Fiber tip based sensor system for measurements of pressure gradient, air particle velocity and acoustic intensity

• US 7,224,465 B2
• Filed: 01/21/2005
• Issued: 05/29/2007
• Est. Priority Date: 10/15/2002
• Status: Expired due to Fees

• Current Assignee: University of Maryland
• Original Assignee: University of Maryland
• Inventors: Yu, Miao, Balachandran, Balakumar, Al-Bassyiouni, Moustafa
• Primary Examiner(s): LEE, HWA S
• Application Number:
  US11/038,093
• Publication Number:
  US 20050146726A1
• Time in Patent Office: 858 Days
• Field of Search: 356/478, 356/480, 356/519, 356/477, 356/506, 385/12
• US Class Current: 356/480
• CPC Class Codes: G01D 5/35303 using a reference fibre, e.g. interferometric devices
• CPC Class Codes: G01H 9/006 the vibrations causing a variation in the relative position of the end of a fibre and another element
• CPC Class Codes: G02B 6/29358 Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator G02B6/29347, G02B6/29349, G02B6/2935, G02B6/29361 take precedence; resonator evanescently coupled to light guide G02B6/29335

Abstract

A fiber optic sensor system for pressure measurements where the design permits multiplexity on the input side of the system and the optical part of the system, which has a sensor Fabry-Perot interferometer and a read-out interferometer, is based on low coherence fiber-optic interferometry techniques. This permits a high dynamic range and low sensitivity to the wavelength fluctuation of the light source as well as to the optical intensity fluctuations. The system includes fiber tip based Fabry-Perot sensors, where each sensor includes a diaphragm as the transducer. A combined pressure gradient sensor, air particle velocity sensor, as well as acoustic intensity sensor is built based on the fiber tip based Fabry-Perot sensors.

Claims

1. A fiber optic sensor system for measuring pressure gradient, air particle velocity and acoustic intensity of an acoustic disturbance, comprising:

at least a pair of substantially identical sensors, each sensor including a diaphragm and a sensing fiber-tip based interferometer having a Fabry-Perot cavity formed between said fiber tip and said diaphragm, said fiber tip and said diaphragm both being optically reflective to form a pair of reflective surfaces of said interferometer, said pair of the sensors being spaced one from the other along a first axis with said diaphragms being oriented in a common direction, the acoustic disturbance deflecting said diaphragm of each said sensor; and

a processor operationally coupled to said at least a pair of sensors for calculating pressure gradient, air particle velocity and acoustic intensity of an acoustic field based on the deflection of said diaphragms affected by the acoustic field.

2. The fiber optic sensor system of claim 1, wherein said processor calculates the pressure gradient based on measurements of acoustic pressure sensed by said pair of sensors, as

$\mathrm{Pr}$

essure ⁢

⁢

Gradient = p ⁡

( l / 2 , t ) - p ⁡

( - l / 2 , t ) 2 ⁢

wherein

p(±

l/2,t) is the dynamic sound pressure to be sensed by said sensors at respective locations l/2 and −

l/2 thereof;

l is a distance between said sensors; and

t is a time of taking measurements.

3. The fiber optic sensor system of claim 2, wherein said processor calculates air particle velocity as

$u$

( 0 , t ) = 1 3 ⁡

[ 4 ⁢

u ⁡

( 0 , t - δ

⁢

⁢

t ) - u ⁡

( 0 , t - 2 ⁢

δ

⁢

⁢

t ) - 2 ⁢

δ

⁢

⁢

t ρ

0 ⁢

l ⁡

[ p ⁡

( l / 2 , t ) - p ⁡

( - l / 2 , t ) ] ]

wherein

u(0,t) is an air particle velocity,

δ

t is time between two measurements,

ρ

₀ is the medium mass density.

4. The fiber optic sensor system of claim 3, wherein said processor calculates the acoustic intensity as

I(0,t)=p(0,t)·

u(0,t),

wherein p(0,t) is the sound pressure at the center between said sensors, and u(0,t) is the air particle velocity.

5. The fiber optic sensor system of claim 1, comprising at least one additional pair of said sensors, each sensor of said additional pair of said sensors being spaced one from the other along another axis, said other axis being disposed in angled relationship with respect to said first axis.

6. The fiber optic sensor system of claim 1, comprising a supporting member, said at least one pair of said sensors being attached to said supporting member at a predetermined distance l one sensor from the other sensor,

at least a pair of optical fibers, each optical fiber being coupled to a respective one of said sensors, and

a directing member attached to said supporting member to direct said fiber optic sensor system towards the acoustic field.

7. The fiber optic sensor system of claim 1, further comprising:

a light source,

an Integrated Optical Circuit (IOC) phase modulator coupled to said light source to modulate the light generated from said light source,

a read-out interferometer built in said IOC phase modulator, said read-out interferometer being path-matched to said sensing fiber-tip based interferometer of each of said at least a pair of said sensors,

at least a pair of photodetectors, each photodetector being coupled to a respective one of said at least a pair of said sensors, and

a phase modulation-demodulator coupled to said IOC phase modulator and said at least a pair of the photodetectors for modulating said light beam in said IOC phase modulator in accordance with a multi-step phase-stepping pattern, and for demodulating data obtained from said at least a pair of the photodetectors in synchronism with said multi-step phase-stepping pattern,

wherein said processor is coupled to said phase modulation-demodulation means for controlling said multi-step phase-stepping pattern and for calculating phase signals of said at least a pair of said sensors based on said obtained data.

8. The fiber optic sensor system of claim 7, further comprising an optical switch coupled between said JOG phase modulator and said at least a pair of said sensors for multiplexing an input side of said fiber optic sensor system.

9. The fiber optic sensor system of claim 1, further including a TiO₂ fiber coating on said fiber tip.

10. The fiber optic sensor system of claim 2, wherein said processor calculates the acoustic pressure based on the optical phase change of an output signal of said at least a pair of said sensors.

11. The fiber optic sensor system of claim 1, wherein said diaphragm is formed of polyester film.

12. The fiber optic sensor system of claim 1, wherein each of said at least a pair of said sensors includes a high reliability connector ferrule, said fiber passing longitudinally through said connector ferrule.

13. The fiber optic sensor system of claim 1, wherein the distance between said fiber tip and said diaphragm is selectively adjustable.

14. A method for forming a fiber optic sensor system for measuring pressure gradient, air particle velocity and acoustic intensity of an acoustical disturbance, the method comprising the steps of:

providing a pair of substantially identical sensors each said sensor including a diaphragm and a sensing fiber-tip based interferometer having a Fabry-Perot cavity formed between said fiber tip and said diaphragm, said fiber tip and said diaphragm both being optically reflective to form a pair of reflective surfaces of said interferometer;

spacing each of said pair of said sensors one from the other along a first axis;

coupling a processor to said pair of said sensors;

calculating pressure gradient, air particle velocity and acoustic intensity based on the deflection of said diaphragms subjected to the acoustic disturbance

outputting the calculated pressure gradient, air particle velocity and acoustic intensity.

15. The method of claim 14, wherein the step of spacing said pair of said sensors is followed by the steps of providing at least one additional pair of said sensors;

spacing each said additional sensor one from the other along another axis, said other axis being angled with respect to said first axis.

16. The method of claim 14, further comprising the steps of:

attaching said pair of said sensors to a supporting member a predetermined distance l one sensor from the other sensor, and

coupling an optical fiber to each said sensor.

17. The method of claim 14, further comprising the steps of:

generating a light beam from a light source;

modulating the light beam generated from said light source by means of an Integrated Optical Circuit (IOC) phase modulator coupled thereto, said IOC phase modulator including a read-out interferometer built therein, said read-out interferometer being path-matched to said sensing fiber-tip based interferometer of each of said pair of said sensors;

coupling a photodetector to each said sensor;

coupling a phase-modulation-demodulation means to said IOC phase modulator and a pair of said photo detectors;

modulating said light beam in said IOC phase modulator by means of said modulation-demodulation means in accordance with a multi-step phase-stepping pattern;

demodulating data obtained from said pair of said photodetectors in synchronism with said multi-step phase-stepping pattern; and

coupling said processor to said phase modulation-demodulation means for controlling said multi-step phase-stepping pattern and for calculating phase signals of said pair of said sensors based on said obtained data.

18. The method of claim 17, further comprising the steps of:

coupling an optical switch between said JOG phase modulator and said pair of said sensors, and

multiplexing an input side of said fiber optic sensor system.

19. The method of claim 14, further comprising the step of:

adjusting the distance between said fiber tip and said diaphragm.

20. The method of claim 15, further comprising the step of:

arranging three said pairs of said sensors on a surface of a spherical supporting member for three-dimensional measurements.

21. A fiber optic sensor system for measuring pressure gradient, air particle velocity and acoustic intensity of an acoustic disturbance, comprising:

at least a pair of substantially identical sensors, each sensor including a diaphragm and a sensing fiber-tip based interferometer having a Fabry-Perot cavity formed between said fiber tip and said diaphragm, said fiber tip and said diaphragm both being optically reflective to form a pair of reflective surfaces of said interferometer, said pair of said sensors being spaced one sensor from another along a first axis with said diaphragms being oriented in opposing directions, the acoustic disturbance deflecting said diaphragm of each said sensor; and

a processor operationally coupled to said pair of said sensors for calculating pressure gradient, air particle velocity and acoustic intensity of an acoustic field based on the deflection of at least one of said diaphragms affected by the acoustic field.

22. The fiber optic sensor system of claim 21, further comprising at least one additional pair of said sensors, each sensor of said additional pair of said sensors being spaced one from the other along another axis, said other axis being disposed in angled relationship with respect to said first axis.