Stitch error effect on group-delay ripple for long chirped fiber Bragg gratings

Different sources of stitch errors are identified and simulated in a 200 mm long chirped grating. In each case, the effect on the group-delay ripple and transmission spectrum is investigated. The anticipated response in a typical 10 Gbit/s transmission system is specifically considered. From the simulations, it is clear that information about error source and magnitude can be gained directly from the transmission spectrum.     

Photothermal effects in fiber Bragg gratings

Writing a fiber Bragg grating in optical fiber generates an intrinsic broadband absorption term that can result in photothermal heating during subsequent use with fiber core guided light. This, in turn, can cause a significant shift of a grating resonance via the thermo-optic coefficient, even at low in-fiber light powers. The magnitude of the absorption term and its dependence on the grating strength are detailed. We further show how the degree of heating can be influenced by the particular environment in which the grating is placed and that, while the shift can be quite deleterious for some applications, its effect can be mitigated. A simple conductive model is developed.     

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).     

Nanofibers with Bragg gratings from equidistant holes

We study nanofibers with Bragg gratings from equidistant holes. We calculate analytically and numerically the reflection and transmission coefficients for a single grating and also for a cavity formed by two gratings. We show that the reflection and transmission coefficients of the gratings substantially depend on the number of holes, the hole length, the hole depth, the grating period, and the light wavelength. We find that the reflection and transmission coefficients of the gratings depend on the orientation of the polarization vector of light with respect to the holes. Such a dependence is a result of the fact that the cross-section of the gratings is not cylindrically symmetric.     

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.     

Polarization analysis of surface-relief D-fiber Bragg gratings

Surface-relief fiber Bragg gratings exhibit substantially more polarization dependence than standard fiber Bragg gratings. Using D-fiber with different core orientations, surface-relief gratings are analyzed and fabricated to determine the polarization dependence. We show that the largest Bragg reflection occurs for the polarization state with a dominant TE field component parallel to the flat surface of the fiber. The polarization dependence is adjusted by changing the index of refraction of the surrounding media and by fabricating the surface relief grating using rotated core D-fiber.     

Effect of fabrication errors in channel waveguide Bragg gratings

The spectral performances of nonideal rectangular Bragg gratings, integrated in a rib waveguide, are analyzed by a multilayer approach based on the effective-index method. The effects of errors on the photolithographic definition of the grating, that is, period and shape, and of errors on the control of etching depth are investigated. Also the influence of the stitching error, which is unavoidable when the grating is realized by means of electron-beam photolithography, is addressed. A novel analytical approach that extends coupled-mode theory to the analysis of real gratings is also presented.     

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 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.     

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.     

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.     

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.     

Luus-Jaakola optimization procedure for phase-only sampled-fiber Bragg gratings

We describe a Luus-Jaakola (LJ) optimization algorithm approach for the design of wavelength division multiplexing (WDM) optical communication systems by using phase-only sampled-fiber Bragg gratings. The LJ method is used to optimize the phase grating as well as the number of segments which form the grating. The numerical example of WDM is studied. The method is easily applicable and shows promising results with low refractive index modulation.     

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.     

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.     

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).