Integrated optical beam spread transformer

Details Integrated optical beam spread transformer Integrated optical beam spread transformer

1998-09-22

2001-05-29

Lee, John D. (Department: 2874)

Optical waveguides

Planar optical waveguide

Thin film optical waveguide

C385S028000, C385S043000, C385S049000, C385S050000

Reexamination Certificate

active

06240233

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an integrated optical field width or beam spread transformer structured of several buffer layers on a substrate, the buffer layers being separated from each other by conductive layers, and an unclad rib wave guide of a constant rib width arranged on the upper buffer layer and having a tapered section at the interface between chip and fiber.
In integrated optics, cross-section changes occur when light guides are connected. The propagating light waves have to be subjected either to a widening or to a compacting change of field width in order to avoid abrupt transitions. In addition, the shape of the propagation images has to be matched when coupling photonic switching circuits to light waveguides. In integrated light waveguides these propagation images are of narrow lying elliptical shape and in fibers they are of rotationally symmetric shape. The elliptical shape cannot be avoided in unclad rib waveguides; but the propagating image in the waveguide may be enlarged by widening the field, so that losses from matching the fibre are low.
A number of publications has dealt with problems of optical waveguide couplings and with the coupling of waveguides to fibers. By way of example, German patent specifications DE 41 03 896 C1 and DE 41 42 850 A1 may be mentioned, see in particular
FIGS. 3 through 5
and the associated description thereof. The state of the art upon which the invention is based, is known from “Proceedings 21st European Conference on Optical Communication ECOC '95, Brussels, Belgium, Paper TU P22, 1995, “Uncladded thickness tapers with InGaAsP/InP rib waveguides for efficient fibre-chip butt coupling”, Mörl et al. This beam spread transformer satisfies requirements as regards a low loss of 1 dB in mode matching as well as a cost-efficient fabrication. Because of the tapered section of the rib waveguide there occurs a shift of the propagation image in a vertical direction and a change in the beam spread in horizontal and vertical directions. This resulted in an optimization in respect of the TE polarization. The coupling loss between chip and fiber amounts to 2 dB as regards the TE component, but is higher by several dB for the TM component.
OBJECT OF THE INVENTION
The technical problem with which the invention is concerned is to achieve independence of polarization of the beam spread transformer in addition to the advantages already achieved.
BRIEF SUMMARY OF THE INVENTION
To this end and in accordance with the invention, there is provided in a beam spread transformer of the kind referred to above a section extending from the fiber interface to the tapered section in which the rib extends to the fiber interface and the waveguide layer of the rib waveguide is totally removed and in which the exposed buffer layer is reduced in symmetry to the continuous rib to a width w2 which is greater than the width w
1
of the rib.
Thus, the beam spread transformer in accordance with the invention functions independently of the direction of light propagation and, therefore, constitutes a bi-directional optical component for couplings between chips and fibers. The formed section which is new relative to the state of the art is provided with an additional rib which will hereafter be referred to as TM focussing rib. It is formed by removing, following the removal of the waveguide layer directly below the rib, marginal portions of the buffer layer exposed on both sides of the continuous rib in symmetry thereto down to the waveguide layer therebelow. The TM focussing rib also leads to optimum matching of the propagation image to the rotationally symmetric shape of the fiber in respect of the TM component. The arrangement is thus independent of polarization while maintaining low mode matching losses. It has been shown that the waveguide remains monomodal over the entire beam spread transformer and that possible radiation losses are without noticeable effect. The additional fabrication effort relative to the state of the art upon which the invention is based is limited to a single additional structuring step and has a negligible effect on fabrication costs.
It is of its own inventive significance, also with respect to its cooperation with the inventive structuring of the section in which the width of the exposed buffer layer is reduced, to structure the tapered section as a ramp having a continuously changing incline in its longitudinal direction as well as a smooth transition into the exposed buffer layer. In this manner, the length of this section may be limited to an absolutely necessary extent and, more particularly, an improvement is achieved in the shift of the propagation image in the vertical direction.
In the context of the invention and its embodiments, it is of essential significance that the height of the buffer layers increases in the direction of the substrate. In such an embodiment the spacing between the waveguide layers between the buffer layers below the TM focussing rib increases and an optimum propagation is assured in respect of the TE as well as of the TM component.
Embodiments of the invention will hereafter be described in more detail with reference to the schematic drawings, in which:


REFERENCES:
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patent: 5278926 (1994-01-01), Doussiere
patent: 5657338 (1997-08-01), Kitamura
patent: 5985685 (1999-11-01), Lealman et al.
patent: 6030540 (2000-02-01), Yamamoto et al.
Mörl et al, “Uncladded InGaAsP/InP rib waveguides with integrated thickness tapers for efficient fibre-chip butt coupling”, Electronics Letters, vol. 32, No. 1, Jan. 4, 1996, pp. 36-38.*
Albrecht et al, “Polarization Independent Integrated Mode Transformer For Uncladded InGaAsP/InP Rib Waveguides Without Epitaxial Regrowth”, 22nd European Conference on Optical Communication (ECOC '96), Oslo Norway, Sep. 1996, paper TuC3.3, 4 pages.*
Zengerle et al, “Low-Loss Fibre-Chip Coupling By Buried Laterally Tapered InP/InGaAsP Waveguide Structure”, Electronics Letters, vol. 28, No. 7, Mar. 26, 1992, pp. 631-632.*
Wenger et al, “Design and Fabrication of Monolithic Optical Spot Size Transformers (MOST's) for Highly Efficient Fiber-Chip Coupling”, Journal of Lightwave Technology, vol. 12, No. 10, Oct. 1994, pp. 1782-1790.

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