the digital signal processor 10 performs preprocessing such as filtering and spectrum domain-frequency domain conversion on the spectrum and then adopts a fast phase demodulation method to realize dynamic measurement of the Fabry-Perot cavity length; further, the digital signal processor 10 performs cross-correlation operation on the measured fabry-perot cavity length value and the reference signal with the same frequency, so as to realize a 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, 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 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 optical fiber microphone 4 is a diaphragm microphone based on an optical fiber Fabry-Perot interferometer structure, and has high responsivity to low-frequency sound wave signals.
The broadband scanning laser light source 5 is a scanning laser light source, and the spectral width is more than 20 nm.
The high-speed wavelength query module 7 is a module with high-precision optical wavelength and quick calibration functions, and works in an external trigger synchronous sampling mode, wherein the sampling frequency is M/N times of the light modulation frequency, M and N are integers, and M/N is more than 2.
The square wave signal generator 9 generates a TTL signal with a duty ratio of 50%, and outputs a frequency range of 10Hz-200 Hz.
The wavelength scanning range of the broadband scanning laser light source 5 is 1528-1563nm, and the scanning speed is 200 Hz.
The sampling rate of the high-speed wavelength query module 7 is 200Hz, and the spectral measurement range is 1528nm-1563 nm.
The invention has the beneficial effects that: the Fabry-Perot cavity length dynamic measurement technology based on the optical fiber scanning laser interferometer adopts a phase demodulation method different from intensity demodulation, and can perform high-precision and high-stability detection on low-frequency photoacoustic signals. By means of synchronous control of high-speed spectrum sampling and the combination of a phase-locked amplification technology, the precision and the ultimate sensitivity of micro-gas detection of photoacoustic spectrum can be greatly improved. The invention provides a very competitive technical scheme for detecting the ultra-low concentration trace gas by using the photoacoustic spectroscopy.
Drawings
FIG. 1 is a schematic diagram of the system architecture of the present invention.
FIG. 2 is a Fabry-Perot interference spectrogram for synchronous measurement of a high-speed wavelength query module.
FIG. 3 is a photoacoustic signal measured simultaneously by a scanning laser interferometer.
Fig. 4 is an amplitude of the photoacoustic signal output from the phase-lock amplification block in the signal processor.
In the figure: 1, a photoacoustic excitation light source; 2 an optical modulator; 3, a photoacoustic cell; 4, a fiber optic microphone;
5 broadband scanning laser light source; 6, a fiber optic circulator; 7, a high-speed wavelength query module;
8 phase-locked loop; 9 a square wave signal generator; 10 a digital signal processor; 11 computer.
Detailed Description
The following detailed description of the invention refers to the accompanying drawings.
The system structure diagram of the invention is shown in fig. 1, and mainly comprises 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, 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.
The photoacoustic excitation light source 1 is subjected to light intensity modulation by the light modulator 2 and then enters the photoacoustic cell 3; after the gas molecules in the photoacoustic cell 3 absorb the light energy, the gas generates periodic motion and forms sound waves by the heat energy generated by radiationless transition; the sound pressure acts on the surface of the diaphragm of the optical fiber microphone 4, so that the length of the Fabry-Perot cavity is periodically changed; the phase-locked loop 8 performs phase locking on the square wave signal output by the optical modulator 2 to generate a same frequency signal and a frequency multiplication signal, wherein the same frequency signal is input to the digital signal processor 10 to be used as a reference signal of the phase-locked amplifier, the frequency multiplication signal provides a main clock for the square wave signal generator 9, and the digital signal processor 10 controls TTL trigger signals generated by the square wave signal generator 9 to perform synchronous wavelength scanning control and synchronous spectrum sampling control on the broadband scanning laser light source 5 and the high-speed wavelength query module 7 respectively; broadband scanning laser emitted by the broadband scanning laser source 5 enters the optical fiber microphone 4 after passing through the optical fiber circulator 6; the interference light reflected from the optical fiber microphone 4 is incident to the high-speed wavelength query module 7 through the optical fiber circulator 6, and the high-speed wavelength query module 7 collects a Fabry-Perot interference spectrum; after reading the spectrum data of the high-speed wavelength query module 7 through the high-speed communication interface, the digital signal processor 10 performs preprocessing such as filtering and spectrum domain-frequency domain conversion on the spectrum and then adopts a fast phase demodulation method to realize dynamic absolute measurement of the Fabry-Perot cavity length; the digital signal processor 10 performs cross-correlation operation on the measured value of the Fabry-Perot cavity length and the reference signal with the same frequency, so that the phase-locked amplification function is realized, and the signal-to-noise ratio of photoacoustic signal detection is improved. The computer 11 sets the operating parameters of the digital signal processor 10, and further processes and displays the photoacoustic signal after acquiring the measured amplitude 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 square wave signal generator 9 generates a TTL signal with a duty ratio of 50%, and outputs a frequency range of 10Hz-200 Hz.
The optical fiber microphone 4 is a diaphragm microphone based on an optical fiber Fabry-Perot interferometer structure, and has high responsivity to low-frequency sound wave signals. The wavelength scanning range of the broadband scanning laser light source 5 is 1528-1563nm, and the highest scanning speed is 200 Hz. The high-speed wavelength query module 7 is a near-infrared high-speed wavelength query module, the highest sampling rate is 200Hz, and the spectral measurement range is 1528nm-1563 nm.
FIG. 2 is a Fabry-Perot interference spectrogram for synchronous measurement of a high-speed wavelength query module. The static length of the fabry-perot cavity is calculated to be about 600 μm by a high-speed phase demodulation method.
FIG. 3 is a photoacoustic signal measured simultaneously by a scanning laser interferometer. The chopping frequency was set at 20Hz and the wavelength sweep and interrogation frequency was set at 160 Hz. The low-concentration acetylene gas molecules in the photoacoustic cell absorb the intensity modulated excitation light to generate photoacoustic signals, the cavity length is changed under the action of sound pressure, and the laser interferometer is scanned to demodulate the cavity length value and perform band-pass filtering processing.
Fig. 4 is an amplitude of the photoacoustic signal output from the phase-lock amplification block in the signal processor. And the cavity length demodulated by the scanning laser interferometer is subjected to phase-locked amplification to obtain the photoacoustic signal amplitude with high signal-to-noise ratio.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (9)
1. A scanning laser interference type optical fiber sound wave phase-locked detection system is characterized by comprising an optical-acoustic excitation light source (1), an optical modulator (2), an optical-acoustic cell (3), an optical fiber microphone (4), a broadband scanning laser light source (5), an optical fiber circulator (6), a high-speed wavelength inquiry module (7), a phase-locked loop (8), a square wave signal generator (9), a digital signal processor (10) and a computer (11);
an optical modulator (2) is arranged between the photoacoustic excitation light source (1) and the photoacoustic cell (3), and excitation light emitted by the photoacoustic excitation light source (1) is modulated by the optical modulator (2) and then enters the photoacoustic cell (3); the optical fiber microphone (4) is arranged on the photoacoustic cell (3) and is used for detecting acoustic wave signals generated by absorption of gas molecules in the photoacoustic cell (3); the output square wave signal of the optical modulator (2) is transmitted to a phase-locked loop (8), and the output signal of the phase-locked loop (8) is respectively input to a square wave signal generator (9) and a digital signal processor (10); the digital signal processor (10) provides a feedback signal for the phase-locked loop (8); the digital signal processor (10) controls the square wave signals generated by the square wave signal generator (9) to be respectively transmitted to the broadband scanning laser light source (5) and the high-speed wavelength inquiry module (7); broadband scanning laser emitted by the broadband scanning laser light source (5) enters the optical fiber microphone (4) after passing through the optical fiber circulator (6); the reflected light of the optical fiber microphone (4) is incident to a high-speed wavelength query module (7) through an optical fiber circulator (6); the digital signal processor (10) reads the spectrum data of the high-speed wavelength query module (7) and then realizes the function of phase-locked amplification; and the computer (11) is connected with the digital signal processor (10) and is used for setting working parameters of the digital signal processor (10) and acquiring, processing and displaying the photoacoustic signal measurement amplitude output by the digital signal processor (10).
2. A scanning laser interference type optical fiber sound wave phase-locked detection method is characterized in that a Fabry-Perot cavity length high-speed synchronous demodulation technology based on an optical fiber scanning laser interferometer is combined with a phase-locked amplification technology to realize high-sensitivity and high-stability detection of weak photoacoustic signals;
the method comprises the following specific steps:
firstly, an optical modulator (2) modulates the intensity of exciting light from a photoacoustic excitation light source (1) and then emits the exciting light into a photoacoustic cell (3); gas molecules in the photoacoustic cell (3) absorb light energy and then generate radiationless transition, and the heat energy generated by the transition enables the gas to generate periodic motion and form sound waves; then the sound wave acts on the optical fiber microphone (4), so that the length of the Fabry-Perot cavity is periodically changed; meanwhile, the phase-locked loop (8) performs phase locking on the square wave signal output by the optical modulator (2) to generate a common-frequency signal and a frequency-doubled signal, wherein the common-frequency signal is input to the digital signal processor (10) to serve as a reference signal of the phase-locked amplifier, the frequency-doubled signal provides a main clock for the square wave signal generator (9), and the digital signal processor (10) controls TTL trigger signals generated by the square wave signal generator (9) to perform synchronous wavelength scanning control and synchronous spectrum sampling control on the broadband scanning laser light source (5) and the high-speed wavelength query module (7) respectively; broadband wavelength scanning laser emitted by the broadband scanning laser source (5) is incident to the optical fiber microphone (4) after passing through the optical fiber circulator (6); interference light reflected from the optical fiber microphone (4) enters the high-speed wavelength query module (7) through the optical fiber circulator (6), and the high-speed wavelength query module (7) collects a spectrum signal of the incident light; after the digital signal processor (10) reads the spectrum data of the high-speed wavelength query module (7) through a high-speed communication interface, the spectrum is subjected to filtering and spectrum domain-frequency domain conversion preprocessing, and then a fast phase demodulation method is adopted to realize dynamic measurement of the Fabry-Perot cavity length; furthermore, the digital signal processor (10) performs cross-correlation operation on the measured Fabry-Perot cavity length value and the reference signal with the same frequency, so that a phase-locked amplification function is realized, and the signal-to-noise ratio of photoacoustic signal detection is improved; the computer (11) sets the working parameters of the 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. The scanning laser interference type optical fiber acoustic wave phase-locked detection method according to claim 2, characterized in that 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 an optical fiber Fabry-Perot interferometer structure, and has higher responsivity to low-frequency sound wave signals; the square wave signal generator (9) generates TTL signals with duty ratio of 50%, and the output frequency range is 10Hz-200 Hz.
4. A scanning laser interference type optical fiber acoustic wave phase-locked detection method according to claim 2 or 3, characterized in that the broadband scanning laser light source (5) is a scanning laser light source with a spectral width larger than 20 nm.
5. A scanning laser interference type optical fiber acoustic wave phase-locked detection method according to claim 2 or 3, characterized in that the high-speed wavelength query module (7) is a module with high precision and fast calibration function of optical wavelength, and works in an external trigger synchronous sampling mode, the sampling frequency is M/N times of the optical modulation frequency, wherein M and N are integers, and M/N is greater than 2.
6. The scanning laser interference type optical fiber sound wave phase-locked detection method according to claim 4, characterized in that the high-speed wavelength query module (7) is a module with high precision and fast calibration function of optical wavelength, and works in an external trigger synchronous sampling mode, wherein the sampling frequency is M/N times of the optical modulation frequency, wherein 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, and the spectral measurement range is 1528nm-1563 nm.
7. The method as claimed in claim 2, 3 or 6, wherein the wavelength scanning range of the broadband scanning laser source (5) is 1528-1563nm, and the scanning speed is 200 Hz.
8. The method as claimed in claim 4, wherein the scanning range of the wavelength of the broadband scanning laser source (5) is 1528-1563nm, and the scanning speed is 200 Hz.
9. The method as claimed in claim 5, wherein the scanning range of the wavelength of the broadband scanning laser source (5) is 1528-1563nm, and the scanning speed is 200 Hz.
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| CN101213438B (en) * | 2005-07-06 | 2010-12-01 | 皇家飞利浦电子股份有限公司 | Photo-acoustic spectrometer apparatus |
| CN101963577B (en) * | 2009-08-10 | 2012-07-04 | 重庆川仪自动化股份有限公司 | Method and system for measuring gas concentration |
| CN106124410A (en) * | 2016-06-08 | 2016-11-16 | 中国科学院合肥物质科学研究院 | Single photoacoustic cell measures the new method of aerosol multi-wavelength absorptance simultaneously |
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