An optical frequency generator could produce radio-frequency (RF) output with both lower noise and lower power consumption than is possible with currently used equipment. The optical RF synthesizer, patented by Ronald Logan Jr. while at the Caltech Jet Propulsion Laboratory (Pasadena, CA; US patent #5,379,309), generates phase-stable, intensity-modulated optical carriers with frequencies ranging from about 1 GHz to more than 100 GHz.1 A host of applications that use optical fibers, ranging from communications to radio astronomy, could benefit from the use of intensity-modulated signals at microwave and millimeterwave frequencies.
Such signals are currently generated both optically and electronically. Optical heterodyne signal-generation systems are based on two CW lasers tuned to slightly different frequencies, f1 and f2. The two laser ouputs are combined, and the difference between the two frequencies results in a beat at a third frequency (f1–f2). The frequency difference between the two lasers can be selected to produce a desired beat frequency. Applications that require RF signals modulated on optical carriers can use this signal directly, whereas applications that require an electronic RF signal use a fast detector to convert the optical signal to an electronic one.
One drawback of this method is that the two source lasers are not in phase, and the phase fluctuations introduce noise to the beat-frequency output. Many applications also require the RF output signal to be in phase with a reference oscillator—but the noisy output from these optical generators is difficult to force into phase coherence at microwave and millimeterwave frequencies. Such problems have limited the application of optical heterodyning for RF generation. Instead, electronic generation is used.
Electronic RF generators are used for many nonoptical applications. Encoding electronically synthesized signals onto optical carriers is, however, a problem. According to Logan, optical intensity modulators are inefficient for frequencies from 20 to 100 GHz. Widely tunable electronic generators also tend to be large and expensive devices. Thus, a commercially available electronic frequency generator from Hewlett-Packard (Santa Clara, CA) with a range from 10 MHz to 50 GHz is a 65-lb, $60,000 device.
Phase coherence
In a paper to be presented at the 1996 SPIE Annual Meeting (paper 2844-40; Denver, CO) this month, Logan describes his refined optical heterodyne signal generator. The two CW lasers are each injection-locked to a different frequency chosen from the available longitudinal optical modes of another modelocked laser. The modelocked laser, in turn, is driven by an electronic reference oscillator. This arrangement creates a “comb” of many frequencies, separated by the reference oscillator frequency, from which the frequencies f1 and f2 can be chosen.
Active modelocking forces coherence between the different longitudinal modes of the reference laser, and the modes are used to injection-lock the other lasers. The phase noise in each of the injection-locked lasers is thus correlated with the other, so much of the noise is canceled when the two optical signals are combined. The beat-frequency output signal and the electronic reference oscillator have a high degree of phase coherence with each other.
All three lasers could be semiconductor devices, so the design is well suited to integration, which could make mass production of the optical generator feasible. Higher-frequency modulation could be used to provide higher-speed fiberoptic telecommunications. The generator could also be used in microwave and millimeterwave test equipment, laser ranging systems, and fiberoptic delay-line stabilized oscillators.
One nonoptical application that could use this device is RF upconversion and downconversion: electronic devices mix a local oscillator signal with a higher-frequency signal but require several stages to downconvert from millimeter waves to DC. His device, says Logan, could provide the conversion range in a single stage.
Logan, who is currently manager of advanced photonic technology at Uniphase Telecommunication Products (Chalfont, PA), says that his refined optical heterodyne signal generator has yet to be demonstrated, but his company has made a funding proposal to the US Advanced Research Projects Agency. Despite the lack of a demonstration, the chair of the conference at which Logan will present his paper, Brian Hendrickson, is enthusiastic about the work. Hendrickson calls it a “revolutionary frequency generator,” adding, “The device has a potential 400 to 1 reduction in size over conventional electronic synthesizers and very low power consumption.”