Cascaded Offset Optical Modulator
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.
Lightweight, Self-Deployable Helical Antenna
NASA's newly developed antenna is lightweight (at or below 2 grams), low volume (at or below 1.2 cm3), and low stowage thickness (approx. 0.7 mm), all while delivering high performance (at or above 10 dBi gain). The antenna includes a novel design-material combination in a helical coil conformation. The design allows the antenna to compress for stowage (e.g., satellite launch), then self-deploy at the desired time in orbit. NASA's lightweight, self-deployable helical antenna can be integrated into a thin-film solar array (or other large deployable structures). Integrating antenna elements into deployable structures such as power generation arrays allows spacecraft designers to maximize the inherently limited resources (e.g., mass, volume, surface area) available in a small spacecraft. When used as a standalone (i.e., single antenna) setup, the the invention offers moderate advantages in terms of stowage thickness, volume, and mass. However, in applications that require antenna arrays, these advantages become multiplicative, resulting in the system offering the same or higher data rate performance while possessing a significantly reduced form factor. Prototypes of NASA's self-deployable, helical antenna have been fabricated in S-band, X-band, and Ka-band, all of which exhibited high performance. The antenna may find application in SmallSat communications (in deep space and LEO), as well as cases where low mass and stowage volume are valued and high antenna gain is required.
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