Steven L. Garrett

Symmetric 3 x 3 coupler-based demodulator for fiber optic interferometric sensors

A method for demodulation of fiber optic interferometric sensors that utilizes a 3 X 3 coupler is described. The passive demodulation scheme does not require carrier (phase) modulation. The demodulation scheme relies on the three outputs of a 3 X 3 coupler and uses all three of its phase modulated output signals to recreate the stimulus inducing the original optical phase modulation. The demodulator scale factor (volts/radian) is stable against fluctuations in both fringe visibility and average received power. Upon initial implementation of the scheme, a dynamic range of 116 dB was obtained (at 600 Hz in a 1 Hz bandwidth with maximum THD at 4%). The minimum detectable signal at 600 Hz was 220 (mu) rad/(root)Hz and the maximum tolerable signal was 140 radians. Both the maximum tolerable signal and the minimum detectable signal (noise floor) was observed to increase with decreasing frequency. Thus, depending on the frequency, the demodulation scheme is capable of detecting phase signals less than a milliradian to in excess of kiloradians.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Interferometric fiber optic accelerometer

A push-pull fiber optic accelerometer using a Michelson interferometer to detect the surface strains produced on a pair of coaxial flexing disks is described. The sensitivity below the disks fundamental resonance of 2.45 kHz is 50 radians/g. The fractional phase change per unit force A4/AF is 5.9 x 106 W1 . A noteworthy feature of this sensor is that the 5 meter sensing coils comprising the legs of the interferometer are totally internal to the sensor structure and thus are not exposed to platform or fluid under investigation.

Fiber optic flexural disk microphone

An interferometric fiber optic microphone consisting of a 10‐m‐long, 4‐cm‐diam, flat‐wound (spiral) single mode optical fiber bonded to a simply supported, 8‐cm‐diam, 3‐mm‐thick aluminum disk will be described. In the presence of a pressure difference across the plate or an acceleration of the plate, the surface strain induced in the plate is transmitted to the optical fiber. The optical phase shift induced by the strain is detected in an all‐fiber Michelson interferometer. The calculated strain [A. E. H. Love, A Treatise on the Mathematical Theory of Elasticity (Dover, New York, 1944), 4th ed., Sec. ♯309] and optical strain measured using static pressure, acoustic pressure, and a calibrated aecelerometer, are in good agreement (± 10%) and yield a sensitivity of 30 milliradians per Pascal per meter of optical fiber below the plate resonance frequency of 3 kHz. [Work supported by the Office of Naval Research and Office of Naval Technology.]

A symmetric analogue demodulator for optical fiber interferometric sensors

Describes a form of passive homodyne demodulation for recovering a signal of interest from a Mach-Zender optical fiber interferometric sensor. The interferometer uses a 2*2 optical fiber coupler at the input, a push-pull arrangement to stretch the fibers comprising the two legs of the interferometer in a different manner, and a 3*3 optical fiber coupler at the output, generating three phase-modulated outputs. The demodulator uses all three outputs in a symmetric manner to reconstruct the signal of interest. A dynamic range of 115 dB in a 1-Hz bandwidth has been measured for a circuit implementing this scheme. The minimum detectable signal at a frequency of 600 Hz is 220 mu rad/ square root Hz. The maximum acceptable signal at 600 Hz is 140 rad, increasing linearly with decreasing frequency. A schematic diagram of an implementation of the symmetric demodulation technique in analog electronics is presented.<<ETX>>

Fiber-optic ellipsoidal flextensional hydrophones

Calculations of circumferential strain, polar and equatorial displacement, lowest natural frequency, buckling pressure, and optical pressure sensitivity for an oblate-spheroidal-shell fiber optic hydrophone are presented using the theory for thin shells. Sample designs based on these calculations are compared to other fiber-optic hydrophone designs of similar dimensions and materials. A prolate-spheroidal-shell hydrophone design is also presented. >

High-sensitivity, fiber-optic, flexural disk hydrophone with reduced acceleration response

Abstract A hydrophone consisting of a hollow cylinder whose flexible, circular end plates are bonded to pairs of pat, spiral wound coils of optical fiber is described. When the end plate/disk is deformed due to a pressure difference, the outer and inner fiber coils experience opposite strains resulting in a “push—pull” optical path length difference which is detected in an all-fiber Michelson interferometer. The close proximity of the interferometric fiber coils, separated by the thin thermally conducting end plate, rejects thermal gradient-induced signals. The addition of a second identical end plate and fiber coil pair at the opposite end of the cylinder will double the acoustic sensitivity while canceling acceleration induced signals. The calculated and measured optical strain of a single simply supported plate, single-coil sensor (8.0 cm diameter, 3.0 mm thickness) using static pressure, acoustic pressure, and acceleration are in good agreement and yield a sensitivity of 0.21 rad/Pa (ΔΦ/ΦΔP = -301 dB ...

Fiber Optic Gradient Hydrophone

A laboratory study has been made of the characteristics of an interferometric type optical fiber pressure gradient hydrophone. The optical system is configured as an all fiber Mach-Zehnder interferometer excited by a single mode gallium arsenide diode laser. A pair of identical fiber coils, one in each arm of the interferometer, forms the sensing portion of the gradient hvdrophone. Each coil consists of 10 meters of polyethylene jacketed, single mode fiber, wound in a toroidal element of mean diameter 4 cm and thickness 3 mm. A standing wave type acoustic calibrator was constructed and used to determine the sensitivity of each individual coil. The pair then was aligned coaxially and separated by 10 cm to operate as a pressure gradient device. Details of the construction of the system, calibration procedure, and hydrophone sensitivity data are presented.

General purpose fiber optic hydrophone made of castable epoxy

A fiber-optic, interferometric, flexural disk hydrophone cast from an epoxy resin is described. This hydrophone is designed in the shape of an enclosed, hollow cylinder with pairs of flat, spiral wound coils of optical fiber embedded in each sensing plate. An all-fiber Michelson interferometer is used to detect the optical phase shift which results from pressure induced strains in the optical fiber. The sensing coils are positioned in the plates in a manner to enhance the acoustic response and provide cancelation of acceleration induced signals. An epoxy resin was chosen for its relatively high tensile strength, its low Youngs modulus, and its ability to cure at room temperature. The acoustic sensitivity of this sensor in both air and water was measured to be 0.277 0.005 rad/Pa (-131.2 dB re rad/μPa) which corresponds to a normalized sensitivity (formula available in paper)re 1 μPa-1 below 1 .0 kHz. This measured result is in excellent agreement with simple elastic theory and the measured epoxy elastic constants. The normalized acceleration sensitivity is (formula available in paper)dB re g-1. The acceleration-to-acoustic sensitivity ratio (figure-of-merit) of -134 dB re g4tPa is the largest reported to date for any fiber-optic hydrophone.

Fiber-Optic Oblate Flextensional Hydrophone

When an oblate spheroidal shell having an aspect ratio aib > (2-v)112, where v Poissons ratio, is subject to hydrostatic compression, the semi-major, a, and semi-minor, b, axes experience strain of opposite sign. If two optical fibers, which comprise the arms of an interferometer, are wound around the equatorial and meridional circumferences of the spheroid, pressure changes induce a differential optical phase shift in the interferometer. Calculations of circumferential strain, polar and equatorial displacement, lowest natural frequency, buckling pressure, and optical pressure sensitivity of such a sensor are presented for thin shells. Sample designs based on these calculations are compared to other fiber-optic hydrophones of similar dimensions and materials. Preliminary designs will be presented.

A fiber‐optic, interferometric, acceleration canceling, flexural disk hydrophone

A fiber‐optic hydrophone consisting of an air backed cylinder with clamped circular end plates bonded to four 8‐m‐long flat (spiral) wound single mode optical fibers will be described. For a given pressure differential between the inside and outside of the cylinder, the interior and exterior fiber coils on each 4.5‐cm‐diam, l‐mm‐thick aluminum disk experience opposite strains. This “push‐pull” design induces an optical path length difference in a Michelson interferometer. When subject to acceleration along the cylindrical axis, both disks are deflected in the same direction, producing a positive strain on the outer surface of one disk and an equal, but negative, strain on the outer surface of the other disk. Since the fiber coils on the exterior of the two disks comprise one arm of the interferometer (the interior coils comprise the second arm), the strains cancel. The calculated strain for this four‐coil sensor [A. E. H. Love, A Treatise on the Mathematical Theory of Elasticity (Dover, New York, 1944), 4...