High efficiency, multi-band mid and long infrared laser beam combining based on grating-polarization prism cascaded structure

Mid- to long-wave infrared lasers have found extensive applications in medical diagnostics, environmental monitoring, and biosensing. These applications require wide spectral coverage, high brightness, and compact size. In this paper, a spectral-polarization cascaded beam combining structure was proposed. Blazed grating and polarization prism were selected as the spectral and polarization combiners, respectively. Both were optimized and fabricated to provide high efficiency in an ultra-broad spectral band. It enabled the coaxial beam combination of three individual quantum cascade lasers (QCLs) with 4.5 μm, 7.5 μm, and 9.2 μm central wavelengths covering mid-to long-wave infrared. The output beams of the 7.5 μm- and 9.2 μm-QCLs were firstly spectrally combined by a geometrically optimized blazed grating, which offered over 90% diffraction efficiency across a broad spectral range from 6.5 to 12.5 μm. After spectral beam combining, the dual-band long infrared laser was polarization-combined via an Hg₂Cl₂ prism with the 4.5 μm-QCL beam. Owing to the crystal's ultra-broad transmission spectrum (0.5 ∼ 15 μm) and high birefringence, the polarization prism exhibited a beam-combining capability spanning the mid- to-long-wave infrared band, which can not be possessed by all existing prisms. Through prism cutting angle design, the reflectivity of 92.2% was obtained by the total internal reflection for the dual-band long-wave infrared laser. Meanwhile, the mid-infrared QCL was transmitted at the Brewster angle. Including both grating and prism, the three-band cascaded beam combining configuration provided an overall beam combining efficiency of 87.3%. By carefully arranging the beam waist positions and the divergent angles of the three incident lasers, the spectral dispersion induced beam quality degradation was partially compensated. A good combined beam quality with M² of 2.07 (slow axis) and 1.57 (fast axis) was obtained. Benefit of the high damage threshold of the grating (250 W/cm² at 1064 nm@CW, 3.5 J/cm²@200 ns pulse) and the prism (3.76 J/cm² at 1064 nm@10 ns pulse, 8.06 J/cm² at 3.5 μm@15 ns pulse), the proposed spectral-polarization cascaded beam combining structure is promising to achieve multi-band, high-power mid- and long-wave infrared laser output.