Low power integrated photonics laser package for optical clocks

The overall objective of this Phase I research is to perform a preliminary design, modeling, and evaluation of a packaged 778.1nm PIC laser with sub-10kHz linewidth and 30dB isolation that provides 40mW of linearly polarized light from an optical fiber for a rubidium two-photon optical clock.  The primary effort in this Phase I will be to investigate the feasibility of a minimally magnetic thin-film waveguide isolator that does not necessitate large external magnets and provides 30 decibels of isolation and low injection loss at 778.1nm.   Given the substantial laser-power and isolation requirements for the 778.1-nm Rb and other considerations, non-magnetic isolators present a significant and somewhat unpredictable technical challenge compared to magnetic isolators. In this effort we propose to pursue minimally magnetic isolators for photonic laser systems that require high output power and isolation with low losses on all PIC components using standard laser diodes chips (which is generally desirable to avoid added the cost and complexity that comes with adding additional amplifier and isolation stages on-chip).  The 778.1nm laser package to be developed in the proposed effort exemplifies such a case, with high but limited output power of available DBR diodes at 778.1nm prior to injection locking, isolation, and fiber output coupling stages on the PIC. To this end, a main focus in this proposed Phase I will be to investigate and develop a minimally magnetic on-chip thin-film waveguide isolator solution on a SiN platform at 778.1nm that addresses the limitations presented by conventional non-magnetic and magnetic isolator types described above and is able to: (1) provide high optical isolation of 30dB or more and low injection loss <1dB (target), (2) eliminate the need for larger bulk magnets on-chip, (3) maintain a relatively simple design and fabrication process, and (4) draw minimal-to-zero power. Advantages and target performance of the proposed minimally magnetic nonreciprocal device include a compact footprint (size 2mm), large isolation (>30dB), low injection loss (<2dB), broadband operation (778.1nm +/- 2nm), and no requirement for RF input power.  Design, modeling, and evaluation of the 778.1nm PIC laser package and key components including DBR diode, self-injection locking, minimally magnetic isolator, polarization-maintaining fiber-coupling stage to meet 778.1nm linewidth, isolation, and power targets will be performed. An evaluation of the PIC power budget, design optimizations of components, thermal controls, hybrid integration, and package SWaP will be performed towards 778.1nm laser PIC package fabrication and prototype development in Phase II.