0.26-Hz-linewidth ultrastable lasers at 1557 nm

Narrow-linewidth ultrastable lasers at 1.5μm are essential in many applications such as coherent transfer of light through fiber and precision spectroscopy. Those applications all rely on the ultimate performance of the lasers. Here we demonstrate two ultrastable lasers at 1557 nm with a most probable linewidth of 0.26 Hz by independently frequency-stabilizing to the resonance of 10-cm-long ultrastable Fabry-Pérot cavities at room temperature. The fractional frequency instability of each laser system is nearly 8 × 10^-16 at 1-30 s averaging time, approaching the thermal noise limit of the reference cavities. A remarkable frequency instability of 1 × 10^-15 is achieved on the long time scale of 100-4000 s.