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图1为同步脉冲诱导的中红外差频产生实验装置图。整个系统主要分成三个部分:高功率泵浦脉冲制备、同步诱导脉冲制备以及高效非线性差频产生。图中,FBG:光纤布拉格光栅,YSF:掺镱单模光纤,OC:光纤耦合器,WDM:波分复用器,PS:移相器,LD:激光二极管,ISO: 隔离器,Combiner:合束器,PCF:光子晶体光纤,Col:准直器,PD:光电探测器,NIR CW laser:近红外连续激光器,EOM:电光调制器,EDFA:掺铒光纤放大器,DM:二向色镜,RM:反射镜,Delay:延时器,Lens 1:消色差透镜;PPLN:周期极化铌酸锂晶体,Lens 2:CaF2平凸透镜,Block:光束终止器,LP:长通滤波镜,Power meter:光功率计。泵浦脉冲源基于掺镱光纤激光器,可以通过非线性放大环形镜机制实现被动锁模。其基本原理在于谐振腔内的Sagnac干涉环起到了可饱和吸收体的作用,由于脉冲正反两向运行积累的相位不同,在特定的条件下只有脉冲中心部分才能低损耗透过,经过多次谐振后获得脉冲的窄化,从而实现超短脉冲的产生。一般来说,这种“9”字腔型的泵浦阈值较高,需要足够的增益才能引入所需的非线性相移。为了降低激光器锁模泵浦阈值,笔者在谐振腔内插入了一个光纤集成的相移器,从而提供π/2的非对易线性相移,有助于获得自启动锁模,并提升输出脉冲的稳定性。此外,整个腔体和所有器件均采用全保偏光纤结构,能够显著提高激光器的偏振稳定性、环境适应性和长期可靠性。在实验中,笔者将LD的功率增加至200 mW,即可获得稳定的锁模脉冲输出,通过激光腔内放置的光纤耦合器,在5%输出端口测量的平均功率为5 mW,脉冲重复频率为21.6 MHz,中心波长为1030.3 nm,光谱半高全宽为0.08 nm。振荡级输出脉冲能量较小,难以满足高效非线性频率转换所需的强泵浦光场。为此,采用级联光纤放大技术大幅提升脉冲功率,先用一级单模光纤预放大器将振荡器输出功率提升至190 mW,后经大模场光子晶体光纤主放大器,输出的平均功率最高可达14 W,脉冲宽度为8.5 ps,对应的单脉冲能量约0.6 μJ,脉冲峰值功率约80 kW,为后续研究中红外差频转换性能对强泵浦光场的依赖关系奠定了基础。主放大器中选用大模场PCF(NKT Photonics,DC-135/14-PM-YB)作为增益光纤。其纤芯直径为14 μm,模场直径约15 μm,内包层直径135 μm,对976 nm泵浦吸收系数为7 dB/m。较高的吸收效率和较大的模场直径可以保障种子光的高效放大,同时有效抑制功率提升中的非线性畸变。此外,通过实现光子晶体增益光纤和合束器尾纤(PM-DCF-10/125)的高效熔接耦合,极大简化了光子晶体增益光纤放大中泵浦光和信号光的耦合,极大提高放大器系统的紧凑性、鲁棒性和稳定性。在光子晶体增益光纤的另一端,为提高系统损失阈值并有效抑制端面反射,制作了斜角输出端帽。实验中用到的光子晶体增益光纤长度约1.5 m。在对信号光进行功率放大的过程中,光谱随之展宽,见图2(a),这是一种典型的自相似脉冲放大过程[27]。光谱展宽的原因主要是高峰值功率导致的自相位调制效应。此外,经优化放大器光纤长度,色散的影响尚未凸显,脉冲宽度无明显展宽。输出功率6.2 W时,光谱带宽约为5 nm,脉冲宽度8.5 ps(见图2(b)),对应的时间带宽积约为12.0。进一步地,对放大脉冲进行色散管理和脉冲压缩,可以获得飞秒量级的泵浦激光脉冲。为避免后续实验中中红外波段非线性晶体损坏,实验中用到的1030 nm波段泵浦光输出平均功率最高为6.2 W。
图 1 同步脉冲诱导的中红外差频产生实验装置图。泵浦光源来自于自制的掺镱光纤激光器,利用非线性放大环形镜被动锁模机制能够实现全保偏的超短脉冲输出,然后利用级联的光纤放大器提升脉冲能量。小部分的放大泵浦脉冲通过高速光电探测器产生同步的电脉冲信号,从而驱动电光调制器,实现通讯波段连续激光器的强度调制。最后,时间同步的泵浦与诱导脉冲合束聚焦进入非线性晶体,通过非线性差频过程获得中红外超短脉冲制备
Figure 1. Experimental setup for the mid-infrared difference-frequency generation based on the synchronous pulse induction. The pump source was provided by an Yb-doped fiber laser, which could deliver all-polarization-maintaining ultrashort pulses due to the passively mode-locked mechanism based on the nonlinear amplifying loop mirror. The pulse energy was improved using cascaded fiber amplifiers. A small portion of the output was detected by a fast photodiode to generate the synchronous electrical signal, which drove the electro-optic modulator to implement the intensity modulation on a continuous-wave laser at the telecom wavelength. Finally, the temporally synchronized pump and induction pulses were combined and then focused into the nonlinear crystal, thus leading the generation of the MIR ultrashort pulses via the nonlinear DFG
图 2 (a) 不同平均功率下掺镱光纤激光器的输出光谱;(b) 平均功率为6.2 W时泵浦脉冲的自相关迹。(c) 连续激光器的输出光谱,可在1527~1565 nm间调谐;(d) 经高速强度调制器产生的诱导脉冲时域包络;(e) 不同泵浦光平均功率下产生的中红外激光光谱,诱导光的平均功率保持在500 mW;(f) 非线性差频过程中泵浦光与诱导光脉冲之间的互相关迹
Figure 2. (a) Output spectra for the Yb-doped fiber laser at various average powers; (b) Autocorrelation trace for the pump pulse at the average power of 6.2 W; (c) Output spectra of continuous-wave laser with a tuning rage of 1527-1565 nm; (d) Temporal envelope of the generated induction pulse from the fast intensity modulator; (e) MIR laser spectra for different pump average powers, the power of the induction pulse was fixed at 500 mW; (f) Cross-correlation trace between the pump and induction pulses via nonlinear difference-frequency generation
下面,笔者将通过“光-电-光”高速转换实现同步诱导脉冲的制备。具体来说,放大后的泵浦光场首先经过一个反射率极低的镜片M(R<0.1%),这对于泵浦功率几乎没有影响。反射光功率大致在毫瓦量级,耦合进入带光纤输入的InGaAs高速光电探测器(Discovery Semiconductors, DSC40S),其3dB带宽为16 GHz,能够产生约70 ps的高速电脉冲信号。然后该电信号被用于驱动高速电光强度调制器(科扬光电, KY-MU-15-20),其3dB带宽为20 GHz,且含有自动偏差控制模块,能够通过实时调整直流偏置电压以保持正确的设定点,从而实现输出功率的长期稳定。该调制器用于调制近红外窄带连续激光器(LaseGen, LTL-1500),最终可以产生与泵浦光脉冲时域同步的光信号。图2(c)为调制光脉冲的光谱,其中心波长在1527~1565 nm范围内可调节。为了测量其脉冲宽度,采用高速探测器(Finisar, XPDV2120RA,50 GHz带宽)及高速示波器(Keysight, DSAV334A, 33 GHz分辨率带宽,80 GS/s采样率),捕获的时域包络如图2(d)所示,其半高全宽为62.5 ps。受限于较小的占空比,连续激光器调制后的光功率仅为3 μW,利用多级掺铒光纤放大器可将平均激光提升至0.5 W,作为后续非线性差频的诱导光。至此,笔者完成了自同步的双色脉冲光源制备。相对于主动同步,该方案无需对激光器的重频频率进行精确锁定,规避了复杂的宽带反馈控制系统。相较于此前基于电调制双色同步脉冲系统[26],该方案结合了超短脉冲锁模脉冲与高速同步调制,差频产生的脉冲宽度取决于泵浦光场,为制备皮秒以下的超快中红外光场提供了可能。值得一提的是,电调制同步技术对于外部扰动具有很强的免疫,且诱导脉冲较宽,降低了双色脉冲相对时间抖动的控制要求,为产生长时稳定的中红外超短光源奠定了基础。
最后,笔者基于上述制备的双色同步脉冲搭建了非线性差频转换系统。泵浦与诱导脉冲通过二向色镜获得空间重合,然后经过焦距为75 mm的消色差平凸透镜(Thorlabs, AC254-075-C)聚焦进入周期性极化的铌酸锂(PPLN)晶体。非线性晶体的厚度和长度分别为1 、25 mm,反转周期为30.3 μm。该晶体两端面均镀有1030~1080 nm、1380~1800 nm、2400~4500 nm三个相关波段的增透膜。PPLN晶体被安装在温控炉(HCP, TC038-PC)中,工作温度控制在(44.8±0.1) ℃。为了优化非线性转换效率,双色脉冲之间的时域重叠需通过延迟器精确调节。产生的中红外信号由焦距为75 mm的CaF2平凸透镜准直,并通过二向色镜及2.4 μm长通滤波镜进行频谱滤波,整体的透过率为79.2%,对泵浦光波长的抑制比为70 dB。图2(e)展示了不同泵浦功率下中红外激光的光谱变换,中心波长为3073 nm,对应了诱导脉冲的中心波长为1550 nm。光栅光谱仪(APE, Wavescan USB)的分辨率设置为3 nm。可见,泵浦光随输出功率增大其光谱出现展宽,所获得的中红外光谱也随之加宽。由于诱导光的光谱很窄(< 0.02 nm),中红外光谱的宽度主要取决于泵浦光,两者光谱宽度之比具有中心波长之比的平方关系。如在泵浦光为6.2 W时,泵浦光的光谱宽度为5 nm,中红外的光谱宽度为45 nm。由于非线性差频泵浦光随输出功率增大,光谱出现展宽,所获得的中红外激光受相位匹配影响其光谱也出现一定的展宽。图2(f) 给出了非线性差频过程中泵浦光与诱导光脉冲之间的交叉相关曲线,其半峰全宽为63.1 ps,满足双色脉冲的卷积关系,即
$$ {\mathrm{\tau }}_{\mathrm{c}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{s}}=\sqrt{{\mathrm{\tau }}_{\rm s}^{2}+{\mathrm{\tau }}_{\rm p}^{2}} $$ (1) 式中:τs和τp为诱导光和泵浦光的脉冲宽度,分别为8.5 ps和62.5 ps。因为泵浦光脉冲宽度远小于诱导光,互相关迹宽度以及产生的中红外脉冲宽度主要由泵浦光决定。理论上,通过制备百飞秒水平的泵浦光场,可以获得更短的中红外脉冲宽度,不受诱导光脉冲宽度的限制。
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首先探究了中红外平均功率和转换效率随泵浦强度的变化规律。实验中,诱导光功率设定为500 mW的条件下,中红外的输出功率由热功率计(Thorlabs,S401C)测定。如图3(a)所示,中红外输出功率随泵浦光功率的增加而单调增加,最大输出平均功率约为1.2 W。对应的单脉冲能量为55.6 nJ,峰值功率约为6.6 kW。非线性转换的量子效率可以定义为产生的总功率(包括中红外产生功率及诱导光放大功率)与入射泵浦光功率的比值:
图 3 (a) 中红外平均功率和转换效率随泵浦光功率的变化;(b) 中红外输出功率的长期稳定性,σ为相对抖动
Figure 3. (a) MIR average power and conversion efficiency as a function of the pump power; (b) Long-term stability for the MIR output power, σ denotes the relative fluctuation
$$ \mathrm{\eta }=\frac{{{P}}_{\mathrm{M}\mathrm{I}\mathrm{R}}+{{P}}_{\mathrm{s}\mathrm{i}\mathrm{g}\mathrm{n}\mathrm{a}\mathrm{l}}}{{{P}}_{\mathrm{p}\mathrm{u}\mathrm{m}\mathrm{p}}}=\frac{{{P}}_{\mathrm{M}\mathrm{I}\mathrm{R}}}{{{P}}_{\mathrm{p}\mathrm{u}\mathrm{m}\mathrm{p}}}\times \frac{{\mathrm{\lambda }}_{\mathrm{M}\mathrm{I}\mathrm{R}}}{{\mathrm{\lambda }}_{\mathrm{p}\mathrm{u}\mathrm{m}\mathrm{p}}} $$ (2) 其中利用到Manley-Rowe光子数守恒关系Psignal/PMIR=λMIR/λsignal,以及能量守恒定律1/λpump=1/λsignal+1/λMIR。结果表明,在输入泵浦光为2.0 W时,对应的最高转换效率达到60.0%,之后随泵浦光功率增加而略微减小。这是由于高泵浦功率下部分中红外光与种子光发生了不可忽略的非线性和频反向转换,该现象此前实验中亦有报道[25]。理论上,超短脉冲之间的二阶非线性相互作用过程可以通过瞬态耦合波方程来描述。在实验中,相互作用的脉冲宽度皆为10 ps量级,可以基本忽略光波群速度间的失配和光波包本身的扩散等因素的影响。此时,非线性转换效率主要受限于泵浦脉冲与种子脉冲的时-频域特性。特别是,随着泵浦光功率的提升,其光谱宽度在不断增大,受相位匹配带宽的限制,导致转换效率有所下降。因此,选用更短的非线性晶体有望获得更高的泵浦转换效率,但对泵浦功率也将提出更高的要求。受限于晶体损伤阈值(通常为0.1~1 GW/cm2)及高功率泵浦下不可忽视的热效应,实验中泵浦光功率被限制在6.2 W,防止损伤非线性晶体。经消色差透镜聚焦,泵浦光在晶体中的束腰半径约为50 μm,计算可知此时的峰值强度为0.43 GW/cm2。得益于脉冲产生和放大全链路的保偏光纤结构,结合高稳定的电脉冲调制技术,所获得的双色同步光源具备优异的功率稳定性。同时,诱导光脉冲宽度远大于泵浦光,对于相对时间抖动的容忍度较大,能够有效克服温度变化和声音震动引起的环境扰动。如图3(b)所示,瓦量级的中红外平均功率能够稳定维持至少1 h,相对抖动仅为0.07%。所述平均功率的相对抖动量表征了功率的不稳定度,其定义为一组测量功率值的标准方差(STD)与平均值(MEAN)的比值。实验中,光功率的采样时间为1 s,总采样时长为3600 s。
随后,笔者研究了不同诱导脉冲功率下,中红外功率随着泵浦强度的变化。如图4(a)所示,在不注入种子光的情况下,泵浦功率需要达到4.5 W,才能明显观察到非线性差频信号,即此时泵浦功率达到自发参量下转换阈值。继续增加泵浦功率至6.2 W时,产生的中红外功率仅为300 mW左右。相比之下,在有诱导脉冲注入时,能够明显降低泵光谱光阈值,提升中红外产生功率。例如,当种子光注入功率低至1 mW,泵浦功率在1.5 W时即可明显观察到中红外参量产生,即将泵浦阈值从4.5 W降低至1.5 W。而当种子光注入功率为500 mW时,非线性差频泵浦阈值继续降低至0.5 W。因此,同步脉冲诱导注入能够放宽中红外参量产生对高功率泵浦场的苛刻要求,同时降低非线性晶体的损伤风险。图4(b)研究了不同泵浦强度下,中红外产生效果与诱导光功率的关系。可以发现,越高的泵浦功率,种子脉冲的诱导作用越明显。在不同的泵浦功率条件下,产生的中红外功率与注入的种子光功率基本呈现出前期快速增长与后期逐渐饱和的趋势,显示了种子光注入对于诱导光参量过程的显著效果。在低功率种子光注入时,通过光学参量产生(OPG)可以获得很高的增益,从而加速了中红外光场的产生。当注入种子光功率达到一定程度时,OPG增益会显著降低,此时产生的中红外功率也会随之趋向饱和。在泵浦光功率为2.4 W时,仅需50 μW种子光便可产生50 mW的中红外激光;在6.2 W泵浦功率下,低至纳瓦量级的种子光功率即能产生明显的诱导作用。相对于此前报道中利用连续激光作为诱导光场[28],这里使用的同步脉冲诱导方案能够显著提升种子光的能量利用率,且在相同平均功率下具有更高的峰值功率,能够展现更优的诱导效果。
图 4 (a) 不同种子光功率下,中红外功率随泵浦强度的变化;(b) 不同泵浦功率下,中红外功率与种子光强度的关系
Figure 4. (a) MIR power as a function of the pump intensity for various seed pulse powers; (b) Dependence of the MIR power on the seed pulse intensity at various pump powers
最后,通过对近红外窄带激光器设置不同的波长以及对周期性极化的铌酸锂晶体设置对应的准相位匹配温度,实现了中心波长在3000~3175 nm范围内可调谐的中红外光源产生。如图5所示,近红外窄带激光器设置波长分别为1530、1535、1540、1545、1550、1555、1560、1560 nm时,在1 W泵浦光下经非线性差频产生的中红外光源对应中心波长为3154、3133、3113、3092、3073、3053、3034、3016 nm。若使近红外窄带激光器设置波长在1527~1565 nm范围内进行扫描,并将单一极化周期的PPLN晶体更换为啁啾PPLN晶体,即可产生中心波长在3000~3175 nm(94.4~99.9 THz)范围内不断扫描的中红外超短脉冲光源,通常该类商售近红外窄带激光器波长扫描全程时间约为1 s,即中红外光源光谱扫描速率为5.5 THz/s。此外,商售近红外激光器波长已能完全覆盖从O~L波段,波长范围可覆盖1300~1675 nm,则产生的中红外光源波长将拓展到2.7~5 μm,后续可广泛应用于激光光谱学、气体分析、肿瘤细胞检测等重要领域。
High-power mid-infrared difference-frequency generation based on synchronous pulse induction (Invited)
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摘要: 提出并实验探究了基于同步脉冲诱导的中红外差频产生技术,利用高速光电探测器将泵浦光脉冲转换为超短电信号,使其驱动宽带的幅度调制器,作用于可调谐连续激光器上,从而实现双色脉冲的稳定时域同步。利用了同步脉冲诱导的非线性差频过程,有效降低了光参量下转换的泵浦阈值,能够获得瓦量级的中红外超短脉冲输出,最大泵浦光转换效率达60%,且中心波长在3000~3175 nm范围内可调谐。得益于全保偏光纤架构,平均功率的不稳定度(STD/MEAN)在1 h内低至0.07%,展现了优异的长期稳定性。此外,该方案利用光-电-光高速调制实现高精度脉冲同步,免除了复杂的反馈电路,具有结构简单、即插即用、鲁棒性强的特点,为拓展中红外光源在野外的应用奠定了基础。Abstract: A mid-infrared (MIR) difference-frequency generation(DFG) technique based on the synchronous pulse induction was proposed and implemented, where a high-speed photodiode detector was used to convert the pump optical pulse into an ultrashort electrical signal for driving a high-bandwidth amplitude modulator on a tunable continuous-wave laser, thus leading to the stable timing synchronization between the dual-color pulses. The nonlinear DFG process with synchronous pulse induction was utilized to effectively reduce the pump threshold for the optical parametric down-conversion. Consequently, watt-level MIR ultrashort pulses could be generated with a maximum pump conversion efficiency up to 60%. The central wavelength could be tuned from 3000 to 3175 nm. Thanks to the all-polarization-maintaining fiber architecture, the instability(STD/MEAN) of the average power was as low as 0.07% in one hour, exhibiting a superior long-term stability. Additionally, the optical-electrical-optical conversion was used to implement the high-precision pulse synchronization with the presented scheme, which eliminated the requirement for the complex feedback circuit. The system was thus featured with simple structure, plug-in operation, and strong robustness, which would pave the way to promoting the MIR light source in field applications.
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图 1 同步脉冲诱导的中红外差频产生实验装置图。泵浦光源来自于自制的掺镱光纤激光器,利用非线性放大环形镜被动锁模机制能够实现全保偏的超短脉冲输出,然后利用级联的光纤放大器提升脉冲能量。小部分的放大泵浦脉冲通过高速光电探测器产生同步的电脉冲信号,从而驱动电光调制器,实现通讯波段连续激光器的强度调制。最后,时间同步的泵浦与诱导脉冲合束聚焦进入非线性晶体,通过非线性差频过程获得中红外超短脉冲制备
Figure 1. Experimental setup for the mid-infrared difference-frequency generation based on the synchronous pulse induction. The pump source was provided by an Yb-doped fiber laser, which could deliver all-polarization-maintaining ultrashort pulses due to the passively mode-locked mechanism based on the nonlinear amplifying loop mirror. The pulse energy was improved using cascaded fiber amplifiers. A small portion of the output was detected by a fast photodiode to generate the synchronous electrical signal, which drove the electro-optic modulator to implement the intensity modulation on a continuous-wave laser at the telecom wavelength. Finally, the temporally synchronized pump and induction pulses were combined and then focused into the nonlinear crystal, thus leading the generation of the MIR ultrashort pulses via the nonlinear DFG
图 2 (a) 不同平均功率下掺镱光纤激光器的输出光谱;(b) 平均功率为6.2 W时泵浦脉冲的自相关迹。(c) 连续激光器的输出光谱,可在1527~1565 nm间调谐;(d) 经高速强度调制器产生的诱导脉冲时域包络;(e) 不同泵浦光平均功率下产生的中红外激光光谱,诱导光的平均功率保持在500 mW;(f) 非线性差频过程中泵浦光与诱导光脉冲之间的互相关迹
Figure 2. (a) Output spectra for the Yb-doped fiber laser at various average powers; (b) Autocorrelation trace for the pump pulse at the average power of 6.2 W; (c) Output spectra of continuous-wave laser with a tuning rage of 1527-1565 nm; (d) Temporal envelope of the generated induction pulse from the fast intensity modulator; (e) MIR laser spectra for different pump average powers, the power of the induction pulse was fixed at 500 mW; (f) Cross-correlation trace between the pump and induction pulses via nonlinear difference-frequency generation
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