Optoelectronic Oscillator With Low Acceleration Sensitivity

NASA's Jet Propulsion Laboratory, Pasadena, California

An optoelectronic oscillator with a nominal operating frequency of 11.763 GHz has been designed and constructed to demonstrate a technique for reducing the sensitivity of the operating frequency to acceleration. Optoelectronic oscillators in general exhibit low sensitivity to acceleration in addition to other attractive characteristics (high spectral purity, low phase noise, capability for generating multigigahertz frequencies, and both electrical and optical input and output capabilities). The practical significance of the present development is that the reduction of acceleration sensitivities to exceptionally low levels would render optoelectronic oscillators even more attractive as signal sources for use on diverse moving platforms, including automobiles, ships, aircraft, and spacecraft.

Figure 1. The Optoelectronic Oscillator (shown here before modification to reduce sensitivity to acceleration) comprises an oscillator module and a fiber-optic delay line.

The optoelectronic oscillator (see Figure 1) includes a distributed-feedback laser and a feedback loop that comprises a semiconductor Mach-Zehnder electro-optical modulator, a delay line that consists of a coiled 2-km-long optical fiber, a photodetector, a microwave amplifier, and a band-pass filter. The oscillator also includes an electronic controller that drives the laser, regulates the temperature of the laser, biases the modulator and the photodetector, and supplies power to the microwave amplifier. All of the oscillator components except the fiber-optic delay line are packaged in a module that amounts to a prototype of "turn-key" (fully operational) optoelectronic oscillator units.

Of course, firm mounting of the components within the module is an essential part of the design for reducing sensitivity to acceleration. Most of the remaining sensitivity to acceleration is attributable to acceleration-induced changes in the length of the optical path along the fiber-optic delay line; therefore, the problem of desensitization to acceleration becomes one of minimizing these changes.

Figure 2. The Fiber-Optic Delay Line Has Been Split into two coils of opposite chirality. In combination with other features of design and construction, this split greatly reduces the sensitivity of the oscillator to acceleration along the coil axis.

Experiments and calculations have shown that if the fiber-optic delay line is coiled tightly on a spool, then the sensitivity to acceleration perpendicular to the spool axis is less than 1/20 of the sensitivity to acceleration along the spool axis. Hence, the problem is reduced further in that it should be possible to eliminate most of the sensitivity to acceleration by concentrating on minimizing the response to acceleration along the spool axis.

The solution of the problem is to split the fiber-optic delay line into two coils that are of opposite chirality but are otherwise identical and that are mounted on opposite faces of the oscillator module (see Figure 2). In principle, when acceleration along the spool axis lengthens the optical path in one coil by a given amount, it should shorten the optical path in the other coil by the same amount, so that the net change in optical-path length should be zero. Measurements have shown that the sensitivity is reduced to about 1/40 of that obtained of a single-coil version of the delay line. The total sensitivity to acceleration along all three axes was found to be less than 1.5× 10^-10g^-1 (≈1.5 ×10^-11 s²/m), whereg (≈9.8 m/s²) is the gravitational acceleration at the surface of the Earth.

This work was done by Shouhua Huang, Meirong Tu, and X. Steve Yao 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