WO2002079759A1

WO2002079759A1 - Error function analysis of optical components

Info

Publication number: WO2002079759A1
Application number: PCT/US2002/009365
Authority: WO
Inventors: Jorge E. Franke; John S. French; Sheldon L. Sun; William J. Thompson
Original assignee: Circadiant Systems, Inc.
Priority date: 2001-03-29
Filing date: 2002-03-27
Publication date: 2002-10-10
Also published as: US7187435B2; US20030112425A1

Abstract

A device performs error analysis of an optical component (25). The device comprises an optical transmitter (50), an optical attenuator (52), a port (80), a receiver (56), a processor and a graphical display. The combination of the optical transmitter (50) and optical attenuator (52) produces a test signal at a plurality of selected optical power levels. The port is configured to output the test signal to the optical component and to receive a version of the test signal from the optical component (25). A receiver (56) determines errors in the received version of the test signal. A processor determines data points of a function associated with an error rate at each of the selected power levels and a line corresponding to the data points. The graphical display produces a visual plot of the data points and the corresponding line.

Description

[0001 ] ERROR FUNCTION ANALYSIS OF OPTICAL COMPONENTS

[0002] This application claims priority to U.S. Provisional Patent Application No.

60/279,550, filed March 29, 2001.

[0003] BACKGROUND

[0004] This invention relates generally to optical communication systems. In particular, the invention relates to error analysis of optical components in such systems.

[0005] Optical components, including fiber optics cables, connectors, transmitters, receivers, switches and routers, have become the backbone of the modern telecommunication infrastructure. Due to their extremely low error rates and high bandwidth, optical communication systems have supported an explosion in the growths of data communication systems. As the need for components in such systems increases, the need for accurate tests on these systems also increases.

[0006] Each component within the system must be tested to ensure that it meets technical standards that have been set in the industry. Additionally, the components must be tested to assess their performance in various real world conditions. This testing can be labor intensive, tedious and time consuming.

[0007] A typical testing scheme 10 is shown in Figure 1. The scheme 10 typically includes an optical transmitter 12, an optical attenuator 14, an optical monitor 16 and a receiver 18, such as an optical or electrical receiver. The device under test 25 (DUT) is placed between the transmitting side, (which comprises the transmitter 12, the attenuator

14 and the optical monitor 16), and the receiving side 22, (which comprises the receiver

18). All of these components are then interconnected with fiber optic cables and connectors.

[0008] In order to test the DUT 25, the technician energizes the optical transmitter

12 which transmits a test signal. The optical test signal is transmitted from the optical transmitter 12, through the optical attenuator 14, through the DUT 25 and is received by the receiver 18. The technician adjusts the gain on the optical attenuator 14 until the optical monitor 16 indicates that the output optical power is at a predetermined level for testing the DUT 25. The DUT 25 is tested at this predetermined optical power and the number of errors in the received signal is measured at the receiver 18. A bit error rate (BER) of the DUT 25 at the predetermined optical power is determined, such as by Equation 1.

BER = . total number of bits received

Equation 1

This value is compared to a specified BER at that power level to determine whether the DUT 25 meets the standard.

[0009] There are drawbacks to this approach. Although the test results at the specified power level may be acceptable, the DUT 25 may perform unexpectedly poor at other power levels, in particular higher power levels. To illustrate, a DUT 25 may be expected to have a BER of 10^"9 at the specified power level. However, at a much greater power level, a well behaved DUT 25 may be expected to have a BER of 10^"16. Although the DUT 25 may test at the specified power level with a BER of 10^"9, it may have a BER of 10^"10 at the higher power level. As a result, the DUT 25 in real world conditions would have an unacceptable performance.

[0010] To evaluate the DUT 25 for such conditions, the DUT 25 may be tested at other optical power levels. Using the BERs at these optical power levels, the BER measurements of the DUT 25 are plotted on log paper. The optical power in decibel milliwatts (dBm), the horizontal axis, is plotted against the logarithm to the base 10 (log₁₀) of the BER, the vertical axis. An example of such a plot is shown in Figure 2. [0011] Constructing these plots is extremely time consuming and tedious.

Additionally, these logarithmic plots, typically, require an engineer to evaluate the plotted relationships. As shown in Figure 2, all of plotted data does not fall on a straight line 28. As a result, an engineer analyzes the raw data to determine whether the error rate versus power relationship is an indicator of poor performance of the DUT 25 or merely an acceptable statistical deviation from the norm. This testing procedure is labor intensive and is susceptible to human error. Accordingly, it is desirable to have alternate approaches for error analysis in optical components.

[0012] SUMMARY

[0013] A device performs error analysis of an optical component. The device comprises an optical transmitter, an optical attenuator, a port, a receiver, a processor and a graphical display. The optical transmitter and optical attenuator transmit a test signal at a plurality of selected optical power levels. The port is configured to output the test signal to the optical component and to receive a version of the test signal from the optical component. A receiver determines errors in the received version of the test signal. A processor determines data points of a function associated with an error rate at each of the selected power levels and a line corresponding to the data points. The graphical display produces a visual plot of the data points and the corresponding line.

[0014] BRIEF DESCRIPTION OF THE DRAWING(S)

[0015] Figure 1 is an illustration of a testing scheme.

[0016] Figure 2 is an illustration of a plot of a logarithm of the bit error rate versus optical power in decibel milliwatts (dBm).

[0017] Figure 3 is an illustration of an error analysis system.

[0018] Figure 4 is an illustration of a control unit.

[0019] Figure 5 is an illustration of a graphical user interface.

[0020] Figure 6 is a flow chart of error analysis.

[0021] Figure 7 is an illustration of a plot of a function associated with the BER versus optical power in dBm.

[0022] Figure 8 is an illustration of a flattening curve. [0023]DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0024] One system for error analysis is shown in Figure 3. The system includes an optical transmitter 50, an optical attenuator 52, an optical power monitor 54, an optical receiver 56, a control unit 58, an optical splitter 92 and a graphical user interface 60. For convenience, these components are preferably located in a unitary housing 62. [0025] Each of the optical components 50-56 has a control input/output (I/O) that couples each optical component 50-56 with the control unit 58. These I/O control connections permit the control unit 58 to control all of the optical components 50-56 at a common point and also permit the output from each of the optical components 50-56 to be monitored by the control unit 58. Having a single control unit 58 also permits calibration of all of the optical components 50-56 from a common point of control, which allows for software instead of manual calibration.

[0026] The control unit 58 also includes an I/O control interconnection (I/O) with the graphical user interface 60 to permit the control unit 58 to communicate with the graphical user interface 60 and also to accept user input via the graphical user interface 60.

[0027] Figure 4 shows a control unit 58 in greater detail. The control unit 58 includes a microprocessor 210, an input/output (I/O) buffer 212, and an associated memory 214. The memory 214 stores error analysis 216 as well as other software and any other information which is required to be stored by the control unit 58. Several data buses 222, 224, 226 facilitate the flow of data between the microprocessor 210, the memory 214 and the I/O buffer 212. Another data bus 228 facilitates the flow of data between the I/O buffer 212 and a control bus 184, which is used to communicate with the graphical user interface 60. Although the microprocessor 210 is illustrated herein as including an I/O buffer 212, the microprocessor 210 could have direct access to the memory 214, to eliminate the need for the I/O buffer 212.

[0028] Figure 5 shows the graphical user interface 160 in greater detail.

Preferably, the graphical user interface 160 comprises a touch-sensitive screen 130, which will change depending upon the graphical buttons 132-142 which are selected. Alternatively, the graphical user interface 160 may comprise a CRT screen and associated mouse (not shown) for selecting the different options on the screen. The graphical user interface 160 may also be a printer or a device for sending an email of the results for display by a user.

[0029] Testing of the DUT 25 is explained in conjunction with the flow chart of

Figure 6. To test a DUT 25, the DUT 25 is connected to the ports 80, 82 by an operator. The operator selects a test button displayed on the screen 130. The control unit 58 initiates a test of the DUT 25 at various optical powers by controlling the optical attenuator 52. The signal returned by the DUT 25 may be optical, electrical or even acoustical. The test range depends on the type of DUT 25. A range of power levels for testing is set either automatically or by a user input. One possible user input range may be 10^"4 or 10^"5 BER to 10^"10 BER. If set automatically, the upper most tested power level is determined by adjusting the power level, until a point is found where some errors are made in a reasonable time period. A lower most tested power level is determined by adjusting the power level just prior to a point where an unreasonably high number of errors is made, such as in the range between 10^"5 or 10^"4 BER.

[0030] The DUT 25 is tested with a test signal at selected optical powers within the range, 30. Although any number of test points can be selected, a typical range is 5-20 test points. The errors produced by the DUT 25 are determined at the receiver 56, 32. The DUT 25 is tested at each of the selected power levels, until a specified number of errors is detected. A typical value for the number of errors is 10 errors. To prevent an extremely long test period at low error rates, a time limit may be set. The test is ended when either the specified number of errors is received or the time limit expires. However, the time limit may be overridden by the user. Alternately, the DUT 25 is tested at each power level for a specified time period, regardless of the measured number of errors. [0031 ] The number of detected errors at each power level and the total number of bits received is stored in the memory 214, 34. The test parameters, such as testing power levels and number of errors detected at each power level, may be selected by a user input, although a default setting for these parameters may be used.

[0032] When the requisite number of errors at each power level is accumulated, the

BER is determined by the microprocessor 210, 36. The microprocessor 210 produces a plot of the information to be displayed on the graphical user interface as shown in Figure 7. The horizontal axis is in units representing the optical power level, such as milliwatts or, preferably, in dBm. Along the vertical axis is a function associated with the BER, which is linear in a "well behaved" DUT 25. Errors in a "well behaved" DUT 25 should be dominated by noise, which exhibits a gaussian distribution. Accordingly, one approach to produce a linear model is a version of a complementary error function associated with the BER. The accumulated data is converted into data points for plotting. The selected power levels and the associated BER function are determined. The resulting data points (associated BER function versus power) are plotted, 38. A line is drawn using a best fit approach, such as a least squares fit, 40.

[0033] Additionally, a linearity test may be performed on the tested results. The result of the linearity test may also be displayed on the graphical user interface 60. [0034] By viewing the plotted data and the line, the technician can verify whether the device is functioning properly. If the data points are distant from the best fit line, this indicates that the device is not well behaved. If the data points are close to the line, this indicates that the device is well behaved. The flattening of the curve as shown in Figure 8 is highly undesirable for a DUT 25. Such a curve suggests the existence of an "error floor." An "error floor" is a lower limit to the number of errors produced by an optical component independent of the optical power . This type of linearity analysis is much more important to a network designer than a sensitivity measurement. A DUT 25 can have an acceptable sensitivity but have an unacceptable "error floor. " Additionally, if the DUT 25 yields a straight line plot, the network designer can have some confidence in its behavior. Adherence to a straight line suggests that the DUT 25 behaves well even at error rates far below those actually tested. [0035] To explain the linear relationship between a preferred version of a complementary error function associated with the BER and the optical power, the following is provided. The effect of noise on a transmitted signal can be modeled statistically. An optical signal has symbols of one of two values, represented by a 0 and 1. When sending a one, the transmitter typically transmits light at a selected power level. When sending a zero, typically minimal or zero light is transmitted. At the receiver 56, the value of each received soft symbol is compared to a threshold value and a hard decision is made whether the received soft symbol is a one or a zero. When noise decreases a symbol representing a one to a level below the hard decision threshold, an error is made at the receiver. Similarly, when noise increases a symbol representing a zero to a level above the threshold, an error is also produced.

[0036] Received soft symbols produce two gaussian distributions. The two means, μ₀ and μ_l5 represent the means of the power levels of the zero and one soft symbols, respectively. The variances, σ₀ ² and σf², represent the quantity of noise present at each level, respectively. The rate at which errors occur is related to the "closeness" of the decision threshold to the noisy zero or one level. This "closeness" is measured by the Q- f actor for each level i, i = 0 or 1, as in Equation 2.

Equation 2

D represents the decision level.

[0037] To determine the proportion of zero soft symbols erroneously identified as a one, P₀₁, the proportion of zero soft symbols above the hard decision value is determined. One approach to predict this proportion for a "well behaved" receiver is to use a gaussian distribution. For all zero symbols coming into the device, the fraction erroneously identified as ones, P₀₁ , is given by the fraction of the gaussian distribution (representing noise on the zeros) above the decision threshold, D. This proportion, P₀₁, is the area under the normalized gaussian between the decision threshold, D, and infinity, ∞. This area can be determined using the complementary error function (erfc). Using the complementary error function, the proportion of erroneously identified ones, P₀₁, is determined such as by Equation 3.

Equation 3

Similarly, the proportion of ones erroneously identified as zeros, P₁₀, is determined such as by Equation 4.

Equation 4

By adding P₀₁ to P₁₀, the proportion of incorrectly identified symbols is determined. When the decision threshold, D, is halfway between the zero and one mean levels, the two Q-factors are equal, Q₀ = Q,. Using Q, defined to equal Q₀ = Q,, the combined probability of an incorrectly identified symbol can be determined such as by Equation 5.

Equation 5

Accordingly, if the true BER performance obeys this theoretical result over a wide range of Q values, it suggests that the DUT 25 is "well behaved."

[0038] When the optical power level is varied during a test of the DUT 25, the mean value of the received one soft symbols, μ,, will vary. The value of μ_λ is proportional to the optical power level. Since the decision threshold, D, and noise variances, σ^ and σ , are relatively fixed, the Q-f actor is directly proportional to optical

power. As a result, a function error probability, g(ErrProb), can be found such that g(ErrProb) versus Q is a straight line. Since the error probability is equivalent to the BER, Equation 6 or an analogous equation can be used.

Equation 6

As a result, the plot of /(BER) versus the optical power in dBm should be linear for a "well behaved" DUT 25. Such a plot is shown in Figure 7. The line in Figure 7 is shown for illustrative purposes and may not actually be displayed.

[0039] The relationship of the logarithm of the BER to optical power in dBm is not a true linear relationship in a "well behaved" DUT 25. Such an approach is a crude approximation of a linear relationship. Accordingly, a function related to a BER function, such as per Equation 6, is a better indicator of a well behaved DUT 25. Equation 6 is one illustrative example for deriving a BER function. Under varying conditions, the theoretical straightness of the plot is robust. Accordingly, this approach to analyzing optical components can be used in a variety of applications, such as electrical and acoustical.

Claims

CLAIMSWhat is claimed is:

1. A device for performing error analysis of an optical component, the device comprising: an optical transmitter and an optical attenuator for transmitting a test signal at a plurality of selected optical power levels; a port configured to output the test signal to the optical component and to receive a version of the test signal from the optical component; a receiver for determining errors in the received version of the test signal; a processor for determining data points of a function associated with an error rate at each of the selected optical power levels based on in part the determined errors and a line corresponding to the data points; and a graphical display for producing a visual of the data points and the corresponding line.

2. The device of claim 1 wherein the function is based on in part a gaussian distribution.

3. The device of claim 1 wherein the function results in a linear relationship for a well behaved optical component.

4. The device of claim 1 wherein the function includes an inverse complementary error function associated with a bit error rate or an approximation thereof.

5. The device of claim 1 wherein the function wherein the function includes a functional or numerical equivalent of an inverse complementary error function associated with a bit error rate.

6. The device of claim 1 wherein the function is modeled using a statistical distribution.

7. The device of claim 1 wherein the processor for performing a linearity test on the determined data points and the graphical display displaying a result of the linearity test.

8. A device for automatically performing error analysis of an optical component, the device comprising: an optical transmitter and an optical attenuator for transmitting a test signal at selectable optical power levels; a port configured to output the test signal to the optical component and to receive a version of the test signal from the optical component; a receiver for determining errors in the received version of the test signal; and a processor for adjusting the selectable optical power levels and determining a testing range of interest based on in part determined errors at various optical power levels.

9. The device of claim 8 wherein an upper range limit of the testing range is determined to be a point where a specified number of errors occur at a predetermined time period.

10. The device of claim 8 wherein a lower range limit of the testing range is determined to be a point where a specified number of errors occur at a predetermined time period.

11. The device of claim 8 wherein the processor for determining test points within the determined testing range.

12. The device of claim 11 wherein the processor for directing testing at the test point until a specified number of errors occur.

13. The device of claim 12 wherein the processor for directing testing at the test points until a specified number of errors occur or a time limit is exceeded.

14. A device for performing error analysis of an optical component, the device comprising: means for transmitting a test signal at a plurality of selected optical power levels to an optical component; means for receiving a version of the test signal from the optical component; means for determining errors in the received version of the test signal; and means for determining data points of a function associated with an error rate at each of the selected optical power levels based on in part the determined errors.

15. The device of claim 14 further comprising means for producing a visual of the data points and the corresponding line.

16. The device of claim 14 wherein the function is based on in part a gaussian distribution.

17. The device of claim 14 wherein the function results in a linear relationship for a well behaved optical component.

18. The device of claim 14 wherein the function includes an inverse complementary error function associated with a bit error rate or an approximation thereof.

19. The device of claim 14 wherein the function wherein the function includes a functional or numerical equivalent of an inverse complementary error function associated with a bit error rate.

20. The device of claim 14 wherein the function is modeled using a statistical distribution.