Simulation-based comparison of noise effects in wavelength modulation spectroscopy and direct absorption TDLAS

Software-switching between direct absorption and wavelength modulation spectroscopy for the investigation of ADC resolution requirements

“…2.2) has some of its passbands in the spectral region where 1/f intensity noise is strong, specifically around DC and around the lower order harmonics of the scanning frequency f r,das . This idea agrees with the simulation results in [8] that suggested that for a white spectral noise distribution (meaning a suppression of 1/f -noise, e.g., by balanced detection), the advantage of larger signal amplitudes in DAS does result in better performance. One important result of this investigation certainly is the indication that none of the two approaches is intrinsically superior.…”

“…The WMS technique is well described in the literature, for instance in [2,8,12]. Figure 1 briefly summarizes the operation principle.…”

“…Further signal processing and baseline removal yields the absorbance spectrum noise. As described previously [8], this averaging process acts as a filter with a comb-like transmission spectrum with passbands around harmonics of f r,das . Increasing the number of averaged scans corresponds to narrowing these passbands.…”

“…The left column shows the results without the optional FIR filter, the right column with additional filtering (LSB) relative to the bipolar full scale ADC input range is 2/2 11 = 9.77 · 10 −4 , which is more than three times larger than the peak absorption in the line center. The measurement of absorptions much below the value of the LSB becomes possible only because of the averaging of many ramp scans, which can be interpreted as a dithering-averaging scheme, which leads to an enhanced resolution [8].…”

“…In this context, the question arises, what the necessary resolution for both techniques is and if quantization noise is likely to be a limiting factor for concentration determination. Recently, this question has been addressed in simulations of DAS and WMS spectrometers [8], but no measurements have been presented. Now, we have implemented a spectrometer that can switch between the two techniques in order to directly compare DAS and WMS performance.…”