A unique challenge in the development of a deep space optical software-defined radio (SDR) transmitter is the optimization of the extinction ratio (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.
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 that degrade the ER of the optical signal.
NASA Glenn Research Center developed the Cascaded Offset Optical Modulator, an electro-optic subsystem used to interface a binary output SDR to an optical transmission system. The 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 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.
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. 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.