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 

Photonic on-chip clock distribution networks (CDNs) and signal processing

Femtosecond mode-locked lasers and frequency combs can be a useful optical source for various microwave photonic applications by coherently linking optical frequencies and microwave frequencies. The application area spans wide, including ultralow-noise microwave generation [1,2], remote microwave phase transfer [3,4], frequency synthesizers for radars [5], and RF signal processing.

Our recent focus has been on-chip clock distribution networks (CDNs) based on optical frequency combs. We are currently working on a new class of low-jitter, low-skew and low-power-consumption on-chip CDN injected by the optical pulse train. In donig so, we recently identified that the excess noise in the optical-to-electronic conversion can be minimized to the tens of attoseconds regime at the rising edges of photocurrent pulses [6]. By using such low-jitter photocurrent pulses as a clock signal, we could operate a CMOS chip with low jitter and low skew and now we are optimizing the operation of this optical comb-driven chip.

Our another focus is the on-chip photonic analog-to-digital conversion (ADC) based on high-speed micro-combs [7]. Silica micro-combs can provide few-fs jitter optical pulse train with high (>20 GHz) repetition rates, which can overcome the current limitations of high-speed and high-bandwidth ADCs. We are currently developing an optimal structure for achieving both high bandwidth and high resolution in ADCs.

Related Publications
[1] K. Jung, J. Shin, and J. Kim, "Ultralow phase noise microwave generation from mode-locked Er-fiber lasers with subfemtosecond integrated timing jitter," IEEE Photonics J. 5, 5500906 (2013)
[2] K. Jung and J. Kim, "All-fibre photonic signal generator for attosecond timing and ultralow-noise microwave," Sci. Rep. 5, 16250 (2015)
[3] K. Jung, J. Shin, J. Kang, S. Hunziker, C. K. Min, and J. Kim, "Frequency comb-based microwave transfer over fiber with 7x10-19 instability using fiber-loop optical-microwave phase detectors," Opt. Lett. 39, 1577 (2014)
[4] J. Kang, J. Shin, C. Kim, K. Jung, S. Park, and J. Kim, "Few-femtosecond-resolution characterization and suppression of excess timing jitter and drift in indoor atmospheric frequency comb transfer," Opt. Express 22, 26023 (2014)
[5] J. Wei, D. Kwon, S. Zhang, S. Pan, and J. Kim, "All-fiber-photonics-based ultralow-noise agile frequency synthesizer for X-band radars," Photon. Res. 6, 12-17 (2018)
[6] M. Hyun, C. Ahn, Y. Na, H. Chung, and J. Kim, "Attosecond electronic timing with rising edges of photocurrent pulses," Nature Commun. 11, 3667 (2020)
[7] D. Jeong, D. Kwon, I. Jeon, I. H. Do, J. Kim, H. Lee, "Ultralow jitter silica microcomb," Optica 7, 1108 (2020)