Ph.D Thesis

Ph.D StudentKeren Shay
SubjectNovel Methods for Interrogation of Fiber Bragg Gratings and
their Applications for Developing New Optical
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Moshe Horowitz


Fiber Bragg gratings are important for various applications in optical communication systems and in optical sensors. The spectral properties of fiber gratings are determined by the spatial profile of the grating structure. In this work we have developed novel experimental and theoretical methods that enable, for the first time, to reconstruct the structure of an almost arbitrary fiber Bragg grating. The new methods were implemented to demonstrate novel distributed fiber Bragg sensors.

Our new measurement technique for interrogation of fiber Bragg gratings is based on low-coherence spectral interferometry. The low-coherence light source is a unique broadband fiber laser that operates in the noiselike mode of operation. By analyzing the spectral interference, generated by a reflection from a grating and a reflection from a mirror, we obtain the complex impulse response function of the grating, with high spatial resolution, on the order of tens microns. Unlike other methods our technique does not requires a slow mechanical scan in order to interrogate the whole grating.

We have shown that by using the Gabor transform we could directly detect nonuniform regions inside a grating and could measure the spatial dependence of the resonance wavelength along the grating. We have demonstrated, for the first time, the reconstruction of a highly reflecting fiber Bragg grating structure by solving the Gel'fand-Levitan-Marchenko inverse scattering method.

Based on our capabilities to measure the structure of an almost arbitrary fiber Bragg grating we have developed new applications. A new distributed fiber Bragg sensor for measuring the temperature profile in high-power optical fiber components and a new distributed refractometer for biochemical sensing were demonstrated. We have also developed a new method to reconstruct 2-D data stored in an optical fiber.