Development of the fiber photosensitivity has opened up new opportunities. The photosensitivity permits fabricating fiber Bragg gratings (FBG), which are now prevalent in many applications such as multiplexers, demultiplexers, and optical add/drop filters, where individual wavelength selection and separation is required.
Fiber Bragg gratings are generated by “inscribing” or “writing” systematic changing of refractive index into the core of a special type of optical fiber using an intense ultraviolet source such as a UV lasers. The type of FBG inscription that is most suitable depends on the type of grating to be manufactured. Generally, a germanium-doped silica fiber is used to inscribe fiber Bragg gratings. The germanium-doped fiber is photosensitive, which means that the refractive index of the core changes with exposure to UV light. The amount of the change depends on the intensity and duration of the exposure as well as the photosensitivity of the fiber.
The methods of FBG inscription consist of two types: holographic (interference) and nonholographic. The first type uses the amplitude or spatial splitting of the beam into two beams, which interfere in the fiber bundle. Nonholographic methods based on the periodic illumination of the fiber with the use of pulsed source through an amplitude mask or point-by-point method.
Interferometer inscription schemes use two-beam interference. Here the UV laser is split into two beams which interfere with each other creating a periodic intensity distribution along the interference pattern. The refractive index of the photosensitive fiber changes according to the intensity of light that it is exposed to. This method allows for quick and easy changes to the Bragg wavelength, which is directly related to the interference period and a function of the incident angle of the laser light.
In the scheme with a Lloyd interferometer (spatial beam splitting), the interference pattern is produced by using a mirror: one-half of the beam is concurred with the other half at the angle. This interferometer comprises a dielectric mirror, which directs half of the UV beam to a fiber that is perpendicular to the mirror.
With a Talbot interferometer by simultaneous rotation of additional mirrors located on rotation stages, wherein the fiber should be placed on a linear stage.This interferometer consists of two plane parallel mirrors and a diffractive element (a phase mask) with its surface aligned perpendicular to the mirrors. The illumination is arranged so as to recombine the first – order beams (or other orders such as the plus first and zero orders) to form the UV interference pattern.
Fiber Bragg gratings inscription using phase mask technique is based on the diffraction of UV light by a mask placed closely to the fiber. The phase mask technique is significantly simpler in comparison with other methods. It has the simple setup and uses low coherence UV laser beam.
The common method in FBG phase-mask inscription uses dedicated 193nm or 248nm excimer laser models with increased coherence. When illuminated with the laser beam, the phase-mask generates a regular interference pattern. The enhanced spatial coherence length of the excimer laser provides good contrast behind the mask which is typically placed in a distance of 100µm to 200µm from the fiber core.
The phase mask methods quickly have prevailed over other techniques due to simplicity and flexibility of the application and great reproducibility of the inscribed gratings.
Optromix offers unique UV laser Magius SF257 supplied as a part of Fiber Bragg Grating writing workstation. Optromix Company has made it possible to inscribe FBG by all methods, mentioned above.
