Basic Principle of TDLAS

TDLAS System Hardware for Water Vapor Measurement 

For TOLAS measurement in PEMFCs, the laser diode as the transmitter and the photodiode as the detector are used. In a typical experimental setup with TOLAS as Basu et al. reported, the laser beam from the distributed feedback diode (DFB) laser at a wavelength of 1470 nm was split using a bifurcated optical fiber. One leg 

TDLAS Signal Calibration for Measurement of Water Vapor Concentration 

In these WMS signals, the wavelength crosses the water absorption feature twice for each modulation cycle. Therefore, when water vapor is in the optical path, the amplitude modulation of the received laser power contains a periodic component having precisely twice the wavelength modulation frequency and fixed in phase relative to the wavelength modulation. 

In WMS, phase-sensitive (i.e. lock-in) amplification demodulates the periodic signal with a very narrow electrical bandwidth to precisely measure the RMS amplitudes of the fundamental sinusoid and its second harmonic component, averaged over a period of time equal to the inverse of the electrical bandwidth. These are called the If and 2/ signals and each amplitude is expressed by the following equations, respectively, 

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