Diffraction characteristics of general dielectric planar (slab) gratings and surface-relief (corrugated) gratings are reviewed. Applications to laser-beam deflection, guidance, modulation, coupling, filtering, wavefront reconstruction, and distributed feedback in the fields of acoustooptics, integrated optics, holography, and spectral analysis are discussed. An exact formulation of the grating diffraction problem without approximations (rigorous coupled-wave theory developed by the authors) is presented. The method of solution is in terms of state variables and this is presented in detail. Then, using a series of fundamental assumptions, this rigorous theory is shown to reduce to the various existing approximate theories in the appropriate limits. The effects of these fundamental assumptions in the approximate theories are quantified and discussed.

Analysis and applications of optical diffraction by gratings

We report the infrared operation of a bulk optical Bragg modulator based on electro-optically induced refractive index gratings in z-cut periodically poled lithium niobate Efficiencies in the first order of 45% for 1064 nm e-polarized light and 30% for o-polarized light were achieved, with maximum on/off ratios of 15:1 and 9:1, respectively Field-induced light scattering effects due to poling are observed at higher drive voltages and compromised device performance due to these scattering effects is predicted to limit long-wavelength operation of these devices

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Demonstration and optical characteristics of electro-optic Bragg modulators in periodically poled lithium niobate in the near-infrared

For the first time, transients are measured for a bulk-optical Bragg-deflector based on an electrooptically (EO) induced grating in periodically poled LiNbO/sub 3/ (PPLN). Optical pulse risetimes of 1.2 ns are reported with a first-order diffraction efficiency of 76% for circular Gaussian beams. The fast risetime is accounted for by a substantially suppressed excitation of acoustic thickness modes and careful electrode design. For many applications, the device could replace both acoustooptical Bragg-deflectors and conventional EO amplitude modulators due to a fivefold reduction in risetime and fourfold reduction in drive power, respectively.

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Nanosecond response of Bragg deflectors in periodically poled LiNbO 3

A new kind of laser-beam modulator is described. An interdigital electrode structure deposited on an electro-optic crystal is used to produce a spatially periodic electric field which diffracts the laser beam within the crystal. Baseband operation is possible, and optical-pulse risetimes of 1.5 ns have been observed. The required drive power is only 7.5 nJ per pulse at 25 V, but this could be improved by scaling. The measured extinction ratio is 25 dB and this varies by less than ±1dB for a 10 K temperature change. The form of the modulator lends itself to inclusion in thin-film optical circuits.

High-speed electro-optic diffraction modulator for baseband operation

An electro-optic diffraction modulator/deflector using tota internal reflection and no external optics is described. The zero-order extinction voltage decreases, and the interorder angle increases, with decreasing interdigitated electrode pitch. With LiNbO3, all the zero-order energy is deflected by less than 60 V. The active-material volume is low, approximately 0.5 mm3. Further optimisation is possible.

Total-internal-reflection electro-optic diffraction deflector/modulator

Latest measurements on an electro-optic diffraction modulator show optical-pulse risetimes of 1.5 ns with a drive of only 25 V and 6 nJ per pulse. The modulator lends itself to integration into thin-film optical circuits and is insensitive to temperature variations. Means of improving the performance still further are available.

M.A.R.P. de barros

Papers

Nanosecond baseband optical-diffraction modulator