A theoretical investigation of wedged capacitive strip-grating output couplers for optically pumped terahertz lasers

The output power for terahertz lasers influenced drastically by slight changes in the transmittance of the output coupler for a certain terahertz frequency has been discussed in the paper. Therefore it is necessary to design an output coupler whose transmittance does not rapidly modulate with frequency. To obtain the flat and accurate transmittance spectrum in a narrow wavenumber interval (say 3 cm(-1)), a wedged output coupler is proposed. This conclusion has been revealed after analyzing the disadvantages of two types of capacitive strip-grating output couplers, and theoretically studying its transmission properties. The theoretical results show that the coupler can not only provide flat and accurate transmittance spectrum and thus effectively suppress the etalon effects occurring frequently in previous experiments but also thoroughly eliminate the transmittance sensitivity to the slight shift of substrate thickness. Moreover, the wedged substrate of the improved output coupler can be used repeatedly in the process of adjusting the strip-grating parameters to meet various transmittance requirements for optically pumped terahertz lasers. (C) 2009 Elsevier B.V. All rights reserved.