Radio Frequency Interferometry Using Optical Fiber Sensing Signal Processing

NASA Goddard Space Flight Center has developed a configurable phase mirror system that can address likely obstacles in space optical communications. Through using miniature adjustable mirrors and programmed phase delays to diffract a single communication beam, numerous diffracted beams can be sent to other satellites in various directions for communication and tracking. The initial laser beams wave profile can be dynamically regulated through a fast Fourier transform (FFT) so that when it reaches its desired destination, it forms an intended illuminated spot at the target satellite. Since all the diffracted beams share the same phase mirror, the antenna gain needed to broadcast these beams does not require a multiplied aperture.

A unique challenge in the development of a deep space optical SDR transmitter is the optimization of the ER. For a Mars to Earth optical link, an ER of greater than 33 dB may be necessary. A high ER, however, can be difficult to achieve at the low Pulse Position Modulation (PPM) orders and narrow slot widths required for high data rates. The Cascaded Offset Optical Modulator architecture addresses this difficulty by reducing the width of the PPM pulse within the optical modulation subsystem, which relieves the SDR of the high signal quality requirements imposed by the use of an MZM. With the addition of a second MZM and a variable time delay, all of the non-idealities in the electrical signal can be compensated by slightly offsetting the modulation of the laser. The pulse output is only at maximum intensity during the overlap of the two MZMs. The width of the output pulse is effectively reduced by the offset between MZMs. Measurement and analysis of the system displayed, for a 1 nanosecond pulse width, extinction ratios of of 32.5 dB, 39.1 dB, 41.6 dB, 43.3 dB, 45.8 dB, and 48.2 dB for PPM orders of 4, 16, 32, 64, 128, and 256, respectively. This approach is not limited to deep space optical communications, but can be applied to any optical transmission system that requires high fidelity binary pulses without a complex component. The system could be used as a drop-in upgrade to many existing optical transmitters, not only in free space, but also in fiber. The system could also be implemented in different ways. With an increase in ER, the engineer has the choice of using the excess ER for channel capacity, or simplifying other parts of the system. The extra ER could be traded for reduced laser power, elimination of optical amplifiers, or decreased system complexity and efficiency.

This invention provides a new method for optical data transmissions from satellites using laser arrays for laser beam pointing. The system is simple, static, compact, and provides accurate pointing, acquisition, and tracking (PAT). It combines a lens system and a vertical-cavity surface-emitting laser VCSEL)/Photodetector Array, both mature technologies, in a novel way for PAT. It can improve the PAT system's size, weight, and power (SWaP) in comparison to current systems. Preliminary analysis indicates that this system is applicable to transmissions between satellites in low-Earth orbit (LEO) and ground terminals. Computer simulations using this design have been made for the application of this innovation to a CubeSat in LEO. The computer simulations included modeling the laser source and diffraction effects due to wave optics. The pointing used a diffraction limited lens system and a VCSEL array. These capabilities make it possible to model laser beam propagation over long space communication distances. Laser beam pointing is very challenging for LEO, including science missions. Current architectures use dynamical systems, (i.e., moving parts, e.g., fast-steering mirrors (FSM), and/or gimbals) to turn the laser to point to the ground terminal, and some use vibration isolation platforms as well. This static system has the potential to replace the current dynamic systems and vibration isolation platforms, dependent on studies for the particular application. For these electro-optical systems, reaction times to pointing changes and vibrations are on the nanosecond time scale, much faster than those for mechanical systems. For LEO terminals, slew rates are not a concern with this new system.

JPL's laser communications transceiver comprises two primary modules: an optics module and an electronics/laser module. The optics module includes a 5-cm diameter telescope, a two-axis coarse-pointing gimbal, monitoring sensors, and thermal control. The electronics module includes a transmitter, processor, controllers, and power conditioning. Keeping optical uplink rate modest and emphasizing downlink, the high-bandwidth downlink transmitter uses coarse wavelength-division multiplexing for operation at four 2.5-Gb/s channels (a total data-rate of 10 Gb/s). Applying this technique enables the use of larger active-area photodetectors at the ground station, which reduces the atmospheric scintillation/turbulence effects on received beam; these effects are further reduced with forward-error correction and deep-interleaver codes. A compact laser communications transceiver with a single transmit/receive aperture has been built using components with traceability to flight qualification (i.e., a flight-qualified version is commercially available). The transmit downlink wavelengths fall within the standard C-band telecom grid of erbium-doped fiber amplifiers (EDFAs): 1530 to 1560 nm. In the interest of eye safety, reduced background light, and reduced atmospheric turbulence effects, the received uplink beacon wavelength is longer (1568 nm).

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.