INTEGRATED LIGHT ABSORBER

Abstract: The integrated light absorber comprises a light chamber (2) integrated in an optical chip, the chamber (2) having upper (3A) and lower walls lying substantially parallel to the chip and peripheral walls (3B) extending therebetween and a light input port in a peripheral wall (3B) for receiving light to be absorbed, e.g. from a rib waveguide (1). At least some of the walls have light absorbing properties and the chamber (2) is shaped so that the majority of light which enters the chamber (2) through the light input port undergoes multiple reflections and so is absorbed by the walls of the chamber (2). Circular and star-shaped chambers (2) are described.

Description: INTEGRATED LIGHT ABSORBER TECHNICAL FIELD This invention relates to an integrated light absorber and in particular to a light absorber formed on a silicon chip BACKGROUND ART Certain features of integrated waveguide circuitry call for the dumping or absorption of light in a controlled manner Light may need to be confined so it does not escape into the waveguide substrate and is not back reflected into the waveguide from which it came Low reflection terminations for waveguides based on anti-reflection (AR) coated angled facets exist and are successful at reducing back reflections into a waveguide They are not suitable, however for beam-dumping because light is released into the substrate rather than being absorbed DISCLOSURE OF INVENTION According to the invention there is provided an integrated light absorber comprising a light chamber integrated in an optical chip the chamber being defined by upper and lower walls lying substantially parallel to the chip and peripheral walls extending therebetween and having a light input port in a peripheral wall for receiving light to be absorbed, at least some of the walls having light absorbing properties the chamber being arranged such that the majority of light which enters the chamber through the light input port undergoes multiple reflections and so is confined within the chamoer and absorbed by the walls of the chamber References to upper" and "lower" used herein are to be understood to be relative to the chip in the orientation shown in Figure 5 of the drawings and are not restricted to directions relative to gravitational forces Preferred and optional features of the invention will be apparent from the following description and from the subsidiary claims of the specification BRIEF DESCRIPTION OF DRAWINGS The invention will now be further described merely by way of example with reference to the accompanying drawings in which Figure 1 is a schematic plan view of a first embodiment of an integrated light absorber according to the invention Figure 2 is a schematic plan view of a second embodiment of an integrated light absorber according to the invention Figure 3 is a schematic plan view of a third embodiment of an integrated light absorber according to the invention Figure 4 is a schematic plan view of a fourth embodiment of an integrated light absorber according to the invention and Figure 5 is a perspective view illustrating how an integrated light absorber such as that shown in Figure 1 may be formed in a substrate BEST MODE OF CARRYING OUT THE INVENTION The figures illustrate integrated light absorbers in which light in an integrated waveguide 1 such as a rib (or ridge) waveguide formed on a silicon-on- msulator chip is allowed to escape from an end of the waveguide 1 into an enclosed light chamber 2 integrated on the chip The light chamber 2 is defined by upper and lower walls lying substantially parallel to the plane of the chip (upper wall 3A being shown in Figure 5) and peripheral walls 3B which extend therebetween in a direction substantially perpendicular to the plane of the chip As will be described below at least some of the walls defining the chamber 2 preferably have light absorbing properties and the chamber is arranged such that the majority of the light wnich diverges into the chamber 2 from the waveguide undergoes multiple reflections therein and so is absorbed by the walls of the chamber 2 The peripheral walls 3B of the chamber are preferably arranged so as to reduce the amount of light which undergoes only a small number of reflections before being reflected back into the waveguide 1 The upper wall 3A and peripheral walls 3B of the chamber are preferably coated with a light absorbing material In a sihcon-on-insulator chip the lower wall comprises the interface between the silicon layer in which the chamber 2 and waveguide 1 are formed and the underlying insulating layer e g of silicon dioxide Light enters the chamber 2 through a light input port in the peripheral wall 3B In the arrangement illustrated this comprises the connection between the peripheral wall 3B and the rib waveguide 1 However in other arrangements (not shown) this may comprise a portion of the peripheral wall 3B which permits entry of light from some other source e g an optical fibre Figures 1 - 4 illustrate four possible forms of the light chamber 2 Figure 1 shows the most basic form of chamber which has a circular shape This is the simplest shape to fabricate but has the disadvantage that light entering the chamber 2 on the optical axis of the waveguide will be reflected back to the waveguide by a single reflection from the peripheral wall of the chamber directly opposite the light input port Other paths involving only three or four reflections by which light can be reflected back to the input port also exist However, most paths require the light to undergo a large number of reflections before returning to the waveguide 1 so the light reflected back to the waveguide 1 is greatly attenuated A chamber 2 of multi-sided polygonal shape may be used instead of a circular chamber Figure 2 shows a similar arrangement to Figure 1 but with an axial spike 4 provided on the peripheral wall of the chamber directly opposite the light input port The spike 4 scatters light incident thereon on the optical axis of the waveguide and so prevents direct back reflection to the waveguide 1 and hence removes the largest cause of light being reflected back to the waveguide 1 This principle can be extended by forming a chamber comprising an array of spikes so the chamber has the shape of a multi-pointed star The spikes point towards a central region of the chamber 2 or towards the light input port Figure 3 illustrates such a chamber in which the peripheral walls making up the star shape are substantially straight With such a chamber the only paths having a small number of reflections by which light can be reflected back to the light input port involve reflections from a point either at the bottom 5 or top 6 of a spike The star shape may have an arbitrary number of spikes but preferably has at least six points and most preferably at least eleven points (as shown in Figure 3) The number of reflections thai a typical light ray must undergo before emerging from between two spikes 6 i e before emerging from one of the points 5 of the star shape, can be further increased by making the peripheral walls 3B of the points 5 curved with a substantially parabolic cross-section as shown in Figure 4 The arrangements shown in Figures 3 and 4 can be further improved by providing smaller spikes between the larger spikes making up the star shape i e at the points 5 of the star shape to further reduce the reflection therefrom This process can be continued by forming further miniature spikes between the smaller spikes and so on substantially in the form of a fractal The peripheral walls of the chamber 2 may comprise projections or irregularities of other shapes besides the spikes referred to above The light absorbing chamber may comprise a series of relatively large irregularities between which smaller irregularities are provided and between which yet smaller irregularities are provided again substantially in the manner of a fractal In all the arrangements described, the intention is to minimise the probability of light being coupled back into the waveguide by arranging for the light to undergo multiple reflections in the light absorbing chamber and being attenuated at each reflection It should be noted that the more sophisticated the shape of the chamber the more difficult it will be to fabricate In the majority of applications there will therefore be a trade-off between the degree of attenuation or absorption required and the simplicity and hence cost of the design Figure 5 illustrates how an integrated light absorber such as that shown in Figure 1 may be fabricated The Figure shows the rib waveguide 1 and a circular light absorbing chamber 2 formed by etching away the surrounding areas of the substrate The chamber 2 and waveguide 1 are thus homogenously formed Having defined the shape of the chamber 2 in this way, any oxide thereon is removed and the chamber is preferably engulfed in light absorbing material, e.g. by depositing the material thereon in direct, intimate contact with the silicon. The upper wall 3A and peripheral walls 3B of the chamber are thus coated with light absorbing material The other forms of chamber shown in Figures 2 - 3 may be formed in a similar manner The light absorbing material may be a metallic coating, e.g. of aluminium typically 1 - 2 microns thick, or some other light absorbing material such as an infra-red absorbing glob top, e.g. formed of an epoxy or polymer material, as used in component encapsulation. As mentioned above, the light absorber may be formed on a silicon-on-insulator chip which comprises an upper layer of silicon separated from a substrate e.g also of silicon, by an insulator layer, typically of silicon dioxide The waveguide 1 and light absorbing chamber 2 are etched in the upper layer of silicon

Claims: CLAIMS I An integrated light absorber comprising a light chamber integrated in an optical chip the chamber being defined by upper and lower walls lying substantially parallel to the chip and peripheral walls extending therebetween and having a light input port in a peripheral wall for receiving light to be absorbed, at least some of the walls having light absorbing properties the chamber being arranged such that the majority of light which enters the chamber through the light input port undergoes multiple reflections and so is confined within the chamber and absorbed by the walls of the chamber ? An integrated light absorber as claimed in Claim 1 in which the peripheral walls are arranged so as to reduce the amount of light which undergoes only a small number of reflections before being reflected back to the input port 3 An integrated light absorber as claimed in Claim 2 in which the chamber has a substantially circular or multi-sided polygonal shape 4 An integrated light absorber as claimed in Claim 3 in which the chamber has a substantially multi-pointed star shape 5 An integrated light absorber as claimed in Claim 4 in which the peripheral walls making up the star shape are substantially planar 5 An integrated light absorber as claimed in Claim 4 in which the peripheral walls making up the star shape are curved preferably with a substantially parabolic cross-section An integrated light absorber as claimed in Claims 4, 5 or 6 in which the star shaoe has at least six points and preferably at least eleven points An integrated light absorber as claimed in any of Claims 4 - 7 in which a spike which points towards a central region of the chamber or towards the light input port is provided within each of the points of the star shape An integrated light absorber as claimed in any preceding claim in which a spike is provided on the peripheral wall of the chamber which lies substantially opposite the light input port so as to reduce the direct back reflection therefrom to the input port An integrated light absorber as claimed in any preceding claim in which the peripheral wall of the chamber comprises a series of relatively large irregularities between which smaller irregularities are provided and between which yet smaller irregularities are provided substantially in the manner of a fractal An integrated light absorber as claimed in any preceding claim in which the chamber is defined by removing the surrounding areas of a substrate in which it is formed An integrated light absorber as claimed in Claim 11 comprising a rib waveguide formed in the same substrate for transmitting light to be absorbed into the light input port of the chamber An integrated light absorber as claimed in Claim 11 or 12 formed in a layer of silicon An integrated light absorber as claimed in Claim 13 formed on a silicon- on-msulator chip An integrated light absorber as claimed in any of Claims 1 1 - 14 in which the chamber is engulfed in light absorbing material An integrated light absorber as claimed in Claim 15 in which the light absorbing material comprises a metallic coating, preferably aluminium

Inventors: BARRETT COLIN PAUL, BAXTER STEPHEN MARK, BARRETT, COLIN, PAUL, BAXTER, STEPHEN, MARK
Applicants: BOOKHAM TECHNOLOGY LTD, BOOKHAM TECHNOLOGY LTD.
Application year: 1998
Main classification: G02B6
IPC codes: G02B6