A prototype ring laser in which a transparent microsphere serves as an electromagnetic-mode selector has been constructed in a continuing effort to develop optoelectronic oscillators for generating light signals amplitude-modulated by microwave signals, all with low phase noise. Optoelectronic oscillators could be used as signal sources in fiber-optic and microwave communication systems and in radar systems.

An optoelectronic oscillator is a hybrid of photonic and electronic components, designed to exploit coupling between optical and electronic oscillations. Optoelectronic oscillators have been described in several previous articles inNASA Tech Briefs, the most recent being "Optoelectronic Generation of Optical and Microwave Signals" (NPO-20090), Vol. 22, No. 9 (September 1998), page 50. An optoelectronic oscillator includes, among other things, a laser that operates in multiple modes, plus a high-speed photodetector that samples the laser output. The laser is designed so that the frequency intervals between its modes include the microwave frequency of interest; thus, the microwave frequency of interest appears as one of the beat notes in the photodetector output.

Figure 1. In the Original Microsphere-Based Optoelectronic Oscillator, an automatic frequency-control circuit was used to lock a pump laser to one of the electromagnetic modes of the microsphere.

In some previously developed optoelectronic oscillators, long fiber-optic feedback loops were used to obtain low phase noise. Undesirably, a long fiber-optic loop adds considerably to the size and weight of an oscillator; it also makes the frequency intervals between modes so small that selection of the desired modes becomes difficult. In some optoelectronic oscillators developed more recently, fiber-optic loops were replaced with transparent microspheres configured as high-Q (whereQ is the resonance quality factor) resonators in conjunction with pump lasers operating under feedback control of frequency (see Figure 1). In a microsphere, propagation in a long fiber is replaced by equivalent orbiting of light by total internal reflection in "whispering-gallery" modes. It has been demonstrated experimentally that in visible light,Q ≈ 1010 can be achieved in a microsphere, corresponding to a propagation delay of about 3 µs in an optical fiber 0.7 km long.

Figure 2. Microsphere-Based Ring Lasers have been built to demonstrate direct laser oscillation in microsphere modes, with microwave sidebands at integer multiples of the free spectral range of "whispering-gallery" microsphere modes.

Feedback control of pump-laser frequency in a microsphere oscillator of the type described above was necessary for locking the pump laser to one of the microsphere modes. The feedback frequency control added complexity and introduced a source of additional frequency and phase noise.

In the prototype ring laser, there is no need for feedback control of laser frequency because the microsphere is an integral part of the laser. The prototype ring laser (shown in the upper part of Figure 2) includes a high-purity silica microsphere and a semiconductor optical amplifier plus ancillary optical components connected in an optical loop. One of the components in the loop is an optocoupler for sampling the laser beam.

In early experiments on the prototype ring laser, the sampled laser beam was analyzed for its optical and microwave-modulation spectral contents. The laser was found to oscillate in multiple whispering-gallery modes of the microsphere. The microwave modulation spectrum included peaks at integer multiples of the whispering-gallery free spectral range of 5.93 GHz. At the time of reporting the information for this article, experiments on the apparatus shown in the lower part of Figure 2 were underway. This apparatus is designed to obtain stable single-frequency operation by introducing (1) optical selection of principal waveguide modes and (2) active microwave feedback as in a standard optoelectronic oscillator.

This work was done by Steve Yao, VladimirIltchenko, and Lute Maleki of Caltech for NASA'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

Intellectual Property group,

JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-20597


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Closed-Loop Microsphere Laser for Optoelectronic Oscillator

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This article first appeared in the September, 2001 issue of Photonics Tech Briefs Magazine.

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