UC Santa Barbara

Visible and near-IR low-phase-noise lasers, laser stabilization cavities, and beam manipulation optics are integral components of atomic, ion, and quantum systems. These systems require low-frequency noise at specific frequency offsets, dictated by the applications for trapping, preparation, and manipulation of the atomic and qubit states. Today, these low-noise sources are provided by tabletop lasers locked to the bulk stabilization cavities. Bulk free-space optics are used to route, modulate, and manipulate the necessary beams to realize these systems. The miniaturization and development of portable atomic and quantum systems will benefit from integrated photonic systems for reducing the size, weight, and power budget consumption (SWaP), as well as improving the new low-frequency noise of direct-drive sources at the required transition wavelengths of atoms, ions, and molecules, and potentially improving the performance and sensitivity of these experiments.Progress towards low loss waveguides and precision lasers at visible wavelengths has been limited because of the high loss introduced by significant scattering in visible waveguides where the loss scales as 1??4?. This work demonstrates the lowest waveguide losses at visible and near-IR wavelengths achieved to date by using a dilute mode, weakly confined Si3N4 waveguide design fabricated using a CMOS foundry compatible process. This platform offers a wide transparency window of 405 nm – 2350 nm. The demonstrated lossesare 2 dB/m at λ = 493 nm, 0.6 dB/m at λ = 674 nm – 698 nm and 0.36 dB at λ = 789 nm with the highest quality factors (Q) demonstrated to date in visible at 39 million at 493 nm, 90 million at 674 nm and 100 million at 698 nm and 145 million at 780 nm. These losses are summarized in Figure 2.1 and compared with other published state-of-the-art waveguide losses. To achieve these losses using dilute modes, the absorption in both the silicon nitride core and the oxide cladding must be minimized, and nitride sidewall scattering must be reduced. The low loss and high-Q of the Si3N4 resonators in Si3N4 enable high precision lasers that utilize the high-Q resonator cavity as to realize a low-phase-noise active laser cavity for mid- to high-offset frequencies and either as an external passive cavity for the reduction of low-frequency phase noise and improved carrier stability or a passive nonlinear cavity for improving high-offset frequency noise. Various metrics are used to characterize the phase or frequency noise of lasers, including fundamental linewidth (FLW), integral linewidth (ILW), and Allan Deviation (ADEV), as discussed in detail in this dissertation. The FLW is due to quantum-driven phase fluctuations (memoryless white noise process) that play a role at high-frequencies, define the ILW and shape the wings of the laser linewidth. The technical noise sources due to both fundamental and environmental fluctuations build up the time-averaged broadened shape, the ILW, and contribute to low-frequency noise, such as flicker noise and fractional frequency noise. Finally, at the longest time intervals, the carrier or line-shape exhibits a long-term drift. These are discussed in detail in Section 1.3. This dissertation focuses on the minimization of these frequency noise contributions and drift over wide ranges of offset frequencies, and various linewidth definitions are employed to summarize the frequency noise characteristics. This work demonstrates integrated stimulated Brillouin scattering (SBS) lasers [1,2] operating in the visible and near-IR regions for the first time. These demonstrations are at key atomic optical transitions: 674 nm for the 88Sr+ clock and qubit optical transition, 698 nm for the 87Sr clock transition, and 778/780 nm for the 87Rb two-photon clock and Rydberg applications. These lasers have a 10/6 mW threshold for 674/698 nm and a 0.8 mW threshold at 780 nm wavelength and achieve a factor of 500x - 3500x reduction in fundamental linewidth with a 12 Hz fundamental linewidth at 674 nm, a 7.8 Hz fundamental linewidth at 698 nm, and an 18 Hz fundamental linewidth at 780 nm along with 2-10x reduction in integral linewidth. To further reduce the integral linewidth of the lasers, we designed and realized integrated stabilization cavity is demonstrated in the form of CMOS foundry-compatible coil resonator (coilR) that operate at visible wavelengths. These coil resonators were 3 m long length on the resonator to establish a low thermo-refractive noise (TRN) [3] floor. Pound-Drever-Hall (PDH) locking [4,5] is a widely used technique to enable the laser to take on the frequency noise properties of an optical reference cavity and is discussed further in Section 1.3 of this dissertation. By locking the SBS laser to these integrated coil cavities, the integral linewidth is further reduced in addition to the fundamental linewidth reduction by the SBS laser. Direct locking of lasers in general (SBS, semiconductor, or any other laser design) to these coilR-integrated stabilization cavities achieves a 2-4 orders of magnitude reduction in frequency noise for frequencies in the lock loop bandwidth and a ~ 20x reduction in integral linewidth at 674 nm. Moreover, these coilRs operate over a wide wavelength range of 670 – 700 nm while maintaining a high Q value, which is required for low noise and strong frequency discrimination. This dissertation also describes demonstration of these direct drive, stable low phase noise sources in actual atomic systems. The 674 nm coil resonators is used as a stabilization cavity for a 674 nm laser to address a collaborator’s 88Sr+ ion trap [6], enabling key measurements and functions, such as ion spectroscopy, drift measurement, state preparation, rabi oscillations, and Ramsey interferometry, with the use of a traditional tabletop optical reference cavity. Cold-atom systems use a cloud of atoms or a chain of ions that are cooled and trapped by laser beams along with a magnetic field. These will benefit from integrated photonics is beam delivery and beam shaping for various operations, such as the large-area cooling and trapping beams required for a neutral atom cooling and trapping in a 3D magneto optic trap (MOT). This work demonstrates the largest waveguide emitted beams from photonic waveguide platform, with beam size of 2.5 mm x 3.5 mm flat top beams which represent a 20 million times increase in area over a waveguide mode. Three beams at 54.7 °from the chip normal placed 120 °from each other provided three orthogonally intersecting beams that provided the correct orientation to form an MOT at ~1 cm above the chip surface. The 780 nm gratings were utilized to demonstrate the cooling of over a million 87Rb atoms in a 3D MOT.