基于差频技术的宽谱中红外飞秒激光的产生

Broadband Mid-Infrared Light Based on Difference Frequency Generators

飞秒差频产生器(DFG)是一种获得宽谱中红外激光的有力工具。为了利用DFG产生更高瞬时带宽的中红外激光,可以使用窄带泵浦光、宽带信号光结合大信号光相位匹配带宽的非线性晶体或使用宽带泵浦光、窄带信号光结合大泵浦接受带宽的非线性晶体。研究表明,对于PPLN晶体,当泵浦光波长为1050 nm,闲频光波长在3.4μm附近时,非线性晶体具有较大的泵浦接受带宽,仅使用均匀极化周期PPLN晶体即可获得宽谱中红外激光。基于高重复频率的掺镱光纤激光放大器系统,通过引入自相位调制效应,获得了中心波长为1050 nm的宽谱光源,将其作为DFG系统的泵浦源。利用飞秒脉冲在负色散光子晶体光纤中的拉曼效应,产生了中心波长为1525 nm的超短脉冲,将其作为DFG系统的信号源。在长度分别为1 mm和3 mm的PPLN晶体中,都获得了宽谱中红外闲频光输出,其-10 dB光谱覆盖范围分别为2.72~4.15μm和2.87~4.08μm。

Objective A difference frequency generator(DFG)is the frequently used platform to generate mid-infrared light.Two conditions need to be met to generate the broadband mid-infrared light without wavelength tuning.Firstly,the phase-matching bandwidth of the nonlinear crystal is wide.Second,the DFG needs to be pumped by a broadband light source.In a traditional DFG,the pump source is narrowband and the signal source is broadband.However,broadband signal waves centering at some special wavelengths are difficult to be obtained.In this paper,however we find that a DFG pumped by a broadband pump source and a narrowband signal source can generate broadband mid-infrared light in some special conditions.Methods A periodically-poled lithium niobate(PPLN)based DFG is investigate.Firstly,we derive the relationship between phase mismatching and the group velocities of the pump wave,signal wave,and idler wave.We find that when the wavelength of the signal wave is constant,if the group velocity difference between the pump wave and the generated idler wave is small,the pump acceptance bandwidth is broadband.When the wavelength of the pump wave is constant,if the group velocity difference between the signal wave and the generated idler wave is small,the signal phase-matching bandwidth is broad.Second,we calculate the group velocity versus wavelength in the PPLN.When the pump wavelength is 1050 nm and the generated idler wavelength is about 3.4μm,the group velocity difference between the pump and the idler is small,the corresponding pump acceptance bandwidth is wide.Third,we establish a fiber-laser-based DFG,in which the seed is spectrally broadened in a 6-cm-long PM980 fiber by self-phase modulation(SPM)and pulse-compressed by apair of SF57 prisms.The generated pulse train has an average power of 2 Wand a full width at half maximum(FWHM)pulse duration of 35 fs.A part of it is used as the pump wave of the DFG,and the average power of the pump wave is 720 mW.The rest is free-spaced-coupled in a10-cm-long photonics crystal fiber(PCF)with negative dispersion at 1050 nm.The output from the PCF is a supercontinuum and subsequently passes through a 3-mm-long silicon(Si)window.A soliton with an average power of 84 mW and an FWHM pulse duration of 67 fs is transmitted,which is used as the signal wave of the DFG.A couple of silver concave mirrors with a focus length of 50 mm are employed to focus both beams inside the PPLN crystal and collimate the output beams.The output beams subsequently pass through a 3-mm-long germanium window,and only the mid-infrared idler beam is transmitted.Results and Discussions When the poling periods of the 1-mm-long PPLN crystal are 30.5μm,30μm,and29.5μm,the-10 dB bandwidths of the output from a DFG based on these PPLN crystals are 2.72-4.15μm,2.77-3.97μm,and 2.82-3.82μm,respectively.The corresponding average powers are measured to be 7.5 mW,15 mW,and 13.6 mW.When the poling periods of the 3-mm-long PPLN crystal are 30.5μm,30μm,and 29.5μm,the-10 dB bandwidths of the output from a DFG based on these PPLN crystals are 2.87-4.08μm,2.77-3.9μm,and 3-3.75μm,respectively.The corresponding average powers are measured to be 23 mW,45 mW,and 39 mW.The conversion efficiency is increased with the increase of the length of the PPLN crystal,meanwhile,the bandwidth of the generated idler wave is decreased.Compressed with the traditional DFGs based on the PPLN crystal,the bandwidth of the output from our well-built DFG is wider.In the simulation,given that the pump wave and signal wave center at 1050 nm and 1525 nm,the intensities of the pump wave and the signal wave are 1 GW/cm^(2) and 0.6 GW/cm^(2),respectively.The FWHM pulse durations of the pump wave and the signal wave are 35 fs and 67 fs.The spectra are shown in Figs.7(a)and 7(b),where the poling period of the 1-mm-long PPLN crystal is 30μm.The spectrum of the idler wave generated from the DFG is shown in Fig.7(c),which is similar to that in our experiment.Conclusions By derivation,we find that when the group velocity difference between the pump wave and the generated idler wave is small,the pump acceptance bandwidth is broad.In a PPLN crystal,when the pump wavelength is 1050 nm and the generated idler wavelength is about 3.4μm,the group velocity difference between the pump wave and the idler wave is small,and thus the corresponding pump acceptance bandwidth is wide.Inaddition,we report an ultra-short-pulse-pumped DFG,by using a 1-mm-long PPLN crystal with a poling period of29.5μm,and we obtain mid-infrared sources with bandwidths covering 2.72-4.15μm and an average power of 7.5 mW.Meanwhile,as indicated in the simulation,when the central wavelengths of the pump wave and the signal wave are 1050 nm and 1525 nm,respectively,increasing the bandwidth of the pump wave can increase the bandwidth of the generated idler wave.