Resonant grating waveguide structures

Under certain conditions, a resonance phenomenon can occur in waveguide grating structures. Such structures have multilayer configuration, the 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 illuminated with an incident light beam, part of the beam is directly transmitted and part is diffracted and subsequently trapped in the waveguide layer. Some of the trapped light is then rediffracted outwards, so that it interferes destructively with the transmitted part of the light beam. At a specific wavelength and angular orientation of the incident beam, the structure ''resonates''; namely, complete interference occurs and no light is transmitted. The bandwidth of the resonance is based on 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 and switch applications, The fabrication of such resonant structures utilizes common planar processing technologies; thin-film deposition, etching, and submicron photolithography. This paper reviews previous investigations on the resonance phenomena and presents analytic and numerical models for evaluating the resonance as a function of the geometric and optical parameters of the structures and incident radiation. The technologies for fabricating the structures are described and experimental procedures and results with passive dielectric structures (Si3N4-SiO2) operating at a wavelength of 0.56 mu m and semiconductor structures (InGaAsP-InP) operating at 1.55 mu m, as well as more complicated active (InGaAsP-InP) modulator structures. The results reveal that spectral resonance bandwidths can range from 0.03 nm to several nanometers, with corresponding finesses ranging from 300-15000, and that the ratio of resonant to nonresonant intensities in transmission or reflection can reach 100, The active structures were modulated at frequencies up to 10 MHz, with potential for reaching even higher frequencies. The results suggest that such structures can be exploited in arrays of optical switches or modulators and narrowband spectral filters, for use in advanced optical signal processing and communication systems.