Optoelectronic oscillators (OEOs) of a proposed type would be based partly on the use of fiber-optic linear or ring resonators in place of the long fiber-optic delay lines that have been used to obtain low phase noise in some previously developed OEOs. Although the proposal to use fiber-optic linear or ring resonators was made prior to the use of microsphere resonators described in the two preceding articles, this article appears as the third in the series because the two microsphere-related articles provide information that is prerequisite for appreciating the technical significance of the proposal.
The two preceding articles discuss two of the disadvantages of long fiber-optic delay lines; excessive weight and size, plus difficulty of selecting desired electromagnetic modes because of smallness of frequency intervals between modes. Two more disadvantages arise in conjunction with the need to prevent temperature-induced frequency drift: (1) it is difficult to stabilize the temperature on a long optical fiber, even when the fiber is coiled on a spool, and (2) optical fibers with low thermal expansion are expensive.
The figure illustrates an OEO with a fiber-optic linear resonator and one with a fiber-optic ring resonator. In the case of the linear resonator, the ends of the resonating length would be defined by Bragg gratings or, alternatively, by highly reflective coatings at the ends of the fiber. In the case of a linear resonator, light would propagate with multiple reflections from the ends; in the case of a ring resonator, light would propagate around the ring many times. Thus, in either case, the effective length of the resonator would be greater than the simple geometric length or circumference.
In either case, the frequency interval between modes would equal the free spectral range of the resonator. In order to obtain oscillation, the frequency of the laser carrier signal must equal that of a resonator mode; the frequencies of the laser-beam modulation sidebands must also equal frequencies of other resonator modes. To provide the necessary alignment of frequencies, the laser frequency must be stabilized at a peak of the resonator transmission spectrum. This can be accomplished by a feedback control subsystem that continually monitors the power of light reflected from the resonator and responds by adjusting the laser frequency to drive the reflected power toward a minimum.
If the resonator could be stabilized, then the absolute frequency of the laser would thus be stabilized. Taking advantage of the relatively small amount of fiber needed to achieve a large effective length, one could then fabricate the resonator from low-thermal-expansion fiber.
This work was done by Steve Yao and Lute Maleki of Caltech for NASA's Jet Propulsion Laboratory.
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This Brief includes a Technical Support Package (TSP).
Unfortunately the TSP Optoelectronic Oscillators Based on Fiber-Optic Resonators (reference NPO-20547-) appears to be missing from our system.