k produced by coherent inc the particular laser and wavelength of light is selected to match the exposure sensitivity of the photoresist a wavelength of about 413 nm is ideal for microposit 1400 27 u00ae or microposit 1400 33 u00ae the exposure of the photoresist to interfering beams of light creates a pattern wherein the photoresist is exposed periodically along the fiber for a spatial separation of exposure peaks or lines of about 0 3 micrometers preferably the exposure peaks each lie in a plane perpendicular to the fiber axis the light from the laser is spatially filtered by means of a pinhole and then expanded and collimated the exposure time is from about 1 to about 5 seconds for a 2 inch diameter beam of about 400 milliwatts total power when varying the grating dimensions a determination is made as to the desired grating size and their air interference angle and a light source is selected which can create such a pattern thereafter a suitable photoresist is selected which is sensitive to light of that wavelength after exposure the fiber and holder are removed from the exposing apparatus and the holder is readjusted so that the fiber is loose thus if the developer should be at a slightly different temperature thereby cooling portions of the holder it will not subject the fiber to additional stress which might cause it to break thereafter the fiber and holder are placed into a developing apparatus the developing apparatus is a fountain which contains a suitable developer for the photoresist such as developer no 351 cd 23 a product of the shipley corporation the fiber is exposed.