摘要
飞秒光学参量振荡器(OPOs)是一种高效灵活的波长转换系统,可以在传统激光增益介质无法有效工作的波段内产生超短脉冲激光。宽光谱超短脉冲激光光源在多分子检测、光学相干断层成像和超快科学等新兴领域具有重要的应用价值。通过相位匹配设计,非线性晶体可以获得极大的参量增益带宽。而如何利用大增益带宽的非线性材料,基于飞秒OPO来产生宽谱激光逐渐成为近几年的研究热点。本文综述了近些年国内外飞秒OPO腔内光谱展宽技术的研究进展,重点介绍了本课题组基于腔内光谱合束、啁啾脉冲振荡、腔内脉冲压缩等技术方案实现飞秒OPO宽谱输出的研究成果。
Significance Femtosecond optical parametric oscillators (OPOs)are efficient and flexible wavelength-conversion devices capable of generating ultra-short optical pulses at wave regions not directly addressable by conventional laser gain medium.Over the past few decades,substantial progress has been made in femtosecond OPOs for generating emission with high output power,high pulse energy,high repetition rate,and wide wavelength-tuning range.Ultrashort-pulsed lasers with a broad bandwidth have unique advantages and are important for many emerging applications,including multicomponent trace gas detection,optical coherence tomography,and multispectral imaging.Many technical schemes focus on producing ultrashort optical pulses with a broad bandwidth,for instance,via laser mode locking,supercontinuum generation,or differential frequency generation.In contrast,femtosecond OPOs,which use nonlinear media as the gain materials,have inherent advantages in wavelength tuning and conversion efficiency.Moreover,extremely wide gain bandwidth of a nonlinear gain medium can be achieved through artificial tailoring with phase-matching techniques;therefore,femtosecond OPOs exhibit significant potential for generating ultrashort optical pulses with a broad bandwidth.However,a femtosecond OPO is usually pumped by an ultrashort pulse train.To achieve an efficient parametric transfer in an OPO,the duration of the oscillating OPO pulses should be similar to that of the pump pulses.Indeed,in a synchronously pumped OPO,the duration of the pump pulse imposes a lower limit on the pulse width of the oscillating signal wave.Therefore,in traditional femtosecond OPOs,apump source with very short pulse width is required for generating signal pulses with a broad bandwidth.The possibility of generating optical pulses with considerably wider bandwidths than the pump bandwidth in femtosecond OPOs is quite intriguing.Progress Intracavity spectral combinations can be used to enhance the instantaneous bandwidth of the output idler light in synchronous-pumped OPOs.Idler light with an instantaneous bandwidth that is significantly larger than that of the pump bandwidth can potentially be obtained using a nonlinear crystal with a wide phase-matching bandwidth and by implementing a multichannel configuration (Fig.1).This scheme transforms conventional synchronously pumped OPOs into devices capable of generating idler light with a very broad instantaneous bandwidth.Chirped-pulse optical parametric oscillators (CPOPOs)allow the generation of ultrashort optical pulses with an instantaneous bandwidth that is considerably wider than the pump bandwidth.Chirped-pulse formation can be achieved by inserting a material with a large nonlinear index coefficient into an OPO cavity or using an aperiodic QPM crystal.By properly managing the pulse dynamics by optimizing the intracavity dispersion and spectral broadening,optical pulses with an octave-spanning instantaneous bandwidth can be obtained.Moreover,spectral broadening in CPOPOs can be enhanced to generate spectrum with an instantaneous bandwidth significantly wider than the parametric gain bandwidth of nonlinear crystals.Our study shows that a relatively high residual second-order-dispersion inside the OPO cavity is required for achieving the maximum signal bandwidth that exceeds the parametric gain bandwidth from a CPOPO (Figs.2 and 3).In addition,the parametric d2eff/n2 of nonlinear crystals (n2and deffare the nonlinear refractive index and effective second-order nonlinear coefficient of the crystals,respectively)will play an important role in determining the amount of spectral broadening that can be achieved (Fig.4).We demonstrated a CPOPO system that generated optical pulses with a bandwidth that was approximately 12 times of the gain bandwidth of the nonlinear gain medium (Fig.5).Pulse-compression in cavity length-detuned OPOs can be used to generate high-quality,soliton-like optical pulses with a considerably shorter duration than the pump pulse duration.This can be achieved in a femtosecond OPO via positive cavity-length detuning(Fig.6).At a relatively high pump rate,the resonating signal wave can evolve into multiple pulses.However,we show that single-pulse operations can be recovered by simply increasing the level of cavity-length detuning.Conclusions and Prospects In this article,we review recent progress on the generation of optical pulses with very broad bandwidths from femtosecond OPOs.We summarize the progress of our work performed at the Huazhong University of Science and Technology.Furthermore,we show that spectral broadening in OPOs can be achieved via intracavity spectral beam combination,by configuring a chirped pulsed OPO,or by simply detuning the cavity length.Ultrashort optical pulses with a broad instantaneous bandwidth may be beneficial for many applications,including optical coherence tomography,ultrashort pulse synthesis,and spectroscopy.
作者
衡家兴
冯玺
刘沛
张兆伟
Heng Jiaxing;Feng Xi;Liu Pei;Zhang Zhaowei(School of Optical&Electronic Information,Huazhong University of Science&Technology,Wuhan 430074,Hubei,China;Wuhan National Laboratory for Optoelectronics,Huazhong Unicersity of Science&Technology,Wuhan 430074,Hubei,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2022年第12期66-78,共13页
Chinese Journal of Lasers
基金
中央高校基本科研专项资金(2018KFYXKJC020)。
关键词
非线性光学
飞秒光学参量振荡器
光谱合束
啁啾脉冲
光谱展宽
脉冲压缩
nonlinear optics
femtosecond optical parametric oscillator
spectral beam combining
chirped pulse
spectral broadening
pulse-compression
