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Transmission filters for the terahertz domain having a shaped bandpass have been modeled and demonstrated. The filter designs were based on the desired filter type and bandwidth, and implemented by cascading quarter wave phase shifted fiber Bragg gratings written in Topas polymer subwavelength fiber. As an example, a 5-pole Chebyshev filter with

The dispersion of picosecond pulses on reflection from efficient photorefractive fiber gratings is explored experimentally. Unlike simple measurements of reflectivity, this approach allows both the amplitude and the phase of the grating response to he probed as a function of frequency.

We report what is to our knowledge the first fiber distributed-feedback laser using a single Bragg grating at 1.5 microm written directly into a 2-cm-long Er(3+)-doped fiber codoped with Yb(3+). We obtained robust single-frequency operation by either using one end reflector or locally heating the center of the grating to create the necessary phase shift.

We report a novel method to increase the UV photosensitivity of GeO2:SiO2 optical fibers based on exposure to CO2 laser irradiation before grating writing. Fibers treated with a CO2 laser can produce gratings with refractive-index modulation two times greater and a Bragg wavelength that can be 2 nm longer than those of untreated fibers. Experiments on GeO2:SiO2 preform samples treated with a CO2 laser in a way similar to the fibers showed a marked increase of the 242 nm absorption band, which is associated with an increase of germanium oxygen-deficient centers and is believed to be responsible for the higher photorefractive response.

Bragg gratings have been written in an optical fibre with a core made of ternary SiO2:SnO2:Na2O glass and a SiO2 cladding. The presence of Na2O allows for higher concentrations of SnO2 which is believed to be responsible for the photorefractive response of this composition. In these preliminary experiments significant refractive index modulations, up to 6.2x10-4, have been achieved using a 248nm excimer laser and a phase mask to write gratings for reflectivity at around 1.5µm. The induced refractive index changes show enhanced temperature stability and there is no sign of any erasure up to temperatures exceeding 600°C.

We report on an apodization scheme for terahertz fiber Bragg gratings. The grating consists of only 90 ablated notches on two opposite sides of a subwavelength polymer fiber. The grating strength can be effectively tuned by controlling the longitudinal shift between the two-sided notches, and apodization can be achieved by applying an envelope profile. Side lobe suppression of 14 dB was experimentally observed when compared with an unapodized grating.

A modulator is reported in which an extensional acoustic wave is launched along a fiber Bragg grating. The acousto-optic superlattice effect causes an enhancement in reflectivity within a narrow spectral region on both sides of the Bragg wavelength. For a fixed acoustic propagation direction, the Doppler shift can be either positive or negative, depending on whether the wavelength of the incident light lies above or below the Bragg condition. The device can function as a Bragg cell and a tunable filter.

Boron- and germanium-doped highly photosensitive cladding is used in a novel design to achieve photosensitive Er/Yb-doped fibers, permitting short, strong gratings (length approximately 1 cm, reflectivity99%) to be written without hydrogenation. The high absorption at 980 nm in Er/Yb fibers permits efficient pump absorption over a short device length, which is ideal for achieving highly efficient single-frequency fiber lasers. Both single-frequency Bragg-grating reflector and distributed-feedback lasers with slope efficiencies of 25% with respect to launched pump power have been realized in such fibers.

We have demonstrated fast formation (~1500 pulses at ~1 J/cm2/pulse) of fibre gratings with high negative index modulations (~-3×10-4). These gratings were found to be far more stable than the gratings with positive index modulations formed at the early stage of the grating growth. We have also found that the maximum negative index modulations achieved do not depend on the pulse intensities, although the inverse of the time taken to reach the negative index modulation maximum varies linearly with the pulse intensities. This prompts us to use a three energy level system to model the photosensitivity in the boron-doped germanosilicate fibre. All the necessary parameters of the model can be determined from a single growth measurement of the average index change and the model's prediction fits well the measured index modulation growth.

A new approach for the interrogation of a large number of fiber-optic grating sensors is proposed and demonstrated for a small number of sensors in which signal recovery is achieved by matching a receiving grating to its corresponding sensor. This technique is demonstrated for both quasi-static and periodic measurands, and the resolution achieved for a single sensor–receiving grating pair for quasi-static strain is 4.12 μ∊.

We report the fabrication and operation of an Er3+-doped silica single-mode fiber laser operating at 1.55 µm and using a distributed fiber grating to provide feedback. The output bandwidth was measured as 0.04 nm, significantly narrower than the conventional cavity design. The laser had a threshold of 13 mW and a slope efficiency of 5%.

Significant improvements in the operation of Er(3+)-doped silica fiber lasers operating at wavelengths between 1.5 and 1.6 microm are reported. The use of 980 nm as the pump wavelength provides an output that is limited mainly by the quantum efficiency of the lasing process. It is thus considerably more efficient than previous results using ~810-nm pumping, where excited-state absorption degrades the lasing performance. Operation at three discrete output wavelengths is observed and is accounted for by studying gain across the lasing bandwidth.