Compact, mid-infrared laser source for remote sensing of gas effluents

Abstract

Remote sensing of gas effluents in the mid-IR wavelength region from 2 microns to 5 microns is preferred due to strong molecular absorption features (10 to 100 times stronger than in the near-IR) and high-transparency atmospheric windows. Currently, long-range mid-IR remote sensing is inhibited by the lack of suitable laser sources. As a result, frequency conversion in nonlinear optical materials has emerged as a powerful method to produce high-power, tunable, mid-IR light. However, compact, high-power narrowband conversion systems suitable for long-range mid-IR spectroscopy are not commercially available. This thesis describes the development of a high-power, narrowband, tunable, compact, mid-IR laser source for long-range remote sensing of gas effluents. Frequency conversion into the mid-IR is achieved by use of a high-peak-power, compact pump laser and optical parametric generation (OPG) in periodically poled nonlinear crystals. This mid-IR laser system was designed and developed for remote sensing at ranges up to and exceeding 100 m in a compact form factor. Such a laser will fill the current scientific need for a hand-held mid-IR laser source capable of long-range mid-IR spectroscopy. Based on theoretical models and experimental demonstrations a compact mid-IR laser source was developed that emits > 1 mJ broadband pulses in the mid-IR. To narrow the linewidth of the broadband OPG output, optical parametric amplification was demonstrated through seeding of the OPG process with a narrowband, continuous-wave, distributed feedback (DFB) laser. Seeding efficiencies exceeding 35% were demonstrated for 1 mJ of output energy, and efficiencies exceeding 65% were demonstrated at lower energies when the pump beam was spatially filtered. The linewidth of the narrowed mid- IR output was inferred to be < 350 MHz based upon heterodyne measurements conducted at the signal wavelength in the near-IR. This linewidth is well within the FWHM bandwidth of typical mid-IR atmospherically broadened molecular absorption features. The demonstrated mid-IR energies and laser linewidths are predicted to be sufficient for detection of low concentration gas effluents (< 1 ppm) at ranges exceeding 100 m. The developed mid-IR laser source was used to successfully demonstrate differential detection of carbon dioxide (CO 2).