Research On The Propagation Characteristics Of Broadband Laser Pulse And Dispersive Wave In Single-mode Fiber

Propagation of laser pulses in the fiber is one of the fundatamental processes in Nonlinear Optics. As the ultra-high speed, ultra-high capacity, long-haul optical fiber communication system continually develope, its importance increases rapidly. At the same time, it also is a highly complex process of involving the effects of group-velocity disersion, high-order dispersion, self-phase modulation, cross-phase modulation, higher order soliton split, Raman-induced self-frequency shift, birefringence and gain (loss) in nonlinear optics. In addition, the parameters of pulse such as wavelength, shape, width and initial chirp have qualitative and quantitative effects on the final result. All this influences lead to not only the complication of pulse propagation in fiber but also the advent of many new physical phenomena. Aim at the up to data development of nonlinear fiber optics; we have done some research on the propagation characteristics of broadband laser pulse and dispersive wave in single-mode fiber. We have obtained some novel and innovative results, the main research results are listed below.Firstly, we have built the physical model of broadband laser pulse propagation in Erbium-doped fiber amplifier which the assumption of gain bandwidth of amplifier is significantly larger than that of signal pulse is not valid and disclosed the propagation characteristics of broadband laser pulse in gain fiber.It leads to a large spectral broadening of input pulse due to high-gain amplifier chain is used to realize high output power. The gain bandwidth of the fiber amplifier is expected to play an increasingly important role when it is comparable to the bandwidth of the pulse.Mostly the prevenient model are based on the assumption that the amplifier gain bandwidth is significantly larger than the signal’s optical spectral bandwidth. It has been shown experimentally that the spectral width of parabolic pulses exceeding the amplifier gain bandwidth leads to chirp nonlinearity and, consequently, to a poor quality of the recompressed pulses. That is too say, the assumption is unfortunately not appropriate in situations of ultrabroad bandwidth amplification. Based on our model, results show that when the gain bandwidth of the fiber amplifier is comparable with the bandwidth of pulse, it limits the efficiency of ultrabroad bandwidth pulse amplification. However, we found that the limitation can be minimized by choosing the corresponding optimal fiber length for different gain bandwidths. It also shows that the effect of gain bandwidth on the amplification of a soliton with the initial chirp depends on the sign of the chirp.Secondly, propagation of broadband pulse in fiber ring cavity is numerically investigated. It is found that even in the Maxwell-Bloch formalism stable solitary waves can still be obtained in the laser due to the cavity pulse peak clamping effect. We report on the generation of superbroad spectrum bunched noise-like pulses from a passively mode-locked erbium-doped fiber laser, in which the cavity is made of purely anomalous dispersion fibers.Conventionally, the soliton operation of a fiber laser is described by the Ginzburg-Landau equation (GLE). However, it is to note that the GLE is derived under the condition that the laser gain profile can be approximated with a parabolic shape and the light-matter interaction follows the laser rate equations. This condition is only valid when the formed soliton pulses have spectrum far narrower than the gain bandwidth. Therefore, to accurately model the pulse shaping in a laser, it is necessary to use the full Maxwell-Bloch formalism. It is found that even in the Maxwell-Bloch formalism stable solitary waves can still be obtained in the laser due to the cavity pulse peak clamping effect. We show that the gain bandwidth plays a significant role in determining the detailed property of the formed solitary pulses. In addition, we report on the generation of superbroad spectrum bunched noise-like pulses from a passively mode-locked erbium-doped fiber laser, in which the cavity is made of purely anomalous dispersion fibers. The maximum3dB spectral bandwidth of the pulse is about98nm. We show numerically that the superbroad spectrum of the pulse is caused by the fiber birefringence.Thirdly, generation and progapagtion characteristics of dispersive wave are investigated when a broadband pulse propagates in the photonic crystal fiber. The laws of effect on generation and propagation of dispersive wave by TOD and self-steeping are obtained.Dispersive wave can realize the energy transfer from the laser pulse to new spectral components, it has attracted considerable attention both theoretically and experimentally as it provides a variety of potential applications. We show, by numerically solving the extended nonlinear Schrodinger equation, that a notable efficient conversion of energy from the soliton to the dispersive waves (DWs) can be acquired in photonic crystal fibers with negative dispersion slopes. When the input pulse is narrow, so that the distance which dispersive wave generation is short, the enhancement of frequency shift by TOD can be suppressec by the spectral recoil effect on the zero-dispersion wavelength of fiber. It is also shown that due to the mechanism is different for the red-shift and blue-shift dispersive wave, the effect of self-steepening on red-shift dispersive wave isn’t significant. At the fiber output, the energy focuses on two regions:near the zero-dispersion wavelength and near the DW wavelength.Finally, we have investigated the effects of initial frequency chirp on dispersive wave in photonic crystal fiber. We propose a convenient and efficient method to manipulate the DW generation by controlling the pre-chirp of soliton in the PCF with negative TOD.We present a detailed numerical study on how the initial frequency chirp of the input pulse, which is unavoidable in realistic conditions, affects the DW generation in PCFs. We then find that:For the higher order solitons, the positive chirp can speed up the DWs generation while the negative chirp will slow it down. For the fundamental solitons, however, both the positive and negative frequency chirps slow down the DWs generation. Further, we find that, for both fundamental and higher order solitons, the efficiency of energy transfer is decreased due to the eigenvalues which represent the ultimate soliton amplitudes are reduced for both positive and negative chirps, but this can be easily overcome by simply adding the fiber length. Knowing how to effectively control the excited frequency with particular desired properties is still a challenging work. From the realistic perspective, pulses emitted from laser sources are often chirped, if we tailor the DW wavelength and ZDW of the PCF to a desired output range, the frequency chirp provides us a convenient and efficient method to manipulate the conversion of energy from the soliton to the DWs.