Bidirectional fiber optic systems

• US 4,709,413 A
• Filed: 06/13/1985
• Issued: 11/24/1987
• Est. Priority Date: 09/10/1982
• Status: Expired due to Term

• Current Assignee: Bell Telephone Laboratories, Inc. (Nokia Corporation)
• Original Assignee: American Telephone & Telegraph Company (AT&T, Inc.)
• Inventors: Panock, Richard L., Forrest, Stephen R., Lieberman, Robert A.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): Telesz, Jr., Andrew J.
• Application Number:
  US06/744,465
• Time in Patent Office: 894 Days
• Field of Search: 455/619, 455/606, 455/607, 455/612, 435/808, 435/291, 128/667, 128/672, 128/673, 128/675
• US Class Current: 398/139
• CPC Class Codes: G02B 6/4204 the coupling comprising intermediate optical elements, e.g. lenses, holograms encapsulated active devices H01S5/02208, H01L33/52
• CPC Class Codes: H04B 10/40 Transceivers

Abstract

A bidirectional fiber optic system is described in which at one terminal the output radiation of a light source (LED or semiconductor laser) is coupled through a small diameter hole in the active area of a photodiode into the core of a larger diameter transmission fiber. On the other hand, radiation propagating through the fiber in the opposite direction exits from the fiber with a large numerical aperture so that most of it is incident on the active area of the photodiode and little is lost through the hole. Also described are a number of photodiode configurations for use in such a system, as well as dual photodiodes to perform both signal detection and output monitoring functions. Further described are fiber optic sensing systems for monitoring such variables as blood pressure.

Claims

1. For use in a bidirectional backscattering detection system in which outgoing and backscattered radiation is transmitted over a transmission fiber, a transceiver comprising:

a semiconductor light-emitting device having a p-n junction for generating said outgoing radiation essentially parallel to said junction,

a photodiode having a first active region of a first diameter and a first hole of a smaller second diameter extending through said photodiode in said first active region, said first active region being responsive to said backscattered radiation for generating a signal photocurrent,

means for coupling said outgoing radiation through said first hole, and

a segment of said transmission fiber having a diameter much larger than that of said first hole and having one end positioned adjacent said first active region so that said outgoing radiation is coupled through said first hole into said transmission fiber segment and said backscattered radiation diverges from one end of said transmission fiber segment and is detected by said first active region to produce said signal photocurrent, said first hole being sufficiently small relative to said first active region so that an insignificant portion of said backscattered radiation is coupled through said first hole, and

said coupling means comprises lens means effective to focus said outgoing radiation from said source to a point that lies approximately centrally within said first hole of said photodiode.

2. For use in a bidirectional backscattering detection system in which outgoing and backscattered radiation occurs over a transmission fiber, a transceiver comprising:

a surface-emitting LED having a p-n junction for generating said outgoing radiation essentially perpendicular to said junction,

a photodiode having an active region of a first diameter and a hole of a smaller second diameter extending through said photodiode in said active region, said active region being responsive to said backscattered radiation for generating a signal photocurrent,

means for coupling said outgoing radiation through said hole, and

a segment of said transmission fiber having a diameter much larger than that of said hole and having one end positioned adjacent said active region so that said outgoing radiation is coupled through said hole into said transmission fiber segment and said backscattered radiation diverges from one end of said transmission fiber segment and is detected by said active region to produce said signal photocurrent, said hole being sufficiently small relative to said active region so that an insignificant portion of said backscattered radiation is coupled through said hole,

said coupling means comprising a coupling fiber which extends through said hole in said photodiode, one end of said coupling fiber being butt-coupled to said LED and the other end of said coupling fiber being butt-coupled to said transmission fiber segment, the diameter of said coupling fiber being much smaller than the diameter of said transmission fiber segment so that essentially all of said outgoing radiation is coupled from said LED into said transmission fiber segment, and so that an insignificant portion of said backscattered radiation is coupled from said transmission fiber segment into said coupling fiber.

3. A bidirectional fiber optic sensing system comprising:

an optical transmission fiber;

a sensing element located at one end of said transmission fiber, said sensing element being adapted to reflect light in a manner related to an external variable to permit monitoring of said external variable; and

a transceiver coupled to the opposite end of said transmission fiber for transmission of outgoing radiation and detection of reflected radiation from said sensing element;

said transceiver including at least one semiconductor light-emitting device for generating said outgoing radiation;

said transceiver further including a photodiode having an active region of a first diameter and a first hole of a smaller second diameter extending through said photodiode in said active region, said active region being responsive to said reflected radiation for generating a signal photocurrent, and means for coupling said outgoing radiation through said first hole;

said transmission fiber being adapted to mix the modes of the radiation propagating therein, having a diameter much larger than that of said first hole, and hving one end positioned adjacent said active region so that said outgoing radiation is coupled through said first hole into said transmission fiber and said reflected radiation diverges from one end of said transmission fiber and is detected by said active region to produce said signal photocurrent, said first hole being sufficiently small relative to said active region so that an insignificant portion of said reflected radiation is coupled through said first hole.

4. The system according to claim 3 wherein said transceiver comprises a plurality of light-emitting devices generating light of different wavelengths.

5. The system according to claim 4 further comprising means for forming the ratio of the signal photocurrents produced by the photodiode in response to the light of different wavelengths.

6. The system according to claim 5 further comprising means for frequency multiplexing the light from the plurality of light emitting devices into the transmission fiber.

7. The system according to claim 5 further comprising means for time division multiplexing the light from the plurality of light emitting devices into the transmission fiber.

8. The system according to claim 3 wherein the sensing element is adapted to reflect light in a manner related to external pressure on the element.