INTEGRATED OPTICAL DEVICE PROVIDING ATTENUATION

Abstract: The application describes the addition of serrations to the edge of the light transmissive layer of an integrated optical device. This enables scattered background light to be coupled out of the device, improving the signal-to-noise ratio.

Description: INTEGRATED OPTICAL DEVICE PROVIDING ATTENUATION TECHNICAL FIELD This invention relates to integrated optical devices such as integrated silicon waveguides for use in optical circuits. BACKGROUND ART Integrated optical devices can include silicon waveguides formed on the upper surface of a silicon wafer. Figure 1 shows such a construction, in which a rib waveguide 10 is formed on a layer of silicon 1 2. The silicon layer 1 2 is silicon-on-insulator, having been grown epitaxially over a silica layer 1 4 within a silicon wafer 1 6. The entire waveguide is coated for protective purposes with the silica layer 1 8. As a result, light propagates within the waveguide 1 0. The actual distribution of optical energy is in fact within the zone 20. This extends within the upstanding waveguide rib 1 0, but is principally within the SOI layer 1 2 and does in fact extend slightly either side of the waveguide rib 10. Some stray light will inevitably be lost from the waveguide. This will normally propagate within the SOI layer 1 2, being retained therein by internal reflection. Eventually it may be reflected into a receiver photodiode present on the chip, thus increasing the cross talk signal and thus increasing the cross-talk signal and decreasing the signal to noise ratio for the device as a whole. The performance of the device could therefore be improved by eliminating such stray light. It is known to provide locally doped areas within the SOI layer. These act as absorbent areas for stray light, which is then dissipated as heat. DISCLOSURE OF INVENTION The present invention provides an integrated optical component comprising a light transmissive layer, the layer having at least one serration formed along an edge thereof. Such serrations will provide alternative angles for the internal light to be reflected, with minimised perpendicular reflections. Serrations also generate multiple reflections for the stray light, at least some of which will be lossy. These effects will reduce the proportion of stray light which is returned to the active region of the device. It is clearly preferred that there are a plurality of such serrations along the edges of the light transmissive layer. It may be possible to design the layer such that the majority of the scattered light is captured by a smaller number of suitably located serrations. However, it is preferred if substantially all edges of the layer include serrations. The serrations can be substantially uniform. However, it may in particular designs be preferable to arrange the serrations non-uniformly, for example at a variety of angles. As the angles subtended by the convergent sides of the serration decreases, so the likelihood of an incoming beam becoming trapped within the serration increases. During such trapping light beams will undergo multiple reflections, all of which will incur some attenuation. For this reason, it is preferred that this angle, hereinafter referred to as a, is minimised. A preferred maximum is twice the critical angle of internal reflection of the material of the light transmissive layer (hereinafter θ c ). When a is below this angle, any beam which succeeds in reflecting internally into the serration must be channelled towards the tip, its angles of incidence at successive reflections decreasing. Eventually, the angle of incidence may decrease sufficiently for near complete refraction to take place, coupling the beam out of the transmissive layer. The usual material for the light transmissive layer is silicon, usually presented as silicon-on-insulator. A typical insulator is silica. The refractive index of silicon is approximately 3.5, giving a critical angle of approximately 1 7 ° (ignoring the effect of any protective layers of silica). BRIEF DESCRIPTION OF DRAWINGS Embodiments of the present invention will now be described, by way of example, with reference to the accompanying figures, in which; Figure 1 is a perspective view of a SOI waveguide structure; Figure 2 is a perspective view of a first embodiment of the present invention; Figure 3 is a top view of the embodiment of Figure 2; Figure 4 is a vertical section through IV-IV of Figure 3; Figure 5 is a top view of a second embodiment of the present invention; Figure 6 is a top view of a third embodiment of the present invention; and Figures 7 and 8 illustrate the operation of the present invention. Figure 1 has been described already, and therefore no further description will be given herein. Figures 2, 3 and 4 show a first embodiment of the present invention. The silicon wafer 1 6 includes a silica insulation layer 14, on top of which is an epitaxial silicon transmissive layer 1 2. For clarity, the silica protective layer 1 8 has not been illustrated. At the edge of the silicon transmissive layer 1 2, a plurality of serrations 22 are provided. These can be formed straightforwardly by etching the upper silicon layer by way of a suitable mask. Figure 3 shows the angle of the convergent sides of the serration, marked as a. Figure 5 shows a second example in which a is very much smaller. The advantage of this will be apparent from the description to follow. Figure 6 shows a third embodiment in which the serrations are rounded, but still subtend an angle a along the roughly straight portion. Whilst this arrangement is likely to be slightly less efficient, it may be easier to manufacture. Figure 7 illustrates a principle by which the invention is considered to work. It is necessary to consider a single serration 22 and an incoming scattered light beam 24, arriving at an angle which is in principle random. This will make an angle of incidence θ with one edge of the serration 22, and the beam will be internally reflected at the edge of the serration. It will then meet the opposite edge of the serration at an angle of θ - a, i.e. significantly less than its original angle of incidence. In the case illustrated in Figure 7, the angle of incidence θ - a is less than the critical angle and a significant proportion of the light beam 24 will therefore escape by refraction. Thus, the power of the reflected part of the light travelling on inside the transmissive layer is strongly attenuated. Some incident angles may result in total internal reflection at both sides and still be reflected back into the transmissive layer. However, these are likely to be substantially refracted out at a further edge of the chip, particularly if the chip edges are non-perpendicular or if the serrations are not all aligned substantially parallel as illustrated in Figures 3, 5 and 6. Figure 8 illustrates the advantage of decreasing a. An incoming beam 24 is still internally reflected at the first edge of the serration 22. Having struck the first edge at a more glancing angle, θ is therefore larger and, a being smaller, θ - a is likely to remain above the critical angle. However, the geometry of this situation means that the next successive reflection leaves the beam 24 still directed toward the tip of the serration 22. As reflection continues, θ - n will eventually become less than the critical angle. Further multiple reflections become steadily more lossy. Most of the beam may therefore eventually couple out of the serration, but by then it will have experienced several lossy reflections such that both the light remaining inside the transmissive layer and the one emitted into the device package will be strongly attenuated. Thus, through the present invention, a beam of stray light will be subjected to multiple reflections at each chip edge that it meets. Undergoing such reflective and (in particular) refractive processes will compound the attenuation effect and result in a high total attenuation.

Claims: CLAIMS 1 . An integrated optical component comprising a light transmissive layer, the layer having at least one serration formed along an edge thereof, thereby to cause multiple internal reflection of light and hence attenuation thereof. 2. An integrated optical component according to claim 1 wherein there are a plurality of such serrations along the edges of the light transmissive layer. 3. An integrated optical component according to claim 2 wherein substantially all edges of the layer include serrations. 4. An integrated optical component according to claim 2 or claim 3 wherein the serrations are substantially uniform. 5. An integrated optical component according to any one of claims 2 to 4 wherein the serrations are arranged non-uniformly. 6. An integrated optical component according to claim 5 wherein the serrations are arranged at a variety of angles. 7. An integrated optical according to any preceding claim wherein the angle subtended by the convergent sides of the serration (α), is less than twice the critical angle of internal reflection of the material of the light transmissive layer. 8. An integrated optical component according to any preceding claim wherein the transmissive layer is of silicon. . An integrated optical component according to claim 8 being a silicon- on-insulator device. 0. An integrated optical component substantially as any one described herein with reference to and/or as illustrated in the accompanying figures 2 to 8

Inventors: BAXTER STEPHEN MARK, MCKENZIE JAMES STUART, BAXTER, STEPHEN, MARK, MCKENZIE, JAMES, STUART
Applicants: BOOKHAM TECHNOLOGY LTD, BOOKHAM TECHNOLOGY LIMITED
Application year: 1999
Main classification: G02B6
IPC codes: G02B6