High Power Erbium-doped Fluoride Fiber Laser Source And Its Multidimensional Manipulation Technology

The 2-5?m mid-infrared(mid-IR)laser regime covers important atmospheric window and molecular fingerprint spectra of various gases,liquids and solids,and has important applications and development prospect in biomedical,gas detection,sensing,photoelectric countermeasures and other fields.Among many schemes to realize mid-infrared laser,fiber laser source has become the research hotspot of mid-infrared laser by virtue of its superior beam quality,wide gain spectrum,outstanding heat dissipation ability,higher brightness and robustness.With the maturity of high brightness semiconductor lasers,the improvement of fiber preparation technology and the in-depth study of laser generation mechanism,mid-infrared fiber lasers have made important progress.However,unlike the laser source in visible and near-infrared spectral region,the materials and opto-electronic components in the mid-infrared regime with low loss and high efficiency are deficient,which limits the improvement of the performance indicators of the laser source and the multi-dimensional manipulation of the light field.At present,researchers focus on the fabrication of mid-infrared fiber and fiber components,the impr ovement of the performance of mid-infrared fiber laser and its manipulation in time domain,spectrum and other dimensions.Under continuous wave operation,the average output power of erbium-doped fluoride fiber laser has reached 41.6 W.The erbium-doped fluoride fiber laser can obtain ultrafast laser output of femtoseconds under pulse operating conditions.In addition,the researchers also achieved a wide tuning range of mid-infrared pulsed laser output.However,the performance of mid-infrared fiber laser source and the multi-dimensional control capability of the light field are far from the actual application demands.In this thesis,inspired by the application requirements of high-power mid-IR lasers,the light field manipulation technology of mid-IR fiber laser is investigated from three aspects of time,frequency and coherence domain.The main research results are as follows:(1)Based on the saturable absorber of gold nanomaterials,stable 2.8?m passively Q-switched erbium-doped fluoride fiber laser is obtained,and the physical mechanism of broadband response of gold nanomaterials is revealed.Finite difference time domain(FDTD)simulation was used to verify that the absorption cross section of gold nanostar material,which can be widened to the mid-infrared spectrum by changing the aggregation numbers.The saturated absorption characteristics of gold nanostar material caused by surface plasmon resonance at 2.8?m band were measured by open aperture Z-scan technique.The 2.8?m passively Q-switched erbium-doped fluoride fiber laser can be obtained based on gold nanostar saturable absorption element.Since the optical properties of gold nanomaterials are greatly affected by their shape and size,we studied the nonlinear optical response of gold nanomaterials hexagonal structure in mid-infrared bands to further explore the feasibility of the application of gold nanomaterials in mid-infrared bands.A 2.8?m passively Q-switched fiber laser with gold nano-hexagonal structure thin film has been successfully realized.The experimental results can not only deepen the understanding of the nonlinear optical behavior of gold nanomaterials,but also provide more options for the development of tunable optical devices.(2)Based on a volume bragg grating(VBG)and an acousto-optic Q-switching element,the high brightness and wavelength tunable 2.8?m pulsed fiber laser is demonstrated.Firstly,Er³⁺-doped fluoride fiber laser based on acousto-optic Q switcher is built to study the repetition rate range required for stable pulse generation under different pump power and repetition rate,further illustrating the pulse generation mechanism of acousto-optic Q modulation.At a repetition rate of 47 k Hz,the pulse duration of the laser obtained is 0.18?s,and the corresponding output power is 0.91W.This is the highest average power level obtained in an acousto-optic Q-switched erbium-doped single-mode fluoride fiber laser.Then,the spectrum of erbium-doped fluoride fiber laser is narrowed and tuned by VBG element.The evolution of the wavelength tunable range under different pump power is experimentally studied.The tunable range of the wavelength obtained under continuous laser operation is 132 nm(2715?2847 nm).Finally,an erbium-doped fluoride fiber laser modulated by acoust-optic Q switch and VBG element is studied.The continuous tuning range of 122 nm(2718?2840 nm)is obtained by pulsed laser operation,which is the widest wavelength continuous tuning range realized by the 2.8?m erbium-doped fluoride fiber laser to the best of our knowledge.(3)A high power superfluorescent fiber source at 3?m band is realized by using the amplification stage structure,and the dynamic evolution mechanism of its spectrum is revealed.We first constructed erbium-doped fluoride fiber superfluorescence seed source,and studied the dynamic evolution of the spectrum of the superfluorescence source under different pump power,and analyzed the spectrum evolution from the energy level transition dynamics in detail.The maximum output power of the superfluorescence is 0.224 W,and the corresponding efficiency is 11%.Then we built a first-stage amplification structure based on the seed source,and the maximum power of superfluorescence output was 1.85 W.We compare the spectra and efficiency of the superfluorescent source before and after amplification,and give a detailed explanation.This is the highest power output level currently obtained in a 3?m superfluorescent fiber source to the best of our knowledge.