Programmable gate array WM-TDLAS gas detection system design and application

Zhang Xin; Qiu Haifeng; Lan Jiaqi; Hu Qin; Zhang He

doi:10.12086/oee.2024.240022

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Zhang X, Qiu H F, Lan J Q, et al. Programmable gate array WM-TDLAS gas detection system design and application[J]. Opto-Electron Eng, 2024, 51(4): 240022. doi: 10.12086/oee.2024.240022

Citation:

Zhang X, Qiu H F, Lan J Q, et al. Programmable gate array WM-TDLAS gas detection system design and application[J]. Opto-Electron Eng, 2024, 51(4): 240022. doi: 10.12086/oee.2024.240022

Programmable gate array WM-TDLAS gas detection system design and application

State Key Laboratory of High Power Semiconductor Laser, Changchun University of Technology, Changchun, Jilin 130022, China

Fund Project: Project supported by Science and Technology Research Project of Jilin Provincial Department of Education (JJKH20240916)

More Information

^*Corresponding author: zhanghe@cust.edu.cn

Received Date 02 January 2024

Revised Date 27 February 2024

Accepted Date 27 February 2024

Published Date 25 April 2024

Abstract

Abstract

An FPGA-based field programmable gate array (FPGA) and wavelength-modulated tunable diode laser absorption spectroscopy (WM-TDLAS) technique have been combined to develop a programmable gate array WM-TDLAS CO₂ concentration detection system. Leveraging the programmable nature of FPGA chips, a digital lock-in amplifier (DLIA) with signal acquisition and modulation, as well as harmonic demodulation functions, was designed to meet the application requirements. To validate its performance, harmonic extraction tests, Q-factor assessments, and anti-noise experiments were conducted. The results revealed a linearity of 99.99% for the target frequency extraction and a Q-factor of up to 45. In the harmonic extraction experiments for signals with different signal-to-noise ratios, the maximum relative error in the mean value was only 0.91% when the signal-to-noise ratio was 43 dB. Using a distributed feedback laser with a center wavelength of 1572 nm as the light source, covering the absorption line at 6360 cm⁻¹, and utilizing an effective optical path of 14 m in a dense multi-pass gas absorption cell, gas concentration detection experiments were carried out. The system demonstrated a fitting linearity of 99.982% between the detected concentration and the amplitude of the second harmonic. By increasing the scanning wavelength frequency, the system was capable of obtaining concentration values in 0.1 seconds. The Allan variance results showed that the detection limit of the system was 1.86 ppm when the integration time was 44 seconds. The experimental results indicate that the developed WM-TDLAS detection system based on an FPGA array features high detection accuracy, rapid response, strong stability, and miniaturization, making it suitable for real-time concentration monitoring in practical applications.
- spectroscopy /
- programmable gate array /
- wavelength modulation spectrum /
- digital lock-in amplifier /
- gas detection

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References

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Overview

Overview

In view of the above situation, this paper designs a digital lock-in amplifier (DLIA) with high-frequency, long-bit-wide signal acquisition, signal modulation, and harmonic demodulation functions based on FPGA, and applies it to the carbon dioxide (CO₂) concentration detection system of WM-TDLAS technology. For the concentration detection system, the hardware parameters of the lock-in amplifier and the key data processing algorithms were optimized. 1) The high-low-pass cooperation scheme is used to filter out the low-frequency clutter signal in the input data to improve the signal-to-noise ratio of the system. To reduce resource consumption and optimize system performance, a Cascaded Integeator-Comb Filter (CIC) filter is designed to perform downsampling of signals with a high sampling rate and reduce the required order of low-pass filter to achieve a lower cut-off frequency. 2) Direct Digital Frequency Synthesis (DDS) technology based on external input clocks is introduced to generate high-frequency synchronous clock reference signals, which can reduce the distortion and harmonic peak jitter caused by clock offset jitter in non-homologous digital systems. 3) The FPGA generates the laser scan signal required for wavelength modulation and the high-frequency clock required for the analog signal acquisition circuit, simplifying the peripheral circuit. This paper designs a digital quadrature lock-in amplifier based on programmable logic gate array (FPGA) programmable features. Harmonic signal demodulation is realized, and the frequency of the high-frequency laser modulation signal can be tuned. The quadrature lock-in amplifier can effectively extract the weak signal in the background noise, through the different signal-to-noise ratio of the signal to be measured under the harmonic extraction and Q value experiments. The signal-to-noise ratio of the signal under test is 43 dB with a maximum error of only 0.91%, and the Q value is 45, indicating that the lock-in amplifier has good frequency response and noise immunity. To test the performance of the designed digital quadrature phase locker in the WM-TDLAS detection, build based on the WM-TDLAS carbon dioxide experimental system to carry out the concentration detection, stability, and response time test, the amplitude of the harmonic signal extracted by the lock-in amplifier and the CO₂ concentration has a good linear relationship (R² = 0.99982), the system acquires the concentration value of the time of 0.1 s, Allen's square indicates that the detection limit is 1.86 ppm. The experimental results show that the WM-TDLAS detection system based on FPGA digital lock-in amplifier has the advantages of digital signal modulatability, high detection sensitivity, and strong noise immunity, and can be used for real-time monitoring of concentration in practical applications.