Two measuring methods of wide absorption spectrum by DFB-LDs are presented in detecting water vapor absorption line. One is subsection scanning method, it takes advantage of wide spectrum tuning range by 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 corresponding to specific temperature of DFB-LD, and scanning every section by current modulation for hundreds times and average the data to raise SNR, 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 absorption line-shape function, the absorption line can be described by fitting with partial measured data. The fitting absorption line is fitted well with the measured data, and the square of correlation coefficient (R-square) is no less than 0.99.
Noise from optical and electronic components of a fiber optic gas sensor system using wavelength modulation of the
DFB laser diode in either transmission or reflection mode were investigated. Our experimental results indicate that
reflective type cells give poorer performance due to interference effects from connectors and joints within the fiber
system compare to transmissive type cell. Intensity noise from optical coupler, collimator, was measured and its
affection to sensing system was discussed. In order to raise the signal to noise ratio (SNR), signal processing methods,
such as data average, low pass filter were used and compared. The results would be a useful engineering tool to design high SNR optical gas sensing system.
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.