快速傅里叶变换在阿秒束线光路稳定控制中的应用

Phase control and stabilization in attosecond beamline with fast Fourier transform

本文报道了将快速傅里叶变换算法应用于抽运探测系统和倍频光谱干涉系统(f-2f interferometry),对光路进行反馈控制的原理和结果,分别得到相对臂长抖动均方根1.24 nm(对应时间为4.1 as)的抽运-探测光路锁定和积分时间3 ms下相对相移均方根227 mrad的慢环载波包络相位(carrier envelop phase,CEP)锁定.这样的锁定精度可以保证产生阿秒脉冲的飞秒激光脉冲拥有稳定的CEP,并且为后续阿秒抽运探测提供了稳定的实验条件.

With the unveiling of molecular and atomic dynamics,scientists crave finer and faster tools tocommunicate with the microworld.Attosecond pump-probe enjoys its reputation as the fastest camera,hintingultrafast movements in the delay graph.To employ this camera,the stability and delay control should havevery great accuracy comparable to the camera resolution.It is also of significant importance for stabilizing thecarrier envelope phase(CEP)in few-cycle laser field.When dealing with a huge quantity of data,conventionalFourier transform algorism is challenging in high-speed control.Here we put forward the efficient calculationmethod,fast Fourier transform(FFT)algorism in Mach-Zehnder interferometer for arm length locking and f-2ffor CEP locking.In the interferometer locking,532 nm continuous wave laser is used in the Mach-Zehnderinterferometer,and the phase of the FFT term corresponding to the delay between the two arms of theinterferometer serves as a feedback signal on piezo transducer(PZT)in the delay line to reduce the change ofthe arm length.In the CEP control experiment,data to be analyzed are the f-2f spectrum interference fringesrecorded by the spectrometer.The CEP values are obtained from the first order of FFT module output of theintegrated spectrum interference fringes,and a labview program examines the relative phase drift and sends afeedback voltage signal to the PZT through the proportion integration differentiation module to compensateslow CEP drift after the chirped pulse amplification system by changing the insert length of a prism pair.Theresults show that the root mean square(RMS)of the arm length difference is 1.24 nm(4.1 attosecond for lightto travel)per meter in the interferometer locking over 12 h,and the RMS of CEP is 227 mrad under 3 msintegration time in the CEP locking over 20 min.These results are able to meet the requirement of the accuracyfor attosecond pulse generation and attosecond pump-probe experiments.We also use FFT to stabilize the CEPand relative time simultaneously in the waveform synthesis for 8 h(Huang P,Fang S,Gao Y,Zhao K,Hou X,Wei Z 2019 Appl.Phys.Lett.115 031102),the phase-locking system results in a CEP stability of 280 mrad anda relative time stability of 110 as at a repetition rate of 1 kHz.These results imply that the FFT is versatileand reliable in ultrafast control.