Delayed differential detection for absorption spectroscopy recovery

An elaborately designed system has been devoted to the recovery of the line shape function of absorption spectrum. Laser power passing through trace gas has been divided into the real-time and delayed components, and their difference, i.e. the equivalent of the first-order derivative spectrum, is recorded and integrated to reconstruct the absorption line profile. Since the real-time and delayed signals are derived from the only gas cell and photodetector, the elimination of background is more effective, relative to the general used double beam detection that involves two gas cells and photodetectors. Compared with the 1st harmonic detection used in the wavelength modulation spectroscopy, here the generation of derivative spectrum is achieved without modulating the injection current of laser. Additionally, the expensive lock-in amplifier working for wavelength modulation spectroscopy is replaced by a homemade device, which is made of an all-pass filter and an instrumentation amplifier. Therefore, the complexity and cost are significantly reduced, and the stability is improved. For the purpose of validation, recovering of the absorption spectroscopy is carried out using a methane sample at its R(3) absorption line of the 2ν₃ overtone, and the obtained data are found to be in a high agreement with theoretical deductions.