GB2391952A

GB2391952A - Optical device with optical filter film

Info

Publication number: GB2391952A
Application number: GB0218759A
Authority: GB
Inventors: Craig John Moir; Jolyon Richard Tidmarsh; Jianguo Yao
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2002-08-13
Filing date: 2002-08-13
Publication date: 2004-02-18
Also published as: GB0218759D0

Abstract

An optical device consists of an integrated optical component such as silicon waveguide 1 and an optical fibre 3 optically coupled to each other, in which an end face 5 of the integrated optical component and/or an end face 7 of the optical fibre has a film of material 9 on it that acts as an optical filter and which is different to that of its respective end face. The optical device may be an optical transceiver in which light transmitted by the transceiver g 1 passes through the film, and light received by the transceiver g 2 is reflected by the film. The device may be integrated on semiconductor optical silicon-on-isolator (SOI) chip 15 with a "transmit" laser diode 19 and "receive" photodiode 17 and monitoring or "back facet" diode 21 which monitors the output of laser diode 19.

Description

1 2391 952

Optical Device The present invention relates to optical devices, and especially to integrated optical devices. The invention has particular relevance to optical connections between optical fibres and the integrated optical components (e.g. waveguides) of such devices. Optical devices according to the invention preferably comprise optical transceivers.

Optical transceivers which are formed from integrated optical devices (optical chips) are known. However, such known transceivers suffer from the disadvantages that due to the need to split the optical signals between the transmitting and the receiving functions on the chip, the size of the chip and the degree of the integration on the chip both generally need to be large. This tends to result in high manufacturing costs, and also in a high risk of device failure due to the complexity of the device. Additionally, the need to connect one or more optical fibres to the optical chip (in order to transmit optical signals to and from the chip) can cause optical alignment problems and can result in high optical losses. Overcoming these problems normally requires active optical alignment techniques during manufacturing, resulting in increased manufacturing costs.

The present invention seeks to provide a technically elegant solution to all of these problems, which is applicable to optical devices generally, but which has immediate applicability to optical transceivers in particular.

Accordingly, a first aspect of the invention provides an optical device comprising an integrated waveguide, an end face of the waveguide having thereon a film of a material different to that of the end face, wherein the film acts as an optical filter.

i A second aspect of the invention provides an optical device comprising an integrated optical component and an optical fibre optically coupled to each other, in which an end face of the integrated optical component and/or an end face of the optical fibre has thereon a film of material different to that of its respective end face, wherein the film acts as an optical filter.

Preferably the integrated optical component of the second aspect of the invention comprises an integrated waveguide.

Accordingly, a third aspect of the invention provides an optical device comprising an integrated waveguide and an optical fibre optically coupled to each other, in which an end face of the integrated waveguide and/or an end face of the optical fibre has thereon a film of a material different to that of its respective end face, wherein the film acts as an optical filter.

Preferably the film acts as a wavelength selective optical filter and/or as an optical power splitter.

The film preferably acts as a thin film filter. Preferably the thickness of the (or each) film is no greater than 1.0 m, more preferably no greater than 0.5 m, especially less than 0.5 Am. Preferably there is a plurality of such films on the end face. Preferably there are at least five films, more preferably at least ten films, for example fifteen films, on the end face.

A fourth aspect of the invention provides an optical transceiver comprising an optical device according to any one of the first, second or third aspects of the invention.

t Preferably the integrated waveguide and/or the integrated optical component referred to herein comprises a planar lightwave circuit waveguide and/or optical component. Such waveguides and/or components may, for example, be formed from silica or silicon nitride or polymer, generally on a supporting substrate, for example a silicon wafer (e.g. silica on silicon).

Additionally or alternatively (and generally preferably), such waveguides and/or optical components may be formed from semiconductor. The semiconductor may, for example, be silicon (e.g. silicon-on-insulator (SOI) -

i.e. a silicon epitaxial layer situated on a "buried" layer of silica), or indium phosphide, but other semiconductor materials could be used. The waveguides are preferably rib waveguides, but other waveguide types may be used. Other preferred and optional features of the invention are described below and in the dependent claims.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 shows, in views (a) and (b), schematic diagrams of embodiments of two optical transceivers according to the invention; Figure 2 is a schematic diagram of another embodiment of an optical transceiver according to the invention; Figure 3 shows, in views (a) and (b) two schematic diagrams of another embodiment of an optical transceiver according to the invention; Figure 4 is a schematic cross-sectional view of another embodiment of an optical transceiver according to the invention; and

i Figure 5 is a schematic plan view of a further embodiment of an optical transceiver according to the invention.

Figure 1(a) is a schematic representation of an embodiment of an optical transceiver according to the invention, in which a semiconductor integrated waveguide 1 (preferably fabricated from silicon) is optically coupled to an optical fibre 3. The semiconductor waveguide 1 is tapered, such that an end face 5 of the waveguide is of similar size to an end face 7 of the fibre 3 so that optical losses (for optical modes propagating between the fibre and the waveguide) due to mismatch between the dimensions of the fibre and the waveguide are minimised. That is, the end face of the waveguide is designed to have similar dimensions to the cross-section of the optical fibre, and the waveguide tapers down to smaller cross-sectional dimensions in order to match the mode profile of a laser (see Figure 2) optically coupled to the waveguide. The waveguide taper may, for example, be substantially as described in international patent application WO 01/27670 (Bookham Technology pie) the entire disclosure of which is incorporated herein by

reference. The end face 5 of the semiconductor waveguide 1 is coated with a plurality of films 9 of dielectric materials. For example, for embodiments in which the semiconductor of the waveguide 1 is silicon, the films 9 coated on the end face 5 of the waveguide preferably comprise silica and/or silicon nitride. More preferably the films 9 comprise alternating films of silica and silicon nitride. The films 9 on the end face 5 of the semiconductor waveguide 1 act as a wavelength selective optical filter, such that they are substantially transparent to a first wavelength of light but are substantially reflective to a second wavelength of light.

The end face 5 of the semiconductor waveguide is arranged in close proximity (preferably within about twenty microns) to the end face 7 of the optical fibre 3 so that there is good optical coupling between the two end

faces. The end face 7 of the optical fibre 3 can be cleaved (as shown) at an angle which is not perpendicular to the axis of the fibre. The axes of the fibre 3 and the semiconductor waveguide 1 should normally be angled with respect to each other (at a non-zero angle of inclination) and the end faces may still be in close proximity to each other.

Adjacent to the optical fibre 3 and the semiconductor waveguide 1 is a photodetector 11, which preferably is a photodiode (a "receive photodiode").

The device shown in Figure 1(a) functions as follows. Light of a first wavelength (A i) from a light source (not shown), preferably a laser (especially a laser diode) which preferably forms part of the device, is propagated along the semiconductor waveguide 1 and exits the waveguide via the end face 5 of the waveguide. Because the film filter on the end face 5 of the waveguide is substantially transparent to the first wavelength, this light passes through the filter, and is refracted into the optical fibre 3 via the end face 7 of the fibre.

This is the "transmit" function of the transceiver.

Light of a second wavelength (A 2) iS received by the device via the optical fibre 3. This second wavelength light exits the end face 7 of the optical fibre 3, and because the film filter on the end face 5 of the semiconductor waveguide 1 is substantially reflective to this second wavelength, it is reflected by the film filter and is directed towards the photodetector 11, which is suitably arranged with respect to the fibre and the waveguide to receive and detect this reflected light. This is the "receive" function of the device. The device therefore functions as a transceiver.

An alternative embodiment of the device is shown in Figure 1(b). This embodiment is identical to that shown in Figure 1(a), and it functions in exactly the same way, except that the films 9 of dielectric material are coated on the end face 7 of the optical fibre 3 instead of the end face 5 of the semiconductor

waveguide 1. However, the transmission of the first wavelength light and the reflection of the second wavelength light occurs in exactly the same way as in the embodiment of Figure 1(a).

Instead of comprising a wavelength selective filter, the films 9 of the devices shown in Figure 1 may comprise an optical power filter. For example, the film filter may comprise a semi-transparent reflector, such that at least a portion of the light transmitted from the semiconductor waveguide propagates through the filter but at least a portion of the light received from the optical fibre is reflected by the filter and is received by the photodetector. All of the film filters described and/or illustrated herein may be wavelength selective filters or optical power splitters.

The films may be formed generally by any film-forming technique, for example sputtering, vapour deposition, evaporation, and the like.

Figure 2 is a schematic representation of an embodiment of an optical transceiver device according to the invention which is similar to that illustrated in Figure 1 (a). The main difference between this embodiment and the Figure 1 (a) embodiment is that the light reflected by the films 9 on the end face 5 of the semiconductor waveguide 1 is received by another integrated semiconductor waveguide 13 and is guided by this waveguide to the receive photodiode 11. The functioning of the transceiver device, and especially the manner by which the film filter determines the transmission and reception of light by the transceiver, is identical to that of the Figure 1 (a) embodiment.

Figure 2 shows (albeit schematically) an integrated semiconductor optical chip 15 on which the waveguides 1 and 13 (and other optical components, described below) are integrated. As already mentioned, the integrated waveguides preferably are fabricated from silicon semiconductor (although other semiconductors could be used instead). The optical chip 15 therefore preferably comprises a silicon chip. More preferably, the optical chip

may comprise a so-called "silicon-on-insulator" ("SOI") chip, comprising an upper layer (often termed an epitaxial layer because it is normally produced epitaxially) of silicon, which is separated from a lower substrate layer (normally also of silicon) by a "buried" layer of silicon dioxide (silica). The silicon dioxide layer is electrically insulating, and this property is useful in integrated electronic devices fabricated from silicon chips, hence the term "silicon-on-insulator". In the present invention, because the silicon chip is an optical chip the silicon dioxide functions primarily as an optical confinement layer. The semiconductor waveguides 1 and 13, and the other optical components, are fabricated (e.g. by photolithography and etching techniques) in the upper (epitaxial) layer of the silicon chip 15. The other optical components comprise the receive photodiode 17, (which performs the "receive" function of the transceiver), the laser diode 19 (which performs the "transmit" function of the transceiver) and a monitoring photodiode 21, which monitors the output of the laser diode 19. As shown in Figure 2, the monitor photodiode 21 is a so-called "back facet" monitor. It may, for example, be as described in international patent application WO 00/79658 (Bookham Technology pie), the entire disclosure of which is incorporated herein by

reference. Other arrangements, including front facet monitoring may, however, be used.

Both integrated semiconductor waveguides 1 and 13 (which are preferably rib waveguides) include tapered regions 23 by which their relatively large end faces (so designed for good optical coupling with the optical fibre 3) taper to relatively narrow regions (which preferably are single mode regions). I The optical fibre 3 extends onto the optical chip 15 via a groove 25 fabricated in the chip, which ensures the correct positioning between the fibre 3 and the integrated waveguides 1 and 13. The groove 25 and the positioning of the fibre 3 may, for example, be substantially as described in international patent application WO 99/57591, the entire disclosure of which is incorporated herein

i by reference. Because the positioning of the fibre is determined by the groove 25, and the groove is positioned with respect to the waveguides 1 and 13 by the groove and the waveguides being fabricated in an integrated manner, for example by photolithographic and etching techniques, active optical alignment techniques generally are not required during manufacturing (or use) of the device. Additionally, because the splitting of the light between the "transmit" and the "receive" functions is achieved by the elegant expedient of the film filter on the end face of one of the integrated waveguides (or on the end face of the optical fibre) optical splitting components (for achieving this function) do not need to be integrated onto the optical chip, thereby reducing the required size of the chip and reducing the manufacturing complexity and associated costs. Also, the number of optical interfaces is reduced, thereby further reducing optical losses.

Figure 3 shows two schematic diagrams of another embodiment of an optical transceiver according to the invention. View (a) is a schematic plan view, and view (b) is a schematic cross-sectional view along cross-section A A indicated in view (a). I The embodiment illustrated in Figure 3 is similar to that illustrated in Figure 2, the main difference being that in the Figure 3 embodiment the receive photodiode 17 is not integrated on the semiconductor optical chip 15.

The optical chip 15 and the receive photodiode 17 are mounted on a common substrate 29, which is preferably formed from a ceramic material (e.g. a ceramic tile). In order for light from the optical fibre 3 reflected by the film filter i 9 on the end face 5 of the integrated semiconductor waveguide 1 to be received unimpeded by the photodiode 17 adjacent to the optical chip 15, a groove 31 is provided in the chip between the end face 5 of the waveguide I and the photodiode. This groove 31 is in addition to the groove 25 in which the optical fibre 3 is received and positioned.

The fact that in the Figure 3 embodiment the receive photodetector (i.e. photodiode 17) is not integrated on the semiconductor chip 15 has the advantages that the size of the optical chip may be reduced and the degree of complexity of the optical chip design is reduced. These advantages are achieved while retaining the advantages described above of simple and accurate optical alignment and the avoidance of the need for complex optical splitting components to be integrated on the optical chip. Providing the receive photodiode adjacent to the optical chip on the substrate 29 on which the chip is mounted is space efficient since it may be situated adjacent to electronics 33 associated with the device, which are also mounted on the substrate. As already mentioned, view (b) of Figure 3 shows cross-section A-A of the device. The optical fibre 3 is shown (schematically) received in the semiconductor optical chip 15. The receive photodiode 17 is shown mounted on a support 35 (preferably a ceramic block) which itself is mounted on the common ceramic substrate 29. Both the chip 15 and the ceramic support 35 are bonded by adhesive 37 (e. g. an epoxy resin) to the substrate 29. I Figures 4 and 5 show schematic representations of further variants of optical transceiver devices according to the invention. In both variants the film optical filter is provided on an end face 7 of an optical fibre 3 (similarly to the embodiment shown in Figure 1(b)). In these variants "free space" optics are; used rather than integrated semiconductor waveguides. In Figure 4, the receive photodiode 17 is situated in a recess 39 in the upper surface of the semiconductor optical chip 15. Light received by the photodiode 17 is reflected downwardly by the film filter 9 at the end face of the optical fibre at an angle of substantially 90 degrees to the axis of the fibre, due to the cleavage angle of the end face 7 of the optical fibre being at substantially 45 degrees to the axis of the fibre. Light transmitted by the laser diode 19 is focussed in a ball lens 41 mounted on the optical chip 15 and received and guided by the optical fibre 3. In the Figure 5 variant, the light reflected by the

film filter 9 at the end face 7 of the optical fibre is directed through substantially 90 degrees in a direction substantially parallel to the plane of the optical chip 15. In this latter variant both the receive photodiode 17 and the monitor photodiode 21 are situated in recesses 39 in the optical chip.

Claims

i CLAIMS

1. An optical device comprising an integrated waveguide, an end face of the waveguide having thereon a film of a material different to that of the end face, wherein the film acts as an optical filter.

2. An optical device comprising an integrated optical component and an optical fibre optically coupled to each other, in which an end face of the integrated optical component and/or an end face of the optical fibre has thereon a film of material different to that of its respective end face, wherein the film acts as an optical filter.

3. An optical device according to claim 2, in which the integrated optical component comprises an integrated waveguide.

4. An optical device comprising an integrated waveguide and an optical fibre optically coupled to each other, in which an end face of the integrated waveguide and/or an end face of the optical fibre has thereon a film of a material different to that of its respective end face, wherein the film acts as an optical filter.

5. An optical device according to any preceding claim, in which the film acts as a wavelength selective optical filter.

6. An optical device according to claim 5, in which the film is substantially transparent to a first wavelength of ilght and substantially reflective to a second wavelength of light.

7. An optical device according to any preceding claim, in which the film acts an optical power splitter.

8. An optical device according to any one of claims 2 to 7, in which the film filter acts to allow light to enter the optical fibre through the end face of the fibre, and acts to reflect light which has propagated along the fibre in a direction towards the end face of the fibre.

9 An optical device according to any preceding claim, in which there is a plurality of said films on said end face(s).

10. An optical device according to claim 9, in which there are at least five said films, preferably at least ten said films, on said end face(s).

11. An optical device according to claim 9 or claim 10, in which said plurality of films comprise layers of the films coated one on top of another.

12. An optical device according to any one of claims 9 to 11, in which each film has a composition which differs to the (or each) immediately adjacent film.

13. An optical device according to any one of claims 9 to 12, in which each film comprises silicon dioxide or silicon nitride.

14. An optical device according to any preceding claim, in which said end face comprises the same material as the rest of the waveguide, component or fibre, respectively.

15. An optical device according to any preceding claim, in which the, or each, film has a thickness no greater than 1.0 Am, preferably no greater than 0.5 m.

16. An optical device according to any one of claims 2 to 15, in which the end face of the optical fibre is not perpendicular to the longitudinal axis of the fibre.

17. An optical device according to any preceding claim, in which the integrated waveguide or optical component is a planar lightwave circuit (PLC) waveguide or component.

18. An optical device according to any preceding claim, in which the integrated waveguide or optical component is a semiconductor integrated waveguide or a semiconductor integrated optical component.

19. An optical device according to any preceding claim, in which the semiconductor is silicon.

20. An optical device according to claim 1, 3 or 4, or any claim dependent thereon, in which the waveguide is a rib waveguide.

21. An optical transceiver comprising a device according to any preceding claim.

22. A transceiver according to claim 21, comprising a light source and a photodetector.

23. A transceiver according to claim 21 or claim 22, arranged such that light transmitted by the light source passes through said film(s).

24. A transceiver according to any one of claims 21 to 23, arranged such that light received by the photodetector has been reflected by said film(s).

25. A transceiver according to any one of claims 22 to 24, in which the = light source is a laser, preferably a laser diode.

26. A transceiver according to any one of claims 22 to 25, in which the photodetector is a photodiode.

27. An optical device substantially as herein before described and/or as illustrated in the accompanying figures.