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Tunable diode laser absorption spectroscopy (TDLAS) is a technique measuring the concentration of trace-gas, the gas concentration is in the sub-ppm (part-per-million) range. Experiment results show that the main part of system noise is interference fringes originating from multiple reflections between surfaces in optical components, so-called etalon effects. This work investigates the basis principle of the measurement of trace-gas concentration using wavelength modulation spectroscopy (WMS) technique and analyses etalon effects from different types of optical surfaces. Fluctuations in temperature or vibrations of the etalon cause the interference fringes to shift in wavelength, and the interference fringes originated from short etalons within laser or photodetector cannot be removed by subtraction of a zero baseline using wedged or dithered components. It is shown that although analytical signal, SAS,nf, for short etalons, is decreasing with increased detection order, the signal-to-background ratio (SAS/SBG)nf is improved significantly. It is more advantageous to detect higher even harmonic signals, e.g. the 4th or 6th, than 2nd to reduce or eliminate the interference fringes originated from short etalons within laser or photodetector.