Atmospheric Lidar Observatory at Utah State University

The Atmospheric Lidar Observatory (ALO) gives rise to the green beam seen above the SER building at Utah State University (USU), Logan, Utah, on most clear nights, as shown in the photograph. It is a National Science Foundation (NSF) supported project involving the principal investigator Professor Vincent Wickwar of the Center for Atmospheric & Space Sciences at USU; graduate students Josh Herron, Karen Nelson and Troy Wynn; and staff member Spencer Nelson.

The lidar (light detection and ranging) consists primarily of a very powerful Nd:YAG laser, a 44-cm diameter telescope, and a sensitive detector for the laser light coming back from the sky. It is a radar system based on green light instead of radio waves. Another laser, alexandrite, will soon be added to our lidar facility. The Nd:YAG laser is shown in Figure 1 (called a "block diagram"). A list of the technical information on the Nd:YAG lidar is given in Table 1. The ND:YAG laser generates pulses of infared light at 1065 nm (1065 x 10^-9 meters) that we then "frequency-double" to produce green light at 532 nm.  Each pulse is 8 nanoseconds long or, equivalently, 2.4 meters long. Moving at the speed of light, each pulse appears to the eye to be a continuous beam. With the laser emitting 30 pulses per second (the same rate at which a TV screen is refreshed), no flicker is apparent to most people. However, if you move your head rapidly from side to side, distinct beams are apparent.

The beam is almost parallel, but it does diverge or spread out a small amount. The angle is 0.5 mrad, which means that the laser beam is 1/2 m in diameter at one km altitude, 5-m in diameter at 10 km, and 50-m in diameter at 100 km. The beam is adjusted to be vertical. If you look straight at it from the side, it will appear to be vertical. However, the eye plays interesting tricks. If you turn 30° to either side of the beam and look up, the beam will appear to bend over toward the direction in which you are looking.

You can see the beam because a very small portion of the light is scattered off the molecules in the atmosphere, mostly N₂. This process is called Rayleigh scattering: It is the same process that makes the sky appear blue and makes the sun or moon appear orange when on the horizon. Occasionally, there will be a particularly bright spot. This is usually scattering off of dust particles, which is Mie scattering.

In this second photograph, we see the laser with a Newtonian telescope behind it. To the right of the laser (not seen), under a plastic bag to keep dust off, is a dichroic to reflect the green laser beam to the left behind black cardboard and to transmit the portion of the original IR radiation that is not converted to green into a beam dump (a safe place to put strong excess light). Behind the cardboard is another dichroic to reflect the beam up a chimney to the roof. The light backscattered from the molecules and dust comes back down the chimney and is reflected by a big flat mirror into the telescope (the long white cylinder) just behind the laser.

Although the laser beam going up the chimney is very intense, so little light is backscattered that a large telescope is needed to capture it. This Newtonian telescope focuses the light to a point up in a chamber just above it. The light is then directed to Optics and Chopper and the EMI PMT detector. The PMT (photomultiplier tube) is a very sensitive detector, and the chopper prevents the very low altitude light from damaging this detector, which is so sensitive it can detect single photons.  To attain this sensitivity, while avoiding "detector noise", we cool the PMT to -20° C. Partially blocking the detector chamber, and above the laser in the photograph, are two big air filters to remove dust from the air blowing across the laser. The green cylinder at the right of the picture provides very clean N₂ gas that is blown across all the optical surfaces. (Considerable attention is paid to keep the optical surfaces clean. Otherwise, the laser beam can burn the surface coating.)

The signal from the PMT is sampled every 250 ns (37.5 m) between 0 and 525 km, accumulated for two minutes, and stored on hard disk for later data reduction. This is done using an EG&G multichannel scaler connected to a 486 computer. The accumulating data are visible in real time on the computer monitor. At the end of the night the data are compressed and then reduced to determine atmospheric density and temperature.

Examples of temperature results are shown in Figure 2. On the left in the top half of the figure are temperature profiles obtained from averaging the data for one-hour intervals. The data from two channels have been combined to cover the altitude range from 35 to 85 km. The error bars are shown. The smooth curves are from an atmospheric model. On the left is the temperature profile obtained by averaging all the data for the whole night. On the whole, the temperature profiles on this day are similar to the model, except between 45 and 65 km and, possibly, near 80 km. The lower part of the figure shows monthly averages of the temperatures for January and March. In both these cases we see much bigger differences between the observations and model profiles. It is these differences that we are interested in learning about. What causes them and why do they vary during the year? Another point of interest is to keep track of the temperatures near 55 km. Over the course of 10 to 20 years, they may give us an indication about climate change.