A proposed method of stabilizing microwave and millimeter-wave oscillators calls for the use of feedback in optoelectronic delay lines characterized by high values of the resonance quality factor (Q). The method would extend the applicability of optoelectronic feedback beyond the previously reported class of optoelectronic oscillators that comprise two-port electronic amplifiers in closed loops with high-Q feedback circuits.
The upper part of the figure illustrates the example of a typical free-running oscillator in a conventional form stabilized with an external metal radio-frequency (RF) resonant cavity. The oscillator could be of any of a variety of types, including those based on Gunn diodes, impact avalanche transit-time (IMPATT) diodes, klystrons, backward-wave tubes, and others. The maximum Q of a typical resonant metal cavity ranges from about 104 at an oscillation frequency of 10 GHz down to 103 at a frequency of 100 GHz and to even lower values at higher frequencies. In contrast, the maximum Q attainable in a resonator based on an optoelectronic delay line is of the order of 106 at a frequency of 10 GHz and increases with frequency.
The proposed method is partly similar to two older patented methods that involve the use of fiber-optic delay lines as RF-phase-noise discriminators. However, unlike those methods, the proposed method does not call for the generation of a low-frequency signal applied to a control port of the oscillator to be stabilized. Instead, the delayed RF signal would simply be returned to the oscillator, as described below.
The lower part of the figure shows an example of the same oscillator as before, but this time stabilized by use of optoelectronic feedback according to the proposed method. The RF signal from the oscillator would be fed through a circulator to an electro-optical modulator, which would modulate the RF signal onto a laser beam. After traveling the length of an optical fiber or other optical delay line, a photodetector would demodulate the signal. The RF output of the photodetector would be returned via the circulator to the oscillator.
The return of the delayed RF signal would enforce a steady phase in an otherwise noisy free-running oscillator, thereby suppressing phase noise in the oscillations. This stabilizing effect is expected as a consequence of the frequency-pulling effect or self-injection locking observed previously in oscillators equipped with high-Q external resonant cavities.
The original tunability of the free-running oscillator would be substantially preserved in the presence of optoelectronic stabilization, except as described next: Upon tuning of the oscillator, the frequency of the oscillator would not change continuously but would jump between successive resonances of the optoelectronic feedback loop . Typical frequency jumps would likely range from a few tens of kilohertz for a kilometer-long fiber-optic delay line up to a few gigahertz for an optical microresonator.
This work was done by Lute Maleki and Vladimir Iltchenko of Caltech for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-40144, volume and number of this NASA Tech Briefs issue, and the page number.