Abstract: A method of producing a planar substrate having waveguide channels, which method comprises: (i) providing a tube (6) of a substrate material; (ii) depositing silica layers (110) on the inside of the tube (6), the silica layers (110) being doped with a photosensitive material; (iii) drawing the tube (6) so that the cross-sectional size of the tube (109) is reduced; (iv) before or after the reducing of the cross-sectional size of the tube (6), causing the tube (6) to collapse into a flat shape by applying a low pressure to the tube, whereby the deposited silica layers together form a photosensitive silica layer (111); (v) cutting to required lengths the tube (6) which has been collapsed and reduced in cross-sectional size; and (vi) using laser writing to define waveguide channels in the cut lengths of the tube (6) and thereby to produce the planar substrate having the waveguide channels.