Silicon Nitride External Cavity Laser with Alignment Tolerant Multi-Mode RSOA-to-PIC Interface

TL;DR

This paper presents a silicon nitride external cavity laser with an alignment tolerant multi-mode RSOA-to-PIC interface, achieving wide tuning, improved alignment tolerance, and narrow linewidth for integrated photonics applications.

Contribution

It introduces a novel alignment tolerant multi-mode waveguide coupler and Vernier ring resonators, enhancing integration ease and performance of silicon nitride external cavity lasers.

Findings

Alignment tolerance increased by a factor of 3.

Achieved laser linewidth of 42 kHz.

Maintains lasing with ±6 μm misalignment.

Abstract

We demonstrate an external cavity laser formed by combining a silicon nitride photonic integrated circuit with a reflective semiconductor optical amplifier. The laser uses an alignment tolerant edge coupler formed by a multi-mode waveguide splitter right at the edge of the silicon nitride chip that relaxes the required alignment to the III-V gain chip and equally splits the power among its two output waveguides. Both the ground and first order mode are excited in the coupler and reach the quadrature condition at the waveguide junction, ensuring equal power to be coupled to both. Two high-quality-factor ring resonators arranged in Vernier configuration close a Sagnac loop between the two waveguides. In addition to wideband frequency tuning, they result in a longer effective cavity length. The alignment tolerant coupler increases the alignment tolerance in the two directions parallel to the chip surface by a factor 3 relative to conventional edge couplers, making it ideal for gain chip integration via pick-and-place technology. Lasing is maintained in a misalignment range of $\pm$6 $\mu$m in the direction along the edge of the chip. A Lorentzian laser linewidth of 42 kHz is achieved.