Method and Apparatus for Inscription

The invention relates to methods and apparatus for inscribing structures within materials such as optically transmissive materials e.g. optical fibres or the like.

The use of laser sources combined with chemical etching for the fabrication of micro- photonic and micro-fluidic devices for modern biomedical, chemistry and sensing applications provides the advantage of obviating a photolithographic step. The fabrication process permits the inscription of arbitrary microstructure shapes and sizes following the path or pattern inscribed by a focus of laser light moving within the material being inscribed. The inscribed parts of suitable materials are believed to respond to the focused laser light by becoming structurally modified and become more susceptible to etching. Subsequent etching of the inscribed material thereby enables selective removal of the laser-modified inscribed regions revealing micro- channel structures within the material following a pattern defined by the path of the earlier inscribing laser focus.

The accuracy and precision of such micro-channel structures depends sensitively upon the accuracy and precision of the inscription process. Consequently, accurate inscription depends sensitively upon control of the inscribing laser light. For example, should inscribing laser light fail to be controlled sufficiently, the resultant intensity of laser radiation may be insufficient to modify the material in question and improperly, incompletely and/or inaccurately inscribe that material at the desired position therein.

The present invention aims to address these deficiencies.

At its most general, the present invention proposes apparatus and methods for inscribing material, such as optically transmissive material, with electromagnetic radiation directed through a non-flat (e.g. curved or irregular/uneven or discontinuous) outer surface area of the material via an optical coupler which reduces the deviation or distorting effects imposed upon the inscribing radiation by the non-flatness (e.g. curvature or irregularity) of the outer surface of the material through which the radiation passes, and which is separate or spaced from the means for directing the radiation. This stems from the observation that inscription of materials using light passed through a non-flat outer surface of that material significantly limits or degrades the ability of the light to inscribe within the material. This is attributed to the distorting or deviating effect of the non-flat outer surface of the material upon the incident inscribing radiation. For example, radiation directed and arranged to come to a focus within the material may significantly de-focus as a result of having passed through the refractive non-flat (e.g. curved) outer surface of the material or may be reflected. It may be suitable to regard the non-flat outer surface of material upon which the radiation is incident as acting in a manner analogous to a distorting lens. The present invention may reduce this de- focusing/distorting or deviating characteristic by presenting to the inscribing radiation an optical surface which is less lens-like, or reflective, than the outer surface provided by the material being inscribed. Furthermore, by employing an optical coupling means separate or spaced from the means for directing the electromagnetic radiation to the material, one is free to easily move the material (and optical coupler) relative to the radiation source or directing means when focussing, or when performing inscriptions. One may separably place the coupling means in contact with a material to be inscribed as desired. One is also free to modify the radiation source without directly affecting the optical coupler and vive versa.

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The invention may provide apparatus for inscribing materials (e.g. optically transmissive material) including an optical coupling means placed, or arranged to be placed, in contact with a non-flat (e.g. curved, irregular or uneven) outer surface area 5 of the material to form an interface (e.g. a non-flat interface which may be a separable interface) therewith, a radiation source (e.g. laser) and convergence (e.g. focussing) means spaced from the optical coupling means and arranged for directing electromagnetic radiation from the radiation source into the optical coupling means and into the material via the (e.g. non-flat) interface to converge (e.g. focus) within

10 the material to provide therein an intensity of radiation sufficient to inscribe the material, wherein the optical coupling means is arranged (e.g. shaped, structured or constructed) to reduce the deviation (e.g. refraction and/or reflection) of electromagnetic radiation from the radiation source directed to enter the material via the non-flat outer surface area thereof (e.g. the non-flat interface when formed). The

15 convergence means may be any suitable optical apparatus such as a lens or lenses. The optical coupling means may possess a refractive index and/or an outer surface area shaped structured to reduce the deviation of radiation directed by the convergence means to the material.

20 In this way, the de-focusing, distorting or "lensing" effect of a non-flat (e.g. curved) outer surface of a material being inscribed may be reduced by reducing the deviating effect of that surface upon incident inscribing radiation using the optical coupling means to form an interface with the material. The reflectivity of the non-flat outer surface, such as resulting from obliqueness of incident radiation caused by non- 25 flatness (e.g. discontinuities, irregularities or curvature) of the surface, may also be reduced in this way. The inscribing light may then pass directly from the optical coupling means into the material via the interface therebetween. The optical

coupling means is most preferably arranged to be in direct physical contact continuously across a non-flat outer surface area of the material such that light passing from the coupling means to the material does not pass through an intermediate gap therebetween. This enables the optical coupling means to be used to control the nature of the optical interface between itself and the material in question so as to provide that the deviation (e.g. reflection and/or refraction) of inscribing radiation incident upon that interface is less than would be the case were the optical coupling means not present. The separation/spacing of convergence means and optical coupling means allows great versatility in suitably controlling the way in which - and the location at which - inscribing radiation converges towards and/or within the material since the one may be freely moved anywhere relative to the other (within reason) without influencing or compromising the function of the optical coupler. As a further result, no requirements are imposed by the optical coupling means on the optical components of the radiation source and convergence means.

The optical coupling means may be arranged (e.g. shaped, structured or constructed) to reduce at the interface the deviation of electromagnetic radiation from the radiation source entering, or directed to enter, the material via the interface. For example, the optical properties of the optical coupling means at the interface, and/or elsewhere, may be chosen to match or more closely match the optical properties of the material to be inscribed, as compared to the optical properties of the immediate environment of the material at the non-flat outer surface were the optical coupling means not present. Optical properties in question may include the refractive index, the optical density or any other optical property suitable for achieving the desired reduction in deviation of inscribing radiation at the interface as the radiation reaches or enters the material from the optical coupling means. This arrangement aims to

reduce the degree of refraction imposed upon inscribing radiation incident upon the interface obliquely (i.e. not perpendicular to the curved surface of the material). The non-flatness (e.g. irregularity or curvature) of the surface in question typically results in the angle of incidence of different parts of the converging electro-magnetic radiation (e.g. different regions of an incident laser beam) differing from each other resulting in different degrees of deviation and therefore reflection or a de-focusing refraction effect can ensue. The degree of refraction is sensitive to the difference in refractive index of the substances either side of the interface. Reducing this difference aims is to reduce the degree of refraction and reflection.

The optical coupling means may be arranged (e.g. shaped, structured or constructed) to reduce or minimise the deviation of electromagnetic radiation from the radiation source entering , the optical coupling means. The optical coupling means may include a body of optically transmissive liquid placed, or placeable, in contact with said non-flat outer surface of the material to form said interface. An outer surface of the body of optically transmissive liquid may be so placed or placeable to form the interface. The interface may be so formed without surrounding, immersing or enveloping the material therewith. An oil or gel, or any other suitable fluid or liquid such as would be readily apparent to the skilled person may be used to this end. The body of liquid may comprise a liquid contained in a container means (e.g. a sac) or it may be open to direct contact with the surface of a material to be inscribed thereby enabling the viscosity and/or surface tension of the liquid to act to form an area of continuous contact with the non-flat surface.

Fluids such as fluorocarbon-based index-matching fluids (IMF), chlorofluorocarbon- based IMF may be used. Suitably, the fluid has a viscosity value in the range of 50 to 200 centistokes (cSt), or any other viscosity value sufficient to enable the fluid to

remain in-place (e.g. "glued") to a solid material with which it is placed in contact in use.

The optical coupling means is preferably dimensioned such that it is placeable upon the body of a material to be inscribed to form an interface which is placeable at, and moveable along a sub-portion of the outer surface of that material which generally faces the direction of incoming inscribing radiation. In this way, through preferably not being required to fully surround, immerse or envelope a sample of material, the optical coupling means may be separable from such a sample material at the interface in question, permitting versatility in movement and positioning of the interface upon the sample during inscription. Where IMF is employed, it may be attached to a solid optically transmissive plate (e.g. a glass plate) forming a radiation input surface. The IMF may attach to the plate through the action of viscosity/surface tension alone, and may attach to a material to be inscribed in the same manner. This enables samples (such as optical fibres) to be mounted/attached to the optical coupling means at an interface so as to be at least partially horizontally oriented, inclined or aligned thereby enabling a partially of substantially horizontal direction of incoming inscribing radiation.

Preferably the range of suitable values of refractive indices of the IMF employed in the optical coupling means is 1.430 to 1.530, for example in increment steps of 0.002.

The optically transmissive liquid may have an index of refraction having a value closer to the value of the index of refraction of the material (e.g. optically transmissive material) than to the value of the index of refraction of the immediate environment of the material. For example, the optical coupling means presents to the inscribing

radiation an optical surface via which that radiation must enter the optical coupling means before subsequently entering the material to be inscribed. In such a case, the outer surface of the optical coupling means presented to the inscribing radiation is most preferably structured and arranged to produce little or no deviation upon inscribing radiation entering it at that surface. Most preferably, any such deviation which is imposed by the outer surface of the optical coupling means in question is less than the deviation which would be imposed upon that radiation by the non-flat outer surface of the material to be inscribed, in the absence of the optical coupling means. That is to say, any deviation of radiation from the radiation source entering the optical coupling means is most preferably at least less than the deviation which would have been imposed on that radiation were it to enter the material via the non- flat outer surface thereof without the presence of optical coupling means. Most preferably, an air gap, or the like, exists between the convergence means and the optical coupling means, the two being generally unattached.

It is most preferable that the optical coupling means is placed in direct and uninterrupted physical contact with the relevant non-flat outer surface area of the material to be inscribed in order to avoid the deviating effects of such air gaps upon inscribing radiation passing from the optical coupling means to the material at or across the interface therebetween. The surface of the optical coupling means presented to the non-flat outer surface area in question is therefore most preferably shaped (or shapeable) and arranged to intimately reciprocally match the surface non- flatness (e.g. curvature and shape of the surface of the material with which it is to interface. The reciprocally shaped (or shapeable) surface of the optical coupling means may be formed in a solid optically transmissive body of material, or may be provided by a deformable surface arranged to deform upon being placed (or pressed) in physical contact with the non-flat outer surface of the material to be inscribed. This

interfacing surface of the optical coupling means may comprise a body of enclosed optically transmissive liquid, such as in a sac, bubble, bag or an exposed quantity of liquid (e.g. located within a recess or attached to a surface), into contact with which may be placed the material to be inscribed thereby to form the separable interface therewith.

The optical coupling means may have a radiation input area in the form of an outer surface area, for receiving radiation from the radiation source, which is more flat than the non-flat (e.g. curved) outer surface area of the material at said interface, or has a curvature or non-flatness less than any curvature, non-flatness or irregularity at the interface, and the convergence means may be arranged to direct said electromagnetic radiation into the coupling means via said more flat or less curved outer surface area. The optical coupling means may have an outer surface area which is substantially flat, and the radiation source may be arranged to direct the electromagnetic radiation into the coupling means via the flat outer surface area. Consequently refractive distortion of converging radiation entering the optical coupling means from the convergence means (e.g. focussing lens) may be reduced or minimised. The radiation input area of the coupling means is preferably arranged to be presented to the convergence means and may be a solid optically transmissive material such as glass (e.g. a glass plate).

The material to be inscribed may be an optically transmissive material and may be any Silica-based material and/or an optical fibre.

The convergence means may be arranged to direct the electromagnetic radiation into the material (e.g. optically transmissive material) to converge to form a focus therein.

A focused or focusable lens or lens system may be used. Mirrors or other optical means may be used for convergence.

The apparatus may include means for moving the material (e.g. optically transmissive material) and the optical coupling means relative to the convergence means and/or the source of electromagnetic radiation, or vice versa, thereby to move the region of convergence e.g. the focus within the material. In this way the converged inscribing radiation may be moved within the material in any desired direction(s) to inscribe therein any desired pattern or structure.

The convergence means may be arranged to direct the electromagnetic radiation to converge within the material to provide therein an intensity, and/or other property (e.g. wavelength) of radiation, sufficient to render material inscribed thereby more susceptible to etching, such as chemical etching. The apparatus may include etching means for etching (e.g. chemically etching) the material (e.g. optically transmissive material) to selectively remove said inscribed material. The result would be to reveal micro-channel structures or the like within the material.

The radiation source may be arranged to produce radiation in pulses of duration of the order of about 150 femtoseconds, or of between about 10 femtoseconds to about

200 femtoseconds. Preferably each pulse conveys an energy of about 0.5 microJoules, or a pulse energy falling within the range of about 10 nanoJoules to

500, 600, 700, 800 or 900 or more nanoJoules. A wavelength of the electromagnetic radiation of 800 nanometres, or of a wavelength falling within the range of about 750 nanometres to about 850 nanometres may be used. The laser may be arranged to produce such pulses at a repetition rate of about 1 kHz, or multiples thereof, although it is to be noted that, in principle, any repetition rate may be used. Preferably the

radiation source is arranged to converge the inscribing radiation to a focal spot having a diameter of the order of about 1 micrometre, or several micrometres.

It will be understood that the invention in its first aspect described above implements a method for inscribing materials. The present invention encompasses such methods within its scope.

In another of its aspects the invention may provide a method for inscribing material (e.g. optically transmissive material) including placing an optical coupling means in contact with a non-flat (e.g. irregular, discontinuous, uneven or curved) outer surface area of the material to form a non-flat interface therewith, directing electromagnetic radiation into the material via a focusing means spaced from the optical coupling means and via the non-flat interface to converge within the material to provide therein an intensity of radiation sufficient to inscribe the material, and using (e.g. the shape, structure or construction of) the optical coupling means to reduce the deviation of the electromagnetic radiation from the radiation source directed to enter the material via the non-flat interface.

The method may include using (e.g. the shape, structure or construction of) the optical coupling means to reduce or minimise at the non-flat interface the deviation of the electromagnetic radiation entering the material via the interface.

The method may include arranging (e.g. using the shape of) the optical coupling means to reduce the deviation of the electromagnetic radiation entering the optical coupling means.

The optical coupling means may include a body of optically transmissive liquid, and the method may include placing the body of liquid (e.g. an outer surface thereof) in contact (direct or indirect) with a non-flat (e.g. curved irregular etc) outer surface of the material to form said interface. The body of liquid may include a liquid contained in a container deformable to match (reciprocally) the non-flat outer surface when in contact with, or pressed, thereto.

The method may include providing said liquid with an index of refraction having a value closer to the value of the index of refraction of the material (e.g. optically transmissive material) than to the value of the index of refraction of the immediate environment of the material. Index matching liquid may be used.

The method may include placing the optical coupling means upon the material to form said interface without fully surrounding, immersing or enveloping the material therewith.

The method may include providing the coupling means with an outer surface area which is more flat than the non-flat (e.g. curved) outer surface area of the material at said interface, and directing said electromagnetic radiation into the coupling means via said outer surface area thereof.

The method may include providing the coupling means with an outer surface area which is substantially flat, and directing said electromagnetic radiation into the coupling means via the flat outer surface area.

The material may be an optically transmissive material and may be any silica-based material such as an optical fibre.

The method may include directing said electromagnetic radiation into the material (e.g. optically transmissive material) to converge to form a focus therein.

The method may include moving the material (e.g. optically transmissive material) and the optical coupling means relative to convergence means and/or the source of said electromagnetic radiation, or vice versa, thereby to move the focus within the material.

The method may include directing said electromagnetic radiation to converge within the material to provide therein an intensity (and/or wavelength) of radiation sufficient to render material inscribed thereby more susceptible to etching.

The method may include subsequently etching (e.g. chemically etching) the material (e.g. optically transmissive material) to selectively remove said inscribed material.

In another of its aspects the invention may provide a material such as an optically transmissive material (e.g. an optical fibre), inscribed according to the method described above. The invention may provide an optical fibre, such as a single-mode optical fibre, inscribed as described above, to provide therein a linear inscription structure within the cladding of the fibre immediately adjacent the core of the fibre and not passing into the core. The invention may provide an optical fibre possessing a micro-channel etched from the aforesaid optical fibre. The micro-channel structure may extend linearly any distance within and/or across the fibre transversely or obliquely to the long axis of the fibre.

The invention may provide a sensor incorporating an optical fibre such as described above. The sensor may be arranged to detect or sense or measure evanescence field of light propagating within the fibre. The invention may provide a photonic device including an optical fibre such as described above. The micro-channel may contain liquid.

There now follows an example of the invention with reference to the drawings in which:

Figure 1 schematically illustrates a single-mode optical fibre possessing ill- defined laser-modified inscribed regions therein, resulting from laser inscription of the optical fibre with laser radiation passed directly through the curved outer surface of the optical fibre;

Figure 2 schematically illustrates apparatus for inscribing an optical fibre; Figure 3A schematically illustrates a single-mode optical fibre possessing a well-defined linear inscription structure therein resulting from laser inscription using the apparatus of Figure 2.

Figure 3B illustrates another view of the optical fibre of 3A in which the fibre is rotated by 90° about its long axis to reveal a view along the long axis of the linear inscription structure;

Figure 4A illustrates an optical microscope image of the optical fibre of Figures 3A and 3B after chemical etching thereof to remove the inscribed material and to reveal a micro-channel;

Figure 4B illustrates another view of the optical fibre of Figure 4A in which the fibre is rotated by 90° about its long axis to reveal a view along the long axis of the micro-channel.

In the figures, like items are assigned like reference symbols.

The fabrication of a micro-channel in a Silica-based single-mode optical fibre is described in the following example. The fabrication process employs inscription of a linear structure within the material of the optical fibre using exposure to focused laser pulses of 150 femtosecond duration. The Silica-based material of the optical fibre responds to this exposure by becoming modified in the sense of becoming more susceptible to etching. Thus, subsequent etching of inscribed parts of the fibre moves the laser-modified material to expose a linear micro-channel.

The mechanism responsible for the increased etching rate of inscribed material may be one or a combination of: an increase in the internal stress level of the material resulting from laser exposure; the presence of laser-induced voids or micro-cracks therein; a laser-induced densification effect in the material. Commentators have suggested that laser-induced modification and increased etching susceptibility of silica-based material, upon inscription, arises from a change in the Si-O-Si bond angle in the Silica molecules. Another apparent effect of inscription is an increase in the refractive index of the inscribed, laser-modified material.

Figure 1 illustrates schematically the view, through an optical microscope, of a single- mode optical fibre 1 , with a fibre core 2, into which has been directed inscribing laser radiation. The radiation was directed into the fibre directly through the cylindrically curved outer surface of the cladding of the fibre exposed to ambient air. Femtosecond laser pulses of 150 femtoseconds duration were generated from a regeneratively amplified Ti:sapphire laser system at a wavelength of 800 nm with a 1 kHz repetition rate. The laser beam was focused into the Silica fibre by a 100x (i.e. 100 times) objective lens with a numerical aperture of 0.55 and a working distance of

13 mm between the objective lens and the outer surface of the optical fibre. The focus spot size was evaluated to be about 1.5 micrometres. The standard single- mode optical fibre was mounted on a dual-axis air-bearing translation stage such that the desired modification structures could be written by translating the fibre along or perpendicular to the direction of propagation of the laser beam. No treatment or sensitisation process was performed on the fibre.

A single-pass exposure, perpendicular to the fibre axis, was carried out in order to inscribe a straight channel through the fibre. Prior to the inscription process, the fibre was positioned such that the focus spot of the femtosecond laser beam was well- away from fibre cladding. The translation speed of the fibre was 50 micrometres per second, moving in a straight line in the direction away from the objective lens over a distance much larger than the fibre diameter. The intensity of the femtosecond laser pulses was measured to be about 1 microJoule per pulse. A high resolution optical microscope was then used to visually inspect the inscribed feature within the optical fibre, and Figure 1 schematically illustrates the view through that microscope.

As shown in Figure 1 , the femtosecond laser-induced refractive index modification 100 in the fibre is clearly visible under the optical microscope. It is evident that the inscribed feature significantly deviates from the desired straight channel and shows significant distortions. The inscription process was repeated whereby the fibre was translated in a straight line in the reverse direction, and a similar ill-defined index modification structure on the near-side of the fibre (with respect to the objective lens) was observed. It is evident that the laser-induced inscription feature expands as it approaches the fibre core and subsequently fades away as the inscribing laser beam moves further into the fibre. This is tentatively attributed to, amongst other things, the curved outer surface of the optical fibre which is believed to de-focus the incident

femtosecond laser beam leading to a decrease in light fluence and intensity. This effect increases with increasing penetration depth into the fibre and the laser light fluence subsequently dropped below the threshold necessary to cause any significant structural modification to the material of the fibre resulting in no increase in the susceptibility of that material to subsequent chemical etching. The curved outer surface of the fibre may also exacerbate the tendency of incident inscribing laser radiation to reflect from that surface at curved regions which are increasingly oblique to the direction of the laser beam.

Figure 2 schematically illustrates a cross-sectional view of apparatus 3 for inscribing the optical fibre 1 in a manner which overcomes the problem of de-focusing (and possibly reflection) of laser light suffered by the methodology resulting in the ill- defined inscription structure 100 of Figure 1.

The apparatus includes an optical coupler (8, 11) placed in direct physical contact with the curved outer surface 5 of the optical fibre separably to form an interface therewith, and a laser 6, and a focussing lens (9) arranged to direct pulses of laser radiation 7 into the optical fibre via the interface. The focussing lens 9 is arranged to cause the pulse radiation to converge to form a focus at a desired region positionable within the optical fibre thereat to generate laser radiation of intensity/fluence sufficient to induce a modification in the Silica-based material of the fibre which renders that material more susceptible to chemical etching.

The optical coupler is placed inbetween the optical fibre and the focusing lens and includes a flat glass plate 8 transparent to radiation provided by the laser and a quantity of index-matching oil 11 located between the glass plate and the optical fibre to be inscribed.

Examples of types of index-matching fluids (IMF) that can be used include fluorocarbon-based IMF, chlorofluorocarbon-based IMF. The index-matching fluid has sufficient viscosity, in the range of 50 to 200 centistokes (cSt) to stay sufficiently "glued" to the optical fibre and the glass plate.

A flat outer surface of the glass plate provides a laser-light input surface 10 and is positioned to face the radiation output of the laser and directed by the focusing lens, preferably generally directly and fully (i.e. not obliquely), so as to permit inscribing laser light to enter the optical coupler in a direction preferably generally perpendicular to the flat input surface.

The quantity of index-matching oil 1 1 is carried upon a surface of the glass plate facing the optical fibre and is in direct physical contact with both the glass plate and a curved outer surface area of the optical fibre forming a continuous interface with each. The index-matching oil is transparent to the radiation provided by the laser and has a refractive index chosen to more closely match the refractive index of the cladding material (typically 1.5) of the optical fibre than does the refractive index of the air (typically 1.0) surrounding the optical fibre. Typically, the refractive index of the index-matching oil is a value between 1.5 and 1.0, such as 1.4.

The range of possible values of refractive indices for index-matching fluid is 1.430 to 1.530, preferably in increment steps of 0.002. A typical value suitable for use with a standard silica fibre is 1.454. Of course, different fibre materials may possess different refractive indices those exemplified here, and different refractive indices of IMF may be more suitable in such circumstances as would be readily apparent to the skilled person.

The refractive index of the index-matching oil is also chosen to more closely match that of the glass plate 8 than does the refractive index of air. The result is that the interfaces between the index-matching oil and both the optical fibre and the glass plate, suppress the degree of deviation of laser light which would otherwise occur were the oil removed and replaced with an air gap. Deviations may be due to refraction across the interface(s) and/or reflection at the interface(s). As will be readily apparent to the skilled person familiar with Snell's Law of refraction, and Fresnel's equations of reflection and refraction, the degree of such deviations at the aforementioned interfaces formed with the index-matching oil is increased when laser light is increasingly obliquely incident on those interfaces, such as results from the curvature of the optical fibre outer surface. However, the extent of deviation is reduced by the presence of the index-matching oil at the interfaces and the flatness and orientation of the surface of the glass plate through which the laser pulses enter the optical coupler. This reduces the extent to which such deviations may reduce the intensity or fluence of the laser light at its focus within the optical fibre. Thus, light loss due to reflection of the laser light and due to the de-focusing effect of refraction at the curved surface of the optical fibre, is reduced enabling accurate and effective inscription of the optical fibre to occur.

Figures 3A and 3B schematically illustrate the result of inscribing the optical fibre illustrated in Figure 1 with the apparatus and procedure referred to in connection with that figure, but employing the optical coupler as described with reference to Figure 2. The glass plate of the optical coupler employed had a length of 20 mm, a width of 20 mm, and a thickness of 125 micrometers. Index-matching oil was employed as described above to form a continuum between the surface of the glass plate facing the optical fibre and the optical fibre itself.

It is evident that index modifications (indicating the inscribed regions) were consistently and generally uniformly induced along the path traversed by the focal point of the laser from one side of the fibre to the other transverse to the long axis of the fibre. The introduction of the optical coupler effectively removed the effect of the fibre curved surface to allow the focused femtosecond laser pulses to propagate transversely (with respect to the fibre axis) into the fibre without suffering large distortion, deviation or scattering. This leads to a uniform straight-line modification zone 200 as shown in Figures 3A and 3B. It is noted that a conically-shaped inscription feature appears at the end of the inscription lying nearmost the laser. This phenomenon has been attributed to the result of a clipping effect of the inscribing laser radiation whose power gradually changes as the beam transits the edge of the surface of the optical fibre. It has been found that this effect is reducible if the refractive index of the index-matching fluid is closely matched to the refractive index of the fibre cladding.

Figures 4A and 4B illustrates optical microscope images of the optical fibre of Figures 3A and 3B subsequent to chemical etching to remove the inscribed fibre material. This chemical etching of the inscribed fibre was performed by placing the fibre in a solution of 5% hydrofluoric acid diluted with water in an ultrasonic bath at ambient temperature for selective removal of the laser-modified (inscribed) regions of the fibre material.

Figures 4A and 4B clearly show a well-defined linear etched micro-channel 300 as a characteristic opaque or dark colour under the bright illumination field used in the optical microscope. Rotating the fibre viewed in Figure 4A by 90°, to provide the view shown in Figure 4B, indicates a micro-tunnel through the fibre with a diameter of the

order of the fibre core diameter. It is to be noted that one compares the dimensions of the etched channel 300 illustrated in Figures 4A and 4B with the dimensions of the inscribed linear structure 200 illustrated in the corresponding Figures 3A and 3B, one can see that the diameter of the laser-induced (etched) linear feature of Figure 3A has a diameter less than the diameter of the optical fibre core. Conversely, the diameter of the subsequently etched channel is approximately equal in diameter to that of the fibre core. These images therefore highlight an observation that the laser- induced inscription regions (those regions of increased etching sensitivity) may extend over distances larger than the apparent dimensions of the inscription track left by the inscribing laser focus. However, these differences are reducible by appropriate tuning of various parameters (e.g. laser energy, etching time etc).

Potential attributes and applications of optical fibres possessing micro-channels of the type discussed herein are many and varied in the field of photonics and sensing applications. It is to be noted that the accuracy with which an optical fibre may be inscribed according to the present invention enables a high degree of proximity of a micro-channel to the optical fibre core as is illustrated in Figure 4B. This proximity enables access to the evanescence field of the light propagating with the fibre core. Consequently, the spectral properties of the fibre waveguide may be modified by the use of fluid placed within the micro-channel without compromising the insertion loss performance thereof. The present invention envisages optical probes and sensors employing this technique such as tuneable photonic devices and integrated micro- fluidic devices and applications. Though illustrated in terms of a curved outer surface of a material to be inscribed (a fibre), the present invention applies to any non-flat, irregular or discontinuous surface in addition or alternatively.

It is to be understood that modifications and variations of the examples of the invention described above, such as would be readily apparent to the skilled person, are encompassed within the scope of this invention.