高功率单晶光纤超短脉冲放大技术研究进展(特邀)

High Power Single Crystal Fiber Ultrashort Pulse Amplification Technology(Invited)

单晶光纤具有细长的晶体结构以及对泵浦光的波导传输特性,使其兼具晶体以及光纤的激光放大介质优点,其细长的晶体结构可以有效地进行散热,保证了在高功率运转下依然可以保证高光束质量,与传统的晶体棒相比,对泵浦光的波导特性使其具有更大的能量提取效率和放大增益,简单的行波放大结构使得系统易于集成。单晶光纤作为放大增益介质已广泛应用于高功率高能量超短脉冲激光放大技术中,并在科研、工业加工等领域具有重要的应用前景。本文重点介绍了单晶光纤的结构和制备方法,以及近年来1μm波段基于单晶光纤的超短脉冲放大技术研究的主要方法及结果,包括本课题组取得的主要进展,探讨和展望了单晶光纤放大技术的前景和发展方向。

Femtosecond laser with high repetition rate and high output power can be used as the driving light source of attosecond pulse generation,which can effectively increase photon flux,obtain enough experimental data quickly,and overcome the existing problem of space charge effect.Therefore,it has important application value in high order harmonics and attosecond pulse generation.At the same time,high repetition rate and high output power femtosecond pulses also have important application prospects in ultrafast laser precision micro-machining field,which can solve the technical problems of hard,brittle and soft materials processing with high-precision and high-quality“cold machining”in aviation and aerospace field.With the rapid development of ultra-fast laser processing market,high power femtosecond laser system with higher cost performance has become the major factor of concern in the industry.Compared with traditional solid-state lasers,fiber amplifiers have many characteristics,such as easy operation,all-fiber fusion,easy integration,excellent heat dissipation performance,excellent beam quality,etc.,which have attracted much attention in the field of ultra-short pulse amplification.However,limited by the mode field area of ordinary single-mode or nearly single-mode fiber,the output energy of the ultrashort pulse based on the structure of optical fiber amplifier system is limited due to the Raman scattering and stimulated brillouin scattering,the self-phase modulation,cross phase modulation and other nonlinear effect in the process of high power pulse amplification.In order to achieve high energy femtosecond pulse amplification,the current fiber amplification system mainly adopts the structure of CPA system.Before amplification,the pulse is stretched in the time domain and the special fiber with large mode area is used to achieve high energy output in the space domain.Slab,disc and other solid amplifying media can output higher pulse energy,however,they are difficult to package,and the amplification structure is relatively complex,which is not conducive to the mass production of domestic lasers.Single crystal fiber as a new type of amplifier gain medium,it has a slender crystal structure and waveguide transmission characteristics for pump beam,which makes it have the advantages of both crystal and fiber laser amplification media.Its slender crystal structure can effectively dissipate heat,ensuring high beam quality under high-power operation.The waveguide characteristics of pump beam make it have greater energy extraction efficiency and higher amplification gain than traditional crystal rods.Compared with the fiber amplifier,the nonlinear accumulation of the single crystal fiber amplifier is much smaller,which is more conducive to the improvement of pulse energy.Meanwhile,the amplification structure of the single-crystal fiber amplifier mainly focuses on the single-pass or double-pass traveling wave amplification structure.Compared with the high complexity of the slab and thin-disk amplification system,the single-crystal fiber has better integration and stability.The waveguide structure of single-crystal fiber amplifier can match the multi-mode pump beam with the signal beam propagating in free space,which can obtain higher amplification efficiency and better beam quality than the traditional crystal rods.Excellent thermal management performance also enables the structure to achieve hundredwatt level ultra-short pulse output with good optical parameters.The hybrid fiber-single crystal fiber ultrashort pulse amplification system combined the high gain property of fiber laser and high peak power property of crystal gain medium.In order to further improve the average power and pulse energy,it has been less effective by simply stretching the pulse duration in time domain and increasing the optical fiber mode area in the space domain,technologies such as coherent beam combination can obtain much higher power and larger energy.When the output power of the laser is less than 200 W,the single crystal fiber can consitute a simple and reliable amplification setup.For higher power output,it can also be used as a stable,cost-effective seeder with good optical parameters for solid state amplifier,such as thin-disk lasers.In the future development of lasers,the research and development of fiber laser with appropriate wavelength for pump source and the continuous optimization of brightness of fiber coupled laser diode can effectively improve the amplification efficiency of ytterbium-doped single crystal fiber,and achieve a wider application in the field of ultrafast laser.The cladding structure of crystal fiber can increase the ratio of surface area to volume of single crystal fiber,by improving the heat transfer performance of cladding and the thermal management ability of fiber can realize the long distance fundamental mode waveguide,and finally realize the high power laser with higher efficiency,which is also an important direction of future development.Therefore,single crystal fiber as amplification gain medium has been widely used in ultrashort pulse amplification and has important application prospects in scientific research,national defense,industrial processing and other fields.This paper mainly introduces the structure and preparation methods of single crystal fiber,and the main research methods and results of ultrashort pulse amplification technology based on single crystal fiber in 1μm waveband,including the main progress made by our research group,the prospect and development direction of single crystal fiber amplification technology are also discussed and prospected.