Fig. 2: External cavity tunable laser performance.

a Single mode laser output from 1520–1580 nm. b Thermo-optic tuning of the top ring of the external cavity tunable laser (ECTL). c ECTL operation at 1560 nm with a measured side mode suppression ratio (SMSR) of ~ 65 dB, measured on an optical spectrum analyser (OSA) with resolution bandwidth (RBW) <0.01 nm. d Fiber-coupled ECTL output power measured across the tuning range. e Frequency noise spectrum of the free-running ECTL at 1550 nm (blue, solid) was measured using a fiber-Mach Zehnder Interferometer (fiber-MZI) as an optical frequency discriminator (OFD). The blue dashed line plots the measured ECTL fundamental linewidth (FLW) of 6.1 Hz, and the shaded purple region shows the area under the curve that contributes to the 1/π integral linewidth (ILW) of 1750 Hz. The green and orange dashed curves are simulated estimates of the thermorefractive noise (TRN) and photothermal noise (PTN) limits of the ECTL rings, and the red dashed curve is the OFD photodetector noise. The high frequency spurs at multiples of 1 MHz correspond to the free-spectral range of the OFD fiber-MZI and do not contribute to the integral linewidth calculation. f FLW of the ECTL measured across the 60 nm tuning range. Higher reflectivity of the Sagnac loop mirror and weaker ring-bus coupling may contribute to the decrease in FLW at shorter wavelengths.