טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentShapira Yuval
SubjectExperimental and Theoretical Study of Nonlinear Pulse
Propagation in Fiber Bragg Gratings Written in a
Fiber Amplifier
DepartmentDepartment of Electrical Engineering
Supervisor Professor Moshe Horowitz
Full Thesis textFull thesis text - English Version


Abstract

Fiber Bragg gratings (FBGs) are quasi-periodic structures of refractive index along an optical fiber. Although most current applications of FBGs are based on their linear properties, the nonlinear properties of FBGs have a very promising potential to generate and manipulate optical pulses in fibers at power levels when the.


In this work we study, both theoretically and experimentally, nonlinear propagation and amplification of optical pulses in fiber Bragg gratings written in fiber amplifiers (FBG-FA). We develop the underlying mathematical model and find both numerically and analytically new types of solitary waves and similaritons in FBG-FA. The newly discovered solitary waves can propagate very long distance along the FBG-FA without any significant change in their parameters by balancing the net gain experienced by their front  Due to high dispersion in such structure, which can be five orders of magnitude higher that in a fiber amplifier, FBG-FA allows to obtain similaritons of width on the order of nanosecond. Group velocity in FBG-FA can be significantly lower than group velocity in a fiber amplifier. As the group velocity is reduced the effective gain coefficient is enhanced. Therefore, high gain can be obtained over a short amplifier length. During the evolution of the input pulse towards FBG-FA similariton pulse break-up is avoided. We address the problem of lasing in long FBG-FA and propose a practical solution based on apodization of the grating.


Our specially built experimental setup allowed us to fabricate non-uniform FBG-FAs written in Ytterbium-doped fiber amplifier (YDFA). Using the obtained gratings, we experimentally studied nonlinear nanosecond pulses evolution and amplification as they propagate along the FBG-FA. The grating dispersion could be tailored by tuning the grating spectrum with respect to the input pulses wavelength. The experimental results are in good agreement with the theory. In previous works it was shown that Raman and four wave mixing (FWM) effects limit the amplification of pulses with a duration of about 400 ps. We show that the high normal GVD of the grating helps reducing the deterioration of such pulses due to those nonlinear effects and demonstrate experimentally associated enhancement of the obtainable amplified pulse energy by a factor of two. Although the obtained grating is too short for formation of the parabolic pulse, we believe that the obtained experimental results show the beginning of evolution of the input pulse towards FBG-FA similariton.


Using a similar setup, we were able to measure refractive index change (RIC) associated with the pumping of YDFA, and to separate between its thermal and optical contributions. The dependence of the RIC on the pumping is important for distributed feedback fiber lasers, fiber laser sensors, and

FBG-FA amplifiers.


The results of this work pave new way for nonlinear generation, amplification, and shaping of optical pulses in fiber amplifiers.