Patent Publication Number: US-6989892-B2

Title: Optical fiber propagation time measurement

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to measurement of optical fibers, and in particular to measurement of optical fiber propagation time measurement. 
   It is well known by those skilled in the art that the time required for light to propagate through the length of an optical fiber is dependent upon the refractive index of the fiber and the wavelength of the light being transmitted. For a commercial grade fused silica optical fiber found in a conventional fiberoptic network, the refractive index is roughly 1.49 and the propagation time is roughly 5.0 ns/m (nanoseconds per meter). There are many well known methods for determining the length of optical fibers and measuring the time of light propagation down a fiber. Instruments such as time-domain reflectometers, for example, are capable of measuring both length and propagation time and displaying a graph of the resultant measurement. Typically, conventional methods are expensive and rely on either high-speed counters or high-speed digital sampling to make the measurements, and therefore there is a limit to the measurement resolution, which in turn contributes to inaccuracies. For example, sampling with a 100-megahertz clock results in 10 nanoseconds between samples. If the propagation time is approximately five ns/m, the sampling uncertainty of ± one clock count means that accuracy of a length measurement is ± two meters. For measurement of long fibers, this may not be a problem. But for short fibers, an accuracy of ± two meters may be unacceptable. 
   It would be desirable to provide a measurement technique having high accuracy for short (&lt;500 meters) optical fibers, and without the expense, complexity and power consumption of known methods. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a simple, low-cost system for measuring optical fibers with a high degree of accuracy is provided. 
   A light pulse is iteratively transmitted into a near end of a fiber under test (FUT) at a predetermined frequency and detected at a far end of the FUT after a propagation time. A repetitive digital signal having a predetermined amplitude and a width corresponding the propagation time is developed by detection circuitry. The d.c. voltage average of the square wave voltage is determined and used to compute the propagation time since the ratio of the d.c. voltage average to the predetermined amplitude is equal to the ratio of the propagation time to the period of the light pulses. 
   The preferred embodiment utilizes the internal clock of a microprocessor to generate stimulus voltage pulses which are simultaneously applied to one input of a logic circuit and to an optical transmitter that is coupled to the near end of the FUT. A light detector at the far end of the FUT is also coupled to the logic circuit, and the logic circuit generates a repetitive (at the clock frequency) signal of predetermined amplitude whose width is equal to the propagation time of the FUT. This signal is applied to a filter to obtain the d.c. average voltage. The average voltage is measured by an analog-to-digital converter, which provides the measurement results to the microprocessor, which in turn computes the propagation time and fiber length. 
   The measurement circuit in accordance with the preferred embodiment may suitably be a portion of a fiber-optic measurement instrument, such as an optical power meter. 
   Other objects, features, and advantages of the present invention will become obvious to those having ordinary skill in the art upon a reading of the following description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a measurement circuit in accordance with the present invention; and 
       FIGS. 2A-2C  are waveforms to provide an understanding of the circuit of FIG.  1   
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1  of the drawings, there is shown an optical fiber propagation time measurement circuit in accordance with the present invention. This circuit may suitably be a portion of a measurement instrument, such as an optical power meter. A microprocessor  10  may control several instrument functions, and it is shown connected to a memory  12  and a display device  14 . A stimulus pulse output of microprocessor  10  is connected to both an optical transmitter  16 , which converts voltage pulses into light pulses, and one input of a logic gate  18 . The output of optical transmitter  16  is coupled via an optical coupler to the near end of a fiber under test (FUT)  20 . The far end of FUT  20  is coupled via an optical coupler to a detector  24 , which converts received light pulses to a voltage signal. The output of detector  24  is coupled through an inverter  26  to a second input of logic gate  18 . The output of logic gate  18  is connected to a filter  30 , which comprises a resistor  32  and integrating capacitor  34 . The output of filter  30  is connected to an input of analog-to-digital converter (ADC)  36 , the output of which is coupled to microprocessor  10 . 
   It should be pointed out that the components of the measurement circuit of  FIG. 1  are commercially available as off-the-shelf parts, and so parts can easily be selected without undue experimentation by one having ordinary skill in the art once the concept of the invention is understood. 
   Microprocessor  10  includes an internal clock which produces repetitive stimulus pulses V 1 , which are shown in FIG.  2 A. In the preferred embodiment, stimulus pulses V 1  are produced at a 200 kilohertz rate. From the well-known period (P)-frequency (F) relationship P=1/F, the time period (TP) of each pulse is five microseconds. The stimulus pulses are applied iteratively to optical transmitter  16 , which in turn iteratively generates light pulses that are transmitted into the near end of FUT  20 . 
   Referring for the moment to the waveforms of  FIGS. 2A and 2B , note that when a stimulus pulse is generated, that is, when V 1  goes high, both inputs of logic gate  18 , which is an AND gate in this embodiment, are both high, causing the output of logic gate  18  to go high in response to stimulus pulse V 1 . It should be pointed out that other logic circuits, such as a flip-flop or other bistable circuit, could easily be substituted for the AND gate to perform the same function, with perhaps added complexity and power consumption. Another alternative would be to implement the logic circuit in a field programmable gate array (FPGA). 
   After some time delay (TD), the light pulse has propagated the length of FUT  20  and arrives at detector  24 . The detected light pulse is amplified and applied via inverter  26  to logic gate  18 . This is seen as a negative-going voltage at the input of logic gate  18 , causing the output thereof to go negative. The resultant rectangular-wave pulse Vmeas having a width TD as shown in  FIG. 2C  is iteratively applied to filter  30 , which produces a d.c. average (V AVE ) of the Vmeas pulses. 
   A predetermined voltage is selected for the maximum output voltage V REF  of logic gate  18 . Because TD is a ratio of TP, that is, the time delay period is a ratio of the period of one cycle of stimulus pulse V 1 , then V AVE  is the same ratio to V REF  Mathematically, TD/TP=V AVE /V REF . This leads to TD=TP (V AVE /V REF ). If V REF  is selected to be same value as the reference voltage for ADC  36 , then time delay TD can be measured directly by ADC  36  as time delay TD=time period TP TD (N COUNTS /N MAX ), where N COUNTS  is the number of counts produced by ADC  36 , and N MAX  is the number of full-scale counts of ADC  36 . In the preferred embodiment, ADC  36  has a reference voltage of +3.3 volts, and so this is the value chosen for V REF  as the maximum output of logic gate  18 . As will be seen, this will allow microprocessor  10  use the ADC  36  results to calculate propagation time without using a scale factor. 
   Also, in this particular embodiment, ADC  36  is a 16-bit converter with a full-scale count of 65,536 counts. Theoretical resolution provided by ADC  36  is (1/65,536)×5 μS=76 picoseconds. As a practical matter, however, noise and filter ripple have to be taken into account. Standard measurement techniques can be used to reduce the effects of noise and ripple. These techniques include averaging multiple readings of the filtered voltage V AVE , and auto-zeroing and calibration to cancel circuit offset voltages. 
   Once the average voltage has been measured, the measurement result may be utilized by microprocessor  10  to compute the propagation time TD from the ratios described above. Also, the length of the fiber may easily be computed using the refractive index of the FUT. These computed values may be stored in memory  12 , and also may be displayed on display device  14 . 
   By increasing the frequency of the stimulus pulses V 1  with attendant appropriate selection of filter components, shorter fiber links may be measured with a high degree of accuracy. 
   While I have shown and described the preferred embodiment of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. It is therefore contemplated that the appended claims will cover all such changes and modifications as fall within the true scope of the invention.