Sub-Hz Fundamental Linewidth Silicon Nitride Integrated Brillouin Lasers and Their Applications
- Author(s): Gundavarapu, Sarat;
- Advisor(s): Blumenthal, Daniel J.;
- et al.
The growing demand for high-performance system-on-chip photonic solutions is pushing for rapid performance advancements in photonic integrated circuits (PICs) and more specifically, spectrally pure chip-scale lasers. The creation of laser sources with low frequency and amplitude noise is critical for the wide range of future on-chip applications that will span operating ranges from the visible to infrared wavelengths, including coherent optical communications, microwave photonics, metrology, sensing, and spectroscopy. Brillouin lasers offer unprecedented sub-Hz linewidth performance with the ability to operate across this wide wavelength range, yet today these lasers are built with fiber-based or micro-optic discrete components. The important next step of realizing these lasers that are compatible for integration with other optical components and wafer-scale processing has remained elusive.
This thesis describes the first demonstration of foundry compatible, sub-Hz (~0.7 Hz) fundamental linewidth photonic-integrated all-waveguide Brillouin laser. The laser comprises a high Q bus-ring resonator fabricated on the ultra-low loss (< 0.5 dB/m) Si3N4 waveguide platform that supports operation from 405 nm to 2350 nm and can be readily integrated with a wide array of other photonic components. The design overcomes issues with other approaches related to phonon confinement, optical cavity losses, storage of a large number of single polarization photons, sensitivity to environmental conditions and compatibility with large scale monolithic photonic integration.
The development of the integrated silicon nitride waveguide Brillouin laser has yielded three major research outcomes that form the contents of this thesis: (1) Observation of significant Brillouin gain in an integrated low optical loss waveguide that is devoid of acoustic guiding with a measured peak Brillouin gain coefficient of ~ 0.1 m-1W-1 and a gain bandwidth of 153 MHz (2) Development of a new theory for power and noise dynamics of single and cascaded order Brillouin lasers and phase noise dynamics of beat notes between cascaded Stokes orders (3) Experimental demonstration of cascaded order Brillouin lasing up to 10 Stokes orders and sub-Hz fundamental linewidth emission in an integrated waveguide laser resonator with a measured loaded Q of ~30 million.
Applications that greatly benefit from the low phase noise properties of the laser include RF (microwave and mm-wave) signal synthesis and optical gyroscopes. This thesis discusses the demonstration of a low phase noise photonic RF oscillator at a frequency of 21.8 GHz realized by photo-mixing the first and third Stokes orders of cascaded order Brillouin laser. The measured phase noise was as low as -84 dBc/Hz at 10 kHz frequency offset. This high spectral purity of the Brillouin laser and generated beat note signal indicates potential for realizing integrated laser gyroscopes. Brillouin laser based optical gyroscopes are theoretically predicted to offer higher sensitivity compared to their interferometric counterparts in a relatively smaller form factor. A qualitative performance comparison of these two gyroscope variants is presented and an interferometric integrated optical gyroscope that uses a 3 m waveguide coil is demonstrated. The results of the interferometric gyroscope indicate a rate grade performance with sensitivity of 8.52 deg/√hr and bias drift of 58.6 deg/hr.