Experimental Observation of 10.6-microm Guided Waves in Ge Thin Films.
Optical guided waves in thin films can potentially be used to combine the advantages of short optical wavelengths with the sophistication of microwaves in communication applications. Recently, a number of workers have reported efficient excitation of optical guided waves in thin films at the visible light wave­ lengths. 6 However, except for a few films, such as the sput­ tered glass film, the attenuation rates of the visible light guided waves in many deposited and sputtered films are very high. Much of the attenuation is attributed to the scattering of light by the grainy structure of the films. Guided-wave propagation at 10.6-μm wavelength is especially important to the future development of CO2 laser communica­ tions. For example, if electrooptical modulation can be achieved in guided-wave form, the modulation voltage can be reduced to the 50-V range from the 1000-V range needed in the bulk modula­ tors. Moreover, the attenuation loss due to scattering experi­ enced at the visible light should also be much reduced at the longer wavelength. However, there has not been any experimental ob­ servation of 10.6-μm guided waves reported, perhaps partly due to the difficulties of experimentation at 10.6 μm. We wish to report here the preliminary results of the first ob­ servation of guided waves at 10.6 μm. The film used is a 5-μmthick Ge film sputtered on a 2-mm-thick and 2.5-cm-diameter IRTRAN I I window prepared by Kodak. Computer calcula­ tion shows tha t this film has three guided T E modes. Their propagation wavenumbers are listed in the second column of Table I. When a 60° right-angle Ge prism is used to excite the guided wave as shown in Fig. 1, we calculated that only the TE1 and TE2 modes can be excited by this method. The TE 0 mode requires a physically unrealizable value of 0. The angle θ at which the inci­ dent laser beam must enter the Ge prism in order to phase-match the βn of the TE1 and TE 2 modes is shown in the third column of Table I. Obviously, only the TE1 mode is easily excitable: con­ sequently, only the excitation of the TE1 mode is reported here. Experimentally, a special mount, shown in Fig. 2, is set up to hold both the prism and the IRTRAN I I window. In this special mount, pressure is applied to the prism via the four springs to con­ trol the coupling, i.e., the gap distance δ, between the prism and the film. The film and the prism are then mounted on a 12.7-cm Lansing gimble mount which is in turn mounted on a Lansing translation stage for precise orientation and position control with respect to the incident laser beam. The linearly polarized laser beam is first reflected through a plane mirror M so that its angle of incidence is approximately θ = 15.5° and so that it would reach the bottom surface of the prism fairly close to the right-angled edge. The micrometers are then used to provide the fine adjustments on angular orientation and prism position while the pressure is adjusted for the appropriate Fig. 1. Schematic illustration of the excitation of guided waves on Ge film at 10.6 μm.