We demonstrate backward-directed continuous-wave (cw) emission at 2.21 μm generated on the 4P3/2-4S1/2 population-inverted transition in Na vapors two-photon excited with resonant laser light at 589 and 569 nm. Our study of power and atom-number-density threshold characteristics shows that lasing occurs at sub-10 mW total power of the applied laser light. The observed 6 mrad divergence is defined mainly by the aspect ratio of the gain region. We find that mirrorless lasing at 2.21 μm is magnetic field and polarization dependent that may be useful for remote magnetometry. The presented results could help determine the requirements for obtaining directional return from sodium atoms in the mesosphere.
The adaptive optics (AO) systems of future Extremely Large Telescopes (ELTs) will be assisted with laser guide stars (LGS) which will be created in the sodium layer at a height of ≈ 90 km above the telescopes. In a Shack-Hartmann wavefront sensor, the long elongation of LGS spots on the sub-pupils far apart from the laser beam axis constraints the design of the wavefront sensor (WFS) which must be able to fully sample the elongated spots without undersampling the non-elongated spots. To fulfill these requirements, a newly released large complementary metal oxide semiconductor (CMOS) sensor with 1100×1600 pixels and 9 µm pixel pitch could be employed. Here, we report on the characterization of such a sensor in terms of noise and linearity, and we evaluate its performance for wavefront sensing based on the spot centroid variations. We then illustrate how this new detector can be integrated into a full LGS WFS for both the ESO ELT and the TMT.
The largest uncertainty on measurements of dark energy using type Ia supernovae is presently due to systematics from photometry; specifically to the relative uncertainty on photometry as a function of wavelength in the optical spectrum. We show that a precise constraint on relative photometry between the visible and near-infrared can be achieved at upcoming survey telescopes, such as at the Vera Rubin Observatory (VRO), via a laser source tuned to the 342.78 nm vacuum excitation wavelength of neutral sodium atoms. Using a high-power laser, this excitation will produce an artificial star, which we term a “laser photometric ratio star” (LPRS) of de-excitation light in the mesosphere at wavelengths in vacuum of 589.16 nm, 589.76 nm, 818.55 nm, and 819.70 nm, with the sum of the numbers of 589.16 nm and 589.76 nm photons produced by this process equal to the sum of the numbers of 818.55 nm and 819.70 nm photons, establishing a precise calibration ratio between, for example, the VRO r and z filters. This technique can thus provide a novel mechanism for establishing a spectrophotometric calibration ratio of unprecedented precision for upcoming telescopic observations across astronomy and atmospheric physics; thus greatly improving the performance of upcoming measurements of dark energy parameters using type Ia supernovae. The second paper of this pair describes an alternative technique to achieve a similar, but brighter, LPRS than the technique described in this paper, by using two lasers near resonances at 589.16 nm and 819.71 nm, rather than the single 342.78 nm on-resonance laser technique described in this paper.
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