CN111948173A - TDLAS signal processing system based on acousto-optic correlation technique - Google Patents
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Abstract
The invention discloses a TDLAS signal processing system based on an acousto-optic correlation technology, which comprises a TDLAS system (1), a preposed amplifier (2), a first laser (3), a first time integral acousto-optic correlator (4), a first photoelectric detector array CCD (5), a first analog-to-digital converter (6), a DSP digital post-processing system (7), a signal generator (8), a second laser (9), a second time integral acousto-optic correlator (10), a second photoelectric detector array CCD (11) and a second analog-to-digital converter (12), so that the TDLAS signal processing system based on the acousto-optic correlation technology is formed. The TDLAS signal processing system based on the acousto-optic correlation technology is simple in structure and suitable for the fields of optoelectronic systems and optoelectronic information processing.
Description
Technical Field
The invention relates to the field of photoelectric systems and photoelectric information processing, in particular to a TDLAS signal processing system based on an acousto-optic correlation technology.
Background
Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology is a spectrum absorption technology, which combines tunable characteristics of a semiconductor laser and absorption characteristics of gas molecules to energy light with specific wavelength, and can simultaneously detect multi-parameter flow field information of gas such as concentration, temperature, pressure, flow rate and the like by virtue of high sensitivity, high reliability, high spectral resolution, short response time, non-contact and the like, and is currently widely applied to the fields of atmospheric environmental pollution monitoring, industrial production, agricultural production, chemical industry and the like. From the detection methods, the TDLAS technique can be classified into direct absorption detection, differential absorption detection, and harmonic detection. Compared with other detection methods, the harmonic detection method has higher detection sensitivity, better selectivity and stability, and is more suitable for detecting gas with weaker absorption spectrum lines. The harmonic detection is combined with a wavelength modulation technology, the characteristic spectral line of the gas to be detected is scanned by using the signal modulated into high frequency, and the harmonic component carrying the gas concentration information is extracted by the digital phase-locked amplifier, so that the gas concentration is calculated.
The phase-locked amplifier is an important device in a TDLAS system, and according to the relevant detection principle, a signal to be detected and a reference signal are subjected to relevant operation through the phase-locked amplifier to obtain a second harmonic signal. In the process of scanning the characteristic spectral line of the measured gas, a first harmonic signal and a second harmonic signal corresponding to each point are demodulated. The sampling frequency in the demodulation algorithm is at least twice the modulation frequency according to the nyquist sampling theorem. In an actual hardware system, the sampling frequency should be increased appropriately to obtain higher detection accuracy, which results in the problems of slow demodulation speed and slow response speed. Also, 1/f noise is generated in the TDLAS gas concentration detection system, which is caused by the potential caused when the two semiconductors are in contact. Since the magnitude of 1/f noise is inversely proportional to the modulation frequency, the influence of 1/f noise can be reduced by setting a higher modulation frequency. Therefore, a new method is needed to achieve the purpose of increasing the demodulation speed and reducing the 1/f noise.
The acousto-optic information processing technology is a novel subject developed in recent ten years, and the important application of the acousto-optic signal processing technology is acousto-optic frequency measurement analysis technology and acousto-optic related processing technology. The acousto-optic signal processing technology has large instantaneous bandwidth, large dynamic range, high processing gain and high-speed parallel processing capability, so that the acousto-optic signal processing technology has wide application prospect in the aspects of real-time signal detection and analysis in electronic warfare, spoofing interference, communication countermeasure, radar ranging and direction finding and the like. The acousto-optic correlator realizes the correlation operation of space signals by using acousto-optic interaction, and can obtain high-speed real-time operation and high correlation gain.
The characteristics of acousto-optic correlation technology are considered, the system is applied to demodulation of a first harmonic signal and a second harmonic signal, a measurement result equivalent to a digital phase-locked amplifier is achieved, detection of gas concentration is achieved, and the system can meet the requirements of high demodulation speed, high precision and 1/f noise reduction.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the time-integration acousto-optic correlator is used for replacing a digital lock-in amplifier to demodulate a first harmonic signal and a second harmonic signal so as to detect the gas concentration.
The technical scheme adopted by the invention for solving the technical problems is as follows: a TDLAS signal processing system based on acousto-optic correlation technique. The device comprises a TDLAS system, a preamplifier, a laser, a time integral acousto-optic correlator, a signal generator, a photoelectric detector array CCD, an analog-to-digital converter and a DSP digital post-processing system.
Wherein, firstly, the tunable semiconductor laser in the TDLAS system is modulated by the modulation signal generated by the signal generator. And then the TDLAS system obtains an electric signal carrying gas concentration information, and the electric signal is pre-amplified and respectively sent into the two lasers to modulate the light intensity of the lasers. The reference signal generated by the signal generator is then used to modulate a radio frequency carrier, which is fed into the acousto-optic device in the time-integrating acousto-optic correlator.
The harmonic signal demodulation unit is composed of a signal generator, a reference signal, a 2f frequency multiplication signal, a laser, a time integral acousto-optic correlator and a photoelectric detector array CCD, and a first harmonic signal and a second harmonic signal are demodulated. The odd harmonic signal is zero at the center of the spectral line, and the even harmonic signal is maximum amplitude at the center of the spectral line. Therefore, the first harmonic signal is commonly used to achieve the locking at the center of the spectral line, and the second harmonic signal to achieve the measurement of the gas absorption signal.
The analog-to-digital converter and the DSP digital post-processing system form a digital signal post-processing unit. The first harmonic signal and the second harmonic signal obtained by demodulating the photoelectric detector array CCD are converted into digital signals through analog-to-digital conversion and sent to a digital signal post-processing system. And then, calibrating the amplitude of the second harmonic signal, finding the relation between the amplitude of the second harmonic component carrying the gas information and the gas concentration, and finally realizing the detection of the gas concentration.
The system can be applied to the field of signal processing such as acousto-optic spectrum analysis and acousto-optic signal processing.
The basic principle applied by the invention relates to the following two aspects:
principle of wavelength modulation method
In the conventional TDLAS detection technique, laser is directly transmitted through a gas to be detected according to the lambert-beer law, and the gas concentration is calculated through a series of parameters such as light intensity detection, line type selection, line intensity and the like, and the measurement process is called as a direct absorption method. The direct absorption method has the characteristics of convenient measurement process, simple calculation, low cost and the like, but in actual operation, because signals acquired by the photoelectric detector are very weak, the signals are often influenced by a large amount of noise, and finally the accuracy of a measurement result is not high. Analysis of these system noises can find that these system noises are mostly low-frequency noises such as background noise, laser noise and photodetector noise. Therefore, the TDLAS technology is combined with the wavelength modulation technology, the modulation wave can well eliminate the interference of system signals by modulating the signals, and when the position of the wavelength of the strongest absorption peak of the gas to be detected is superposed with the output wavelength of the laser, the required absorption spectrum can be obtained. Compared with the direct absorption method, the method has higher sensitivity and obviously improves the signal to noise ratio.
② principle of acousto-optic correlation detection
Mathematically, the cross-correlation function of the two signals a (t) and b (t) is defined as follows:
R12(τ)＝∫tA(τ)B(t-τ)dt (1)
the integral variable in equation (1) is the time t and the variable of the correlation function is the time delay τ.
Since diffracted light is a function of time and time delay. Therefore, the acousto-optic correlation method can be used for completing the multiplication of the two signals within the time delay range, and then the product is subjected to time integration. Therefore, the acousto-optic architecture that can implement equation (1) is referred to as a time-integrating acousto-optic correlator.
In the invention, an optical signal is converted into an electric signal through a photoelectric detector in a TDLAS system, and then the electric signal is divided into two paths which are respectively sent to a harmonic signal demodulation unit, wherein the input of one path of signal is the electric signal carrying gas information and a reference signal generated by a signal generator; the other path of signal is input by an electric signal carrying gas information and a double frequency signal of a reference signal, and then the demodulation of a first harmonic signal and a second harmonic signal is realized through a photoelectric detector array CCD.
The invention has the beneficial effects that:
(1) the invention can achieve the equivalent measurement result with the digital phase-locked amplifier by applying the acousto-optic correlation technology to the demodulation of the fundamental frequency signal and the second harmonic signal.
(2) In the system, the time integral acousto-optic correlator is adopted for demodulating harmonic signals, so that the traditional digital phase-locked amplifier can be replaced. Because the traditional digital phase-locked amplifier has the problems of large calculated amount, low demodulation speed and the like, the system of the invention realizes the fast demodulation of the first harmonic signal and the second harmonic signal under high frequency by introducing the time integral acousto-optic correlator. Compared with the traditional demodulation mode of the digital phase-locked amplifier, the digital phase-locked amplifier reduces 1/f noise and greatly improves demodulation speed.
(3) The system has simple structure and is easy to adjust.
Drawings
Fig. 1 is a schematic diagram of a TDLAS signal processing system based on acousto-optic correlation.
FIG. 2 is a block diagram of a DSP digital post-processing flow.
The system comprises a TDLAS system 1, a preposed amplifying system 2, a first laser device 3, a first time integral acousto-optic correlator 5, a first photoelectric detector array CCD 6, a first analog-to-digital converter 7, a DSP digital post-processing system 8, a signal generator 9, a second laser device 10, a second time integral acousto-optic correlator 11, a second photoelectric detector array CCD 12 and a second analog-to-digital converter.
Detailed Description
The invention will be further described with reference to the accompanying drawings.
As shown in fig. 1, includes: 1. TDLAS system, 2, preposition amplification, 3, a first laser, 4, a first time integral acousto-optic correlator, 5, a first photoelectric detector array CCD, 6, a first analog-to-digital converter, 7, a DSP digital post-processing system, 8, a signal generator, 9, a second laser, 10, a second time integral acousto-optic correlator, 11, a second photoelectric detector array CCD, 12 and a second analog-to-digital converter.
The tunable semiconductor laser in the TDLAS system is driven by a driving signal generated by a signal generator, wherein the driving signal is formed by superposing a low-frequency sawtooth wave and a high-frequency sine wave. The tuned laser enters a collimator and a gas absorption cell to scan the characteristic spectral line of the detected gas, and the laser with attenuated energy enters a photoelectric detector to be converted into an electric signal. After pre-amplification, the electric signals carrying gas information are respectively sent to a harmonic signal demodulation unit for signal demodulation, and the signals demodulated by a photoelectric detector CCD are respectively a first harmonic signal and a second harmonic signal. And then the demodulated first harmonic signal and second harmonic signal are converted into digital signals through an analog-to-digital converter, and finally the digital signals are sent to a DSP digital signal post-processing system for analysis and processing.
According to the detection principle, the concentration of the detected gas and the finally output second harmonic signal are in a direct proportion relation. The relational expression of the two is calculated and substituted into the related data to obtain the concentration of the measured gas.
The DSP digital post-processing system is realized by the following steps:
the first harmonic signal and the second harmonic signal which are demodulated are converted into digital signals through an analog-to-digital converter, the modulation amplitude of the first harmonic coefficient is zero at the central frequency, the modulation amplitude of the second harmonic coefficient is maximum at the central frequency, the two harmonic signal graphs are corresponded, and the maximum modulation amplitude of the second harmonic coefficient is correspondingly found by means of the central frequency position of the first harmonic. Due to the current sweep, different wavelengths of power have an effect on the detection. After the influence is eliminated, the concentration of the gas to be measured can be obtained according to the calculation relation of each harmonic component and the gas concentration.
Claims (5)
1. A TDLAS signal processing system based on acousto-optic correlation technology comprises a TDLAS system, a preamplifier, a laser, a time integral acousto-optic correlator, a signal generator, a photoelectric detector array CCD, an analog-to-digital converter and a DSP digital post-processing system, and therefore the TDLAS signal processing system based on acousto-optic correlation technology is formed.
Wherein, firstly, the tunable semiconductor laser in the TDLAS system is modulated by the modulation signal generated by the signal generator. And then the TDLAS system obtains an electric signal carrying gas concentration information, and the electric signal is pre-amplified and respectively sent into the two lasers to modulate the light intensity of the lasers. The reference signal generated by the signal generator is then used to modulate a radio frequency carrier, which is fed into the acousto-optic device in the time-integrating acousto-optic correlator.
The analog-to-digital converter and the DSP digital post-processing system form a digital signal post-processing unit. The first harmonic signal and the second harmonic signal obtained by demodulating the photoelectric detector array CCD are converted into digital signals through analog-to-digital conversion and sent to a digital signal post-processing system. And then, calibrating the amplitude of the second harmonic signal, calculating a relational expression between the amplitude of the second harmonic component carrying the gas information and the gas concentration, and finally realizing the detection of the gas concentration.
In the TDLAS signal processing system based on the acousto-optic correlation technology, a time-integration acousto-optic correlator is used for replacing a digital lock-in amplifier to complete the demodulation of a first harmonic signal and a second harmonic signal, and further the detection of the gas concentration is realized.
The TDLAS signal processing system based on the acousto-optic correlation technology applies the acousto-optic correlation detection principle and realizes the fast demodulation of the first harmonic signal and the second harmonic signal under high frequency by introducing the time integral acousto-optic correlator. Compared with the traditional demodulation mode of the digital phase-locked amplifier, the digital phase-locked amplifier reduces 1/f noise and greatly improves demodulation speed.
2. The acousto-optic correlation based TDLAS signal processing system as claimed in claim 1, wherein: the signal generator, the reference signal, the 2f frequency doubling signal, the laser, the time integral acousto-optic correlator and the photoelectric detector array CCD form a harmonic signal demodulation unit, and a first harmonic signal and a second harmonic signal are demodulated. The locking of the center of the spectral line is realized through the first harmonic signal, and the measurement of the gas absorption signal is realized through the second harmonic signal.
3. The acousto-optic correlation based TDLAS signal processing system as claimed in claim 1, wherein: the time integral acousto-optic correlator is used for replacing a digital phase-locked amplifier, and is applied to the demodulation of a first harmonic signal and a second harmonic signal, so that the equivalent measurement result of the digital phase-locked amplifier is achieved, the detection of the gas concentration is realized, and the system can simultaneously meet the requirements of high demodulation speed, high precision and 1/f noise reduction.
4. The acousto-optic correlation based TDLAS signal processing system as claimed in claim 1, wherein: the analog-to-digital converter and the DSP digital post-processing system form a digital signal post-processing unit. The first harmonic signal and the second harmonic signal obtained by demodulating the photoelectric detector array CCD are converted into digital signals through analog-to-digital conversion and sent to a digital signal post-processing system. And according to the fact that the modulation amplitude of the first harmonic coefficient is zero at the central frequency and the modulation amplitude of the second harmonic coefficient is maximum at the central frequency, two harmonic signal graphs are corresponding to each other, and the maximum modulation amplitude of the second harmonic coefficient is correspondingly found according to the central frequency position of the first harmonic. Then, the amplitude of the second harmonic signal is calibrated, the relational expression between each second harmonic component and the gas concentration is calculated, and the related data are substituted, so that the concentration of the gas to be measured can be obtained.
5. The acousto-optic correlation based TDLAS signal processing system as claimed in claim 1, wherein: the system can be applied to the field of signal processing such as acousto-optic spectrum analysis and acousto-optic signal processing.
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- 2020-08-12 CN CN202010805101.1A patent/CN111948173A/en active Pending
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