Bengt Sahlgren

A writing technique for long fiber Bragg gratings with complex reflectivity profiles

We have developed and demonstrated a novel technique to manufacture very long gratings with a nearly arbitrary reflectivity profile. The technique is based upon writing a set of consecutive subgratings with interferometric control of the relative position of each subgrating.

Fabrication of advanced fiber Bragg gratings by use of sequential writing with a continuous-wave ultraviolet laser source

We present a novel scheme based on sequential writing for fabrication of advanced fiber Bragg gratings. As opposed to earlier sequential methods this technique uses a cw UV laser source and allows for very precise control and repetitivity of the formation of the gratings. Furthermore it is possible to use high average irradiances without destroying the fiber, resulting in considerable reduction in fabrication time for complex gratings. The method has been applied to several test gratings, which proved its versatility and quality.

Characterization of fiber Bragg gratings by use of optical coherence-domain reflectometry

A method based on optical low coherence reflectometry for complete characterization of fiber Bragg gratings (FBGs) is presented. It is shown that the measured signal corresponds to the impulse response of the grating filter, and the measurement therefore yields all information about the device. Experiments have been carried out with a novel dual channel interferometer. The results are in excellent agreement with the theory, demonstrating the versatility of the method for characterization of fiber gratings.

Characteristics of fiber grating sensors and their relation to manufacturing techniques

This work examines the effects of different manufacturing techniques of fiber optic intracore Bragg gratings on their sensing and system parameters. Specifically, we have looked at tolerance with respect to center wavelength, strain, and temperature sensitivity. Our test results indicate a slight variation in the strain and temperature sensitivity of different gratings. In an attempt to quantify the effect of transverse loading on fiber gratings, three differently manufactured sensors were diametrically loaded and their spectrums studied. Our experimental results indicate a splitting of the grating spectrum with small loads for an optical fiber grating manufactured in either boron doped fiber or hydrogen loaded fibers. On the other hand, very small transverse sensitivity was observed for a fiber optic grating formed in a bend insensitive fiber suggesting some degree of control on the transverse sensitivity of these sensors.

High-reflectivity fiber-optic bandpass filter designed by use of the iterative solution to the Gel’fand–Levitan–Marchenko equations

A fiber Bragg grating bandpass filter has been investigated. The profile of the 3.1-cm-long grating was synthesized for high reflectivity and low dispersion by use of the iterative solution to the Gelfand-Levitan-Marchenko equations. A grating designed according to this synthesis was written into a boron-codoped germanosilica fiber. The measured spectral response was in good agreement with simulations. The achieved in-band reflection was 97%, and the passband ripple was only 0.06 dB.

Evanescent field refractive index sensor utilizing a narrow fiber Bragg grating and a tunable DBR laser

We have demonstrated an evanescent field refractive index fiber sensor comprising a 42 mm Bragg grating in an etched fiber together with a tunable DBR laser. Characterization of different aqueous sucrose solutions resulted in a resolution of roughly 10 mM sucrose. The sensor in the presented form has a theoretical sensitivity of higher than 10-5 refractive index units (riu) in a refractive index region close to the cladding index of the fiber. However, the technique allows for an even higher sensitivity than 10-6 riu with a proper signal processing scheme.

Phase distribution measurement of fibre Bragg gratings with white light interferometry

A white light interferometer, interrogating fibre Bragg gratings scanning simultaneously at two positions, has been developed to measure the phase and the amplitude of the refractive index modulation throughout the grating.

Phase-based Bragg intra-grating sensing of strain gradients

Light reflected from chirped fiber Bragg gratings possess both phase and amplitude information which is unique. In this paper we report for the first time the use of the relative phase to determine the strain-induced linear chirp. By measuring this intramodal (chromatic) dispersion induced on the incident wave, the wavelength dependent penetration depth and therefore the strain gradient can be found by using a closed-form approximation. Moreover, the error can be reduced by selecting the appropriate grating properties like the writing induced chirp and the index variation parameters.

Experimental system considerations for distributed sensing with grating Fabry-Perot interferometers

In this paper we present an experimental evaluation of a quasi-distributed fiber-optic strain and temperature sensor system based on time division multiplexing. The system relies on an advanced dual-core fiber to which a set of Fabry-Perot interferometers is introduced with fiber Bragg gratings used as weak reflectors.

Fiber gratings for distributed sensors

Techniques for manufacturing of Bragg fiber gratings as well as the sensor system based on in fiber Fabry-Perot interferometers are presented.