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An optical element (3) is mechanically retained within a slot of (2) in a rigid substrate (1), e.g. of a ceramic or crystalline material, by contact with the sides of the slot. Where the difference between a dimension of the optical element and the slot width at ambient temperature is a very small

An optical element (3) is mechanically retained within a slot of (2) in a rigid substrate (1), e.g. of a ceramic or crystalline material, by contact with the sides of the slot. Where the difference between a dimension of the optical element and the slot width at ambient temperature is a very small and positive, it may be removed by heating the substrate and/or cooling the element (differential thermal expansion) to allow insertion of the element. Then the temperature difference is removed so that the element is mechanically retained under compression by the slot sides. Alternatively, where the element may be directly inserted so that it is in contact with or immediately adjacent the slot wall, the material of the substrate and/or the optical element is the oxidized in the region of contact so as to increase the dimension of the oxidized material and mechanically retain the element. Either technique is applicable to parallel sides slots, but the element may be additionally or alternatively mechanically retained by shaping the edges of the slot. The fibre (3) shown may be butted against a second fibre, or coupled to a guide or other element within the substrate.

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The invention claimed is: 1. A method for mounting an optical element in a substrate comprising the steps of taking a substrate having a parallel sided slot formed therein the difference between a dimension of the optical element and the slot width at ambient temperature being a very small positive

The invention claimed is: 1. A method for mounting an optical element in a substrate comprising the steps of taking a substrate having a parallel sided slot formed therein the difference between a dimension of the optical element and the slot width at ambient temperature being a very small positive amount, producing a positive temperature differential between the substrate in the region of the slot and the optical element such that said dimension is now no more than the slot width, inserting the optical element therein in the presence of said temperature differential between the substrate and the optical element and thereafter allowing the temperatures of the optical element and the substrate to equilibrate. 2. A method according to claim 1 wherein the step of producing a positive temperature differential includes cooling the optical element. 3. A method according to claim 1 wherein the step of producing a positive temperature differential includes heating the first substrate. 4. A method according to claim 1 and comprising the further step of bonding the optical element to the first substrate. 5. A method according to claim 4 wherein the bonding step comprises applying adhesive to the optical element and first substrate. 6. A method according to claim 1 and comprising the further step of thermally oxidizing one or both of the first substrate and the optical element in the region of contact. 7. A method according to claim 1 wherein the substrate is rigid. 8. A method according to claim 7 wherein the substrate is of a ceramic or crystalline material. 9. A method according to claim 1 wherein the optical element is an optic fibre and said dimension is the or a diameter of the fibre, the slot depth being greater than half said dimension. 10. A method according to claim 9 and comprising the further step of mounting the first substrate to a second substrate with the fibre in optical alignment with an optical component on or in the second substrate. 11. A method according to claim 10 wherein the first substrate is mounted so that the fibre abuts the said component. 12. A method according to claim 10 comprising the further step of providing a depression in the said surface of said second substrate, wherein a surface of said second substrate includes the optical component recessed or formed therein and the mounting step comprises mounting said first substrate in said depression with the optic fibre in alignment with said optical component. 13. A method according to claim 12, the depression having vertical opposed walls, and wherein the mounting step comprises arranging said first substrate between the vertical opposed walls such that it is gripped mechanically there-between under compression. 14. A method according to claim 12 comprising the step of arranging an end of the optic fibre to lie substantially flush with an edge of the second substrate. 15. A method according to claim 10 comprising the step of providing the first and second substrate with co-operating projections and holes or slots for relative alignment thereof. 16. A method according to claim 15 wherein the step of providing the co-operating projections and holes or slots comprises arranging at least one projection to be a slightly oversize fit in its hole or slot in at least one dimension of the substrate surface, such that it may be cooled relative thereto for insertion, and is gripped thereby on temperature equalisation. 17. A method according to claim 15 wherein the step of providing the co-operating projections and holes or slots comprises arranging at least one projection to be an accurate fit in its hole in at least one dimension of the substrate surface for precise location in said at least one dimension. 18. A method according to claim 15 wherein the step of providing the co-operating projections and holes or slots comprises arranging the size of the projections relative to the holes or slots so that there is latitude for adjustment of alignment in one or both dimensions of the second substrate surface. 19. A method according to claim 1 wherein the depth of the slot is less than the said dimension so that a portion thereof projects above the surface of the first substrate. 20. A method according to claim 19 and comprising the further step of mounting the first substrate to a second substrate with the fibre in optical alignment with an optical component on or in the second substrate and wherein a groove is formed in the second substrate to house the projecting portion of the fibre. 21. A method according to claims 1 wherein the depth of the slot is equal to or greater than said dimension. 22. A method according to claim 1 wherein the optical element is an optic fibre, the substrate includes or is provided with an optical component with an input surface or aperature, and the inserting step comprises butting an end of the optic fibre against said aperature. 23. A method according to claim 22 wherein the slot forming step is controlled so that the fibre contacts the bottom of the slot when in alignment with said aperature. 24. A method according to claim 22 wherein said component is formed integrally within the substrate, adjacent or spaced below its surface. 25. A method according to claim 22 wherein said optical component is a separate component mounted within the substrate. 26. A method according to claim 25 wherein said component is mounted within a section of said slot. 27. A method according to claim 26 wherein said component is mounted within said section by producing a positive temperature differential between the slot section and the component, inserting the component, and allowing the temperature differential to disappear. 28. A method according to claim 25 wherein said component is another optic fibre. 29. A method according to claim 25 wherein said fibre and said component are mounted simultaneously to said substrate. 30. A method according to claim 25 wherein said fibre and said component are mounted sequentially to said substrate. 31. A method of coupling two optical fibres comprising the steps of taking a first substrate having a parallel vertical sided slot formed in a surface thereof, the difference between the diameter of the fibres and the slot width at ambient temperature being a very small positive amount, producing a positive temperature differential between the substrate in the region of the slot and the optical fibres such that the fibre diameter is now no more than the slot width, inserting the optical fibres therein with their ends butting in the presence of said temperature differential between the substrate and the optical element, and thereafter allowing the said temperature differential to disappear. 32. A method according to claim 31 wherein the substrate is rigid. 33. A method according to claim 32 wherein the substrate is of a ceramic or crystalline material.