Investigation Of Ultra-short Pulse Propagation Characteristics In The Nonlinear Optical Loop Mirror Constructed From Dispersion Decreasing Fiber

The study of fiber laser is a very important area for the nonlinear optical research, which is regarded as one of the key technologies for the generation of high quality ultra-short optical pulse that is needed for the modern long distance, ultra-high bit rate optical communication systems. Although it is possible to obtain picosecond or sub-picosecond optical pulse in a dispersion decreasing fiber by adiabatic soliton compression, the output pulse is not satisfying due to the wide pedestal accompanied with the pulse. The pedestal is harmful because it can lead to cross talk between pulses in a communication system. However, Nonlinear Optical Loop Mirror (NOLM) can be used to suppress that pedestal and compress pulses more properly. Moreover, NOLM, which can be used as a saturable absorber, has many important applications of passive mode lock technologies in optical fiber lasers, especially in Er-dropped fiber lasers. In this thesis, NOLM constructed by dispersion decreasing fiber (DDF-NOLM) is studied. The propagation properties of the optical pulse in DDF-NOLM are discussed detailedly, including the pulse evolution in the fiber loop mirror, the compression factors, the pedestal energy and the peak power of the output pulse. A new type of NOLM is also presented that constructed by a segment of conventional single mode fiber and a segment of dispersion decreasing fiber. The optical switching properties and the propagations of optical pulses in this new NOLM are also studied. Some useful conclusions are achieved.The thesis consists of 5 chapters:Chapter 1: Shows the basic structure of NOLM, presents the applications and the research history of the NOLM.Chapter 2: Analyzes the mechanisms of the NOLM, including thepropagation equations of optical pulse in fibers and the mechanism of optical coupler and the optical switch.Chapter 3: Proposes the new type of NOLM based on the previous studies and finds out the optimal structure of the new NOLM from several schemes, which is constructed by a segment of dispersion decreasing fiber and a segment of conventional single mode fiber. From this new type of NOLM, a compressed pulse without pedestal and nearly un-chirped, which can propagated stably in a conventional fiber, is obtained. In comparison with the traditional DDF-NOLM, the output pulse has higher compression factor with shorter fiber length.Chapter 4: Studies the propagation of femtosecond pulse in a DDF-NOLM. The results show that there exists a time delay between input pulse and the output pulse in DDF-NOLM due to the Stimulated Roman Scattering (SRS) effects. Moreover, it is shown that the third order dispersion of the DDF fiber contributes a lot to the time delay and that the negative third order dispersion is more favorable than positive one in depressing this time delay and in getting higher compression factor, higher peak power of the pulse and higher transmission ratio. In short, negative third order dispersion is better than positive one for pulse compression in DDF-NOLM.Chapter 5: Draws the conclusion and makes some prospective views.