Research Overview 
ultralow-noise fs lasers 
and comb sources 
microwave photonics 
timing control for ultrafast 
electron and X-ray sources 
industrial applications and 
commercialization of fs lasers 

Timing Metrology and Control for Ultrafast Electron and X-ray Sources

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 tested 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 challenging problems in 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]. This result suggests that laser-based RF phase distribution and synchronization can operate well even in harsh accelerator environment with strong vibrations and high voltage and current pulsed signals.

We have been also working 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 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 noisy commercial Ti:sapphire photocathode laser and an S-band microwave oscillator [4], which is successfully installed and operating at the KAERI-UED Facility. This simple and low-cost method may find many applications for small- to mid-scale ultrafast X-ray and electron sources as well.

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)