We have developed a wideband tunable optical filter that uses a long-period fiber grating (LPFG) in which both resonance wavelength and its signal attenuation can be adjusted. We create the grating mechanically by pressing a spring coil to an optical fiber. We achieve continuous fine tuning of wavelength and attenuation by varying the temperature of the LPFG. The adjustable ranges of the LPFG are more than 200 nm in resonance wavelength and more than 10 dB in signal attenuation.

The temperature distribution in an optical fiber during the fabrication of fiber Bragg gratings (FBGs) using a KrF excimer laser with a phase mask has been analyzed experimentally for typical fabrication conditions. The fluence of UV laser light at the fiber surface has been varied up to 420 mJ/cm 2. These experiments show that (1) scanning electron microscope (SEM) images of the fiber surface facing the excimer laser beam reveal partial physical damage apparently owing to partial melting, so that the surface temperature has increased beyond the softening point of silica glass, which is approximately 1200 C, (2) the optical spectrum transmitted through the FBG during laser irradiation contains spikes coincident in time with the laser pulses that correspond to a near instantaneous shift of the FBG spectrum to a higher temperature spectrum representing fiber core heating of approximately 8 C, and (3) analysis of the energy absorbed by the fiber indicates a bulk temperature rise of approximately 3 C in the fiber. The resulting large variation in the temperature rise over the fiber cross section from a few C to 1200 C, along with partial physical damage on the surface, will certainly induce large internal stresses in the fiber material and reduce the mechanical strength of the FBG.

The shift in the central wavelength of fiber Bragg gratings (FBGs) during annealing of hydrogen-loaded optical fiber exposed to ultraviolet (UV) laser irradiation through a phase mask is studied and shown to be caused by thermal diffusion of hydrogen out of the fiber for both short-period and long-period FBGs. Reloading FBGs with hydrogen followed by a second annealing cycle without exposing the fiber to the UV laser reproduces the details of the wavelength evolution observed during first annealing following UV irradiation. This shows that the wavelength shift of the grating during annealing is determined by diffusion of hydrogen gas out of the optical fiber for both short-period and long-period FBGs.

A fiber Bragg grating (FBG) is normally made with a length of 10 to 30 mm. Depending on the application, however, a short length may be desired, especially in sensor applications for local detection. Under the most common fabrication conditions using the phase mask method, we have fabricated FBGs with lengths ranging from 0.35 to 15 mm using different excimer laser irradiation times. Although 0.35-mm-long FBGs can be made with laser exposure times exceeding 1 h, the practical minimum FBG length appears to be around 3 mm with exposure times in the range of 3 to 5 min.

A very effective apodization, in which side lobes are suppressed to below a particular noise level, is found to operate automatically in a two-beam interference device for fabricating fiber Bragg gratings when a pair of mirrors deflecting two beams are off parallel to each other. This is considered to be a consequence of the short temporal coherence length of the KrF excimer laser used.