Pressure Tolerant Fiber Optic Hydrophone Design
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Solution Overview
Problem
Fiber optic interferometric hydrophones are limited by high laser costs due to the need for low frequency noise lasers and are sensitive to broad acoustic frequencies, with previous designs experiencing unacceptable increases in laser phase noise from static pressure changes and sensitivity to low frequencies.
Innovation Solution
The design incorporates an inner mandrel open to the surrounding fluid with optical fiber wound around it, and a larger outer mandrel with a pressure-compliant material, featuring a high pass filter that maintains a small path mismatch of the interferometer, allowing the use of higher frequency noise lasers and enhancing sensitivity by filtering out low frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a fiber optic interferometric hydrophone uses a traditional design with fiber wound around a pressure-compliant mandrel, then it can detect acoustic signals, but static pressure changes cause path length mismatches that increase laser phase noise and require expensive low frequency noise lasers
Solution Approach 1:
The hydrophone is divided into multiple independent sensing elements (first hydrophone element and second hydrophone element) with separate interferometers. Each element has its own fiber optic path, allowing independent path length management. This segmentation enables each interferometer to maintain its own path match independently, preventing static pressure-induced path mismatches from affecting the entire system.
Solution Approach 2:
A non-compliant mandrel is introduced as an intermediary structure between the pressure-compliant mandrels and the fiber optic paths. The non-compliant mandrel remains dimensionally stable under static pressure changes, providing a reference structure that prevents path length mismatches. This intermediary element mediates between the pressure-sensitive sensing regions and the fiber optic interferometer paths.
2Measurement precision
If the hydrophone uses a sealed air-filled mandrel to increase sensitivity, then acoustic detection sensitivity improves, but the device becomes sensitive to broad frequency ranges including low frequencies that may cause problems
Solution Approach 1:
Different mandrels are assigned different compliance properties: the first and second mandrels are pressure-compliant to enhance acoustic sensitivity, while the third mandrel is non-compliant to provide dimensional stability. This local differentiation of mechanical properties allows the hydrophone to achieve both high sensitivity and controlled frequency response, filtering out unwanted low-frequency signals while maintaining sensitivity to acoustic frequencies.
3Device complexity
If the path mismatch between interferometer branches is increased to a few meters as in traditional Michelson designs, then the interferometer structure is simplified, but laser phase noise increases significantly under static pressure changes
Solution Approach 1:
The hydrophone design accommodates dynamic pressure changes by using pressure-compliant mandrels that can deform with static pressure while maintaining the interferometer path match. The compliant mandrels dynamically adjust their dimensions with pressure changes, preventing path length mismatches. This dynamic adaptation allows the use of shorter fiber paths without increasing laser phase noise.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration minimizes system laser phase noise, reduces costs by enabling the use of less expensive diode lasers, and increases sensitivity, particularly at great depths, while maintaining performance across varying operational depths without path mismatch issues.
Implementation Method 1
fiber optic interferometric hydrophone
Implementation Method 2
cylindrical shell of pressure compliant material
Implementation Method 3
small orifice 68 in outer case 60
Data Source
AI summary
An interferometric hydrophone is disclosed that comprises a first mandrel defining an interior that is open to surrounding fluid. A sensing optical fiber is wound upon the first mandrel. A second mandrel is positioned in surrounding relationship with respect to the first mandrel. The first and second mandrels define a first chamber therebetween. A case encloses the first and second mandrels and first chamber. The cylindrical case and the second cylindrical mandrel define a second chamber therebetween, which is sealed and filled with gas or vacuum.