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Space-qualified fiber optic link for radar altimeter applications

This paper reports on the system design of an rf-modulated optical link for spaceborne radar altimeter applications and presents results of rf carrier phase noise and phase stability measurements. Our study involved the transmission of rf-modulated optical signals at 834 nm wavelengths using Bias-T and Mach-Zehnder modulators. The rf/microwave signals' phase coherence, modulation levels, and insertion loss are reported. Phase noise measurements revealed a noise floor of at least -118 dBc/Hz at frequencies greater than 100 Hz from a 5-MHz carrier with direct modulation. The phase noise was degraded by about 10 dBc/Hz for external modulation techniques. The rf insertion losses appear smallest for the Bias-T intensity modulators (approximately 30 dB at an 834 nm optical carrier with 5 MHz modulation). The results of modulation experiments with 320 MHz radar altimeter chirps are also presented with an emphasis on coherence, stability, and rise and fall time. The linear FM chirp signal ramps down at a rate of -3.125 MHz/microsecond(s) (+/- 0.5%) and is flat to within +/- 1 dB. Measurements show that this type of FM chirp modulated on optical carriers at 834 nm meets radar altimeter system requirements.