in fig1 a it is schematically illustrated , as viewed from the side or from the top , the central parts of a conventional fiber splicing machine 1 . the splicing machine 1 has plates or chucks 2 which are attached to retainers 3 , which can be displaced , by not shown mechanisms and driver devices , generally in three coordinate directions in relation to each other . further there are electrodes 5 connected to a voltage source , not shown , for generating an electric arc 6 between the points of the electrodes 5 . the electric arc 6 passes through a region centrally located between the chucks 2 , in the conventional case for heating the ends of two optical fibers which are to be welded to each other . for production of a strain sensor a single conventional optical fiber 7 , for instance a quartz glass fiber , is placed in the two chucks 2 , so that the fiber passes in the space between the chucks 2 and through the region between the points of the electrodes 5 , where the electric arc 6 is generated . the chucks 2 contain , as is conventional , guides indicated at 8 for the optical fibers which are to be fusion welded or only heated , and they can , in an initial state in the method discussed here , be placed opposite and aligned with each other , so that the optical fiber 7 passes from one chuck to the other without essentially any bending thereof . an electric arc 6 is then generated between the points of the electrodes 5 and it will heat a region of the optical fiber 7 centrally located between the front sides or end surfaces of the chucks 2 . this region is rather local , as seen in the longitudinal direction of the fiber , and can comprise for instance about 5 fiber diameters at most for standard fibers of single mode type . the heating is made all around the whole fiber 7 and penetrates rapidly into the inner portions of the fiber 7 . when the heating is sufficient , so that the glass in the optical fiber 7 within the locally heated region is suitably soft or semimelted , the retainers 3 are displaced a distance d laterally in relation to each other and hereby , also the chucks 2 are displaced the same distance d laterally in relation to each other . owing to the rigidity of the not heated free portions of the fiber 7 and the softness of the heated region it will produce a bending of s - type of the optical fiber , as is schematically illustrated in fig2 . after this heating and the displacement of the retainers 3 in relation to each other , the current supply to the electrodes 5 is discontinued , whereby the electric arc 6 disappears . the optical fiber 7 has now got a permanent bend induced which is of the type illustrated in fig2 . a preferred alternative of this production method is illustrated in fig1 b . here the guides 8 in the chucks 2 are initially displaced the distance d from an aligned state , after which the optical fiber 7 is positioned in the guides 8 , so that it extends from one chuck 2 to the other one . a weak bending of the fiber is then obtained in the region between the end surfaces of the chucks 2 . thereafter a suitable electric high voltage is supplied to the electrodes 5 for creating an electric arc in the same way as above . hereby a region of the fiber 7 is heated locally as above and becomes soft and plastic . owing to the elasticity of the not heated region of the optical fiber 7 the fiber portions between the heated region 4 and the end surfaces of the chucks 2 are straightened out in order to be aligned with the adjacent portion of the fiber in the respective chuck 2 , whereby the material in the heated region 4 is bent to the same shape of s - type as is illustrated in fig2 . the produced microbend of an optical fiber 7 illustrated in fig2 is characterized by the fact that the bend consists of two arcs or curved segments composed to an s - like shape , so that the portions of the fiber 7 outside the heating region are essentially parallel to each other but displaced laterally approximately the distance d . the core 9 of the fiber 7 and the surface of the fiber cladding 11 has essentially the same profile . in the production of the microbends according to fig2 the fiber is heated , as has been stated above , over a region in its longitudinal direction having a length corresponding to some multiple of the diameter of the cladding of the fiber 7 , for instance longer than a fiber diameter and shorter than about 5 fiber diameters . the parallel displacement d of the optical fiber has the magnitude of order of one or a few fiber diameters at most , for instance less than three times the fiber diameter , in the preferred case less than 1 fiber diameter , in the range comprising 0 . 5 - 1 fiber diameter . an optical fiber having double cores comprises two parallel cores which extend close to each other and are enclosed by the same cladding . if the cores have the same geometry and are located adjacent each other , light can be coupled between the two cores . such a fiber can be bent in the same way as the fiber illustrated in fig2 and is illustrated in fig3 . here the bend is made in a suitable direction in relation to the plane passing through the two fiber cores 9 &# 39 ;. the bend can advantageously be made so that the fiber cores 9 &# 39 ; after this bending will extend in the same plane . the optical characteristics of the two cores are then changed and the coupling of light between the two cores is thereby changed . the method described above for producing a microbend in an optical fiber can also be used for other fibers , for instance a longitudinal cavity fiber 7 &# 39 ;. such a fiber is illustrated schematically in fig5 and comprises a longitudinal cavity 13 extending inside the fiber cladding 11 in parallel to the fiber core 9 . in order to make a microbend of a cavity fiber 7 &# 39 ; also the device schematically illustrated in fig4 can be used . here a carbon dioxide laser is arranged producing a light beam 17 which is made to locally heat an optical longitudinal cavity fiber 7 &# 39 ;. when the material , which is located in the cladding of the fiber 7 &# 39 ; around the cavity 17 , is heated locally , it collapses , so that the cavity is not preserved within a portion of the heated region . when the material collapses no visible bend of the cladding 11 of the fiber 7 &# 39 ; is formed but instead a microbend of the fiber core 9 is produced which is visible at 17 in fig5 . the microbend of the core is rather small and the parallel displacement of the core comprises a fraction of the radius of the fiber core , typically having the magnitude of order 0 . 1 of the core diameter for an optical standard fiber having a core diameter of about 10 μm and generally for instance not more than 0 . 3 of the core diameter . also , a heating without any parallel displacement of the retainers 3 or the chucks 2 in an ordinary welding machine of the type shown in fig1 can produce a corresponding local heating of the fiber causing thereby a corresponding collapsing or closing of the longitudinal cavity 13 passing through the heated region . when the fibers 7 , 7 &# 39 ;, which are described above and which have microbends produced therein of at least the core 9 of the fiber , are subjected to a change of their lengths , i . e . a strain , the geometrical conditions of both the core and the cladding are changed , for instance by a straightening of the microbend , and further a change is obtained of the distribution of the refractive index in the core 9 and the cladding 11 . for a strain of an optical fiber having double cores with microbends according to fig3 the bend may for instance be straightened so that the ratio of tensile stress and compressive stress is reduced resulting in a change of the coupling of light between the two cores . in the collapsing or closing of the cavity of a longitudinal cavity fiber according to fig4 and 5 it may in extreme cases occur , that no visible bend of the fiber core 9 is obtained . the cavity fiber can still , when it is subjected to a strain , give changed refractive indices in the region adjacent the fiber core 9 and this effect can be detected . a schematic picture of the use of an optical fiber 7 for measurement of the strain of a substrate or support 19 is shown in fig6 . a piece of a fiber 7 treated as above , for instance having a multitude of microbends 20 , is moulded to and rigidly attached to the support 19 by means of an adhesive layer 21 of a plastics material , for instance a thermosetting adhesive such as an epoxy adhesive . in applying the adhesive care should be taken that the layer between the fiber 7 and the surface of the substrate 19 is as thin and as rigid as possible , in order to produce a direct transfer of strains of the support to the fiber . the ends of the optical fiber 7 are connected to suitable detection devices . in the case shown an equipment for transmission measurement is used and it can then comprise a light source 23 and a light detector 25 . from the light source 23 light having a suitable wave length is issued through the fiber 7 and the transmitted light is measured by the light detector 25 . by arranging , as is illustrated in fig6 several microbends close to each other at a spacing of for instance about 1 mm an increased sensitivity is obtained when the region having microbends is subjected to a strain . also other detection methods can be used . such an arrangement is shown in fig7 where two regions of a fiber 7 &# 39 ; having microbends 20 of the core have been placed attached to two different supports 13 &# 39 ;. in this case each region is attached to the respective support by means of adhesive bumps or some other mounting means 27 at each sides of the region having a microbend 20 , so that the region having the microbend is free from , detached from the substrate . one end of the fiber is connected to an otdr instrument 27 operating both as a transmitter and receiver of light . the receiver part can detect the magnitude of received light and distinguish backwards diffused light from different points of the optical fiber 7 &# 39 ; and in particular from the two microbends 20 . thereby the strain of each one of the two substrates 19 &# 39 ; and 19 &# 34 ; can be determined .