Study on the cavity solitons in fiber lasers
Date of Issue2018-02-09
School of Electrical and Electronic Engineering
Since it was first proposed in 1960s, fiber laser has attracted consistent focus both from industry and scientific research for these decades. In industry, fiber laser is an ideal source of high power CW /pulse beam output for the fine machining. In scientific research, pulse fiber lasers can provide picosecond or femtosecond optical pulses which are needed in physics, chemistry, medical and material research. On the other hand, the pulse propagation in optical fiber is mathematically described by the nonlinear Schrodinger equation (NLSE), and pulse propagation in a fiber laser is mathematically described by the Grinzburg-Laudau Equation (GLE). Both equations are very important in nonlinear science, describing a conservative and dissipative nonlinear system, respectively. Therefore, fiber laser is also an excellent testbed for experimentally studying the nonlinear physics. As a typical nonlinear cavity, fiber laser has nonlinear properties as those externally pumped nonlinear fiber ring cavities. Moreover, without the polarization selective components, fiber laser is a vectorial cavity which has two orthogonal polarization modes. Two orthogonal polarization modes will couple together through either coherent or incoherent interaction. Recently, the temporal cavity solitons in the externally pumped nonlinear fiber ring cavities have received much attention. As a nonlinear cavity, in principle temporal cavity solitons should also can be generated in fiber laser cavity. In this thesis, I'll focus on the study of cavity solitons in fiber laser. First of all, inspired by modulation instability induced by cavity boundary conditions in the externally pumped nonlinear fiber ring cavities, the cavity induced modulation instability (CIMI) is both theoretically and experimentally confirmed existing in a fiber ring laser. ClMI origins from the cavity detuning. It's theoretically shown that when the total cavity phase delay (linear phase delay with nonlinear phase delay) is not 2mn, the non-zero cavity detuning will cause a new type of modulation instability. Compare with the traditional modulation instability appeared in single mode fibers, CIMI can occur both in normal and anomalous dispersion regime, and the modulation frequency is related with the cavity detuning. In experiment, we studied the ClMl lasing state of fiber laser. Stable periodical pulse train emjssion can be achieved, whose repetition rate can be altered by either tuning the cavity or changing the light intensity. The experimental results match well with theoretical conclusions. CIMI makes the fiber laser as a promising pulse source with tunable repetition rate, without any external modulation methods. Under the CIMI induced pulse train emission state, if increasing the light intensity, we experimentally found the pulse train will be finally shaped into dissipative solitons. Through numerical simulate the operation of fiber laser, the gain bandwidth limitation plays a key role in the shaping mechanism. Under sufficient narrow gain bandwidth limitation, the CIMI modulated CW beam can evolve into dissipative solitons. We named this kind of spontaneously generated solitons as the temporal cavity solitons in fiber laser, which distinguish from the previously reported solitons in the mode-locked fiber lasers, nevertheless they are experimentally found having similar properties. The vector temporal cavity solitons are also experimentally observed. In an anomalous dispersive fiber laser, the induced vector solitons under either coherent or incoherent coupling between two polarization modes are observed. The numerical simulation also supports the experimental conclusion. The coherently coupled vector solitons are also observed in the normal dispersive fiber laser. The coherent energy exchange between vector solitons is first time experimentally observed in normal dispersion regime. In the vector fiber laser cavity, the temporal cavity solitons will be coupled with the light on their orthogonal polarization. Under the incoherent coupling, when the cavity birefringence is strong, an interesting phenomenon call "soliton-dark pulse pair" is experimentally observed. It can be interpreted as an extreme case of polarization domain formation, where one polarization mode are bright solitons. On the other hand, under weak birefringence and coherent coupling, a novel state of induced dark solitons is experimentally observed. Through numerical simulation, it finds that the formation of induced dark solitons is due to the coherent coupling between soliton and a dark pulse with phase jump. Without phase jump, a bright soliton will be induced instead In the research of dark solitons, an interesting phenomenon of dark envelope emission of a fiber ring laser is discussed. When a high frequency modulation and a dark-soliton-like dark pulse coexist in a fiber laser, they will be shaped into a high repetition rate dark pulse train and a dark soliton, respectively. Due to the superposition of their optical fields , the narrow dark soliton will form a wide dark envelope embedded in the dark pulse train background. The formed dark envelope makes the narrow dark soliton easily detectable. This finding brings a new understanding on the dark soliton formation in fiber lasers, especially on their experimental detection. Based on this understanding , the formation mechanism of the experimentally observed bright-dark vector solitons also becomes clear. In a weak birefringence cavity fiber laser, as a result of the strong coherent coupling between the two orthogonal polarization components, either the phase locked bright-bright vector cavity solitons or the bright-dark vector cavity solitons will be formed, which type of the vector cavity solitons will be formed depends on the phase relationship between the solitons. The coherent polarization coupling could lead to novel types of vector solitons formation.
DRNTU::Engineering::Electrical and electronic engineering