Michigan Aerospace Corporation - GroundWinds Overview

The GroundWinds LIDAR, while being a prototype for a space LIDAR, is optimized for operation from the ground to produce the best quality wind, temperature, density, and aerosol profiles in the vicinity of Mt. Washington. The space LIDAR will undoubtedly use ultra-violet light to allow the greatest molecular return at high altitudes, but GroundWinds would suffer from high attenuation near the telescope if a UV laser light source were used since it is observing from the ground at a relatively low elevation. Thus, a decision was made to use visible light for the GroundWinds LIDAR. This decision resulted in an optimized ground instrument; reduced cost and fabrication time and will provide a visible demonstration to visitors at Mt. Washington of the operation of the LIDAR system.

The GroundWinds system uses a commercial Nd:YAG laser that meets most of the optical requirements of the space LIDAR, but at considerably lower cost than would be required to procure a copy of a space-qualified engineering model. This laser also has considerably lower maintenance and operational costs associated with it. A custom Continuum 8010 laser was chosen for this system. The selected laser provides ~4 watts of output power at 532 nm. The laser is seeded to obtain a single frequency output, with a bandwidth comparable to that required for a satellite system. The pulse length of this laser was chosen to be somewhat longer than was actually required. The laser's oscillator cavity was modified to provide a 50 ns pulse. Initial assessments indicated that this would provide error margin in spectral resolution at a minimal cost and risk to the project. A pulse length of 8 - 10 ns is typical with this type of laser. In practice, the interferometer is not able to take advantage of the additional resolution that the laser provides with this stretched pulse (see section 1.4.4). The laser is fired at a repetition rate of 10 Hz.

Components of the Continuum 8010 laser:

1. Rear mirror (master oscillator)
2. Pockels cell
3. 1/4 wave plate
4. Dielectric polarizer
5. Master Oscillator pump chamber
6. Gaussian output coupler
7. Turning mirrors
8. Amplifier pump chamber
9. Harmonic generator
10. Dichroic mirrors for beam separation
11. Injection seeder
12. 1064 nm output
13. 532 nm output
14. Residual 1064 (dumped to a liqui-cooled heat sink)

The telescope subsystem consists of one transmitter telescope and one receiver telescope. The telescopes are mounted together on a common gimbal, which rotates 290 degrees in azimuth and be fixed in zenith at a 45 degree elevation. The limitation on azimuth angles is a device that has been designed to protect the fiber optic receiver. This device may be disabled to allow 360-degree operation. To meet the performance requirements, the receiver telescope has a usable diameter of 0.5 meters and the transmitter telescope has have a clear aperture sufficient to beam expand the laser to the desired beam divergence (see table 6.10). The expansion ratio for this beam expander is a function of the field of view of the telescope and the divergence of the laser beam. The main receiving telescope is coupled to the rest of the instrument using fiber optics (see section 6.6). We have chosen the gimbaled configuration because it is analogous to that required by a LIDAR system flown on a satellite.