<title>Advances In Integrated Optical Spectrum Analyzers</title>

Abstract

AbstractThe operating characteristics of a hybrid integrated optical spectrum analyzer and the results of a design analysis performed to permit the development of an improved device are discussed. The existing device incorporates a LiNbOs substrate, which contains a Ti-indif- fused optical waveguide, two near-diffraction-limited geodesic waveguide lenses, and a two- element surface acoustic wave transducer array, and a butt-coupled photodiode array. The optical source is either an end-fire or butt-coupled laser. This unit has been shown to operate over a 400 MHz bandwidth with a resolution which varies from 5.3 MHz for an optical wavelength of 0.6328 urn to 4.0 MHz for 0.83 ym.IntroductionDuring the past several years considerable effort has been focused upon the development of an integrated optical RF spectrum analyzer (IO SA) . This first 10 device demonstrates the feasibility of applying integrated optics to complex signal processing. 1 Initially, this effort was devoted to the development of individual 10 components such as waveguides, waveguide couplers, broadband surface acoustic wave (SAW) transducers, waveguide lenses, and high speed detector arrays capable of operating over a large dynamic range. Two differ ent waveguide technologies were explored; one using silicon as the substrate material, and the other, lithium niobateThe use of silicon as the substrate material is desirable in that detector arrays can be fabricated such that the waveguided radiation is coupled directly into the detector ele­ ments . 2 , 3 in addition, waveguides on silicon substrate are formed by sputtering a higher index film over a thermally grown Si02 buffer layer grown over the polished face of the silicon. Since such waveguides are fabricated of materials with relatively low optical re­ fractive indices, it is possible to utilize one of many available materials of significant­ ly greater refractive index to fabricate Luneburg waveguide lenses. ^ Such lenses were thought to be advantageous in that the contour and thickness of the sputtered lens material could be controlled to yield a near-diffraction-limited image. Even though analyses indi­ cated extremely tight tolerances of the order of tens of angstroms on the thickness and contour of such sputtered films must be met, this technology was considered desirable since it involved standard processing techniques employed in integrated circuits.The major disadvantage of silicon-based integrated optics technology is the need for a separate piezoelectric material to form the broadband SAW transducer. Several experimental efforts use ZnO as such a piezoelectric material in conjunction with interdigitated metal finger patterns to form electrically efficient SAW transducers . 6 , 7 one such effort result­ ed in transduction which approached the theoretical efficiency limit but it was found that the SAW propagation through both the ZnO and the optical waveguide was significantly more lossy than that observed on single crystal LiNbOs surfaces.As a result of the reported high SAW propagation losses in ZnO and in waveguides sput­ tered on Si substrates, the emphasis shifted toward the use of LiNbO3 as the preferred sub­ strate material. Waveguides are formed in LiNbO3 by Ti indif fusion or by Li20 outdif fusion; the former has been found to provide a more tightly confined waveguide with optical propa­ gation losses of the order of 0.5 dB/cm. Transducers can be easily fabricated on LiNb03 by depositing an interdigitated finger pattern on the waveguide surface because the material is piezoelectric. Problems associated with LiNbO3~based integrated optics center around the needs to butt-couple a detector array to the waveguide, and the need to utilize other lens fabrication techniques. Significant progress has been made in recent years in devel­ oping high quality geodesic lenses and techniques to couple efficiently from the waveguide into a detector array. 9,10 These advances have led to the recent successful demonstration of working spectrum analyzers utilizing LiNb03 waveguides.11/ 12 Emphasis is now being placed on the optimization of the performance of the 10 SA and on the development of other integrated optical devices.Components for integrated optical spectrum analyzers in LiNbO3The development of the integrated optical spectrum analyzer on a LiNbO3 substrate re­ quired the fabrication of the following high quality components: