The brightness of a laser-generated guide star is determined not only by the power of the laser, but also by the spectral and temporal properties of the laser. We show that a guide star laser pulsed at the Larmor frequency of the sodium atoms enhances guide star brightness by up to 2X, compared to an optimized cw laser at the same average power. We describe a frequency-addition source of optical radiation that can provide such pulsed light, while providing any desired spectral shape.
Using a stable single frequency (Δυ < 1 MHz) cw fasor we have characterized the guide star radiance under several conditions,
including routinely measuring the radiance at various launch powers and simultaneously illuminating the same spot
with a second fasor with a range of different frequency separations. Making use of sodium's hyperfine energy diagram and
allowed transitions it is shown that some transitions do not contribute to the radiance after a short time period thus greatly
reducing the number of states whose populations need to be tracked in a simple rate equation model. An offshoot of this
view is the importance of the pump source's spectral content for efficient sodium scattering. Accounting for atomic recoil,
which causes atoms to be Doppler shifted out of resonance, we obtain model curves for photon return flux versus launch
power for both linear and circular polarization, both agree with measurements; the only free parameter being the sodium
column density on the single night both sets of data were taken. We attempted to measure the sodium velocity distribution
due to recoil using two Fasors in a pump-probe arrangement. We have measured some subtle phenomena that this simple
model does not explain and these will be discussed. These may imply the importance of understanding the collision rates
for sodium atoms to re-equilibrate through velocity changing collisions, spin relaxation and coherent beam propagation
under various atmospheric conditions.
This report will describe the progress towards modeling the radiance of a mesospheric atomic sodium guidestar pumped
with a continuous-wave, narrow-linewidth source. We will model the cases of pumping only the D2a line and pumping
both the D2a and D2b lines simultaneously. The simulation is named the sodium guidestar simulation or SGS.
A computer-automated cw sodium guidestar FASOR (Frequency Addition Source of Optical Radiation) producing a
single frequency 589-nm beam with up to 50 W for mesospheric beacon generation has been integrated with the 3.5-m
telescope at the Starfire Optical Range, Kirtland AFB, New Mexico. Radiance tests have produced a peak guidestar V1
magnitude = 5.1 (~7000 photons/s/cm2 at zenith) for 30 W of circularly polarized pump power in November 2005. Estimated
theoretical maximum guidestar radiance is about 3 times greater than measured values indicating saturation due
to atoms possibly becoming trapped in F'=1 and/or atomic recoil. From sky tests over 3.5 years, we have tracked the
annual variation of the sodium column density by measuring the return flux as a function of fasor power and determining
the slope at zero power. The maximum occurs on October 30 and the minimum on May 30, with corresponding predicted
returns of 8000 (V1 = 4.8) and 3000 (V1 = 5.8) ph/s/cm2 with 50 W of fasor power and circular polarization. The
effect of the Earth's magnetic field on the radiance of the sodium laser guidestar (LGS) from various azimuths and elevations
has been measured. The peak return flux over our observatory occurs at [az=198o; el=+71o], compared with the
direction of the magnetic field lines at [190o; +62o], and it can vary by a factor of 3 over the sky above el = 30o. First
results for non-optimized sodium LGS adaptive optics (AO) closed-loop operation have been obtained using binary
stars. Strehl ratios of 0.03 have been measured at 850 nm and a 0.14 arc second binary star has been resolved during
first closed loop observations. Guidestar characteristics, including radiance, size, and Rayleigh backscatter, the sodium
LGS wavefront sensor (WFS) AO system, and recent closed-loop results on binary stars are presented.
Mesospheric sodium guidestar radiance is plotted vs. wavelength, fasor power, fasor polarization and date. Peak radiance for circular polarization was about 7000 photons/sec/cm2 (V1 magnitude = 5.1) for 30 watts of pump power in November of 2005. Pumping with circular polarization at high power produces about 2 times more return than linear polarization. Pumping D2a at high power produces about 12 times more return than pumping D2b. A lidar equation is used to determine column density. Estimated maximum possible guidestar radiance is about 3 times greater than measured values. Guidestar radiance may be saturated by atoms becoming trapped in F'=1 and atomic recoil.
A CW Na guidestar excitation source has been constructed and installed on the 3.5-m telescope at the Starfire Optical Range. This device is comprised of injection-locked Nd:YAG ring lasers operating at 1064 nm and 1319 nm and a doubly resonant cavity where sum-frequency generation of these wavelengths in LBO produces a diffraction-limited linearly-polarized 589-nm beam. Up to 50 W of 589-nm light for mesospheric guide-star generation has been produced. The injection-locked Nd:YAG lasers are capable of operating at up to 100 watts at 1064 nm and 60 watts at 1319 nm.
We report on the development of a 50-W, continuous-wave, sodium wavelength guidestar excitation source for installation on the azimuth gimbal structure of the 3.5-m telescope at the Starfire Optical Range. The laser is an all solid-state design employing two diode-pumped Nd:YAG sources operating at 1064 and 1319 nm that are combined to generate 589-nm radiation using a lithium triborate non-linear crystal. Key features of the system include single-frequency, injection-locked high-power oscillators, a doubly resonant sum frequency generator cavity, a short-term 10 kHz wide 589 nm spectrum, excellent beam quality and power stability, and turn-key operation using computer control and diagnostics. The laser beam is projected from the side of the 3.5-m telescope. A novel elevation beam dither approach is employed to determine range to the centroid of the guidestar formed in the column of mesospheric sodium and maintain focus of the wave front sensor.
Three sets of sky tests have been conducted at the Starfire Optical Range with a continuous-wave, single-frequency, 20-W laser in preparation for a 50-W facility-class laser. Brightness measurements were made of the sodium guidestar produced with and without adaptive optics (AO) correction to the outgoing laser beam when it was either linearly or circularly polarized. Correcting for the transmission of our V filter at the sodium wavelength, a circularly polarized laser beam of 12 W out the telescope produced a guidestar of V=7.1 (1015 ph/s/cm2 at the top of the telescope). In general, a circularly polarized beam produces a guidestar between 75 and 100% brighter than a linearly polarized beam, indicating a significant degree of optical pumping of the sodium D2-line magnetic sublevels. However, guidestars produced with beams launched with tip-tilt correction only were 11% brighter than with beams launched with full AO correction. From deconvolved images of the guidestar taken with the 3.5-m telescope, the smallest spot, produced from a beam with 8.5 W of power out the telescope, circular polarization, and launched closed loop, had a Gaussian FWHM of 0.85 arcsec, or 38 cm at an altitude of 92 km. This corresponds to a peak Gaussian intensity of 3.8 mW/cm2.
An analytical/numerical approach, based on the Van Vleck Green function, was developed in order to analyze spectrally multiplexed beam combining of a linear array of fiber lasers, in which a blazed diffraction grating located in an external cavity plays the role of a spatially dispersive element. The focus is on the laser-external cavity coupling, which determines excess cavity loss, as affected by primary aberrations of the transform lens. Other issues are also touched upon, however, such as beam quality and bandwidth dependence on element location, and the optimization of the latter. Analytical results were supported by a more general numerical implementation. Typical values for bandwidth were found optimally as low as a few GHz, which, being substantially narrower than the free-running fiber laser line-width, maximum power limits are foreseeably determined by stimulated light scattering. A rough but encouraging degree of agreement of the resonator theory with independent lens aberration calculations and preliminary experiments performed at MIT/LL using triplet and quadruplet transform lenses was encountered so far.
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