Theodor W.Hänsch

(Max Planck Institute for Quantum Optics, Ludwig-Maximilian University, Munich, Germany)

Abstract: With the rapid development of femtosecond laser technology and nonlinear optics at the beginning of this century, optical frequency comb technology has become an important precision frequency measurement technology.It connects the radio and optical frequency realm by inching through the vast frequency gap via the coherent relationship between their separated frequencies and phases, which has a significant application value in the fields of time-keeping and frequency standards, precision spectral metrology and constants measurement in fundamental physics.

Keywords：optical frequency comb；single photon；Nobel prize in physics；precision spectroscopy

Editor’s note: This paper was organized and translated by Professor Yong Ma and Professor Guangyu Wang of Chongqing University of Posts and Telecommunications, based on the academic presentation of Professor Theodor W.Hänsch on the 70th anniversary of Chongqing University of Posts and Telecommunications in October 2020.The paper was revised and authorized by Professor Hänsch for publication in this journal.Since 2015, Chongqing University of Posts and Telecommunications has established extensive academic exchanges and cooperation with the Nobel Prize Laboratory led by Professor Hänsch.

编者按：本文由重庆邮电大学马勇教授、王光宇教授，根据西奥多·W·汉希 于2020 年 10 月重庆邮电大学建校70周年庆典的学术报告整理、翻译，并经汉希教授修订后授权本刊发表。重庆邮电大学从2015年开始和汉希教授领导的诺贝尔奖实验室建立了广泛的学术交流及合作。

Visible light has approximately 500,000 billion oscillations per second.These sampling oscilloscopes create a new type of spectroscopy where the radiation emitted by complex molecules over a broad spectral range can be analyzed.This approach is based on the laser frequency comb, invented 20 years ago[1-6].This tool measures the optical frequencies of hundreds of thousands of terahertz.Further, it provides a phase-coherent link between the optical and radio frequency regions and offers a clockwork mechanism for optical atomic clocks, where the pendulum is formed by atoms or ions oscillating with the frequency of light.

众所周知，可见光每秒钟振荡500 000亿次。光波的高速振荡采样可以实现一种新的光谱分析技术，能够在很宽的光谱范围内对复杂分子的荧光信息进行分析，其核心技术是20年前发明的光频率梳技术[1-6]。光频率梳技术可用于测量频率为104 THz的光学振荡。它提供了光学和射频区域之间的相位相干链接，并基于此技术制造光学原子钟，其“钟摆”由以光频率振荡的原子或离子形成。

A frequency comb is explained in the time domain as follows.Fig.1 shows the spectrum of a laser that emits an ultrashort pulse.A broad spectrum is observed for ultrashort pulse, i.e., the shorter the pulse, the broader the spectrum.If two identical pulses are considered, instead of one, then they interfere and produce fringes in the spectrum.Interestingly, this is comparable to Young’s double-slit experiment, where two slits separated in space produce spatial interference fringes on the screen.Here two laser pulses separated in time produce interference fringes in the spectrum.The separation between two interference maxima precisely equals the inverse time interval between the two pulses.The further apart the two pulses, the closer the fringes become, resembling a comb.Notable, two pulses only can produce a frequency comb; however, with blunt teeth.

Fig.1 Two pulses separated in time and their spectrum

时域分析可以用来阐述光频梳的性质。图1给出了激光器发射的超短脉冲的光谱。超短光脉冲的光谱很宽，且脉冲越短产生的光谱就越宽。在发射2个相同且性质一致的脉冲时，脉冲之间会产生干涉。这种现象类似于杨氏的双缝实验，杨氏实验中，光经过2个在空间上分开的狭缝后会在空间屏幕上产生干涉条纹。2个时间分开的脉冲也会在光谱上产生干涉条纹。2个干涉条纹最大值之间的频率间隔等于脉冲时间间隔的倒数。2个脉冲相距越远，干涉条纹就越相互靠近。光谱干涉条纹的形状类似于常见的梳子。2个时域脉冲就可以产生光频梳，只是其“梳齿”不是很锋利。

Nevertheless, if a whole train of regular pulses, such as produced by a mode-locked femtosecond laser, is used rather than two pulses, then an interference pattern with sharp comb lines can be produced.The longer the interval between pulses, the sharper these lines become.Comb lines as sharp as continuous-wave radiation from a well-stabilized laser can be obtained with precisely controlled timing only.Any small random fluctuations in timing or phase will disrupt the spectrum.

如果使用一整列规则脉冲，如锁模飞秒激光器产生的脉冲，而不是2个脉冲，那么就可以产生具有尖锐梳状线的干涉图案。……