GB2381769A

GB2381769A - Treatment of laser lightwave circuits

Info

Publication number: GB2381769A
Application number: GB0126560A
Authority: GB
Inventors: Ian Flint; Paul Jerram
Original assignee: Marconi Optical Components Ltd
Current assignee: Marconi Optical Components Ltd
Priority date: 2001-11-06
Filing date: 2001-11-06
Publication date: 2003-05-14
Also published as: GB0126560D0

Abstract

Lasers can be used to machine glassy planar light wave circuits, and to alter performance by local heating or exposure to UV radiation. Features such as facets may be machined by first ablating the feature using a laser and then polishing using a longer wave length laser. Local heating may be used to expand the mode of the waveguide in the region of a facet. Local exposure to UV radiation may be used to alter the refractive index of core glass in the region of a facet to expand and condense the mode. The waveguide may be silica on a silicon substrate.

Description

TREATMENT OF LASER LIGHTWAVE CIRCUITS The invention relates to the treatment of planar lightwave circuits (PLCs). It is particularly concerned with modifying the performance of PLCs.

PLC manufacture involves the creation of an optical waveguide structure in a core material surrounded by two layers of lower refractive index. Typically, the waveguide may be manufactured in silica (glass) on a silicon substrate. The buffer layer (or undercladding) and the top cladding are pure silica and the core layer is silica doped with an impurity such as Germanium, which raises the refractive index by between approximately 0.3% to 1.3%. While it is possible to fabricate a number of features in the waveguide, the process is entirely planar.

Thus, light can only be coupled in or out of the waveguide by creating facets at the edge. Furthermore, the performance of some components made in the waveguide require tolerances that are outside the waveguide fabrication process.

There is, therefore, a need to modify the performance of those components.

A number of techniques for laser based processing of waveguides are known. US 4,886, 538 assigned to Polaroid Corp. discloses the use of lasers to heat a waveguide substrate locally, or non-uniformly, to spread the dopant away from the waveguide core. This has the effect of spreading the core and increasing, or tapering, the mode field within the waveguide.

US 5,368, 900 (assigned to Motorola, Inc. ) discloses laser ablation of polymeric waveguide material to produce a facet at 45% to the vertical to reflect an optical signal

from the plane of a waveguide to the normal to that plane.

The disclosure is specific to polymeric materials such as epoxy acrylate and is not appropriate to other materials such as glassy, non-crystalline non-polymeric materials.

The use of F2 short wavelength lasers operating at 157 nm and below is disclosed in"Manufacturing of Micro-optical component using UV and VUV excimer laser radiation"by Tonshoff et al (Proc 10th ECIO, April 2001, Paderburn, Germany, ISBN 3-00-007634-4 p 265ff). However, this document is concerned with arrays of lenses in the other plane and does not contemplate the application of similar techniques to thin individual layers, such as for example, silica waveguide structures grown onto silicon or some other substrate of dissimilar material. The materials processing disclosed in this article involves substantially larger features than are required for PLCs.

EP-A-0803747 (NGK Insulators, Ltd) discloses the production of optical waveguides using laser ablation and is concerned specifically with problems in achieving good waveguide definition and vertical sidewalls in materials such as lithium niobate and lithium tantalate and resolving these with Excimer laser machining. It does not consider the applicability of such processes to glassy non-crystalline materials.

WO 0048024 (The University of Sydney) discloses use of a CO2 laser to affect the properties of waveguide materials.

Laser ablation is used to modify the effective refractive index of a waveguide, and effect stress relaxation, thereby modifying the material bi-refringence. The disclosure only removes material in the locale of waveguide core.

EP 1067409 (Lucent Technologies, Inc. ) discloses the use of CO2 lasers to affect the properties of a silica-on-

silcon waveguide. Irradition with CO2 laser light modifies the waveguide by any of stress reduction (and hence bi-refringence modification), by softening the waveguide core or cladding, modification of the materials refractive index, and modification of the waveguide's transmission class, dependent on the irradiation level.

None of the above documents disclose techniques which are suitable for machining PLCs for operation in the plane of the light.

In its broadest form, the invention provides for the formation of features in a PLC by laser ablation. The invention may be used with any glassy material, that is one that is non-crystalline and non-polymeric. Typically, these materials are mounted on crystalline substrates such as silicon.

The laser used for ablation may be a short wavelength, short pulse length laser or a long wavelength laser.

The invention also provides for modification of the refractive index of a portion of a waveguide core by exposure to radiation from an ultraviolet laser.

More specifically, there is provided a method of processing a planar lightwave circuit (PLC) comprising a glassy non-crystalline non-polymeric material, the method comprising laser ablating the PLC to form a feature.

The invention also provides a method of processing a glassy non-crystalline non-polymeric waveguide structure comprising the step of: ablating the waveguide structure to form a feature using a short wavelength short pulse length laser.

The invention further provides a method for forming a facet on a glassy crystalline non-polymeric waveguide structure, comprising: ablating the waveguide structure to form the facet using a short wavelength short pulse length laser; and polishing the facet by exposing the facet to long wavelength long pulse length laser light to reflow material at the surface.

The invention also additionally provides a method for forming a lens on an optical waveguide comprising ablating an end of the waveguide using a short wave length short pulse length laser; and reflowing the ablated waveguide end with a long wavelength long pulse length laser.

The invention also provides a method of processing an optical waveguide wafer to form a step to which a component is to be mounted, the method comprising: ablating waveguide structure material from the wafer to produce a step having a height chosen to match the component to be matched, the ablation being performed using a short wavelength short pulse length laser.

The invention further provides a method of forming holes in a silica waveguide structure mounted on a silicon substrate, comprising ablating one or more holes through the silica waveguide structure at least partially into the silicon substrate using a short wavelength laser having a short pulse length.

The invention additionally provides a method of modifying the performance of an optical waveguide having a doped core glass layer, comprising altering the refractive index of at least a portion of the doped core glass layer by exposing the portion of the doped core glass layer to ultraviolet radiation from an ultraviolet laser.

Embodiments of the invention enable features to be formed on planar optical waveguides that cannot be formed using conventional techniques. Laser ablation is used to remove material. Short pulses of short wavelength radiation may be used, for example generated by a KrF Excimer laser.

Alternatively, long wavelength lasers, such as a CO2 laser operating at lopin may be used. Preferably, the ablated surface, which may be rough, is smoothed or polished using a long wavelength laser such as a CO2 laser, preferably long pulses are used. The radiation is absorbed to a depth roughly equal to the size of the features causing the surface roughness, these are melted and reflowed to produce a smooth surface. This is useful in improving optical coupling, for example, between a waveguide facet and a device dropped into a waveguide such as a laser diode.

Preferably, local heating may be used to disperse core layer dopants to widen the mode of a waveguide. This is advantageous in the vicinity of a facet and can be used to improve light collimation and improve coupling with optical devices. Localised heating may also be used to attach components.

In one aspect of the invention, localised exposure to UV radiation is used. This has the effect of altering the refractive index of the core glass in the exposed area to condense the mode, or, with suitable core glass, expand the mode. This again is advantageous and may be used to improve coupling across gaps.

Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings in which: Figure 1 shows a silicon substrate waveguide having a vertical facet and an angled facet;

Figure 2 shows a waveguide machined to form a convex lens; Figure 3 shows how the height of a waveguide can be controlled by laser ablation; Figure 4 shows how waveguide facets can be laser polished; Figure 5a and 5b show mode expansion; Figure 6 shows mode condensation; and Figure 7 shows how components may be fixed by local soldering.

The following description details a variety of techniques for laser machining, local heating, and waveguide performance modification by UV exposure. The techniques are particularly suited, and intended for, glassy noncrystalline non-polymeric PLCs. They are particularly suited to silica glass devices as well as fluoride, silicate, telluride and chalcogenide devices. They are not intended for silicon or other crystalline materials or epoxy acrylate or other polymeric materials.

Figures 1 to 4 shows examples of laser machining techniques. In each of the scenarios to be described, laser ablation removes material and may be followed by reflow using a long wavelength laser.

Furthermore, in each of the examples to be discussed, light travels in the plane of the device. In the prior art discussed previously, light is normal to the devices which makes operating on the device more easy.

Laser ablation may be performed using short wavelength lasers with a short pulse length. The exact wavelength will depend on the material used in the PLC. A short pulse is one of duration of about 200nS or less.

Alternatively, long wavelength lasers may be used.

In the following discussion, reference is made to laser ablation using short wavelength lasers with a short pulse length. However, in each of the examples to be discussed,

long wavelength lasers may be used. An example of such a long wavelength laser is a CO2 laser operating at 10pm.

In this specification the term long or longer wavelength refers to wavelengths of about lum and longer. The term short wavelength refers to wavelengths of about near UV wavelength or shorter. That is a wavelength of about 400nm or less.

Referring now to Figure 1, a silica waveguide structure 10 is shown mounted on a silicon substrate 12. The waveguide structure has a pair of waveguides 14,16 each of which is made up of an underclad layer 18, a core layer 20 and an overclad layer 22. The waveguides are separated by a gap 24 and in the gap, a portion of the substrate is removed to form a step 26. One waveguide 14 has an angled facet 28 and the other waveguide has a vertical facet 30. A photodiode 32 having an active area 34 above the angled facet 26 is mounted above the gap by solder bumps 36 applied to the overclad 22 of the two waveguides 14,16.

The purpose of the angled facet 28 is to enable light to be deflected through substantially 900 out of the plane of the waveguide, for example to be detected by a monitor photodiode 32 such as that shown in Figure 1, or for a variety of other reasons, such as the creation of monitor points in the waveguide.

In the example of Figure 1, light is emitted from the vertical facet 30, across the gap 24 and deflected upwards through approximately 90 by an angled mirror formed on the angled facet 28. Although this structure is very simple, it is very difficult to manufacture.

The angled facet may be formed by laser ablation using short pulses of high frequency light. This may be achieved using a fluorine Excimer laser. The surface

produced by laser ablation is often rough. Smoothing can take place by using a long wavelength laser for relatively long pulses on the ablated surface of the facet. Long pulses are pulses that have a duration of about 200nS or more. Long wavelength light is suitable, for example using a CO2 laser, as it penetrates a few microns, sufficient to melt the surface layer and smooth the surface to provide a polished surface. The absorption length is similar to the height of the features to be smoothed. The actual wavelength of laser light will depend on the exact type of waveguide being used.

Once the facet has been formed and polished it is metallised to produce a mirror finish typically using vacuum deposition: either evaporation or sputtering.

The vertical facet 30 may also be created by laser ablation. Again, it is preferred that ablation is performed using a short wavelength short pulse duration laser to produce good physical definition. Ablation may be followed by laser polishing using a long wavelength long pulse duration laser such as a CO2 laser to locally heat and reflow the surface to produce a polished facet.

This is illustrated in Figure 4.

Polishing of facets is known in the art. However, it is achieved by chemical or mechanical means. This is only feasible if the facets are at the edge of the waveguide.

In practice, in devices such as PLCs, facets may be formed elsewhere within the waveguide, particularly if laser ablation described above is used. An unpolished facet may give adequate performance but performance can be increased by polishing. This may, for example, improve coupling between the waveguide and a device dropped into the gap 24 of Figure 1 which in turn may remove the need for additional components such as lenses.

Figure 2 shows how laser ablation can be used to form a convex lens at the end of the waveguide. Again, this removes the need for an additional drop-in component such as a separate lens in the gap. The lens is heretofore required when coupling optical waveguides to devices such as laser diodes as there is usually a significant loss in intensity due to the mismatch in mode shapes, unless a mode expanded laser is used. Additionally, lensing the end of the waveguide is also useful to affect the beam divergence of the launched beam. Usually, but not exclusively, the beam divergence is required to decrease as shown in Figure 5 (a). A typical lens on the waveguide will have a radius of curvature of 3-10pm. The lenses may be any shape, spherical or cylindrical, regular or irregular, concave or convex and are again machined using a shorter wavelength laser. A fluorine Excimer Laser, or CO2 laser using short pulse lengths are suitable lasers.

After formation of the lens by ablation a long wavelength long pulse length laser such as a CO2 laser may be used to reflow the lens surface after ablation to polish the surface. The formation of a concave lens is useful in improving the coupling between the waveguide and an optical fibre.

Figure 3 shows how laser ablation may be used to control height in a waveguide wafer. During the processing of the waveguide wafer to add drop-in components such as lasers, it is sometimes desirable to create a layer at a slightly different height within the wafer. This may be produced by standard semiconductor techniques such as etching but it is then not possible to adjust each step to match tolerances in individual components. Laser ablation may be used to produce individual steps for very accurate height control of components. A UV laser of short pulse duration is suitable.

It is known to cut holes in silicon using lasers.

However, cutting holes using a laser in glassy noncrystalline, non-polymeric materials on a crystalline substrate, for example a silica waveguide structure on a silicon substrate is not known. Such holes may be used to receive drop in components such as filters. Typically, the dimensions of these holes are of the order of 2mm by lmm and have a depth between 0. 5mm and lmm. The holes may extend through the thickness of the entire wafer or may only extend part way through. Typically, in the known art, this technique is performed by dry (plasma) etching.

The use of laser ablation to form the holes is advantageous as it is quicker, if only a small number of holes are required, and it may be performed with other laser machining that is to be performed in the waveguide, for example as described above.

Thus, the use of laser ablation to machine PLCs is highly advantageous and may be applied to glassy, noncrystalline, non-polymeric devices.

A short wavelength laser should be used, preferably with short pulses. F2 Excimer lasers and UV lasers are suitable for various applications. The ablation step is advantageously followed by a reflow stage which can smooth or polish the ablated surface. This is performed by a long wavelength laser, preferably using long pulses.

Figures 5 (a), 5 (b) and 6 show how lasers may be used to vary the characteristics of a waveguide through local heating.

In Figure 5 (a) a component 40 is shown dropped in between the waveguides 54,56. Light exiting the core layer 20 of waveguide 54 will spread out causing a loss of coupling efficiency with the device 40. In Figure 5 (a) the semiangle of divergence is shown as 6. This is unsatisfactory

and may require components such as lenses to be placed between the component 40 and the waveguides.

Spreading the mode size of the waveguides locally in the region of the facet is desirable to give a better match, for example for coupling to fibres. This is illustrated in Figure 5 (b) where the core layers 20 are shown diverging at 42 to the facets. If the mode size is increased sufficiently it will enable the light to traverse a gap in the waveguide structure with reduced loss, such as when a drop in component is inserted. This is illustrated in Figure 5 (b) in which the semi-angle of divergence P of the expanded mode waveguide upon exit is less then the semi-angle of divergence 6 in the Figure 5 (a) example.

The expanded mode size cannot conveniently be large over the entire area of the waveguide device as this would increase the dimensions of all components significantly which is undesirable. Additionally, a smaller waveguide normally corresponds to a large refractive index difference between the core and cladding, a smaller mode and more easily controlled confinement, lower bend losses, or equivalently tighter bend radii, and more effective use of material. However, the mode size can be expanded locally, in the region of the facet by heating the waveguide core with a laser until the dopants diffuse into a larger diameter to give a larger size waveguide.

Mode size may be varied by exposure to UV radiation. The core glass may change its refractive index if exposed to UV radiation. The mode size may accordingly be varied.

Figure 6 shows how the mode size may be condensed. The refractive index can be changed locally by exposure with a UV laser. This can be used, as illustrated in Figure 6, to compress the mode at the end of the waveguide. This improves the coupling of the emitted light to devices such

as lasers from which light is emitted at highly diverging angles. A KrF Excimer laser operating at 193nm is suitable for refractive index modification.

A modified core glass may be used that decreases in refractive index when exposed to UV radiation. The UV laser can be used with this core glass to weaken the waveguide and so expand the mode size. Thus, UV exposure is used in both cases to change the mode shape.

Figure 7 shows how local heating techniques may be used for local soldering of components. It is often necessary to attach components onto the waveguide. Examples of these components include local hermetically sealing rings (lids), lenses and filters. When processing whole wafers of devices it is advantageous to attach these devices by local heating with a laser. The attachment may be made by heating a metal alloy solder or a glass solder.

Typically, this may be done using a long wavelength laser with long pulses. A CO2 laser is suitable.

In summary, the embodiments of the invention described provide a variety of techniques for modification of planar lightwave circuits which are advantageous in terms of both speed and the ability to process areas of circuits which present techniques are unable satisfactorily to process.

The examples given are not exhaustive and many other possible applications of the techniques described are within the scope of the invention and will occur to those skilled in the art.

Claims

CLAIMS 1. A method of processing a planar lightwave circuit (PLC) comprising a glassy non-crystalline non- polymeric material, the method comprising laser ablating the PLC to form a feature.

1. A method according to claim 1, wherein laser ablation is performed using a short wavelength laser.

1. A method according to claim 2, wherein the ablation is performed using a short laser pulse duration.

1. A method according to claim 1, wherein laser ablation is performed using a long wavelength laser.

1. A method according to any of claims 1 to 4, comprising reflowing the ablated feature using a long wavelength laser.

1. A method according to claim 5, wherein reflow is performed using a long pulse duration.

1. A method according to any of claims 1 to 6, wherein the feature is in the plane of the PLC.

1. A method according to any preceding claim, wherein the feature comprises a facet.

1. A method according to claim 8, wherein the facet is a vertical facet.

1. A method according to claim 8, wherein the facet is at an acute angle to the direction of emission of light from the PLC.

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1. A method according to any of claims 1 to 7, wherein the feature is a lens formed at an end of a waveguide of a PLC.

1. A method according to claim 11, wherein the lens is concave.

1. A method according to claim 11, wherein the lens is convex.

1. A method according to any of claims 1 to 7, wherein the feature comprises one or more holes.

1. A method according to claim 14, wherein the PLC comprises a glassy non-crystalline non-polymeric structure mounted on a crystalline substrate and the holes extend through the structure into the substrate.

1. A method according to any of claims 1 to 7, wherein the feature comprises a step for receiving a component to be mounted on the PLC.

1. A method according to any preceding claim, further comprising the step of locally heating with a laser a region of the core layer of a waveguide of the PLC in the region of a waveguide facet to diffuse core layer dopants thereby to increase the size of the core layer in the heated region.

1. A method according to any preceding claims 1 to 16, comprising attaching a component to the PLC by local heating with a laser.

1. A method according to any of claims 1 to 16 comprising varying the refractive index of a core layer of a waveguide of the PLC by exposing a region

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of the core layer to ultraviolet radiation generated by a laser.

1. A method according to claim 19, wherein the laser is a KrF Excimer laser.

1. A method according to claim 19 or 20, wherein the refractive index is increased to compress the waveguide mode.

1. A method according to claim 19 or 20, wherein the core layer of the waveguide comprises a material that decreases in refractive index when exposed to UV radiation and wherein on exposure to the UV radiation the mode of the waveguide is expanded.

1. A method according to any preceding claim, wherein the glassy, non-crystalline, non-polymeric material is selected from the group comprising silicas, silicates, fluorides, chalcogenides and tellurides.

1. A method of processing a glassy non-crystalline non- polymeric waveguide structure comprising the step of: ablating the waveguide structure to form a feature using a short wavelength short pulse length laser.

1. A method according to claim 24, further comprising smoothing the ablated feature by reflowing waveguide material at the surface using a long wavelength long pulse length laser.

1. A method for forming a facet on a glassy crystalline non-polymeric waveguide structure, comprising: ablating the waveguide structure to form the facet using a short wavelength short pulse length laser; and

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polishing the facet by exposing the facet to long wavelength long pulselength laser light to reflow material at the surface.

1. A method for forming a lens on an optical waveguide comprising ablating an end of the waveguide using a short wavelength short pulse length laser; and reflowing the ablated waveguide end with a long wavelength long pulse length laser.

1. A method according to claim 27, wherein the lens is a convex lens.

1. A method according to claim 27, wherein the lens is a concave lens.

1. A method of processing an optical waveguide wafer to form a step to which a component is to be mounted, the method comprising: ablating waveguide structure material from the wafer to produce a step having a height chosen to match the component to be matched, the ablation being performed using a short wavelength short pulse length laser.

1. A method of forming holes in a silica waveguide structure mounted on a silicon substrate, comprising ablating one or more holes through the silica waveguide structure at least partially into the silicon substrate using a short wavelength laser having a short pulse length.

1. A method of modifying the performance of an optical waveguide having a doped core glass layer, comprising altering the refractive index of at least a portion of the doped core glass layer by exposing the portion

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of the doped core glass layer to ultraviolet radiation from an ultraviolet laser.

1. A method according to claim 32, wherein the laser is an Excimer laser.

1. A method according to claim 32, wherein the step of exposing at least a portion of the core glass layer to ultraviolet radiation compresses the waveguide mode.

1. A method according to claim 33, wherein the core layer glass is a material having a refractive index that decreases on exposure to ultraviolet radiation and the step of exposing a portion of the core glass layer to ultraviolet radiation expands the waveguide mode.