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 

Low-noise and integrated ultrafast lasers, frequency combs, and frequency snythesizers

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 (ADCs), clock distribution networks (CDNs), RADARs and LIDARs, time-of-flight (TOF) sensors, and signal analysis instruments, to name a few [1].

Using extremely sensitive timing detection method and understanding the impacts of pulse dynamics on timing jitter in fiber lasers [2], we showed that passively mode-locked fiber lasers can 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 [3,4]: 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 [5] and intra-cavity filtering [6,7] can be used for the reduction of timing jitter to the sub-femtosecond regime. We also worked on more compact, more robust and lower-noise comb sources for more widespread applications of such lasers outside laboratory environment. We demonstrated an all-polarization maintaining (PM) Er-fiber laser using multi-functional planar lightwave circuit (PLC) device [8] and a nonlinear amplifying loop mirror (NALM) with bandpass filter [9].

We are currently working on noise characterization and optimization of microresonator-based Kerr frequency combs (micro-combs). While mode-locked fiber lasers can achieve sub-fs jitter, its repetition rates are limited to sub-GHz range. Micro-combs can achieve tens GHz repetition rates, facilitating their use for telecommunications and signal processing. We recently showed that 2.6-fs jitter, 22-GHz repetition rate soliton pulse train can be generated from a chip-scale silica micro-comb [10]. We are currently developing frequency-stabilized compact micro-comb sources and applying them for microwave synthesizers and photonic ADCs.

Another topic we are currently working on is all-fiber stabilization of frequency combs [11,12,13]. Multiple combs could be simultaneously stabilized to 10-15 level instability with a compactly packaged 1-km-long telecommunication fiber spool with a diameter of only a few cm. The demonstrated method constitutes a mechanically robust and reconfigurable tool for generating multiple ultra-stable combs that are highly suitable for various high-precision field applications, including molecular spectroscopy, photonics-based RADARs and LIDARs, high-stability microwave generators for 5G and 6G telecommunication systems, and compact frequency stabilizer for atomic reference-based quantum sensors.

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] Y. Song, K. Jung, and J. Kim, "Impact of pulse dynamics on timing jitter in mode-locked fiber lasers," Opt. Lett. 36, 1761 (2011)
[3] 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)
[4] 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)
[5] 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)
[6] 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)
[7] 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)
[8] 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)
[9] 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)
[10] D. Jeong, D. Kwon, I. Jeon, I. H. Do, J. Kim, H. Lee, "Ultralow jitter silica microcomb," Optica 7, 1108 (2020)
[11] K. Jung and J. Kim, "All-fibre photonic signal generator for attosecond timing and ultralow-noise microwave," Sci. Rep. 5, 16250 (2015)
[12] D. Kwon and J. Kim, "All-fiber interferometer-based repetition-rate stabilization of mode-locked lasers to 10-14-level frequency instability and 1-fs-level jitter over 1-s," Opt. Lett. 42, 5186 (2017)
[13] D. Kwon, I. Jeon, W. Lee, M. Heo, and J. Kim, "Generation of multiple ultra-stable optical frequency combs from an all-fiber photonic platform," Science Advances 6, eaax4457 (2020)