RAYTUM PHOTONICS LLC — Department of Energy SBIR Phase I: 34e

Description

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 (<1014 cm-3) working at cryogenic temperature. Researchers at MIT Lincoln Lab reported 120W CW operation at 10.6μm in the 1979 publication and the achieved power level is not exceeded by later efforts. This is a lab experiment applying high magnetic field from superconductive coil and in liquid Helium filled dewar whose size limits the operation time. The achieved isolation was only 23dB limited by the polarization extinction ratio (PER) of wire grid polarizers which also have excessive insertion loss. We propose these innovations to build a ready-to-deploy cryogenic Faraday isolator product at CO2 wavelength: Faraday rotator inside a cryogenic cell based on low vibration cryocooler. The Faraday rotator has a reflective structure of small incident angle (~2°). A sandwiched rotator construction allows efficient heat dissipation from both the faces and the edge of the InSb rod, significantly reduces the radial temperature gradient. All-reflective system layout, minimizing the loss from optical components besides the Faraday rotator and minimize the effective insertion path even at the presence of the bulky cryogenic cell. Ultra-low loss and >35dB PER, broadband ZnSe Brewster polarizer is made possible by novel polarizing coating design. Ge windows of R<0.2% over 10% bandwidth are realized by multiple- layer Antireflection (AR) coating. The coatings feature unique thin film material system transparent at MWIR/LWIR range and has strong bond to substrate and excellent durability against moistures environment. Composite rare earth permanent magnet assembly which can provide >1.5T (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.