the following description is of a method of jointing both an input and an output fibre to a single optical waveguide on a lithium niobate substrate and resulting optical fibre to waveguide connections . referring now also to the drawings , fig1 and 2 are schematic diagrams of a joint connecting an optical single mode fibre 1 to a waveguide 2 of a waveguide substrate 3 . the optical fibre 1 is a single - mode fibre comprising a cladding 9 , around 0 . 1 mm in diameter , which surrounds a single mode core 5 , of around 0 . 005 mm diameter , forming the light - guiding structure of the fibre 1 . the optical waveguide substrate 3 consists of a rectangular slab of lithium niobate which is a few cm long , about one cm wide , and of the order of one mm thick . embedded in one of the two largest sides are one or more waveguides 2 ( only one of which is shown in the drawings ), which typically run parallel to the long sides of the slab and terminate across the width of the slab . the waveguides 2 are formed by narrow strips which have a higher refractive index than the neighbouring substrate material . methods of forming the waveguides are known in the art and , not being pertinent to the present invention as such , will not be described here . secured to that side of the substrate 3 which carries the waveguide 2 is a support block 4 , also of lithium niobate . the block 4 as shown in the figures straddles a single waveguide . in practice the block will often extend the full width ( i . e . the side shown face - on in fig2 ) of the waveguide substrate 3 and often straddle more than one waveguide . the support block 4 is secured to the substrate by means of a layer 8 &# 39 ; of adhesive . the optical fibre 1 is butt - jointed to the substrate 3 and the block 4 by a layer 8 of adhesive . making the fibre to waveguide joint with correct optical alignment of the core 5 with the waveguide 2 is achieved as follows : the fibre endface 6 ( fig1 ) is prepared by cleaving the fibre such that the fibre endface is substantially normal to the fibre axis . methods by which fibre endfaces may be cleaved as required are known and are not therefore described herein . the block 4 of lithium niobate is mounted on the waveguide substrate 3 adjacent to the termination 7 of the relevant waveguide 2 . one of the sides of the block 4 is brought into alignment with that side of the waveguide substrate 3 at which the waveguide terminates , and bonded to the substrate 3 with adhesive layer 8 &# 39 ;. next the core 5 of the fibre 1 is aligned with the waveguide 2 , and the endface of the fibre 1 is glued to both the waveguide substrate 3 and the block 4 , as follows . the block 4 is mounted on the waveguide substrate 3 by means of an adhesive 8 &# 39 ; which is curable by exposure to white light . the adhesive 8 &# 39 ; used was durafill bond , manufatured by kulzer and co ., wehrheim , a photocuring intermediate liner for tooth fillings . the adhesive , when cured , has a refractive index of less than that of the lithium niobate substrate . the block 4 is also constructed out of lithium niobate and its cross section is rectangular . it is disposed such that a first face lies in parallel contact with the waveguide substrate 3 , and a second face is coplanar with the endface of the substrate 3 . a convenient means of aligning the endface of the substrate 3 and the second face of the block 4 is to bring them both into abutment with a common planar surface before curing of the adhesive 8 &# 39 ;. referring to fig2 the block 4 is positioned so that it straddles the waveguide 2 , being in contact not only with the waveguide surface but also with the surface of the substrate 3 to either side of the embedded waveguide 2 . referring now specifically to fig3 and 4 , the end of input fibre 31 is first clamped to a micropositioner 34 with three translational directions of adjustment ( i . e ., it permits fibre movement up and down , left and right , and along the fibre axis ) and two rotational or angular directions of adjustment ( i . e ., it permits the rake and the tilt of the fibre to be adjusted ). the angular adjustments are used to bring the endface of the fibre 31 to lie perpendicular to the beam of a hene laser ( not shown ), which also defines an optical axis for the equipment . the fibre 31 is mode stripped to remove the cladding modes . the waveguide substrate 3 is mounted in a package which allows access to the terminations of the waveguide 2 at both ends of the substrate . the package itself is mounted on a goniometric head 35 which provides precise angular orientation . the endfaces of the waveguide substrate 3 are also brought to lie perpendicular to the beam of the laser . an optical source 32 is coupled to the end of the input fibre 31 which is remote from the waveguide substrate 3 . the optical source 32 is a packaged semiconductor laser with a single mode fibre tail . the fibre 31 is passed through a polarization controller 33 which enables either te or tm fields to be selected . the end of the fibre 31 is then moved into alignment with the waveguide 2 by using the linear adjustment facilities of the micropositioner 34 and viewing the output termination of the waveguide 2 with a × 5 microscope objective 36 when the optical source 32 is activated . the image obtained from the objective is displayed using a video camera and monitor 37 . the position of the input fibre 31 is varied until an intense near field mode pattern of the waveguide 2 is observed . to prevent fabry - perot cavity effects between the end of the fibre 31 and the waveguide 2 and to increase the stability of the launch conditions , a coupling gel is used . the refractive index of the gel is matched to that of the fibre 31 . after the above alignment step the imaging equipment , i . e . the objective 36 and camera / monitor 37 , is removed . referring to fig4 next , one end of the output 41 is mode stripped and coupled to a calibrated photodiode 43 . the other end is attached to another micropositioner 42 and angularly aligned with the optical axis provided by the he ne beam mentioned above . the linear adjustments of the micropositioner 42 are then used to align the fibre 41 roughly with the waveguide 2 and a small quantity of light curable adhesive is then introduced between the end of the fibre 41 and the waveguide substrate and support block about the waveguide output termination . the refractive index of the adhesive closely matches that of the fibre 41 . the two fibres 31 , 41 are then maneuvered in turn until the power measured on the photodiode 43 is at a maximum . at this state , the adhesive between the output fibre 51 and the waveguide 2 is exposed to white light , so fixing the fibre 41 to the termination of the waveguide 2 and to the surfaces of the substrate 3 and of the block 4 which lie adjacent to that termination . hence a joint of the type shown in fig1 and 2 is produced . the power measured on the photodiode 43 may be monitored during curing of the adhesive and additional adhesive may be applied around the joint to improve its strength . finally , adhesive is applied remote from the joint to attach the fibre 41 to the base of the package carrying the waveguide substrate 3 . this ensures that any strain on the fibre 41 is not transferred directly to the joint . the index matching gel is then cleaned from the input fibre 31 and the waveguide 2 , and replaced by a small quantity of white light curable adhesive . after again realigning the joint to maximise the power meausred on the photodiode 43 , the adhesive is cured and the joint attached in the same way as the output joint . finally package end walls together with a protective fibre sheath ( not shown ) are added . to check the effect of mounting the block 4 of lithium niobate on the waveguide substrate 3 , particularly with regard to the guiding propertiets of the waveguide 2 , near field light intensity distribution measurements were made on a 10 μm wide guide both before and after the lithium niobate block 4 was attached . referring to fig5 which shows the results , it can be seen that there was no significant difference between the before and after plots , indicating that the block 4 did not modify the guiding properties of the waveguide 2 . fig6 shows the excess loss introduced by the fixing of the joints for a sample of 24 ends . the loss is calculated from the measurements of the power coupled from a waveguide into a fibre before and after the curing of the adhesive . the mean excess loss for the joints was found to be 0 . 06 db with a standard deviation of 0 . 05 db . it should be noted that the waveguide in question was not optimised for use with the associated fibres e . g . in terms of mode patterns . the mechanical strength of the joint was assessed by a pull test on the fibre . a joint was made in the normal way but with no additional strain relief on the fibre . the fibre was loaded until the joint broke at a force of 0 . 7 newtons . the present invention provides a simple and practical technique for attaching fibres to waveguides on planar substrates . the technique has been used in the packaging of waveguide components , and has been shown to be capable of producing robust joints which introduce only a small additional coupling loss . although an embodiment of the invention has been described with reference to only one input fibre 31 and one output fibre 41 , in many cases it may be that the waveguide substrate 3 concerned comprises a plurality of waveguides embedded in parallel in one surface of the substrate . in such a case it may be convenient to use a single support block adjacent the endfaces of the substrate and mounted so as to straddle all the waveguides of the substrate which terminate at the end . the refractive index of the adhesive used to mount the block 4 on the waveguide substrate 3 will generally be selected to be less than or equal to the refractive index of the substrate . the refractive index of any material adjacent to the waveguide may affect its optical guiding properties . because of the small dimensions relevent to a waveguide which is generally about 10 μm in width , it will normally be the refractive index of the adhesive rather than that of the block 4 itself which will dictate the effect on the waveguide &# 39 ; s properties of mounting the block 4 to straddle the waveguide 2 . if the adhesive &# 39 ; s refractive index is less than that of the substrate , light will not generally be coupled out of the waveguide into the adhesive . however in certain circumstances such coupling may be desirable and the refractive index of the adhesive , and possibly also that of the block 4 , may then be selected to be greater than that of the substrate . acceptable coupling losses have been achieved with joints in which there is an offset between the polished face of the block 4 and the termination of the waveguide 2 of up to 5 μm . the extent to which such an offset is likely to be acceptable may affect the means used to mount the block 4 on the waveguide substrate 3 . although both the block 4 and the substrate are described as being of lithium niobate material , they may be of other suitable material . also , instead of using white light curable adhesive as described , other types of adhesive may be employed , provided it complies with the requirements as to refractive index and , possibly , strength . the above described process may be at least partly automated by using suitable control circuitry , for example microprocessor control for performing the alignment operations . also , the method can be used to attach more than one input or output fibre to substrates with two or more waveguides .