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Non-resonant optical modulation of quantum cascade laser and its application potential in infrared spectroscopy

Based on the nature of ultra-fast carrier life time in semiconductor quantum well, optical modulation of quantum cascade laser offers an unique way to control intersubband transition through interband transition. This method circumvents the problem of parasitic effects associated with electrical modulation, resulting in a high modulation bandwidth. In addition it allows for fast wavelength modulation on standard type quantum cascade lasers by directly injecting charge carriers to laser active region with near-infrared optical excitation. Here, we demonstrate the first infrared spectroscopic measurement conducted with this all-optical modulation approach. Using wavelength modulation spectroscopy, a 1st order derivative spectrum of methanol vapor gas is observed. Optically based wavelength modulation up to 200 MHz is purely induced by pumping the front facet of quantum cascade laser with an intensity-modulated 1550 nm DFB laser. Compared with conventional direct absorption approach, the noise equivalent sensitivity is improved by a factor of 10 by adding optical modulation in a non-optimized system.