Rare-earth-doped lithium niobate waveguide structures

Details Rare-earth-doped lithium niobate waveguide structures Rare-earth-doped lithium niobate waveguide structures

1993-07-28

1995-12-05

Ullah, Akm E.

Optical waveguides

Planar optical waveguide

Thin film optical waveguide

372 20, G02B 600

Patent

active

054737222

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a waveguide structure with diffused rare-earth doping in a light-guide channel arranged in a lithium niobate crystal surface.
From R. Brinkmann et al, "Annealed erbium-implanted single-mode LiNbO3 waveguides", 1990, OSA Techn. Digest Series, Vol. 5, post deadline paper, PD1, an erbium-doped lithium niobate optical fibre waveguide is known which is manufactured by means of erbium ion implantation and subsequent tempering. This involves planar arrangement of the erbium doping close to the surface. The erbium ion beams destroy the crystal structure close to the surface, and this destruction has to be healed again in a separate stage of the process, so that the crystal becomes optically useful once more. So long as the healing is by means of a heat treatment, the greater mobility which they enjoy in the destroyed region means that the embedded erbium atoms prefer to migrate towards the surface of the crystal and less so into the depths, which is where the light-guide channel requires effective erbium doping. Besides, no narrow lateral doping boundary to the narrow light-guide channel is provided, and this gives rise to erbium fluorescence losses due to reabsorption in the lateral ridges and troughs of the waveguide channel, since the .sup.4 I.sub.14/2 -.sup.4 I.sub.15/2 erbium fluorescence junction is part of a three-level system and results in the ground state.


SUMMARY OF THE INVENTION

It is the object of the invention to disclose rare-earth-doped waveguide structures of enhanced optical quality, viz. having lower dispersion losses and higher amplification constants, as well as exhibiting increased absorption of pumping radiation, to be used for integrated optical amplifiers and lasers, manufactured using a production process simplified for implantation engineering.
This object is achieved by the fact that the rare-earth doping is arranged in an effective rare-earth doping region approximately coaxially with the light-guide channel, said doping region is diffused directly from the crystal surface and its expansion is laterally limited.
Advantageous embodiments are indicated in the subsidiary claims.
It is especially advantageous if the effective expansion of the rare-earth doping is less than that of the light-guide channel laterally or depthwise or, in particular, on all sides. It is especially advantageous if the maximum doping is located beneath the surface of the crystal and preferably coincides more or less with the focus of the light distribution in the light-guide channel.
Using erbium as the rare earth has proved to be especially advantageous, since its small ion diameter allows relatively good mobility in the diffusion process. Other rare earths, e.g. neodyme, require longer diffusion operations or much higher diffusion temperatures.
The waveguide structures can be produced overlapping the erbium doping by using a conventional titanium diffusion doping technique. Because the titanium diffusion temperature and the diffusion time of titanium sufficing to form a waveguide also allow the rare earth to penetrate deeper into the crystal, it is best to take this tempering process into account when calculating the final diffusion depth of the rare earth. At the same time, titanium's more rapid diffusion provides the desired greater expansion of the light-guide channel than of the rare-earth doping region.
The light-guide channel may also be produced surrounding the rare-earth doping region in the form of a proton exchange waveguide, with lithium ions being replaced in conventional manner by hydrogen ions.
The rare-earth-doped waveguide can advantageously be used for an optical travelling-wave amplifier in which an input light wave and a pumping wave are supplied to said amplifier. It is advantageous if the light-guide channel is bounded to the rear by a wavelength-selective mirror. This mirror reflects the pumping wave into the active region, thereby making double use of said wave to amplify the input light wave. The amplified light wave passes through

REFERENCES:
patent: 5128948 (1992-07-01), Papuchon et al.
E. Lallier et al.: "Nd:MgO: LiNbO.sub.3 waveguide laser and amplifier", Optics Letters, vol. 15, No. 12, (15 Jun. 1990), pp. 682-684.
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K. Komatsu et al.: "Titanium magnesium double diffusion method for efficient fibre-LiNbO.sub.3 waveguide coupling", Electronics Letters, vol. 22, No. 17, (14 Aug. 1986), pp. 881-882.
H. Toda et al.: "Optical integrated circuit for a fiber laser doppler velocimeter", Journal of Lightwave Technology, vol. LT-5, No. 7 (Jul. 1987), pp. 901-905.
R. Brinkmann et al.: "Absorption, fluorescence and stimulated emission in Ti-diffused Er: LiNbO.sub.3 waveguides", Proceedings of the SPIE--the International Society for Optical Engineering, vol. 1362, Pt. 1, pp. 377-382. (1990) Conference Date Oct. 28-Nov. 2, 1990.
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