Can amplified spontaneous emission produce intense laser guide stars for adaptive optics?

Abstract

Adaptive optics is a key technology for ground-based optical and infrared astronomy, providing high angular resolution and sensitivity. Systems employing laser guide stars can achieve high sky coverage, but their performance is limited by the available return flux. Amplified spontaneous emission could potentially boost the intensity of beacons produced by resonant excitation of atomic or molecular species in the upper atmosphere. This requires the production of a population inversion in an electronic transition that is optically thick to stimulated emission. Mesospheric metals have insufficient column density for amplified spontaneous emission, but atomic oxygen and nitrogen are potential candidates. They could potentially be excited by a high-energy chirped femtosecond pulsed laser, making visible-wavelength transitions accessible. Such lasers can also generate a white-light supercontinuum in the atmosphere. In addition to providing high intensity, the broadband emission from such a source could facilitate the sensing of the tilt component of atmospheric turbulence.

Atomic transitions for adaptive optics

“…The LGS return flux can be computed from the equation of radiative transfer. For an optically-thin medium (optical depth τ 1) the specific intensity I ν (W m −2 Hz −1 sr −1 ) of the backscattered radiation is given by [8]…”

“…Also of interest is the dimensionless product Nσ 21 , which determines the maximum possible optical depth. Specifically, τ is bounded by −Nσ 21 ≤ τ ≤ Nσ 12 [8]. Therefore, ASE is not possible if Nσ 21 < 1.…”

“…Several new techniques that might potentially achieve highintensity LGS are discussed in [8] . One possible approach is amplified spontaneous emission (ASE) [9].…”

Investigation of Numerical Simulation for Adaptive Optics System

“…In ground-based telescope applications, atmospheric turbulence can drastically decrease resolution for pupil diameters much bigger than Fried coherence diameter (r o ) [3]. A telescope equipped with a functional adaptive optics system enhances the spatial resolution of the image [4]. Light collected by ground-based telescopes has propagated through turbulent Earth atmosphere before arriving at the Earth's surface.…”

“…Zernike polynomials were used to describe the residual error and figure out how much the compensation changed the measured turbulence values. The results of the computer simulation involving atmospheric turbulence reveal that elevating the r o values (4,8,12,16,20,24,28,32) cm resulted in a 3.4% rise in S. However, when the adaptive optics system operated with a constant r o (20 cm), augmenting the Zernike aberration modes led to a remarkable 44% increase in S, signifying a substantial enhancement in the compensation procedure.…”