Fiber optic sensors

• US 4,924,870 A
• Filed: 01/13/1989
• Issued: 05/15/1990
• Est. Priority Date: 01/13/1989
• Status: Expired due to Term

• Current Assignee: C.R. Bard, Inc. (Becton, Dickinson & Co)
• Original Assignee: Fibergel Technologies Incorporated (Agritech Worldwide, Inc.)
• Inventors: Tomasko, Douglas G., Coletta, Luciano, Campbell, James A., Wlodarczyk, Marek T.
• Primary Examiner(s): Sykes, Angela D.
• Application Number:
  US07/297,069
• Time in Patent Office: 487 Days
• Field of Search: 128/664-667, 128/672-673, 128/675, 128/748, 128/633-634, 73/705, 73/708, 73/715
• US Class Current: 600/480
• CPC Class Codes: A61B 5/02154 by optical transmission
• CPC Class Codes: G01L 9/0077 for measuring reflected light

Abstract

Serveral improvements in fiber optic sensing systems are disclosed. One improvement incorporates a dielectric filter applied directly to the sensing end of an optical fiber detector which has the characteristic that it reflects back a reference light beam of one wavelength while passing a sensing signal of a different wavelength which is modulated in some predetermined fashion beyond the filter. Both light signals are reflected back through the fiber and are, accordingly, attenuated in the identical or nearly identical fashion in response to various noise sources. The ratio of the intensity of the two signals is proportional to the sensed parameter and renders the system essentially self-compensating. The dielectric filter is preferably directly coated onto the exit end of the fiber through vapor deposition techniques. Time division multiplexing is used to sequentially fire plural light sources and a single photodetector is used for measuring the intensity of the reflected back signals of the various wavelengths. Another improvement encompasses the detection unit in which the input and reflected signals are separated using an off-axis parabolic mirror which provides excellent separation efficiency. According to another principal feature of this invention, a concave surface is formed at the fiber end which increases the numerical aperture of the fiber which provides greater sensitivity to the deflection of a pressure sensitive diaphragm spaced from the confronting the fiber end.

Claims

1. A fiber optic sensing system comprising:

an optical fiber having first and second terminal ends,

first light emitting means for producing an input reference light signal,

second light emitting means for producing an input sensing light signal of a wavelength different from that produced by said first light emitting means,

optical means for inputting said reference and sensing light signals into said first fiber end,

a sensing tip fastened to said fiber second end and having measuring means for modulating said sensing light signal in response to a parameter to be measured,

a wavelength selective filter adjacent said second terminal end which allows said sensing light signal passing through said filter to be modulated by said measuring means while returning said reference signal,

photodetector means for sensing the intensity of said reference and sensing light signals returned through said second fiber end, and

controller and signal processing means for controlling said first and second light emitting means to produce alternate pulses of light from each of said light emitting means and for processing signals from said photodetector means to separate said returned reference light signal from said returned sensing light signal, and for comparing the intensity of said returned reference and sensing signals to provide an output related to said parameter to be measured.

2. A fiber optic sensing system as set forth in claim 1 wherein said measuring means comprises a diaphragm overlying said fiber second end which deforms in response to pressure differentials and varies the intensity of said returned sensing light signal returned into said fiber dependent on its spacing from said fiber second end.

3. A fiber optic sensing system as set forth in claim 1 wherein said wavelength selective fiber is a dielectric filter applied onto said fiber second end.

4. A fiber optic sensing system as set forth in claim 1 wherein said reference signal is of a shorter wavelength than said sensing signal.

5. A fiber optic sensing system as set forth in claim 1 wherein said wavelength selective filter comprises alternate layers of silicon dioxide and titanium dioxide directly coated onto said fiber second end.

6. A fiber optic sensing system as set forth in claim 1 wherein said controller and signal processing means is further for measuring the intensity of light outputted by said first and second light emitting means before said light signals are injected into said fiber for controlling the output of said first and second light emitting means.

7. A fiber optic sensing system as set forth in claim 1 wherein said photodetector means comprises a single photodetector element which receives both said returned reference and sensing light signals.

8. A fiber optic sensing system comprising:

an optical fiber having first and second terminal ends,

a wavelength selective filter adjacent said second terminal end including numerous layers of dielectric materials directly deposited onto said fiber second end having the characteristic that said filter transmits light within a first range of wavelength and reflects light within a second range of wavelengths,

first light emitting means for producing a reference light signal within said second range of wavelengths,

second light emitting means for producing a sensing light signal of a wavelength different than that produced by said first light emitting means and within said first range of wavelengths,

optical means for injecting said reference and sensing light signals into said first fiber end,

a sensing tip fastened to said fiber second end and having measuring means for modulating said sensing light signal in response to a parameter to be measured,

photodetector means for sensing the intensity of said reference and sensing light signals returned through said fiber, and

controller and signal processing means for evaluating the intensity of said returned reference and sensing light signals and providing an output related to said parameter to be measured.

9. A fiber optic sensing system as set forth in claim 8 wherein said controller and signal processing means further comprises means for controlling said first and second light emitting means to produce alternate pulses of light and for processing signals from said photodetector means to separate signals related to said returned reference light signal from signals related to said returned sensing light signal, and for comparing the intensity of said returned reference and sensing signals to provide an output related to said parameter to be measured.

10. A fiber optic sensing system as set forth in claim 8 wherein said reference signal is of a lower wavelength than said sensing signal and is reflected back by said filter.

11. A fiber optic sensing system as set forth in claim 8 wherein said controller and signal processing means is further for measuring the intensity of light outputted by said first and second light emitting means before said light signals are injected into said fiber for controlling the output of said first and second light emitting means.

12. A fiber optic sensing system as set forth in claim 8 wherein said measuring means comprises a diaphragm overlying said fiber second end which deforms in response to pressure differentials and varies the intensity of said returned sensing light signal dependent on its spacing from said fiber second end.

13. A beam separator assembly for a fiber optic sensing system of the type having a sensing tip with measuring means for sensing a parameter to be measured and modulating the intensity of a sensing light signal which is returned back through an optical fiber, comprising:

light source means for generating a sensing light signal,

first optical means for inputting said sensing light signal into an end of said optical fiber,

a parabolic mirror having an aperture through which said sensing light signal passes before being injected into optical fiber,

second optical means for enlarging the returned light signal exiting said optical fiber end and directing said returned signal onto said parabolic mirror around said aperture, said parabolic mirror having an optical axis tipped from that of said input and returned sensing light signal, and

photodetector means for receiving and measuring the intensity of light focused by said parabolic mirror.

14. A beam separator assembly as set forth in claim 13 wherein said light source means includes at least two discrete light sources generating light of different wavelengths and wherein the light from said two sources is combined coaxially by a beam splitter to define said input beam.

15. A beam separator assembly for a fiber optic sensing system of the type having a sensing tip with measuring means for sensing a parameter to be measured and modulating the intensity of a sensing light signal which is returned back through an optical fiber, comprising:

light source means for generating a sensing light signal,

first optical means for inputting said sensing light signal into an end of said optical fiber along an optical axis,

second optical means for receiving said returned signal emitted from said fiber end along said optical axis and for expanding said returned beam to a diameter larger than said input light beam,

a parabolic mirror positioned along said optical axis for receiving and focusing said returned light beam, said mirror having an optical axis inclined from said beam optical axis, and

photodetector means for receiving and measuring the intensity of light focused by said parabolic mirror.

16. A beam separator as set forth in claim 15 wherein said input beam passes through an aperture within said mirror.

17. A fiber optic sensing system comprising:

an optical fiber having first and second terminal ends,

a wavelength selective filter adjacent said second terminal end including numerous layers of dielectric materials directly deposited onto said fiber second end having the characteristic that said filter transmits light within a first range of wavelength and reflects light within a second range of wavelengths,

first light emitting means for producing a reference light signal within said second range of wavelengths,

second light emitting means for producing a sensing light signal of a wavelength different than that produced by said first light emitting means and within said first range of wavelengths,

optical means for injecting said reference and sensing light signals into said first fiber end,

a sensing tip fastened to said fiber second end and having measuring means for modulating said sensing light signal in response to a parameter to be measured,

photodetector means for sensing the intensity of said reference and sensing light signals returned through said fiber, and

controller and signal processing means for evaluating the intensity of said returned reference and sensing light signals and providing an output related to said parameter to be measured, and further having means for controlling said first and second light emitting means to produce alternate pulses of light and for processing signals from said photodetector means to separate signals related to said returned reference light signal from signals related to said returned sensing light signal, and for comparing the intensity of said returned reference and sensing signals to provide an output related to said parameter to be measured.

18. A fiber optic sensing system comprising:

an optical fiber having first and second terminal ends,

a wavelength selective filter adjacent said second terminal end including numerous layers of dielectric materials directly deposited onto said fiber second end having the characteristic that said filter transmits light within a first range of wavelength and reflects light within a second range of wavelengths,

first light emitting means for producing a reference light signal within said second range of wavelengths,

second light emitting means for producing a sensing light signal of a wavelength different than that produced by said first light emitting means and within said first range of wavelengths,

optical means for injecting said reference and sensing light signals into said first fiber end,

a sensing tip fastened to said fiber second end and having measuring means for modulating said sensing light signal in response to a parameter to be measured,

photodetector means for sensing the intensity of said reference and sensing light signals returned through said fiber, and

controller and signal processing means for evaluating the intensity of said returned reference and sensing light signals and providing an output related to said parameter to be measured, and further for measuring the intensity of light outputted by said first and second light emitting means before said light signals are injected into said fiber for controlling the output of said first and second light emitting means.