Broadband Mid-infrared Optical Frequency Comb Based On Difference Frequency Generation Via Lithium Niobate Crystal

An optical frequency comb is a stable laser pulse train in the time domain and a wide spectrum composed of equidistant narrow lines in the frequency domain.Initially optical frequency combs were invented for frequency metrology.Its wide spectrum,high precision,high resolution and fast acquisition speed provide a new measurement method for spectroscopy.A broadband comb can simultaneously stimulate different samples,and dual-comb technique enables fast spectrum measurements with a high resolution.A large number of molecules have strong vibrational absorption in the midinfrared region,which makes mid-infrared spectroscopy a method for identifying and quantifying molecules.Mid-infrared frequency combs not only provide high precision,high precision and high speed for molecule detection,but also high sensitivity.Midinfrared frequency combs have potential applications in environmental monitoring,breath analysis and industrial safety.There are many methods to generate mid-infrared combs.Among these methods,difference frequency generation enables simpler controlling system,higher power and widest spectrum covering mid-infrared region.In order to realize mid-infrared optical frequency combs covering the atmospheric window of 3～5 μm,this thesis focuses on the broadband mid-infrared combs based on the difference frequency generation via periodically poled lithium niobate crystals.The main research includes the seed source,tunable mid-infrared combs,octave-spanning mid-infrared combs.Finally,a broadband mid-infrared comb with a spectral coverage from 2.6 μm to 5.3 μm is achieved.And the coherence and gas absorption spectrum are demonstrated in this system.Therefore,the broadband mid-infrared frequency comb will have potential applications in precision spectral measurement.The details and innovations are summarized as follows:1.Seed sources of mid-infrared combs are achieved.The models of nonlinear amplifying loop mirror and nonlinear polarization evolution mode-locking with polarization-maintaining fiber are established with theoretical analysis.And saturable absorption mechanism is demonstrated using numerical calculation and simulation.Experimentally five different mode-locked lasers are realized.The robust oscillator based on nonlinear amplifying loop mirror with broad spectrum is selected as seed source of mid-infrared comb.2.A mid-infrared optical frequency comb with tunable spectrum is achieved.Based on a near-infrared mode-locked fiber laser,the mid-infrared comb with tunable spectrum of 3.0～4.4 μm is generated via periodically poled lithium niobate crystal using frequency control,chirped pulse amplification,nonlinear spectrum broadening and difference frequency generation.In order to broaden tunable spectrum,higher average power and narrower pulse duration is achieved via a self-similar amplifier.The system can emit pulses with 56.8-W average power,33-fs pulse duration and 22.95-MW peak power.The mid-infrared comb with tunable spectrum of 3.3～5.2 μm is generated.3.An octave-spanning width mid-infrared optical frequency comb is achieved.Optimizing the bandwidth of near-infrared laser system and the crystal structure,a midinfrared comb with direct spectral coverage of 2.6～5.3 μm is achieved via a chirped periodically poled lithium niobate crystal.The coherence of mid-infrared comb and the potential of multi-gases measurement are also verified.In order to develop more applications,a chirped periodically poled lithium niobate waveguide is used to simplify the comb system.And a broadband mid-infrared comb with 2.5～5.0 μm spectral coverage is also achieved via intrapluse difference frequency generation.