Raoul Stubbe

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.

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.

Position weighting of fiber Bragg gratings for bandpass filtering

A fiber Bragg grating bandpass filter with tailored refractive-index modulation is investigated. The writing method includes a position weighting technique, which provides flexible and accurate control of the index modulation along the fiber. This weighting is accomplished by adjusting the positions of a large number of overlapping subgratings to control the local grating strength. A 10-cm grating with a truncated sin(x)/x refractive-index modulation function was written and characterized with this method, showing good agreement with simulations. These types of filter are well suited for wavelength-division-multiplexing applications and can be made to have steep edges, low sidelobes, large bandwidth, and low stopband ripple.

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.

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.

Wavelength stabilization of DBR laser using a fiber with multiple gratings

Wavelength stabilization at multiple wavelengths of a DBR laser using fiber gratings has been achieved. The wavelength is stabilized to approximately 0.1 nm even though the laser temperature is varied 10/spl deg/C. We demonstrate the use of multiple UV-written gratings in a single fiber to control the wavelength stablity of different modes of a three-section DBR laser. The work demonstrates a principle which can be extended to control widely tunable four-section lasers.

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.

Distributed strain sensing using long intracore fiber Bragg gratings

We report on two possible techniques of short distributed strain measurements along a length of a single fiber Bragg gratings (FBG). The first technique relies on the spectral shape, the second is based on the group delay response of the FBG. A theoretical comparison of the two techniques were made and later applied to determine the strain distribution around a hole in a loaded aluminum plate. Monotonically varying strain measurements were made with a spatial resolution that was dependent on the strain gradient at that point. A spatial resolution as small as approximately 0.8 mm was obtained over the steep parts of the strain variation.

A pseudo-heterodyne fiber optical gyro, using integrated optics

A hybrid fiber-optical gyro using a novel scheme for pseudo-heterodyne detection is presented. The large bandwidth of integrated optical phase modulators allows for the use of a triangular-wave for dynamical biasing of the interferometer, while still being able to choose the modulation frequency to be the eigenfrequency of the fiber-loop. It is shown that this, together with the use of a dual-channel approach, minimizes both bias and scale factor errors. It is proposed that the fourth harmonic can be used to keep the modulation index of the applied triangular-wave phase modulation at the correct height and thus guarantee a good scale factor linearity. >