Integrated optics and new wave phenomena in optical waveguides

The theory and recent applications of waves in periodic structures are reviewed. Both the Floquet and coupled waves approach are analyzed in some detail. The theoretical part of the paper includes wave propagation in unbounded and bounded active or passive periodic media, wave scatterring from periodic boundaries, source radiation (dipole, Cerenkov, transition, and Smith-Purcell) in-periodic media, and pulse transmission through a periodic slab. The applications part covers the recent development in a variety of fields: distributed feedback oscillators, filters, mode convertors, couplers, second-harmonic generators, deflectors, modulators and transducers in the fields of integrated optics and integrated surface acoustics. We also review the work on insect compound eyes, mehanical structures ocean waves, pulse compressions, temperature waves, and cholestric liquid crystals. Particles interaction with crystals is briefly reviewed, especially in the case of zeolite crystals and supelattices. Recent advances in fabrication techniques for very fine gratings me also covered. Finally, speculations about future problems and development in the field of waves in periodic structures are given.

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Waves in active and passive periodic structures: A review

This paper deals with the theory and device applications of periodic thin-film waveguides. Topics treated include mode solutions, optical filters, distributed feedback lasers (DFB), distributed Bragg reflector (DBR) lasers, grating couplers, and phase matching in nonlinear interactions.

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Periodic structures for integrated optics

Wave propagation in periodic waveguides is analyzed by decomposing the eigen Bloch waves into traveling-wave components. It is shown that the principal components consist of a primary forward wave, a primary backward wave, and their Bragg-scattered secondary waves. One important parameter is the coupling constant s due to Bragg scattering, which relates the secondary wave to the respective primary wave. Laser threshold condition is then obtained by applying the continuity of tangential E and H at the two boundaries. The results thus obtained are general and applicable to thin-film lasers with various waveguide structures. The laser threshold condition of thin-film Bragg lasers is expressed in terms of two effective reflection coefficients for easy comparison with conventional lasers. For appreciable reflection, a significant change either in the propagation constant or in the coupling constant is required. Two basic types of thin-film Bragg lasers are distributed-feedback (DFB) lasers in which Bragg scattering is confined to the active medium and distributed-Bragg-reflector (DBR) lasers in which Bragg scattering is limited to regions beyond the active medium. The threshold gain, frequency control, and mode selectivity for both types are analyzed and the analyses are applied to GaAs and Nd lasers. It is shown that DBR lasers should have a lower threshold gain and a better mode selectivity than DFB lasers. For distributed-feedback effect to play a significant role in thin-film Bragg lasers, the product kL int must be greater than unity where K is the distributed-feedback coefficient and L int is the interaction length. Advantages for having periodic structures outside the active medium so as to relax constraints on k and L int are also discussed.

Principles of distributed feedback and distributed Bragg-reflector lasers

The fabrication and evaluation of corrugation grating filters with reflectivities better than 75% and bandwidths less that 2 A is reported. Grating filters were fabricated on glass waveguides using standard interferometric and ion‐beam machining techniques and evaluated with a tunable dye laser. Measured and calculated filter responses are found to be in excellent agreement. Tolerances for realizing ideal filter performances are given.

Grating filters for thin‐film optical waveguides

Distributed feedback of a thin‐film optical guided mode has been produced by a periodic perturbation of the guide height. Using photoresist and ion‐milling techniques, gratings with a period of ∼0.2 μ and a peak‐to‐peak height of ∼0.1 μ have been formed on fused silica substrates. A film of polyurethane doped with Rhodamine 6G pumped by an N2 laser formed the gain medium. Data presented on the oscillation threshold of several lasers are in agreement with calculated feedback parameters. This technique is capable of producing high‐resolution gratings in durable materials which can provide feedback in low‐gain laser media.

M.F. Galvin

Papers

Thin‐film distributed‐feedback laser fabricated by ion milling