Extended fiber optic sensor using birefringent fibers

• US 4,405,198 A
• Filed: 08/25/1981
• Issued: 09/20/1983
• Est. Priority Date: 08/25/1981
• Status: Expired due to Fees

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Taylor, Henry F.
• Primary Examiner(s): Lee, John D.
• Application Number:
  US06/295,989
• Time in Patent Office: 756 Days
• Field of Search: 350/96.15, 350/96.29, 350/96.30, 350/96.33, 367/140, 367/141, 367/169, 73/649, 73/653, 73/655, 73/657, 73/658, 340/380, 340/850, 250/227, 250/231 P, 181/110
• US Class Current: 385/13
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125

Abstract

An optical technique for detecting acoustic waves of selected frequency and determining their angle of arrival in a medium such as water. The technique utilizes one or more lengths of single mode optical fiber having a birefringence whose orthogonal axes are helically disposed throughout the length of the fiber at a predetermined uniform pitch. Sound pressure waves of certain frequencies incident upon the fiber throughout its length change its birefringence which affects the relative phase of polarized light components propagating from one end to the other by an amount proportional to the amplitude of the acoustic wave. The twisted optical fiber may be arranged in parallel with other like fibers and axes twisted at different pitches thereby enabling detection of sound waves over a range of frequencies and their angles of incidence.

Claims

1. In a sensor for sensing waterborne acoustic pressure waves of selected frequency, a sensor region comprising:

an elongate pressure compliant tubular member adapted to be disposed in water for receiving incident acoustic pressure waves; and

,

plural parallel birefringent optical fibers fixedly secured longitudinally to the wall of the tubular member with their orthogonally disposed fast and slow axes twisted at different helical pitches of length L along their longitudinal extends which define plural response periods, respectively, of L/2 distance apart where their fast axes are disposed normal to the tubular member outer surface;

said wall adapted to apply squeeze pressure along the fibers in a direction generally only parallel to the tubular member outer surface in response to an incident acoustic pressure wave on the tubular member;

whereby acoustic sound waves whose wavelength maxima apply squeeze pressure along all the optical fibers, but, only when the maxima arrive in substantial unison at a plurality of response periods of a particular fiber, do they additively change the birefringence of that particular fiber to cause maximum relative phase shift in polarized light components passing through the fiber to identify an acoustic pressure wave of a particular frequency.

3. The invention according to claim 1 or 2 wherein the plural optical fibers are located in the wall of the elongate pressure compliant tubular member.

4. The invention according to claim 1 or 2 wherein the pressure compliant tubular member carries the longitudinally disposed optical fibers both out and return along its elongate extent.

5. The invention according to claim 4 wherein the optical fibers are embedded in the wall of the elongate pressure compliant tubular member.

7. The invention according to claim 5 wherein the tubular member wall thickness dimension is substantially less than its radius dimension.

2. A sensor for sensing waterborne acoustic pressure waves of selected frequency comprising:

an elongate pressure compliant tubular member adapted to be disposed in water to receive incident acoustic pressure waves and comply thereto;

plural parallel birefringent optical fibers fixedly secured longitudinally to the wall of the tubular member with their orthogonally disposed fast and slow axes twisted at different helical pitches L along their longitudinal extents;

means for launching polarized light into one end of each of the optical fibers;

means for detecting relative phase shift of the polarized light components emanating at respective opposite ends of the optical fibers;

said pressure compliant tubular member wall being relatively thin and adapted for applying unidirectional squeeze pressure against opposite longitudinal sides of the optical fibers in response to incident omnidirectional acoustic pressure waves for additively changing birefringence of the optical fibers at spatial periods of L/2 distance apart where the fibers'"'"' fast axes are perpendicular to the wall outer surface;

whereby, when a maximum phase shift in polarized light components is detected in one of the optical fibers, there is identified an acoustic pressure wave frequency whose wavelength maxima are arriving in concert at the spatial periods L/2 along that fiber.

6. The invention according to claim 2 wherein the polarized light launching and detecting means are both located at one end of the elongate pressure compliant tubular member, and the optical fibers extend both out and return along the member.

8. A method of sensing waterborne acoustic pressure waves of selected frequency comprising the steps of:

twisting an optical fiber so that its birefringent orthogonally disposed fast and slow axes are helically disposed therealong at a predetermined pitch L;

launching polarized light into one end of the optical fiber and measuring relative phase shift of its components emanating from the other end in response to a change in the optical fiber birefringence; and

,

converting waterbore acoustic pressures to squeeze pressures on the optical fiber which cause additive birefringence in the optical fiber at spatial periods of L/2 therealong where the fiber'"'"'s fast axis is normal to the squeeze pressure;

whereby measured maximum relative phase shifts of polarized light components indicate the presence of acoustic pressure waves of a frequency whose wavelength maxima are arriving in unison at the spatial periods.

9. A method of sensing the presence of waterborne acoustic pressure waves of selected frequency comprising the steps of:

twisting birefringent optical fibers so that their orthogonal fast and slow axes are helically disposed at different pitches L along their lengths;

arranging the fibers longitudinally parallel with each other;

passing polarized light into one end of each optical fiber and measuring relative phase shifts of components emanating from respective opposite ends in response to changes in birefringence in any of the fibers; and

,

converting waterborne acoustic pressure maxima to squeeze pressures on all of the optical fibers along their longitudinal extents;

whereby acoustic pressure waves having wavelengths such that their pressure maxima, arriving in substantial unison on any of the optical fibers having spatial periods of L/2 distance apart, additively change its birefringence where the fast axes are normal to the squeeze pressures to cause a detectable phase shift in polarized light components to identify the presence of an acoustic wave having a particular frequency.

10. In a sensor for sensing waterborne acoustic pressure waves of a selected wavelength, a sensor region comprising:

an elongate pressure compliant tubular member adapted to be disposed in water for receiving incident acoustic pressure waves; and

,

a birefringent optical fiber fixedly secured longitudinally to the wall of the tubular member with its orthogonally disposed fast and slow axes twisted at a helical pitch on length L along its longitudinal extent to define a response period of L/2;

said wall adapted to apply squeeze pressure along the fiber in a direction generally only parallel to the tubular member outer surface in response to an incident acoustic pressure wave on the tubular member;

whereby acoustic sound waves that apply squeeze pressure along the optical fiber with substantial periodicity of the response period along the fiber additively change the birefringence of the fiber to cause a relative phase shift between polarized light components passing through the fiber in order to identify an acoustic pressure wave of a particular wavelength.

11. A sensor for sensing waterborne acoustic pressure waves of a selected wavelength comprising:

an elongate pressure compliant tubular member adapted to be disposed in water to receive incident acoustic pressure waves;

a birefringent optical fiber fixedly secured longitudinally to the wall of the tubular member with its orthogonally disposed fast and slow axes twisted at a helical pitch L along its longitudinal extent;

means for launching polarized light into one end of the optical fiber;

means for detecting a relative phase shift between the polarized light components emanating at the opposite end of the optical fiber;

said pressure compliant tubular member wall being relatively thin and adapted for applying unidirectional squeeze pressure against opposite longitudinal sides of the optical fiber in response to incident acoustic pressure waves for additively changing the birefringence of the optical fiber at spatial periods of L/2 where the fiber'"'"'s fast axes are perpendicular to the wall outer surface;

whereby, when a relative phase shift between polarized light components is detected in the optical fiber, there is identified an acoustic pressure wave whose maxima are arriving in concert at the spatial periods L/2 along the fiber.