Research On Novel Mode-locked Fiber Lasers And Their Dynamics

Passively mode-locked fiber lasers(PMLFLs) have widespread applications in high-speed optical fiber communications, laser micromachining, optical frequency comb, lidar, biomedicine and nonlinear optics because they can produce ultrashort pulses with high peak power and broad optical spectrum. Compared with solid-state lasers, fiber lasers exhibit the advantages of compact size, high gain efficiency, good heat dissipation and no need for alignment. The development of new materials, new devices and new pulse formation mechanisms has motivated research on novel PMLFLs, which is an important global research frontier. Recently, Chinese researchers have made a great progress in this field. It is of great scientific and practical importance to research PMLFLs. In this thesis, we have studied novel mode-locked fiber lasers and their dynamics based on some new materials, new devices and new pulse forming mechanisms. The main research contents of this thesis are as follows:1. By utilizing two-dimensional(2D) nanomaterial molybdenum disulfide(MoS2) as a saturable absorber, a passively mode-locked erbium-doped fiber laser(EDFL) is first presented. The few-layer MoS2 is grown by the chemical vapor deposition(CVD) method. The nonlinear optical absorption in the material is measured and the results show that few-layer MoS2 exhibits obvious saturable absorption at around 1.55 μm. The mechanism for the intriguing saturable absorption observed at such waveband can be attributed to the bandgap reduction induced by introducing suitable S atomic defects in few-layer MoS2. By using the few-layer MoS2 saturable absorber, the mode-locked EDFL produces soliton pulses with central wavelength, spectral width, pulse duration, and repetition rate of 1568.9 nm, 2.6 nm, 1.28 ps, and 8.288 MHz, respectively. In addition, harmonic mode-locked soliton pulses with orders from 2nd to 20 th are obtained from another EDFL with an optimized cavity.2. By using graphene saturable absorbers with different layers, the mode-locked femtosecond and nanosecond pulses are achieved in EDFLs with different cavities. Monolayer graphene is prepared by the CVD method and transferred onto the end face of a fiber connector to form a graphene saturable absorber. With the monolayer graphene saturable absorber, the mode-locked EDFL emits femtosecond pulses. The 11th-order harmonic mode locking is achieved by properly adjusting on the pump power level and state of polarization controller(PC). Secondly, we obtain nanosecond pulses in an EDFL mode-locked by a four-layer graphene saturable absorber. The nanosecond pulses have a pulse width of 24 ns and a spectral width of 0.96 nm. The pulse repetition rate and signal-noise ratio(SNR) are 5.78 MHz and 65 dB, respectively. The formation mechanism of nanosecond pulses emitted from the laser could be attributed to the pulse duration broadening induced by the graphene SA with large lumped normal dispersion. Such nanosecond pulse duration as well as megahertz repetition rate makes this mode-locked all-fiber laser a suitable seed oscillator for chirped pulse amplifications.3. By using a new integrated fiber component, a high-repetition-rate femtosecond EDFL mode-locked by nonlinear polarization rotation(NPR) technique is designed and constructed. Firstly, we achieve stable conventional solitons with a repetition rate of 169 MHz and pulse duration of 0.64 ps. The dynamics of bound solitons in the laser are studied comprehensively. Bound solitons consisting of multi-pulses up to seven pulses are obtained, in which the adjacent pulses have an equal or unqual pulse separation. With further optimization on the laser cavity, soliton pulses with a repetition rate up to 384 MHz are achieved in the all-fiber ring laser. The mode-locked pulses are externally amplified and compressed. As a result, femotosecond pulses with an average power of 207 mW and pulse duration of 340 fs are achieved.4. The characteristics of dual-wavelength solitons are investigated in an EDFL passively mode-locked by NPR technique in the anomalous dispersion regime. With proper adjustments on the pump power and PC, conventional solitons at 1566.5 and 1594.5 nm are achieved simultaneously. Both the two spectra have Kelly sidebands, which is a typical characteristic of conventional solitons generated in the anomalous dispersion regime. The central wavelength separation between the two spectra is about 28 nm. The net cavity dispersion and fiber dispersion can be measured based on the relationship between the central wavelength separation and RF separation of the dual-wavelength solitons. The formation mechanism of the dual-wavelength solitons is attributed to the combination of the relatively broad gain spectrum of EDF and the birefringence-induced cavity filtering effect. In addition, mode-locked states with spectral sidebands induce by modulation instability are also observed in the laser.5. The characteristics of dissipative soliton are experimentally investigated in EDFLs mode-locked by NPR technique. In the laser with large normal cavity dispersion, typical dissipative solitons with the spectral width of 27 nm and pulse duration of 12.5 ps are obtained. In the near-zero positive dispersion regime, dissipative solitons with the spectral width of 66.3 nm and pulse duration of 62 fs are achieved. The formation mechanism and dynamic evolution of dissipative solitons are numerically analyzed in fiber lasers with the large and near-zero net normal cavity dispersion, respectively. Moreover, we numerically investigate the output characteristics of dissipative solitons in a thulium-doped fiber ring laser operating at 2 μm by using a semiconductor saturable absorber mirror(SESAM). The features of vector dissipative solitons are analyzed. Finally, noise-like pulses with 3-dB spectral width of 60.2 nm are generated from a mode-locked EDFL with net normal cavity dispersion. Supercontinuum generation in a highly nonlinear ?ber pumped by the noise-like pulses is investigated. As a result, supercontinuum generation with a 20-dB spectrum bandwidth over than 500 nm is achieved.