What is Coherent Lidar? Coherent lidar is a particular technique for measuring winds by determining the Doppler shift of backscattered radiation that has been originally transmitted (by the primary laser). The procedure is very photon-efficient in that it does not require the construction of a full spectrum of the returning light, rather only the frequency shift of the peak or centroid of the spectrum. The collected, scattered light is mixed with that from another laser (called the local oscillator) onto a light-sensitive detector. (This technique is called heterodyning.) The two frequencies mix such that the wave peaks reinforce each other, peaks with opposite signs cancel each other out, etc. The resulting signal is analyzed to obtain the difference frequency, which is typically in the radio-frequency regime, and which contains the information about the frequency shift, or winds. The advantage of the coherent technique, over one which must construct a frequency spectrum, is that it needs to collect just a few photons in order to get a measurement, rather than to collect enough light to obtain an intensity versus wavelength measurement. One of the challenges with this technique is that the only returning light that is useful is that which has remained coherent, which means that the wavefronts remain aligned in the same manner as which they left the instrument; thus, very precise optical alignments are crucial. Although this is a challenge, it is a very reasonable expectation with today’s state of engineering expertise. A disadvantage of the technique is that it relies on backscatter from aerosols in the atmosphere. In very clean air, then, one needs a powerful system in order to get a wind measurement. See the figure below for the expectation of SPARCLE and of a future space instrument. The color bars indicate the band of backscatter capability for various parts of the atmosphere, which is an indication of how “clean” they are. A future system would be able to acquire wind data even in the cleanest atmosphere. Lidar Tutorial Bibliography   Comments regarding our web service may be e-mailed to: paul.meyer@msfc.nasa.gov Responsible Official: Dr. James E. Arnold (jim.arnold@msfc.nasa.gov) Page Curator: Diane Samuelson (diane.samuelson@msfc.nasa.gov) Last Updated: July 8, 1998