Fully planar proton-exchanged lithium niobate waveguides with grating taps.

A two-step proton-exchange process to produce waveguides and gratings in lithium niobate is described. In both steps, mixtures of lithium benzoate and benzoic acid are used. The fabricated components are characterized at 0.633 and 0.422 µm. In particular, we observe that the light intensity diffracted into the air by the grating is ~0.5% of that in the waveguide.

Arbitrarily oriented SAW gratings: network model and the coupling-of-modes description.

A network model for surface-acoustic-wave (SAW) structures fabricated along arbitrary and hence nonsymmetric orientations, including natural single-phase unidirectional transducer (NSPUDT) directions, is described. From the predictions of this model for SAW gratings. the frequently used coupling of modes (COM) phenomenological description is evaluated. The network model consists of the usual sequence of mismatched transmission lines with susceptance loading at discontinuities to account for energy storage, plus a new additional reactive element calculated using a perturbation formula, to account for the asymmetry that exists for arbitrary orientations. The circuit elements are determined by geometrical and material parameters. It is demonstrated that the incremental COM description can be derived from the unit-cell-based network model, and analytical formulas quoted in the literature for the COM coefficients are compared with the values calculated from the ABCD network model description. An analytically tractable approximation for the network model is also described, from which rather simple explicit formulas for the COM coefficients that predict their dependence on material parameters and on frequency are derived. The analysis and numerical calculations indicate that these formulas may yield results that are valid over almost 30% relative bandwidths.

Stopband characteristics of shallow gratings.

A numerical method of rapidly computing the width of the stopband of a shallow grating in a SAW filter and the dependence of this bandwidth on fabrication parameters is presented. The method is based on the properties of the equivalent circuit of the unit cell at the transition frequencies. Numerical results illustrate the variation of the relative bandwidth with respect to the element reflection coefficient, the mark-to-space ratio, and the velocity perturbation parameter.

Modeling, simulation, and design of SAW grating filters.

A systematic procedure for modeling, simulating, and designing SAW (surface acoustic wave) grating filters, taking losses into account, is described. Grating structures and IDTs (interdigital transducers) coupling to SAWs are defined by cascadable transmission-matrix building blocks. Driving point and transfer characteristics (immittances) of complex architectures consisting of gratings, transducers, and coupling networks are obtained by chain-multiplying building-block matrices. This modular approach to resonator filter analysis and design combines the elements of lossy filter synthesis with the transmission-matrix description of SAW components. A multipole filter design procedure based on a lumped-element-model approximation of one-pole two-port resonator building blocks is given and the range of validity of this model examined. The software for simulating the performance of SAW grating devices based on this matrix approach is described, and its performance, when linked to the design procedure to form a CAD/CAA (computer-aided design and analysis) multiple-filter design package, is illustrated with a resonator filter design example.