OPTOELECTRONIC APPLICATIONS
Hassaun Jones-Bey
Optical pendulum regulates multigigahert¥clockResearchers at the Jet Propulsion Laboratory (JPL; Pasadena, CA) are preparing to place a 60-GH¥optical pendulum on a semiconductor chip, and it may someday keep time for a new generation of cellular phones. At present, the researchers joke about having one of their chips in every cellular phone 10 years from now, but they are quite serious in discussing the current need for higher-frequency capability.
"Right now cellular phones are at 900 MHz, and the bandwidth is already used up," said X. Steve Yao, a technical staff member in the JPL time and frequency systems research group. "And to have more bandwidth we need to go to higher frequencies. Sixty gigahert¥is the future for wireless communication, and right now there is no good oscillator to generate those high- frequency signals."
Under the direction of group supervisor Lute Maleki, the JPL team has developed such an oscillator using optoelectronic techniques. They expect the optoelectronic oscillator (OEO) to eventually fill several applications, such as low-noise oscillators for radar systems and flywheel oscillators for atomic clocks, in addition to wireless-communication applications such as cellular phones.
OEO design
The heart of the OEO might be described as an optical pendulum. "The essence of an oscillator is that you start with some phenomenon that is periodic," Maleki said. "You have a physical oscillator, such as a pendulum, and there`s a relationship between the frequency and the length of the pendulum." In an electromagnetic oscillator, he continued, periodicity is often generated by an inductance/capitance (LC) circuit. The LC circuit has resonance but does not resonate in and of itself. "You have to add a source of energy," he said.
For a pendulum in a grandfather clock, for instance, the source of energy is basically the initial push. In a vacuum-tube oscillator, the source of energy is the voltage source that pushes electrons off the heated cathode in the direction of the plate (see figure). The periodicity is provided by the LC feedback circuit that senses the plate current and modulates the grid current to set up oscillation, in accordance with the time-delay, or Q-value, of the LC circuit.
In the OEO, the power source and cathode are replaced by a diode-pumped Nd:YAG laser (Lightwave Electronics; Mountain View, CA). The grid is replaced by an electro-optic modulator, and the plate is replaced by a photodiode. The LC circuit is replaced by a long length of optical fiber. The key factor in replacing mechanical or electronic elements with optical ones, however, is the elimination of dispersive losses that limit oscillation frequency.
"The fiber is dispersive to the laser, but not to the modulation," Maleki said. "That`s the beauty of it." So while dispersive losses increase with frequency in mechanical and electromagnetic oscillators, performance actually improves with frequency in the OEO.
"The optoelectronic oscillator works even better if you go to higher fre- quencies, because the storage time multiplied by the frequency is the equivalent Q," said Vladimir Ilchenko, a visiting researcher from Moscow State University. "So as you go to higher frequencies, you get higher Qs."
Typical frequencies obtainable with the vacuum-tube design are on the order of kilohertz, and in the commonly used electromechanical quart¥oscillators one can reach tens of megahertz, Maleki said. But, he says, with the OEO, "I can set the frequency as high as I wish, limited only by the operation of the modulator. And you can get modulators nowadays at frequencies up to 100 GHz."
Applications
So far, the JPL team has used the OEO to demonstrate a device that "can recover a clock signal or carrier with a frequency up to 75 GH¥and that can be interfaced with a photonic communication system both electronically and optically."1 The researchers have also demonstrated "ultrastable, spectrally pure microwave reference frequencies as high as 75 GH¥with a phase noise below -140 dBc/H¥at 10 kHz, independent of oscillation frequency."2, 3
Currently the researchers are working to further improve the noise performance of these devices, but the key in moving them from the status of laboratory marvels to use in actual systems will be in integration of all of the components onto a semiconductor chip. The researchers have already fabricated a semiconductor laser and electro-absorption modulator on chip and are currently in the process of replacing the optical-fiber delay line with a microsphere that will delay the optical signal by circulating it in a "whispering gallery" mode, Maleki said.
"We know that losses can be 0.2 dB or less per kilometer in fiber, and if we are able to reproduce the same small attenuation in the sphere, the light would travel there for about 100 µs," Ilchenko said. "That`s equivalent to a propagation of many kilometers in fiber, which is why this miniaturization is very intriguing and promising."
REFERENCES
1. X. S. Yao and G. Lutes, IEEE Photon. Technol. Lett.8(5), 688 (May 1996).
2. X. S. Yao and L. Maleki , Opt. Lett. 21(7), 483 (April 1, 1996).
3. X. S. Yao and L. Maleki, J. Opt. Soc. Am. B 13(8), 1725 (August 1996).