A Precise Photometric Ratio via Laser Excitation of the Sodium Layer II:
  Two-photon Excitation Using Lasers Detuned from 589.16 nm and 819.71 nm
  Resonances
  This article is the second in a pair of articles on the topic of the
generation of a two-color artificial star (which we term a "laser photometric
ratio star," or LPRS) of de-excitation light from neutral sodium atoms in the
mesosphere, for use in precision telescopic measurements in astronomy and
atmospheric physics, and more specifically for the calibration of measurements
of dark energy using type Ia supernovae. The two techniques respectively
described in both this and the previous article would each generate an LPRS
with a precisely 1:1 ratio of yellow (589/590 nm) photons to near-infrared
(819/820 nm) photons produced in the mesosphere. Both techniques would provide
novel mechanisms for establishing a spectrophotometric calibration ratio of
unprecedented precision, from above most of Earth's atmosphere, for upcoming
telescopic observations across astronomy and atmospheric physics.
  The technique described in this article has the advantage of producing a much
brighter (specifically, brighter by approximately a factor of 1000) LPRS, using
lower-power (<30 W average power) lasers, than the technique using a single 500
W average power laser described in the first article of this pair. However, the
technique described here would require polarization filters to be installed
into the telescope camera in order to sufficiently remove laser atmospheric
Rayleigh backscatter from telescope images, whereas the technique described in
the first article would only require more typical wavelength filters in order
to sufficiently remove laser Rayleigh backscatter.
