0.26-Hz-linewidth ultrastable lasers at 1557 nm

Spectrally-narrow laser sources are central to many applications, such as gravitational wave detection, optical atomic clocks, precision spectroscopy and quantum computation1,2,3,4,5. To meet those requirements, lasers with a linewidth of a few Hz or sub-hertz have been constructed3,6,7,8,9,10,11. Among them, many have focused on the particular wavelength of 1.5 μm. Since it is low-loss window of optical fiber, many groups have developed narrow-linewidth ultrastable laser systems at 1.5 μm for coherent transfer of light through fiber12,13,14,15. When compact and low-cost Er:fiber-based laser frequency comb is used to transfer the coherence from 1.5 μm lasers to other optical wavelengths or microwave region16,17, it enables many novel applications such as direct frequency comb spectroscopy and low noise microwave generation4,18. Furthermore, 1.5 μm narrow-linewidth lasers could also be used to study two-electron interactions via precision spectroscopy of helium19. For those attractive applications, narrow-linewidth ultrastable laser systems at 1.5 μm have been constructed based on different techniques. By frequency-stabilizing to the resonance of room-temperature high-finesse ultrastable Fabry-Perot (F-P) optical cavities with the Pound-Drever-Hall (PDH) technique20, lasers at 1.5 μm with a linewidth of ∼1 Hz or sub-hertz have been constructed14,21. A 40-mHz-linewidth laser has been demonstrated by stabilizing to a silicon single-crystal cavity operated at 124 K11 for low thermal noise22. By directly phase-locking to a cavity-stabilized laser in the visible region via an optical frequency comb as a transfer oscillator, a 1 Hz-linewidth laser at 1.5 μm has been constructed23. In this paper, the frequencies of two 1557 nm diode lasers are independently stabilized to two 10-cm-long ultrastable F-P cavities with the PDH technique. To achieve a thermal-noise-limited performance, each cavity, operated at room temperature, is deliberately designed for better isolation from environmental vibration and temperature fluctuation. By comparing two similar laser systems, each laser system has a most probable linewidth of 0.26 Hz and fractional frequency instability of 8 × 10−16 at an averaging time of 1–30 s. Compared to those using cryogenic cavities, the laser systems described in this paper are relatively simple while they still provide excellent frequency stability on both short-term and long-term scale of a few thousand seconds.