Analog Phase Modulation for Avionics Applications

Radio-frequency (RF) photonics and fiber optics play an important role in future airborne applications, leveraging advantages over traditional RF cabling such as decreased size and weight, increased flexibility, lower signal loss in fiber, larger bandwidth, and invulnerability to electromagnetic interference. In military electromagnetic warfare applications, RF links can be used route microwave signals throughout the airframe where analog-to-digital conversion is not practical or possible. Traditionally, intensity modulation with direct detection architectures have been proposed for such links, but here we suggest phase modulation with interferometric detection as an alternative. Phase modulation with interferometric detection (La Gasse and Thanivyavarn, 1997) offers numerous advantages over intensity modulation with direct detection. For example, a phase modulator requires no electronics as opposed to a Mach-Zehnder modulator (MZM), which requires a quadrature bias voltage. This is particularly advantageous in airborne applications where the modulator may be required in a harsh environment such as on the wingtip or in a sensor pod. In addition, the ruggedization of a phase modulator should be more practical than that for the multi-path MZM architecture, owing to a phase modulator's relative simplicity. The authors show that phase modulation with interferometric detection can outperform intensity modulation with direct detection in terms of all RF performance metrics. While not of immediate concern in avionic applications, where the transmission lengths do not exceed a few hundred meters, the constant-intensity phase modulation format is also less susceptible to fiber nonlinearities. The tradeoffs for the benefits afforded by phase modulation with interferometric detection are reduced bandwidth and increased complexity at the terminal end of the RF link.