Patent Publication Number: US-8538207-B2

Title: Method of fabricating waveguide devices which use evanescent coupling between waveguides and grooves

Description:
This application is a national phase of International Application No. PCT/GB2008/000995 filed Mar. 20, 2008 and published in the English language. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to evanescent field optical waveguide devices and methods for the manufacture of such devices. 
     A class of optical devices is known in which an optical channel waveguide is formed in a planar substrate comprising a core layer sandwiched between a lower cladding layer and an upper cladding layer. The channel waveguide is defined by a higher refractive index channel in the core layer. A window is formed in the substrate surface such that it overlies part of the waveguide, and has a depth through the upper cladding layer down to, and possibly into, the core layer. A sample of material can be placed in the window, and part of the evanescent field of light propagating in the waveguide will extend out of the waveguide and into the material in the window, where the light and the material interact to modify the optical field. 
     This can be utilized in a number of ways. The modification of the optical field can be used for frequency or amplitude modulation of the light. Alternatively, changes in the light can be used to infer properties of the material. Hence, these devices are generally modulators or detectors. 
     Often, a Bragg grating is included in the waveguide under the window. This provides a mechanism for determining the refractive index of the material via a spectral measurement. The presence of the sample affects the effective modal index experienced by light propagating in the grating and hence modifies the wavelength filtering response of the Bragg grating (it shifts the Bragg wavelength). The wavelength shift in light transmitted or reflected by the grating that is produced by the sample can be measured, and the refractive index of the material calculated from the size and direction of the shift. 
     Examples of sensors of this type can be found in WO 2006/008447 [1] and WO 2006/008448 [2]. 
     A particularly useful technique for forming the channel waveguide is that of direct ultraviolet (UV) writing, as described in WO 2004/049024 [3]. In this technique, the core layer of the substrate is photosensitive to ultraviolet light. A spot of ultraviolet light is formed having a width the same as the desired width of the waveguide, the spot is positioned in the core layer, and the spot and the substrate are moved relative to each other to trace out the path of the waveguide. The ultraviolet light causes an increase in refractive index of the photosensitive material, thereby defining the waveguide. The spot may have a periodic intensity pattern of high and low intensity fringes, produced for example by intersecting two beams of light at an angle or by exposure through a phase mask. If the movement of the light spot relative to the substrate is at a constant velocity and exposure of the substrate to the spot is continuous, a uniform change in index is produced, giving a conventional waveguide. If exposure is discontinuous, a Bragg grating can be produced. Thus, the method provides a simple technique for creating waveguides having gratings therein in a single fabrication step. 
     Once the waveguide (including gratings if desired) has been written in the substrate, the sample window for the optical device can be formed. Conventionally, this is done by etching with hydrofluoric acid, which can remove the cladding and core layers to a desired depth [1, 2, 4-6]. Use of hydrofluoric acid is undesirable from safety, environmental and industrial points of view. Also, it is necessary that the etching be carried out after the waveguide has been written. Hence any error in the etching stage will ruin the otherwise completed substrate and waste the effort expended in writing the waveguide. Etching is also slow and relatively costly, and the versatility of the device is limited by the fact that the window must be positioned on top of the waveguide, so that only a limited range of configurations is possible. Also, the etched window has sharply defined edges across the waveguide which present abrupt changes to the propagating light, giving rise to undesirable back reflections and cavity effects. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is directed to a method of fabricating an optical waveguide device, comprising: providing a planar substrate comprising at least a lower cladding layer, a core layer and an upper cladding layer; and forming in the substrate: a groove having a depth extending at least into the core layer; and a waveguiding channel in the core layer; wherein at least a part of the waveguiding channel is sufficiently proximate to the groove in the plane of the substrate for an evanescent field of light propagating in the waveguiding channel to extend laterally into the groove. 
     This provides an evanescent optical field device with a lateral geometry, in that the waveguide and the groove are arranged side-by-side in the plane of the substrate, giving a horizontal configuration assuming the device is oriented with the substrate horizontal. This offers a range of advantages over the conventional vertical configuration of a surface window over the waveguide. For example, under the invention, the waveguide can have a curved path through the core and hence be smoothly brought into proximity with the groove. This gives an adiabatic change in the geometry and avoids the abrupt edge changes of a window that produce undesirable reflection effects. Also, the lateral configuration gives great flexibility to the relative arrangement of the waveguide and the groove. The waveguide can access the groove at multiple points, and it is straightforward to provide any amount of proximity. Further, the use of a groove instead of a surface window removes the need to use acid etching, because the groove can be formed by a variety of other simpler and less hazardous techniques. 
     There is substantial flexibility available for manufacturing the device. The lateral geometry allows either the waveguide or the groove to be made first. In particular, the groove may be formed before the waveguiding channel is formed. This is different from conventional window arrangements where the window must be made after the waveguide, so that the intricate effort of making the waveguide is wasted if an error occurs in making the window. The present invention allows the simple step of making the groove to be performed first so that an already written waveguide is not wasted in the event of an error. 
     The method may further comprise forming a Bragg grating in the part of the waveguiding channel proximate to the groove. The inclusion of a grating allows the device to perform a range of functions, such a refractive index measurement of a sample material in the groove, or tuning of the grating by modification of an active material in the groove. The method is thus versatile in allowing many different devices to be fabricated in a simple manner. 
     For example, the core layer may be, photosensitive, and the waveguiding channel and any Bragg grating may be formed by exposing parts of the core layer to a spot of ultraviolet light to produce a change in refractive index. Further, the spot of ultraviolet light may have a periodic intensity pattern of high and low intensity fringes. Direct writing of a waveguide with a writing spot of ultraviolet light is an attractively simple technique that allows complex waveguide configurations incorporating gratings to be created in a single fabrication step. Precise positioning of the waveguide is readily achievable, so that if the groove is formed first, using a low precision process, subsequent ultraviolet writing of the waveguide can provide the appropriate level of positional accuracy between the groove and the waveguide. 
     For example, if the groove is formed before the waveguiding channel is formed, the position of the spot of ultraviolet light relative to the groove in the plane of the substrate may be tested during formation of the waveguiding channel by measuring the amount of light transmitted from the spot into the groove. This is a simple technique by which the separation between the waveguide and the groove can be accurately controlled during waveguide formation. 
     The groove may be formed using a cutting device, such as a semiconductor wafer dicing or milling saw. Cutting a groove into the substrate is much quicker, cheaper and safer than etching a window using acid. Although cutting with a saw is a relatively coarse technique, the manufacturing tolerances on the groove are relatively low if it is formed first, because the waveguide can be subsequently written relative to the groove with appropriate accuracy. Cutting a groove after formation of the waveguide is an alternative, but the positional accuracy is more difficult to achieve. 
     Alternatively, the groove may be formed using a lithographic technique and etching. Lithography can be used before or after the waveguide is formed, but is a more complex process than cutting. 
     The groove may have a depth extending to, into or through the lower cladding layer. This gives a groove which exposes the whole thickness of the core layer. Hence the extension of the optical field into the groove is unimpeded and the interaction with material in the groove can be maximised for a given proximity. Also, the lateral geometry allows the depth of the groove to specified with a large tolerance compared to the depth of an etched window, which determines proximity to the waveguide and hence must be very accurately defined. 
     The device may be made more complex by including additional grooves, waveguides, gratings and/or positions of proximity between a groove and a waveguide. For example, the method may further comprise forming one or more further waveguiding channels in the core layer, wherein at least a part of the or each further waveguiding channel is sufficiently proximate to the groove in the plane of the substrate for an evanescent field of light propagating in the further waveguiding channel to extend laterally into the groove. Thus, a single groove can be accessed by multiple waveguides. Additionally, the method may further comprise forming a Bragg grating in the part of each waveguiding channel proximate to the groove. 
     In other embodiments, the method may further comprise forming one or more further grooves in the substrate, the or each groove having a depth extending at least into the core layer, wherein the waveguiding channel has parts sufficiently proximate in the plane of the substrate to every groove for an evanescent field of light propagating in the waveguiding channel to extend into the grooves. This gives a device in which a single waveguide can access multiple grooves, each of which may contain a different material or have a different surface treatment to react with the material. The method may further comprise forming a Bragg grating in each part of the waveguiding channel proximate to a groove. 
     Further, the waveguiding channel may have at least two parts sufficiently proximate to the groove in the plane of the substrate for an evanescent field of light propagating in the waveguiding channel to extend into the groove, each proximate part having a different proximity to the groove. 
     Also, the waveguiding channel may have at least two parts sufficiently proximate to the groove in the plane of the substrate for an evanescent field of light propagating in the waveguiding channel to extend into the groove, each proximate part having a different width. 
     In embodiments in which a grating is formed in the waveguiding channel, the Bragg grating may be a chirped Bragg grating, and the waveguiding channel may be formed such that the part proximate to the groove has a proximity that varies as a function of the grating period. This can be engineered so that a temperature independent refractive index measurement of a material in the groove can be obtained. 
     In other embodiments, the optical waveguide device may be an optical modulator, and the method may further comprise filling the groove with liquid crystal and providing the substrate with electrodes by which an electric field can be applied across the liquid crystal to modify the Bragg wavelength of the Bragg grating. 
     In alternative embodiments, the optical waveguide device may be a refractive index sensor, and the method may further comprise: forming a second groove in the substrate, filling the groove with liquid crystal and providing the substrate with electrodes by which an electric field can be applied across the liquid crystal; forming a second waveguiding channel arranged to collect light reflected from or transmitted by the Bragg grating in the said waveguiding channel, the second waveguiding channel having a portion sufficiently proximate to the second groove in the plane of the substrate for an evanescent field of light propagating in the second waveguide to extend laterally into the second groove; and forming a second Bragg grating in the proximate part of the second waveguiding channel such that application of an electric field across the liquid crystal can modify the Bragg wavelength of the second Bragg grating. This gives a device in which measuring the light from the second grating while that grating is tuned in wavelength allows a spectral analysis to be performed of the light from the first grating that is measuring the refractive index of any material in the groove. 
     Alternatively, the optical waveguide device may be a laser, and the method may further comprise: forming a pair of Bragg gratings in the waveguiding channel, one on each side of the part proximate to the groove, to define a resonant cavity in the waveguiding channel; forming a second waveguiding channel in the core layer for propagation of pump light, the second waveguiding channel having a part sufficiently proximate to the groove in the plane of the substrate for an evanescent field of pump light propagating in the waveguiding channel to extend laterally into the groove; and filling the groove with a material capable of population inversion when exposed to pump light propagating in the second waveguiding channel and which produces stimulated emission at the resonant wavelength of the resonant cavity. Additionally, the method may further comprise: forming a pair of further grooves in the substrate such that the parts of the waveguiding channel having the pair of Bragg gratings are sufficiently proximate in the plane of the substrate to the pair of further grooves for an evanescent field of light propagating in the waveguiding channel to extend into the grooves; filling the pair of further grooves with liquid crystal; and providing electrodes by which electric fields can be applied across the liquid crystal to modify the Bragg wavelength of the Bragg gratings, and thereby tune the resonant wavelength. 
     In further embodiments, the optical waveguide device may be an amplifier, and the method may further comprise: forming a second waveguiding channel in the core layer for propagation of pump light, the second waveguiding channel having a part sufficiently proximate to the groove in the plane of the substrate for an evanescent field of pump light propagating in the waveguiding channel to extend laterally into the groove; and filling the groove with a material capable of population inversion when exposed to pump light propagating in the second waveguiding channel and which produces light by stimulated emission that couples into the said waveguiding channel for amplification of light propagating therein. 
     A second aspect of the present invention is directed to an optical waveguide device comprising: a planar substrate comprising at least a lower cladding layer, a core layer and an upper cladding layer; a groove formed in the substrate and having a depth extending at least into the core layer; and a waveguiding channel in the core layer; wherein at least a part of the waveguiding channel is sufficiently proximate to the groove in the plane of the substrate for an evanescent field of light propagating in the waveguiding channel to extend laterally into the groove. 
     The device may further comprise a Bragg grating in the part of the waveguiding channel proximate to the groove. The groove may have a depth extending to, into or through the lower cladding layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: 
         FIGS. 1A and 1B  show respectively a cross-sectional side view and a plan view of an optical waveguide device according to an embodiment of the invention; 
         FIGS. 2A and 2B  show respectively a side view and a plan view of a sample optical waveguide device fabricated in accordance with an embodiment of the invention; 
         FIG. 3  shows a plan view of a sensor device according to an embodiment of the invention; 
         FIG. 4  shows a plan view of an alternative sensor device according to a further embodiment of the invention; 
         FIG. 5  shows a plan view of a further alternative sensor device according to a yet further embodiment of the invention; 
         FIG. 6  shows a plan view of a still further alternative sensor device according to a still further embodiment of the invention; 
         FIG. 7  shows a cross-sectional side view of a modulator device according to an embodiment of the invention; 
         FIG. 8  shows a graph of wavelength shift produced by varying a voltage applied to the modulator device of  FIG. 7 ; 
         FIG. 9  shows a plan view of a device according to an embodiment of the invention, incorporating a sensor and a modulator; and 
         FIG. 10  shows a plan view of a laser device according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Conventionally, optical devices that exploit the effect of a sample of material on the evanescent optical field of light propagating in a waveguide comprise a window etched into the top surface of a planar substrate containing a channel waveguide. The window is positioned over the waveguide and extends down to or into the core layer of the substrate so that the evanescent field can extend into material contained in the window. The presence of a material in the window modifies the optical field. A material placed in the window can thereby be interrogated if the modification is measured; the amount of modification is representative of a property of the material which can hence be determined. A device operated in this way is therefore a sensor. Alternatively, the material properties can be used to deliberately modify the light in a desired manner, so that the device operates as a modulator. In all cases, however, the geometry of the waveguide and window combination is orthogonal to the plane of the substrate. 
     The present invention seeks to overcome some of the drawbacks of these devices and their geometry and fabrication by proposing an alternative geometry, in which the channel waveguide and the “window” are arranged side-by-side in the plane of the substrate and in close proximity so that the evanescent field extends into the “window” and the material therein in a lateral direction, through the side wall of the “window”. Thus, assuming that the substrate is oriented horizontally, the device geometry is also horizontal, in contrast to the vertical configuration of existing devices. 
     The shallow surface window of the known devices is replaced according to the present invention by a groove, trench or channel formed in the substrate, which extends sufficiently deeply into the substrate that the optical field of light carried by a waveguiding channel formed in the core layer of the substrate and adjacent to the groove can extend into the groove (which will contain sample material or material to modify the propagating light). Thus, the groove will have a depth that extends at least part of the way into the core layer, and more probably all the way through the core layer and possibly into the underlying cladding layer or further. Complete exposure of the full thickness of the core layer by the side wall of the groove increases the amount of the evanescent field that can penetrate into the groove and therefore maximises the interaction between the light and any material in the groove. The waveguiding channel and the groove are sufficiently close together that a sufficient amount of the evanescent field overlaps into the groove to create a discernable optical path change for light propagating in the waveguide. 
     The proximity of the groove and the waveguide determines the sensitivity of the device; the sensitivity is higher if the core of the waveguide is in direct contact with the groove than if it is some distance away. To give an idea of scale, an example of a larger spacing between the edge of the groove and the centreline of the waveguide is 15 micrometers. Also, the sensitivity can be enhanced by reducing the width of the waveguide in the proximate region. The amount of reduction may depend in part on the ability of the waveguide to support an optical mode. Further, the optimal proximity of the waveguide relative to the groove is also determined by the absolute index and loss of the material in the groove, the interaction length between the groove and the waveguide, and the system used to analyse the light detected from the waveguide. 
       FIG. 1A  shows a cross-sectional view through a substrate configured as an optical device in accordance with an embodiment of the invention. The device  10  is formed from a layered planar substrate comprising a base layer  12 , a lower cladding layer  14  arranged over the base layer  12 , a core layer  16  arranged over the lower cladding layer  14 , and an upper cladding layer  18  arranged over the core layer  16 . The core layer  16  has a higher refractive index than the cladding layers, to confine propagating light in accordance with the well-known principles of optical waveguiding. The device may comprise additional layers if desired. 
     A waveguiding channel or waveguide  22  (shown in end view in  FIG. 1A ) comprises a path of higher refractive index defined in the core layer  16  so that light is confined to the waveguide  22  in the plane of the core layer  16 . Light propagating along the waveguide  22 , although largely confined within the high refractive index region, also has an evanescent optical field  24  that extends beyond the waveguide  22 . 
     The device  22  further comprises a groove or trench  20  defined in the substrate material, and in this example extending down through the upper cladding layer  18 , the core layer  16  and the lower cladding layer  14  to the base layer  12 . The groove  20  is roughly parallel to the waveguide  22  in the plane of the substrate. At the plane of the cross-section shown in  FIG. 1 , the waveguide  22  approaches very close to the groove  20  so that the optical field  24  extending out from the waveguide  22  reaches into the groove  20 . Therefore, light propagating along the waveguide  22  can interact with any material contained in the groove  20  (none is shown in  FIG. 1A ). 
       FIG. 1B  shows a plan view of the device  10  shown in  FIG. 1A . In this example, the groove  20  is straight and extends across the substrate. The waveguide  22 , although generally parallel to the groove  10 , has a slightly serpentine path which brings only a part  26  of the waveguide  22  into sufficiently close proximity with the groove  20  for the evanescent field to reach into the groove  20 . The waveguide  22 , initially more remote from the groove  20 , curves towards the groove  20 , runs parallel to it in close proximity for a short distance, and then curves away. Thus, optical interaction with material in the groove is limited to a small length  26  of the waveguide  22 , giving a similar arrangement to that offered by a delimited overlying window in a conventional device. 
     However, the smooth curving relationship between the waveguide  22  and the groove  20  offers a significant advantage over the conventional surface window. The curves give a gradual adiabatic increase and decrease to the extension of the evanescent field into the groove and the material it contains. A window, in contrast, has sharply defined edges across the path of the waveguide, giving abrupt changes which cause unwanted back reflections and cavity effects. Thus, better quality light signals with lower loss can be obtained using a device according to the invention. 
       FIG. 1B  also shows a Bragg grating  28  defined in the waveguide  22  at the part  26  proximate to the groove  20 . Although not essential to the invention, a grating in the interaction region between the waveguide and the groove can provide various functions. For example, the device can be used to measure refractive index. The characteristic Bragg wavelength of the Bragg grating, being the peak wavelength at which the grating reflects, varies with the effective modal index experienced by light propagating in the grating. A material in the adjacent groove will modify that effective modal index, thereby altering the Bragg wavelength. This alteration or shift can be detected from light reflected from or transmitted by the grating, and used to calculate the refractive index of the material. Window-based sensors using this principle are known [1, 2]. Devices according to the present invention are not limited to including a grating in the position shown in  FIG. 1B , and may or may not include Bragg gratings in the proximate region of the waveguide or elsewhere. 
     The groove and the waveguide may be formed by any convenient fabrication technique, with either formed first. A disadvantage of existing devices with surface windows is that the window must necessarily be formed after the waveguide, because of its position over the waveguide. The present invention is free from this constraint, and allows the groove to be fabricated before the waveguide if desired. In fact, this arrangement is highly advantageous because it allows the less intricate fabrication step to be performed first. Thus any errors that occur in making the groove do not result in the waste of previous effort in making the waveguide, as can occur for a window-based design. 
     The groove may conveniently be fabricated using a cutting device such as a saw or mill of the type used to dice and otherwise prepare wafers in the semiconductor industry. The saw cuts into the substrate to the required depth. The groove depth is by no means as critical as the depth requirement for a surface window (which defines the proximity to the waveguide), so less accuracy is needed and the comparatively coarse method of cutting or milling is sufficient. Cutting in this way is both quick and inexpensive compared to the time-consuming and costly acid etching technique for forming windows. Also, the highly corrosive and therefore dangerous hydrofluoric acid is eliminated. A further advantage is that the groove cutting can be combined with dicing of a large wafer into smaller device substrates to allow two fabrication steps to be performed together. 
     The spacing between the groove and the waveguide is important for good device operation. The positional errors likely in defining a groove or trench by cutting with a saw means that it is less appropriate for this technique to be used in cases where the waveguide is fabricated first, because it is more difficult to locate the groove with sufficient accuracy relative to the waveguide. Therefore, if the waveguide has already been fabricated, other methods of making the groove are more suitable. For example, lithographic techniques allow sufficiently accurate indexing of the groove position relative to the waveguide. Formation of the trench by lithography and etching may also be used in situations where the groove is made before the waveguide. 
     Similarly, any method can be used for forming the waveguide and any Bragg gratings therein. However, a particularly advantageous technique, and one which has been found to be highly compatible with using a saw to cut the groove, is that of direct ultraviolet writing [3]. 
     This technique uses a beam of ultraviolet (UV) light, focussed to form a spot having a width matching that of the desired width of the waveguide, to trace the intended path of the waveguide through a region of photosensitive material. The exposure to UV light causes the refractive index of the photosensitive material to permanently increase, giving the higher refractive index value needed for the core of a waveguide. Thus, if the core layer of a substrate is made of photosensitive material, the UV spot can write a waveguide in the core layer. If the spot has a “constant” intensity distribution (such as Gaussian) and moves through the photosensitive material at a constant velocity, a uniform refractive index change is produced along the waveguide, giving a uniform waveguide. A similar effect results if the spot has a pattern of high and low intensity fringes but moves at a constant velocity. However, a fringed spot moved in a discontinuous manner, such as by exposing consecutive parts of the material in sequence, can produce a Bragg grating in the waveguide. Very fine control of the relative movement of the spot and the substrate is possible, allowing a complex pattern of waveguides with integral gratings to be written with a high degree of positional accuracy in a simple one-stage fabrication process. 
     The combination of the two simple techniques of cutting a groove with a saw and directly writing a waveguide with a UV spot gives a highly advantageous method for forming optical waveguide devices according to the present invention. In particular, the groove can be formed before the waveguide, which is preferable from the point of view of performing the least delicate fabrication step first. UV writing allows the waveguide to be formed second, which is more difficult with other waveguide fabrication techniques because the presence of the groove would prevent the successful spinning of resist onto the substrate surface. UV writing after groove formation also offers the ability to achieve sufficiently accurate positional alignment between the groove and the waveguide. 
     However, at first sight, it may appear that the combination of cutting a groove with a saw followed by direct UV waveguide writing would not produce good results. For example, the groove fabrication should not produce significant mechanical weakness, delamination, degradation or stress-induced birefringence in the substrate that could interfere with subsequent waveguide fabrication and operation. Cutting with a saw might cause these problems. Also, the close proximity of the waveguide path to the groove could produce interference with the UV writing beam or cause catastrophic material ejection. The vertical edge of the groove needs to be of sufficient quality not to generate scattering of the UV writing beam. Sufficiently accurate alignment between the groove and the waveguide needs to be achievable, preferably to a submicron level. 
     A useful method for alignment has been found to be that of monitoring the amount of light transmitted sideways into the groove from the UV writing beam. The magnitude of the measured light is proportional to the distance of the beam from the wall of the groove, so the UV spot can positioned accordingly. This has been found to provide a good level of positional accuracy. While the monitoring can be carried out throughout the writing process, it can also be performed initially at each end of the groove (by, for example, intersecting the foci of the two UV beams used to produce a fringed spot with the edge of the groove) to ensure that the axis of the groove and the axis of the translation stage used to produce relative movement between the substrate and the writing spot are collinear. 
     Sample devices have been fabricated to test the compatibility of cutting a groove followed by UV writing of a waveguide with gratings. A substrate with an area measuring 10 mm by 20 mm consisted of base layers supporting a thermally oxidized silicon lower cladding layer overlaid by two layers of silica deposited using flame hydrolyse deposition (FHD) providing the core layer and the upper cladding layer. The core layer was doped with germanium oxide to promote photosensitivity for the UV writing. Other dopants were also included to match the refractive indices as required. The photosensitivity of the core layer was further enhanced by leaving the substrate in a high pressure (approximately 120 bar) hydrogen environment for several days to load the substrate with hydrogen. This substrate is merely an example, however. The invention is applicable to any substrate in which a waveguide can be fabricated. 
     Grooves were cut into the substrate to a depth extending past the lower cladding layer using a semiconductor dicing saw. The grooves were 300 μm deep and 300 μm wide. This is relatively large, to allow adequate testing for mechanical weakness and any delamination problems. 
     Waveguides and Bragg gratings were then formed using the UV direct writing technique described above and in WO 2004/049024 [3]. A frequency-doubled argon ion laser generated 244 nm wavelength writing light with a power range of 50 to 100 mW and fluencies in the range 1 to 20 kJ/cm 2 . 
       FIGS. 2A and 2B  show a side view and a plan view of the substrate after groove and waveguide fabrication. The side view shows the lower cladding layer  14 , the core layer  16  and the upper cladding layer  18 . Five grooves or trenches  20  are cut into the substrate, and five waveguides  22  (shown in end view in  FIG. 2A ) are defined, one next to each groove. 
     The plan view  FIG. 2B  shows the grooves  20  extending the length of the substrate (not essential but this makes for easier fabrication with a saw), with the associated waveguides  22  running alongside the grooves  20  and each having a portion  26  that approaches closely to the associated groove  20 . In this example, each proximate portion  26  contains a Bragg grating  28 . Also in this example, each waveguide  22  includes a second Bragg grating  30  defined in a part of the waveguide  22  that is not proximate to the groove  20 . These second gratings  30 , which are unaffected by any material in the nearby groove, can be used as reference gratings to compensate for any environmental factors such as temperature which may cause wavelength shifts in the primary Bragg gratings which would be otherwise indistinguishable from the shift caused by the material. Each groove-grating pair could, for example, be used as a refractive index sensor. A single pair might be provided on one substrate, but the invention is flexible in allowing multiple waveguides, gratings and grooves to be easily fabricated on the same substrate. The configuration of  FIGS. 2A and 2B  could therefore be used for simultaneously measurements of five different samples, or for simultaneous measurements at five different wavelengths if the gratings have different Bragg gratings. 
     Characterisation tests on the sample of  FIGS. 2A and 2B  showed no stress-induced birefringence, and further suggested that the grooves actually reduce stress-induced birefringence as compared to devices with surface windows. Similar effects have been observed elsewhere [7, 8]. Also, it was concluded that the reduction in stress via the definition of the grooves may increase the UV-induced refractive index change. Forming grooves in optoelectronic samples is not unknown, however, including by sawing and milling, and is carried out for functions such as alignment (so-called V-grooves), stress relief and waveguide formation [9-11]. 
     It has been previously observed that UV writing into an absorbing layer such as the photosensitive core layer can cause surface ablation which can be catastrophic. It is known experimentally that defects on silica-on-silicon surfaces can cause UV absorption that leads to damage. Prior to fabrication of the substrate in  FIGS. 2A and 2B , it was unclear if UV writing close to the side wall of the groove would cause material ejection into the groove. Low optical loss was measured from the waveguides  22 , which suggests that any such ablation does not occur. 
     A concern relating to the use of a polishing/dicing saw is that of surface roughness in the walls of the grooves. It might be expected that scatter from the sawn surface would make UV waveguide writing untenable. The fact that this is not so is surprising. Also, as the waveguide approaches closer to the groove, the optical power reflected from the grating is reduced owing to the power lost into the groove, but the grating sensitivity increases. Optimisation of the proximity to balance these effects is dependent on the device requirements. However, the loss observed from the substrate of  FIGS. 2A and 2B  is comparable to that observed in devices with windows fabricated using surface acid etching. This indicates that any surface roughness and chipping of corners of the groove is not so significant as to prevent useful device operation. 
     Therefore, a quality device having a lateral geometry according to the present invention can be conveniently fabricated using groove formation with a saw followed by waveguide formation by direct UV writing. 
     Devices according to the present invention offer further advantages, including the adiabatic change offered by curving waveguides mentioned above. In addition, there is the ability to change the optical mode size in the lateral direction, by varying the width of the waveguide which can be easily achieved using UV writing. This also allows multimode operation, from which further information can be obtained about a sample material in the groove if the device is configured as a sensor. Also, it is possible for a single waveguide to have multiple regions that are proximate to a single groove, each region having a different proximity. This allows different penetration depths of the evanescent mode into the groove, so that multiple measurements can be simultaneously made on a single sample of material. Different grating wavelengths can be used to distinguish the different proximities. Additionally, the lateral arrangement of waveguide and groove allows separate waveguides to access the same groove (and any material therein), by placing a waveguide on each side of the groove. Indeed, the lateral configuration is highly flexible, allowing complex groupings of individual grooves and waveguides. A wide variety of sensor and modulator devices can thereby be achieved, including the desirable design of multiple sensors on a single chip. 
     The example device shown in  FIGS. 2A and 2B  is very simple. The invention can be readily extended to a wide variety of devices. 
       FIG. 3  shows a plan view of an alternative device. In this example, the device  32  comprises a single groove  20  and two waveguides  22 . The waveguides  22  are arranged one on each side of the groove  20 , and each has one region  26  that is proximate to the groove  20 , and includes a Bragg grating  28 . Also, each waveguide  22  includes a secondary or reference grating  30  located in a part of the waveguide remote from the groove, as described with respect to  FIG. 2B . The proximate gratings  28  may be used for refractive index measurements, with compensation for environmental disturbances provided by the reference gratings  30 . The provision of two separate waveguides to access the same groove, and hence the same sample of material, allows redundancy and confirmation for the refractive index measurement. The width of the groove  20  should be sufficient to avoid coupling of the optical fields between the waveguides  22 . 
       FIG. 4  shows a plan view of a further example device. The device  34  again features a single groove  20  extending across the substrate and two waveguides  22 , one on each side of the groove  20 . Again, each waveguide  22  includes a reference grating  30  in a part of the waveguide remote from the groove  20 . In this example, however, each waveguide  22  includes a plurality of gratings  28  that are proximate to the groove  20 . One waveguide  22 A has a width that varies with distance along the waveguide/groove. This allows higher order optical modes to propagate. Each grating  28  is arranged in a portion of the waveguide  22 A having a different width, and each grating  28  has a different Bragg wavelength. This provides a way of interrogating the refractive index of a sample in the groove  20  at each of the modes. The larger cross-section of the higher order modes may give additional information about the index variation of the sample material. 
     The other waveguide  22 B has a constant width, but is arranged such that each of its plurality of gratings  28  has a different proximity to the groove  20 . The evanescent field can therefore penetrate further into the groove for each successive grating. The gratings  28  have different Bragg wavelengths so that they can be distinguished, and in addition the combination of different period gratings with different proximities/penetration depth provides a high level of sensitivity for samples of different refractive index. This configuration also reduces the mechanical tolerances on fabrication. 
     Although the device in  FIG. 4  includes two differently configured waveguides each with several gratings, which together offer a wide range of index measurements from a single device, a device including just one or the other of the waveguides may instead be provided. 
       FIG. 5  shows a plan view of another example device. The device  36  comprises a single waveguide  22  having a reference grating  30  at one end. In contrast to the previous examples, a plurality of individual separate grooves  20  is formed in the substrate. The waveguide  22  is shaped to come into proximity with each of the grooves, so that the waveguide  22  follows a serpentine or undulating path. The waveguide  22  has a plurality of gratings  28 , one in each waveguide section that is proximate to a groove  20 . This allows a plurality of samples to be analysed at the same time from a single waveguide, by a different sample being placed in each groove  20 . Alternatively or additionally, each groove may be provided with a different surface treatment to interact with the sample material; this provides information on different components of a sample material. In an alternative arrangement, the waveguide may be substantially straight, and each of the grooves may be curved into and out of proximity with the waveguide. 
       FIG. 6  shows a plan view of another device  38 . This includes just one groove  20  and one waveguide  22 . However, the portion  26  of the waveguide  22  that is proximate to the groove  20  is arranged at an angle to the axis of the groove so that the waveguide tapers along its length, with a decreasing width. Thus, the proximity of the waveguide, and the penetration of the evanescent wave, increases with distance along the proximate part  26  of the waveguide  22 . In addition, the proximate part  26  includes a Bragg grating  28 , which in this example is chirped, i.e. it has a grating period that varies with distance along the grating. The grating period and the proximity are related along the grating  28 . A change in index yields a change in the bandwidth of the light reflected by the grating  28 , making it insensitive to temperature fluctuations. Monitoring of the bandwidth of the reflected spectrum  28  therefore gives a temperature-insensitive measurement of the refractive index of a sample material in the groove  20  (hence the device  38  does not include a reference grating  38 ). The use of chirped gratings to obtain temperature-independent measurements of strain is known [12], but it is believed that a refractive index sensor using chirp to compensate for temperature as proposed herein is novel. 
     The use of a continuously varying evanescent wave penetration depth is made possible by the lateral arrangement proposed by the present invention. Such variation is difficult if not impossible to achieve with any precision using a surface window overlying a waveguide grating. The increased control over the spacing between the waveguide and the groove that is made possible by a lateral configuration is highly advantageous and offers much flexibility in device design. 
       FIG. 7  shows a cross-sectional view of a further alternative device, which can be operated as a modulator, in contrast to the sensors described so far. The device  40  again comprises a waveguide  22  in close proximity to a groove  20  so that the optical field  24  of the waveguide  22  extends into the groove  20 . A Bragg grating is included in the proximate region of the waveguide  22 . In this example, however, the groove  20  is filled with an active material rather than a sample analyte material. Liquid crystal  42  or another tunable dielectric material may be used, for example, to provide modulation of an optical signal propagating in the waveguide  22 . To achieve modulation, an electric field is applied across the liquid crystal  42 . The resulting change to the refractive index of the liquid crystal  42  changes the Bragg wavelength of the grating via the effect on the evanescent optical field  24 . Thus, the grating can be tuned. A pair of electrodes arranged across the liquid crystal  42  is required for application of the electric field. This can be facilitated by use of the silicon substrate layer as an electrode. As shown in  FIG. 7 , the groove  20  containing the liquid crystal  42  extends down into the substrate base layer  12 , which is made of silicon, and is provided with an aluminium contact  44  on its lower surface so that it can function as an electrode. A second electrode is provided by a thin film  46  of indium tin oxide (ITO) on the lower surface of a sheet of glass  48  or similar that is placed over the upper cladding layer  18  of the device  40  to contain the liquid crystal  42 . 
       FIG. 8  shows a graph of results obtained from the modulator  40  of  FIG. 7 , illustrating the wavelength shift obtained as a function of applied voltage. 
     The modulator of  FIG. 8  may be extended by providing a second waveguide on the other side of the groove  20 , with a similar geometry to that of  FIG. 3 , and a groove width in the range of about 5 to 20 μm. This provides a controllable coupler, in which the amount of optical coupling from one waveguide to the other across the groove (which is narrow enough for this to occur) is modifiable according to the voltage applied across the liquid crystal. A similar device based on side-polished fibres has been proposed [13], but is extremely time-consuming and expensive to fabricate. 
       FIG. 9  shows a plan view of a device  50  that combines a refractive index sensor and a tunable modulator. The substrate is provided with two grooves  20 A,  20 B spaced apart from one another. One groove  20 A receives a sample of analyte material, and the other groove  20 B is filled with liquid crystal or another tunable dielectric, suitable electrodes being provided for application of an electric field across the liquid crystal. An input waveguide  22 E has a section proximate to the analyte groove  20 A which includes a grating  26  so that the refractive index of the analyte material can be sensed. Light reflected from the grating  26  is directed along an output waveguide  22 F that branches off from the input waveguide  22 E. The output waveguide  22 F has a section including a grating  52  which is proximate to the liquid crystal groove  20 B. The response of the grating  52 , in particular its transmission wavelength, can be tuned by varying the voltage applied across the liquid crystal, thereby providing a tunable filter. Tuning this output grating  52  in a known manner allows the reflection spectrum of the sensor grating  26  to be analysed, i.e. by tuning the output grating  52  and measuring the amount of light reflected from the sensor grating  26  that is transmitted, the full spectrum of the reflected light can be easily measured, from which the refractive index of the analyte material can be determined. The device may also include a temperature reference grating  30  in a part of the input waveguide  22 E that is remote from the analyte groove  20 A but from which reflected light can travel to the output waveguide  22 F; this may also be analysed by tuning the output grating  52 . 
     The modulator thus acts as an analyser of the index sensor. Butt-coupling a broadband diode optical source  60  to the waveguide input and a photodiode  62  to the waveguide output provides a cheap, compact refractive index sensor. 
       FIG. 10  shows a plan view of a device that can be operated as a laser. The device  54  includes a single groove  20 , and two waveguides  22 C,  22 D arranged one on each side of the groove  20 , and each having a region that approaches proximate to the groove  20 , similar to the arrangement of the device  32  of  FIG. 4 . In this example, however, one waveguide  22 C has no Bragg gratings. The other waveguide  22 D has a pair of Bragg gratings  56  with substantially the same Bragg wavelength. These gratings  56  are positioned in the waveguide  22 D one on each side of the proximate region  26 , i.e. the gratings are not proximate to the groove. 
     If the groove  20  is filled with a material capable of population inversion, such as Rhodamine 6G or another laser dye, the device  54  can operate as a laser. The waveguide  22 C without gratings can propagate pump light that couples into the groove at the proximate region, and excites the laser material to produce an inversion. The stimulated emission from the laser material then couples across into the second waveguide  22 D, where the two Bragg gratings  56  form a resonant laser cavity. The Bragg wavelength is therefore selected for the gratings to be suitably reflective at the laser wavelength. One grating  56 A is a high reflector, and the other grating  56 B has a lower reflectivity so as to act as an output coupler and transmit some of the laser light out along the waveguide  22 D. An additional Bragg grating  58  that is reflective at the pump wavelength can be provided in the second waveguide  22 D beyond the output coupler  56 B to filter out any pump light that may couple across from the pump waveguide  22 C to the laser waveguide  22 D. 
     Tuning of the laser output wavelength can be achieved by altering the cavity grating wavelengths by localised heating or modulation by liquid crystal in a proximate groove as described with regard to  FIG. 7 . Alternatively, the cavity gratings  56  can be replaced by broadband reflectors on the end facets of the waveguide  22 D to provide a larger tuning range. Also, the gain medium material in the groove could be replaced by a material with a strong Raman effect to provide a Raman-shifted laser. 
     Laser devices such as these are potentially very useful as building blocks for so-called “lab on a chip”-type experiments. 
     Moreover, if the cavity gratings  56  are omitted, a similar arrangement can be used as an optical amplifier to amplify light at the “laser wavelength” that is already propagating in the second waveguide  22 D. 
     For devices configured as sensors, samples of fluid can be delivered to the groove or grooves by a microfluidic system. The grooves can be connected by channels in the substrate surface to form a fluid network, though which a fluid can flow. The network be closed (to contain the fluid) by a sealing layer (or glass or Perspex, for example) laid over the top surface of the sensor, with input and output access points for fluid to be injected into the network and then removed therefrom. A gasket layer under the sealing layer can improve the sealing of the fluid network, for example to enhance suction of fluid through the network or to reduce sample contamination. 
     Thus, a wide variety of optical waveguide devices can be made that exploit the inventive concept of a proximate and laterally arranged waveguide and groove in the same substrate. The invention is not limited to the examples described herein; many other examples will be apparent to the skilled person. 
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