Small Spacecraft Electric Propulsion (SSEP) Technology Suite

communications
Cascaded Offset Optical Modulator (LEW-TOPS-148)
For Optical Communications
Overview
Innovators at the NASA Glenn Research Center have developed the Cascaded Offset Optical Modulator, an electro-optic subsystem used to interface a binary output Software Defined Radio (SDR) to an optical transmission system. Traditionally, this task is accomplished through the use of a Mach Zehnder Modulator (MZM), a primarily analog component. In order to generate a high-fidelity optical signal, the input electrical signal must be of equally high fidelity, which is a difficult task due to the competing requirements of the digital components and the analog modulator. The digital components required to generate the waveform contain non-idealities which degrade the extinction ratio (ER) of the optical signal. The Cascaded Offset Optical Modulator corrects these non-idealities in the electro-optic subsystem during modulation by relieving the SDR of the extreme fidelity requirements imposed by the optical modulator. This approach is valid in any optical transmission system that requires high fidelity binary pulses without a complex component.

The Technology
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.
FIG. 1: A modulator/controller circuit board of the Cascaded Offset Optical Modulator
Benefits
  • Reduced complexity for optical modulators in Pulse Position Modulation (PPM) systems: The Cascaded Offset Optical Modulator reduces the signal quality requirement for the SDR, resulting in a less complex and more cost effective implementation
  • Increased channel capacity and reduced SWaP: This innovation simplifies the electrical portion of an electro-optic system, which can significantly increase channel capacity and reduce size, weight, and power (SWaP)
  • High-quality PPM signal: The Cascaded Offset Optical Modulator has achieved a signal with a 40 dB extinction ratio for PPM32 and a 48 db extinction ration for PPM256
  • Compatible with current communications infrastructure: Drop-in upgrade to many existing optical transmitters in free space and fiber

Applications
  • Free-space optical communications
  • Fiber-optic communications
Technology Details

communications
LEW-TOPS-148
LEW-19698-1
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