Stable fiber-optic hydrophone

• US 4,525,818 A
• Filed: 07/28/1982
• Issued: 06/25/1985
• Est. Priority Date: 08/09/1979
• Status: Expired due to Fees

• Current Assignee: Her Majesty The Queen In Right of Canada As Represented By The Minister of National Defence of Her Majesty's Canadian Goverment
• Original Assignee: Her Majesty The Queen In Right of Canada As Represented By The Minister of National Defence of Her Majesty's Canadian Goverment
• Inventors: McMahon, Garfield W., Cielo, Paolo G.
• Primary Examiner(s): TUDOR, HAROLD JAY
• Application Number:
  US06/402,808
• Time in Patent Office: 1,063 Days
• Field of Search: 367/141, 367/149, 350/96.15, 350/96.29, 356/44, 356/28.5, 73/655, 250/461 R
• US Class Current: 367/149
• 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
• CPC Class Codes: G01V 1/201 Constructional details of seismic cables, e.g. streamers integrated optoseismic systems G01V1/226; line connectors in general H01R, transducer mountings in general G10K11/004
• CPC Class Codes: G02B 6/4457 Bobbins; Reels

Abstract

An optical fiber hydrophone system in which a single optical fiber is used for all of the acoustical sensors in the system. A signal source and detector provides an optical signal in selected form, such as continuous or pulsed and detects and extracts an identifiable output signal. Each sensor is in the form of a sensing portion of the single optical fiber. Each sensing portion includes two optical reflectors separated one from another by a predetermined length of said optical fiber. Variations in acoustical pressure incident on the sensing portion causes a change in the predetermined length. This causes reflected portions of the optical signal to interfere with one another. Such interference is detectable for extraction of the identifiable output signal. In one form each sensing portion has two terminal branches of a mechanically deformable material, deformable in response to the fluctuations in acoustical pressure. Preferably, the optical fiber has two portions, a sensing portion thereof underwater and having a first optical cavity, and another portion thereof on board a vessel and having a second optical cavity, typically tunable with respect to the optical length thereof to maximize the interference in the detected optical signal.

Claims

1. An optical fiber hydrophone system adapted to process variations in local acoustical pressure in a fluid body and provide an identifiable output signal, comprising:

signal source and detecting means for providing an optical signal in a selected form, and for detecting and extracting said indentifiable output signal;

a single optical fiber connected at one end to said source and detecting means and operable for conducting optical signals from said source and to said detecting means;

a sensing means in said optical fiber remote from said source and detecting means, said sensing means being adapted for immersion in said body and including two optical reflector means incorporated into said fiber and spaced one from the other by a predetermined length of said optical fiber, at least one of said reflector means being partially reflective and partially transmissive, said variations in acoustical pressure incident on said sensing portion causing a change in said predetermined length thereof and thus a phase and intensity modulation in the portion of said optical signal reflected back and forth between said two optical reflector means and,

means connected to said source and detecting means for processing the intensity modulated optical signal transmitted back to said source and detecting means in said optical fiber for extraction therefrom of said identifiable output signal.

2. The optical fiber hydrophone system defined in claim 1, wherein said single optical fiber is provided with at least one terminal branch in the form of a section of a second optical fiber, the two fibers being optically coupled together so as to share said optical signal of selected form, the second optical fiber also containing reflector means to provide a reflected optical signal, mechanical means being provided so that the two fibers are subject to different changes in length in response to the variations in acoustical pressure incident on said sensing portion.

3. The optical fiber hydrophone system defined in claim 1 or 2 wherein each said reflector means is in the form of one of a microscopic taper in the optical fiber, a low quality fiber connector whose reflectivity depends on misalignment of the ends of the optical fibers being connected together and with the refractive index of a matching liquid used in said connector, or a periodic disturbance generated through exposure to an optical standing wave.

4. The optical fiber hydrophone system defined in claim 1 wherein a multiplicity of sensing portions are provided, each sensing portion being spaced from adjacent sensing portions by a predetermined length of said optical fiber.

5. The optical fiber hydrophone system defined in claim 1 wherein a multiplicity of sensing portions are provided, each sensing portion being spaced from adjacent sensing portions by a preselected length of said optical fiber, each sensing portion comprising two terminal branches of optical fiber with a fiber connector incorporated into each extremity of the terminal branches, said branches being separated one from another over a major portion of their length by a mechanically deformable material deformable in response to said variations in acoustical pressure incident thereon, deformation of said material causing said modulation in the reflected signal.

6. The hydrophone system defined in claim 1, or 2, wherein said optical fiber has a multi-branch terminal portion and one of said reflector means is incorporated into an extremity of each branch, said branches having different coefficients of shrinkage, the variations in acoustical pressure incident thereon causing variations in said predetermined length of fiber to modulate the intensity of the resultant reflected optical signal transmitted back to said source means thereby to produce said identifiable output signal.

7. The optical fiber hydrophone system defined in claim 1, wherein said single optical fiber includes two sections, a first section, in use, being disposable underwater with said sensing portion defining a first optical cavity, the second section of the optical fiber typically being located aboard a vessel, and including a second optical cavity therein delimited by optical reflector means;

said first and second optical cavities having lengths of said optical fiber therein that are in predetermined relation to one another, whereby said variations in acoustical pressure cause proportional variations in the predetermined length of the sensing portion on which it is incident, this causing intensity modulation of the reflected signal from the first optical cavity and enabling said detector means to extract said identifiable output signal from the intensity modulations.

8. The optical fiber hydrophone system defined in claim 7, wherein said second optical cavity is electronically tunable.

9. The optical fiber hydrophone system defined in claim 8, wherein said second optical cavity comprises a length of said optical fiber wound about a piezoelectric element, whereby a small tuning voltage applied across the piezoelectric element controls the length of said second optical cavity.

10. The optical fiber hydrophone system defined in claim 9, wherein a servo-control circuit is provided for applying said tuning voltage to the piezoelectric element, whereby variations made to said tuning voltage to maintain said cavities tuned, correspond to the modulations in the reflected signal caused by variations in the length of said first optical cavity, as created by the variations in acoustical pressure incident thereon, said variations to the tuning voltage providing said identifiable output signal.

11. The optical fiber hydrophone system defined in claim 1, wherein a multiplicity of sensing portions is provided and said optical signal is a series of time-separated optical pulses;

each sensing portion having two short terminal branches of optical fiber optically coupled to said single optical fiber by a directional tee-coupler, each terminal branch terminating with a total reflector whereby a fraction of the optical signal of said time-separated pulses is coupled into each terminal branch, with the modulated reflected signals from those sensing portions on which said variations in acoustical pressure are incident also being time-separated to enable extraction of the identifiable output signal.

12. The optical fiber hydrophone system defined in claim 1, wherein a multiplicity of sensors are provided and each sensor selects a narrow optical frequency band;

the various frequency bands being separately detected at the end of the fiber by a dispersive element such as a prism.

13. The optical fiber hydrophone defined in claim 5, 11 or 12 wherein the sensing portions are provided in a plurality of planes each of which contains an underwater section of said single optical fiber.

14. The optical fiber hydrophone system defined in claim 5, or 12 wherein the sensing portions form a two dimensional array.

15. An optical fiber hydrophone system adapted to process variations in local acoustical pressure in a fluid body and provide an identifiable output signal, comprising:

signal source and detecting means for providing an optical signal in a selected form, and for detecting and extracting said identifiable output signal;

a single optical fiber connected at one end to said source and detecting means and operable for conducting optical signals from said source and to said detecting means;

a sensing means in said optical fiber remote from said source and detecting means, said sensing means being adapted for immersion in said fluid body and including a predetermined length of said optical fiber;

two optical reflector means incorporated into said fiber and spaced one from the other by said predetermined length of said optical fiber, at least one of said reflector means being partially reflective; and

anisotropical means secured to the predetermined length of fiber for stressing the predetermined length of fiber anisotropically in response to variations in acoustical pressure incident on said sensing means, thus causing a change in the dimensions of said predetermined length of fiber and a phase and intensity modulation in the portion said optical signal reflected back and forth between said optical reflector means and

means connected to said source and detecting means for processing the intensity modulated signal transmitted back to said source and detecting means in said optical fiber for extraction therefrom of said identifiable output signal.

16. An optical fiber hydrophone system defined in claim 15, wherein said predetermined length of said optical fiber is in the form a coil and said anisotropical means comprise a hollow end-capped cylinder in which the coil is embedded.

17. An optical fiber hydrophone system defined in claim 15 wherein the anisotropical means comprise a body of an anisotropically deformable material.