Janet L. Brooks

Coherence multiplexing of fiber-optic interferometric sensors

This paper describes a method of multiplexing several optical signals onto a single spatial channel (e.g., a single-mode fiber) using a short coherence length continuous wave light source. Several system configurations which utilize this technique are proposed, and some design considerations are discussed. Experimental results for a single sensor and receiver are presented and compared with theoretical predictions.

Two-mode fiber modal coupler

When a single-mode fiber is used at a wavelength below the cutoff wavelength, the fiber guides second-order modes, which travel at different phase velocities from the fundamental mode. Periodically stressing this two-mode fiber once per beat length can cause coherent coupling between the modes. Such a modal coupler has been developed and is described here. Coupling to one of the second-order modes has been achieved, leaving less than -40-dB residual power in the fundamental mode. Two couplers have been mounted on a single strand of fiber to construct a Mach-Zehnder interferometer with better than a 30-dB on/off ratio. The coupler is polarization sensitive and can be used as an in-line polarizer. A 36-dB extinction ratio between polarizations has been observed.

Time-domain addressing of remote fiber-optic interferometric sensor arrays

This paper describes and analyzes a particular application of high duty-cycle time-division multiplexing to the separation and identification of signals from an interferometric sensor array. Using the method discussed here, the coherence length of the laser is no longer a severe design constraint. Also, the source phase-induced intensity noise which limits some other multiplexing methods may be overcome, leading to a higher sensitivity. The arrays of all-passive remote sensors exhibit minimal crosstalk between sensors, and have downlead insensitivity. A synthetic heterodyne demodulation technique prevents environmentally induced signal fading. Analysis includes coupling ratios for all directional couplers in the system, signal and noise spectra, minimum detectable phase shift, and the effect of ac coupling on noise and crosstalk. An experimental all-fiber implementation of a two sensor array has yielded a measured sensitivity of approximately 10 μrad/ \sqrt{Hz} over a range of signal frequencies, and a crosstalk level of better than 55 dB.

Birefringent-fiber polarization coupler

Periodically stressing a birefringent fiber once per beat length can cause coherent coupling to occur between polarization modes. Such a birefringent-fiber polarization coupler is described here. More than 30 dB of power transfer between polarizations has been achieved. The device has been used as the output coupler of an in-line Mach-Zehnder interferometer, and better than 25-dB on/off extinction has been measured. The device is wavelength selective and can be used as a multiplexer or as a notch filter. A notch of 9-nm full width at half-maximum has been achieved with a 60-period comb structure.

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.

Active polarization coupler for birefringent fiber

Static coupling between polarization modes achieved by periodically stressing birefringent fiber once per beat length was recently reported. The same scheme is now used to obtain coupling modulation at kilohertz-to-megahertz frequencies by applying pressure to the fiber with an oscillating piezoelectric ceramic. An amplitude of 30-50 V (peak to peak) was found to be necessary to modulate the polarization coupling from a minimum to a maximum. Polarization modulation is also achieved by applying stress along one fiber polarization axis between the two static couplers of a Mach-Zehnder interferometer.

Sensitive Fiber-Optic Interferornetric Sensor Arrays

This paper describes a passive interferometric sensor array which uses high duty-cycle time-domain addressing to separate and identify the signals from each sensor. Using the array configuration described here, the source phase-induced intensity noise which limits some other multiplexing methods is substantially eliminated, resulting in high sensitivity. The array configuration facilitates the use of synthetic heterodyne demodulation techniques to prevent environmentally induced signal-fading. An experimental all-fiber implementation of a dual-sensor array was used to investigate practical sensitivity and cross talk levels. A measured sensitivity of approximately 10 μrad/√Hzhas been achieved over a range of signal frequencies.