WCL Key Features and Hardware

   An Ultra Pulsed Nd:YAG Laser from the Big Sky Laser Technologies Inc. providing a 20 Hz 16 mJ output at 355 nm, is used for the WCL-I. Operating at 355 nm not only makes it easy to achieve eye safe operation, it also provides a stronger molecular backscattering signal than a lidar operating at 532 or 1064 nm with the same laser energy. This is important for calibrating backscattering coefficients. The laser beam is expanded 5 times to a diameter of 15 mm before emittance into atmosphere, making the system eye safe beyond a distance ~65 m. To improve lidar linear depolarization measurements, a 1/2 λ wave plate is placed after the beam expander and this is coupled with a cubic polarization beam splitter in the receiver path.

   The receiver in WCL-I is based on 75mm refractive lens with 12.5mm collimated beam that enters into the cubic polarization beam splitter. The field of view is controlled by a pin hole located at the focal plane of the receiving lens. The PMT packages include narrow band filters (0.3 nm), a focus lens, and a compact PMT. To provide needed ruggedness and stability for WCL to operate in a vibration environment, the receiver is designed to share the same optical bench with the transmitter. The gain of the PMT can be easily adjusted with bias control voltage. Signals from the PMTs are sent to the LICEL data acquisition system. The data system has a combined A/D and photon counting capability. To provide high spatial measurements, only strong signals digitized by A/D at 40 MHz are saved at single shot or averaging of number of shots. Thus, the WCL can provide measurements at ~4.5 m horizontal and 3.75 m vertical resolutions from the UWKA (average cruise speed of ~90 m/s).

   In the WCL-II, an air-cooled diode pumped lasers from Photonics Industries International Inc. is used instead. Which can provide a 1 KHz 200 uJ laser pulse output at 355 nm.  A Newtonian reflective telescope with 108 mm effective aperture is employed as the receiver.  A high speed PCI based digitizer card is used for the data acquisition, which can provide 1.5m vertical resolution.