All-optical flip-flop based on nonlinear effects in fiber Bragg gratings

We demonstrate a novel design for a passive all-optical R-S flip-flop with separate set-rest ports based on nonlinearity in the fiber Bragg gratings (FBGs). The spectrum of FBGs in the presence of the Kerr effect is investigated, and cw and transient characteristics of nonlinear-induced bistability of the passive distributed feedback structures are studied by numerical solving of coupled-mode equations. It is shown that a pulse with a pulsewidth of 0.15 ns can switch the state of the flip-flop.     

Second-harmonic generation in second-harmonic fiber Bragg gratings

We consider the production of second-harmonic light in gratings resonant with the generated field, through a Green's function approach. We recover some standard results and obtain new limits for the uniform grating case. With the extension to nonuniform gratings, we find the Green's function for the second harmonic in a grating with an arbitrary phase shift at some point. We then obtain closed form approximate expressions for the generated light for phase shifts close to π/2 and at the center of the grating. Finally, comparing the uniform and phase-shifted gratings with homogeneous materials, we discuss the enhancement in generated light and the bandwidth over which it occurs, and the consequences for second-harmonic generation in optical fiber Bragg gratings.     

Acousto-optic superlattice modulation in fiber Bragg gratings

The superposition of a long-period grating and a fiber Bragg grating, which we call an optical superlattice, causes high-efficiency narrow-band reflections to be induced on either side of the Bragg wavelength. This effect was recently observed experimentally in a fiber-based acousto-optic superlattice modulator. We develop in detail the theory of optical superlattices in fiber Bragg gratings, treating both the acousto-optic and the fixed-grating cases. Applications include reconfigurable wavelength division multiplexers, fiber lasers and sensors, tunable filters, modulators, and frequency shifters.     

Spectra of dual-overwritten chirped fiber Bragg gratings

The coupled-mode equations for dual-overwritten chirped fiber Bragg gratings (CFBGs) are obtained and numerically calculated. By simulating the reflection spectra of dual-overwritten CFBGs, we have investigated the effects of chirp coefficient, period difference, grating length and index modulating coefficient, on the reflection spectrum and its intensity and full-width-at-half-maximum (FWHM).     

Surface-relief fiber Bragg gratings for sensing applications

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface-relief FBGs are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. We describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors.     

Fabrication of self-apodized short-length fiber Bragg gratings

A new technique for writing extremely short-length Bragg gratings in optical fibers is demonstrated. A physical model describes the diffraction effects on the spatial and wavelength spectra of the Bragg gratings. Selection of appropriate diffraction patterns and related parameters permits self-apodized Bragg gratings with a typical spatial length of several hundred micrometers and a bandwidth of several nanometers to be obtained. These gratings with well-defined spectra are suitable for use as miniature distributed strain sensors and other applications requiring small physical dimensions and broadband spectra.     

Thinned fiber Bragg gratings as refractive index sensors

In this work, highly sensitive refractive index measurements have been experimentally demonstrated by using thinned fiber Bragg grating (FBG) sensors. When the cladding diameter is reduced, significant changes in the effective refractive index occur due to surrounding medium refractive index modifications, leading to Bragg wavelength shifts. Uniformly thinned FBGs have been obtained by using wet chemical etching in hydrofluoric acid solutions. In order to prove sensor sensitivity, experimental tests have been carried out by using glycerine solutions with well-known refractive indices. Obtained results agree well with the numerical analysis carried out by using the three-layer fiber model. If the cladding layer is completely removed, resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for the outer refractive index around 1.450 and 1.333, respectively, are possible. Finally, a novel approach based on the selective etching along the grating region has been analyzed, leading to high-sensitivity refractive index sensors based on intensity measurements.     

Analysis of chirped-sampled and sampled-chirped fiber Bragg gratings

We analyze sampled fiber Bragg gratings that have chirp in the grating period, in the sampling function, or in both. In the last-named case the sampling period can be chirped, the sample length can be chirped, or both. We explain the features in the spectral and group-delay (dispersion) responses for gratings that have a single chirp parameter. We then show how the response of sampled gratings that have multiple chirp parameters can be explained in terms of the single-chirp parameter cases.     

Analysis of nonuniformities of sampled fiber Bragg gratings

We have analyzed the effects of various normally distributed nonuniformities of sampled fiber Bragg gratings on the reflective spectra and group time delay. Through numerical simulations we have drawn the following conclusions: (1) the magnitude of nonuniformity with normal distribution of the fiber's average refractive-index modulation deltaneff greatly influences the characteristics of both reflective spectra and group time delay, whose suggested precision varies from 20% to 10%; (2) the nonuniformities of sampling periods P and sampling lengths L are important factors that influence the characteristics of the group time delay, and the accepted tolerance of dimensional precision of both P and L are +/- 4 microm; and (3) the tolerance of nonuniformity of the sampling period's chirp coefficient is high, and its precision can be as great as 100% with few adverse effects.     

High-strength fiber Bragg gratings for a temperature-sensing array

We have successfully demonstrated a one-step laser process of fabricating fiber Bragg grating arrays directly through fiber buffer. A new polysiloxane-based buffer provides high 244-nm transmission and showed no degradation due to UV irradiation or thermal annealing as verified by Weibull analysis of tension tests. The FBG array can withstand over 400-kpsi tension tests and 220/spl deg/C temperature. The spectral quality of the FBG is compatible with the FBGs currently used by the sensing industry and, therefore, can be interrogated with existing instruments. The laser fabrication process is robust with no coating, stripping, or recoating required.     

Long-term stable device for tuning fiber Bragg gratings

It is demonstrated that with a proper choice of embedding material, the composite beam bending method constitutes an effective and reliable approach for tuning fiber Bragg gratings. A long-term stable device is presented with a dynamic range of 80 nm, which exhibits insertion losses smaller than 0.28 dB and small variations of the full width at half-maximum.     

Efficient Bragg gratings in phosphosilicate and germanosilicate photonic crystal fiber

We present ArF laser-induced dynamics of Bragg grating (BG) growths in phosphosilicate-doped or germanosilicate-doped core photonic crystal fibers (PCFs). To this end, we have adapted the technique of H2 loading, usually used in conventional fiber, to the case of microstructured fiber, allowing both the concentration of hydrogen in the PCFs to be kept nearly constant for the time of the exposure and the BG spectra to be easily recorded. We compared the characteristics of BG growths in the two types of PCF to those in conventional step-index fibers. We then conducted a study of the thermal stability of the BGs in PCFs through 30 min of isochronal annealing. At the same time we discuss the role played by the microstructuration and the doping with regard to the grating contrast and the Bragg wavelength stability.     

Response of fiber Bragg gratings to longitudinal ultrasonic waves

In the last years, fiber optic sensors have been widely exploited for several sensing applications, including static and dynamic strain measurements up to acoustic detection. Among these, fiber Bragg grating sensors have been indicated as the ideal candidate for practical structural health monitoring in light of their unique advantages over conventional sensing devices. Although this class of sensors has been successfully tested for static and low-frequency measurements, the identification of sensor performances for high-frequency detection, including acoustic emission and ultrasonic investigations, is required. To this aim, the analysis of feasibilty on the use of fiber Bragg grating sensors as ultrasonic detectors has been carried out. In particular, the response of fiber Bragg gratings subjected to the longitudinal ultrasonic (US) field has been theoretically and numerically investigated. Ultrasonic field interaction has been modeled, taking into account the direct deformation of the grating pitch combined with changes in local refractive index due to the elasto-optic effect. Numerical results, obtained for both uniform and Gaussian-apodized fiber Bragg gratings, show that the grating spectrum is strongly influenced by the US field in terms of shape and central wavelength. In particular, a key parameter affecting the grating response is the ratio between the US wavelength and the grating length. Normal operation characterized by changes in wavelength of undistorted Bragg peak is possible only for US wavelengths longer than the grating length. For US wavelengths approaching the grating length, the wavelength change is accompanied by subpeaks formation and main peak amplitude modulation. This effect can be attributed to the nonuniformity of the US perturbation along the grating length. At very high US frequencies, the grating is not sensitive any longer. The results of this analysis provide useful tools for the design of grating-based ultrasound sensors for meeting specific requirements in terms of field intensity and frequencies.     

Inverse scattering algorithm for reconstructing lossy fiber Bragg gratings

We demonstrate an inverse scattering algorithm for reconstructing the structure of lossy fiber Bragg gratings. The algorithm enables us to extract the profiles of the refractive index and the loss coefficient along the grating from the grating transmission spectrum and from the reflection spectra, measured from both sides of the grating. Such an algorithm can be used to develop novel distributed evanescent-wave fiber Bragg sensors that measure the change in both the refractive index and the attenuation coefficient of the medium surrounding the grating. The algorithm can also be used to analyze and to design fiber Bragg gratings written in fiber amplifiers. A novel method to overcome instability problems in extracting the parameters of the lossy grating is introduced. The new method also makes it possible to reduce the spectral resolution needed to accurately extract the grating parameters.     

Optical fiber Bragg gratings. Part I. Modeling of infinitely long gratings

We present an accurate numerical method based on the Floquet-Bloch formalism to analyze the propagation properties and the radiation loss in infinitely long uniform fiber Bragg gratings. The model allows us to find all the propagation characteristics including the propagation constants, the space harmonics and the total field distribution, the guided and radiated power, and the modal loss induced by the periodic structure. The influence of the geometrical and physical parameters on the performance of the Bragg gratings has been established. A clear explanation of the physical phenomena related to the index modulation amplitude changes is presented, including the photonic bandgap effect, which is not easily described by the finite-difference time-domain method and cannot be described by the widely used coupled-mode theory.     

Temperature-induced reversible changes in the spectral characteristics of fiber Bragg gratings

It is reported that reversible changes in the reflectivity of Bragg gratings can be induced by a change in the temperature of the grating (77 K < T < K). The changes have proved to be greater in highly doped Ge fibers than in standard fibers, whereas they could hardly be detected in hydrogenated fibers. The sign of the change for type I gratings was opposite that for type IIA gratings. The changes are likely due to a temperature-induced increase (or a decrease) in the amplitude of the refractive-index modulation. Possible mechanisms for these changes in modulation are discussed. Interestingly for the purpose of correcting data of isothermal accelerated aging experiments, a numerical relation that accounts for the temperature-induced changes in type I grating reflectivity is given.     

Dynamic two-axis curvature measurement using multicore fiber Bragg gratings interrogated by arrayed waveguide gratings

We describe the use of arrayed waveguide gratings (AWGs) in the interrogation of fiber Bragg gratings (FBGs) for dynamic strain measurement. The ratiometric AWG output was calibrated in a static deflection experiment over a +/-200 microepsilon range. Dynamic strain measurement was demonstrated with a FBG in a conventional single-mode fiber mounted on the surface of a vibrating cantilever and on a piezoelectric actuator, giving a resolution of 0.5 microepsilon at 2.4 kHz. We present results of this technique extended to measure the dynamic differential strain between two FBG pairs within a multicore fiber. An arbitrary cantilever oscillation of the multicore fiber was determined from curvature measurements in two orthogonal axes at 1125 Hz with a resolution of 0.05 m(-1).     

Characterization of apodized fiber Bragg gratings for rejection filter applications

Apodized fiber Bragg gratings with >/=30-dB sidelobe suppression +/-1 nm from the Bragg center wavelength are spatially characterized with side-scatter and side-diffraction techniques. Resonant side scatter is suppressed, indicating that little dispersion arises from reflections off the grating boundaries.     

倾斜切趾光纤光栅的理论与实验研究

针对飞秒激光刻写均匀光纤布拉格光栅的反射主峰存在较高旁瓣,容易对主峰形成串扰的问题,开展基于倾斜切趾光纤光栅的理论与实验研究。根据飞秒激光诱导产生的折射率特点,修正耦合模模型中的交流耦合系数,并利用有限元仿真的形式,验证了倾斜切趾法的切趾类型为高斯型。之后对影响切趾效果的关键参数进行实验研究,结果表明：实验结果与仿真计算基本一致;初始横向位移为5~10 μm,且不对称偏移量小于±2 μm时,边模抑制比可提高到15 dB。研究成果为提高光纤光栅边模抑制比提供了重要理论支撑与实践指导,对于推动光纤光栅制备工艺技术发展具有重要意义。