Kenneth A. Fesler

Fiber-optic interferometric sensor arrays with freedom from source phase-induced noise.

We propose arrays of all-passive remote sensors with freedom from both source phase-induced intensity noise and cross talk between sensors. These arrays employ high-duty-cycle time-domain addressing, utilize laser diodes, and have downlead insensitivity. A synthetic heterodyne demodulation technique is used to prevent environmentally induced signal fading. An experimental all-fiber implementation of a single remote sensor yielded a measured sensitivity of below 40 microrad/ radicalHz at signal frequencies above 600 Hz.

Stable fiber-source gyroscopes

We describe two new configurations of fiber gyroscopes that use superfluorescent fiber laser sources. In the first configuration we introduce a backward-pumping technique that achieves an order-of-magnitude increase in the power that can be launched into the gyroscope without instabilities. In the second configuration we employ the optical gain of the superfluorescent fiber laser source to amplify the signal returned from the gyroscope, obtaining an additional three-orders-of-magnitude increase in the optical power incident upon the detector, with an implication for simplification of the electronics. In both configurations, source beat-noise-limited rotation sensitivity is achieved.

Reentrant fiber Raman gyroscope

An experimental demonstration of an active reentrant fiber gyroscope is reported. Raman amplification is used to increase the number of signal recirculations in the rotation-sensing loop, which improves system sensitivity to rotation rate. A theoretical analysis of the wave mixing between counterpropagating pump and signal fields, interacting through Raman scattering in a polarization-preserving fiber, is presented. >

Digital integrating fiber-optic gyroscope with electronic phase tracking

A novel demodulation scheme for interferometers with optical phase modulation is described. The optical phase shift is measured by mixing a train of square digital pulses with a photodetector current and adjusting the pulse spacing by using an electronic closed loop. The optical phase shift is tracked with deviation less than 0.007 rad, which can be easily corrected by using a look-up table. An experimental optically open-loop fiber-optic gyroscope that uses this demodulation shows a linear scale factor in good agreement with theory.

Behavior Of Broadband Fiber Sources In A Fiber Gyroscope

Two fiber laser sources, a resonant fiber laser (RFL) and a superfluorescent fiber laser (SFL), have been given initial tests as gyro sources using a medium quality gyro test bed. The RFL reacted strongly to optical feedback from the gyro circuit resulting in very large unstable errors in the gyro output. These were suppressed substantially by an optical isolator which reduced feedback from the gyro, or by a phase modulator within the laser cavity. The SFL was found to be free of errors of this type when operated at output power levels below a threshold level (about 0.5 mw) for self oscillation due to external optical circuit reflections.

Spectrum thermal stability of Nd- and Er-doped fiber sources

The spectral thermal stability of broadband rare-earth-doped fiber sources makes them attractive for fiber sensor applications. We quantify the mean wavelength variation of both Nd- and Er-doped fiber sources operating as superfluorescent fiber lasers. Besides the intrinsic variation of such sources the effects of pump power and pump wavelength are also considered since both the power and wavelength of conventional laser diode pump sources are temperature sensitive. Other types of rare-earth-doped sources are also briefly considered.

Selective interferometric sensing by the use of coherence synthesis

A technique is discussed and demonstrated whereby the sensitivity of an imbalanced fiber-optic interferometric sensor is changed by as much as 70 dB by modifying the power spectrum and thus the coherence function of the input optical source. This phenomenon is then extended and used to monitor selectively single sensors in a small fiber-optic ladder array by synthesizing coherence functions that result in a high sensitivity for the sensor to be observed but a low sensitivity for all the others. Limitations of the demonstrated implementation are discussed and possible future designs presented.