CH683802A5 - Integrated optical light coupling device with reduced losses - has expansion horns on ends of transmission and return optical waveguides for monomode signal, and planar reflector directing signal from transmission guide to return guide - Google Patents

Abstract

The integrated optical coupler, for permitting a monomode optical signal to propagate via a leading or transmission waveguide (2) towards a return guide (7), has expansion horns (3,6) on the ends of the transmission and return guides, respectively, at the junction of the coupling. A planar reflector (5) is located a distance d from the horns, which are oriented at an angle alpha to the normal to direct an optical signal from the transmission horn to the return horn. The waveguides support monomode signal propagation with lateral confinement. the reflector comprises a mirror arranged on a substrate, or a back-coupling network. ADVANTAGE - Avoids reflection back into transmission waveguide. Reduced optical attenuation, by increasing difference index delta-n between guide and gain, and by increasing thickness of waveguide or fibre without risk of multimode operation.

Description

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Description

The present invention relates to optical coupling means between light guides in general and relates more particularly to an integrated optical device for retro-coupling of light.

The presentation by M. Papuchon, entitled "Integrated Optics" and published in the reports of the Journées d'Electronique 1982, pp. 26 and following. (Presses Polytechniques Romandes) dedicated to Optoelectronics in Telecommunications and Metrology, is partially reproduced below.

Bell Telephone researchers suggested in 1969 a new generation of components by showing that it was possible to produce optical elements in thin layers. The most numerous interesting embodiments more particularly use electrooptic waveguides produced in LiNb03- crystals

In general, a waveguide consists of a zone of high refractive index surrounded by mediums of lower refractive indices. In the case of the plane waveguide, this zone is infinite in the two directions other than the thickness. In the case of the two-dimensional waveguide, the high index region is only infinite in one direction.

The author shows that the characteristics of the guided mode of such structures depend on the optical and geometric properties of the structure. Thus, said characteristics can be modified by changing the thicknesses, widths, refractive indices of the media constituting the integrated structure.

In the case which concerns us, the main function sought is the turning back of a wave guided in an integrated optical guide. The realization of curved guides integrated in a substrate supposes a significant modification of the parameters mentioned above. Therefore, such an embodiment poses problems in particular as for the writing of masks by electron beam imposing the drawing of curves in the form of straight line segments. A curved guide, even perfectly reproduced, undergoes a loss of light limiting the tolerable radius of curvature. For example, an optical guide integrated on a glass substrate by K-Na ion exchange and forming a quarter of a circle with a radius of curvature of the order of 7 mm leads to losses of approximately —4 dB / it / 2.

The text “Guided-Wave Optoelectronics” edited by Théodor Tamir and published by Springer editions concerns, more particularly in paragraph 6.1.6, the question of losses in curved optical guides.

The ability of an optical wave propagating in an integrated guide to change direction is limited by the phenomena of conversion from guided modes to radiated modes. The theory of coupled modes teaches that a curved optical guide induces a coupling of the guided mode to the radiated modes.

One solution to limit this attenuation of the wave propagating in a curved single-mode guide is to increase the difference in index An between the guide and its sheath, or to increase the width w of the guide or to obtain a high value of the thickness to width ratio of the guide.

A disadvantage of these solutions is that beyond a certain value of the width w of the waveguide, the latter ceases to be single-mode to become multimode.

Also an object of the present invention is an integrated optical device making a substantial change in direction of the light wave not having the drawbacks and / or limitations of the devices of the prior art.

Another object is an optical device for retro-coupling the light avoiding the retro-reflection of the wave in the supply guide.

The features of the invention are defined in the claims.

The advantages of the invention consist mainly of an optical device with low losses and providing good tolerance as to the arrangement of the different elements.

Other objects, characteristics and advantages of the present invention will become clear on reading the following description of an exemplary embodiment, description made in relation to the accompanying drawings in which: FIG. 1 shows an optical device for retro-coupling the light according to the invention.

The integrated optical device 1 for retro-coupling the light according to the invention comprises, by traversing the optical path, an optical supply guide 2, a first expansion horn 3, a planar optical guide 4, a reflective element 5, said planar optical guide 4, a second expansion horn 6 and finally a return optical guide 7.

Said optical feed guide 2 opens onto said first expansion horn 3. Said return optical guide 7 follows on from said second expansion horn 6. The optical feed and return guides are single-mode guides with lateral confinement. As for the planar optical guide, it is monomode in depth and not laterally confined. Said first and second expansion horns are located between said optical supply and return guides and said planar optical guide 4. The longitudinal axes associated with each of the expansion horns form an angle 2a between them. The planar optical guide 4 is itself located between said first and second expansion horns and said reflective element 5.

The integrated optical device of the invention is used to inject an optical wave coming from a first optical guide, called the supply optical guide, to a second optical guide, called the return optical guide. These two guides can be substantially parallel to each other.

In other words, the optical device allows a single-mode optical wave to propagate from a single-mode guide with lateral confinement to another single-mode guide also with lateral confinement, through a planar guide and via the two horns d and the reflective element. Said optical back-coupling device also allows a change in direction of the wave which, depending on the value assigned to a, can go as far as the turning back.

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The fundamental mode of the optical feed guide propagates in the first expansion horn without giving rise notably to transverse modes of higher order. The optical wave is collimated by the first expansion horn and is reflected on the reflective element.

Stray reflections in the first expansion horn are canceled by positioning the reflector element so that the incident wave is fully reflected in the second expansion horn. The wave thus reflected is collected by the second expansion horn to be focused in the return guide.

The geometrical structure of each expansion horn allows the adaptation of the mode conversion rate, such as the propagation of said fundamental mode between the optical guide of supply and the planar optical guide then between said planar guide and the return optical guide is done without losses.

One technology making it possible to produce the lateral confinement optical guides of the integrated optical device is in particular the ion exchange in the glass as described by J.-E. Gortych et al., "Fabrication of planar optical waveguides by K + ion exchange in BK7 glass", Optics Letters / Vol. 11, No. 2 February 1986, pp. 100-102.

The expansion horns are described by W. K. Burns et al. in US Patent 4,678,267. The production of said expansion horns follows from the compromise "length - efficiency". The length of the expansion horns can be 10 mm for a width of the mouth of Wmax = 35 | im. The incident beam is well collimated and has an opening of about 10 bn in the case of a propagation of the wave in the glass.

The reflective element can consist either of a retro-coupling network, or of a mirror placed at the edge of the substrate. Said back-coupling network has a pitch slightly greater than A = X / 2ne, X being the working wavelength and not the refractive index of the glass in which the guided wave propagates. In the case of an integrated network on a glass substrate and for working wavelengths x of 0.63 μm and 1.5 μm, the pitch A is typically 0.21 μm and 0.5 (im, respectively.

A technology enabling these back-coupling networks to be produced is described by M.T. Gale et al. in the publication "Waveguide Gräting Fabrication and Optimization for Integrated Optic Polarity Interferometry," IOOC'89, July 18—21th 1989, KOBE, Japan.

If the angle formed by the longitudinal axis of the expansion horns and the perpendicular, in the plane of the planar guide, of the reflective element is small, then we are close to the retro-coupling. The condition d> w / 2a ensures that there is no retro-reflection of the wave coming from the supply guide in this same guide, w is the width of the mouth of the expansion horns and d the distance between the reflector element and the expansion cones.

Claims (5)

Claims 1. Integrated optical device (1) for retro-coupling the light allowing an optical wave to propagate from an optical supply guide (2) to a return optical guide (7), characterized in that: - Said optical feed guide (2) opens onto a first expansion horn (3); - Said return optical guide (7) extends a second expansion horn (6); - said first and second expansion horns open onto a planar guide (4) and - Said planar guide (4) is arranged between said first and second expansion horns, on the one hand, and a reflective element (5), on the other hand. 2. Integrated optical device according to claim 1, characterized in that said supply and return waveguides are single-mode optical waveguides with lateral confinement. 3. Integrated optical device according to claim 1, characterized in that said planar guide is an optical waveguide monomode in depth and not confined laterally. 4. Integrated optical device according to claim 1, characterized in that the reflective element is: - either a mirror placed at the edge of the substrate, - or a back-coupling network. 5. Integrated optical device according to claim 1, characterized in that the difference d between said reflective element and said first and second expansion horns is greater than w / 2a. 5 10 15 20 25 30 35 40 45 50 55 60 65 3