Shared Aperture Diffractive Optical Elements for LIDAR Reception

Conical scanning LIDAR systems and laser ranging systems that measure atmospheric backscatter, clouds, aerosols, winds, trace species, and sub-surface ocean layers require ruggedized construction and easy alignment. We are proposing to reduce the complexity and alignment chores by building five telescope objectives into one large shared aperture using holographic volume multiplexing techniques similar to those used in some holographic data storage systems. The holographic objectives will then be stationary with five simultaneous look angles that may be turned on and off with a smaller coaxial out going beam scanner or a separate laser in each channel. This simple scheme can eliminate all moving parts which simplifies alignment while providing a more rugged and transportable system. The addition of a holographic aberration correction plate is now possible in a static position near the detector. This small HOE gets illuminated by incoming LIDAR signals and outputs a collimated wave which is easily focused by a simple doublet to a near diffraction limited spot. The benefit is that the field of view can be greatly reduced, thereby reducing background noise levels. The additional diffractive surface squares the wavelength dependant filtering function provided by diffractive optics, thereby further reducing out of band noise.