Tunable, Highly Stable Lasers for Coherent Lidar

Practical space-based coherent laser radar systems envisioned for global winds measurement must be very efficient and must contend with unique problems associated with the large platform velocities that the instruments experience in orbit. To compensate for these large platform-induced Doppler shifts in space-based applications, agilefrequency offset-locking of two singlefrequency Doppler reference lasers was thoroughly investigated. Such techniques involve actively locking a frequency- agile master oscillator (MO) source to a comparatively static local oscillator (LO) laser, and effectively producing an offset between MO (the lidar slave oscillator seed source, typically) and heterodyne signal receiver LO that lowers the bandwidth of the receiver data-collection system and permits use of very high-quantum-efficiency, reasonably- low-bandwidth heterodyne photoreceiver detectors and circuits. Similar techniques are being applied in atmospheric CO2 differential-absorption lidar work, where MO sources need to be actively offset-locked to CO2 reference cells for continuous absolute-calibration purposes. Active MO/LO offset- locking is also highly applicable to lidar problems involving very high target velocities with respect to a static or moving lidar platform.