Abstract
Today’s narrowest linewidth lasers are limited by mirror motion in the reference optical resonator used to stabilize the laser’s frequency. Recent proposals suggest that superradiant lasers based on narrow dipole-forbidden transitions in cold alkaline earth atoms could offer a way around this limitation. Such lasers operating on transitions with linewidth of order mHz are predicted to achieve output spectra orders of magnitude narrower than any currently existing laser. As a step towards this goal, we demonstrate and study a laser based on the 7.5-kHz linewidth dipole-forbidden to transition in laser-cooled and tightly confined . We can operate this laser in the bad-cavity or superradiant regime, where coherence is primarily stored in the atoms, or continuously tune to the more conventional good-cavity regime, where coherence is primarily stored in the light field. We show that the cold-atom gain medium can be repumped to achieve quasi-steady-state lasing. We also demonstrate up to an order of magnitude suppression in the sensitivity of laser frequency to changes in cavity length, verifying a key feature of the proposed narrow linewidth lasers.
- Received 23 October 2015
DOI:https://doi.org/10.1103/PhysRevX.6.011025
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Published by the American Physical Society
Physics Subject Headings (PhySH)
Synopsis
Bad Cavities for Precise Lasers
Published 9 March 2016
The frequency of a laser based on trapped ultracold atoms can be made insensitive to fluctuations in the laser cavity’s length.
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Popular Summary
Current state-of-the-art frequency-stable lasers are used in optical clocks as well as in searches for gravitational waves and precision tests of relativity. These lasers are limited by length fluctuations in the reference cavity to which the laser is stabilized, which are imprinted on the laser’s frequency. One way to overcome these fluctuations is to operate a laser in the superradiant regime, where a narrow-band gain medium sets the lasing frequency and renders the laser output insensitive to cavity-length fluctuations. Here, we take a step toward realizing such a laser by demonstrating and characterizing a cold-atom laser that can be tuned between the conventional “good-cavity regime” and the superradiant regime.
Our laser operates on a narrow, dipole-forbidden optical transition in an ensemble of roughly 100,000 strontium atoms cooled to 9 millionths of a degree above absolute zero and optically confined within a high-finesse optical cavity. We demonstrate pulsed lasing that exhibits interesting oscillatory behavior, as well as quasi-steady-state lasing when pump lasers are applied to maintain population inversion. The pump power can be used to tune the laser from the superradiant regime, where the laser’s phase coherence is maintained by the atoms, to the good-cavity regime, where coherence is stored by the cavity light field. We measure the sensitivity of the laser output frequency to changes in cavity length across these two regimes, and we demonstrate an order-of-magnitude reduction in sensitivity to cavity-length fluctuations in the superradiant regime.
We expect that our findings, which can be extended to even narrower transitions in the same type of system, will lead to the development of lasers with frequency stability far better than what is achievable today.