Sub-Hz Linewidth Photonic-Integrated Brillouin Laser (1802.10020v1)

Abstract: Photonic systems and technologies traditionally relegated to table-top experiments are poised to make the leap from the laboratory to real-world applications through integration. Stimulated Brillouin scattering (SBS) lasers, through their unique linewidth narrowing properties, are an ideal candidate to create highly-coherent waveguide integrated sources. In particular, cascaded-order Brillouin lasers show promise for multi-line emission, low-noise microwave generation and other optical comb applications. Photonic integration of these lasers can dramatically improve their stability to environmental and mechanical disturbances, simplify their packaging, and lower cost. While single-order silicon and cascade-order chalcogenide waveguide SBS lasers have been demonstrated, these lasers produce modest emission linewidths of 10-100 kHz. We report the first demonstration of a sub-Hz (~0.7 Hz) fundamental linewidth photonic-integrated Brillouin cascaded-order laser, representing a significant advancement in the state-of-the-art in integrated waveguide SBS lasers. This laser is comprised of a bus-ring resonator fabricated using an ultra-low loss Si3N4 waveguide platform. To achieve a sub-Hz linewidth, we leverage a high-Q, large mode volume, single polarization mode resonator that produces photon generated acoustic waves without phonon guiding. This approach greatly relaxes phase matching conditions between polarization modes, and optical and acoustic modes. Using a theory for cascaded-order Brillouin laser dynamics, we determine the fundamental emission linewidth of the first Stokes order by measuring the beat-note linewidth between and the relative powers of the first and third Stokes orders. Extension to the visible and near-IR wavebands is possible due to the low optical loss from 405 nm to 2350 nm, paving the way to photonic-integrated sub-Hz lasers for visible-light applications.

• The paper demonstrates a photonic-integrated Brillouin laser achieving a fundamental linewidth of ~0.7 Hz using cascaded-order stimulated Brillouin scattering in Si3N4 waveguides.
• It details an innovative design and fabrication of an ultra-low loss ring-bus resonator leveraging high-quality Si3N4 waveguides and a broadband Brillouin gain bandwidth.
• Experimental beat-note analyses validate the laser's coherence across various pump wavelengths, highlighting its potential in communications, metrology, and spectroscopy.

Analysis of a Photonic-Integrated Sub-Hz Linewidth Brillouin Laser

The paper presents the development and characterization of a novel photonic-integrated Brillouin laser with a sub-Hertz linewidth. This advancement signifies a pivotal stride in the miniaturization and integration of highly coherent light sources for a myriad of applications. The authors have successfully demonstrated a fundamental linewidth of approximately 0.7 Hz leveraging cascaded-order stimulated Brillouin scattering (SBS) within a silicon nitride (Si3N4) waveguide platform.

Key Contributions and Methodology

1. Design and Fabrication:
   • The laser architecture employed consists of a ring-bus resonator fabricated using an ultra-low loss Si3N4 waveguide. The design capitalizes on high-quality factors (Q) and a large mode volume to enable high intra-cavity intensities.
   • A unique aspect of this design is the large broadband Brillouin gain bandwidth achieved by utilizing unguided phonons, which significantly contributes to the narrow linewidth by minimizing the phase noise transfer from the pump.
2. Experimental Approach:
   • Measurement of the laser linewidth was conducted through beat-note linewidth analyses between the first and third Stokes orders. This method allows for precise determination of the linewidth based on relative Stokes power measurements without relying on uncertain cold-cavity parameters.
   • The authors utilize a detailed theoretical model that correlates the noise dynamics inherent to cascaded Brillouin lasers with the emitted optical powers.
3. Performance Results:
   • The reported laser achieved a first Stokes order linewidth of ~0.7 Hz, setting a new benchmark for photonic-integrated SBS lasers.
   • The setup demonstrated the ability to maintain coherence across a wide range of pump wavelengths, broadening the potential applicability of the technology.

Implications and Potential Applications

The sub-Hz linewidth achieved in this work holds significant implications for applications demanding high frequency stability and low phase noise, such as:

• Coherent Optical Communications: The miniaturization and integration potential of this laser can lead to more compact and energy-efficient coherent communication systems.
• Metrology and Sensing: With inherent low noise characteristics, this laser is well-suited for precise metrological instruments and sensitive optical sensors.
• Atomic Clocks and Spectroscopy: Extension of the system to visible and near-infrared bands could transform atomic clocks and spectroscopic tools by reducing size and cost while maintaining high coherence.

Future Prospects

The successful demonstration of a wafer-scale compatible, sub-Hz SBS laser invites further exploration. Future research can focus on:

• Extension to Visible Wavelengths: The large transparency window of Si3N4, spanning 405 to 2350 nm, suggests that these lasers could be adapted for visible light applications, enhancing the capability of lasers in optical clocks and spectroscopy.
• Integration with Photonic Circuits: This technology's compatibility with existing photonic components points towards the creation of integrated photonic systems-on-chip merging active and passive components for advanced functionality.
• Enhanced Dispersion Engineering: Although the current design minimizes the need for complex dispersion engineering, future work might aim to further refine these mechanisms for even higher performance.

In summary, this paper elucidates an important advancement in photonic integration, demonstrating a sub-Hz linewidth in a Brillouin laser. Through strategic design and comprehensive theoretical understanding, the work reveals a path towards deploying miniature, coherent light sources in a broad range of precision applications.