Design and fabrication of efficient collimation and focusing optics for mid-IR quantum cascade lasers

Abstract

Quantum cascade lasers (QCL) are a new class of solid-state lasers capable of delivering mid-infrared (mid-IR) radiation wavelengths from 3.5 μm to 25 μm. QCLs are finding extensive use in chemical sensing applications due to the abundance of absorption features in the molecular fingerprint region spanned by these sources. They are also being exploited in the field of electro-optical infrared countermeasures. QCL devices exhibit an elliptical emission profile that is highly divergent in the fast axis of the laser waveguide. The far-field profile of the QCL emission, 62° and 32° ± 2° for the fast and slow axes, respectively, places stringent demands on the design of efficient collimation lenses. Because of the current lack of commercially available mid-IR optical components, QCL users must design and fabricate custom micro-optics to efficiently collect, collimate, and focus the QCL emission. In this paper, we report the design, fabrication, and characterization of germanium aspheric collimating and focusing optics designed for mid-IR Fabry-Perot QCLs with an emission wavelength of 8.77 μm. Custom aspheric collimating and focusing lenses with a numerical aperture (NA) of 0.85 and 0.20, respectively, were designed and fabricated using single-point diamond turning. Measurements of the transmitted wavefront error at mid-IR wavelengths showed diffraction-limited performance with Strehl ratios >0.94 and >0.99 for the collimation and focusing lenses, respectively. A beam propagation figure of merit (M 2 ) of 1.8 and 1.2 was measured for the fast and slow axes, respectively, of a Fabry-Perot QCL using a confocal optical system comprised of these lenses.