Functional Filtering Characteristics Of Fiber Gratings And The Applications In Optical Pulse Grneration

As one classic of passive optical devices, fiber Bragg gratings are widely used in many practical applications, especially in fiber communications and optical sensing domain for their characteristics of compact size, compatibility to fiber and low transmission loss, etc. Detailed analysis on FBGs is extraordinarily important for the design and performance promotion, which would make the FBGs more useful in the optical communication systems. So far, lots of theoretical and experimental results have been presented on this topic. However, more and more challenges and opportunities for the FBGs will arise with the technolgy development and commercial demands. Based on the existing research results, we are going to study the novel performances and the applications mechanism of the FBGs, including the filtering characteristics, the design of the reflection spectrum, and the pulse compression and pulse train generation.The equivalent phase, the Fourier transform, the transfer matrix, and the Taylor series expansion have been employed to analyze and verify the proposed schemes. Firstly, according to the equivalence in the phase domain, a general phase condition for the Talbot effect is obtained to develop the dense multi-channel effect by combining the linear chirp and phase shift. On the basis of the Fourier transform relationship between the index modulation of the grating and the reflection spectrum, cosine-shape, quadratic-function shape and right-angle-trapezoid-shape reflection spectra have been achieved. These proposed schemes are numerical demonstrated by the transfer matrix method, which can provide certain theorical reference for the pratical applications in optical signal processing. Secondly, the frequency chirp of phase modulated ligtht is analyzed using the Taylor series expansion method. Furthermore, we propose a frequency chirp compensation scheme by using high order dispersion compensation (e.g. second order and fourth-order dispersions), resulting in a effective pulse compression and generation. The content and results of this thesis are listed as follows.Firstly, the thesis focuses on the novel filtering characteristics of FBG, including multi-function optical comb filter and the special spectrum response. The general phase condition can be derived to be the foundation of the hybrid technique, in which the chirp and the phase shift are equivalent. Thus, a comb filter with different channel spacings and dispersions is available by adjusting the distribution of the chirp and the phase shift. Futhermore, by tuning the phase shift of FBG dynamically, a tunable dispersion compensator can be realized. After analysis on the index modulation based on the Fourier transform, five special reflection spectra, such as quadratic function, right angle trapezoid function, and so on, are achieved, respectively. Moreover, special group delay responses have also been investigated based on the same approach, including linear shape, quadratic-function shape and so on. Using transfer matrix method, the numerical results agree well with the theoretical analysis results. This kind of filter can easily find applications in the signal generation and sensor area.Secondly, a novel generation scheme for rate-multiplication pulse train (i.e. rate-doubled and rate-quadrupled) is presented based on the phase modulation and chirped FBG. After passing through the phase modulator, a beam will be separated into two orthogonally polarized signals which are then injected into the same chirped FBG in opposite directions. The odd-order side-lobes will be eliminated due to the interference, from which a return zero pulse train with double rate is generated with a duty ratio of33%. Furthermore, by applying the sine signal to modulate a linear polarized light, the polarization angle relative to the axis of the polarization modulator is set as45°. When the signal is transmitted in the chirped FBG, the reflected light then results in rate-quadrupled pulse sequence after passing through the polarizer.Finally, the pulse compression and generation based on the phase modulation and high order dispersion compensation is investigated. Through compensating the second order and the fourth order dispersions of an external modulated light, the pulse compression is realized with a larger peak power and a smaller time-bandwidth products. Based on the reconstruction-equivalence chirp theory, a pulse compression scheme is proposed by using an SFBG including the second order and the fourth order dispersions. Simulation results show that the scheme can offer a time-bandwidth product of only0.79for pulse compression.