Research progress of optical H2O sensor with a DFB diode laser

Abstract

In the field of near infrared H 2 O sensing, the acquisition of the absorption signal usually is from a noisy background, thus it is important to adopt an effective signal demodulation method. This study introduced the research progress in the field of trace water vapor detection, covering different individual gas detection techniques. On the basis of the conventional double-beam differential absorption, the division method in voltage and the dual-peak method based on the differential value of two adjacent absorption lines have been studied. Voltage division has an excellent stability to temperature variation, mechanical extrusion, and fiber bend loss. The dual-peak method proved a linear relation with the water vapor concentration, and this method provided a way to measure the concentration at high pressure. Furthermore, the so called balanced ratiometer detection (BRD) was introduced. It has an outstanding self-adjusting capability, and it can also avoid an excess phase difference caused by the current-to-voltage converting circuit, thus this method has a high sensitivity. In addition, the second harmonic technique applied to gas detection was introduced, and for the high-frequency modulation via driving current, 1/f was suppressed apparently; as a result, this technique realized a better sensitive detection by one to two orders of magnitude.

A coal mine multi-point fiber ethylene gas concentration sensor

“…Given that parameters such as the pressure, line intensity, and length of the gas absorbing light path are known, the integral value of ) ln( 0 I I  in the frequency domain is substituted into (3), and the final gas concentration can then be obtained [10].…”

IR detection module with integrated real-time FIR filter implemented in FPGA

“…For example, a stabilization technique with lock-in amplification can be used to This work was supported by Military University of Technology grant no. UGB 22-871. extract a signal from the noise, but the modulation frequency and phase must be known [7,8]. Moreover, the lock-in instrumentation is complex and expensive [9].…”