Uniform Transmission Si Fabry Perot Couplers for Optically Pumped FIR Lasers

An ideal output coupler for CW optically pumped FIR laser must present : i) a low adjustable and uniform FIR transmission coefficient (≲ 0.1), ii) a weak absorption at FIR wavelengths, iii) a good reflection at the pump wavelength. The condition of uniform transmission is essential to obtain a slowly diverging FIR output beam. Hybrid couplers which satisfy in part the properties i), ii), iii), have been successfully developed beyond 100 μm [1], [2]. Different versions of Fabry-Perot (F-P) couplers have been recently proposed at shorter FIR wavelengths; a space fixed triple plate system specially designed for 70 μm [3] and a variable double wire-mesh interferometer [4]. We have realized a variable double silicon plate F-P coupler which fulfils the conditions i), ii), over a large FIR frequency range and avoids the damage caused by intense pump beams in mesh systems [5].