PhD defence by Aleksandr Razumov

Abstract

Optical frequency combs are remarkable laser-based tools capable of producing evenly spaced “teeth” of light across a broad range of colors. Their high precision and stability make them indispensable in applications such as timekeeping (e.g., atomic clocks), advanced communication networks, and sensitive environmental sensors. Despite their promise, fully characterizing the tiny fluctuations, or phase noise, across each “tooth” of a frequency comb remains a significant challenge. Traditional methods typically focus on only a few key noise sources and may overlook subtler sources that can degrade a comb’s overall performance.

In this PhD project, a novel strategy called “subspace tracking” is applied to tackle this issue. By treating the measured phase noise from multiple comb lines as a mixture of signals, the method mathematically separates them into distinct, independent sources - even if we do not know ahead of time how the noise is generated. Numerical simulations and subsequent experiments on several types of frequency combs (electro-optic, quantum dot mode-locked, Cr:ZnS, and solid-state femtosecond lasers) show that subspace tracking can successfully isolate each source’s contribution and determine exactly how the noise scales from one comb line to the next. These findings enable more accurate ways to stabilize and optimize frequency combs, potentially improving ultra-precise clocks, telecommunications, and spectroscopic measurements used in areas ranging from astrophysics to environmental monitoring. Over time, the insight drawn from subspace tracking may open new designs of lower-noise lasers and pave the way for advanced sensing systems that rely on detailed knowledge of tiny fluctuations in light.

Supervisors

• Main supervisor: Professor Darko Zibar, DTU Electro, Denmark.
• Co-supervisor: Francesco Da Ros, DTU Electro, Denmark
• Co-supervisor: Principal Research Scientist Poul Varming, NKT Photonics, Denmark.
• Co-supervisor: Principal Optical Engineer, Jens Pedersen, NKT Photonics, Denmark.

Assessment committee

• Professor Karsten Rottwitt, DTU Electro, Denmark (chair)
• Professor Jungwon Kim, Korea Advanced Institute of Technology (KAIST), South Korea
• Professor Arnaud Mussot, University of Lille, France.

Master of the Ceremony

• Michael Galili, Associate Professor, DTU Electro, Denmark.