Study On Temporal Soliton And Optical Frequency Comb Generation In Brillouin Laser Cavity

Optical frequency comb?OFC?is frequency components with equal frequency interval in frequency domain and pulse light with fixed repetition rate in time domain.It has important applications in precise measurement of physical parameters,optical clock,coherent optical communication,generation of microwave and optical arbitrary wave,spectroscopy,calibration and ranging of astronomical spectrometer,etc.At present,the main methods to realize optical frequency combs include frequency combs based on mode-locked laser,frequency combs based on electro-optic modulation technology,frequency combs based on traveling wave four wave mixing and frequency combs based on optical microcavity.Recently,temporal solitons have been obtained by precisely adjusting the pump laser frequency injected into the optical microcavity which provides a way of not only generating low-noise optical frequency comb spectra with smooth spectral envelopes in the frequency domain,but also optical ultrashort pulses at tens of gigahertz repetition rates in the time domain.Microresonator-based soliton frequency combs have achieved great breakthroughs,such as soliton microcomb range measurement,self-referenced chip-scale soliton frequency comb and integrated-photonics optical-frequency synthesizer.However,the generation conditions of the above-mentioned temporal solitons are harsh,the stability is poor and the mode spacing is difficult to achieve large-scale tunable repetition rate,which is necessary in many applications such as optical communication,optical sensing,spectroscopy,tunable microwave and terahertz wave generation.In order to solve the above problems,the author has carried out systematic research on the generation of temporal soliton and optical frequency comb with large-scale tunable repetition frequency,as well as the generation of broadband and flattened frequency comb,and obtained the following research results:?1?In this paper,a method is proposed to generate temporal soliton and optical frequency comb with tunable repetition rate in Brillouin laser cavity.By introducing the Brillouin gain into the cavity,the generated intra-cavity Brillouin lasers,instead of using external pump light,are used as the pump light to drive the cavity coherently for generating temporal solitons.n contrast to previous works,the soliton pulses form spontaneously via intra-cavity pumping without the need of fine tuning of the external pump light?to match the cavity resonance?,even if the cavity resonance is affected by thermal effects and other environmental perturbations,and enables the generation of soliton frequency comb via cascaded four wave mixing between multiple wavelength intra-cavity Brillouin lasers.Furthermore,by varying the frequency separation of multiple wavelength intra-cavity Brillouin lasers,the soliton pulses with a pulse width of hundreds of femtoseconds and a repetition rate of GHz-THz are achieved via intra-cavity pumping,corresponding to the generation of soliton frequency combs with step tunable mode spacing from GHz to THz.?2?The influence of nonlinear fiber length and pump laser linewidth on the generation of optical frequency comb in Brillouin laser cavity is studied.First of all,the number of pump wavelengths in the installation of frequency comb generation is reduced to two.The dual wavelength Brillouin laser generated in the cavity has a fixed frequency interval,so that the frequency space of the combs generated by the cascade four wave mixing effect is strictly equal.Then,the influence of high nonlinear fiber(?=10 W^-1km^-1)length in Brillouin laser cavity on the bandwidth of optical frequency comb is studied.When the length of high nonlinear optical fiber is 100 m and the repetition frequency is 200 GHz,the optical frequency comb with spectrum range of1450¹650 nm is obtained;when the length of high nonlinear optical fiber is 20 m,the optical frequency comb with single longitudinal mode output is obtained.At the same time,the influence of the linewidth of the pump laser on the generation of frequency comb in Brillouin laser cavity is studied.Under the same pump power,the narrower the linewidth of the pump laser is,the better the spectrum range and flatness of the frequency comb can be obtained.?3?All-solid W-type index fluorotellurite fibers?AWFTFs?with near-zero-flattened dispersion?NZFD?profiles are designed for optical frequency comb?OFC?generation.The core and cladding materials of the optical fiber are fluorotellurite glass and aluminum fluoride based glass respectively.By optimizing the structural parameters of W-type fluorotellurite high nonlinear optical fiber,the fiber with the chromatic dispersion value between-0.2 and 0.35 ps/nm/km in the wavelength range of 1500¹600 nm is achieved.The simulated results show that flat-top OFC spectrum expanding from 1450 to 1700 nm with tunable mode spacing from 25 to 100 GHz can be generated in 2-m-long fiber by using a 1550-nm laser with a pulse width of 0.825 ps and a peak power of 60 W as the pump source.