Research Overview 
ultralow-noise fs lasers 
and comb sources 
timing and synch for 
ultrafast science 
ultrafast, ultra-precise 
TOF sensing and imaging
photonic on-chip 
clock distributions 
optical metrology for 
semiconductor manufacturing 

Ultrafast and ultra-precise time-of-flight (TOF) sensing and imaging

Laser-based ranging and displacement measurements have been widely used in various areas of science and technology, from gravitational-wave detection to connected sensors for advanced manufacturing. The three important performance parameters for displacement measurements are precision, measurement speed and non-ambiguity range (NAR). So far, improvement in one parameter has been achieved at the expense of degradation in other parameters.

By combining a low-noise mode-locked laser (frequency comb) [1] and an electro-optic sampling-based timing detector [2,3], we are working on an ultrafast, ultra-precise and multi-functional time-of-flight (TOF) detection method [4]. Our new method can uniquely combine ultrafast measurement speed, sub-nm precision and NAR of more than several mm. Starting from 24 nm precision for 4 ns acquisition time, the precision can reach 180 pm for 5 ms acquisition time.

By changing the sensor head, various types of TOF sensor can be realized. As representative examples, we showed 3D surface profile imaging [4], step-height measurement [5], and high-precision, large-range strain sensing [4,6]. Using this method, we could also show real-time detection of single-event, fast and high-dynamic-range mechanical displacements. This capability can lead to the realization of new measurement and analysis platforms for studying broadband, transient and nonlinear mechanical dynamics in real time.

We are currently working on real-time measurement of mechanical dynamics in micro devices (such as MEMS cantilevers, bridges and membrances), rapid and precise 3D imaging of semiconductor devices, and large-scale absolute ranging and LiDAR imaging.

Related Publications
[1] J. Kim and Y. Song, "Ultralow-noise mode-locked fiber lasers and frequency combs: principles, status and applications," Adv. Opt. Photon. 8, 465-540 (2016)
[2] K. Jung and J. Kim, "Subfemtosecond synchronization of microwave oscillators with mode-locked Er-fiber lasers," Opt. Lett. 37, 2958 (2012)
[3] C. Jeon, Y. Na, B. Lee, and J. Kim, "Simple-structured, subfemtosecond-resolution optical-microwave phase detector," Opt. Lett. 43, 3997-4000 (2018)
[4] Y. Na, C. Jeon, C. Ahn, M. Hyun, D. Kwon, J. Shin, and J. Kim, "Ultrafast, sub-nanometre-precision and multifunctional time-of-flight detection," Nature Photon. 14, 355-360 (2020)
[5] X. Lu, S. Zhang, C. Jeon, C. Kang, J. Kim, and K. Shi, "Time-of-flight detection of femtosecond laser pulses for precise measurement of large microelectronic step height," Opt. Lett. 43, 1447-1450 (2018)
[6] X. Lu, S. Zhang, X. Chen, D. Kwon, C. Jeon, Z. Zhang, J. Kim, K. Shi, "Ultrasensitive, high-dynamic-range and broadband strain sensing by time-of-flight detection with femtosecond-laser frequency combs," Sci. Rep. 7, 13305 (2017)