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 

Engineering Ultralow-Noise Femtosecond Lasers and Comb Sources

The availability of signal sources with extremely low timing jitter and phase noise enables new scientific and industrial applications requiring ultrahigh time-frequency precision, including novel instrumentation for advanced ultrafast X-ray sources, analog-to-digital converters, clock distribution and communication networks, radars and lidars, and signal analysis instruments.

Using extremely sensitive timing detection method and understanding the impacts of pulse dynamics on timing jitter in fiber lasers [1], we showed that passively mode-locked fiber lasers can, in fact, generate extremely low-jitter optical pulse trains that were not possible before. We were the first to show sub-100-attosecond timing jitter from mode-locked Er-fiber and Yb-fiber lasers [2,3]: this result was also featured in Research Highlights of February 2012 issue of Nature Photonics. We further investigated the reduction of timing jitter in various types of fiber lasers, and identified that dispersion engineering [4] and intra-cavity filtering [5,6] can be used for the reduction of timing jitter to the sub-femtosecond regime. We have also invented ultrahigh-sensitive and accurate measurement methods to characterize timing jitter of free-running mode-locked lasers and frequency combs [7,8]. In particular, we recently demonstrated a reference-free and ultrahigh-resolution timing jitter measurement method using a long fiber delay line [8], which can be very useful for characterizing the timing jitter of supercontinua and Kerr comb sources.

Our recent focus is on more robust and more compact low-noise comb sources for more widespread applications of such low-noise lasers outside laboratory environment. One direction is all-polarization-maintaining (all-PM) fiber lasers. We recently demonstrated a robust, compact, and low-noise all-PM Er-fiber laser using multi-functional planar lightwave circuit (PLC) device [9]. We are also investigating the noise reduction in all-PM nonlinear amplifying loop mirror (NALM)-based fiber lasers by proper spectral filtering [10]. Another direction is optimizing the timing noise in Kerr-frequency comb sources, which have high potential for integrated photonic signal processing and communication systems.

Our works on low-noise mode-locked lasers have been widely recognized on international level. In the last few years, we gave several invited and tutorial talks on ultralow-noise mode-locked lasers at high-quality international conferences including Conference on Lasers and Electro-Optics (CLEO) 2015, IEEE Photonics Conference (IPC) 2016, and Conference on Lasers and Electro-Optics Pacific-Rim (CLEO-PR) 2015. Recently, we also published an invited review article on low-noise mode-locked fiber lasers for OSA's prestigious review journal Advances in Optics and Photonics [11], which underlines our leading position in low-noise mode-locked lasers and their applications.

Related Publications
[1] Y. Song, K. Jung, and J. Kim, "Impact of pulse dynamics on timing jitter in mode-locked fiber lasers," Opt. Lett. 36, 1761 (2011)
[2] Y. Song, C. Kim, K. Jung, H. Kim, and J. Kim, "Timing jitter optimization of mode-locked Yb-fiber lasers toward the attosecond regime," Opt. Express 19, 14518 (2011)
[3] T. K. Kim, Y. Song, K. Jung, C. Kim, H. Kim, C. H. Nam, and J. Kim, "Sub-100-as timing jitter optical pulse trains from mode-locked Er-fiber lasers," Opt. Lett. 36, 4443 (2011)
[4] C. Kim, S. Bae, K. Kieu, and J. Kim, "Sub-femtosecond timing jitter, all-fiber, CNT-mode-locked Er-laser at telecom wavelength," Opt. Express 21, 26533 (2013)
[5] P. Qin, Y. Song, H. Kim, J. Shin, D. Kwon, M. Hu, C. Wang, J. Kim, "Reduction of timing jitter and intensity noise in normal-dispersion passively mode-locked fiber lasers by narrow bandpass filtering," Opt. Express 22, 28276 (2014)
[6] W. Chen, Y. Song, K. Jung, M. Hu, C. Wang, and J. Kim, "Few-femtosecond timing jitter from a picosecond all-polarization-maintining Yb-fiber laser," Opt. Express 24, 1347 (2016)
[7] K. Jung and J. Kim, "Characterization of timing jitter spectra in free-running mode-locked lasers with 340 dB dynamic range over 10 decades of Fourier frequency," Opt. Lett. 40, 316 (2015)
[8] D. Kwon, C. Jeon, J. Shin, M. Heo, S. Park, Y. Song, and J. Kim, "Reference-free, high-resolution measurement method of timing jitter spectra of optical frequency combs," Sci. Rep. 7, 40917 (2017)
[9] C. Kim, D. Kwon, D. Kim, S. Choi, S. Cha, K. Choi, D. Yeom, F. Rotermund, and J. Kim, "Robust, low-noise, polarization-maintaining mode-locked Er-fiber laser with a planar lightwave circuit (PLC) device as a multi-functional element," Opt. Lett. 42, 1472 (2017)
[10] D. Kim, S. Zhang, D. Kwon, R. Liao, Y. Cui, Z. Zhang, Y. Song, and J. Kim, "Intensity noise suppression in mode-locked fiber lasers by double optical bandpass filtering," Opt. Lett. 42, 4095 (2017)
[11] 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)