By transforming the optical fiber span into an ultralong cavity laser, we experimentally demonstrate quasilossless transmission over long (up to 75 km) distances and virtually zero signal power variation over shorter (up to 20 km) spans, opening the way for the practical implementation of integrable nonlinear systems in optical fiber. As a by-product of our technique, the longest ever laser (to the best of our knowledge) has been implemented, with a cavity length of 75 km. A simple theory of the lossless fiber span, in excellent agreement with the observed results, is presented.

We report a near-ideal in-fiber polarizer implemented by use of 45°tilted fiber Bragg grating structures that are UV inscribed in hydrogenated Ge-doped fiber. We demonstrate a polarization-extinction ratio of 33 dB over a 100-nm operation range near 1550 nm. We further show an achievement of 99.5% degree of polarization for unpolarized light with these gratings. We also theoretically investigate tilted grating structures based on the Green's function calculation, therein revealing the unique polarization characteristics, which are in excellent agreement with experimental data. 3 However, the polished fiber is fragile and must be housed in a bulk substrate, thus somewhat neutralizing the advantages of a fiberbased system. In recent years, fiber Bragg grating devices have been extensively exploited as wavelengthdivision multiplexing filters and polarization compensators for telecommunications applications and as strain or temperature sensors for smartstructure applications. With advances in UVinscription technology and demands for its application, a variety of grating structures have been developed. One of these structures is the tilted fiber Bragg grating (TFBG), which exhibits strong polarization-dependent loss (PDL) effects 4 when the tilted angle is large and has been implemented as a PDL equalizer 5 and an in-line polarimeter. 6 We report, for the first time to our knowledge, in-fiber polarizers based on 45°TFBGs, achieving a polarization-extinction ratio higher than 33 dB covering a 100-nm operation range. We also present a theoretical investigation of the characteristics of TFBGs, which provides effective design guidance for achievement of high-performance in-fiber polarizers and polarization splitters.As an alternative to coupled-mode theory, the spectrum of a TFBG may be simulated by the Green's function method (also known as the volume current method). 7 The loss of a core mode by a small section,

For the first time to the authors' knowledge, fiber Bragg gratings (FBGs) with Ͼ80°tilted structures have been fabricated and characterized. Their performance in sensing temperature, strain, and the surrounding medium's refractive index was investigated. In comparison with normal FBGs and long-period gratings (LPGs), Ͼ80°tilted FBGs exhibit significantly higher refractive-index responsivity and lower thermal cross sensitivity. When the grating sensor was used to detect changes in refractive index, a responsivity as high as 340 nm/refractive-index unit near an index of 1.33 was demonstrated, which is three times higher than that of conventional LPGs.

Using an optical biosensor based on a dual-peak long-period fiber grating, we have demonstrated the detection of interactions between biomolecules in real time. Silanization of the grating surface was successfully realized for the covalent immobilization of probe DNA, which was subsequently hybridized with the complementary target DNA sequence. It is interesting to note that the DNA biosensor was reusable after being stripped off the hybridized target DNA from the grating surface, demonstrating a function of multiple usability. © However, some of these demonstrated optical biosensors have limitations for real-time hybridization studies and monitoring hybridization kinetics. Here we implement an optical biosensor based on a dualpeak long-period fiber grating (LPFG) for detecting biomolecular interactions at a silica-liquid interface with the advantages of high sensitivity, real-time monitoring, and reusability. To achieve high-sensitivity detection of the designed biomolecular interaction, the most sensitive LPFG structures should be used. It has been reported that the coupling condition of LPFGs with relatively short periods are close to the dispersion turning points, resulting in conjugate dual-peak cladding modes that are extremely sensitive to external perturbations [9,10]. Several dual-peak LPFGs with a relatively small period ͑ϳ160 m͒ were UV inscribed in H 2 -loaded SMF-28 fibers employing the point-by-point method and a frequency-doubled Ar laser. All the gratings were annealed at 80°C for 48 h to stabilize their optical properties. Figure 1 shows the spectral evolution of a 30 mm long LPFG with a period of 161 m during the UV inscription process. It is clear that with increasing UV exposure the conjugate dual peaks were increasing in strength and moving close to each other as the coupling condition approached the dispersion turning point.The ability of LPFGs to couple light from the fiber core mode to cladding modes allows optically detecting the change in refractive index at the grating surface. This thus provides an optical detection method to monitor biochemical and biomolecular interactions. Figure 2 shows the basic scheme of the functionalization of a LPFG as a DNA-array biosensor. Silanization is a process for modification of the glass surface to adsorb biomolecules. The LPFG surface is first silanized, followed by the activation of cross linkers to facilitate the immobilization of probe DNA and then to be used to monitor in situ the hybridization of targeted DNA. The DNA hybridization process modifies the refractive index of the LPFG surface, thus resulting in its spectral shift. By demodulating the spectral shift, the designed DNA hybridization can be monitored with high sensitivity.All the biochemical experiments were performed in a fume cupboard. To minimize the bend cross sensitivity, the dual-peak LPFG with a 161 m period was placed straight in a V-groove container on a Teflon plate, and all the chemicals and solvents were added and withdrawn from the container by carefully pipetting.Prior ...

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We report on an optical bend sensor based on a Bragg grating inscribed in an eccentric core polymer optical fiber. The device exhibits the strong fiber orientation dependence, the wide bend curvature range of ± 22.7 m-1 and high bend sensitivity of 63 pm/m-1 .

Abstract:We describe a detailed investigation on tilted fiber Bragg grating (TFBG) structures with tilted angles exceeding 45°. In contrast to the backward mode coupling mechanism of Bragg gratings with normal and small-tilting structures, the ex-45°TFBGs facilitate the light coupling to the forward propagating cladding modes. We have theoretically and experimentally examined the mode coupling transition of TFBGs with small, medium and large tilt angles. In particular, we have conducted experiments to investigate the spectra and far field distribution, and temperature, strain and refractive index sensitivities of ex-45° devices. It has been revealed that these ex-45°gratings exhibit ultra-low thermal sensitivity. As in-fiber devices, they may be superior to conventional Bragg and long period gratings when the low thermal cross sensitivity is required.