in the following description of the embodiments , references to the accompanying drawings are by way of illustration of an example by which the invention may be practiced . it will be understood that other embodiments may be made without departing from the scope of the invention disclosed . referring to fig1 , a small recess 12 , or a notch , has been carved on the side of an optical fiber 11 . the fiber diameter is typically 125 μm . hence the recess is very small : typically 30 μm deep and 50 μm wide . many techniques can be used to make this recess : laser ablation , chemical etching and others . but one of the simplest ways is using a dicing saw with a thin diamond blade . if the blade has been properly worn out , the blade &# 39 ; s edge forms a small radius . so when cutting in the direction perpendicular to the axis of the fiber , one can obtain a shallow cylindrical cut on the fiber surface . good results have been obtained with a 55 mm diameter , 100 μm thick resin blade with 46 μm particle size turning at 18 000 rpm , but similar or better results could be obtained with a different blade . a small bead of adhesive 13 is then deposited in the recess . of course , one has to make sure that the bead size is smaller than the recess and also that no adhesive has been deposited outside the recess . here again , different techniques can be used . for example , a small drop of fresh adhesive can be first deposited and thereafter , the adhesive partially cured . an alternative and preferred method uses a drop of partially cured epoxy on the tip of a very fine needle . one can simply put the drop in contact with the recess and heat the fiber so that the adhesive becomes liquid and wets the recess with the proper amount of material . the needle is then removed and the fiber is cooled down so that the adhesive bead becomes hard again . also , a great number of adhesives can be used for this purpose . by way of non - limiting example , one such suitable adhesive is aremco 526n two - part , high temperature epoxy . a partial cure of 15 minutes at 100 ° c . is sufficient to gel the epoxy but insufficient to fully cure it . heating it at approximately 175 ° c . for short periods of time brings it to a liquid state and back to a gelled state when cooled down to room temperature . but this method is not restricted to epoxies other adhesives , like solder glass or thermo plastics can also be used . among the factors in choosing the adhesive is that it is hard or highly viscous at room temperature , but able to flow at a higher temperature . another desirable , feature is that it can be fully cured at an intermediate temperature so that further exposition to higher temperature will not put it back into liquid state . the next step consists in inserting the fiber inside a micro - capillary 14 . fig2 shows the resulting assembly during heating and before swelling of the adhesive 13 . here , the micro - capillary 14 is shown with a protective polyimide coating 15 but this method will also work with an unprotected capillary . the capillary is preferably made of fused silica since it is the same material as the fiber . other materials could be used . in one embodiment of the present invention , the capillary material coefficient of thermal expansion matches that of the fiber . the focused beam 16 of a co2 laser can be used to locally heat the capillary around the adhesive . in one embodiment of the invention , the laser power is very low , less than one watt , and the beam width is approximately 300 μm . this suffices to sufficiently heat by optical absorption the capillary to a temperature where it will heat by conduction the adhesive enough to bring it to a liquid state . at that point , the adhesive expands and immediately tacks the capillary inner wall . just a few seconds of co2 laser exposure is enough to obtain a small bonded area that is approximately 50 μm wide . this situation is illustrated in fig3 where the bonded area 17 is shown . a cross - sectional view along the plane 4 - 4 defined in this figure is shown in fig4 . here , it is seen how the adhesive has swollen out of the recess 12 , between the capillary 14 inner wall and the fiber 11 round surface . of course , other heat sources than the radiation of a laser could be used for the same purpose . for example , one could use heated air flow or a loop of electric current heated wire . the last step is to completely cure the adhesive so that further heating will not bring the adhesive back to a liquid state . for the adhesive mentioned above , curing for four hours at 100 ° c ., two hours at 150 ° c ., two hours at 200 ° c ., two hours at 250 ° c . and two hours at 325 ° c . sequentially gives satisfactory results . this is best accomplished in a computer - controlled oven so it can be done overnight . as mentioned earlier , this final step depends heavily on the adhesive used and , in some cases , could be altogether omitted if the heating in the step before has been sufficient to fully cure the adhesive or if the device is not expected to be stored or used at elevated temperatures . furthermore , heat could not be needed if the adhesive used is a room temperature curable adhesive or a light - curable adhesive . a fiber - optic fabry - perot strain or displacement sensor using the bonding method described hereinabove is schematically illustrated in fig5 . two pieces of fiber , the incident fiber 19 and the reflection fiber 20 , are fixed by adhesive bonds spots 17 inside a capillary 14 with the method described earlier . the facing fiber ends are cleaved or polished and coated with partially - reflecting mirrors 21 and 22 . these two mirrors form a fabry - perot interferometer of which the cavity length is shown here as d . the distance between the two bonding areas 17 defines the gage length , lg . when this sensor is used as a strain measuring device , it is either bonded on the surface or embedded inside a body from which one wishes to measure the deformation . as the body is stretched , the capillary 14 is also stretched with an equal strain . hence , the distance between the two bonding points will change according to where ε is the strain , lg ( ε ) is the gage length for a given strain ε and lg ( 0 ) is the gage length when no strain is applied . since no strain is applied on the incident fiber 19 and the reflection fiber 20 , the distance between their ends , and consequently the cavity length d , will change by the same amount : hence , the initial gage length is representative of the sensitivity of the sensor : the longer is lg ( 0 ), the more the cavity length d ( ε ) will change for a given strain ε . methods known in the art can be used by the signal conditioner to demodulate and extract the cavity length information from the optical signal of the sensor . suffice it to say that light enters from the incident fiber 19 and is reflected by the fabry - perot cavity 21 - 22 . light in and out of the incident fiber is coupled from and to a third fiber , the input / output fiber 24 , which is connected to the optical conditioner . the input / output fiber 24 is attached in the capillary by way of an adhesive 25 , which can be different than the adhesive used in the beads 13 . another purpose of the adhesive 25 is that it seals this end of the capillary from liquids or liquid intrusion . in an embodiment , the opposite end of the capillary is sealed by a drop of adhesive or , as illustrated here 23 , by melting the glass at this end with a co2 laser or using an arc - fusion splicer . this way , the interferometer is completely sealed , which is advantageous if it is intended to be glued on or embedded in a body . in an embodiment , a third fiber , the input / ouput fiber 24 , is used instead of simply extending the incident fiber 19 all the way to the signal conditioner because it permits free movement of the fibers between the sealing adhesive 25 and the incident fiber 19 bonding point . alternatively , a continuous length of fiber between these two points could be used . strain applied to the capillary or to the input / output fiber 24 would then be transferred to the incident bonding point , a situation that could possibly result into non - linearities in the sensor response or eventually into breakage of the bonding point . in an embodiment , the facing ends 26 , 27 of the input / output fiber 24 and the incident fiber 19 are cleaved at an angle to minimise reflection losses . similarly , the far edge 18 of the reflection fiber 20 is shattered or cleaved at an angle to prevent reflected light to pass a second time through the fabry - perot cavity . another way to achieve the latter result is to use a reflection fiber 20 that is either non - guiding , or that has a significantly different core than the other fibers 19 and 24 . one should understand that the reflection fiber 20 does not have to be an optical fiber at all since no useful light is recovered from this second fiber . for instance , one could use a high thermal expansion glass fiber or even a metallic fiber instead of an optical fiber . this could be useful for example as a strain sensing device that would have its thermal response adjusted so as to counteract the expansion of the material to which the sensor is attached to . furthermore , in particular optical devices , the reflection fiber 20 could be replaced by a reflective surface attached to one end of the tube 14 and facing the partially - reflecting mirror 21 to provide the fabry - perot cavity . only one fiber bonding would than be used ( to bond the incident fiber 19 to the tube 14 ) and the distance between this bonding area 17 and the far end reflective surface would define the gage length , lg . the reflective surface could be attached to the end of the tube 14 using an adhesive or any other bonding method known in the art . the core size of the incident fiber 19 and the input / ouput fiber 24 and the reflectivity of the mirrors 21 and 22 depends largely on the light source and signal conditioner used . in one embodiment , a 50 μm core fiber with 0 . 22 numerical aperture and 30 % reflectivity mirrors are used along with a filament white light bulb light source and a white - light interferometry signal conditioner . but this is not the only possible configuration . for example , acceptable results could also be obtained using single - mode fibers with a led source and an optical spectrum analyzer . the method presented here is not restricted to strain measurement but can also be applied to other fiber - optic sensing devices such as for temperature or pressure measurement . also , this method as well as the fabry - perot interferometer presented here can be used in other fields such as optical telecommunications and for other optical devices such as fiber - optic filters or modulators where it would be desirable to have a fiber assembled inside a microcapillary . while this invention has been described in terms of specific embodiments with some variations in their construction , those skilled in the art will recognize that the invention can be practiced in other embodiments that are within the spirit and scope of the invention . the scope of the invention is therefore intended to be limited solely by the scope of the appended claims .