S. S. Zhang

Application of wavelength scanning for measuring water vapour concentration by distributed laser diode

A technique which takes advantage of distributed feedback laser diode (DFB-LD) wavelength scanning to measure water vapor concentration is presented. Concentration is gotten by peak absorption rate according to Beer-Lambert law and absorption coefficient of water vapor in HITRAN database. Theoretical work on the pressure affection to light intensity absorption rate has been done, a scheme is presented to cope with the affection of overlap of two adjacent lines, it takes advantage of the peak absorption difference between 1368.597nm and 1367.862 nm, and the difference value is used to calculate the water-vapor concentration.

The theoretical and experimental exploration of a novel water-vapor concentration measutrment scheme based on scanning spectrometry

According to the near infrared absorption spectrum, a novel water-vapor concentration detection scheme based on scanning spectrometry is proposed. The experiment device adopts narrowband illuminant distributed feedback laser diode (DFB - LD) as the source and uses the method of taking the average value of multiple stacking signals to improve the signal-to-noise ratio. The accuracy and resolution which are verified by the micro-water generator reach 20ppm and 10 ppm respectively. In addition, the variation of the absorption coefficient of the water-vapor spectral center wavelength with different pressures is analyzed, and based on that, we get the conclusion that the line-shape function can be fitted by Lorenz line-shape and Gaussian line-shape when the pressure is more than 0.4atm and less than 0.005atm respectively. Besides, the method of using the integral area of absorption spectrum to measure gas concentration is elaborated for the first time. Especially when the pressure is higher than atmospheric pressure, this method which could improve the resolution and accuracy is a feasible one to calculate gas concentration in engineering.

Measurement methods in trace water vapor concentration detection system based on direct absorption spectroscopy

In trace water vapor direct absorption spectroscopy, the absorption signal is buried in noise and up and downs of the light intensity, an effective signal extraction method is vital. In the basis of double-beam differential absorption, division method in voltage and an approach based on balanced ratiometer detection (BRD) were studied. Voltage division has an excellent stability to temperature variation, mechanical extrusion and fiber bend loss. As to the BRD method, it has an outstanding self-adjusting capability and it can also avoid an excess phase difference caused by current-to-voltage converting circuit, thus this method has a high sensitivity. Furthermore, a so called dual-peak method based on the differential value of two adjacent absorption lines is introduced, the differential value proved has a linear relation with water vapor concentration, and this method provides a way to measure the concentration at high pressure. In addition, the influence of water vapor inside the optical components has been discussed.

Novel demodulation method for eliminating Rayleigh scattering in Raman distributed temperature sensors using anti-Stokes light only

A novel temperature demodulation method which eliminates the impact of Rayleigh scattering on Raman distributed temperature sensors (RDTS) using anti-Stokes light only is presented. This method utilizes two sections of reference fiber which are placed into temperature control chambers with different temperatures, such that the impact caused by the variation of laser’ power and the Rayleigh scattering is eliminated by the two reference temperatures. In the experiment, the temperature error caused by the Rayleigh scattering was decreased by 0.6℃ and 1.7℃ at 30℃ and 50℃compared with conventional method respectively.

Applications of asymmetric distributed-feedback fiber laser in sensor array system

A 4-element wavelength division multiplexed linear array of asymmetric distributed feedback fiber lasers (DFB-FL), pumped at 1480nm is reported. A very desirable feature of asymmetric DFB-FL is unidirectionality, and relevant principle is presented. Larger output powers are obtained from shorter ends of all the four asymmetric DFB-FLs which are fabricated in our laboratory by phase mask moving method. This obvious advantage has important applications to design sensor array. And furthermore, many experiments are completed to confirm it. Output flatness of the sensor array system presents a good performance with the applications of asymmetric DFB-FLs.