Research On High-repetition-rate Passively Mode-locked Fiber Lasers At 1.0?m

Pulse trains with high repetition rate,feature of short temporal interval?<1 ns?and large longitudinal mode spacing?>1 GHz?,have been widely used in a variety of applications,including high-capacity optical communication networks,high signal-to-noise ratio bio-optical imaging,high-precision material processing,precision spectroscopy,and optical arbitrary waveform generation.Therefore,it has prompted how to obtain high repetition rate laser pulse sources as a hot spot in the field of pulsed laser research.High repetition rate passively mode-locked fiber lasers have attracted much attention in many implementations due to their compact structure,compatibility with fiber systems,and stability of output pulse.This paper focuses on the all-fiber high-repetition frequency passively mode-locked fiber lasers at 1.0?m wavelength range and the improvement of output pulse performance.The detailed research on fiber lasers includes the analysis of pulse formation process in the all normal dispersion high repetition rate passively mode-locked fiber lasers,and the construction of the all normal dispersion high repetition rate passively mode-locked laser cavity.Output pulse performance enhancements include nonlinear optical pulse amplification systems,spectral broadening in photonic crystal fiber,and output pulse repetition frequency synchronization.The results achieved are as follows:?1?A complete numerical model of the high-repetition rate passively mode-locked laser cavity was established by combining the steady-state rate equation of the uniformly broadening two-level of the gain fiber and general nonlinear Schr?dinger equation?GNLSE?.The temporal domain and frequency domain evolution in the simulation results indicate that the gain filtering effect of the gain fiber has a key effect on the formation of stable mode-locked pulse output in the ultra-short fiber laser cavity.In experiment,mode-locked laser cavities with GHz-level fundamental frequency repetition rate were constructed by using the highly Yb³⁺-doped phosphate fiber.And the characteristics of the output pulse were analyzed while shortening the laser cavity length.It is worth noting that mode-locked pulse trains with a fundamental frequency repetition frequency up to 12.5 GHz was obtained based on a section of 7.8-mm long gain fiber,which is the highest achievable repetition rate in the passively mode-locked fiber laser at the 1.0?m wavelength range.?2?Based on the steady-state rate equation of the uniformly broadening two-level of the gain fiber and the GNSEL,the numerical model of the nonlinear optical amplifier was established.With the index of the narrowest pulse duration of the compressed pulses,the influence of the center wavelength of the launched pulse,the length of the gain fiber,and the pump power on the amplified chirped pulse was analyzed.Based on the numerical simulation results,a nonlinear pulse amplifier with a 1.27-GHz mode locked fiber laser as the seed source is built.The amplifier has a light-to-light conversion coefficient of 35.7%,and an output pulse of single pulse energy of⁷ nJ can be obtained.The achievable narrowest pulse width of the compressed pulse trains is 184 fs.With the amplified pulse as the laser pump source,an optical supercontinuum with a longitudinal mode spacing of 1.27 GHz and a spectral width exceeding an optical octave is obtained in the photonic crystal fiber.?3?Using the phase-locked loop technology,the fundamental repetition rate of an all-fiber 1.27-GHz passively mode-locked laser was synchronized with the highly stable microwave signal.Under the stabilization,the timing jitter?integrated from 1 Hz to 1 MHz?of the 1.27-GHz repetition rate was reduced with a compression factor over 300.Otherwise,the Allan deviation of the stabilized pulse trains is⁵ mHz recorded by a frequency counter at1-s averaging time,and the corresponding relative Allan deviation is 3.9×10^-12.