RESONANT GRATING WAVE-GUIDE STRUCTURES

Under certain conditions, a resonance phenomenon can occur in waveguid e grating structures. Such structures have multilayer configuration, t he most basic of which is comprised of a substrate, a thin dielectric layer or semiconductor waveguide layer, and an additional transparent layer in which a grating is etched. When such a structure is illuminat ed with an incident light beam, part of the beam is directly transmitt ed and part is diffracted and subsequently trapped in the waveguide la yer. Some of the trapped light is then rediffracted outwards, so that it interferes destructively with the transmitted part of the light bea m. At a specific wavelength and angular orientation of the incident be am, the structure ''resonates''; namely, complete interference occurs and no light is transmitted. The bandwidth of the resonance is based o n parameters such as the grating depth and duty cycle, as well as the thickness of the waveguide layer, The bandwidth can be designed to be very narrow (on the order of 0.1 nm) which is of interest for filter a nd switch applications, The fabrication of such resonant structures ut ilizes common planar processing technologies; thin-film deposition, et ching, and submicron photolithography. This paper reviews previous inv estigations on the resonance phenomena and presents analytic and numer ical models for evaluating the resonance as a function of the geometri c and optical parameters of the structures and incident radiation. The technologies for fabricating the structures are described and experim ental procedures and results with passive dielectric structures (Si3N4 -SiO2) operating at a wavelength of 0.56 mu m and semiconductor struct ures (InGaAsP-InP) operating at 1.55 mu m, as well as more complicated active (InGaAsP-InP) modulator structures. The results reveal that sp ectral resonance bandwidths can range from 0.03 nm to several nanomete rs, with corresponding finesses ranging from 300-15000, and that the r atio of resonant to nonresonant intensities in transmission or reflect ion can reach 100, The active structures were modulated at frequencies up to 10 MHz, with potential for reaching even higher frequencies. Th e results suggest that such structures can be exploited in arrays of o ptical switches or modulators and narrowband spectral filters, for use in advanced optical signal processing and communication systems.