| High speed frequency modulation of infrared light source is a crucial technique in the areas of chemical sensing, atmospheric science and optical communication. Over the last twenty years, Quantum Cascade Lasers (QCLs) have become the dominant coherent light source in the mid-infrared region with its compact, powerful and wavelength-tailored features. Due to its unique intersubband lasing mechanism, fast wavelength tuning is quite challenging. Until now, most QCL-based systems still rely on conventional injecting current modulation technique, in which both modulation bandwidth and depth are adversely affected by low RF coupling efficiency and strong parasitic effects.;Recently, a non-resonant all-optical modulation technique was proposed to control QCL emission wavelength, which essentially circumvents the problem of parasitic effects associated with electrical modulation. Within this thesis, its application potential in ultra-sensitive gas sensing is explored through combination with frequency modulation spectroscopy (FMS) technique for the first time. The main work consists of two parts.;In the first part, an optimization of the non-resonant all-optical modulation scheme is presented. By matching the excitation photon energy of near-infrared pump laser with interband energy gap of cascade quantum well structures, the maximal wavelength tuning range under continuous pump is nearly tripled compared with the old scheme. The benefit is attributed to the competing effects of carrier heating and index of refraction change.;In the second part, the optimized all-optical modulation is employed into the application of infrared spectroscopy. A FMS gas tracing system is built upon a traditional direct absorption spectroscopic (DAS) system by adding an optically-induced frequency modulation to the probe QCL light source. Fast frequency modulation up to 200 MHz is induced by front facet illumination with an intensity-modulated 1550 nm telecommunication laser. Using phase-sensitive detection, typical 1st order derivative spectra of several methanol nu8 bands around 10 &mgr;m are clearly observed, which allows the determination of actual modulation index under modulated pump. Compare the modulation spectrum with direct absorption spectrum, all-optically-induced FMS results in a more than 10 times improvement of the system sensitivity, which confirms the huge application potential of all-optical modulation towards sensitive infrared spectroscopy. |
