One or more photonic crystals 11, 22, 40, 58, 59 are formed directly in the path of light within an optical fiber 13, 23, 42, 56. Light processed by the photonic crystal may be transmitted out of the fiber by means of a lens 48 or it may be measured by a photoresistive device 51, 60. The photonic crystal may be formed in a trench 12 as an array of dielectric rods 16 having one or more selective defects 17, or the crystal may be formed by providing holes 20 directly in the optical fiber. Filling the interstices between rods 16 with non-linear optical material, and subjecting the crystal to a varying electric field applied by electrodes or to a varying optical radiation can produce a tunable photonic crystal within an optical fiber.