Low Insertion Loss High Power Cryogenic Faraday Isolator for CO2 Wavelength

The fundamental challenge for a high power Faraday isolator at CO2 wavelength is the high absorption coefficient of materials with significant magneto-optic Faraday effect at the upper MID IR wavelength range. A few watts of CO2 laser beam will cause thermal runaway for Faraday isolator at room temperature. There were extensive research works on Faraday material at CO2 wavelength back in the 1970’s. There have been continued efforts for developing Faraday isolator at CO2 wavelength in the following decades. No breakthrough on better Faraday material was reported. N-doped InSb is the most promising Faraday rotator material for general purpose Faraday isolators at CO2 wavelength. Free-carrier type of InSb with high doping concentration greater than 1016 cm-3 has high value of Verdet constant and is typically used for commercial MID-IR isolators. However the figure of merit of free carrier InSb at optimized doping concentration is below the minimum requirement for a Faraday rotator having less than 10% insertion loss. The only viable approach of having a Faraday isolator at CO2 wavelength compliant with the topic specifications is Interband InSb rotator of low doping centration (35dB PER, broadband ZnSe Brewster polarizer is made possible by novel polarizing coating design. Ge windows of R1.5T (15KG) uniform magnetic field in ϕ20mm cross section. Raytum Photonics will establish COMSOL model to analyze the thermal dynamics of the cryogenic rotator. We will design and demonstrate the cryogenic Faraday Isolator with the proposed innovative optical components. A 9.15μm QCL laser diode will be used to establish baseline of lower power performance and then we will collaborate with national lab for testing with high power CO2 lasers at pulsed and CW operation. During the Phase 2, we are going to deliver specs conforming units for CO2 wavelength as well as establish a manufacturing process of building the high power isolators in volume. All development work will conduct in Raytum Photonics facility. We are also going to collaborate with Brookhaven National Lab to have a preliminary test with the isolator we develop in Raytum Photonics facility.