Thermally Controlled Comb Generation and Soliton Modelocking in Microresonators (1603.08017v2)

Abstract: We report the first demonstration of thermally controlled soliton modelocked frequency comb generation in microresonators. By controlling the electric current through heaters integrated with silicon nitride microresonators, we demonstrate a systematic and repeatable pathway to single- and multi-soliton modelocked states without adjusting the pump laser wavelength. Such an approach could greatly simplify the generation of modelocked frequency combs and facilitate applications such as chip-based dual-comb spectroscopy.

• The paper introduces a thermal control technique using integrated heaters to achieve soliton modelocking in microresonators.
• It employs fixed-frequency lasers and Si3N4 microring resonators to enable repeatable generation of single- and multi-soliton states.
• The approach simplifies comb generation by eliminating pump wavelength adjustments, enhancing stability for precision spectroscopic and metrological applications.

Thermally Controlled Comb Generation and Soliton Modelocking in Microresonators

The paper by Joshi et al. presents a significant advancement in the paper of optical frequency comb generation using microresonators by proposing a technique for thermally controlled soliton modelocking. This approach utilizes the thermo-optic effect to achieve comb generation via integrated heaters without the need for pump laser wavelength adjustment. This technique could potentially simplify current methodologies and facilitate more widespread applications across diverse fields, including dual-comb spectroscopy and precision metrology.

Key Findings

The research illustrates the successful demonstration of soliton modelocking in silicon nitride (Si$_3$N$_4$) microresonators through thermal control. By integrating resistive heaters into the microresonators, the authors achieve a systematic and repeatable pathway to soliton states. The modulation of electric current through the heaters alters the microresonator’s refractive index, which in turn adjusts the cavity's resonant frequency, allowing for the generation of both single- and multi-soliton states.

Methodology

The authors employed an experimental setup that includes the use of fixed-frequency lasers with narrow linewidths in conjunction with Si$_3$N$_4$ microring resonators. The experimental configuration allowed for the monitoring of comb spectrum evolution, RF amplitude noise, and transmitted pump power. By thermally tuning the resonance frequency through current modulation, the paper navigated the microresonator system into various comb states until achieving low-noise soliton states.

Implications and Applications

The results exhibit notable implications for frequency comb technologies. The use of thermally controlled modelocking circumvents the limitations associated with laser frequency tuning, such as noise and broader linewidth, thus enhancing the overall performance and stability of the frequency combs. These improvements are crucial for applications requiring high precision, such as time and frequency metrology, optical communication, and dual-comb spectroscopy.

Moreover, this approach facilitates the generation of multiple modelocked combs from a single pump source on a chip scale, providing a pathway for integrated and miniaturized frequency comb devices. This feature is particularly advantageous for dual-comb spectroscopy applications that demand two frequency combs with slightly different free spectral ranges (FSRs).

Future Developments

The paper opens several avenues for future research and technological development in the domain of optical frequency combs. The integration of this thermally controlled method into a broader range of microresonator materials and geometries could further extend its applicability and efficiency. Moreover, the prospect of achieving complete frequency stabilization through locking the FSR paves the way for the development of entirely integrated systems capable of more robust and practical field applications.

Continued research into improving the repeatability and control of soliton state formation, as well as minimizing environmental influences such as thermal fluctuations and photonic packaging, will be paramount in advancing these integrated photonic technologies. As the precision and control of these systems improve, they could become indispensable tools across diverse scientific and industrial fields.