James F. Lotspeich

Electrooptic light-beam deflection

The impetus given display, recording, computer, and space technologies by the revolutionary discovery of the laser has created an ever-increasing need for controlled optical-beam deflection techniques. Those that have been developed thus far, usually employing mechanical or acoustical methods, have had either high insertion loss or limitation on bandwidth. Electrooptic methods have not only overcome these problems, but have successfully met the demands of systems requiring aperiodic and rapid-access modes of operation. After the basic theory of optical deflection, resolution, and control are outlined, two experimental models of electrooptic beam deflectors, as well as the two major difficulties that have been encountered, are described.

Iso-index coupled-wave electrooptic filter

A new optical filter, first demonstrated more than a decade ago and recently extended to other frequencies and materials, is based on the accidental isotropy of refractive indexes that occurs in certain uniaxial semiconductors near the band edge. This paper shows how coupling of light energy can take place between ordinary and extraordinary polarizations at the isotropic point by application of a dc electric field. When placed between crossed polarizers, these materials can thus act as narrow-band filters. The field-of-view characteristics have been analyzed; and it is concluded that this type of filter can, in principle, accommodate a 2π field with less than 20 percent increase in passband over that of the narrow-field condition. It is noted, in particular, that AgGaS 2 exhibits the required change of sign of its birefringence at a wavelength of 4970 A (blue-green), with a rate of change that could provide a passband of only 0.2 A in a 1 cm sample.

Explicit General Eigenvalue Solutions for Dielectric Slab Waveguides

A formula is developed for dielectric slab waveguides that yields general solutions of the TE eigenmode propagation constants as explicit functions of a composite independent variable involving the guide layer thickness, operating wavelength, and refractive indexes of guide and cladding media. This provides a convenient means for circumventing the transcendental eigenvalue equations. The maximum error is less than 0.6%. A method for deriving the corresponding TM eigenvalues with an accuracy of about 99% or better is shown.

Coupled-mode analysis of light propagation in optically active crystals

A new mathematical formalism, based on a coupled-mode approach, is presented to describe light propagation in birefringent optically active crystals. An arbitrary light wave in the medium is represented as an expansion in terms of linearly polarized eigenmodes; explicit solutions of the equations for the coupled-wave amplitudes show how power is transferred between the principal components in the presence of optical activity. The (generally elliptically polarized) eigenmodes of the optically active crystal are also derived by a diagonalization of the coupled-mode equations. The analysis is extended to include electro-optic crystals, yielding a more general, complex coupling parameter. The predictions of the theory are in excellent agreement with spectral-transmission measurements in AgGaS2 near its isoindex point at 497 nm.

Electrooptic diffraction modulation in Ti-diffused LiTaO 3

The design and fabriction of electrooptic Bragg diffraction modulators in Ti-diffused LiTaO(3) waveguides are reported. The modulators developed have demonstrated 98% deflection efficiency for visible and near-ir operation with extinction ratios of at least 250:1 for both deflected (m = 1) and nondeflected (m = 0) beams.

A perturbation analysis of modes in diffused optical waveguides with Gaussian index profile

Abstract Inclusion of a quartic term with adjustable coefficient in the Taylor expansion approximation to gaussian index profiles of diffused optical waveguides is shown to provide an improved approximation to the true gaussian curve to beyond the e-folding depth. A first-order perturbation to the square-law “harmonic oscillator” solutions is applied to improve the calculations of mode indexes and to illustrate the manner in which the corrected wave functions are extended deeper into the substrate.

A multilayer AgGaS2 structure for infrared (2-10 μm) electro-optic tunable filters: fabrication and performance

Alternating layers of high‐ and low‐resistivity silver thiogallate (AgGaS2) were grown sequentially by liquid‐phase epitaxy on AgGaS2 substrates using the vertical dipping technique. High‐resistivity layers were grown by using KCl‐AgGaS2 solutions while low‐resistivity layers were grown from Ge‐AgGaS2‐KCl solutions. Layer growth of such a multilayer device and demonstration of its electro‐optic response for filter application are described.

Single-Crystal Electrooptic Thin-Film Waveguide Modulators for Infrared Laser Systems

Wideband, low power electrooptic modulators of optical waveguide structure have been developed for infrared laser applications. They allow a reduction in driver power of two orders of magnitude below that of conventional devices. The modulators are composed of very thin layers of single-crystal GaAs, bounded on both sides by evaporated films of lower refractive index material: CdTe or As(2)S(3). Minimum propagation loss, measured at 10.6 microm, was less than 1 dB/cm for TE modes and less than 5 dB/cm for TM modes. A 20-pF modulator exhibited a pulse response rise time of 3 nsec and showed useful frequency response to beyond 200 MHz. The basic capability for advanced design modulators of this type to operate at 10 microm with driver powers of less than 25 mW/MHz for 50% modulation depth is shown.

Transfer efficiencies in guided wave electrooptic filters/mode converters

The influence of a spatially nonuniform electric field on the polarization Conversion efficiency of single- and multimode guided wave electrooptic filters is analyzed quantitatively, using coupled-mode theory. A single-mode graded-index surface waveguide filter is capable of essentially 100-percent transfer efficiency, with overlap integrals of field and a single TE, TM mode pair approaching 0.6 in Ti: LiNbO 3 . The thicker multi-mode slab waveguides require that spatial periods of applied field be considerably greater than the guide thickness to achieve high transfer efficiency, though never quite 100 percent; The best performance also requires focused coupling into the lowest order mode only. Experimental results with a LiTaO 3 slab waveguide filter show reasonable agreement with a multiple-coupled-mode solution.

Modulation-sensitivity enhancement in electro-optic slab waveguides

Analysis of the phase modulation characteristics of electro-optic or acousto-optic waveguide modulators reveals that a significant increase of modulation sensitivity, per unit of applied field, above that of conventional, bulk devices can occur in multimode waveguides with tight confinement. In the limit, an enhancement of as much as a factor of 3 is theoretically attainable. Explicit formulas for relative modulation sensitivity are given for both TE- and TM-mode polarizations, and illustrative curves are presented for a wide variety of structural parameters. Semiquantitative experimental verification of the effect is presented, using results of a GaAs electro-optic thin-film modulator at 10.6 μm.