A. M. Yurek

Preparation of amorphous metallic glass transducers for use in fiber optic magnetic sensors

An important figure of merit for fiber optic magnetometers is the effective C parameter Ceff relating magnetostriction e to applied field H: e=CeffH2. We describe both the annealing and the characterization after annealing of cylindrical metallic glass transducers and present the results of measurements of Ceff for various anneal temperatures, times, and magnetic field strengths for flat Metglas 2605‐S2 samples. Ceff values of (2.0–2.5)×10−6 Oe−2 are observed in the annealed flat strip and cylindrical samples studied here.

Very high responsivity fiber optic hydrophones for commercial applications

The design and performance of single element fiber optic hydrophones for shallow water applications is described. Hydrophone responsivities up to -113.8 dB re rad/(mu) Pa were measured while maintaining a usable depth capability. A system with this design of hydrophone is being used to monitor the deterioration of water carrying siphons in Arizona.

Fringe visibility and phase noise in superluminescent diodes

A Michelson interferometer has been used to characterize fringe visibility and noise due to phase fluctuations in a GaAlAs superluminescent diode. The superluminescent diode was chosen to have very little longitudinal mode structure. The fringe visibility decreases from unity at zero path-length difference (PLD) to 50 percent at 13 μm PLD and 10 percent for a PLD of 26 μm. Noise in the interferometer increases monotonically with PLD until a level 9 dB above that for zero PLD is reached at a total photodetector current of 60 μA. An expression relating this limiting noise level and the fringe visibility data to an amplitude correlation function is derived. A numerical value of 0.74 is determined for this correlation function in the limit of large PLD.

Hydrophone transduction mechanism

A sensing portion for an interferometric planar hydrophone is disclosed which comprises: a support assembly; a plurality of hollow compliant mandrels mounted to the support assembly in a fixed planar relationship with respect to each other to form a planar configuration; a sensing optical fiber sequentially wound around each of the plurality of mandrels to form a single, serial, optical sensing arm or path between an input position on a first one of the plurality of mandrels and an output position on a last one of the plurality of mandrels; and an acoustically transparent material encapsulating the sensing portion to form a sealed compliant planar hydrophone. In an operational system, an interferometric planar hydrophone is disclosed which comprises: the sensing portion in which the sensing arm is operative to measure a predetermined measurand in a preselected environment and which sensing portion further includes a reference optical fiber environmentally isolated from the predetermined measurand; a light source for transmitting coherent light to both of the sensing and reference optical fibers; and circuitry responsive to light received from the sensing and reference optical fibers for developing an output signal indicative of phase shift differences between the sensing and reference optical fibers to determine the predetermined measurand.

High-sensitivity fiber optic planar ultrasonic microphone

We describe the design and performance of a novel planar ultrasonic fiber sensor for detection of air acoustic signals. The sensor is formed from a potted single layer of spirally wound optical fiber, and configured such that the static backing pressure can be varied. Experimentally we demonstrate normalized sensitivities ((Delta) (phi) /(phi) (Delta) P) of -295 to -265 dB re. 1 (mu) Pa-1.

Fiber Optic Magnetometers Using Planar And Cylindrical Magnetostrictive Transducers

Fiber optic magnetometers which require high sensitivity at low frequencies (dc-10 Hz) rely on the nonlinear magnetostriction of materials such as amorphous metallic glass alloys. Typically, fiber is bonded to a magnetostrictive sample to convert strain in the sample to phase shift in a fiber interferometer. We present the results of measurements of the frequency dependence and dc and ac magnetic field sensitivity of both planar and cylindrical transducing elements, and discuss the practical advantages and disadvan-tages of each configuration.

Performance of 3 x 3 couplers in fiber optic sensor systems

The performance of fiber optic interferometers constructed with 3 X 3 couplers is investigated. Deviations from ideal behavior are noted at low fringe visibilities. The use of Faraday mirrors in a 3 X 3 Michelson configuration is shown to eliminate these effects.

Demodulator optimization for the interrogation of fiber optic hydrophones in real-world environments

Real world operational environments place stringent requirements upon the fidelity of the demodulator output of the acoustic signal which is present. This paper discusses the optimization of a demodulator circuit for hydrophones in time-varying hydrostatic pressure environments as well as in environments which contain large amplitude acoustic signals.

Fiber optic gradient hydrophones

Two fiber optic pressure gradient hydrophones have been designed, constructed and tested. These devices both exhibit good sensitivity and appropriate directionality. Their responsivities are linear in the 100 Hz to 1000 Hz frequency band.

A High Frequency Fiber Optic Hydrophone

Recently there has been interest in fiber optic hydrophones for high frequency applications. A fiber optic hydrophone with flat frequency response to 50 kHz has been designed and built at NRL and demonstrated at the USRD calibration facility.