Gain flattened and improved EDFA using microbending long-period fibre gratings

A new method for flattening the gain spectrum of erbium-doped fibre amplifiers (EDFAs) is proposed. This method is based on microbending long-period fibre gratings, that are simply fabricated by bending the fibre. By adjusting the spectral profiles of the filters mechanically, a gain flatness of <1 dB has been obtained, and gain improvement of <5.32 dB over the 33 nm wavelength range at two different pump powers of 34 and 86 mW, respectively.     

Generation of high-repetition-rate optical pulse using cascaded long-period fibre gratings

The generation of high-repetition-rate optical pulses using cascaded long-period fibre gratings is proposed. The pulse multiplication is realised by exploiting the difference in the propagation constant of the core and a cladding mode in a standard singlemode fibre. Two, four and eight times pulse multiplication have been successfully implemented with a minimum repetition period of 2.4 ps.     

Electric-arc-induced long-period gratings in fluid-filled photonic bandgap fibre

Electric-arc-induced long-period gratings in a fluid-filled photonic bandgap fibre (PBGF) is demonstrated. Gratings of varying pitch were fabricated and excellent agreement between the measured resonances and numerically simulated phase matching curves is obtained. The PBGF geometry allows the detection of very small arc-induced structural deformations of the fibre and their correlation with the grating strength.     

Gain-flattening filter using long-period fiber gratings

We propose a novel structure for a gain-flattening filter, in which a conventional long-period fiber grating (LPFG) and a phase-shifted LPFG are written closely. The transmission characteristics are investigated in the case of a closely arranged configuration. For the fabrication of a phase-shifted LPFG, a UV-trimming method to control the amount of the phase shift is examined theoretically and experimentally. Finally, a 67-mm-long LPFG gain-flattening filter designed for an erbium-doped fiber amplifier is fabricated and the measured spectrum is shown     

Nonlinear pulse switching using long-period fiber gratings

We show that the intensity required to achieve all-optical switching in long-period fiber gratings can be reduced by orders of magnitude through the use of uniform phase-shifting regions between gratings. Predicted switching intensities are of the order 100 MW/cm² compared to 10 GW/cm² for normal device configuration     

Simultaneous measurement of temperature and strain based on arc-induced long-period fibre gratings

A study on arc-induced long-period fibre gratings (LPFGs) revealed that their strain sensitivity depends on the electric current of the arc discharge. Based on that property, a sensor scheme comprising two concatenated LPFGs was implemented for discrimination of temperature and strain effects. This sensor presented resolutions of /spl plusmn/0.1/spl deg/C//spl radic/Hz and /spl plusmn/35 /spl mu//spl epsiv///spl radic/Hz, respectively.     

Annealing time dependence at very high temperature of electricarc-induced long-period fibre gratings

Long-period fibre gratings fabricated with an electric arc have been annealed at high temperatures. The time dependence of transmitted spectra are reported and related to the evolution of internal stresses in the fibre     

Axial rotation dependence of resonances in curvedCO2-laser-induced long-period fibre gratings

The magnitude and wavelength of transmission resonances in curved CO₂-laser-induced long-period fibre gratings (LPFGs) are shown to exhibit a strong dependence on the axial rotational orientation of the LPFGs     

Bragg gratings in multimode and few-mode optical fibers

Bragg gratings in optical fibers in multimode propagation are investigated experimentally and theoretically. Bragg gratings formed in optical fibers in multimode propagation show multiple reflection peaks or multiple transmission dips in the reflection or transmission spectra, respectively. For standard graded-index multimode fiber, the number of reflection peaks of a Bragg grating depends on excitation condition of propagating modes. The number of reflection peaks of a Bragg grating at around 1.55 μm is 19 for highly multimode excitation and 3-4 for lower order mode excitation. We analyze the phase-matching conditions of the propagating modes and identify half of the reflection peaks as the reflection to the same mode and the rest as the reflection to the neighboring modes. In dispersion-shifted fiber, a Bragg grating at around 0.8 μm in three-mode propagation shows three reflection peaks in the reflection spectrum. The temperature dependence of each reflection peak is similar to that of a conventional Bragg grating in single-mode fiber. Polarization dependence measured on a Bragg grating in multimode graded-index fiber is negligible. An advantage of Bragg gratings in multimode fiber (MMF) and the applications are discussed     

Optical fibre for fabricating long-period gratings with low temperature sensitivity within selected temperature range

Reported is a photosensitive fibre for making long-period gratings with temperature sensitivity of the resonance wavelength as low as 0.25 nm over 0-70/spl deg/C. Furthermore, a convenient method for selecting the temperature of minimum sensitivity, by controlling the boron concentration in the core and monitoring it with attenuation at 1550 nm, is presented.     

Bend measurement using Bragg gratings in multicore fibre

The first measurements of curvature made using Bragg gratings written in separate cores of a multicore optical fibre are described. The gratings act as independent, but isothermal, strain gauges. The difference in Bragg wavelength between the gratings provides a temperature-independent measurement of the local curvature     

Polymer waveguide notch filter using two stacked thermooptic long-period gratings

A polymer waveguide notch filter using two stacked thermooptic long-period gratings is proposed. It is a long-period waveguide grating whose transmission spectrum can be dynamically controlled. Thermooptic index perturbation, i.e., grating is temporarily induced by two groups of periodic heaters stacked vertically on the filter surface. The periods of the two groups are determined such that two different resonance bands are generated in the spectrum by the respective groups. Thus, the overall spectrum may be actively shaped by controlling the individual power applied to the two groups. Its feasibility is demonstrated experimentally by fabricating it with thermocurable polymers.     

Fabrication of chirped fibre gratings using etched tapers

A technique for fabricating fibre gratings with an accurately controlled chirp profile is demonstrated. The technique involves writing a grating over a section of fibre, the cladding of which has been tapered by etching it in HF acid. A strain gradient along the taper is used to produce the chirp either during or after writing. Linearly chirped gratings with bandwidths up to 4.8 nm have been demonstrated. Transmission filters have also been demonstrated when a step is made in the taper     

Cladding-mode extraction from long-period fibre grating using self-aligned core-mode blocker

A novel method is presented of fabricating a core-mode blocker for use in long-period fibre gratings by forming a self-aligned absorber on the end of the fibre core. The absorber efficiently suppresses the core-mode power and allows the cladding-mode radiation to pass through, thereby resulting in a bandpass output at the resonant wavelengths     

High birefringence fibre Bragg gratings written in tapered photonic crystal fibre with femtosecond IR radiation

High birefringence Bragg gratings were written with ultrafast 800 nm radiation in non-birefringent pure silica core photonic crystal fibres (PCF) that were tapered with an oxy-hydrogen flame. The grating spectra indicate that a birefringence above 10^-3 was created in the tapered PCF     

High-temperature sensitivity of long-period gratings in B-Gecodoped fiber

We report an investigation of thermal properties of long-period fiber gratings (LPFGs) of various periods fabricated in the conventional B-Ge codoped fiber. It has been found that the temperature sensitivity of the LPFGs produced in the B-Ge fiber can be significantly enhanced as compared with the standard telecom fiber. A total of 27.5-nm spectral shift was achieved from only 10°C change in temperature for an LPFG with 240-μm period, demonstrating a first ever reported high sensitivity of 2.75 nm/°C. Such an LPFG may lead to high-efficiency and low-cost thermal/electrical tunable loss filters or sensors with extremely high-temperature resolution. The nonlinear thermal response of the supersensitive LPG was also reported and first explained     

Corrugated long-period fiber gratings as strain, torsion, andbending sensors

We present a novel corrugated long-period fiber grating whose transmission spectra are highly sensitive to the applied tensile strain, torsion, and bending due to the periodical index modulation created and changed by these mechanic forces. The induced index modulation can also be experimentally characterized by using a built-in fiber Bragg grating (FBG). The long period fiber gratings possess the following unique properties when used as sensors. As a tensile strain sensor, its resonance loss varies but resonance wavelength remains stable. As a torsion sensor, the wavelength varies with the applied twist rate. As a bending sensor, the cladding-mode resonance grows with the bending curvature     

Reflectively tapped fibre optic transversal filter using in-fibreBragg gratings

A novel optical transversal delay line signal processor based on the use of in-fibre Bragg grating reflectors as tapping elements is presented. The requirements on reflectivity mismatch are analysed, and this is accurately controlled using postprocessing techniques. Results on frequency discriminator filters using reflectively tapped optical transversal filter structures demonstrate uniform and level responses >20 GHz with notch depths >44 dB