Thermally-Adaptive Solid State Laser Crystal Mount

The new NASA LaRC Pulsed 2-Micron Laser Transmitter for Coherent 3-D Doppler Wind Lidar Systems is an innovative concept and architecture based on a Tm:Fiber laser end-pumped Ho:YAG laser transmitter. This transmitter meets the requirements for space-based coherent Doppler wind lidar while reducing the mission failure risks. A key advantage of this YAG based transmitter technology includes the fact that the design is based on mature and low-risk space-qualified YAG host crystal. The transmitter operates at a 2096 nm wavelength using Ho:YAG, resulting in high atmospheric transmission (>99%), versus a transmitter operating at 2053 nm using co- doped Tm:Ho:LuLiF, which suffers limited transmission (90%) due to water vapor interference. In-band pumping through Tm:Fiber pump Ho:YAG architecture offers lower quantum defect from 1908 to 2096 nm (9.1%) compared to traditionally used co-doped Tm:Ho:LuLiF of 792 to 2051 nm (61%). The transmitter has an efficient pump compared to LuLF, since YAG has 27% higher pump absorption and 52% lower reabsorption of the emitted 2-micron, resulting in higher efficiency and lower heat load. Being isotropic, YAG is amenable for spatial-hole burning mitigation which supports linear cavity architecture without compromising injection seeding quality. This attribute is important in designing a compact, stable, high seeding efficiency laser. A folded linear cavity for long pulse (>200 ns), transform limited line-width (2.2 MHz) and high beam quality (M2 = 1.04) - the most critical parameters for coherent detection - are easier to achieve using YAG compared to LuLF. Lower heat load results in high repetition rate (>300 Hz) operation, which allows higher probability of wind measurements through broken clouds, off clouds, and below clouds, thus reducing errors and increasing science data product quantity and quality.

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.

Below an MPL’s minimum overlap range, the return signals are not completely in the instrument’s field of view, so the receiver only captures a portion of the backscatter laser pulse. MPL overlap ranges vary, but is usually between 4-8 km, encompassing the lower atmosphere where most aerosols reside. Commonly, correction entails recording horizontal profiles that require a ~10 km clear line-of-sight and homogenous atmospheric conditions, limiting the solution’s practicality. In contrast, NASA’s WFR device corrects for the overlap using a second receiver co-aligned with the MPL that captures the same backscattered laser pulses as the MPL receiver, but with a ~20x wider FOV that enables a much shorter overlap range from ~6 km down to 250 m. Thus, the combination of the WFR and MPL can capture accurate signals from near surface to the stratosphere. The WFR utilizes the same detector as the MPL, enabling it to connect to the MPL data system for synced data acquisition. By eliminating the need for homogeneous horizontal measurements to determine the MPL overlap function, overlap corrections are more easily and more frequently obtained. Further, the WFR mount base was designed to easily integrate with MPLs. NASA originally developed this device to improve accuracy of MPLs in the MPLNET, ensuring data collected are both accurate and reliable, thereby enhancing our understanding of atmospheric processes and contributing to more informed climate research and environmental modeling. The technology’s operational ease, flexibility, and cost savings are relevant to a wide range of scientific, environmental, and industrial applications. Companies that manufacture and sell MPLs may wish to offer this advanced calibration device as a product to enhance accuracy of MPL-based measurements. This NASA technology is at TRL 8 (Actual system completed and "flight qualified" through test and demonstration.) and is available for patent licensing.

This new methodology selectively scans an area of interest and effectively pre-compresses the image data. Instead of using LiDAR resources to gather redundant data, only the necessary data is gathered and the redundancy can be used to fill in up-sampled data using intelligent completion algorithms. The system utilizes a unique LiDAR system to collect a pattern of specific points across a given area by modulating the incoming light, creating a pattern that can be decoded computationally to reconstruct a scene. By designing specific coding patterns, the system can strategically skip certain measurements during the scanning process to create an under-sampled image area. The system reconstructs the under-sampled area to recreate an accurate representation of the original object or area being scanned. As a result, redundant data is prevented from being collected by reducing the number of required measurements and data condensed in post-collection to reduce power consumption. By selectively skipping certain pixels during the scan and using sophisticated recovery algorithms to reconstruct the omitted information, the system makes more efficient use of the available photons, thereby enhancing overall data collection. This technology represents a significant advancement in LiDAR systems, offering a more useful method for data collection and processing and addresses the challenges of power consumption and data redundancy, allowing for more sustainable and effective remote sensing applications. This technology can offer advantages in applications such as mapping for construction, surveying, forestry, or farming as well as computer vision for vehicles or robotics.

A new method is described for optical data transmissions from satellites using laser arrays for fine pointing of laser beams that use body pointing. It combines a small lens system and a VCSEL/Photodetector Array in a novel way to provide a fine pointing capability for laser beams that are pointed by body pointing of a CubeSat. As Fig. 1 shows, an incoming laser beam (green or blue, with rightward arrows), transmitted from a ground terminal, enters the lens system, which directs it to an element of the pixel array (gray rectangle). Each element, or pixel, consists of a VCSEL component/photodetector pair. The photodetector detects the incoming beam, and the VCSEL component returns a modulated beam to the lens system (green or blue, with leftward arrows), which sends it to the ground terminal. As the incoming beam changes direction, e.g., from the blue to the green incoming direction, this change is detected by the adjacent photodetector, and the laser paired with that photodetector is turned on to keep the outgoing laser beam on target. The laser beams overlap so that the returning beam continues to point at the ground terminal. The VCSEL component may consist of a single VCSEL or a cluster of VCSELs. Figure 2 shows the propagation of two overlapping laser beams. The system can very accurately point finely focused diffraction-limited laser beams. Also, simultaneous optical multiple access (OMA) is possible from different transceivers within the area covered by the laser array. For this electro-optical system, reaction times to pointing changes and vibrations are on the nanosecond time scale, much faster than mechanical fine pointing systems.