High Power Broad Band Isolator for kilowatt-class ultrafast laser systems

With the accelerator applications in many areas demanding higher fluxes of particles and radiation, there are increasing requirements on developing high average power, high peak power, large-aperture low-loss Faraday isolators capable of handling kW-class, sub-picosecond ultrafast laser systems at either Yb or Tm wavelength. Tremendous effort has been made in many DOE national labs like Lawrence Berkeley National Lab (LBNL) and Lawrence Livermore National Laboratory (LLNL) to attain the pulse energy toward Joule level with repetition rates at and beyond kHz. The reliable operation of optical components like isolator becomes extremely important in building kilowatt-class ultrafast laser systems. Raytum Photonics proposes an innovative KW class high power isolator platform based on terbium gallium garnet (TGG) crystal for Yb wavelength and Bismuth iron garnet (BIG) for Tm wavelength, which features: Liquid cooled isolator outer case, serving both as the frame of the isolator body as well as heat sink to dissipate the heat from the optical elements and light of the rejected polarization. Specially coated Brewster plates are used as both input or output polarizer, being able to handle power density better than 20kW/cm2 (CW) and 2GW/cm2 (Pulsed) respectively. Built-in rejected light trapper removed the need for escape window for easier layout of laser system. Composite Neodymium rare earth magnets formation for large aperture and uniform field. Built-in effective heat dissipation path from inner rotator material to isolator case. Introduction of compensator for the thermal lensing effect. A mean to improve the temperature uniformity at the boundary condition of the rotator materials, therefore to minimize the heat induced birefringence. Including optical rotator (such as quartz rotator) to compensate for wavelength dependency of the Faraday rotator for the wide isolation bandwidth which is desirable for the ultrafast laser operation. Dual rotator stage. We identify that facets of the rotator material are the most vulnerable area exposed to high laser power density. For both TGG and BIG rotator, we propose an innovative defect-free antireflection coating (AR) prepared by Atomic Layer Deposition (ALD) technique to serve for facet passivation & protection. During the Phase I, we are going to establish numerical model to analyze the isolation, insertion loss, heat dissipation pattern and magnetic field. The model will become a tool for detailed isolator design and failure analysis, design and build key components, demonstrating the components and assembled unit can handle higher power level up to 3KW (CW) by extrapolating experimental data in lab. We will also demonstrate the system can handle up to 100KW by scaling up the beam size and design refinement. During the Phase 2, we are going to deliver specs conforming units for both Yb and Tm 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 Lawrence Berkeley National Lab to have a preliminary test with the isolator we develop in Raytum Photonics facility.