Broadband electro-optic frequency comb generation in an integrated microring resonator (1809.08636v1)

Abstract: Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communications and for precision metrology, timing and spectroscopy. To date, wide-spanning combs are most often generated by mode-locked lasers or dispersion-engineered resonators with third-order Kerr nonlinearity. An alternative comb generation method uses electro-optic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability. Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. In this work, we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large electro-optic response, ultra-low optical loss and highly co-localized microwave and optical felds, while enabling dispersion engineering. Our measured EO frequency comb spans more than the entire telecommunications L-band (over 900 comb lines spaced at ~ 10 GHz), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 Hz to 100 MHz), and utilize this feature to generate dual frequency combs in a single resonator. Our results show that integrated EO comb generators, capable of generating wide and stable comb spectra, are a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy to optical communications.

• The paper shows an integrated LN platform achieving broadband EO comb generation with over 900 comb lines spaced ~10 GHz apart.
• It employs a high-Q microring design to ensure strong second-order nonlinearity and efficient overlap of microwave and optical fields.
• It highlights tunable frequency spacing from 10 Hz to 100 MHz, paving the way for advances in spectroscopy and high-precision optical communications.

Overview of Broadband Electro-Optic Frequency Comb Generation in an Integrated Microring Resonator

The authors present a significant advancement in the field of optical frequency comb (OFC) generation by utilizing a lithium niobate (LN) integrated photonic platform. This paper explores the generation of broadband electro-optic (EO) frequency combs using microring resonators engineered for strong second-order ($\chi^{(2)}$) nonlinearity. Such developments are poised to enhance applications in metrology, communications, and spectroscopy, offering a compelling alternative to the traditional Kerr comb systems.

Key Findings and Methodological Approach

The paper details the integration of EO comb generation on a thin-film LN platform, which succinctly addresses limitations in previous EO comb systems regarding bandwidth and dispersion engineering capabilities. A noteworthy achievement is the generation of an EO frequency comb that spans the entire telecommunications L-band, presenting over 900 comb lines with ~10 GHz spacing.

The primary advantage of leveraging the second-order nonlinearities via the EO effect lies in the achievable stability and control over the generated combs. The authors meticulously employ a high-$Q$ microring resonator design to ensure the substantial overlap of the microwave and optical fields, thereby enabling the efficient broadening of EO combs. This approach also facilitates dispersion engineering crucial for the enhancement of bandwidth beyond what conventional methods allow.

Implications and Theoretical Underpinning

One significant contribution of the paper is the demonstration of the tunable control of frequency spacing over broad orders of magnitude—from 10 Hz up to 100 MHz—validating an essential characteristic for EO combs in integrated photonic systems: configurability. Such control is vital for applications like dual-comb spectroscopy and high-precision optical communications. This tunability is contrasted with the inherent frequency constraints found in Kerr-based systems, emphasizing the EO approach's flexibility.

Theoretical modeling is utilized to predict and verify the comb spectra, considering both microwave detuning effects and dispersion-induced phase variations. This comprehensive model provides insights into the comb’s stability and how dispersion limits traditional EO comb generation while advocating for the potential of more sophisticated dispersion engineering in integrated platforms.

Future Directions

The research opens pathways for further refining EO comb generators with extended bandwidths, potentially spanning an octave. Such enhancements would be realized through advanced dispersion engineering and increased microwave modulation frequencies. The authors propose that the integration of complementary components such as filters within the same chip architecture could significantly enhance the signal-to-noise ratio and facilitate application-specific photonic circuits.

Importantly, leveraging LN's transparency range could broaden the scope of the EO comb systems, accommodating a wider spectrum from visible to near-infrared wavelengths. Future systems could conflate numerous optical functionalities onto a single chip, advancing complete photonic systems for Tb/s optical communications, on-chip spectroscopy, and high-resolution ranging.

Conclusion

The authors show that integrating EO frequency combs on a thin-film LN platform significantly enhances performance compared to traditional bulk systems and prior integrated iterations. By addressing EO combs’ stability, control, and potential for broadened bandwidths, this work underscores the role of integrated EO comb generators as a powerful complement to existing Kerr-based solutions, offering distinct advantages in terms of spectral control and tunability. This development harbors the promise to not only refine existing applications but also broaden the scope of integrated photonics solutions.