WO2003025649A2

WO2003025649A2 - Optical coupling mount with spot size converter

Info

Publication number: WO2003025649A2
Application number: PCT/GB2002/004152
Authority: WO
Inventors: Yee Loy Lam; Kian Hin Victor Teo
Original assignee: Denselight Semiconductors Pte Ltd.; Finnie, Peter, John
Priority date: 2001-09-17
Filing date: 2002-09-12
Publication date: 2003-03-27
Also published as: AU2002321647A1; GB0122426D0; EP1428054A2; WO2003025649A3

Abstract

An optical coupling mount for use in coupling light between a semiconductor waveguide device and an optical fibre comprises a silica based spot size converter located on an optical bench with fiducial marks and protrusions, whereby the semiconductor device can be positioned in close alignment with the spot size converter. The spot size converter comprises a tapered upper waveguide located above a non-tapered lower waveguide. The dimensions of the spot size converter are such that a semiconductor device emitting a small, astigmatic optical beam can be efficiently coupled to a single mode fibre requiring a larger, concentric beam. Also provided is a thermal backplate with electrical routing patterns and which, when assembled with the optical bench, contact both the semiconductor device and corresponding electrical routing patterns on the optical bench, thereby providing a mechanically robust device with provision for simple connection to an external electrical power supply.

Description

AN OPTICAL COUPLING MOUNT

Background to the Invention

In the production of laser diode based, fibre deliverable light sources, there is a great need for the construction and assembly process to be concomitant with efficient manufacturability, in a high yield manner. Currently, a multitude of technologies and techniques exist to enable laser diodes to be assembled and integrated in a packaged device. Solder bumping techniques are commonly used to assemble semiconductor opto-electronic dies to substrates. The assembly, having to achieve optical functionality, often requires fibre integration for signal input and output. Consequently, the substrates, also commonly fabricated from silicon material, are fashioned to receive a prepared fibre channel.

In the case of semiconductor emitters, the emitted light beam is often required to be shaped by the emitter structure. An example is the incorporation of a spot size converter (SSC) with the semiconductor substrate, i.e. monolithically integrating an SSC and a semiconductor laser. However, this approach requires additional, costly semiconductor material and does not allow the laser and SSC to be tested separately. The alternative is to use other optical methods to focus the emitted light beam into the fibre channel. In the reverse situation, such as fibre light detectors, methods such as shaped, lensed fibres and optical lens arrangements are used to launch the light into the semiconductor detector device.

Models of these assemblies are often reported to provide good structural and signal integrity in the research laboratory. However, in the mass-manufacturing environment, these arrangements necessitate long assembly times and high material costs. The cost of rejects is consequently also very high. Passive methods of fibre attachment have been proposed, but these assembly models still require light beams to be intrinsically shaped and fashioned to a beam profile acceptable to the fibre connectivity modes. The fabrication of the associated parts demands ultra-high precision and it is usually not possible to test the components individually and in subassemblies, prior to final assembly and packaging.

Summary of the Invention

In accordance with one aspect of the present invention, an optical bench for coupling light between an optical device and an optical fibre, the optical bench comprising a substrate with an integral optical spot size converter and optical alignment means for fixing the position of an initially separate optical device relative to the spot size converter so that, in use, light is coupled between the optical device and the spot size converter. In the present invention, we provide an optical bench with provision for alignment and mounting of a separately formed optical device, such that on assembly the device is in close alignment with an optical spot size converter, that is integral to the optical bench. Accordingly, in a preferred embodiment the present invention provides a simple means for efficient and stable coupling of light between a semiconductor waveguide device and spot size converter that provides for the conversion of a small and astigmatic spot shape to one that is well matched to a fibre. An accurate assembly technique is included to assist in the alignment of the semiconductor waveguide device relative to the spot size converter leading to an overall inexpensive optical package. Preferably, the optical bench is formed of a silicon material.

Preferably, the optical device is a semiconductor edge emitting waveguide device. Examples of such devices include laser diodes, light emitting diodes, array waveguide gratings and semiconductor optical amplifiers.

Preferably, the optical alignment means is adapted to receive the optical device. More preferably, the optical alignment comprises one or more protrusions or pads that can cooperate with corresponding pads or protrusions on the optical device. Preferably, the size and shape of said protrusions can be adjusted such that, on assembling the optical device with the optical bench, the optical device is brought into close alignment with the spot size converter. An example would be protrusions comprising a material that can be preferentially heated such that, as the optical device and bench are brought together, the protrusions deform and can be fixed so as to achieve the required alignment.

Preferably, the protrusions comprise micro-solder bumps.

It is preferred that the optical device is assembled with the optical bench by means of flip chip packaging procedures, as used in the assembly of integrated electronic circuits such as multi-chip modules (MCM). It is therefore preferred that the optical device and optical bench feature fiducial marks to aid in the alignment. If the optical device requires an electrical supply to operate, as is the case for active optoelectronic devices such as laser diodes, amplifiers and modulators, then appropriate electrical connectivity can be integrated with the optical bench.

Preferably, the optical bench includes electrical routing patterns for the conduction of electrical power to an optical device located on the optical bench. In order to provide access, it is preferred that the electrical routing patterns on the optical bench extend to an area unobstructed by the optoelectronic device. If such routing patterns are provided, it is preferred that the optical bench substrate, on which the electrical routing patterns are fabricated, is substantially electrically isolating. Preferably, the electrical routings contact the optoelectronic device by means of corresponding pads and protrusions. More preferably, the said electrically conducting pads and protrusions comprise one or more of the pads and protrusions previously described for the accurate positioning of the optical device relative to the spot size converter. Preferably, the optical device is mounted p-side down on the optical bench.

The light emerging from a semiconductor edge emitting device is typically not well matched in shape and size to the optical mode that can be efficiently coupled into an optical fibre, particularly a single mode fibre. Thus, a preferred embodiment of the present invention includes an integral spot size converter to permit substantial reshaping of an optical beam.

Preferably, the spot size converter comprises a pair of waveguides, at least one of which is dimensioned so as to cause light preferentially to couple from one waveguide to the other as light propagates along the length of the waveguide. More preferably, the spot size converter comprises an upper waveguide having a reducing lateral taper along at least part of its length, vertically spaced a distance above a non- tapering lower waveguide. Preferably, the upper waveguide and lower waveguide are separated by a cladding region.

In the present invention, light from a semiconductorwaveguide device mounted on the optical bench enters the spot size converter via the facet of the non-tapering end of the upper waveguide. The dimensions of the upper waveguide at the facet are such that its mode and distribution is well matched to that of the device to be coupled. Similarly, the dimensions and extent of the taper are such that the optical mode propagating in the upper waveguide is efficiently coupled into the lower waveguide. Light exiting the lower waveguide can be coupled into an optical fibre, preferably a single mode optical fibre. Again, the dimensions of the lower waveguide are selected such that its mode and distribution is well matched to that of the fibre into which the light is to be coupled. The output light from the spot size converter can be launched into an optical fibre by a conventional butt-coupling technique. It is preferred that the optical bench includes an integral v-groove dimensioned to allow for the location of an optical fibre adjacent a facet of the spot size converter.

According to a second aspect of the present invention, an optical assembly comprises the combination of an optical bench in accordance with the one aspect of the present invention, an optical device located on the optical bench, and an optical fibre, each of the optical device and optical fibre being aligned with the spot size converter to provide coupling of light between the optical device and the optical fibre.

Thus the present invention provides a means for assembling a laser diode based, fibre deliverable light source with high accuracy and with provision for the independent testing of components and subassemblies.

Preferably, the optical assembly further comprises a backplate having a substrate with electrical routing patterns and a means for positioning the backplate from the optical bench in the optical assembly such that at least part of an electrical routing pattern on the backplate is in electrical contact with the optical device.

It is preferred that the substrate of the backplate comprises a material which is substantially electrically isolating and substantially thermally conducting so as to provide a route for heat transportation from the optical device to a cooling element or heat sink. Preferably, the assembly comprises electrically conducting pads or protrusions on the backplate, in contact with electrical routing patterns thereon, which contact corresponding protrusions or pads on the optical bench. Such protrusions will typically comprise solder bumps. Thereby, separate portions of the electrical routing patterns on the backplate can be in electrical contact with one each of the two sides of the optical device.

To provide access, it is preferred that electrical routing patterns on the backplate that are in electrical contact with the optical device extend to an area unobstructed by the optical device. Accordingly, the said optical device can be supplied electrical power by means of external electrical connections to the backplate. The backplate should be affixed to the optical bench at a spacing such that there is substantial electrical connectivity between the optical device mounted on the optical bench, and part of the electrical routing pattern on the backplate.

Preferably, the means for spacing the backplate from the optical bench comprises one or more protrusions or pads on the backplate that can cooperate with corresponding pads or protrusions on the optical bench. More preferably, the said pads and protrusions comprise one or more of those previously described for contacting electrical routing patterns on the backplate to electrical routing patterns on the optical bench. The complete assembly of components will sandwich the optical device between the optical bench and the thermally conducting backplate, providing mechanical rigidity and protection for the optical components, whilst providing simple connection points for delivering electrical power to the device. The complete assembly can then be packaged using a standard technique such as butterfly packaging.

Brief Description of the Drawings

Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is a perspective of an example of an optical coupling mount, in accordance with the present invention;

Figure 2 is a schematic cross sectional view showing the construction of a spot size converter, as shown integrated with the optical coupling mount in Figure 1 ;

Figures 3A and 3B are schematic cross sectional views showing the arrangement of an example of a spot size converter at the input and output facets of the spot size converter, respectively;

Figure 4 is a perspective of an optical assembly, constructed on an optical bench, and a separate backplate ;

Figure 5 is a perspective of a complete optical assembly, comprising an optical system sandwiched between an optical bench and a backplate; and, Figures 6A and 6B show, respectively, a side view and a fibre-end view of the assembly depicted in Figure 5. Detailed Description

As shown in Figure 1, an optical bench 1 , for use with a semiconductor edge emitting waveguide device 2, is provided with an integrated spot size converter 3 including an upper waveguide 4, featuring a reducing lateral taper along part of its length 5,and a non-tapering lower waveguide 6 vertically separated by a cladding region 7.

Using flip-chip technology, the waveguide device 2 can be accurately positioned on the optical bench 1 , with respect to the spot size converter 3, by means of fiducial marks 8 and by micro-solder bumps 9 which cooperate with assembly pads 10 on the waveguide device 2. The micro-solder bumps 9 are located on electrical routing patterns 11 , by which electrical power can be delivered to the p-side 12 of the waveguide device 2. Also shown is an integral v-groove 13 which provides for the accurate positioning of an optical fibre 14 with respect to a facet of the spot size converter 3.

The cross sectional view of Figure 2 shows an example of the layer construction of a spot size converter 20 of the type depicted in Figure 1. The fabrication process requires two levels of masking.

During fabrication a 2 μm thick layer of Si0₂ 21, with a refractive index of 1.475, is deposited and etched on a substrate of grown silica-on-silicon (SOS) 22, with a refractive index of 1.46. This Si0₂ layer, which acts as the lower waveguide for the spot size converter, is fabricated by a plasma-enhanced chemical vapour deposition (PE-CVD) process. A 5 μm thick layer of a sol-gel glass 23, with a refractive index of 1.46 (equal to that of the substrate), is spin-coated across the wafer to surround the lower waveguide 21. A 1 μm thick layer of silicon oxynitride (SiON) 24, with a higher refractive index of 1.56, is deposited and etched on the sol-gel glass 23 to form the upper waveguide of the spot size converter 3. A photolithography process is used to define the tapered structure of the upper waveguide 24. A final layer of a similar sol-gel glass 25, with refractive index of 1.46, is spin-coated across the wafer to surround the upper waveguide 24 and to act as a passivation layer.

Figures 3A and 3B are schematic cross sectional views showing a particular arrangement of the spot size converter of Figure 2, designed to couple a ridge laser at the input facet to a single mode optical fibre and the output facet of the spot size converter, respectively. As shown, the upper waveguide tapers from 6 μm to 0.5 μm.

Figure 4 shows the assembled chip 30 of Figure 1 , whereby the waveguide device 31 , spot size converter 32 and optical fibre 33 are located on the optical bench 34, in close optical alignment. Also shown is a separate thermal backplate 35 with an electrical routing pattern 36 that will contact with the n-side 37 of the waveguide device 31. Standoff solder bumps 38 are provided for affixing the thermal backplate 35 to the optical bench 34, with the appropriate spacing. The standoff solder bumps 38, which are located on electrical routing patterns 39 on the backplate 35, can contact with electrical routing patterns 40 on the optical bench 34. Thus, when assembled, the chip 30 and backplate 35 provide a completed electrical circuit, whereby electrical power can be delivered to the waveguide device 31 via the electrical routing patterns 36 and 39 on the backplate 35.

Figure 5 shows an assembled device 50, comprising the optical bench 51 and backplate 52 of Figure 4. The optical fibre 53 can be seen protruding from the device. Also shown are electrical conducting wires 54 bonded to the accessible parts of the electrical routing patterns 55 on the backplate 52.

Figures 6A and 6B show, respectively, a side view and a fibre-end view of the assembled device 60 depicted in Figure 5. The relative positioning of the optical bench 61 and thermal backplate 62 is clearly shown, with respect to the integrated spot size converter 63, waveguide device 64 and optical fibre 65. Also clearly shown are the relative proportions of the alignment/conducting bumps 66 for the waveguide device 64 and the standoff/conducting bumps 67 separating the optical bench 61 from thermal backplate 62. In addition, Figures 6A and 6B show external electrical connections to the electrical routing patterns 68 and also the location of the entire device on a thermoelectric cooler 69, such that heat is conducted from the active waveguide device 64 via the thermal backplate 62 to the thermoelectric cooler 69.

Thus the present invention provides a means for cheaply and accurately fabricating an optically aligned assembly of waveguide device and optical fibre on an optical bench with integral spot size converter in a high yield manufacturing environment, whilst facilitating the independent testing of components and subassemblies prior to final assembly. Further, when the waveguide device is an active optoelectronic device, such as a diode laser, the addition of a backplate provides mechanical rigidity and protection, whilst providing a route for heat removal and easy delivery of electrical power via integral electrical routing patterns.

Claims

1. An optical bench for coupling light between an optical device and an optical fibre, the optical bench comprising a substrate with an integral optical spot size converter and optical alignment means for fixing the position of an initially separate optical device relative to the spot size converter so that, in use, light is coupled between the optical device and the spot size converter.

2. An optical bench according to claim 1 , formed of a silicon material.

3. An optical bench according to claim 1 or claim 2, in which the spot size converter comprises a pair of waveguides, at least one of which is dimensioned so as to cause light preferentially to couple from one waveguide to the other as light propagates along the length of the waveguide.

4. An optical bench according to claim 3, in which the spot size converter comprises an upper waveguide having a reducing lateral taper along at least part of its length, vertically spaced a distance above a non-tapering lower waveguide.

5. An optical bench according to claim 3 or claim 4, in which the two waveguides are separated by a cladding region.

6. An optical bench according to any preceding claim, in which the optical alignment means is adapted to receive the optical device.

7. An optical bench according to any preceding claim, in which the optical alignment means is keyed for engagement with the optical device.

8. An optical bench according to any preceding claim, in which the optical alignment means comprises at least one trench in the optical bench within which the optical device is to be located and one or more alignment grooves or ridges that cooperate with corresponding alignment ridges or grooves, respectively, formed on the optical device.

9. An optical bench according to any preceding claim, further comprising an integral V-groove dimensioned to allow for the location of an optical fibre adjacent a facet of the spot size converter.

10. An optical bench according to any preceding claim, further comprising electrical routing patterns for the conduction of electrical power to an optical device located on the optical bench.

11. An optical assembly comprising an optical bench according to any preceding claim in combination with an optical device located on the optical bench, and an optical fibre, each of the optical device and the optical fibre being aligned with the spot size converter to provide coupling of light between the optical device and the optical fibre.

12. An optical assembly according to claim 11 , in which the optical device is a semiconductor edge emitting waveguide device.

13. An optical assembly according to claim 11 or claim 12, further comprising a backplate, the backplate comprising a substrate vertically spaced from the optical bench and affixed to the optical bench, each of the optical device, spot size converter and optical fibre being disposed between the optical bench and the backplate.

14. An optical assembly according to claim 13, in which the backplate is thermally conducting.

15. An optical assembly according to claim 13 or claim 14, in which the backplate further comprises electrical routing patterns for the conduction of electrical power to the optical bench or optical device located thereon.