Signal averaging for optical time domain relectometers

• US 4,928,232 A
• Filed: 12/16/1988
• Issued: 05/22/1990
• Est. Priority Date: 12/16/1988
• Status: Expired due to Fees

• Current Assignee: Laser Precision Corp. (GN Store Nord A/S)
• Original Assignee: Laser Precision Corp. (GN Store Nord A/S)
• Inventors: Gentile, John R.
• Primary Examiner(s): Lall, Parshotam S.
• Assistant Examiner(s): Melnick, Steven A.
• Application Number:
  US07/285,026
• Time in Patent Office: 522 Days
• Field of Search: 356/73.1, 364/473, 364/525, 364/554, 364/487
• US Class Current: 356/73.1
• CPC Class Codes: G01M 11/3145 Details of the optoelectronics or data analysis

Abstract

Means for digitizing and averaging the signals in an optical time domain reflectometer are disclosed in which a comparator is used to output a 1 or 0 into an arbitrarily large number of successive time-bins. In each comparator choice, the analog voltage signal from the OTDR receiver is compared to a selected analog voltage value. The selected value is chosen randomly from the range of available signals in the interval of interest; and each such valve is used to provide a comparator input into every time-bin during one waveform recovery.

Each comparator output signal (1 or 0) is directed to a separate time-bin (counter), representing the desired time interval (resolution). Subsequent comparator output signals are added to each time bin. The waveform recovery runs are repeated until an acceptable signal-to-noise result is achieved. Each waveform run uses a different voltage for comparison to the receiver voltage. Such comparison voltages may be selected by a random generator; or an ordered series of comparison voltages may be used.

Claims

1. In an optical time domain reflectometer which includes (a) a laser for firing signals into a fiber strand, (b) means for receiving and measuring continuously varying returning signals representing reflection values during a waveform sweep along the fiber strand, and (c) a computer controller, means for extracting useful waveform-determining values from the noise in which they are embedded, comprising:

a comparator which receives two analog signals as first and second inputs, and outputs a 1 or 0 signal indicating which of the received signals is higher;

means for directing the varying analog voltage values from the receiving/measuring means into the comparator as its first input signal;

means for providing a large number of comparison voltage values, which are varied so as to be substantially evenly distributed in the range of possible values;

each such comparison signal providing the second input signal to the comparator during a waveform sweep representing the length of the fiber strand;

means for providing a series of time-bins, each of which represents a certain time, and thus a certain distance along the length of the fiber strand;

clocking means for determining the frequency of output signals from the comparator; and

means for causing each time-bin to accumulate the comparator output signals from a given segment of the waveform during successive waveform sweeps caused by successive laser firings.

2. The apparatus of claim 1 in which each time-bin comprises:

multiple registers for temporarily storing successive output signals from the comparator;

multiple binary counters, each connected to one of the registers; and

clocking means for simultaneously transferring the stored information from each register into its counter after each waveform sweep.

3. The apparatus of claim 2 which also comprises:

means controlled by the computer controller for interrogating each counter after a desired number of successive laser firings/waveform sweeps have been accomplished; and

means responsive to the interrogating means for transferring to the computer controller, from each counter in succession, the word accumulated therein representing a binary number obtained by co-adding the values transferred to the counter from its connected register during the desired number of laser firings/waveform sweeps.

4. The apparatus of claim 1 in which:

a random number generator is used to supply the comparison voltage values to the second input of the comparator.

5. The apparatus of claim 1 in which:

an automatically varying ordered series of values is used to supply comparison voltage values to the second input of the comparator.

6. The apparatus of claim 2 which also comprises:

one or more integrated circuit chips, each of which contains a large number of counters, and an equal number of shift register stages.

7. The apparatus of claim 2 in which:

the clocking means which times each output signal from the comparator also times the change from one temporary storage register to the next.

8. The apparatus of claim 7 which also comprises:

means for changing the frequency of the comparator/register clock, in order to change the resolution, in time and distance, of the waveform digital signal intervals.

9. In an optical time domain reflectometer which includes (a) a laser for firing signals into a fiber strand, (b) means for receiving and measuring continuously varying returning signals representing reflection values during a waveform sweep along the fiber strand, and (c) a computer controller, a method for extracting useful waveform-determining values from the noise in which they are embedded, comprising:

directing to the first input of a comparator an analog voltage signal representing the values returning from the fiber strand during a given waveform sweep;

directing to the second input of the comparator an analog voltage signal selected from a large number of signal values which are substantially evenly distributed in the range of possible values;

maintaining the same signal at the second input of the comparator during one complete waveform sweep;

outputting from the comparator at clocked intervals a digital signal indicating which of its input signals is instantaneously higher;

storing each digital signal from the comparator in a different storage register stage;

clocking the comparator output signals and the storage register stage inputs at a frequency which provides the desired waveform resolution;

transferring the signal stored in each register stage to a separate counter at the end of a waveform sweep;

changing the signal at the second input of the comparator;

repeating during the next waveform sweep the steps taken during the previous waveform sweep;

adding the values from successive sweeps into the respective counters; and

repeating the foregoing process the number of times needed to obtain a desired signal-to-noise ratio in the recorded waveform.