Strip gratings on dielectric substrates as output couplers for submillimeter lasers

This paper describes the use and advantages of metallic strip gratings on dielectric substrates as output couplers for both optically pumped and discharge-excited submillimeter lasers. Formulas are presented for the calculation of transmittance and loss of such couplers, taking account of loss in the strip grating as well as loss and multiple reflections in the substrate. Included are expressions for the phase shifts on reflection and transmission by an arbitrary lossy grid on a plane boundary between two dielectrics according to a transmission-line model that is applicable for wavelengths in both dielectrics longer than the grid period. In relation to these phase shifts attention is drawn to an important sign convention. The theory is shown to agree well with measured transmittance of a typical device between 500 and 1600 GHz as well as spot measurements at 891 (337-microm HCN laser), 1540, and 1578 GHz (195- and 190-microm DCN laser). Finally, the theory is used to design a low-loss coupler for the low-gain 119-microm line of discharge excited H2O.