A small amount (2 %) of each transmitted laser pulse is directed into a pair of 100 m long optical fibers, delayed, and injected back to the receiver for system calibrations (see Figure 5). Since some of the laser shots are unseeded, the delay is necessary so that the measured Q-switch buildup time of each pulse can be used as a quality control to trigger the data system only for seeded shots. The length of the fibers is set so that the time-separated pulses can be recorded in the data profile. The other ends of the calibration fibers are connected to a diffuse cylinder, which is located at the receiver such that the calibration light signals can be used to monitor system performance during data taking. Another 2 % of the laser light is taken into an energy monitor. The energy normalization of the received signal is realized by storing the energy of each laser pulse into the data record. A 4 cm long iodine cell is used for frequency locking of the laser. This cell provides 50 % absorption when the laser is tuned to the absorption peak. The frequency locking to the iodine peak is described later in Chapter 5.2.
Because the HSRL measures depolarization of the clouds and uses this information to separate between water and ice clouds, the polarization stage of the outgoing laser beam has to be well controlled. The output of the laser is linearly polarized and the orientation of the polarization vector is set perpendicular to the plane of the hypotenuse of the polarization cube. The use of the polarization cube guarantees that the residual cross-polarized component is cleaned out from the outgoing laser beam. In order to be able to use the same receiver optics and the same detector for both polarization components, the polarization of the outgoing laser pulse is rotated by 90 degrees for alternative laser pulses by a Pockels cell. The accuracy of the Pockels cell rotation is measured by observing the light coming from the Pockels cell through a polarizing cube with a photodetector. These calibrations show that the residual cross polarization can be reduced to 0.1 % of the parallel component. The calibration accuracy is limited by the 0.1 % rejection accuracy of the polarizing cube. The timing of the voltage switching between laser pulses is synchronized with the Q-switch signal and the controller electronics assures that the proper Pockels cell voltage is applied in time before the next laser shot. The final alignment between the transmitter and receiver polarization axes is performed by adjusting a half-waveplate and by using the atmosphere as a calibration target.
The use of small receiver field of views is made possible by decreasing the divergence of the outgoing laser beam with a beam expanding telescope (magnification 4x).