Tunable Phase-Matching Techniques For Submillimeter Waves

Abstract We report a new tunable phasematching technique for submillimeter and far-infrared difference-frequency generation by optical lasers. We use a multiple waveguide principle based on an optical waveguide embedded in a low-loss modified H-guide. The same phasematching technique also permits the development of a Gigahertz electro-optic modulator design. In order to use uniaxial electro-optic materials, the theory of the H-guide was extended to determine the appropriate crystal orientations and dispersion equations. Numerically generated design curves are presented for tunable electro-optic modulators designed for 3-10 Gigahertz operation. A brief discussion of other possible designs and applications based on the multiple-waveguide method of phasematching is also given.IntroductionOptical parametric amplification and optical difference frequency generation - are two techniques suitable for generating tunable coherent submillimeter (SMM) and far-infrared radiation.1 However, most past develop­ ments have been hindered by a dependence on crystalline birefrigence or dispersion for phasematching the optical pump to the lower sidebands, although some work based on overmoded optical waveguides has been reported.2" 6 Better methods of phasematching would make it possible to use electro-optic materials with large dielectric constants or cubic structure. Other applications would also benefit from new phasematching techniques: Gigahertz electro-optic travelling wave modulators, tunable submillimeter spectral filters, parametric amplification using far-infrared lasers as pumps, and possibily even four-wave acousto-optic generation of submicron sound waves. This paper will describe a new approach to phasematching which is based upon the properties of open surface-wave dielectric submillimeter guides.Double-Waveguide Principle and Specific DesignsThe basic principle is to use an open surface-wave dielectric guide to confine the propagating submillimeter wave. By controlling the evasnescent field extent in the surrounding lower index medium, the phase velocity can be adjusted over a larger range determined by the relative dielectric constants. Since the waveguide is an open structure, it is relatively simple to embed an optical dielectric waveguide into the submillimeter dielectric medium . The optical waveguide can be ion-implanted, inserted as a thin lamination, or deposited as a thin film. The optical guide confines the pump beam so that the nonlinear efficiency, which is proportional to the pump intensity, is greatly increased. It also reduces beam divergence and increases the effective interaction length.An additional advantage of this method is that it lends itself to the use of layered dielectrics. By making one of these layers an adjustable gap, the phasematching SMM wavelength can be tuned in a simple manner, even pizeoelectrically.We have developed some specific device designs using this double-waveguide principle. Two examples are shown in Figure 1. Both designs are based upon modified versions of the H-guide originally developed by Tischer.7 This is a surface-wave guide consisting of a horizontal dielectric slab between two vertical metal planes. This structure is especially appropriate to SW[ wavelengths because it supports a hybrid PM mode which has the desirable property of decreasing conduction losses at short wavelengths. This property arises from the absence of a magnetic field component perpendicular to the dielectric so that there is no longitudinal current in the metal walls. If the dielectric is thick enough, it can support an anti-symmetric mode PM mode where the electric field tangential components are zero in the center of the dielectric. A metal ground plane can be placed at this position without altering the field configurations. This is the trough guide, which is also shown in Figure 1. This guide will, however, have conduction losses in the ground plane. 8Besides the low-loss hybrid modes, the H-guide also supports TE, TM and PE hybrid modes. The TE mode is of interest because it lends itself to the design of electro-optic modulators which should be effecient even at frequencies as high as 30 GHz. The lowest order symmetrical TE mode has a zero frequency cutoff, and can be made the dominant mode by choosing narrow wall spacing and thin dielectrics.