Propagation Characteristics Of Chirped Pulses In Optical Fiber Communication Systems

Due to limits of experimental conditions, the experiments of pulse propagation and measurement are normally performed by employing the autocorrelation technology so that the temporal waveforms and other characteristics of the pulses can not be exactly determined. The pulses generally have frequency chirp which can be controlled by pre-chirped technology, etc. The second-harmonic generation frequency-resolved optical gating (SHG-FROG) analyzer can be used to exactly measure the pulses characteristics. In this thesis, the linear and nonlinear propagation characteristics of chirped pulses in the optical fiber communication systems are experimentally and theoretically studied by using the SHG-FROG analyzer and the numerical method. As a result, the theoretical and experimental bases are established for design and optimization of optical communication systems and their photoelectric devices.The linear propagation characteristics of 10 GHz short pulse are experimentally studied by employing SHG-FROG analyzer. The pulse from the laser is near transform-limited Gaussian pulse with negative linear chirp. After the pulse propagates over 12.7 km of DFF, the spectral width is almost unchanged, the temporal width is about three times as wide as that of input pulse, the chirp is about four times as large as that of the pulse. The experimental data are consistent with the numerical results and the predictions of the theory.The linear propagation characteristics of the hyperbolic secant pulse and the double-side exponential pulse with initial linear and nonlinear frequency chirp are numerically studied by using the split-step Fourier method (SSFM). It is found that the effect of the negative linear chirp on the two pulses broadening is greater than that of the positive chirp. The effect of the nonlinear chirp on the temporal waveform is greater than that of the linear chirp. The temporal waveform splitting of the two pulses with nonlinear chirp is respectively more obvious than that of Gaussian pulse during linear propagation. Furthermore, the expressions of the spectral width and time-bandwidth product of the two pulses with the linear chirp C are given.The evolution of 10 GHz pulse with frequency chirp into the soliton is experimentally investigated by employing the SHG-FROG analyzer. Formation and propagation of chirped soliton are numerically studied according to the nonlinear propagation theory by using the SSFM. The numerical results are consistent with the experimental data. The nonlinear propagation characteristics of the double-side exponential pulse with initial linear frequency chirp are numerically studied by use of the SSFM. The greater the pulse amplitude A is, the less chirp parameter |C| is, the shorter the propagation distance evolving into a soliton is. The amplitude A of exponential pulse which evolves into a soliton is greater than that of hyperbolic secant pulse at the same propagation distance.The effect of the linear frequency chirp on the supercontinuum spectrum generation of Gaussian pulse is numerically investigated in the dispersion-flatted fiber with convex dispersion profile by using the SSFM. The relevant parameters of the supercontinuum spectrum of Gaussian pulse are compared with those of the hyperbolic secant pulse. The supercontinuum spectrum of the unchirped Gaussian pulse is wider than 200 nm, its characteristics are better than those of the hyperbolic secant pulse. The fiber length corresponding best supercontinuum spectrum decreases with the increase of chirp parameter C. The characteristics of supercontinuum spectrum for negative chirp are worse than those for positive chirp. The characteristics for chirp case are worse than those for unchirped case, gradually become severe with the increase of C.The collision characteristics of the orthogonally polarized solitons with initial linear frequency chirp are numerically studied in a linear birefringent fiber. It is found that initial chirp changes the threshold value of solitons forming into bound state in the birefringent fiber. The effect of initial positive chirp on the threshold value is more obvious than that of negative chirp.The equation of soliton propagation is modified with the Raman amplification. The effects of Raman amplification on propagation characteristics of soliton are experimentally investigated by employing SHG-FROG analyzer. The experimental data are consistent with the numerical results by using the SSFM. Raman amplification can compress the soliton, compensate the fiber loss, while not change the temporal waveform of the soliton. Raman amplification can completely compensate the fiber loss when the propagation distance is less than the effective fiber length of Raman amplification, can partially compensate the fiber loss when the propagation distance is more than the effective fiber length. Compression of the soliton and compensation of the fiber loss increase with the increase of the Raman pumping power. Experimental results show that soliton is not sensitive to the polarization of Raman pumping source. It is consistent with the theory that soliton maintains a high degree polarization in the fiber.The expressions of autocorrelation characteristics of the super-Gaussian pulse are derived form the principle of the SHG-FROG analyzer. The autocorrelation characteristics of the super-Gaussian pulse and their variations affected by the edge sharpness parameter m, linear chirp parameter C, pulse noise and random noise which are compared with the relevant parameter of the pulse, are investigated by using the numerical method. A useful method of filtering the random noise which is validated by the experiment is given.