A Facility Laser Beacon System for Astronomy

F. Shi, M. F. Smutko, M. Chun, J. Larkin, V. Scor, W. Wild, & E. Kibblewhite

(This is a partial "reprint" of a poster paper presented at the January 1996 meeting of the American Astronomical Society.)


More laser photos


ChAOS sodium laser beacon. In December 1995, our sodium laser was launched from the Apache Point telescope through a 12 inch lens mounted on the corner of the secondary mirror support structure. This lens is visible in the lower right-hand corner of the photo. Three flat mirrors and a field lens guide the beam from the laser to the lens. Boresighting and aligning the laser with the 3.5m telescope are done by tilting these three mirrors. Note that the sodium spot is not visible in this photo; the bright spot at the end of the beam is caused by reflection off of a thin layer of clouds.

Diode-laser mounted on the telescope. This photo shows the laser mounted on the Nasmyth port of the 3.5m telescope. A thermally insulated "house" was constructed to prevent heat leakage into the dome. Additionally, the entire optical bench of the laser was placed into a thermally controlled box to prevent temperature fluctuations. This photo was taken right after the laser was first lowered onto the Nasmyth port (by removing the roof of the house).

Diode-pumped sum frequency laser. Left: Two mode locked Nd:YAG lasers, one tuned to 1.064Ám and the other to 1.319Ám, are mixed in non-linear crystals to generate light at the sodium D2 line (0.589Ám). The laser outputs 7-9 Watts of high-quality sodium light with a FWHM line width of about 1GHz. Right: In this photo, the 1.064Ám laser is on the left (seen as green because of frequency doubling), the 1.319Ám laser is in the middle (fairly invisible), and the summed-frequency sodium laser is visible on the right side of the bench.

Laser Pulses. Left: Transform-limited spectral profile of the 0.589Ám sodium laser. Free spectral range of the spectrum analyzer is 8 GHz. FWHM of the spectrum is about 1 GHz. The laser line width can be changed from about 1 to 3 GHz using mode lock "chirping". Center: The laser operates at a pulse rate of 400 Hz. This plot shows the laser's micropulse temporal profile. Right: This plot shows the laser's macropulse temporal profile (which lasts about 150Ás).

Laser Profiles Theoretical instantaneous laser beam profile at the sodium layer for an optimized 0.5 meter launch telescope. The three curves correspond to seeing of 1, 2/3, and 1/2 arcsecons.

Lincoln Labs laser group. Pictured here are some of the people at Lincoln Labs who created our sum frequency laser.


Advantages and Disadvantage of the Sum Frequency Laser

Advantages:

Disadvantage:


Saturation Effects

Radiative saturation becomes significant (i.e. effective cross-section reduced by a factor of two) at: I ~ 6mW/cm^2/line-width

Usable line widthdelta nu ~ 3 GHz
Sodium natural line widthdelta nu ~ 10 MHz
Saturation effects atI ~ 2W/cm^2
Spot size expected at good site20 cm FWHM
Laser peak powerP < 1000W
Laser mean powerP ~ 1000W x duty cycle
Laser power required for high order AOP ~ 10W

Past Laser Experiments

MMT Experiment. Left: In 1993, in a joint experiment with the University of Arizona adaptive optics group, we took our CW laser to the MMT. The laser was projected out of a 75cm guide telescope in the center of the MMT. The U of A group was able to "close" their adaptive optics system with this beacon. Right: This is a contour plot of the sodium beacon produced in the MMT experiment. The beacon was imaged through a single mirror of the MMT. In this 2.6 sec exposure with a 1.2 Watt CW laser, the artificial star is equivalent to a star of V magnitude 12.5 and has a FWHM of 1.0 arcsec x 1.3 arcsec. The slight elongation is due to the 2.5m offset between the beam projector and the receiving telescope.

Yerkes Experiments. Left: In 1991, we projected our dye laser through a five inch telescope at Yerkes Observatory and viewed the sodium beacon with the famous 40 inch refractor. Right: In this false-color and highly foreshortened view of the return, the streak at lower right is produced by low-altitude Rayleigh backscatter and becomes most intense (red spot) at 23km where the beam encounters volcanic dust from an eruption of Mt. Pinatubo. Farther up, the backscatter fades as the air thins. At upper left is the 12th magnitude artificial sodium star at an altitude of 90km.

Our CW dye laser. Nestor Farmiga adjusts our continuous wave dye laser in the basement of Yerkes Observatory.


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