The system can be used to generate coherent signals at multiple remote locations by duplicating the assembly labeled Reference Generator Module (RGM) in Figure 1 and driving all RGMs with the same master synthesizer signals. If all RGMs and photodetectors are identical, and if each pair of fibers is reciprocal (same electrical length in both directions), then the output signals at all remote locations have equal phase, even if the fiber pairs to the various locations have vastly different lengths.
This system has an advantage over those that rely on manipulation of the fiber because it can correct variations in the fiber length much faster. In some applications, the dominant cause of changes in fiber length is temperature variation, but there are also circumstances where the fiber is subject to varying mechanical stress, such as when it must traverse the rotation axes of a movable reflector antenna. Temperature changes are often slow (and can be further slowed by adding insulation), but mechanical changes can be rapid. Here the correction speed is limited only by the round-trip signal time; for the loop to be stable, it must have a time constant at least several times the round-trip time.
It is also possible to vary the transmitted frequency f0 while maintaining constant phase differences among multiple remote locations. The rate of frequency change need only be slow enough that all of the correction loops remain phase locked. This system was implemented for operation in the range f0 = 437 to 454 MHz and was used to provide coherent reference signals to the antennas of a 5-element microwave phased array. In this frequency range, the RGM can be made small and inexpensive, as shown in Figure 2. Before deploying it in the array, the innovators conducted tests using a 305-m long spool of multi-fiber cable where both ends of the cable were in the laboratory so that the input and output phases could be compared. The spool was kept outdoors and subjected to diurnal temperature cycling over several days. With the correction loop disabled, phase changes of 26.5° peak-topeak were observed at 450 MHz with 23.9 °C peak-to-peak temperature change; this corresponds to a delay coefficient of 6.2 ps/°C, which is about as expected for this length of standard single-mode fiber. With the correction loop enabled, no variation with fiber temperature was detectable, and the measurements placed an upper limit on the coefficient of 0.054 ps/°C.
This work was done by Larry R. D’Addario and Joseph T. Trinh of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-46711.
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