We present a portable broadband photoacoustic spectroscopic system for trace gas or aerosol detection using distributed feedback quantum cascade laser arrays. By sequentially firing 128 lasers, our system acquires a photoacoustic spectrum covering 565 cm-1 (935-1500 cm-1) with a normalized-noise-equivalent-absorption coefficient of 2.5×10-9 cm−1WHz−1/2. In addition to sequential firing mode, the laser source can be operated in multiplexing mode. The firing sequence that determines when and which laser to be activated is programmable, which enables frequency-multiplexing excitation. For demonstration, we modulate 12 lasers at 12 distinct frequencies and a photoacoustic spectrum is acquired within 13 ms.
Realization of microelectromechanically wavelength tunable Fabry-Perot filters using high index-contrast distributed Bragg reflectors (DBRs) comprising GaAlAs/AlOx and Si/SiO2 material system are reported. Due to the broadband nature of these high index-contrast DBRs, the 3dB transmission bandwidth of the cavity resonance is narrow and stable over the tuning range. While the three different sets of GaAlAs/AlOx based filters with different number of DBRs exhibited linewidths of 0.5nm, 2.0nm and 0.47nm with tuning ranges of 59nm, 83nm, and 60nm respectively, the silicon-based filter exhibited a linewidth of 0.3nm and a tuning range of 12nm. Transmission spectra from these devices displayed varying magnitudes of higher order spatial modes were attributed to lensing effect caused by partially oxidized AlGaAs layers within the mirror layers. One of the GaAlAs filters showed a frequency response of 500 KHz at 3dB cutoff point indicating a switching time of 2 microseconds.
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