PMMA Based Holographic Optical Elements for Free Space Optical Communication Systems on Mobile Platforms

A Free Space Optical Communication Systems (FSOCS) enables broadband inter-connectivity among mobile platforms.  A key feature of an FSOCS is the low probability of intercept and the low probability of detection (LPI-LPD).  Furthermore, it is unhindered by RF jamming or frequency contention.   When augmented by a passive range detection capability, it also enables precise location knowledge, and continuous tracking during dynamic maneuvers.  However, the current technology used for FSOC involves large mirrors and lenses, thus making it bulky.  Holographic Optical Elements (HOEs) can be used to realize the same functionalities, but at a significant reduced SWaPC.    The work done during Phase I Base has demonstrated the feasibility of using HOEs to enhance the capabilities of an FSOCS.  Specifically, we have demonstrated the required functionality of all the relevant HOEs.   First, we developed a stream-lined process for making multiple holographic substrates based on PQ doped PMMA in parallel, with excellent optical quality.  Second, we developed a code for calculating the precise angles of the object and the reference beams to be used for writing multiple gratings in a single substrate for the task of combining a set of lasers at different  frequencies into the same direction.  We then demonstrated a three-channel holographic multiplexer/demultiplexer (HMD) which can combine lasers at 760 nm, 780 nm and 795 nm into a single direction, and can split incoming light into these three wavelengths.  Using a pair of these HMDs, we have also demonstrated the operation of a three channel FSOCS.  We have also developed a concrete design for a passive monocular ranging system (MRS) based on using one of these HMDs.  Finally, we have developed a set of converging as well as diverging holographic lenses.  Using a converging holographic lens, we have demonstrated collimation of a highly divergent input beam. Under the work proposed here, we will build on this work to achieve the following objectives.  During the Phase II Base, we will first demonstrate a WDM enhanced FSOCS with 10 channels, by using a pair of HMDs, for telecom band lasers ranging from 1545 nm to 1554 nm.  Next, we will demonstrate a passive Monocular Ranging System (MRS), using a three-channel HMD, for wavelengths of 760 nm, 780 nm, and 795 nm, a pointing camera and high-efficiency photodetectors.  This will be followed by the integration of a scannable converging holographic lens with the FSOCS and the MRS to produce a field of view as large as practically achievable, for each system.  During Phase II Option, we will first build a portable, compact and low SWaPC version of the receiver stage and the transmitter stage for a wide field of view 10-channel WDM  enhanced FSOCS, and carry out field tests of this system.  This will be followed by building a portable, compact and low SWaPC version of the wide field of view Monocular Ranging System (MRS), and carrying out field tests of this system.