Mid-infrared Quantum Cascade Laser Gas Detection System

To meet the requirement of mid infrared gas detection based on continuous Quantum Cascade Laser (QCL), a board level QCL driver and a lock in amplifier are developed. High precision direct current (DC) bias signal, low frequency saw tooth scan signal and high frequency sine wave modulation signal are generated by a signal generation circuit to control the laser current and scan/modulate laser output wavelength. The developed lock-in amplifier includes a multiple frequency circuit, an orthogonal conversion circuit, and a data-conversion circuit, which detects the second harmonic (2f) signal from the trace gas absorption signal and gains a high signal-to-noise ratio. In addition, in order to improve the stability and reliability of the system, a linear power supply with high stability and a high-performance protection circuit are designed. A QCL with a wavelength of 4.76 mu m developed by the Semiconductor Institute of Chinese Academy of Science is used as a light source, and function verification and gas detection experiments of the electrical system are carried out. The experimental results show that the linearity of laser driver is as high as 0.006 3% , the long-term current stability is 5.0X10(-5), and the long-term power stability of QCL is 5.07 X 10(-4). The lock-in amplifier has an average detection error on the first harmonic (1f) signal of less than 2.4% and an error on the 2f signal of less than 5.5%. Low-concentration carbon monoxide (CO) detection is carried out, and there is a high linearity (R-2 > 0.99) between the amplitude of 2f signal and CO gas concentration in the concentration range of 0-100 ppm. These results confirm that the developed electrical system has good stability and reliability, assuring a safe and reliable mid-infrared CO gas detection.