Electro-optic waveguide device

Details Electro-optic waveguide device Electro-optic waveguide device

1990-05-22

1992-01-21

Ullah, Akm

Optical waveguides

Directional optical modulation within an optical waveguide

Electro-optic

357 17, 372 44, G02B 610, H01L 29161, H01L 29205, H01L 29225

Patent

active

050823426

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to an electro-optic waveguide device of the kind appropriate inter alia for optical beam steering applications.
2. Discussion of Prior Art
Optical beam steering devices based on electro-mechanical movements are well known in the prior art. A typical arrangement comprises a mirror mounted on a current-actuated moving coil. Electro-mechanical systems are inherently limited to low response frequencies up to the order of 1 kHz.
Acousto-optic beam steering devices are also known, such as for example that described by Suhara, Nozaki and Nishihara in the Proceedings of the IVth European Conference on Integrated Optics. This comprises a Ti-doped LiNbO.sub.3 optical waveguide having an interdigital acoustic transducer and a light focussing diffraction grating coupler on its upper surface. The grating coupler is curved and has varying (chirped) spatial frequency to provide light output focussing. A radio frequency (RF) signal is applied to the transducer, which produces surface acoustic waves in the waveguide transverse to the light propagation direction. The acoustic waves modulate the waveguide refractive index and thereby interact with light propagation. The output focus from the grating coupler is raster scanned or beam steered by frequency sweeping the RF signal applied to the transducer, deflection angle being approximately proportional to frequency. The transducer had a centre frequency of about 500 MHz and a bandwidth of 330 MHz. This beam steering apparatus is of high optical resolution. However, the maximum rate at which the output beam can be steered depends on the rate of propagation of acoustic waves from the transducer across the waveguide, and is in the order of 1 MHz. Moreover, frequency swept RF signal sources are expensive, and are difficult to interface with digital electronic circuitry. In order to achieve beam positioning in response to digital signals, it would be necessary to provide circuitry for converting the signals into RF frequencies within the acoustic transducer bandwidth.
An electro-optic beam steering device is described by R A Meyer in Applied Optics, Vol. 11, pages 613-616, March 1972. It comprises an LiTaO.sub.3 crystal in the form of a rectangular block 0.1 mm thick, 23 mm wide and 15 mm long. One 23 mm.times.15 mm face of the crystal bears 46 parallel electrodes each 0.2 mm wide and with a centre-to-centre spacing of 0.5 mm. The electrodes length dimensions are parallel to that of the crystal. Parallel light is input to one 23 mm.times.0.1 mm crystal face and propagates parallel to electrode length. The crystal has electro-optic properties, and consequently its refractive index is a function of electrode voltage. Light output from the crystal is variable in optical phase in accordance with electrode voltage also. The light output from a crystal region immediately beneath the centre of an electrode depends on electrode voltage. Because of electrical field non-uniformity at electrode edges and in inter-electrode regions, light propagating in corresponding parts of the crystal is non-uniformly phase modulated and changes its polarisation state. The crystal is masked to restrict light output to central areas under electrodes. The mask comprises a linear array of 100 .mu.m square apertures.
Meyer does not provide easily accessible information on the beam steering performance of his device, but it would appear that it is capable of steering a beam through about 0.2.degree.. The limit of beam steering is set by the distance between grating lobes, these being individual diffraction maxima within the main lobe of a diffraction pattern arising from one element or mask aperture alone. The angular beam steering limit is inconveniently small. To obtain a useful degree of linear shift in beam position, a light reception surface would require location at a substantial distance from the device. Furthermore, the device is of inconveniently large dimensions, of the order of centimetres in extent. It is therefore a bulk

REFERENCES:
patent: 4685763 (1987-08-01), Tada et al.
patent: 4778235 (1988-10-01), Fujiwara
patent: 4923264 (1990-03-01), Langer et al.

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