Rayleigh scattering (contd...)

An important application of Rayleigh scattering is in the measurement of the temperature profile in the middle atmosphere of the earth. The instrument used for this purpose is called LIDAR ( light detection and ranging ). A schematic drawing of a hand-assembled LIDAR is shown in Figure 7.8. A brief description of the instrument is given here. A powerful pulsed Nd:YAG laser, 10 MW peak power within a pulse width of less than 10 ns, 532 nm wavelength, 30 Hz repetition frequency is used. The high intensity of the laser permits measurements during daytime as much as night-time, not being influenced by the glare of the sun. Scattered radiation is collected in the back-scatter mode with a narrow field-of-view within 0.1 mrad, thus enabling a high signal-to-noise ratio. The telescope aperture is quite large, being around 1.2 m. The laser line is isolated using a Fabry-Perot etalon and a narrow-band interference filter.

The lidar operation can be described as follows. Pulses from the laser transmitter are coupled into the telescope so that, in effect, a collimated beam is transmitted into the sky. Light back-scattered from the atmosphere is collected back by the telescope. The scattered light is collected by the detector through a transmit-receive switch that prevents light from returning to the laser. The switch relies on the change in polarization of the scattered light with respect to the original. Specifically, in the transmit phase, the plane polarized light from the laser passes through the polarizing beam splitter and is converted into circular polarization by a quarter wave plate. The back-scattered light, still circularly polarized, is collected by the receiver, converted into s-polarized light by the quarter wave plate, and directed into the detector by the polarizing beam splitter.

The low altitude signals tend to be strong band and can be removed by a rotating mechanical shutter. The background light at wavelengths different from that of the laser are rejected by using a tiltable Fabry-Perot interferometer (10 GHz band pass) and an interference filter (IF). The signal detector is a photomultiplier tube and a fast counter. Differences in altitude that act as sites of backscatter can be distinguished because of the delay in photons returning to the detector. As per Equation 7.5, the intensity ratio between the outgoing and scattered radiation is a measure of the number density, and indirectly, from the equation of state, the local temperature in the earth's atmosphere.