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We propose a new pH-sensing scheme using a methylene blue adsorption on an optical fiber cladding surface. Interactions between the silica and hydroxyl ions of a base solution induce the surface of the silica negatively charged. The charged surface attracts the positively charged chromophores of methylene blue. As the pH of the solution is reduced, the electrostatic attraction will also be reduced. This electrostatic attraction can change the transmitted light intensity of the cladding mode, since the boundary condition changes. We also carried out a simulation to verify the effect from external refractive index change around a long-period fiber grating. Our results confirm that the wavelength shift by external refractive index change is negligible compared to the transmitted light intensity variation of the cladding mode. By using a long-period grating pair, we can detect the cladding mode transmittance variations. Experimentally, we showed the possibility of pH sensing in the $1.5{\mu}m$ infrared region.

An electrically controllable fiber Bragg grating inscribed in a hydrogen-loaded standard single-mode fiber with liquid-crystal cladding is presented. Control of the optic axis of liquid crystals by means of external electric fields results in the change of reflectivity and Bragg wavelength of the grating. The increase of surrounding refractive index of a fiber makes effective refractive index of a propagation mode higher, which results in high field confinement and longer Bragg wavelength. The reduction of the fiber diameter by chemical etching process improves the long-range ordering of liquid-crystal molecules and reduces controlling voltage. The tunable ranges of reflectivity and Bragg wavelength of the liquid crystal-cladding fiber Bragg grating were $\~4.6dB\;and\;\~0.3nm$ , respectively.

We propose a new method for tuning the dispersion of a linearly chirped fiber Bragg grating with fixed center wavelength and bandwidth. We polished one side of the grating and attached it to a cantilever metal beam. By contacting an index matching block to the polished face of the grating, we could shorten the effective grating length through which light can propagate in the core of fiber. Decrease of grating length results in reflection bandwidth narrowing. However, by bending the grating attached on the cantilever beam, we could restore the bandwidth. Because the grating is shortened without bandwidth narrowing, the amount of dispersion is decreased. By using this method, we tuned chromatic dispersion of the linearly chirped fiber Bragg grating from −687.24 to − 256.11 ps / nm for 0.75 nm fixed bandwidth without center wavelength shift.

An electrically tunable dispersion slope compensator using a chirped fiber Bragg grating tuned by quadratic temperature distribution is demonstrated. A fan-shaped thin metallic heat channel was used to generate a quadratic temperature gradient along the grating. By controlling the temperature of inner and outer boundaries of the heat channel, we achieved a dispersion slope from -60.82-65.89 ps/nm/sup 2/ for 1.2-nm bandwidth. The dispersion slope compensation in 160-Gb/s return-to-zero transmission is demonstrated by simulation.

A new method to adjust the dispersion of a fiber grating with enhanced flexibility and accuracy is described. The proposed method involves bending a fiber grating attached on a metal beam with multisectional bending scheme. The device is composed of several pairs of bolts which hold the metal beam at designated deflection positions. Because the curvature induced by bending defines the local strain, the strain distribution along the fiber can be tailored by positioning the pairs of bolts according to designated deflection curve. We demonstrated linear and quadratic dispersion tuning by using the proposed method.

Simultaneous measurement of strain and temperature was demonstrated by using a long-period fiber grating inscribed on a polarization-maintaining fiber (PM-LPG). The spectral sensitivities of the two adjacent resonant dips of the PM-LPG were measured with respect to strain and temperature. The sensitivities of each resonant dip for strain and temperature were different in magnitude and sign, which makes this PM-LPG have a capability of simultaneously sensing strain and temperature. Experimental results show root mean square deviations of 15.6 /spl mu//spl epsiv/ and 0.7/spl deg/C for strain and temperature, respectively.

A novel method for dispersion control of a fiber grating is proposed and experimentally demonstrated. A super-imposed fiber Bragg grating (SFBG) is attached on an elastic plate and wound around two columns, which are rotated on a disk. The system induces strain gradient for chirping that can be controlled by the rotation of the disk. The central reflection wavelength can be either maintained or changed by controlling the position of the grating with respect to the two columns. Various group delay curves can be obtained by altering the shape of the plate on which the SFBG is attached. In addition, the proposed device can also be used as a broad-band tunable filter in optical communication systems.

A new method for tuning the dispersion profile of a fiber grating is described. The method involves compressing the fiber grating attached to a structure in which the cross-sectional area varies with its position. Because the cross-sectional area defines the compression rate, the local Bragg reflection wavelength that determines the dispersion profile varies along the position. A structure in which the cross-sectional area can be simply altered by inserting-extracting a part of structure was designed. Therefore, the dispersion order of the fiber grating attached to the structure can be selected. Using the method, a linearly-quadratically chirped fiber grating was made and various group delay slopes depending on the applied stress were obtained.

A new method to induce a strain gradient on a fiber Bragg grating is described. The method can be used to generate arbitrary strain distributions along the fiber. The objective of the method was to apply strain to a fiber grating that is attached on an appropriately tapered elastic plate. Using this method, linearly/nonlinearly chirped fiber gratings were prepared. Various group delay slopes were obtained depending on the plate shape and the applied strain. In addition, the repeatability could be maintained by adopting a feedback control method for the strain adjustment.

A novel intensity-based temperature-independent strain sensor using a chirped grating is demonstrated. The longer reflection wavelength part of the chirped grating is fixed to a glass tube and, hence, the reflected power from the grating decreases linearly with strain and is independent of temperature perturbation.

We propose a simple method to tune dispersion profile of chirped fiber Bragg grating by compressing the grating attached on a tapered structure. The order of dispersion was selectable by inserting/extracting some part of the structure.

A chirped fibre Bragg grating (CFBG) fabrication method, which enables us to tailor a dispersion compensator with an arbitrary group delay profile, has been proposed. This method is very simple to realize because it takes advantage of strain gradient induced by the attached plate rather than the optical fiber itself. The chirp profile can be adjusted arbitrarily by changing the plate shape and the applied strain. Thus, the method is best suitable for high order dispersion compensation that needs nonlinear dispersion property. We demonstrated two types of CFBG's that have linear and hyperbolic group delay profiles. The group delay ripple was less than 20 ps. In the experiment we used acryl plates that have low elasticity, and thus, the material is not eligible for a dynamic control of dispersion compensator. For use as a dynamic dispersion compensator, we should use a high elastic material, which is under study now.

We propose a new technique realizing a tunable multi-channel dispersion equalizer with the super-imposed fiber Bragg grating. Our simple method for linear chirp control is based on the strain gradient technique by sticking the FBG to the cone (or tube) of a changing circular curvature. This method allows us to control chirp and center wavelength of a broadband chirped grating. The dispersion characteristics can be tuned by applying displacement on the cone or changing the curvature of the tube. This method will be appropriate for future dispersion control in optical transmission systems, and can also be used as an optical filter with very large pass band or rejection band.

We propose a temperature-independent intensity-based strain sensor which uses a chirped grating as a sensor head. The longer wavelength part of the grating is fixed to a glass tube, and hence, the reflected power from grating decreases linearly with strain and is independent of temperature perturbation.

Summary form only given. Using a nonlinear phase shift in a signal wave by means of cross-phase modulation (XPM) from a synchronized pump wave, we can detune transmission spectra of long-period fiber gratings (LPFG's). Thus, the transmission loss fringe shifts to longer wavelengths when the effective index for a signal wave increases by a nonlinear phase shift due to XPM from a pump wave. Utilizing a narrow spectral loss band from an LPFG pair, we could reduce the operation intensity to less than 5 GW/cm/sup 2/. We demonstrated for the first time to the best of our knowledge that a signal wave can be gated all-optically by means of a pump wave in LPFG's.

We propose a novel fiber Bragg grating (FBG) sensor demodulator using fiber birefringence. The demodulator is composed of a polarizing beam splitter (PBS), a polarization controller (PC), and a chirped fiber grating (CFG). In the sensor application of this paper, the CFG is used as a broadband reflection mirror. The experimental results showed that the demodulator has good performance for measuring static and dynamic strain.

A fiber Bragg grating (FBG) laser sensor using a semiconductor optical amplifier (SOA) as gain medium was proposed. Experimental results showed good performance of the proposed laser sensor for multi-point sensing.

A novel fiber Bragg grating sensor system using a fiber laser with a semiconductor optical amplifier as a gain medium is proposed. The inhomogeneous broadening property of the semiconductor optical amplifier is used to generate multiple wavelength operation, which was not possible in an erbium-doped fiber laser due to the homogeneous broadening property. Using a semiconductor optical amplifier, the sensor system can involve multiple point sensor heads, which is an eight-point sensor in the initial experiment.

A novel application of an unapodized chirped fiber Bragg grating inscribed on a high-birefringence fiber is reported. We utilized a dispersionless band of the unapodized grating. The characteristics of the dispersionless band are analyzed using simulations and experiments. We confirm that the band originates from broadband reflections at the input edge of the grating. If an unapodized chirped fiber Bragg grating is inscribed on a high-birefringence fiber, the reflection spectra and the group delays are separated depending on the polarization of the light, hence each group delay curve has a dispersionless band at a different wavelength. In this paper, we show the feasibility of polarization-dependent chromatic dispersion compensation with a single chirped fiber Bragg grating operating at the bandwidth where the dispersion is zero for one state of polarization but not zero for the other polarization state.

A novel multiwavelength-switchable laser based on a semiconductor optical amplifier (SOA) within a fibre ring cavity with a sampled high-birefringence fibre grating is proposed. Four channels with 0.8 nm spacing have been generated at room temperature and switched to other channels with 0.4 nm wavelength-shift by adjusting an inserted polariser. The laser can be used in hierarchical cross-connect WDM networks employing an interleaved waveband concept.

We have analyzed the effects of the fiber cladding radius on the characteristics of long-period fiber gratings. By etching the cladding of a common single-mode fiber, we verified the characteristics experimentally. When by etching we reduce the cladding radius of a common single-mode fiber on which a long-period refractive-index modulation has been imposed, the coupling strength of the core and the cladding modes increases. In addition, the difference in the propagation constants (for a fixed wavelength) between the core mode and the cladding modes increases; hence the resonant transmission dip wavelengths shift to longer wavelengths. The proposed method can be useful in making and detuning long-period fiber grating filters.