| High repetition rate(>10MHz),high power(>10m W)long-wave mid-infrared(6-20μm)femtosecond frequency comb plays a very important role in the fields of biomedical molecular detection,ultrafast spectroscopy and condensed matter physics.In this thesis,we present theoretical and experimental research aimed at addressing this international challenge of how to generate tunable long-wave mid-infrared femtosecond pulses with high repetition rate and high power.A novel scheme was proposed using high-power pump pulse centered at 1.55μm generated from Erbium-doped fiber laser,which effectively avoids the two-photon absorption.The development of self-phase modulation-enabled spectral selection pumped by the erbium-doped fiber laser,was also pursued.This technology enabled the generation of a high-power tunable pulse with a wavelength of 1.6 μm-1.94 μm,which was used as a signal.Finally,Ga Se was chosen as the crystal to do difference frequency generation,overcoming the power bottleneck caused by its low figure-of-merit.Thus,a high-power tunable long-wave mid-infrared femtosecond optical comb light source is obtained.This breakthrough enabled the development of a high-power tunable long-wave mid-infrared femtosecond optical comb light source,expected to contribute to the mid-infrared dual optical comb system and advance the development of ultrafast spectroscopy.The main innovative achievements are summarized as follows:1.Based on dispersion-managed soliton mode-locking technology,research has been conducted on broadband stretched pulse mode-locked ring cavity oscillators.This is achieved by employing a low-doped gain fiber to compensate for intracavity dispersion and adjusting the length of the passive fiber PM1550 to precisely control intracavity dispersion.As a result,we have successfully obtained a mode-locked pulse output centered at 1.55 μm with a spectral bandwidth of more than 20 nm.The oscillator output has a high signal-to-noise ratio exceeding 80 d B,and the power RMS variation is only 0.31%.Furthermore,the oscillator maintains a stable mode-locked state throughout the year in the laboratory,making it an ideal front-end for subsequent applications.Based on chirped pulse amplification technology,research has been conducted on a high-power femtosecond Erbium-doped fiber laser.To achieve this,we have selected dispersion compensating fiber with large dispersion as the stretched fiber and adjusted the polarization controllers at both ends of the stretched fiber to obtain a pulse output with a high polarization extinction ratio of 30 d B.We have then used Er/Yb co-doped gain fiber to amplify the stretched pulse and compressed the amplified pulse using a grating pair.As a result,we have obtained output pulses with pulse duration of 265 fs,average power of 7.71 W,pulse energy of 247.1 n J,and corresponding peak power of nearly 1 MW.This Erbium-doped fiber laser serves as a high-power laser front-end for subsequent wavelength conversion,as well as providing a high-power pump pulse for subsequent difference frequency generation.2.Based on SPM-enabled spectral selection technology,research on wavelength conversion using high-power Erbium-doped fiber lasers has been carried out.For the first time,high-energy(>10 n J)near-conversion limit pulses(<100 fs)with a wide tunable range between 1.28-1.95 μm have been produced in dispersion-shifted fiber with a large mode field,with the highest pulse energy reaching 12.6 n J.Additionally,a dualwavelength laser with simultaneous output of 1.55 μm and 1.04 μm has been developed,achieving,for the first time,high-energy pulse(10 n J)output with a tunable wavelength range of 0.8-1.7 μm.This laser provides high-power wide-range tunable signal pulse for subsequent difference frequency generation and an ideal light source for multi-photon microscopy imaging.3.Using the time-dependent coupled wave equation,we conducted a theoretical study on the difference frequency generation process in Ga Se and BGSe crystals.By pumping a 2 mm thick Ga Se crystal with 50 n J energy pulses at 1.55 μm,we were able to generate mid-infrared pulses at a center wavelength of 9.83 μm with an expected average power exceeding 100 m W.With the maturity of fiber optic devices,it is possible to use2.88 μm laser to pump the difference frequency generation process,so we simulated the difference frequency generation process of 2.88 μm pulse-pumped Ga Se crystal.The results of the simulation indicate that there is a smaller group velocity mismatch between pulses,and thicker crystals can be used to obtain higher energy mid-infrared pulses.Furthermore,we investigated the difference frequency generation process of a novel BGSe crystal,which revealed its potential to produce high-energy mid-infrared pulses when pumped with 1.55 μm pulses.These simulations provide valuable guidance for future difference frequency experiments and offer a feasible approach to achieve high-power long-wave mid-infrared pulses.4.Based on difference frequency generation,high-power mid-infrared pulse experiments were conducted using Ga Se and BGSe crystals.A 1.55 μm pulse from a high-power erbium-doped fiber laser was used as the pump pulse,and a 1.6-1.94 μm tunable side-lobe generated by 1.55 μm-driven SPM-enabled spectral selection is used as the signal pulse.This setup enables the realization of long-wave mid-infrared pulse output with a wavelength range of 7.7-17.3 μm in a 2 mm Ga Se crystal,with a maximum average power of 58.3 m W,which is an order of magnitude increase in power.In this experiment,we observed for the first time the process of high repetition rate difference frequency generation operating under the regime of optical parametric amplification.To further expand the tunable range of mid-infrared pulses,a new scheme is proposed to generate mid-infrared pulses based on the leftmost and rightmost side lobe difference frequency generation.This new scheme enables the tunable mid-infrared pulse output with a wavelength of 6-13.6 μm,with a maximum average power of 9.5 m W.These experiments on mid-infrared pulses provide strong support for the subsequent development of long-wave mid-infrared dual-comb systems. |
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