Optical fault locator

• US 6,381,011 B1
• Filed: 02/04/2000
• Issued: 04/30/2002
• Est. Priority Date: 02/19/1999
• Status: Expired due to Fees

• Current Assignee: Wilcom, Inc.
• Original Assignee: Wilcom, Inc.
• Inventors: Nickelsberg, Paul, Rapoza, Joseph, Craig, Dean R., Kalin, Walter F.
• Primary Examiner(s): Pham, Hoa Q.
• Assistant Examiner(s): Nguyen, Tu T.
• Application Number:
  US09/499,107
• Time in Patent Office: 816 Days
• Field of Search: 356/73.1, 385/147, 700/12, 700/286, 700/79, 324/522, 324/509, 324/628, 324/107, 702/188, 702/59, 702/127, 702/64
• 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

A hand held optical fault locator used to determine the distance to a discontinuity in a fiber optic cable, such as a break, nick, cut, scrape, indentation, or the like, a splice, a connector, or the end of the cable. The locator includes an analog front end, a complex programmable logic device (CPLD), a laser diode and drive, a microcontroller, at least one input, and at least one output. The preferred locator allows a user to select a specific index of refraction, to measure the distance to a fault directly in feet or meters, to measure fiber lengths at counting frequencies in the 100KHz range, to determine fiber reflection events in the digital-time domain, and to make accurate measurements without the use of a sensitivity adjustment feature.

Claims

1. An optical fault locator for locating reflective events within a fiber optic cable, said optical fault locator comprising:

at least one user input for accepting information from a user;

a laser assembly for sending a pulsed laser light through the fiber optic cable;

a front end for receiving a reflected portion of the pulsed laser light from the fiber optic cable, comparing, a value of the reflected portion of the pulsed laser light with a threshold value, and outputting a digital pulse corresponding to a result of the comparison;

a complex programmable logic device in communication with the front end for sending the threshold value to the front end accepting the digital pulse from the front end, and sending at least one output corresponding to a distance to a reflective event;

a microcontroller in communication with said user input, said laser assembly, and said complex programmable logic device, said microcontroller being programmed to accept said information from said user input, calculate the threshold value, simultaneously send a threshold signal, corresponding, to the threshold value, to said complex programmable logic device and a laser signal, corresponding to the threshold value, to said laser assembly to launch a laser pulse, receive said at least one output corresponding to a distance to a reflective event from said complex programmable logic device, calculate an actual distance based upon said at least one signal, and send a distance output signal; and

at least one user output for accepting said distance output signal from said microcontroller and providing a corresponding distance value to the user.

2. The optical fault locator as claimed in claim 1, wherein said information from the user comprises an index of refraction of the fiber optic cable, wherein said complex programmable logic device comprises a counting circuit having a programmable oscillator for varying a counting frequency, and wherein said microcontroller is further programmed accept said index of refraction and to send a control signal, corresponding to said index of refraction, to said programmable oscillator.

3. The optical fault locator as claimed in claim 2 wherein said information further comprises a unit of measure, wherein said microcontroller is further programmed to reference a look up table corresponding to said unit of measure and said index of refraction and to send a control signal to said programmable oscillator, said control signal corresponding to said unit of measure and said index of refraction.

4. The optical fault locator as claimed in claim 3 wherein said unit of measure is chosen from a group consisting of meters and feet, and wherein said control signal causes said counting frequency of said programmable oscillator to be varied such that one oscillation corresponds to one unit of said chosen units of measure.

5. The optical fault locator as claimed in claim 1, wherein said complex programmable logic device comprises a counting circuit comprising:

a digital event detector;

a clock gate in communication with said digital event detector and said microcontroller;

a programmable oscillator for varying a counting frequency;

a prescale circuit in communication with said programmable oscillator and said clock gate; and

a counter in communication with said clock gate and said microcontroller;

wherein said microcontroller sends a control signal to said programmable oscillator and simultaneously sends an opening signal to open the clock gate, said programmable oscillator sends a counting signal through said prescale circuit and said clock gate to said counter and said counter receives and counts a number of prescaled counting signal oscillations until said digital event detector sends a closing signal to close said clock gate and sends a count output signal to said microcontroller once said clock gate is closed.

6. The optical fault locator as claimed in claim 5 wherein said prescale circuit prescales said counting signal in two stages such that said prescaled counting signal has a sixteen bit length.

7. The optical fault locator as claimed in claim 5 wherein said microcontroller is further programmed to receive a plurality of count output signals from said counter and to calculate and store a distance to said reflective event based upon said plurality of count output signals.

8. The optical fault locator as claimed in claim 7 wherein said microcontroller is further programmed to calculate and store distances to up to eight reflective events.

9. The optical fault locator as claimed in claim 1 wherein said analog front end comprises:

an infrared detector for detecting the reflected portion of the pulsed laser light from the fiber optic cable and providing a current output corresponding to said detected light;

a trans-impedance amplifier for receiving the current output from the infrared detector and outputting a voltage output signal;

a digital to analog converter for accepting a digital representation of the threshold value from the complex programmable logic device and converting said digital representation to an analog threshold signal; and

a comparator for accepting said voltage output signal from said trans-impedance amplifier, receiving said analog threshold signal from said digital to analog converter, comparing said voltage output signal to said analog threshold signal, and sending a digital pulse to said complex programmable logic device when said voltage output signal exceeds said analog threshold signal.

10. The optical fault locator as claimed in claim 9 wherein said trans-impedance amplifier outputs a saturated voltage output signal.

11. The optical fault locator as claimed in claim 9 wherein said trans-impedance amplifier outputs an unsaturated voltage output signal and wherein said microcontroller is programmed to vary the threshold value until a reflective event is located.

12. The optical fault locator as claimed in claim 1 wherein said at least one input comprises at least one membrane type button, and wherein said at least one output comprises a liquid crystal display.

13. The optical fault locator as claimed in claim 1 wherein one of said at least one input is chosen from a group consisting of an RS232 communication link and an infrared communication link.

14. The optical fault locator as claimed in claim 1 further comprising a circuit board upon which said laser assembly, said front end, said complex programmable logic device, and said microcontroller are disposed.

15. A method of detecting a reflective event in a fiber optic cable comprising the steps of:

(a) inputting an index of refraction into a microcontroller;

(b) instructing said microcontroller to begin a test;

(c) launching a pulse of laser light into the fiber optic cable;

(d) opening a clock gate simultaneously with said launching step;

(e) causing a programmable oscillator to oscillate at a predetermined frequency based upon said index of refraction;

(f) counting a number of oscillations passing through said clock gate;

(g) receiving a reflected portion of the pulse of laser light from the fiber optic cable;

(h) comparing a value of the reflected portion of the pulsed laser light with a threshold value;

(i) closing the clock gate when the reflected portion of the pulsed laser light exceeds the threshold value;

(j) summing the number of oscillations counted between the opening of the clock gate and the closing of the clock gate;

(k) repeating steps (c) through (j) a predetermined number of times; and

(l) calculating a distance to the reflective event based upon the results of the summing steps.

16. The method as claimed in claim 15 further comprising the step of inputting a unit of measure into a microcontroller prior to said step of instructing said microcontroller to begin a test, and wherein the step of causing a programmable oscillator to oscillate at a predetermined frequency comprises the step of causing said programmable oscillator to oscillate at a predetermined frequency of oscillation corresponding to said unit of measure and said index of refraction.

17. The method as claimed in claim 16 wherein said predetermined frequency of oscillation is such that one oscillation corresponds to one of said unit of measure.

18. The method as claimed in claim 15 further comprising the steps of determining that a reflective event has not been detected and altering the threshold value until the reflective event is detected.

19. The method as claimed in claim 15 wherein steps (c) through (l) are repeated up to eight times and wherein said method further comprises the step of storing a result of each calculating step.

20. The method as claimed in claim 15 further comprising the step of sending an output corresponding to the distance.