Effect of noise-induced wavelength fluctuation in tunable diode lasers on narrow-linewidth absorption measurements

Tunable diode laser absorption spectroscopy is being widely used to make sensors for diagnostic purposes in various engineering applications. Since the wavelength of many diode lasers used in such sensors is sensitive to the driving current, even noise as small as a few μA_rms in the driving current can cause a wavelength fluctuation of ∼ 0.5 pm, which is large enough to interfere with sensitive absorption measurements. Although these fluctuations are small, they can cause significant systematic error in measured absorption spectra in applications where the absorption line probed is narrow, as is the case for low-density hypersonic flows. As an example, at a pressure of 300 Pa and 297 K, the error in the full width at half-maximum was 6.5% in an absorption spectrum obtained using a system based on a vertical-cavity surface-emitting laser scanned at 10 kHz. This paper analyzes the effect of such systematic errors on measured temperature and velocity and suggests some remedial measures.