Coated optical fibers typically have at least three components: a core, a cladding surrounding the core, and a protective coating on the cladding. Both the core and the cladding are typically made of silica glass, but the core typically has germanium doping to increase its refractive index and, thus, minimize loss of light from the core.
Coated optical fibers can also contain at least one Bragg grating, a segment of the fiber core having a periodic modulation in refractive index. Optical fibers containing Bragg gratings (fiber Bragg gratings) have proven useful in a wide variety of applications, including telecommunications, sensors, and sensor arrays. For example, fiber Bragg gratings are widely used in telecommunication components and devices, such as wavelength stabilizers for pump lasers, narrowband add/drop filters for wavelength division multiplexing, and gain-flattening filters. Fiber Bragg gratings are also widely used in fiber optic sensors, particularly sensors for strain, pressure, and temperature measurements.
Fiber Bragg gratings are typically fabricated by exposing an optical fiber core to an ultraviolet laser beam to produce periodic changes in the refractive index of the core in the exposed region. However, because most protective coatings are not transparent to ultraviolet light at the wavelengths (e.g., 193 nm and 240 nm) commonly used to write gratings, the coating must be removed before exposing the core to ultraviolet light. The optical fiber must then be recoated to prevent damage to the fiber and to preserve its mechanical strength. These stripping and coating operations are problematic, time-consuming, and expensive. For example, once stripped of the protective coating the fiber is susceptible to irreversible environmental degradation caused by humidity and debris. Also, the material used to recoat the fiber must have good adhesion to the silica surface of the cladding.
Various approaches to solving the problems associated with stripping and recoating optical fibers have been reported. For example, Patent Application Publication No. 2003/0152352 A1 to Starodubov discloses a refractive index grating fabricated in an optical fiber having a multilayer coating and a method for making refractive index patterns such as gratings in optical fibers such that the mechanical properties of the original fiber are preserved. The patterns are written into the optical fiber by partially stripping away the outer coating of the fiber, exposing the core of the fiber through the remainder of the coating with an actinic radiation to form the pattern in the photosensitive core of the fiber, followed by recoating the fiber in the stripped area to provide protection of the newly formed pattern from corruption and to preserve the mechanical properties of the fiber.
U.S. Patent Application Publication No. US 2004/0228594 A1 to Andre et al. discloses an optical fiber having at least one Bragg grating, the fiber comprising a core surrounded successively by cladding and by a coating, said rating being obtained by being written directly in the core and/or the cladding of the fiber through the coating which is made of a material that is substantially transparent to ultraviolet type radiation used for writing said grating, wherein the material of said coating contains a first polymer network interpenetrated by a second polymer.
U.S. Pat. No. 6,240,224 B1 to Reekie et al. discloses a coated optical fiber comprising a coating, an optical fiber, at least one waveguiding region and an index grating. The waveguiding region contains at lest one photosensitive region, the index grating is formed by writing through the coating using UV light, and the coating transmits UV light at the wavelength at which the index grating is written. The '224 patent also teaches the coating should not contain a photoinitiator, since photo-initiators are often UV sensitive.
Chao et al. (Electronics Letters, 27 May 1999, Vol. 35 (11), 924-926) disclose fiber Bragg gratings fabricated by writing through the fiber coating using both 244 nm frequency-doubled Ar+-ion and 248 nm KrF excimer lasers.
Although, the aforementioned references disclose various methods of fabricating fiber Bragg gratings, there is a continued need for a fast, low cost method of fabricating gratings that is suitable for large scale manufacture.