A DEVICE FOR REFLECTING LIGHT

Abstract: A device for reflecting light waves to be used when building optical systems for various applications and special devices, is built on a substrate and has a planar structure. It comprises a light power coupler (1) of the MMI-type, configured as a rectangular plate at or in the surface of the substrate. The coupler splits light incoming on an input terminal into two equal portions, each portion delivered on an output terminal. Also the coupler combines light incoming on two output terminals into combined light delivered on the input terminal. A loop is connected to the two output terminals for conducting light delivered on each one of these output terminals back into the other one of these output terminals. The loop is a planar waveguide built on the substrate, connected to an edge surface of the coupler. The loop has an outer contour (7) comprising a multitude of linear segments and it can also have an inner contour (9) having a polygon shape. The shape of the loop can be formed by folding a strip having a uniform width at least twice, the folds being symmetrically made.

Description: A DEVICE FOR REFLECTING LIGHT TECHNICAL FIELD The present invention relates to a device for reflecting light also called an optical reflection plug. 5 BACKGROUND OF THE INVENTION When building optical systems for various applications and special devices, there may be a need for providing a total reflection of light. At present there is no good established method to produce a total reflection of light independently of the wavelength of the light. Metallizing the end of the waveguide by aluminum has been mentioned as a possible way, see ιo M.V. Bazylenko, M. Gross, E. Gauja, and P.L. Cchu, "Fabrication of Light- Turning Mirrors in Buried-Channel Silica Waveguides for Monolithic and Hybrid Integration", J. Lightwave Technol. Vol. 15(1), pp. 148 - 153, 1997. When making optical integrated circuits having integrated waveguides metallization of vertical end surfaces is a costly and complicated process. Also the lifetime can be questioned. A method of producing total i5 reflection for a relatively narrow wavelength range is to use Bragg gratings. Using special designs such as varying the grating period in the propagation direction a total reflection over a wider range could possibly be achieved. However, it is relatively costly to produce Bragg gratings and the production thereof requires costly equipment as well as a large area on the chip in the case where the totally reflecting structure is to be incorporated in an integrated 2 o circuit structure. In L.V. Iogansen et al. , "Multimode fiber interferometers" , Optics and Spectroscopy, Vol. 58, No. 5, May 1985 optical reflectors using resonant tunnel loop reflection are disclosed. Simple loop reflectors are shown in fig. 5 in this document. There are some losses in such a loop reflector comprising loss in the waveguide, loss in the tunnel-coupling section, 2 5 and loss of radiation in the loop. In the Japanese patent application JP 8-274398 a semiconductor laser module is disclosed. It has a loop mirror comprising a four- terminal fiber coupler and a fiber loop connected to the coupler. SUMMARY OF THE INVENTION 3 o It is an object of the invention to provide a device for reflecting light waves to be easily incorporated in integrated optical circuits. A device for reflecting light waves is built on a substrate and has a planar structure. It comprises a light power coupler operating as a power splitter, preferably of the MMI-type, and configured as a rectangular plate at or in the surface of the substrate. The coupler splits 3 & light incoming on an input terminal into at least two equal portions, each portion delivered on an output terminal. Also the coupler combines light incoming on at least two output terminals into combined light delivered on the input terminal. A loop is connected to two output terminals for conducting light delivered on each one of these output terminals back into the other one of these output terminals. The loop is a planar waveguide built on the substrate, connected to an edge surface of the coupler. The loop has preferably an outer contour comprising a multitude of linear segments and it can also have an inner contour comprising a multitude of linear segments. The shape of the loop can e.g. be formed by folding a strip having a uniform width at least twice, the folds being symmetrically made. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail by way of non-limiting embodiments with reference to the accompanying drawings, in which: Fig. 1 is a schematic view from above illustrating the principle of a loop mirror structure, Fig. 2 is a view from above of a loop mirror made as a planar structure on a substrate, Fig. 3 is a view from above of an alternative embodiment of a loop mirror made as a planar structure, Fig. 4a is an enlarged view of the loop waveguide of Fig. 2 illustrating the reflection conditions, s Fig. 4b is a sectional partial view of the loop waveguide as seen in the direction of the arrow A of Fig. 4a illustrating an embodiment of construction of the loop, in particular at the reflecting surfaces, Fig. 5 an enlarged view of the loop waveguide of Fig. 3 illustrating the reflection conditions, o Fig. 6 is a view from above of a loop mirror having a loop integrated in a coupler, and Fig. 7 is a view from above of an alternative embodiment of a loop mirror having a loop integrated in a coupler. DESCRIPTION OF PREFERRED EMBODIMENTS In Fig. 1 the basic construction of a loop mirror is illustrated. A loop mirror comprises 5 a 1x2 optical coupler 1 , in the preferred case a 1x2 MMI (Multi Mode Interference) waveguide coupler. The coupler operating as an equal power splitter having one input terminal and two output terminals. The two output terminals connected to each other through a light waveguide loop 3. MMI devices are described in e.g. L.B. Soldano and E.C.M. Pennings, "Optical Multi-Mode Interference Devices Based on Self-Imaging: Principles and o Application" , J. Lightwave Technol. Vol. 13(4), pp. 615 - 627, 1995, and the published International patent application WO97/35220. The waveguide 3 may as conventional be an optical fiber but in the embodiment considered herein it is made as a planar structure to be used e.g. in PLCs (Planar Lightwave Circuits). The waveguide 3 is then made as a strip of a first material embedded in a layer of a 5 second material having a different refractive index, the layer and strip being produced at or in the surface of a substrate, basically like electronic integrated circuits. The second material can be air and then the strip can be produced by etching a layer applied to the surface of the substrate. Typically however, the waveguide is made as a rectangular waveguide comprising a core having a rectangular cross-section made by etching a layer having a suitable refractive index and further comprising an undercladding layer and an overcladding layer, these different layers being formed of e.g. doped silicon oxide material or polymer layers. The waveguides used, both for conducting light at the input side of the coupler 1 and in the reflecting loop are preferably always designed to be single-mode or monomode type. The loop 3 as illustrated in Fig. 1 is configured as a curved strip having a smooth path joining the terminal portions at the outputs of the coupler, these terminal portions being parallel to each other. The curved smooth portion of the loop has over a substantial part thereof the shape of part of a circle and thus has there a constant curvature. The maximum curvature must always be smaller than a value set by the difference of the refractive index of the waveguide and that of the surrounding material, since larger curvatures and thus a smaller radius of the loop would give unacceptable losses. For a small radius a too large portion of the energy of light propagating in the reflecting loop would leak out of the loop. Furthermore, if the difference of the refractive index n of the waveguide and that (Π Q ) of the surrounding material is large the loop will have a reasonably small diameter. For instance, s for a relative refractive index difference of 1.5 %, the diameter can be smaller than 4 mm. For smaller refractive index differences the circular loop structure will have a larger diameter and can in some cases take a too large space on a substrate. Instead the waveguide of the loop can be made to have substantially a polygon shape, as is illustrated by the embodiments illustrated in the plan views of Figs. 2 and 3. The polygon o loop 5 is also here single-mode and comprises a curved strip-shaped region having parallel and identically shaped connection regions connected to the two outputs of the coupler 1. The waveguide 5 is limited by an exterior contour line 7 and an inner contour line 9, each being a non-closed polygon line comprising a plurality of straight linear segments. The start and end line segments of each polygon line 1. 9 are thus parallel to each other and extend from the 5 output side of the coupler 1 at right angles thereto. The two contour lines 7, 9 are, like the loop of Fig. 1, symmetrical about a symmetry line extending from the center of the line connecting the outputs of the coupler 1 , i.e. about the longitudinal axis of the coupler in the case where it is an MMI-device. In the loop waveguide structure 5 of Fig. 2 the exterior contour line 7 of the waveguide 0 comprises straight linear pieces having directions deviating from each other by 45° and thus forming angles of 135° to each other, at the four comers of the totally three straight line segments required to connect the parallel start and end segments. The inner contour line 9 has only a total of three linear segments, one segment connecting the start and end segments at angles of 90°. The comers of the inner contour line are symmetrically located in relation to 5 two corresponding comers of the exterior contour line 7, these two comers connecting the line segments connected directly to the start and end segments. The propagation of light in such a waveguide is illustrated in Fig. 4a. It appears from Fig. 4a, that for the wavelength of the used light, the material of the waveguide 3 should have such a refractive index n in relation to the refractive index the surrounding material, e.g. air, that the light propagating in the waveguide will be totally reflected for all incident angles η larger than about 45°, i.e. practically for all incident angles which are larger than an angle somewhat smaller than 45°, see A. Stano, L. Faustini, C. Coriasso, D. Campi, C. Cacciatore, "OPTIMIZED WAVEGUIDE-INTEGRATED MIRRORS", Proc. ECIO '97 EWF3, Stockholm, 1997, for a detailed discussion. In Figs. 4a and 4b also a construction of the loop waveguide 5 comprising layers on a substrate, not shown, is illustrated. The waveguide is constructed of an undercladding 11, a core or waveguide layer 13 and an overcladding 15. The layers forming the undercladding, the waveguide core and the overcladding are etched to have the required shape. At the reflecting surfaces 17 of the loop etched rectangular recesses or vents 19 are provided which have accurately flat surfaces at the waveguide, these flat surfaces also being accurately perpendicular to the large surfaces of the structure. If the total reflection condition cannot be fulfilled in the waveguide layout of Figs. 2 and 4a, a structure requiring a total reflection only for all incident angles η larger than s somewhat more than 30° can be used, as is illustrated by the embodiment of Fig. 3. Here the exterior contour line 7 comprises seven linear segments located in angles of 150° to each other what corresponds to the condition that a linear segment of the contour has a direction deviating by an angle of 30° from those of the line segments connected to the considered linear segment. The interior contour line 9 comprises five linear segments having directions o deviating from each other by 60° or located in angles of 120° to each other. The comers of the interior contour line 9 are also here located symmetrically in relation to segments of the exterior contour line, on the line passing such a segment centrally at right angles. The propagation of light in this loop structure is illustrated in Fig. 5. The waveguide can be made to include reflective surfaces to air like the embodiment of Fig. 4a by providing recesses 19 5 from the surface of the structure. The contour lines of the structures of Figs. 2 and 3 can be obtained by folding a strip of material having a uniform width two and three times respectively. This can easily be generalized to folding more times, say n times. Then the resulting structure will comprise an outer contour line of 2n+ l line segments and an interior contour having n+ 1 line segments. 0 The segments of the exterior contour have directions deviating from those of the adjacent segments by angles of 90°/n and the segments of the interior contour have directions deviating by angles of 2-90/n. The used light must be totally reflected at the border surface between the material of the waveguide and the surrounding material at all incident angles about 90°/n or larger than somewhat more than 90°/n. 5 A simplified reflecting structure can also be incorporated directly in a MMI-coupler, see Figs. 6 and 7. The MMI-coupler 21 is a planar structure and comprises a thin layer of a material having a refractive index adapted to be higher than that of the surrounding material. The layer has for a simple coupler a rectangular shape and forms the main part 23 of the reflecting structure. The rectangular body 23 is adapted so that for light incoming into the layer centrally at a short side, two images of the incoming light wave will appear at the opposite short side at places symmetrically located in relation to the longitudinal direction of the rectangular shape. In Figs. 6 and 7 these pictures appear at the line 25. In order to make such a simple reflecting structure the rectangular shape 23 is supplemented with a polygon shape 27 at said opposite short side. The polygon has a free polygon line 29, which is symmetrical in relation to the longitudinal direction of the rectangular body 23. The polygon shape is in Fig. 6 an isosceles triangle having a 90° -angle at the free comer. In the embodiment of Fig. 7 it is a symmetric trapezium having its oblique sides located in directions deviating from the direction of the long sides of the rectangular body 23 by angles of 30°, i.e. from the longitudinal direction. For suitable refractive index conditions, as outlined above, light from the two images at the line 25 will be reflected by the oblique sides of the outer polygon shape 29, as is illustrated in Figs. 6 and 7.

Claims: CLAIMS 1. A device for reflecting light waves propagating in an optical waveguide, the device comprising a light power coupler having an input terminal and at least two output terminals, splitting light incoming on the input terminal into at least two shares of light, each share delivered on one of the at least two output terminals and combining light incoming on the at least two output terminals into combined light delivered on the input terminal, the device further comprising a loop connected to two of the output terminals, conducting light delivered on each one of these output terminals back into the other one of these output terminals, characterized in that the device is a planar device built on a substrate, the coupler being a substantially rectangular plate at or in the surface of the substrate and having a refractive index adapted to the refractive index of surrounding material and an adapted size so that at an input terminal is formed at a side of the rectangular plate and at least two output terminals are formed at an opposite side and the loop being formed by a planar waveguide at the surface of the substrate having an adapted refractive index and having an end segment at one of the s output terminals and an end segment at another one of the output terminals. 2. A device according to claim 1, characterized in that the coupler is an MMI coupler. 3. A device according to any of claims 1 - 2, characterized in that the loop is a strip having smooth curved outlines, a major portion of the loop being formed as part of a circle. 4. A device according to any of claims 1 - 2, characterized in that the loop is a strip o having polygon outlines comprising only straight line segments located in angles to each other. 5. A device according to any of claims 1 - 2, characterized in that the loop has a convex polygon outline comprising only straight line segments located in angles to each other, one of the straight line segments being a large side directly connected to that side of the 5 rectangular plate forming the coupler at which the output terminals are formed, the convex polygon being symmetric in relation to a center line of the rectangular plate. 6. A device according to claim 5, characterized in that the rectangular plate forming the coupler and the planar waveguide forming the loop are integrated into one unit plate having a uniform refractive index. o 7. A device according to claim 6, characterized in that the unit plate has an outline corresponding to a rectangle having an isosceles triangle at one of its sides. 8. A device according to claim 6, characterized in that the unit plate has an outline corresponding to a rectangle having a trapezoid at one of its sides. 9. A device for reflecting light waves propagating in an optical waveguide, the device 5 comprising a light power coupler having an input terminal and at least two output terminals, splitting light incoming on the input terminal into at least two shares of light, each share delivered on one of the at least two output terminals and combining light incoming on the at least two output terminals into combined light delivered on the input terminal, the device further comprising a loop connected to two of the output terminals, conducting light delivered on each one of these output terminals back into the other one of these output terminals, characterized in that the loop is a planar waveguide at or in the surface of substrate having a refractive index adapted to the refractive index of material surrounding the planar waveguide and that the loop has the shape of a strip extending from one of the output terminals to another one of the output terminals. 10. A device according to claim 9, characterized in that the strip has smooth curved outlines, a major portion of the strip being formed as part of a circle. 11. A device according to claim 9, characterized in that the strip has polygon outlines comprising only straight line segments located in angles to each other. 12. A device according to any of claims 9 and 11, characterized in that the strip has outlines corresponding to the outlines of a strip which has a uniform width and is folded at least twice.

Inventors: AUGUSTSSON TORSTEN, AUGUSTSSON, TORSTEN
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1999
Main classification: G02B6
IPC codes: G02B5, G02B6