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 

Timing and synchronization for ultrafast science facilities

Ultrafast electron-based coherent radiation sources, such as X-ray free-electron lasers (XFELs) and ultrafast electron diffraction (UED) sources, are becoming more important sources in today's ultrafast science. To fully exploit the potentials of these sources, especially for pump-probe experiments with atomic spatial resolution and femtosecond temporal resolution, it is important to achieve high-precision timing synchronization between various mode-locked lasers and microwave sources up to kilometers length scale. During his PhD study at MIT, Prof. Kim led the first demonstration of long-term-stable, sub-10-femtosecond-precision remote synchronization of mode-locked lasers and microwave sources [1]. Through technology transfer, the demonstrated timing synchronization system is now being actively installed and used in large-scale accelerator facilities, including FERMI@Elettra (Trieste, Italy), FLASH (Hamburg, Germany), PSI-XFEL (Villigen, Switzerland), and European-XFEL (Hamburg, Germany).

Since Prof. Kim joined KAIST, we keep working on timing metrology and control for FEL and UED facilities around the world. The first project was the design and implementation of high-stability S-band RF signal distribution system for the Pohang Accelerator Laboratory (PAL). We showed sub-10-fs-stability remote laser-RF synchronization over 1-km fiber links in the PAL Injector Test Facility (PAL-ITF), which was the first demonstration of mode-locked laser-based, femtosecond-stability remote RF distribution in the klystron gallery [2].

More recently, we have worked on timing metrology and control between Ti:sapphire photocathode lasers and microwave sources for UED facilities. We collaborated with the Ultrafast Electron Imaging Group of the Max-Planck Institute of Quantum Optics (MPQ) (Graching, Germany) for demonstrating stable laser-electron synchronization for single-electron diffraction apparatus. Our FLOM-PD technology was employed for accurate timing metrology between a 5-MHz Ti:sapphire laser and a 6.2-GHz microwave signal, which served as an important sub-system for few-femtosecond laser-electron timing maintained over hours [3]. We are also working on precise laser-microwave timing control for the RF photogun in the MeV UED Facility of the Korea Atomic Energy Research Institute (KAERI). We demonstrated a modular method that achieves long-term (>1 day) stable 10-fs-level synchronization between a Ti:sapphire photocathode laser and an S-band microwave oscillator [4], which is successfully installed and operating at the KAERI-UED Facility [5]. We are currently working on drift-free laser-electron synchronization using THz streak camera-based active control [6].

Related Publications
[1] J. Kim, J. A. Cox, J. Chen, and F. X. Kaertner, "Drift-free femtosecond timing synchronization of remote optical and microwave sources," Nature Photon. 2, 733 (2008)
[2] K. Jung, J. Lim, J. Shin, H. Yang, L. Chen, F. X. Kaertner, H. Kang, C. Min, and J. Kim, "Remote Laser-Microwave Synchronization Over Kilometer-Scale Fiber Link With Few-Femtosecond Drift," J. Lightwave Tech. 32, 3742 (2014)
[3] M. Walbran, A. Gliserin, K. Jung, J. Kim, and P. Baum, "5-fs laser-electron synchronization for pump-probe crystallography and diffraction," Phys. Rev. Appl. 4, 044013 (2015)
[4] H. Yang, B. Han, J. Shin, D. Hou, H. Chung, I. Baek, Y. Jeong, and J. Kim, "10-fs-level synchronization of Ti:sapphire photocathode laser with S-band RF-oscillator for ultrafast electron and X-ray sources," Sci. Rep. 7, 39966 (2017)
[5] H. Kim, N. Vinokurov, I. Baek, K. Oang, M. Kim, Y. Kim, K. Jang, K. Lee, S. Park, S. Park, J. Shin, J. Kim, F. Rotermund, S. Cho, T. Feurer, and Y. Jeong, "Towards jitter-free ultrafast electron diffraction technology," Nature Photon. 14, 245-249 (2020)
[6] J. Shin, H. Kim, S. Park, H. Bark, I. Baek, K. Oang, K. Jang, K. Lee, F. Rotermund, Y. Jeong, and J. Kim, "Sub-10-fs timing for ultrafast electron diffraction with THz-driven streak camera," Laser Photon. Rev. 15, 2000326 (2021)