Optoelectronic Generation of Optical and Microwave Signals

NASA's Jet Propulsion Laboratory, Pasadena, California

The figure schematically illustrates an optoelectronic apparatus that generates both an optical output and an electronic output (typically at frequencies in the range from hundreds of megahertz to tens of gigahertz). The apparatus is denoted a "coupled opto-electronic oscillator" (COEO) because its optical and electronic oscillations are coupled to each other.

The COEO is the latest product of a continuing effort to develop photonic/electronic frequency synthesizers with low phase noise, wide tuning range, and high resolution in frequency. Previous developments in this effort were described in a number of articles in NASA Tech Briefs, including "Light Induced Microwave Oscillator" (NPO-19440), Vol. 20, No. 9 (September 1996), page 4a; "Electro-Optical Clock- and Carrier-Recovery Device" (NPO-19573), Vol. 20, No. 9 (September 1996), page 6a; and "Self-Injection-Locked Electro-Optical Microwave Oscillator" (NPO-19568), Vol. 20, No. 8 (August 1996), page 17a.

This Coupled Opto-Electronic Oscillator generates stable microwave and pulsed optical outputs simultaneously.

The COEO is based partly on concepts described in yet another prior NASA Tech Briefs article; namely, "Multiloop Photonic/Electronic Frequency Synthesizers" (NPO-19825), Vol. 21, No. 6 (June 1997), page 10a. To recapitulate: A rudimentary but impractical photonic/electronic oscillator would include a single optical feedback concatenated with an electrical feedback loop. By including a radio-frequency (RF) filter with narrow pass band in the electrical feedback loop, one could allow oscillations to occur in one electromagnetic mode while suppressing oscillations in other modes. To reduce phase noise, the length of the optical feedback loop must be increased; this would reduce the frequency interval between modes, necessitating a reduction in the bandwidth of the RF filter to ensure the selection of only the desired modal frequency. However, at a typical frequency that one seeks to generate (of the order of 10 GHz), it is difficult to make an RF filter with pass band narrow enough to discriminate between modes. In addition, the inclusion of a narrow-band RF filter would sacrifice the tunability of the oscillator.

A multiple-loop apparatus could satisfy the need for both a longer optical feedback loop to reduce phase noise and a shorter optical feedback loop to facilitate discrimination against unwanted modes without need for a narrow-band RF filter, while providing broad frequency tunability. The multiple-loop apparatus proposed in the cited prior article would include a pump laser, a longer and a shorter fiber-optic delay line (longer and shorter optical feedback loops), a photodiode followed by an amplifier at the output end of each optical fiber, and a dual-drive electro-optical modulator that would be common to both fiber-optic delay lines and would be driven by the outputs of the amplifiers.

The COEO is a multiple-loop photonic/electronic oscillator, but it differs from the apparatus of the cited prior article in some important ways. Here, one does not use an external laser to pump the optoelectronic feedback loops; instead, the laser is an integral part of both the shorter feedback loop and of a semiconductor optical amplifier (SOA), the gain of which can be modulated electrically. The laser is a ring laser, so that its optical cavity (the ring) constitutes the shorter optical feedback loop. The laser has many longitudinal modes at integer multiples of a frequency interval that depends on the length of the loop. A typical value is 23.3 MHz, corresponding to a loop length of 8.58 m. The longitudinal modes of the longer loop are separated by much smaller frequency intervals.

A 50/50 optical coupler draws optical power from the shorter optical feedback loop, and a subsequent 90/10 optical coupler feeds optical power into the longer optical feedback loop. At other end of the longer loop, the optical signal is fed to a photodetector. The electronic output of the photodetector is amplified, delayed, band-pass-filtered, and attenuated as needed, and some of the resulting RF signal is fed to the modulator port of the SOA.

The midband frequency of the band-pass filter is chosen to be an integer multiple of the frequency interval between laser modes, and the bandwidth is chosen to be less than this interval, so that the band-pass filter effectively picks out one of the beat frequencies between modes. Without the RF feedback to the SOA, the phases of the longitudinal modes of the ring laser would be independent of each other, so that the optical output would be nearly steady, with superimposed random power fluctuations caused by interference among the modes. However, in the presence of the band-pass-filtered RF feedback at an integer multiple of the laser modal frequency interval, the sidebands of the modulated modes coincide with the frequencies of other modes, so that all the modes become injection-locked by the RF feedback. Many modes of the longer feedback loop compete to oscillate within the pass band, and the winner is the one with a frequency closest to the beat frequency, because only this one can mode-lock the ring laser. The superposition of locked modes causes the optical output to consist mostly of a train of pulses.

This work was done by Xiaotian Steve Yao and Lutfollah Maleki of Caltech forNASA's Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to