2kHz Linewidth C-Band Tunable Laser by Hybrid Integration of Reflective SOA and SiO2 PLC External Cavity

We present a 35nm tunable laser by hybrid integration between a reflective semiconductor optical amplifier and an external cavity based on silica ring resonators and a Mach-Zehnder interferometer. We demonstrate a record low 2kHz line width.

Tunable Laser Design and Simulations

The planar lightwave circuit (PLC) (Fig 1a) is made of low-loss SiO₂ waveguides. The ring resonators are designed with coupling coefficients between the bus waveguide and the ring and with free spectral ranges (FSRs) around 25GHz. In any waveguide section, phase can be locally modified via thermo-optic heaters on the waveguides, controlled by a voltage. A Sagnac loop provides about 25% optical feedback into the cavity, and drives the remaining light to the output port for coupling into an optical fiber. A commercially available 1mm-long RSOA with high reflectivity on the rear facet, and anti-reflection coated 10°-tilted front facet is butt-coupled to the PLC and controlled by current injection.