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The NDL uses homodyne detection to obtain changes in signal frequency caused by a target of interest. Frequency associated with each segment of the modulated waveform collected by the instrument is positive or negative, depending on the relative range and direction of motion between the NDL and the target. Homodyne detection offers a direct measurement of signal frequency changes however only the absolute values of the frequencies are measured, therefore additional information is necessary to determine positive or negative sign of the detected frequencies. The three segmented waveform, as opposed to conventional two-segmented ones, allows for resolving the frequency sign ambiguity. In a practical system, there are times when one or more of the three frequencies are not available during a measurement. For these cases, knowledge of the relative positions of the frequency sideband components is used to predict direction of the Doppler shift and sign, and thus make correct range and velocity measurements. This algorithm provides estimates to the sign of the intermediate frequencies. The instrument operates continuously in real time, producing independent range and velocity measurements by each line of sight used to take the measurement. In case of loss of one of the three frequencies, past measurements of range and velocity are used by the algorithm to provide estimates of the expected new range and velocity measurement. These estimates are obtained by applying an estimation filter to past measurements. These estimates are used during signal loss to reduce uncertainty in the sign of the frequencies measured once signals are re-established, and never to replace value of a measurement.

The innovation expands the range of signals that coherent optical receivers can detect. Unlike traditional systems with fixed LOs, this approach allows real-time adjustments to an LO’s properties, such as frequency, phase, polarization, amplitude, spatial mode, or timing, to better match incoming signals. These adjustments improve measurement accuracy and signal recovery in various scenarios, such as shifting heterodyne frequencies into the receiver’s bandwidth or adapting to different signal polarizations. The innovation lies in the ability to switch an LO's configurations on the fly using technologies like fiber-optic or integrated photonic switches, as well as other methods like optical modulation or tunable delay lines. This dynamic capability allows coherent receivers to switch seamlessly between range-Doppler and Doppler-only modes. As a result, a single system can track both nearby, slow-moving targets and distant, high-velocity objects (up to 20+ km/s) while operating with a compact, low-speed receiver (<1 GHz). This versatility significantly enhances the performance and adaptability of advanced optical sensing and communication systems. While initially developed for NASA’s Navigation Doppler LiDAR project, this invention can support advanced signal detection applications across several industries, including optical communications, aerospace, structural health monitoring, and autonomous vehicles. By enabling real-time reconfiguration, the invention offers improved signal recovery, enhanced sensitivity, and broader system adaptability for challenging detection environments. The technology has gone through prototype development and is currently at TRL 3 (proof-of-concept), and the reconfigured LO is available for patent licensing.

The NASA receiver is specifically designed for use in coherent LiDAR systems that leverage high-energy (i.e., > 1mJ) fiber laser transmitters. Within the receiver, an outgoing laser pulse from the high-energy laser transmitter is precisely manipulated using robust dielectric and coated optics including mirrors, waveplates, a beamsplitter, and a beam expander. These components appropriately condition and direct the high-energy light out of the instrument to the atmosphere for measurement. Lower energy atmospheric backscatter that returns to the system is captured, manipulated, and directed using several of the previously noted high-energy compatible bulk optics. The beam splitter redirects the return signal to mirrors and a waveplate ahead of a mode-matching component that couples the signal to a fiber optic cable that is routed to a 50/50 coupler photodetector. The receiver’s hybrid optic design capitalizes on the advantages of both high-energy bulk optics and fiber optics, resulting in order-of-magnitude enhancement in performance, enhanced functionality, and increased flexibility that make it ideal for long-distance or low-backscatter LiDAR applications. The related patent is now available to license. Please note that NASA does not manufacturer products itself for commercial sale.