Development of ultrastable lasers at 1. 5μm

We report the development of ultrastable lasers at 1.5μm. Through PDH method, the fiber lasers are locked to two ultra-stable optical cavities with finesse of 483000 and 148000 respectively. Through comparing two similar ultrastable laser systems, the linewidth of 0.68Hz and the fractional frequency instability of 2.7×10
^-15
at 1s averaging time are achieved.