PhD defence by Anamika Nair Karunakaran

Abstract

Microresonator-based frequency combs have emerged as a promising technology with diverse applications in fields such as metrology, spectroscopy, and communication systems. This thesis investigates comb generation in silicon nitride microresonators, focusing primarily on minimising noise in the comb lines and enhancing overall performance and stability.

The investigation begins by exploring novel methodologies to reduce the noise inherent in comb generation processes. The thesis addresses a critical aspect of noise reduction by utilising a lownoise pump laser to minimise frequency fluctuations of comb lines attributed to pump laser noise. By employing the low frequency noise laser along with temperature stabilisation and packaging of the resonator, highly stable frequency combs are generated in dual ring normal dispersion microresonator. These combs can operate for several hours without the need for active stabilisation. This thesis also explores an innovative approach to tackle thermo refractive noise, a significant source of instability in microresonator systems, by implementing an alloptical servo loop. Through extensive experimentation and analysis, this technique demonstrates a 50% reduction in comb line noise over the bandwidth of 10 kHz to 1 MHz of the generated soliton comb.

Following this, an in-depth study of the dynamics of solitons produced in a dual ring resonator and their variation as a function of pump resonance detuning is conducted. The thesis reports the experimental demonstration of the variations in spectral envelope, repetition frequency, bandwidth, conversion efficiency and frequency noise of the comb lines with changes in the detuning. Moreover, our findings with regard to frequency noise and repetition rate stability of microcombs emphasise their great technological potential, especially for applications in the fields of microwave generation. The findings and knowledge acquired from this research are also valuable in the stabilisation of these combs. In addition to the advancements in dual ring microresonator technology, this thesis explores frequency comb generation in the mid-infrared (IR) spectrum using silicon nitride single ring microresonator. This investigation opens up new avenues for leveraging microresonator-based frequency combs in IR spectroscopy and sensing applications.

In summary, this thesis presents significant advancements in microresonator-based frequency comb generation and noise reduction techniques. By conducting a thorough analysis of dual ring microresonator systems, along with methods to address noise issues and investigating new possibilities for frequency comb generation in the mid-infrared frequencies, this study enhances the development of optical frequency comb technologies, which have broad applications in various scientific and engineering fields.

Supervisors

• Principal supervisor: Professor Kresten Yvind, DTU Electro, Denmark
• Co-supervisor: Senior Researcher Minhao Pu, DTU Electro, DenmarkCo-supervisor: Patrick Bowen, NKT Photonics

Evaluation Board

• Associate Professor Michael Galili, DTU Electro, Denmark
• Professor Christelle Monat, Ecole Centrale de Lyon, France
• PhD. John Jost, Enlightra, Switzerland

Master of the Ceremony

• Senior researcher Mikkel Heuck, DTU Electro, Denmark