GB2376755A

GB2376755A - Integrated optical device

Info

Publication number: GB2376755A
Application number: GB0115441A
Authority: GB
Inventors: Andrew Michael Tomlinson; John Paul Drake; Haydn Frederick Jones
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-06-23
Filing date: 2001-06-23
Publication date: 2002-12-24
Also published as: GB0115441D0; WO2003001174A1

Abstract

An integrated optical device comprising a waveguide 16 can be tested by forming a recess 23 on the surface across the waveguide, and providing a discrete reflective device 22 insertable into the recess. It is preferred that the reflective device is a prism, insertable into the recess so as to reflect light leaving the waveguide. Suitable design of the prism permits the light to leave vertically if this is desired. The recess is preferably etched by any suitable etching process. It is preferred that the arrangement is employed for selective testing, in which the reflective device will be inserted into and out of the recess as desired or necessary. Accordingly, the reflective device can be supported on a moveable arm to allow its position to be controlled. This could be driven by servomechanical or piezoelectric means, or the like. A testing apparatus is also described, comprising a stage for an integrated optical device, a reflective device moveable towards the stage, and a light sensing device disposed to detect light reflected from the reflective device.

Description

Integrated Optical Device The present invention relates to integrated optical devices, and in particular to arrangements for coupling light out of the device. This is necessary both to extract the output signal, and also during manufacture in order to test the device.

To date, tests of the optical fields in waveguides have been carried out by detection at the edge or facet of the device, or by placing optical fibres in a suitable location so as to receive light from the waveguide, such as in an etched hole.

It is also known to etch a V-groove at the end of the waveguide, transverse thereto, and coat the face of the V-groove opposite the waveguide with a reflective coating such as a metallic or dielectric layer. This is used to deflect light into a detector fixed over the device. Such arrangements for coupling light out of a waveguide to a fixed photodiode placed above the device, are described in US 6, 108, 472.

The present invention therefore provides a method of testing a planer integrated optical device comprising the steps of activating the device such that an optical signal is conveyed through a waveguide in the device, causing the optical signal to be deflected out of the plane of the device, and detecting the deflected

optical signal with a discrete optical detector. Thus, the device can be tested on the fly, preferably prior to dicing the wafer into individual devices.

It is preferred that the waveguide is a branch carrying a minority of the signal present in the waveguide from which it branches. In this way, the testing apparatus will not interfere with the eventual functioning of the device. The optical signal can be deflected by a reflective device insertable into a recess to which the waveguide leads, or the waveguide can lead to a recess with at least one etch

0 which is at an angle to the waveguide of less than 90, thereby to deflect the optical signal out of the waveguide.

It is particularly preferred that the deflected optical signal is monitored whilst another part such as an optical fibre is aligned with the integrated optical device.

This allows active alignment in that the optical power in the waveguide can be monitored against the position of the incoming optical fibre thereby to place the optical fibre in an optimum position. This is particularly useful where two fibres need to be aligned with the integrated optical device, such as one input fibre and one output fibre. In this case, it is notoriously difficult to move both fibres around and locate an ideal position. According to the invention, the input optical fibre can be manipulated until the optical power in the waveguide is at a maximum, following which the output fibre can be moved until the optical power therein is also at a maximum. This allows an easy way to align multiple optical fibres leading to and from the same device.

The present invention therefore proposes an integrated optical device comprising a waveguide leading to a recess on the surface thereof, and a discrete reflective device insertable into the recess.

It is preferred that the reflective device is a prism. This is thus insertable into the recess so as to reflect light leaving the waveguide. Suitable design of the prism permits the light to leave vertically if this is desired.

The recess is preferably etched. Any suitable etching process can be used, as reflection occurs at the discrete reflective device and the location and orientation of the recess sides is therefore unimportant. This also means that the etch need not be accurate.

It is preferred that the arrangement is employed for selective testing, in which the reflective device will be inserted into and out of the recess as desired or necessary. Accordingly, it is preferred that the reflective device is supported on a moveable arm to allow its position to be controlled. This could be driven by servomechanical or piezoelectric means, or the like.

The present invention also relates to a testing apparatus comprising a stage for an integrated optical device, a reflective device moveable towards the stage, and a light sensing device disposed to detect light reflected from the reflective device.

Embodiments of the present invention will now be described by way of example, with reference to the accompanying figures, in which; Figure 1 shows a vertical section through an integrated optical device being tested according to the present invention; Figure 2 shows a second embodiment according to the present invention; Figure 3 shows a perspective view of a third embodiment; Figure 4 shows a perspective view of a fourth embodiment; Figure 5 shows, in schematic form, a test apparatus according to the present invention;

Figure 6 shows a structure for allowing access to the waveuide Figure 7 shows a collection of devices being tested prior to separation; and Figure 8 shows an enlarged part of figure 7.

Figure 1 shows a silicon-on-insulator (SOI) chip comprising a silicon substrate 10 on which is formed an epitaxial silicon layer 12 separated from the substrate by an insulating layer 14 of silica. A waveguide 16 is defined in the epitaxial layer 12.

This is usually by way of trenches etched on either side of the waveguide location.

In this way, light 18 can be confined in the waveguide and directed as desired.

A recess 20 is etched through the epitaxial layer 1 2, silica layer 14 and the substrate 10, transverse to the waveguide 16. This allows a reflective prism 22 to be inserted in front of the new end of the waveguide formed by the intersection of the waveguide 16 and the recess 20. Thus, when light leaves the waveguide 16 it disperses somewhat to form a divergent beam 24 which enters the prism 22 and is internally reflected off the angled face 26 thereof into an upwardly directed beam 28. A reflective coating 30 can be provided on the angled face to maximise the reflected energy. This can be detected by a suitable sensor such as a P- !-N device or other photodiode.

When the prism 22 is aligned appropriately, as shown, the light enters the prism perpendicularly to the entry face 32 and is reflected into a vertical direction, leaving the prism perpendicularly to the exit face 34.

In this way, the prism 22 can be inserted into the recess 20 to test the optical energy in the waveguide 16, as desired. Testing can be carried out in the middle of the device, as opposed to necessarily being at the edge. Thus, a wider selection of waveguides can be tested.

The test waveguide 16 could be a relatively short waveguide section into which optical energy is diverted temporarily for test purposes. Y-branch constructions are well known as are 1x2 switches which can selectively divert optical energy into one of two waveguides. After testing and verification are complete, the diverting of the optical energy into the waveguide 16 can be discontinued, allowing the device to function normally without the recess 20 interfering.

Figure 2 shows a second embodiment. The silicon substrate 100 carries a waveguide 102 in the epitaxial layer 104. A V-groove trench 106 is formed, the waveguide 102 leading to 108 side of the trench. The other edge 110 of the trench, facing the end of the waveguide 102, is formed with a reflective layer 112 which therefore deflects the optical signal 114 out of the plane of the device and along a different path 116. This deflective optical mode 118 can then be detected by a suitably located detector. Otherwise, the device operates as per the first embodiment.

Figure 3 shows a third embodiment. Part of an optical device 200 is shown, with a waveguide 202 formed thereon. An optical signal 204 propagates in the waveguide. The area to be tested has a grating 206 formed from a series of individual trenches 208 in the waveguide, although other methods of defining gratings exist. As the optical mode 204 meets the grating, a portion is deflected upwards as 210 and can be detected as described above.

Figure 4 shows a fourth embodiment. In this case, the optical device 250 and waveguide 252 end at a dovetail etch 254. The optical mode 256 will be partially reflected at 258 or refracted at 260, either of which can be detected as described above.

The third and fourth embodiment described with respect to figures 3 and 4 involve a permanent deflection structure and are thus better suited to use in

conjunction with a stub waveguide as described earlier.

Figure 5 shows a schematic arrangement for testing an optical device. It will be apparent that the features of this apparatus will normally be buiit into a larger apparatus capable of carrying out testing in this manner as well as a range of other functions. However, figure 5 serves for illustrative purposes.

A stage 50 supports the optical device 10 on an x-y positioning device 52 which is as shown in figure 1. The prism 22 is supported over the device 10 by an arm 54 which is in turn supported in a drive column 56 which contains servomechanical means adapted to move the arm 54 and hence the prism 22 in a vertical direction. The x-y positioning device 52 can move the optical device 10 so that the recess 20 (or the desired recess if several are provided) is beneath the prism 22. Of course, the arm 54 could be made movable in three dimensions instead of one, but the illustrated arrangement is advantageous in that the horizontal disposition of the prism 22 and hence the upwardly directed beam 28 remains fixed.

An optical sensor 58 is located over the prism 22 to detect the upwardly directed beam 28. In arrangements as illustrated where the horizontal location of the prism 22 is fixed, the optical sensor 58 can also be fixed. The sensor can be of any suitable type, such as a P-l-N detector, photodiode, phototransistor, etc.

Other reflective devices can be used to replace the prism 22. For example, a simple plane mirror or a concave mirror can be substituted. A concave mirror will also serve to arrest the divergence of the upwardly directed beam 28 or even focus it on the optical sensor.

Figure 6 shows an arrangement for employing the invention with potentially no loss in use. The device 60 with the waveguide 62 to be tested would lead to an end region 64 in which there is a recess 66 in the waveguide 62. This recess

66 would be employed as discussed above to derive signal from the waveguide 62 for testing. It could of course be replaced by the grating of figure 3.

Once testing was complete, the end region 64 is removed. It can be cut off using known methods or snapped off via the break line 68. Figure 6 also shows an optional taper 70 which would then be at the edge of the chip to allow coupling with other elements such as optical fibres.

To minimise the use of space on the wafer, it may be possible to design the layout of the individual devices on the wafer such that the end region 64 in fact occupies a spare region on the edge of the adjacent device. This will allow the devices to be tested until the wafer is diced into separate components, following which the waveguides will lead to the edge of the chip and can be used in the normal way without losses resulting from the test arrangements. In this way, each device will comprise waveguides for performing their function and also a remnant test recess 66 left over from a formerly adjacent device. This minimises optical loss and also the economic cost of the wafer space occupied by the test recess (or grating) etc.

Through the use of this invention, the device can be tested whilst in situ on a wafer or in a package, where access to the edges is likely to be limited. This could be used to verify the correct operation of the device or in the active alignment of other components during assembly, such as optical fibres or lasers. This is shown in figures 7 and 8 where a wafer 80 has been processed to form a number of devices 82 which are yet to be diced into individual chips. Each is formed with a recess 84 at one edge, to which the output (and possibly the input) waveguides lead. The devices can then be tested by activating them and inserting a probe 86 into the appropriate recess 84. Electrical tests could be carried out at the same time. Many designs of probe are possible but we prefer one fabricated of silicon with ridge waveguides 88 to convey the light from the recess to optical fibres 90.

Light could also be injected into the device by such a probe where necessary. A

set of micro-machined mirrors or grating structures 92 can be provided at the end of the probe to couple light into or out of the probe in the appropriate direction.

A silicon probe will be very lightweight and (as it would be fabricated on a silicon wafer) its thickness would be 5001im, or less if required. With appropriate automation, the probe would be scanned in the x-y plane of the wafer and then lowered into the recesses 84 to launch or collect light into or from the device under test. Two probes would be required for an in-line device. In this way, many devices could be tested in a short period as only one alignment step would be required, that of the wafer itself. Individual devices on the wafer would be aligned to the accuracy of the projection system used to fabricate the devices, typically 20nm, and re-alignment for each device would be unnecessary.

It will, of course, be apparent to the skilled reader that many variations can be made to the described embodiment without departing from the scope of the present invention. For example, although the example is an SOI structure with Si waveguides, the invention is applicable to any integrated waveguide structure. Equally, the coupling of light out of the waveguide has been illustrated but the arrangement could also be employed to inject light into a waveguide for testing purposes.

Claims

CLAIMS : 1. A method of testing a planar integrated optical device comprising the steps of activating the device such that an optical signal is conveyed in a waveguide on the device, causing the optical signal to be deflected out of the plane of the device, and detecting the deflected optical signal with a discrete optical detector.
2. A method according to claim 1 in which the waveguide is a branch carrying a minority of the signal present in the waveguide from which it branches.
3. A method according to claim 1 or claim 2 in which the optical signal is deflected by a grating formed in the waveguide.
4. A method according to claim 1 or claim 2 in which the waveguide leads to

0 an edge which is at an angle to the waveguide of less than 900 thereby to deflect the optical signal.
5. A method according to claim 4 in which the edge is defined by a recess.
6. A method according to claim 1 or claim 2 in which the optical signal is deflected by a discrete device insertable into a recess to which the waveguide leads.
7. A method according to any one of the preceding claims in which the deflected optical signal is monitored whilst another part is aligned with the integrated optical device.
8. A method according to claim 7 in which the other part is an optical fibre.

<Desc/Clms Page number 10>
9. A method according to claim 8 in which the optical signal is monitored whilst two fibres are aligned with the integrated optical device.
10. A method according to any one of the preceding claims in which the integrated optical device is one of a plurality formed on an intact wafer.
11. A method according to claim 10 as dependent on claim 6 or any claim dependent thereon, in which the recess is defined on a device adjacent the device under test.
12. An optical apparatus being a combination of an integrated optical device comprising a waveguide leading to a recess on the surface thereof, and a discrete device insertable into the recess adapted to couple light out of the recess.
13. A testing apparatus comprising a stage for supporting an integrated optical device, a discrete device moveable towards the stage, and a light sensing device disposed to detect light coupled from the integrated optical device by the discrete device.
14. An apparatus according to claim 12 or claim 13 in which the discrete device is reflective.
15. An apparatus according to claim 14 in which the discrete device is a prism.
16. An apparatus according to claim 15 as dependent on claim 12 in which the prism is insertable into the recess so as to reflect light leaving the waveguide.
17. An apparatus according to any one of claims 1 2 to 1 6 in which the recess is etched.

<Desc/Clms Page number 11>
18. An apparatus according to any one of claims 14 to 17 in which the discrete device is supported on a moveable arm.
19. An apparatus according to claim 18 in which the arm is driven by servomechanical or piezoelectric means.
20. An integrated optical device substantially as herein described with reference to and/or as illustrated in the accompanying figures.
21. A testing apparatus for an integrated optical device substantially as herein described with reference to and/or as illustrated in the accompanying figures.
22. A testing method substantially as herein described with reference to and/or illustrated in the accompanying figures.