Wide absorption spectrum measuring methods by DFB-LDs in water vapor detection system

Y N Liu,W Wei,X Z Liu,W J Wang,S Jiang,G P Lv,Q Wang,Z L Wang,J Chang,C G Zhu,J Lian,S S Zhang,B N Sun,Z Liu

doi:10.1007/s13320-013-0152-1

Abstract

Two measuring methods of the wide absorption spectrum by distributed feedback laser diodes (DFB-LDs) are presented in detecting the water vapor absorption line. One is the subsection scanning method, and it takes advantage of the wide spectrum tuning range by the temperature modulation and fast spectrum tuning speed by current modulation. Specifically, this method is realized by dividing a target spectral region into several sections which correspond to the specific temperature of DFB-LD, scanning every section by current modulation for hundreds times, and averaging the data to raise the signal to noise ratio (SNR), then combining all sections to get the whole spectrum. An accuracy of 10 ppmv had been obtained in the measurement of water vapor with a 10-cm path length by this method. Another is data fitting method, based on the absorption line-shape function; the absorption line can be described by fitting with partial measured data. The fitting absorption line was fitted well with the measured data, and the square of correlation coefficient (R-square) was no less than 0.99.

Highlights

SF6 performs excellent in insulation and arc-extinction, given such advantages, SF6 gas insulated switchgears (GIS) are widely applied in the electrical power systems
The adopted distributed feedback laser diodes (DFB-LDs) wavelength tuning range is less than 320 pm under current modulation, obviously, it is impossible to scan the whole absorption line by current modulation; temperature modulation can tune a wider range of 1800 pm, but its modulation frequency is only about 1/30 Hz, which is too low that common filters can hardly fit it, the effective average process would cost a overlong time, temperature modulation is not a proper way either
We propose two measuring methods of the wide absorption spectrum by commercially available DFB-LDs for water vapor concentration detection

Summary

Introduction

SF6 performs excellent in insulation and arc-extinction, given such advantages, SF6 gas insulated switchgears (GIS) are widely applied in the electrical power systems. In these applications, it is vital to ensure the SF6 purity, because trace water vapor inside could affect the insulation and the resultants corrode the equipments, both could cause serious accidents [1, 2]. The traditional TDLAS technique usually drives a distributed feedback laser diode (DFB-LD) scanning over a whole gas absorption line and gets the peak value in the electronic method, only in a low gas pressure condition it works well, where the absorption line is not Article type: Regular. The adopted DFB-LD (such as WSLS-137010C1424-20) wavelength tuning range is less than 320 pm under current modulation (corresponding to a 60 mA current range), obviously, it is impossible to scan the whole absorption line by current modulation; temperature modulation can tune a wider range of 1800 pm (corresponding to a 20-°C temperature range), but its modulation frequency is only about 1/30 Hz, which is too low that common filters can hardly fit it, the effective average process would cost a overlong time, temperature modulation is not a proper way either

Methods

Findings

Discussion

Conclusion