Monolithic photonic circuits for Kerr frequency comb generation, filtering and modulation (1809.08637v1)

Abstract: Microresonator Kerr frequency combs, which rely on third-order nonlinearity ($\chi^{(3)}$), are of great interest for a wide range of applications including optical clocks, pulse shaping, spectroscopy, telecommunications, light detection and ranging (LiDAR) and quantum information processing. Many of these applications require further spectral and temporal control of the generated frequency comb signal, which is typically accomplished using additional photonic elements with strong second-order nonlinearity ($\chi^{(2)}$). To date these functionalities have largely been implemented as discrete off-chip components due to material limitations, which come at the expense of extra system complexity and increased optical losses. Here we demonstrate the generation, filtering and electro-optic modulation of a frequency comb on a single monolithic integrated chip, using a thin-film lithium niobate (LN) photonic platform that simultaneously possesses large $\chi^{(2)}$ and $\chi^{(3)}$ nonlinearities and low optical losses. We generate broadband Kerr frequency combs using a dispersion-engineered high quality factor LN microresonator, select a single comb line using an electrically programmable add-drop filter, and modulate the intensity of the selected line. Our results pave the way towards monolithic integrated frequency comb solutions for spectroscopy data communication, ranging and quantum photonics.

• The paper demonstrates on-chip integration of Kerr comb generation, filtering, and modulation using dual χ(3) and χ(2) nonlinearities on lithium niobate.
• The paper achieves broadband frequency comb generation with a 700 nm span and a loaded Q factor of 6.6×10^5, along with 47 dB pump suppression.
• The paper realizes high-speed modulation at up to 500 Mbit/s, indicating scalable potential for advanced photonic applications in LiDAR, telecommunications, and quantum processing.

Monolithic Photonic Circuits for Kerr Frequency Comb Generation, Filtering, and Modulation

The paper "Monolithic photonic circuits for Kerr frequency comb generation, filtering and modulation" addresses a significant challenge in photonic integration by demonstrating the generation, filtering, and modulation of Kerr frequency combs on a single lithium niobate (LN) chip. This integration is crucial for improving the complexity, size, and efficiency of photonic systems used in a plethora of applications such as optical clocks, telecommunications, LiDAR, and quantum information processing.

Key Contributions

The authors employ a thin-film lithium niobate photonic platform that exhibits both large third-order nonlinearity ($\chi^{(3)}$) and second-order nonlinearity ($\chi^{(2)}$). This dual capability allows for the on-chip integration of both Kerr frequency comb generators, which rely on $\chi^{(3)}$, and components such as electro-optic modulators, which exploit $\chi^{(2)}$. Specifically, the paper details the successful generation of broadband Kerr frequency combs using dispersion-engineered high-Q LN microresonators, spectral line filtering through an electrically programmable add-drop filter, and subsequent modulation of the comb line.

Technical Specifications and Results

1. Frequency Comb Generation: The integrated microresonator exhibits a loaded Q factor of $6.6 \times 10^5$ for TE polarization, supporting the generation of frequency combs with spans of up to 700 nm. The combinational management of the pump power and microresonator dispersion results in a comb spanning most of an octave.
2. Filtering: The integration of an add-drop filter with an over-coupled microring resonator enables the selective targeting of a single comb line. The system demonstrates a significant extinction ratio, achieving approximately 47 dB suppression of the pump, indicating efficient targeting capabilities.
3. Modulation: Using the $\chi^{(2)}$ electro-optic effect, the authors achieve high-speed modulation of the selected comb line, operating at data rates of up to 500 Mbit/s. This capability is mainly limited by the photonic lifetime of the resonator, with potential for improvement through integration with Mach-Zehnder modulators.

Implications and Future Prospects

This research posits significant implications for the development of more efficient and scalable photonic circuits. The monolithic integration on a LN platform reduces the need for off-chip components, enhancing the practicality and scalability of applications requiring frequency combs. Moreover, the potential to leverage $\chi^{(3)}$ nonlinearity in optimizing power thresholds and integrating dense wavelength division multiplexing (DWDM) functionalities presents avenues for future exploration. Importantly, the research aligns with efforts to integrate higher levels of functionality on a single chip, critical for advanced LiDAR applications, programmable pulse shaping, and quantum photonics.

The demonstrated platform could inform future developments where both passive and active photonic components coexist, providing a stepping stone toward more compact, effective, and dynamically controllable photonic devices. Such advancements are imperative in the pursuit of high-speed, low-power communications and high-precision sensing technologies.