MHz-rate scanned-wavelength direct absorption spectroscopy using a distributed feedback diode laser at 2.3 µm

This paper demonstrates scanned-wavelength tunable diode laser absorption spectroscopy (TDLAS) with a scan rate up to 2 MHz using a distributed feedback (DFB) diode laser. The 2.3-µm DFB laser was used to probe the R(11) line present in the first overtone band of carbon monoxide (CO). Detailed tuning characteristics of the DFB laser were investigated over a wide range of scan rates from 20 kHz to 2 MHz. The tuning range was found to decrease exponentially with the scan rate below 1 MHz, and then slightly decrease up to 2 MHz. By applying a sinusoidal waveform to the injection current, the phase shift (ϕ) between intensity and frequency modulations was measured to be 1.11π at the scan rate of 20 kHz, which increases to 1.61π at 2 MHz. Due to the significant variation of ϕ with the scan rate, the position of the target absorption profile on the transmitted laser intensity curve must be adjusted accordingly based on the simultaneously recorded etalon signal. All the measurements obtained at different scan rates show excellent consistency in the CO spectrum. This study reveals that MHz-rate scanned-wavelength direct absorption spectroscopy can be developed for ultrafast gas sensing that may find promising applications in many fields.