WO2002077688A1

WO2002077688A1 - Optical wavelength dispersive device

Info

Publication number: WO2002077688A1
Application number: PCT/GB2002/001058
Authority: WO
Inventors: Daniel John Kitcher
Original assignee: Bookham Technology Plc
Priority date: 2001-03-21
Filing date: 2002-03-08
Publication date: 2002-10-03
Also published as: GB0107111D0

Abstract

An optical wavelength-dispersive device integrated on a planar substrate comprising a dispersive array of of waveguides of different optical path lengths on the substrate and a curved reflector in an output path from the array, the wavelength-dispersive device having an input focal position at which light may be inputted to the array and an output focal line at which the dispersed light is focussed, said curved reflector being located in the output path of the light from the output end of the array and arranged to reflect the dispersed light along a path to an output location on the substrate and form at the output location an enlarged image of the dispersed light.

Description

OPTICAL WAVELENGTH DISPERSIVE DEVICE

The invention relates to optical wavelength dispersive devices and particularly to such devices including a dispersive array integrated on a planar substrate.

Integrated chip optical dispersive devices are known including devices which have a plurality of opto/electric transducers such as photodiodes to provide output signals corresponding to the demultiplexed optical signals. When using a row of photo diodes to sense the output from a plurality of output channels, the optical signals of the output channels need to be separated to a spatial extend corresponding to the spacing of the photo diodes. Due to the limitation of small size of photo diode, the optical signals in the output channels need to be separated accordingly. If the optical array is formed by semiconductor waveguides the spatial separation of the optical output channels can be much closer than can be achieved for an array of photo diodes. Known proposals for such devices include detecting the output channels from the array in a plurality of output waveguides which collect the demultiplexed channels and deliver the light to output locations which are sufficiently spaced to match the array of photo diodes.

It is an object of the present invention to provide an improved optical dispersive device in which the spatial dispersion of the output may be controlled by means other than diverging output waveguides.

The present invention provides an optical wavelength-dispersive device integrated on a planar substrate comprising a dispersive array of optical paths of different optical path lengths on the substrate and a curved dispersive reflector in an output path from the array, wavelength-dispersive device having an input focal position at which light may be input to the array and an output focal line at which the dispersed light is focussed, said curved reflector being located in the output path of unfocussed light from the output end of the array and arranged to reflect the dispersed light along a path to an output location on the substrate and form at the output location an enlarged image of the dispersed light.

In one embodiment the dispersive array is substantially symmetrical at opposite ends.

In one embodiment, the curved dispersive reflector comprises a convex mirror.

In one embodiment, a plurality of photodiodes are located at the output focal line for detecting the power of a respective portion of the dispersed light. The photodiodes may, for example, be located at one edge of the substrate.

In one embodiment, the dispersive array includes an array waveguide grating wherein the waveguides are tapered inwardly towards each other at the input end of the array so that the directional axes of the waveguides at the input end intersect at the said input focal position, and/or the waveguides are tapered inwardly towards each other at the output end of the array so that the directional axes of the waveguides at the output end intersect at a position beyond said curved reflector.

In one variation, the waveguides are arranged parallel to each other at the input end of the array and/or at the output end of the array.

In one embodiment, the ends of the waveguides in the array terminate in part circular arcs at each end of the array.

The invention also provides an optical signal demultiplexer integrated on a planar substrate comprising a dispersive array of optical paths of different optical path length on the substrate and a curved dispersive reflector in an output path from the array, said demultiplexer having an input focal position at which light may be input to the array and an output focal line at which output signals representing the demultiplexed signals are focussed, said curved reflector being located in the path of unfocussed light from the output end of the array and arranged to reflect demultiplexed signals along a path to the focal line at an output location on the substrate and form at the output location an enlarged image of the demultiplexed signals.

Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a diagram of a prior art optical signal multiplexer using output waveguides,

Figure 2 is a view corresponding to that of Figure 1 but showing an embodiment of the present invention,

Figure 3 shows more detail of the location of channel output signals in use of the device of Figure 2, and

Figure 4 is a schematic diagram of a further embodiment of the invention.

In the schematic prior art arrangement shown in Figure 1, a dispersive waveguide array 11 consists of a plurality of curved waveguides 12. Each of the waveguides has a straight input section 15 and a straight output section 19. Line 13 indicates the junction between the straight input sections 15 and the curved sections 12. Similarly the line 14 indicates the junction between the curved sections and the straight output sections 19. In this case the input and output ends of the array 11 are symmetrical. The straight input sections 15 incline inwards towards each other so as to point to the focus position 17 at the end of the input waveguide 16. Similarly the straight output sections 19 are inclined towards each other so as to form a focus in region 20 adjacent the entrance to an array of output waveguides 21. The geometry of the input and output ends of the array each form part of a similar Rowland circle arrangement. The input ends of the straight waveguide sections 15 lie on an arc forming part of a larger circle 22 having its centre coincident with the end 17 of the input waveguide 16. Point 17 lies on the circumference of an inner circle 23 having half the radius of the larger circle 22. Similarly at the output end of the array 11 , the ends of the straight waveguide sections 19 terminate on an arc forming part of a larger circle 24 having its centre coincident with region 20 forming a focus for the output of the dispersive array. The output waveguides 21 are also arranged to terminate in an arc lying on the smaller inner circle 25 which has half the radius of the outer circle 24. Due to the dispersion within the array 11 being dependent on wavelength, the demuliplexed output channels are focussed on an arc of the circle 25 adjacent the output waveguides 21. The channels are closely spaced at the focal line and are too closely positioned for effective detection by respective photo diodes in the output detectors 26. For this reason the array of output waveguides 21 detect the output channel images formed on circle 25 and transmit the optical signals to more spaced locations at the edge 27 of the chip where the spacing is sufficient to match the separate diode locations in the array of diodes 26.

In the first embodiment of the invention shown in Figure 2, similar reference numerals have been used for similar parts. The demultiplexer is formed as an integrated chip on a planar substrate. The substrate may be formed with silicon on insulator and the waveguides may be ridge waveguides of the type shown in US Patent 5757986. The array 11 is a dispersive array of ridge waveguides formed on the chip 30 with an input arrangement similar to that already described for Figure 1. In this case the dispersive array 11 is symmetric in that the waveguide regions 15 and 19 at opposite ends of the array are inwardly inclined in similar fashion and each terminate in an arc lying on part of the larger Rowland circle 22 at the input end and a similar diameter Rowland circle 24 at the output end. In this case a concave mirror 31 is arranged in the output path of the array 11 and is positioned partway between the end of the array 11 and the point 20 referred to in Figure 1 and shown in Figure 2 on the circumference of the smaller circle of the Rowland circle arrangement. In this way the dispersive convex mirror 31 is located in the path of the demultiplexed signals where the light is still unfocussed. The unfocussed light is reflected by the mirror through free propagating regions of the chip 30 onto a further planar mirror 32 which is arranged to redirect the output light onto the line of output detectors 26. The inward inclination of the output waveguides 19 and the curvature of the mirror 31 is such as to focus the output light channels close to the row of detectors 26 which are located on an edge 33 of the chip 30. The output detectors in this example comprise a row of photo diodes arranged to provide output signals corresponding to light detected in the output channels. It will be understood that by using a convex mirror 31 to increase the distance between the end of the array 11 and the focal line formed on the edge of the chip 30, the image of the output channels has an increased spacing at the focal line thereby enabling output channels to be detected by corresponding photo diodes in the array. The optical spacing of the focussed channels can be made to correspond with the desired physical sizing of the photo diodes used. In the example shown in Figure 2 light is input to the input waveguide 16 through an optical fibre 34 along which the multiplexed optical channels are transmitted. The photodiodes 26 are arranged to provide output signals on electrical lines to further electrical circuitry.

Figure 3 shows more detail of the output image formation. The demultiplexed signals form a plurality of channels extending between a "first" channel and a "last" channel shown in Figure 3. The channel outputs are focussed on the arc of the Rowland circle 25 with the first and last channels being focussed at the opposite edges of the array where line A forms a tangent to the Rowland circle 25. The photodiodes 26 are positioned on line B midway between lines A and C. Figure 4 shows a further variant in accordance with the invention. Similar reference numerals have been used for similar parts. Again the array of ridge waveguides 11 forms a symmetrical dispersive array. A free propagating region is located at each end of the array 11 and the ends of the waveguides forming the array 11 are tapered inwardly towards each other and terminate in an arcuate line forming part of the circumference of a large Rowland circle 22 at the input end and 24 at the output end. Again a convex mirror 31 is located in the path of the output light from the array 11. The convex mirror is located in a position where the output light is still unfocussed between the end of the array and the point 20 on the smaller Rowland circle 25. This unfocussed light is reflected in a dispersed manner through free propagating regions of the silicon chip including the free propagating region between the input waveguide 16 and the input end of the array 11. This reflected light is directed by the mirror 31 directly onto an output edge 35 of the chip on which is located a row of photo diodes 26. The curvature of the mirror 31 is arranged in combination with the taper of the waveguides in the array 11 to focus the demultiplexed output channel onto the photo diodes 26. By choice of the curvature of the mirror and tapering of the waveguides, the separation of the focussed output channels is made to correspond with the required separation needed for the photo diode array.

It will be understood that in both these examples the use of dispersive mirrors in the optical output path at a position where the light remains unfocussed, it is possible to form the focussed image of the output channels at a position sufficiently remote to provide the required spacing between the channels. In both the examples of Figure 2 and 4 the waveguides in the dispersive array may be arranged to maintain constant spacing between adjacent waveguides throughout their length along the array. In such a case the input and output waveguides of the array may be straight and parallel with each other so that the output light representing the demultiplexed channels has a parallel light path on reaching the dispersive mirror 31. The curvature of the mirror will then determine the focussing position of the output channels. It will be appreciated that in each of the above embodiments, the increased distance between the dispersive array and the output focal line enables sufficient spacing of the output channel images to enable the light to be incident directly on the photo diode array even allowing for the physical size necessary for the discrete photo diodes in the array. Furthermore, all light from the dispersive arrays is directed onto the line of photodetectors. In the case of using output waveguides such as those marked 21 in Figure 1 , some losses inevitably occur due to light which forms part of the output image but which is not conveyed through the waveguides due to the physical size, mode filed shape and physical separation between adjacent waveguides in the output array. It is not possible for an array of waveguides side by side to detect the entire light forming the image of the output channels at position 20 in Figure 1. However in the embodiment described above the entire light output from the array is directed onto the photo diodes thereby resulting in a much reduced loss of light intensity and light signal data which can be detected and used by the photodiodes in generating electrical signals indicating the result of the signal demultiplexing.

The invention is not limited to the details of the foregoing examples. Although a single input waveguide 16 is shown in each of Figures 2 and 4, a plurality of input waveguides may be used. In that event, the input waveguides may be connected to respective light sources off chip and the ends of the input waveguides adjacent the dispersive array will terminate on the arc of the smaller Rowland circle with the point 17 lying in the mid point of that arc. The ends of the input waveguides will be inclined towards each other so as to point to the mid point of the facing ends of the dispersive waveguide array 11. When using a plurality of waveguides 16, any one may be selected either by selective operation of light sources off chip or by including selectively operable attenuator switches on chip or by including selectively operable attenuator switches on chip in the input waveguides 16. The photodiode 26 may be on chip or located off chip adjacent the edge of the demultiplexer chip 30. The photodiode may comprise a photodiode array or a plurality of separate photodiode chips located along the edge of chip 30.

Although the example shown in Figures 2 and 4 use convex mirrors 31 , other embodiments may use concave mirrors to form the enlarged image of the demultiplexed signals. Such concave mirrors may be located further from the output end of the dispersive array so that the focus point or region 20 lies between the concave mirror and the output end of the dispersive array.

The applicant draws attention to the fact that the present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, without limitation to the scope of any definitions set out above. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

CLAIMS:

1. An optical wavelength-dispersive device integrated on a planar substrate comprising a dispersive array of optical paths of different optical path lengths on the substrate and a curved dispersive reflector in an output path from the array, wavelength-dispersive device having an input focal position at which light may be input to the array and an output focal line at which the dispersed light is focussed, said curved reflector being located in the output path of unfocussed light from the output end of the array and arranged to reflect the dispersed light along a path to an output location on the substrate and form at the output location an enlarged image of the dispersed light.

2. A device according to claim 1 in which the dispersive array is substantially symmetrical at opposite ends.

3. A device according to claim 1 or claim 2 in which the curved dispersive reflector comprises a convex mirror.

4. A device according to any one of the preceding claims in which a plurality of light detectors are located at the output focal line for detecting the power of a respective portion of the dispersed light.

5. A device according to claim 4 in which the light detectors are located on the integrated planar substrate.

6. A device according to claim 4 or claim 5 in which the light detectors^"comprise a line of photo diodes at one edge of the substrate.

7. A device according to any one of the preceding claims in which the dispersive array comprises a plurality of optical waveguides.

8. A device according to claim 7 in which the waveguides are .inclined inwardly towards each other at the input end of the array so that the directional axes of the waveguides at the input end intersect at the said input focal position.

9. A device according to claim 8 in which a free light propagating region is provided on the substrate between the input end of the dispersive array and said input focal position.

10. A device according to any of claims 7 to 9 in which the waveguides are inclined inwardly towards each other at the output end of the array so that the directional axes of the waveguides at the output end intersect at a position beyond said curved reflector.

11. A device according to any one of claims 7 to 9 in which the waveguides at the output end of the array are arranged parallel to each other.

12. A device according to claims 10 or 11 in which a free light propagating region is provided between the output end of the dispersive array and said curved reflector.

13. A device according to any one of claims 10 to 12 in which the waveguides of the dispersive array are parallel to each other at the input end of the array.

14. A device according to any one of claims 7 to 13 in which the ends of the waveguides in the array terminate in part circular arcs at each end of the array.

15. A device according to any one of the preceding claims formed as an integrated semiconductor chip.

16. A device according to claim 15 comprising a plurality of silicon on insulator waveguides.

17. A device according to claim 16 comprising a plurality of ridge waveguides.

18. A device substantially as hereinbefore described with reference to and as shown in Figures 2 or 3 of the accompanying drawings.