WO2009007712A1

WO2009007712A1 - Optical structures

Info

Publication number: WO2009007712A1
Application number: PCT/GB2008/002342
Authority: WO
Inventors: Mykhaylo Dubov; Seriampalayam Ramaswamy Natarajan; John Williams; Ian Bennion
Original assignee: Aston University
Priority date: 2007-07-06
Filing date: 2008-07-04
Publication date: 2009-01-15
Also published as: GB0713182D0

Abstract

Apparatus and methods to ablate a desired region, part or pattern in a metal layer (8) formed on a substrate (7), to reveal the substrate at that region (23), and to process the revealed region to form an optical structure (24) there (e.g. a waveguiding structure or other optical component). This obviates the need for a mask, for exposure and subsequent development of a photoresist through the mask, and for chemical etching of an exposed underlying metal layer to form a desired pattern of exposed substrate.

Description

Optical Structures

The present invention relates to optical structures and methods for their fabrication, and particularly, though not exclusively, relates to optical waveguiding structures, integrated optical circuits and components therefor.

Optical integration is an attractive method for not only- reducing the cost and size of communication and sensing systems, but also for efficient and optimal performance. Planar lightwave circuit technology is often preferable to equivalent bulk-optic optical components employing non-integrated hermetical packages which require manual alignment of optical components.

The fabrication of integrated optical circuitry, and components therefore, typically comprises the formation in an optically transmissive substrate (e.g. glass) of selected regions of higher refractive index delineating a desired structure or pattern. The desired structure or pattern conforms to the desired shape or structure of an optical circuit or circuit component such that light within the substrate may be guided through it along those delineated regions as required. In this way, a waveguiding structure or optical component may be integrated into the body of the optic transmissive substrate. Multiple optical components may be formed in this way, in a common substrate in optical communication with one another without the need for the alignment and optical coupling necessary when building optical circuits comprising bulk-optic components.

Existing methods of delineating circuit patterns in a substrate for integrated optical circuit fabrication either involve a photo lithographic process requiring delineation of optical circuit patterns in a mask, or the direct writing of circuit patterns into the photo sensitive material of the substrate using a suitably directed laser beam. The former method necessarily involves the time, complexity and attendant cost involved in the prefabrication of a necessary mask, the deposition of a photoresist layer to be suitably exposed through the mask, and the subsequent development of the photoresist to delineate the corresponding mask pattern within it. All of these steps must proceed the subsequent steps of forming the waveguide structure within the substrate at the regions of the substrate defined by the pattern in the photoresist.

While the direct laser-writing technique obviates the need for a mask and photoresist, it requires the material of the substrate to optimally photosensitive to the optical characteristics of the laser used to delineate the optical circuit pattern. Furthermore, direct laser- writing often imparts stresses to the written material which may be detrimental to the quality of the written structure.

The present invention aims to address these limitations of the prior art.

At its most general, the invention proposed is to ablate a desired region, part or pattern in a metal layer formed on a substrate, to reveal the substrate at that region, and to process the revealed region to form an optical structure there (e.g. a waveguiding structure or other optical component) . This obviates the need for a mask, for exposure and subsequent development of a photoresist through the mask, and for chemical etching of an exposed underlying metal layer to form a desired pattern of exposed substrate. The result is a gain in the speed and efficiency of fabrication of optical structures, and a reduction in the complexity and cost of fabrication. This also provides optical structures of high resolution, and permits flexibility to change shapes and geometries of optical structures (e.g. optical circuits) rapidly and easily. In a first of its aspects, the present invention may provide a method of fabricating an optical structure (e.g. waveguide) including, providing a work piece including an optically transmissive substrate with a layer of metal formed upon a surface thereof, ablating a selected part of the layer of metal to reveal a selected part of the substrate, changing the refractive index of the material of the selected part of the substrate thereby to define an optical (e.g. waveguide) structure.

The inventors have recognised that suitably concentrated light impinged upon a metal-coated substrate may ablate the metal coating to reveal the substrate underneath. By suitably controlling the spatial and/or temporal intensity of the impinging light, sharply-resolved and well-defined patterns, structures or features may be delineated in the metal layer to reveal the underlying substrate accordingly. The refractive index of the material of the revealed substrate may then be exclusively altered (e.g. increased) to define a waveguiding structure, or other optical structure. For example, the revealed pattern of substrate material may be caused to undergo structural and/or compositional changes by action of a suitable agent or process applied to the work piece to this end. Parts of the substrate not revealed by ablation of the metal layer may be substantially shielded or protected from the action of the agent or process in question, such that a refractive index change is generally confined to those parts of the substrate revealed by ablation.

The ablation of the metal layer may be performed using focussed light.

The method may include ablating the selected part of the layer of metal using radiation from a laser.

The ablation of the metal layer is preferably performed without causing consequential change (e.g. measurable change) in the structure or optical characteristics (e.g. refractive index) of the optically transmissive substrate. For example, the ablation of the metal layer may render unchanged the integrity or shape of the surface of the substrate revealed by metal ablation, and may render unchanged the refractive index of the material of the substrate so revealed. In this way, ablation of the metal layer is preferably done without ablating or damaging or altering the material of the underlying substrate. Suitable choices, for a given circumstance, of one or more of the following parameters of the ablating radiation may be made to assist in achieving this end, such as may be readily performed by the skilled person: wavelength, pulse duration of pulsed radiation, energy or energy density per pulse of pulsed radiation. Similarly, suitable choices of one or more of the following factors may assist in achieving this end: speed of scan movement of the focus of radiation across the metal layer, focal spot size, intensity profile transverse to a pulse of radiation, thickness and/or material of metal layer. There follows a list of parameter values suitable for achieving these ends. Parameters relating to radiation pulses refer to circumstances where pulsed radiation is employed. Continuous radiation may be used.

Parameters for avoiding substrate change

Parameter Range Comment

Wavelength [micrometers, μm]: 10 ^'1 - 10 ¹ E.g. a wavelength of ablating radiation from 800nm (O.βμm) to 267nm (0.267μm) is suitable. Wavelengths beyond these bounds are also suitable.

Pulse duration [picoseconds, ps]: 10 ^"2 - 10 ¹ E.g. a pulse duration of 1.1x10 ^'1ps (100fs) with a wavelength of ablating radiation of 800nm, and a pulse duration in the range {2.5 - 3.5}x10 ^"1DS (250 - 350fs) may be suitable with a wavelength of ablating radiation of 267nm. Pulse Energy [nanoJoules, nJ]: 10 ^"1 - 10 ³ E.g., with a wavelength of ablating radiation of 267 nm, the lowest suitable laser pulse energy may be about 9 nJ per pulse (at 1 kHz repetition pulse rate), and is suitably in the range 20 to 30 nJ per pulse. At a wavelength of ablating radiation of 800 nm, typically a pulse energy of 100 nJ, or thereabouts, is suitable.

Focal Spot Area [cm² {3 - 5}x10^'9 Density of Energy [J/cm ] 10 ^"2- 10 ² E.g. in the case of 267nm laser and 1OnJ pulse energy a suitable energy density may be ~2 J/ cm² (this may be per single pulse).

Peak Intensity [W/cm²] 10¹¹ - 10¹⁴ Typically, {5 - 7}x10¹² W/cm² is suitable.

Focal spot area [μm 2, ] 10 ^"1 - 10 ¹ E.g. 0.5 μm ² (5x10^"9cm²) with a wavelength of ablating radiation of 267nm, and when a x 74 microscopic objective lens is used to focus laser pulses.

Metal Layer Thickness [nm] 10 ¹ - 10 ³ E.g. 100nm to 1000 nm (e.g. when Al is used). Optimal thickness for Al may be, for example, about 90 nm. Metal(s) any Suitable examples are: Al, Ag, Cu, Au, Cr, Ni, Zn, Ti, Fe, Mg, Co, Mb, Nb, Ta, W, V, Pt and others.

The ablating radiation may be continuous or pulsed. The duration of a pulse of the ablating radiation may be controlled to control the dimensions (e.g. width or area) of the part of the substrate revealed by ablation of the metal layer caused by the pulse. It has been found that reducing the duration of such a pulse of radiation generally reduces the quantity of ablated metal and so reduces the dimension or area of the metal ablated by the pulse. Consequently, the dimension or area of the selected part of the substrate revealed by ablation may be suitably controlled by controlling the pulse duration.

The wavelength of ablating radiation has been found to have a significant influence upon the extent, or rate, of ablation for a given pulse duration and intensity. The dimension or area of the selected part of the substrate revealed by ablation may be controlled by suitably controlling or selecting the wavelength of the radiation. Preferably, the wavelength of the radiation is selected from the range 200nm (or thereabouts e.g. 300 nm) to 10 microns (or thereabouts) , though this range may alternatively extend down to 500nm, 400nm or 300nm and into the Ultra-Violet (UV) range. It is proposed that, in the process of ablating a metal layer, the metal of the layer initially absorbs a fraction of the radiation incident upon it and reflects some of that radiation. At a certain combination of wavelength and intensity of radiation (or range thereof) , it is proposed that the extent of reflection may become sufficiently low to achieve ablation at a maximum efficiency. This condition has been found to be sensitively dependent upon the wavelength of ablating radiation employed. The invention may include the step of selecting wavelength of the radiation to be that which achieves maximally efficient ablation.

For example, ablating radiation with a wavelength of 800nm has been found able to ablate in an Aluminium layer a hole of diameter as small as 40nm. This is approximately 20 times below the diffraction limited size of the focal spot of the ablating radiation. The ablating radiation may be focussed onto the metal layer and the focus moved/scanned across the layer at a speed variable to vary the flux of energy per unit area of the metal layer irradiated by the radiation.

Generally, reducing the scanning speed of the focus increases the flux of energy to a given area impinged by the focus and, consequently, increased the amount of ablation there. This enables the scanning speed to be controlled to control the dimensions of the ablated structure (e.g. hole or channel etc) formed in the metal layer by ablation.

The intensity of the ablating radiation may be selected or controlled to select of control the dimensions (e.g. depth and/or width) of structures, such as channels or holes etc, formed by ablating regions of the metal layer.

When sub-picosecond pulse durations are employed, the flux of the incident energy per pulse may be a value selected from the range of about 0.01 Joules/cm² to about

10 Joules/cm². The wavelength of the radiation may be at or about 800nm in these circumstances.

The duration of the pulse of ablating radiation may be about one to several hundred or more nanoseconds in duration, or may be about one to several hundred or more picoseconds in duration, or may be about one to several hundred or more femtoseconds in duration.

A range of pulse durations which may be suitable in producing ablating patterns in the metal layer is from about 10 femtoseconds to about 10 picoseconds.

For example, a pulse duration having a value selected from the range of about 50 femtoseconds to 150 femtoseconds is suitable.

The ablating radiation may be repetitively pulsed with a pulse repetition rate of between about 1 Hz and 10 MHz or more. For example, a rate of about 1 kHz or, several kHz may be employed.

A suitable pulse repetition rate may be about one, or up to several, kHz or more. The ablating radiation may be arranged to deliver a controlled amount of energy in each pulse. The amount of energy delivered per pulse may be controlled to control the amount of metal ablated from the metal layer by each pulse.

The energy delivered per pulse may be about one nanojoule (nJ) to several microjoules or more, and may be a value chosen from the range of about InJ to about 10 nJ (or several 10s of nJ) , or from the range of about 100 nJ to several microjoules or more, or from the range between these two ranges. A range of suitable energies per pulse may be about 10 nJ (or several tens of nJ) to about 10 (or several tens of) microjoules. For example, the energy per pulse may be chosen from the range about 50OnJ to 800 nJ to about 1 microjoule. The order of 10 nJ to 20 nJ is possible, and 30 nJ to 40 nJ is suitable. These values may be employed in respect of ablating radiation having a wavelength of 800 nm, or thereabouts.

Pulse durations may be controlled to control the intensity of radiation delivered by each pulse and this may be controllably varied to range from one or several femtoseconds to up to one or several, or several hundred nanoseconds. It has been found that pulse durations of between one and several hundred femtoseconds are suitable to ablate a sufficiently focused region of a layer of metal to reveal a selected part of the substrate having dimensions of the order of several nanometres.

Diameters or widths of channels, holes or spaces ablated in the metal layer may be as small as about lOOnm, and have been found to vary in proportion to the thickness of the metal layer being ablated, and to reduce as the wavelength of ablating radiation is reduced. The thickness of the metal layer (e.g. Al) may be about lOOnm.

The parameters and properties of the ablating radiation may be adjusted according to the type of metal used to form the layer of metal, and/or according to the thickness of the layer of metal at that part of the work piece from which metal is to be ablated. The thickness of the layer of metal may vary over different parts of the surface of the substrate on which it is formed and the method may include suitably adjusting the parameters (e.g. pulse duration, energy, intensity and/or repetition rate or wavelength) in order to control the rate of ablation. For example, parameters may be adjusted to increase the rate of ablation of regions of the layer of metal thicker than other regions of the layer of metal at which a lower rate of ablation was sufficient. This enables the rate at which the substrate is revealed (e.g. the rate at which a pattern is "written" onto the work piece) to be adjusted or held substantially uniform across varying thicknesses of the layer of metal.

The parameters of the ablating radiation (e.g. pulse duration, energy conveyed per pulse, intensity or wavelength etc) may be controlled to achieve ablation of metal from the layer of metal substantially without causing changes to the structure or optical characteristics of the underlying optically transmissive substrate material.

It has been found that the greater the energy delivered by the radiation used to ablate the metal layer, the greater the possibility of changing the optical properties of the material of the substrate revealed by the ablation. Preferably, the step of ablation of the metal layer is controlled to reduce the quantity of energy delivered, and rate of energy delivery (e.g. energy and power) , by the ablating radiation while still enabling significant ablation of the metal layer to occur. This may include the steps of repeatedly scanning a given part of the work piece with ablating radiation

(e.g. the focus thereof) at successive times (e.g. two or three times or more) to remove from the previously scanned part/area any metal remaining there not ablated by the previous ablation step at that part. This may be done by either re-scanning selected parts of a complete scan pattern, or by repeating the whole scan pattern.

Alternatively, the parameters of the ablating radiation may be selected and controlled to ablate substrate material from the underlying substrate and/or to change the refractive index of that material. To this end, the ablating radiation can be used not only to "write" a pattern in the metal layer of the work piece, but also to directly "write" a pattern in the material of the substrate (e.g. pattern comprising selected regions of increased refractive index) or form a surface structure on the revealed substrate. While the radiation employed to ablate the metal layer may be employed to this end, or the same source of radiation may be employed but with radiation parameters adjusted to achieve optimum ablation of the substrate material, in alternative arrangements a different source of ablating radiation may be employed to ablate the substrate material in this way.

The parameters of the radiation used for ablating the substrate material (e.g. glass) may differ from those of the radiation used to ablate the metal layer so as to increase the energy delivered, or rate thereof, to the substrate material by the ablating radiation (e.g. per pulse thereof as appropriate) . These, and/or other changes in the radiation parameters may be made to optimise the process of ablation of the substrate.

The method may include ablating at least some of the selected part of the substrate to form a structure in the surface of the substrate. The method may include ablating at least some of the selected part of the substrate using radiation from a laser.

The structure may be an optical grating structure.

Other types of optical structure which could be ablated into the surface of the substrate include any periodically varying surface structure and any 2- dimensional or 3-dimensional surface structure.

The method may include irradiating the selected part of the layer of metal and/or the selected part of the substrate with radiation from a laser a plurality of separate successive times. For example, the ablating radiation may be directed to initially ablate a desired region of the metal layer or substrate, to subsequently ablate another desired region, and to then be directed back to the initial desired region to ablate any metal remaining there or to ablate additional material from the substrate where such ablation has already occurred. The radiation may be directed to repeat its writing pattern.

For example, the location at which the ablating radiation impinges upon the work piece may be controlled so as to move across the work piece (e.g. the layer of metal) to impinge at different locations on the work piece (e.g. the metal layer) . This may be done by a continuous scanning of radiation across the surface of the layer of metal in a scan pattern substantially replicating the pattern to be written into the layer of metal by ablation, or may be according to some other scanning procedure such as a raster scanning methodology. The point at which ablating radiation impinges upon the work piece (e.g. layer of metal) may be moved or scanned across the surface of the work piece by moving the ablating beam of radiation relative to a stationary work piece, or by moving the work piece relative to a stationary radiation beam. In the latter case, the work piece may be mounted upon a translation stage operable to translate the work piece in a plane. The plane of translation may be perpendicular to the direction of the impinging beam of ablating radiation thereby to minimise the focal spot size upon the work piece. The translation stage is preferably operable to move the work piece through controlled displacements of as little as several, or several tens of nanometres. The method may include moving the work piece relative to light from said laser impinging upon the work piece thereby to ablate different selected parts of the work piece. The speed of movement/translation of the work piece may have a value selected from the range of about 100 mm/sec to about 0.01 mm/sec (e.g. 3 mm/sec to 0.01 mm/sec). For example, with a pulsed source of ablating radiation delivering about 800 nJ per pulse, a channel about 12 microns in width may be ablated into a metal layer. This width may be reduced simply by increasing the translation speed. The speed of translation may be about O.Olmm/sec or more, and the ablating radiation may be delivered in pulses of 30 nJ to 1 microjoule per pulse preferably.

The thickness of the metal layer (e.g. Al) may be a value from 1 micron to 10 nm (e.g. 100 nm)

The translation stage may be a motorised translation stage controlled by a computer. The translation stage may have a positioning accuracy of about 50 nm or better (i.e. the ability to return the work piece to a previous position to an accuracy of 50 nm) , and/or a sensitivity of about 2 nm (i.e. the ability to detect/register a movement of the work piece a distance as low as about 2 nm) .

The step of changing the refractive index of the selected part of the substrate may employ a process of ion exchange therewith. Alternatively, the step of changing the refractive index of the selected part of the substrate may include directing into the selected part optical radiation to which the substrate material is photosensitive to change the refractive index of the material irradiated thereby. It is preferable that a process of ion exchange is employed due to the versatility inherent in this process in not only controlling the extent of change of the refractive index in question, but also in controlling the position and extent of the ion-exchanged material of the substrate below the surface of the substrate exposed by ablation of the overlying layer of metal. It is to be understood that processes such as ion implantation or thermal diffusion may be employed to this end as an alternative to a process of ion exchange. Accordingly, references to ion exchange herein may be replaced with references to thermal diffusion or to ion implantation in alternative examples of the invention. For example, a process of indiffusion or of outdiffusion may be employed. Methods of ion exchange, ion implantation or thermal diffusion such as would be readily apparent to the skilled person may be employed to this end.

The refractive index of materials, such as glass, is related to the density of the material, chemical composition and/or permittivity of the material. The refractive index of a material, such as glass, may be controlled by changing its composition. For example, glass is typically composed of SiO₂ forming a network of bonds. Typically, glasses also include minor proportions of oxides such as Na₂O, K₂O and CaO. Some of these cations may become mobile when the glass is heated to a high temperature. The ions produced acquire a thermal mobility allowing them to diffuse through the structure of the glass. By exposing the surface of the glass to other mobile ion the process of ion exchange can occur at the surface of the glass whereby external mobile ions from within the glass are exchanged with mobile ions at the surface of the glass.

In this way suitable external ions can be diffused into the structure of the glass at the interface between the glass and the external ion-bearing substance. The effect is to locally change the composition of the glass where ion exchange occurs and, consequently, change the refractive index of the glass at that region.

Accordingly, the present invention may include the step of conducting such an ion exchange process whereby the selected revealed parts of the substrate are exposed to an ion donor substance and caused to exchange ions from the material of the substrate with ions from the ion donor medium thereby to increase the refractive index of the material of the substrate at the selected parts.

The ion donor substance may be a molten salt providing dopant ions in the form of alkali metal ions for exchange with ions from the material of the substrate. Suitable dopant ions may include Lithium, Potassium, Cesium, Rubidium. Other dopant ions may be ions of Silver, Thallium, or Copper.

Methods of ion exchange such as would be readily apparent to the skilled person may be employed accordingly. For example, purely thermal ion exchange methods may be employed in which the molten salt and the substrate material are heated to suitable temperatures to increase ion mobility therebetween to enhance the rate of ion exchange .

The process of ion exchange may include a process of field-assisted ion exchange.

The method may include applying an electric field across the workpiece during the process of ion exchange in which the electric field is directed into and across the substrate in the direction of diffusion of dopant ions into the substrate material. This may be achieved by placing the selected parts of the substrate between electrodes supporting a common electric field extending between them and through the selected part of the substrate material. For example, the anode of the electrode pair may be placed nearest to the selected parts of the substrate material exposed to the ion donor substance such that an electric field extending from the anode to the cathode acts to urge dopant ions towards the cathode and into the body of the substrate. The value of the electric field may be a value selected from the range 10 V/mm to 100 V/mm, or a value selected from the range 20 V/mm to 60 V/mm.

The process of the field-assisted ion exchange may employ an electrode pair (i.e. cathode and anode) provided independently of the workpiece. Where the ion donor substance is a molten salt, at least a part of one of each of the electrodes of the electrode pair may be immersed in the molten salt together with the selected part of the substrate to which ion exchange is to be applied.

The method may include supporting at the layer of metal an electric field which extends through the selected part of the substrate thereby to use the layer of metal as an electrode in the process of field-assisted ion exchange. The anode of the electrode pair may be provided by the layer of metal formed on the substrate surface.

For example, the layer of metal may be connected to an anode of a voltage source such that the electric field supported by the layer of metal extends through the selected part of the substrate to a cathode mutually supporting the electric field. This obviates the need to provide a separate anode and also ensures that the anode is in intimate proximity to the selected part of the substrate during the process of ion exchange.

It will be appreciated that, due to the ablation applied to the metal layer to reveal the selected part of the substrate, the metal layer, when used as an electrode in field-assisted ion exchange, does not directly overlay or oppose the selected part of the substrate undergoing ion exchange. Rather, the parts of the layer of metal immediately adjacent (i.e. bounding) the selected part of the substrate support an electric field which has components extending parallel to the direction of ion diffusion and also components extending transversely to that direction. Opposing edges of the layer of metal bounding the revealed selected part of the substrate, and created by the process of ablation, may support electric field lines which extend obliquely from the metal edges into the body of the substrate and partially towards the opposing edge of the layer of metal surrounding the selected part of the substrate revealed by ablation. The effect of such obliquely extending field lines is to urge dopant ions diffused into the surface of the substrate material to converge or focus, at least to some extent, as they diffuse into the body of the substrate material. This converging effect has the advantage of to some extent counteracting the propensity of diffusing dopant ions to thermally diverge and outwardly spread as they diffuse into the material of the substrate. This can result in a loss of definition or resolution in the edges and structure of optical components (e.g. waveguides) formed by ion exchange. Using the ablated layer of metal as an electric field electrode may assist in reducing these adverse effects, and improves control over the lateral width of the optical structure produced.

In certain circumstances it may be desired to form an optical structure, such as an optical waveguiding structure, which is wholly submerged within the body of the substrate below the substrate surface. The present invention may include the step of changing the refractive index of the selected part of the substrate using dopant ions, such as described above, and subsequently applying an electric field across the substrate to urge the dopant ion further into the substrate material without adding further dopant ions. The effect is to cause the region of higher refractive index produced by ion doping, and the optical structure it defines, to move further into (e.g. submerse) into the body of the substrate.

The method may include applying an electric field across the optical waveguide structure to induce at least a part of the waveguide structure to migrate further into the substrate thereby to bury that part of the waveguide structure below the surface of the substrate.

The electric field used to induce migration may be supported by the layer of metal (e.g. used as an electrode, such as an anode) and the electric field may^¬ be arranged to extend through the selected part of the substrate generally in the direction of desired migration. It is to be understood that this step of field-assisted ion migration preferably occurs after the step of dopant ion implantation into the substrate material, such as by ion exchange, so as to take place without the concurrent insertion of further dopant ions into selected parts of the substrate surface. Use of the layer of metal as an electrode in this process of field-induced ion migration has associated with it the advantageous effect of urging the convergence or focusing if dopant ions within a substrate by action of the mutually opposing transverse components of the electric field supported by the metal layer either side of the ablated regions, as discussed above.

The method may include a step of removing the remaining parts of the layer of metal from the substrate after the refractive index of the selected part of the substrate has been changed. This may occur after the layer of metal has been employed in field-assisted migration of dopant ions within the substrate as discussed above. The end result is a fabricated optical component.

The method may include a step of forming a layer of metal on a surface of an optically transmissive substrate thereby to provide said work piece. The layer of metal may be any suitable thickness. Suitable thicknesses may be selected from the range 5 nm to 1000 nm.

Metals other than Al could you be used and include Titanium, Chromium and Nickel. The optical structure may be an optical waveguide structure and may be an optical grating structure, or any- periodic or effectively 1-dimensional or 2-dimensional structure. Straight or curved structures (e.g. waveguides, loops or rings etc) may be fabricated and waveguiding structures may be arranged to be single-mode or multi-mode in their properties at given operating wavelengths. Such optical structures may include a layer, or layers, of additional material formed on the substrate at the selected parts of the substrate, or elsewhere,

(e.g. a metal) in an additional step after the refractive index of those selected parts has been changed as described above.

The method may include the step of irradiating with optical radiation (e.g. Ultra-Violet light) some or all of the selected parts of the substrate (e.g. exclusively those parts) defining the optical structure thereby to further modify the refractive index of the material thereof. In this way, after the optical structure has been initially formed on terms of a selected region of changed refractive index in the substrate material, that modified material may be further modified by applying refractive-index-modifying radiation to it to further adjust/tune the value of the refractive index of any chosen part(s) of the optical structure. It has been found that when the refractive index of the selected parts of the substrate material is changed by means of e.g. ion-exchange, the material of the optical structure so formed is particularly responsive to further refractive index modification by suitably chosen optical radiation (e.g. UV radiation). This fine-tuning allows the optical properties and structure of the optical structure to be adjusted in this way.

Further or additional changes in the refractive index of the material of the substrate may be performed by a step forming upon a part of the surface of the substrate (e.g. at an optical structure therein, or elsewhere on the substrate) a layer of metal (e.g. Copper), and changing the refractive index of the material of the substrate covered by the layer of metal by a process of ion exchange (e.g. thermal ion exchange) therebetween. This may include the step of heating the substrate and metal layer to a suitable temperature to assist the process of thermal ion exchange.

In a second of its aspects, the invention may provide a method of fabricating an integrated optical circuit including fabricating an optical structure, such as a waveguide structure, according to a method described above . In a third of its aspects, the invention may provide an optical structure such as a waveguide structure, fabricated according to a method described above.

In a fourth of its aspects, the present invention may- provide an integrated optical circuit including an optical structure, such as a waveguide structure, fabricated according to a method described above. An integrated optical circuit, or circuit part, may be fabricated in this way. A planar lightwave circuit may be provided or fabricated according to the invention.

In a fifth of its aspects, the invention may provide an optical sensor comprising an optical structure, such as a waveguide structure, or an integrated optical circuit as described above.

The method of fabrication described above is particularly suited to providing structures and components for bisensors and for physical and chemical sensors (e.g. gas sensing, pH sensing, environmental sensing etc) in being able to accurately and simply fabricate waveguiding components and other optical structures in, and at, the surface of a substrate within the structure/material of the substrate. The optical sensor may, for example, include a substrate containing an optical structure as described above and having a layer of metal (e.g. Gold or Silver) arranged on a surface thereof covering at least a part of the optical structure to collectively define a structure arranged for generating surface plasmon resonances (SPR) at the outwardly-presented surface of the metal layer. The optical sensor may use SPRs generated thereby, using light guides through the optical structure covered by the metal layer, to probe/sense substances adjacent the outwardly-presented surface of the metal layer.

The fabricated optical component is not only ideally suited for direct contact with the environment to be sensed, but is also robust to adverse environments in not requiring a bond between different materials or components to provide the optical structure. Such bonds are vulnerable to damage in use of a sensor.

In a sixth of its aspects, the present invention may provide a method of fabricating a channel in a substrate, such as a micro-fluidic channel, including fabricating an optical structure in a substrate as described above, and removing selected parts of the substrate material (e.g. optical structure and/or elsewhere on the substrate) by an etching process to form said channel. This may be a post-processing method step subsequent to the steps taken to fabricate the optical structure in the substrate as described above. The substrate material may be a glass. Hydrogen Fluoride, or other suitable etching substance, may be used to remove the selected parts of the substrate by etching it from the substrate. It as been found that a glass in which properties of the matrix of the glass have been changed by irradiation with laser light, is much more susceptible to being etched than is the glass which has not been irradiated in this way. The method may include directing laser light (e.g. pulsed, such as with femtosecond pulse durations) on to and/or in to the substrate at selected regions to modify the structure of the substrate so irradiated to render it more susceptible to removal from the substrate by etching. The pulse energy suitable for achieving this effect is typically in excess of 1 micro joule, and may be a value of up to about 10 micro joules or a few tens of micro joules.

An integrated optical circuit with micro-fluidic channels may be fabricated in the substrate as describes above. The result may comprise a substrate possessing an optical structure formed by the ion-exchange process described above, and possessing one or more 3D micro-fluidic channels. These channels may be formed in proximity (e.g. close proximity) to optical structures in the substrate. Preferably, the channels are formed after the metal layer of the work piece, employed in fabricating the optical structures in the substrate, has been removed.

Existing technologies rely on conventional lithography methods which involve an elaborate mass fabrication process, these being typically complex, expensive and time-consuming. Existing methodologies require the need for chemical etching or a lift-off process to remove layers or portions of material from a substrate being processed. These lead to the possibility of inaccuracies in, or damage to, the structure, integrity or resolution of the pattern being produced. Furthermore, the geometry and pattern to be delineated using existing such techniques is necessarily defined a priori which makes it impossible or very difficult to change the structure being delineated during the production process.

Conversely, the present invention permits better flexibility to change the structure, pattern or geometry being delineated in a workpiece while being capable of complex and high-resolution patterns without the requirement for intermediate processing steps inherent in the prior art. The method of the present invention is well suited for real-time and automated feedback patterning of optical structures which permits changes in the delineation of such structures to be modified "on- the-fly". This may be particularly useful for the purposes of prototyping of optical components and devices in addition to being well suited to small-volume production.

Non-limiting examples of the invention shall now be described with reference to the accompanying drawings in which:

Figure 1 schematically illustrates the process steps involved in fabrication of a waveguide structure according to the prior art; Figure 2 illustrates a method of fabricating a waveguide structure in a substrate according to the prior art;

Figure 3 schematically illustrates steps in fabricating an optical waveguide structure; Figure 4 schematically illustrates apparatus for use in fabricating an optical structure in a substrate;

Figure 5 schematically illustrates a process of field-assisted ion exchanger- Figure 6 schematically illustrates a process of field-assisted ion migration; Figure 7 illustrates the pattern of a micro-ring optical resonator structure ablated into a metal layer formed on a substrate of optically transmissive material;

Figure 8 illustrates an expanded view of straight and bent channels ablated in the metal layer illustrated in Figure 7 at closest approach;

Figure 9 illustrates straight and bent channel patterns of a micro-ring resonator structure ablated in a metal layer formed on an optically transmissive surface, such as illustrated in Figures 7 and 8, in which the width of the ablated channels is less than that of the channels illustrated in Figures 7 and 8.

Figure 10 illustrates a periodical structure comprising holes ablated into a layer of metal formed on an optically transmissive substrate forming a periodical structure;

Figure 11 illustrates a photograph of optical waveguide channels forming a part of an optical micro- ring resonator structure formed in an optically transmissive substrate and conforming to the pattern illustrated in Figure 7;

Figure 12 graphically illustrates the contrast in refractive index of the material of a substrate measured transversely across the optical waveguide channels illustrated in Figure 11 at the point of closest approach; Figure 13 schematically illustrates apparatus with which the images of Figures 7 to 11 and 14 to 15 were obtained;

Figure 14 illustrates a photograph of branching optical waveguides formed in a substrate of optically transmissive material in the process of guiding optical radiation;

Figures 15A and 15B each photographically illustrate the optical outputs of the output ends of the branching waveguides illustrated in Figure 14;

Figure 16 schematically shows the convergence of electrical field lines resulting from use of an ablated metal layer as an electrode, and the focussing effect of that convergence upon dopant ions within the substrate material through which the field lines pass.

In the figures like items are assigned like reference symbols .

Figure 1 schematically illustrates the six basic processing steps required in lithographically fabricating an optical waveguide structure in an optically transmissive substrate according to the prior art.

The initial step (step 1) involves providing an optically transmissive substrate 7 upon a surface of which is formed a layer of aluminium 8 with a photo-resist layer 9 formed upon the outermost surface of the aluminium layer. A mask 10 is provided within which is formed a pattern 1OA defining the pattern of the waveguide to be formed in the optically transmissive substrates 7 by subsequent processing steps.

In a second step (step 2) the photo-resist layer 9 is exposed to ultraviolet radiation through the pattern 1OA formed in the mask 10. The photo-resist is subsequently developed to remove those parts of the photo-resist exposed to ultraviolet radiation thereby to form channels or gaps 11 in the photo-resist revealing an underlying region of the layer of aluminium 8.

In a third step (step 3) the exposed region of aluminium layer is subjected to chemical etching which etches a channel pattern 12 into the aluminium layer revealing the underling optically transmissive substrate 7 at the etched regions.

In a subsequent step (step 4), the photo-resist layer 9 is removed such that only the etched layer of aluminium 8 remains on the surface of the optically transmissive substrate 7. In a fifth and penultimate step (step 5) , the regions of the optically transmissive substrate 7 exposed through the pattern-etched layer of aluminium 18 are subject to ion exchange in order to increase the refractive index of the material of the substrate at and immediately below those surface parts of the substrate revealed through the etched channels of the aluminium layer 8. This results in optical waveguiding structures 13 formed at and immediately under the surface of the optically transmissive substrate 7 in a pattern conforming to the pattern 1OA formed in the mask 10 employed at step 1.

The ultimate step of removing the remaining portions of the aluminium layer 8 from the optically transmissive substrate 7 completes the fabrication process and results in an optical component comprising the optical waveguide structure/pattern 13 formed in the optically transmissive substrate 7.

It will be appreciated that this six-step process is complex, lengthy and expensive. It requires multiple layers to be formed and multiple steps in the processing of each layer in order to reach the final product.

An alternative method employed in the prior art for forming optical structures is schematically illustrated in Figure 2. This employs use of a laser 14 to form a focused and directed laser beam 15 for use in directly inscribing a region of increased refractive index 16 in the material of an optically transmissive substrate 7. The material of the substrate 7 must be photosensitive to the inscribing laser radiation 15 in order for the process to be effective. Regions of increased refractive index 16 are produced when the inscribing radiation changes the structure of the material upon which it impinges and in so doing increases the refractive index of the material at those places. The material subject to this type of process is typically the subject of stresses induced by the action of the inscribing laser beam which can be detrimental to the quality of the optical structure produced by this method.

Figure 3 schematically illustrates an example of the method for the fabrication of an optical structure according to an embodiment of the present invention.

In a first step (step a) a workpiece is provided as follows. A glass substrate 7, comprising soda-lime glass is systematically cleaned to remove dirt, grease and organic/inorganic impurities by treating the substrate 7 with cleansing agents in an ultrasonic bath. The cleansed soda-lime glass substrate is then coated with a layer of aluminium having a generally uniform thickness of between 1 micron and 15 nanometres. The aluminium layer may be formed by evaporation of aluminium in a vacuum on to a planar surface of the soda-lime glass substrate 7.

A focused beam of laser light 21 is provided by a

Ti: Sapphire laser 20 and is caused to impinge upon the outwardly-presented surface of the aluminium layer 8 at selected parts of the metal layer so as to ablate aluminium from the layer to produce "windows" through the layer revealing the surface of the soda-lime glass substrate 7 immediately underneath. In this way windows/channels 22 are formed in the aluminium layer 8 by laser ablation at selected parts of the aluminium layer so as to reveal correspondingly selected parts 23 of the underlying glass substrate which conform to a predetermined desired pattern.

In a subsequent step (step b) the revealed selected parts of the glass substrate 7 are subjected to a process of ion exchange thereby to cause dopant ions 24 to diffuse into the structure of the glass substrate 7 at and immediately below those selected parts of the surface of the substrate revealed by ablation of the overlaying aluminium layer 8. The result is to increase the refractive index of the glass substrate at those ion- exchanged regions.

A final step (step c) includes removal of the remaining parts of the aluminium layer 8 to provide a soda-lime glass piece at and immediately below the surface of which is formed a patterned region of higher refractive index defining a waveguiding structure delineating a pattern conforming to the pattern originally written in the previously overlying layer of aluminium by an ablating laser beam 21. The result is an integrated optical circuit, or circuit component.

Figure 4 schematically illustrates an apparatus for ablating selected parts of the layer of aluminium 8 (or other metal) as discussed in connection with step a of Figure 3.

A workpiece comprising a glass substrate 7 coated with a layer of aluminium 8, is placed on a motorised translation stage 25 with the aluminium coated surface of the workpiece facing upwardly. The translation stage is operable to controllably move the work piece in any direction in a plane parallel to the aluminium layer 8. The translation stage may move back and forth (direction 26 indicated in Figure 4) and side-to-side (direction 27 indicated in Figure 4), or any concurrent combination of those mutually perpendicular directions.

The motorised stage is controlled by a computer 100 and is arranged to receive control signals 101 from the computer generated to control movement of the stage to displace the workpiece (7, 8) mounted upon it along a path defining a predetermined pattern.

A laser system (including a Ti: Sapphire laser) 20 is arranged to generate a laser beam 21 comprising pulses of laser radiation each conveying about 0.75 mJ of energy in pulses of about 150 fs duration with a repetition rate of 1 kHz. The laser operates at a wavelength of 800 nanometres. The pulsed beam 21 is directed to a mirror 29 fixed in position relative to the laser 20 and arranged to deflect the laser beam 21 in a direction substantially perpendicular to the plane of the aluminium layer 8 coating the upper surface of the workpiece. The deflected laser beam is passed through a focusing apparatus including a focusing lens system 28 which focuses the radiation into a beam diameter of about 2 microns. The focusing lens system 28 is arranged to bring the laser beam 21 to this focus at the surface of the aluminium layer 8 of the workpiece. The parameters of the laser beam 21 may be controllably adjusted by suitably controlling the operation of the laser 20. This control may be achieved by computer 100, and the laser 20 may be responsive to laser control signals 102 issued from the computer to the laser for this purpose.

With the focal point of the laser beam 21 in location, the computer 100 controls the motorised translation stage 25 to move the workpiece (7, 8) along a predefined track defining the pattern of the optical structure (e.g. waveguide) to be formed in the glass substrate 7.

Aluminium is ablated from the aluminium layer 8 where the focus of the laser beam impinges upon that aluminium. The result is to reveal selected regions of the glass substrate by removal of selected parts of the aluminium layer 8.

It has been found that the speed of translation of the workpiece must be selected according to the rate of energy delivery to the aluminium layer by the laser beam. The computer 100 is arranged to mutually control these parameters in order to produce consistently reproducible line widths/channels by ablation of an aluminium layer of a given thickness. It has been found that higher rates of energy delivery by the laser beam produce deeper ablation spots which can result in unwanted damage to the workpiece. This may be avoided by increasing the speed of translation of the workpiece to reduce the dwell time of the laser beam at a given region of the workpiece. Conversely, lower rates of energy delivery by the laser beam could result in insufficient penetration/ablation by the laser beam though the aluminium layer. Additionally, increasing the speed of translation of the workpiece, without regard to the rate of energy delivery by the laser beam, may result in a discontinuous line of ablation in which parts of the pattern/line are incompletely ablated due to insufficient delivery of energy at those parts by the laser beam.

The computer 100 may be arranged to control the translation stage to retrace some or all parts of the translation track through which the translation stage moves the workpiece in order that the ablating laser beam 21 may retrace those regions of the workpiece over which it has previously passed. This enables the laser beam to remove, by ablation, those portions of aluminium which it had previously incompletely ablated.

In an implementation of this multiple-scanning/retracing procedure, the system scan an entire pattern once and then rescan the entire pattern a second time. Alternatively, the system may retrace separate portions of a pattern separately and successively before moving on to the next portion of a pattern. This re-scanning enables removal of remaining particles/areas of metal which are intended to be removed but were not so removed by a previous ablation step at their location.

An implementation of the process of ion exchange, as referred to at step b in Figure 3, is schematically illustrated in Figure 5. The workpiece 7, subsequent to ablation of a desired pattern upon it, is placed in a bath 32 containing a dilute melt of NaNO₃ and AgNO₃ indicated by fluid 33 of Figure 5. The workpiece is positioned in the bath 32 so as to immerse within the dilute melt 33 those selected surface parts of the glass substrate 7 revealed by the previous ablation of aluminium from the aluminium layer 8 of the workpiece. A cathode 30 is placed in contact with the surface of the glass substrate 7 of the workpiece opposite to the surface of the substrate upon which the aluminium layer 8 is formed. The effect is to "sandwich" the glass substrate 7 between the cathode 30 and the layer of aluminium 8. The negative terminal (-V) of a voltage source 31 is electrically connected to the cathode 30, and the positive terminal (+V) of the voltage source is electrically connected to the aluminium layer 8. Consequently, the aluminium layer 8 serves as an anode supporting an electrical field which traverses the glass substrate 7 and terminates at the cathode 30. This arrangement is then heated to a temperature of about 360⁰C for a duration of about 45 minutes. During this time, with the voltage source supplied, a process of field-assisted ion exchange occurs between the silver ions of the dilute melt 33 and sodium ions of the glass substrate 7. The rate of diffusion of dopant silver ions into the body of the glass substrate is a process of thermal diffusion assisted by the electrical field supported by the aluminium layer 8 (anode) and the cathode 30 which sandwich the glass substrates. The polarity of the electric field is such as to urge dopant silver ions 24 into the body of the glass substrate towards the cathode 30 as indicated in Figure 5. Of course, this process of ion exchange is limited to those selected areas of the glass substrate revealed by the previous ablation of selected parts of the aluminium layer 8. The value of the voltage V may be selected, in accordance with the lateral dimension of the glass substrate 7, so as to generate across the glass substrate an electric field having field strength of 20-60 V/mm.

Though the present example employs soda-lime glass as the material of the glass substrate 7, other glasses may be used, such as lithium niobate or other speciality glasses, including active ion-doped materials. As shown in figure 16, parts of the layer of metal 22 immediately adjacent (i.e. bounding) the selected part of the substrate support an electric field E which has components extending parallel to the direction of ion diffusion and also components extending transversely to that direction. Opposing edges of the layer of metal bounding the revealed selected part of the substrate, and created by the process of ablation, support electric field lines which extend obliquely from the metal edges into the body of the substrate and partially towards the opposing edge of the layer of metal surrounding the selected part of the substrate revealed by ablation. The effect of such obliquely extending field lines is to urge dopant ions diffused into the surface of the substrate material to converge or focus, at least to some extent, as they diffuse into the body of the substrate material. This converging effect, in a direction indicated by arrows 100 in figure 16, has the advantage of to some extent counteracting the propensity of diffusing dopant ions to thermally diverge and outwardly spread as they diffuse into the material of the substrate. This can result in a loss of definition or resolution in the edges and structure of optical components (e.g. waveguides) formed by ion exchange. Using the ablated layer of metal as an electric field electrode may assist in reducing these adverse effects, and improves control over the lateral width of the optical structure produced.

Once sufficient ion exchange has occurred, the workpiece may be removed from the dilute melt 33 and both the aluminium layer 8, and cathode 30, may be removed from the glass substrate 7 as discussed in conjunction with step c of Figure 3 above. The result is a glass piece containing at and immediately below selected parts of its surface a pattern defining regions of ion-doped glass of refractive index higher than the surrounding un-doped glass of the block. These doped regions are, of course, confined to a pre-selected pattern previously defined by the scanning pattern of the laser beam 21 employed to ablate the aluminium layer 8 of the workpiece. The selected regions of higher refractive index glass define waveguiding structures and other optical components in the glass block.

The glass block 7 may then be cut and edge-polished to enable light and/or other optical components to be coupled to the waveguiding structures formed within it.

In an alternative procedure, subsequent to the process of ion exchange illustrated schematically in Figure 5 and discussed above, the workpiece (7, 8) may be removed from the dilute melt 33 once a suitable concentration of dopant ions 24 have doped the glass substrate 7.

Continuing to apply the voltage V to the anode 8 and cathode 30 attached to the glass substrate, continues to urge dopant ions 24 to migrate through the material of the glass substrate 7 towards the cathode 30. Since the supply of dopant ions has been removed, by removing the workpiece from the dilute melt 33, the effect is to draw the concentrated region of dopant ions 24 into the body of the glass as a group thereby to submerge or sink the group of dopant ions into the internal region of the glass substrate as is schematically illustrated in Figure 6.

The same voltage may be employed for the process submerging dopant ions as was used in the preceding process of field-assisted ion exchange. Use of the ablated metal layer 8 as an electrode in this ion- submerging process has the ion-focussing effect discussed above with reference to figure 16.

The effect is to produce waveguiding structures and optical components which are fully immersed within the glass substrate as an alternative to, or in combination with partially buried optical structures which extend to the surface of the glass such as schematically illustrated in Figure 3 in connection with step C.

Subsequently, the aluminium layer 8 and the cathode 30 will be removed from the glass substrate 7 to provide a glass block inside of which an optical waveguiding structure is formed. The glass block may be cut and edge-polished to prepare the optical structure for optical coupling to other optical components and/or light.

There now follow examples of patterns ablated in an aluminium layer formed upon a glass substrate in accordance with the methodology discussed above in connection with Figure 3, and examples of optical structures fabricated accordingly. Having provided a workpiece (7, 8) as described above, a Ti-Sapphire laser 20 was employed to ablate selected regions of aluminium from the aluminium layer 8 in the form of a desired pattern suitable for producing windows through which ion- exchange was later to be carried out.

The laser system 20 consisted of a Kerr-lens mode-locked Ti: Sapphire oscillator and a regenerative chirp-pulse amplification system. The laser system 20 generated laser pulses of 150 fs pulses at a pulse repetition of 1 kHz with each pulse delivering approximately 0.75 mJ of energy at a wavelength of 800 nm. The laser pulses were initially coarsely attenuated using a zero-order half waveplate followed by one (or two) Brewster angle thin- film polariser (s) . Subsequent attenuation was achieved using an additional waveplate and a Glan laser prism. The femto second pulses produced in this way were focused by a long working distance x 36 acromatic micro-objective 28 with a numerical aperture of 0.5. This resulted in a focused beam diameter of about 2 microns.

The focused laser beam was caused to impinge upon the layer of aluminium 8 on the workpiece as the workpiece was displaced along a predefined path perpendicular to the direction of the laser beam 21, using the motorised translation stage 25 as discussed above.

In this way, a patter of channels was ablated into the aluminium layer 8 in order to reveal the underlying surface of the glass substrate 7.

Figure 7 illustrates a photograph of a completed channel pattern comprising a linear channel 32 adjacent a circular channel 33 each ablated into a layer 36 of aluminium formed on an underlying substrate of glass (not shown) . The ablated channel pattern defines a micro-ring resonator structure having a ring diameter (item 33) of 0.6 mm.

Figure 8 illustrates an exploded view of the channel pattern where the circular channel and the linear channel are nearmost. The width of the linear channel is approximately 4.72 microns while the width of the circular channel is approximately 4.6 microns. The separation of the nearmost edges is approximately 3.71 microns.

The speed of translation of the work piece was 0.33 mm/sec using pulses of light each delivering 623 nano- Joules of energy. Three successive scans of the focus of the ablating light across the metal layer were performed, each repeating the same pattern, to remove metal from the work piece along the pattern.

Figure 9 illustrates a photograph of a similar micro-ring resonator structure pattern ablated in an aluminium layer 36 formed using the same laser beam parameters as employed in the ablation of the pattern illustrated in Figure 8, but employing a higher speed of translation of the workpiece such that the amount of radiant energy delivered by the focal point of the laser beam to a given point on the aluminium layer was reduced. The effect is to reduce the width of the channel ablated by the focused laser light. Ablated channels 34 and 35 illustrate this, in having respective widths of 1.88 microns, 1.86 microns and a separation of nearmost edges of about 6.07 microns.

The speed of translation of the work piece was 0.33 mm/sec using pulses of light each delivering 590 nano- Joules of energy. Three successive scans of the focus of the ablating light across the metal layer were performed, each repeating the same pattern, to remove metal from the work piece along the pattern.

Figure 10 illustrates a photograph of a periodic structure ablated into a layer of aluminium 80 formed upon an underlying glass substrate (not shown) . The periodic structure 81 comprises a series of holes varying in diameter from about 0.94 microns to much less than a micron. The ablated hole indicated by a circle in Figure 10 has a diameter much less than 1 micron although this dimension is not accurately represented as the dimension is below the resolution limit of the optical microscope used to produce the photograph of Figure 10.

Figure 11 illustrates a photograph of optical waveguiding structure forming a part of a micron-ring resonator structure formed in a glass substrate 40 by subjecting the ablated pattern (32, 33) of Figure 7 to a process of ion exchange, and subsequent to removal of the patterned aluminium layer 36 after completion of the ion-exchange process. Figure 11 clearly shows waveguide structures (41, 42) appearing as well defined regions of higher refractive index. The widths of each of the waveguide structures (41, 42) is approximately 9 microns, and the separation between the centres of the waveguiding structures at their point of closest approach is approximately 9.41 microns.

Figure 12 graphically illustrates the change in contrast of refractive index of the glass substrate 40 as measured in a line transversely crossing both of the waveguide structures (41, 42) at their point of closes approach.

Clear and well-defined refractive index increases can be seen. This is as a result of implantation of dopant ions during the ion-exchange process described above with reference to Figures 3 and 5.

Figure 13 schematically illustrates apparatus employed to obtain the photographic images of Figures 7 to 11, and subsequent Figures 14 and 15. After fabrication of a waveguiding structure in a glass substrate as described above, the glass piece was cut and polished to enable the coupling of light into the fabricated waveguide structure. This prepared glass piece 63 was placed on a motorised translation stage 62 and an optical fibre 61 coupled to an optical input of the waveguide structure. The optical fibre 61 was also mounted on a translation stage 62 in order to enable accurate positioning and coupling with the fibre structure in the glass piece 63. A laser 60 was employed to direct laser light through the optical waveguide 61 and into the waveguide structure in the glass piece 63. At an output end of the waveguide structure, a x 25 microscope objective lens was mounted, on a translation stage 62, to couple light transmissive from the waveguide structure and to create an image of the output on a CCD array 66. The near-field pattern of the output end of the waveguide structure was captured by other CCD in this way. The optical fibre 61, glass piece 63, and objective lens 64 were all mounted on respective translation stages such that their relative positions could be accurately manipulated to achieve optimum coupling results.

A microscope 65 was placed to both the glass piece to observe the path of radiation from the laser 60 guided through the waveguide structure, and to observe scattering processes. Figure 14 illustrates a photograph of a glass piece 63 in which are fabricated two branching waveguide structures (71, 72) in the process of guiding optical radiation from a laser 60 at a wavelength of 532 nm. Figures 15A and 15B show photographic images of the output ends of the two branching waveguide structures (71, 72) obtained via the objective lens 64 and the CCD array 66 of Figure 13. The terminal edge of the glass piece 63 can be seen in each of the photographs illustrated in Figures 15A and 15B, together with bright points representing the guided light output by respective branching waveguide structures .

Refractive index contrast measurements illustrated in Figure 12 were made using the Quantitative-Phase- Microscopy (QPM) method, and contrast values ranging from 0.005 to 0.08 are possible in waveguiding structures fabricated as described above. The waveguiding structures were found to display multi-mode propagation at 532 nm and 633 nm, and single-mode propagation at 1550 nm is also possible.

Claims

CLAIMS :

1. A method of fabricating an optical structure including, providing a work piece including an optically transmissive substrate with a layer of metal formed upon a surface thereof, ablating a selected part of the layer of metal to reveal a selected part of the substrate, changing the refractive index of the selected part of the substrate thereby to define an optical structure.

2. A method according to any preceding claim including ablating the selected part of the layer of metal using radiation from a laser.

3. A method according to any preceding claim including ablating at least some of the selected part of the substrate to form a structure in the surface of the substrate.

4. A method according to Claim 3 including ablating at least some of the selected part of the substrate using radiation from a laser.

5. A method according to Claim 3 or 4 in which the structure is an optical grating structure.

6. A method according to any preceding claim including irradiating the selected part of the layer of metal and/or the selected part of the substrate with radiation from a laser a plurality of separate successive times.

7. A method according to any of preceding claims including moving the work piece relative to radiation from said laser directed to impinge upon the work piece thereby to ablate different selected parts of the work piece using said radiation.

8. A method according to any preceding claim including changing the refractive index of the selected part of the substrate by a process of ion exchange therewith.

9. A method according to Claim 8 in which the process of ion exchange includes a process of field-assisted ion exchange.

10. A method according to Claim 9 including supporting at the layer of metal an electric field which extends through the selected part of the substrate thereby to use the layer of metal as an electrode in the process of field-assisted ion exchange.

11. A method according to any preceding claim including applying an electric field across the optical waveguide structure to induce at least a part of the waveguide structure to migrate further into the substrate thereby to bury that part of the waveguide structure below the surface of the substrate.

12. A method according to any preceding claim including the step of removing the remaining parts of the layer of metal from the substrate after the refractive index of the selected part of the substrate has been changed.

13. A method according to any preceding claim including the step of forming a layer of metal on a surface of an optically transmissive substrate thereby to provide said work piece.

14. A method according to any preceding claim in which the optical structure is an optical waveguide structure .

15. A method of fabricating an integrated optical circuit including fabricating an optical structure according to the method of any preceding claim.

16. An optical structure fabricated according to the method of any preceding claim.

17. An integrated optical circuit including an optical structure fabricated according to the method of any preceding claim.

18. An optical sensor comprising an optical structure or an integrated optical circuit according to Claim 16 or Claim 17.