Heterodyne optical time domain reflectometer

• US 5,082,368 A
• Filed: 10/03/1990
• Issued: 01/21/1992
• Est. Priority Date: 11/06/1987
• Status: Expired due to Fees

• Current Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Brennecke, Wolfgang, Hoppe, Wolfgang, Brinkmeyer, Ernst, Beckmann, Friedrich-Karl, Fuchs, Manfred H.
• Primary Examiner(s): McGraw, Vincent P.
• Application Number:
  US07/599,253
• Time in Patent Office: 475 Days
• Field of Search: 356/73.1
• US Class Current: 356/73.1
• CPC Class Codes: G01M 11/3109 Reflectometers detecting the back-scattered light in the time-domain, e.g. OTDR

Abstract

Heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a monomode waveguide (test waveguide) by measurement of the backscattered parts of light pulses transmitted in said waveguide. An acousto-optic modulator (AOM) which deflects the transmission beam from a laser into the waveguide to be tested at a light frequency which is modulated with the acoustic frequency when the AOM is acoustically energized in a pulse mode. The light pulses backscattered from the test waveguide are superimposed on a local oscillator beam (LO) constituted by the laser beam which traverses the AOM when the AOM is not energized. The optical system losses are reduced in that the superposition is realized in a passive coupler, particularly a fibre coupler (10, 26).

Claims

1. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behaviour of a monomode waveguide by measurement of the backscattered parts of light pulses transmitted in said waveguide, comprising:

an acousto-optic modulator (AOM) which deflects the transmission beam from a laser into the waveguide to be tested at a light frequency which is modulated with the acoustic frequency when the AOM is acoustically energized in a pulse mode, and a fibre coupler superimposing the light pulses backscattered from the test waveguide on a local oscillator beam (LO) constituted by the laser beam which traverses the AOM when the AOM is not energized.

2. An arrangement as claimed in claim 1, wherein the backscattering signals from the test waveguide are guided into a monomode waveguide leading to an opto-electronic transducer via the non-energized AOM to a low-attenuation direct path of said coupler, and the LO beam is guided into said monomode waveguide via the cross-coupling path of said coupler.

3. An arrangement as claimed in claim 1, wherein the beam diffracted by the AOM is guided into the test waveguide through the low-attenuation direct path of an asymmetrical coupler, and the backscattering signals are superimposed on the LO beam through the cross-coupling path of the coupler, and the mixed signals are guided into a multimode waveguide leading to an opto-electronic transducer via the non-energized AOM.

4. An arrangement as claimed in claim 1, wherein the diffracted beam pulses are transmitted into the test waveguide via the direct path of a symmetrical coupler, and the LO beam, on which the backscattering signals are superimposed through the cross-coupling path of the coupler, is guided in the reverse direction, via the other direct path of the symmetrical coupler, into a monomode waveguide connected to an opto-electronic transducer.

5. An arrangement as claimed in claim 1, wherein the diffracted beam pulses are transmitted into the test waveguide through a cross-coupling path of a symmetrical coupler, and the LO beam is mixed in the reverse direction with the backscattering signals via a cross-coupling path of the coupler, said backscattering signals being guided on a direct path of the coupler into a monomode waveguide leading to an opto-electronic transducer.

6. An arrangement as claim in claim 1, wherein the AOM includes ports having lenses and the connections to the two ports of the AOM into which the LO beam and the transmission pulses leading to the test waveguide are guided comprise two parallel monomode waveguides whose end faces are directed towards the AOM, at least substantially equidistantly from the optical axis of the lens of the port, which axis is disposed in line with the transmission axis of the AOM, the axes of the beams between the end faces of the monomode waveguides and the lens of the port extending parallel to the optical axis of the lens of the port and the beams between the lens of the port and the AOM are inclined at the Bragg angle α

relative to the optical axis of the lens of the port and intersect one another substantially at the centre of the AOM.

7. An arrangement as claimed in claim 1, wherein the transmission beam of the laser passes through an optical isolator in front of said AOM.

8. An arrangement as claimed in claim 1, wherein the LO beam passes through a polarization scrambler.

9. An arrangement as claimed in claim 1, wherein the laser is a distributed feedback laser.

10. An arrangement as claimed in claim 1, wherein the laser beam passes through a low-attenuation direct path of an asymmetrical coupler having its cross-coupling port connected to a section of a monomode waveguide with a reflective end face so that a small part of the transmission energy is fed back to the laser for the purpose of stabilization.

11. An arrangement as claimed in claim 1, wherein the AOM includes ports and waveguides comprise the port connection thereto, the end faces of the waveguides constituting the port connections to the AOM are inclined relative to their axes at an angle which differs by approximately β

=6°

, from a right angle, and the axes of the waveguides extend at such an angle, in particular δ

=2.7°

, to the lens axis that the axes of the beams emerging from the end faces of the waveguides extend parallel to the optical axis of the lens.

12. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a monomode waveguide comprising:

an acousto-optical modulator for receiving on a first port a laser generated light beam, and supplying said laser beam to second and third output ports during first and second switching conditions, respectively, said second port being connected to a waveguide which is to be tested, and a fourth port which receives backscattered radiation from said waveguide during said second switching condition;

an asymmetrical coupler having first and second coupled paths, a first of said paths being connected to said third output port for receiving said laser beam during said second switching condition, and said second path being connected to said fourth port, said second path delivering a mixed signal comprising said backscattered signal and a local oscillator signal from said third port; and

a photodetector connected to said second path of said asymmetrical coupler for receiving said mixed signal.

13. An arrangement as claimed in claim 12, wherein the optical isolator is arranged between a lens and the AOM.

14. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a monomode waveguide by measurement of the backscattered parts of light pulses carried by said waveguide comprising:

an acousto-optical coupler having a first port for receiving a laser beam, second and third ports for transmitting said laser beam during first and second switching states, respectively, of said acousto-optical coupler, and a fourth port for transmitting backscattered light pulses from said waveguide;

an asymmetrical coupler having first and second direct paths coupled to each other, said first path serially connecting said second port to said monomode waveguide, and said second path connected to said third port; and

a photodetector connected to said fourth port for receiving backscattered light pulses from said waveguide mixed with a local oscillator signal.

15. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a monomode waveguide comprising:

a source of laser light;

a coupler having first and second coupled paths, said first path connected to said source of laser light, said second path connected to a waveguide for stabilizing said source of laser light;

an optical isolator connected to said first path for receiving light from said laser source;

an acousto-optical coupler connected to said isolator, having first and second output ports, said acousto-optical coupler being connected to receive a signal for frequency shifting said laser beam, and switching said light beam between said first and second output ports;

a four port coupler having first and second coupled paths, said first path being connected to said first and second output ports; and

detection means connected by said four port coupler second path to said monomode waveguide, whereby backscattered light from said monomode waveguide is directed to said detection means.

16. The heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a monomode waveguide of claim 15 further comprising a polarization scrambler connected to one of said paths of said four port coupler.

17. The heterodyne optical time domain reflectometer (OTDR) of claim 15 wherein said detection means includes:

a photodetector connected to said four port coupler second path;

a filter for filtering a signal from said photodetector; and

,

a diode biased to operate in its quadratic characteristic curve, connected to receive a signal from said filter producing a voltage proportional to the power of said backscattered light.

18. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a waveguide, comprising:

an acousto-optic modulator (AOM) which, when acoustically energized, serves to diffractively deflect a light beam from an optical source into a waveguide to be tested; and

means, including an asymmetric fiber coupler, capable of optical communication with said AOM, for interacting light pulses backscattered from the test waveguide with a local oscillator (LO) beam constituted by a light beam from the optical source which traverses the AOM when the AOM is not acoustically energized, said light beam diffracted by the AOM being guided into the test waveguide through a low-attenuation direct path of said asymmetric coupler and said backscattered light pulses being interacted with said LO beam via a cross-coupling path of said asymmetric coupler, the resulting mixed signals being guided into a multimode waveguide leading to an opto-electronic transducer via the non-energized AOM.

19. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a waveguide, comprising:

an acousto-optic modulator (AOM) which, when acoustically energized, serves to diffractively deflect a light beam from an optical source into a waveguide to be tested; and

means, including a symmetric fiber coupler, capable of optical communication with said AOM, for interacting light pulses backscattered from the test waveguide with a local oscillator (LO) beam constituted by a light beam from the optical source which traverses the AOM when the AOM is not acoustically energized, said light beam diffractively deflected by the AOM being transmitted into the test waveguide via a first direct path of said symmetric coupler and the LO beam being guided through said symmetric coupler via a second direct path of said symmetric coupler, said LO beam being interacted with said backscattered light pulses via a cross-coupling path of said symmetric coupler, the resulting mixed signals being guided via a monomode waveguide to an opto-electronic transducer.

20. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a waveguide, comprising:

an acousto-optic modulator (AOM) which, when acoustically energized, serves to diffractively deflect a light beam from an optical source into a waveguide to be tested; and

means, including a symmetric fiber coupler, capable of optical communication with said AOM, for interacting light pulses backscattered from the test waveguide with a local oscillator (LO) beam constituted by a light beam from the optical source which traverses the AOM when the AOM is not acoustically energized, said light beam diffractively deflected by the AOM being transmitted to the test waveguide via a first cross-coupling path of said symmetric coupler and said LO beam being interacted with said backscattered light pulses via a second cross-coupling path of said symmetric coupler, the resulting mixed signals being guided via a monomode waveguide to an opto-electronic transducer.

21. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a waveguide, comprising:

an acousto-optic modulator (AOM) which, when acoustically energized, serves to diffractively deflect a light beam from an optical source into a waveguide to be tested;

means, including a fiber coupler, capable of optical communication with said AOM, for interacting light pulses backscattered from the test waveguide with a local oscillator (LO) beam constituted by a light beam from the optical source which traverses the AOM when the AOM is not acoustically energized;

a lens, positioned adjacent an output end of said AOM and

first and second substantially parallel, monomode waveguides, capable of optical communication with said lens, which serve to guide, respectively, said LO beam and said diffractively deflected light beam, said first and second waveguides being substantially equidistant from an optical ax of said lens, said optical axis being substantially co-linear with a transmission axis of said AOM, said LO and diffractively deflected light beams having axes which, between said lens and said first and second waveguides are substantially parallel to, and between said AOM and said lens are inclined at the Bragg angle relative to, said optical axis of said lens and intersect one another substantially at the center of the AOM.

22. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a waveguide, comprising:

an acousto-optic modulator (AOM) which, when acoustically energized, serves to diffractively deflect a light beam from an optical source into a waveguide to be tested;

means, including a fiber coupler, capable of optical communication with said AOM, for interacting light pulses backscattered from the test waveguide with a local oscillator (LO) beam constituted by a light beam from the optical source which traverses the AOM when the AOM is not acoustically energized; and

a polarization scrambler through which said LO beam is passed.

23. A heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behavior of a waveguide, comprising:

an acousto-optic modulator (AOM) which, when acoustically energized, serves to diffractively deflect a light beam from a laser into a waveguide to be tested;

means, including a fiber coupler, capable of optical communication with said AOM, for interacting light pulses backscattered from the test waveguide with a local oscillator (LO) beam constituted by a light beam from the optical source which traverses the AOM when the AOM is not acoustically energized; and

an asymmetric coupler capable of optical communication with said laser, said asymmetric coupler including a cross-coupling port in optical communication with a monomode waveguide having a reflective end face, whereby a portion of the light produced by said laser is fed back to the laser for the purpose of stabilization.