WO2002086467A1

WO2002086467A1 - Automated system and method for determining the sensitivity of optical components

Info

Publication number: WO2002086467A1
Application number: PCT/US2002/012819
Authority: WO
Inventors: John Sargent French; William Joseph Thompson
Original assignee: Circadiant Systems Inc
Current assignee: Circadiant Systems Inc
Priority date: 2001-04-23
Filing date: 2002-04-23
Publication date: 2002-10-31
Anticipated expiration: 2003-10-23

Abstract

A system and method for testing the sensitivity of optical components comprises an optical testing unit including an optical transmitter (150), optical attenuator (152), optical power meter (154) and optical receiver (156) which are commonly controlled by a single control unit (158). The sensitivity testing is automated by the control unit (158). A common user interface permits user friendly control of the system and provide clear system readout.

Description

[0001 ] AUTOMATED SYSTEM AND METHOD FOR

DETERMINING THE SENSITIVITY OF OPTICAL COMPONENTS

[0002] BACKGROUND

[0003] This invention relates generally to optical communication systems.

More particularly, the invention pertains to a system for determining the sensitivity of optical communication systems and components.

[0004] Optical components, including fiber optic cables, connectors, transmitters, receivers, switches, routers and all other types of optical components have become the backbone of the modern telecommunication infrastructure. Due to their extremely low error rate and wide bandwidth, optical communication systems have supported an explosion in the growth of data communication systems, such as the Internet. With the Internet in its infancy, it is expected that the reliance on optical components and systems will only increase as the Internet becomes more closely intertwined with mainstream business and consumer applications.

[0005] Although the technology associated with optical communication systems and components has greatly advanced over the last decade and the use of such technology has accelerated, the technology associated with testing optical communication systems and components has greatly lagged.

[0006] Bit error rate (BER) measurements are a standard tool in verifying the performance of any digital optical communication system. Nevertheless, such tests remain an underutilized resource in understanding and diagnosing issues with such systems; particularly with respect to the receive-side optical front end. There are many contributing factors to this situation; chief among them are a lack of hardware and software resources, the time consuming nature of such measurements, and a lack of appreciation and understanding of the information content such measurements can provide.

[0007] The principle of BER measurements is simple: send digital data through a device and compare the digital result to the input data. The BER is given by the ratio of incorrectly identified bits to the total number of bits processed. In optical systems, BER tests are most commonly associated with determining the sensitivity of the optical receiver. Clearly, if the input optical power decreases enough, the receiver will begin generating errors. Receiver sensitivity is the input optical power required for a particular BER. Sensitivity is typically measured in dBm where:

P_άBm ≡ 10 Equation (1)

such that 0 dBm corresponds to 1 mW. The result depends strongly on the measurement conditions including the quality of the transmitter, the amount of input optical noise, the BER required, the data rate and the data being transmitted. A typical measurement might involve a high quality transmitter, no added input noise, a well-defined pseudorandom bit sequence and a required BER of IE- 10. [0008] For a network designer, sensitivity is often regarded as the most important figure of merit for a receiver since it suggests a minimum input operating power for the device. A designer would ordinarily plan to operate the receiver with an input power high enough above the quoted sensitivity such that the expected error rate will not impact the reliability of the link. But how high above the sensitivity power level the receiver should be operated at is one of the fundamental questions that careful BER measurements can answer.

[0009] It is clear that for a receiver operating with an input power near the measured sensitivity, the error rate increases as input optical power decreases; higher power reduces the error rate. What is less obvious is that for a well-behaved receiver, there is a theoretical relationship between input power and error rate. [0010] A typical prior art testing scheme 10 is shown in Figure 1. The scheme

10 typically includes an optical transmitter 12, an optical attenuator 14, an (optional) optical power monitor 16 with an optical splitter and a optical receiver 18. The device under test 25 (DUT) is placed between the transmitting side 20, (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, 12, 14, 16, 18 are interconnected with fiber optic cables 24 and connectors 26. [0011] In order to test the sensitivity of the DUT 25, the technician energizes the optical transmitter 12 and starts transmitting at a level of optical power that is sufficient for the DUT 25 to process without error. The technician verifies that error free transmission is observed at the receiver 18. The technician also temporarily detaches the input to the DUT 25 and measures the power exiting the fiber normally attached to the input side of the DUT 25 and, using a calibrated power monitor, verifies that this power agrees with that measured by the power monitor 16. The technician then reattaches the fiber to the input of the DUT 25. (These first steps establish the basic functionality of the DUT 25 and provide some confidence in the associated power measurement.) In order to arrive at the correct sensitivity power (the power at which the specified bit error rate is observed), the technician uses the optical attenuator 14 to adjust the input optical power to the DUT 25 to a power near the lower limit of the operating range of the DUT 25 where errors would expect to be observed. If there is power overshoot or undershoot, the technician adjusts the optical attenuator accordingly and then remeasures. Once the desired optical power is achieved, the technician begins measuring the error rate observed in the optical receiver 18 due to data transmission errors generated in the DUT 25 due to a low input power level. This could take from a few seconds to many minutes depending on the actual error rate. If the measured error rate is greater or less than that defined by the sensitivity error rate, the optical power is increased or decreased (respectively), the optical power is noted, and the error rate is remeasured. The technician continues the process interactively until the desired error rate is achieved. The resulting power level is the sensitivity for the DUT 25. [0012] As described above, it should be recognized that not only are the testing schemes cumbersome, they are extremely time-consuming and tedious due to the exacting nature of the work that is required. For example, since the sensitivity of an optical component is unknown prior to testing, it is generally required to test an optical device over a broad range of optical powers.

[0013] What is needed is a simple and effective system and method for efficiently testing the sensitivity of optical components.

[0014] SUMMARY

[0015] The present invention is a system and method for testing the sensitivity of optical components. The invention is an unitary optical testing unit including an optical transmitter, optical attenuator, optical power meter and optical receiver which are commonly controlled by a single control unit. The sensitivity testing routine is automated by the control unit. A common user interface permits user-friendly control of the system and provides clear system readouts.

[0016] Objects and advantages of the system and the method will become apparent to those skilled in the art after reading a detailed description of the presently preferred embodiment.

[0017] BRIEF DESCRIPTION OF THE DRAWLNG(S)

[0018] Figure 1 is a prior art testing scheme.

[0019] Figure 2 is a block diagram of the preferred embodiment of the present invention.

[0020] Figure 3 is a block diagram of a control unit.

[0021] Figure 4 is a flow chart of the sensitivity test procedure.

[0022] Figure 5 is a flow chart of the standard comparison procedure.

[0023] Figure 6 shows the front view of a graphical user interface. [0024]DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) [0025] The preferred embodiment of the present invention will be described with reference to the drawing figures where like numerals represent like elements throughout.

[0026] Referring to the block diagram of Figure 2, the preferred embodiment of the system 200 of the present invention includes an optical transmitter 150, an optical attenuator 152, an optical power monitor 154, an optical receiver 156, a control unit 158, an optical splitter 192 and a graphical user interface 160. Of course, it should be recognized that the optical components are not drawn to scale. All of the optical components are housed in a housing 204. The system 200 also includes the fiber optic cables 186, 188, 190, 194, 196 and optical output and inputs 198, 199 between the system 200 and the DUT 205.

[0027] The system of the present invention is optimized by including a common control bus 184 and a common power bus 182, which is coupled to a central power supply 180. Each active component within the system 150-160 is coupled to both the control bus 184 via a control (C) interconnection and to the power bus 182 via a power (P) interconnection. Although the common control bus 184 and common power bus 182 are not required, they permit the elimination of redundant power supplies and power feeds to each separate component; permit a single control bus to control all of the components 150-160; and eliminate all redundant user interfaces with each optical component. Having a single control unit 158 to provide selective control of each optical component 150-156 greatly simplifies the sensitivity testing procedure as will be explained in further detail hereinafter.

[0028] Each of the optical components 150-156 also includes one or more optical interfaces. For example, the optical transmitter 150 includes an optical output 70. The optical attenuator 152 includes an optical input 72 and also an optical output 74. The optical power monitor 154 includes an optical input 76, and the optical receiver 156 includes an optical input 78. The optical splitter 192 includes an optical input 93 and two optical outputs 95, 97. The housing 204 includes an optical output port 198 and an optical input port 199. The optical output port 198 is coupled with the input of the DUT 205 and the optical input port 199 is coupled with the output of the DUT 205.

[0029] Referring to Figure 3, the control unit 158 is shown in greater detail.

The control unit 158 includes a microprocessor 210, an input/output (I/O) buffer 212, and an associated memory 214. The memory 214 permits storage of all of the individual software modules, predetermined standard success criteria (optional) and any other information which is required to be stored by the control unit 158. For example, the memory 214 includes a sensitivity test module 216 and standard success criteria 218. The memory 214 may also include enough room for future modules 220. Although these modules 216-220 have been graphically illustrated as separate components for ease of explanation in the present application, it should be recognized by those of skill in the art that these modules are resident in software and the software may be stored, and the memory 214 partitioned, as desired by the technician.

[0030] Several data buses 222, 224, 226 facilitate the flow of databetween 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 the control bus 184. Although the microprocessor 158 is illustrated herein as including an I/O buffer 212, in an alternative embodiment of the present invention, the control unit 158 provides for direct access to the memory 214 such that the I/O buffer 212 is not required. Of course, any accessing of the memory 214 in that embodiment will be monitored and/or controlled by the microprocessor 210.

[0031 ] The process implemented by the sensitivity test module 216 will now be described in greater detail with reference to Figure 4. The foregoing discussion assumes that all of the optical components 150-158 have been energized for a predetermined duration, which permits the electronic components therein to reach steady state. It should be noted that the sensitivity test procedure 300 may be fully automated, whereby the technician initiates the process by pushing a button which permits the control unit to fully carry out the sensitivity test procedure 300. In this embodiment, the control unit 158 will select a default error rate and a default "error range" within which the sensitivity must be measured. Alternatively, the sensitivity test procedure 300 may be selectively automated, whereby the technician may set certain parameters for the system to implement the test. For example, the technician may set specific power levels for the system to test and may also set a specific error range within which the sensitivity must be measured.

[0032] In any event, the sensitivity test procedure 300 begins by the technician initiating the procedure (step 302). This can be as simple the technician pressing a "start button" on the graphical user interface 160, or by the technician setting forth all of the individual testing parameters and pushing a "start test" button. The microprocessor 210 retrieves the desired bit error rate from memory (step 304), whether the desired BER is a default BER or a BER that has been previously input by the technician. The control unit 158 controls the optical transmitter 150 to transmit at an initial power level ( step 306). This may be a parameter set by the technician, or a predefined default parameter. Alternatively, the control unit 158 may selectively control the optical attenuator 152 to increase or decrease the amount of optical attenuation in order to output a desired optical power level. The optical power monitor 154 measures the output of the optical transmitter 150 (step 308) and the optical receiver measures the number of bit errors at that power lever (step 310). [0033] It should be noted that the actual measured power level need not to be exactly the same as the desired power level. In this regard, it is critical to ensure that the system accurately measure the number of errors at the current power level. It is the relationship between the actual power level and the number of errors measured at that actual power level that is important. It should be kept in mind that since the sensitivity test procedure 300 is an iterative process that seeks to determine an optical power level at which a certain bit error rate has been achieved, the individual power level measurements which are made in order to ultimately achieve the sensitivity are merely indicators of whether the optical power level should be increased or decreased.

[0034] Accordingly, the optical receiver 156 measures the number of bit errors at that power level (step 310). The control unit 158 then compares the desired BER to measured BER (step 312). If it has been determined that the measured bit error rate is equal to the desired bit error rate (step 314), the sensitivity of the DUT has been found and the control unit 158 stores the current power level in memory and/or outputs the current power level to the GUI as the sensitivity value (step 322). The sensitivity test procedure 300 is then terminated.

[0035] If the measured BER is not equal to the desired BER, the control unit

158 then determines whether the measured BER is greater than the desired BER (step 316). If so, the optical power level is increased (step 320) and steps 308-314 are repeated. If the measured BER is not greater than the desired BER, the control unit 158 decreases the optical power level (step 318) and then steps 308-314 are repeated.

[0036] The sensitivity test procedure 300 provides a simple and effective method for automatically measuring the sensitivity of an optical component. [0037] It should be noted that in addition to determining the sensitivity of an optical component, the system of the present invention may also determine whether that measured sensitivity meets or exceeds nationally or internationally recognized optical standards. This provides the technician with an additional tool for comparing the particular component to objective norms. Preferably, the "success criteria" such as the sensitivity and the desired BER are previously stored in memory 214. The criteria may be downloaded into the memory 214 via a plurality of different methods which will not be described in detail hereinafter. Such methods may include providing external control port 301, (as shown in Figure 2), which permits LAN interconnectivity and or connectivity to the World Wide Web. Other interfaces such as a CD drive, floppy disk drive or other information storage and or input/output devices may be used to provide a set of success criteria for each standard. [0038] Once the success criteria are stored in memory, the procedure for comparing the sensitivity of the optical component to standard success criteria may be implemented as shown in Figure 5. The standard criteria procedure 400 as it will be hereinafter referred to is shown in the flow diagram on Figure 5. It should be noted that as with the sensitivity test procedure 300, the standard comparison procedure 400 may be fully automated, whereby the technician initiates the procedure by pushing a button, and the control unit 158 fully carries out the standard comparison procedure 400 against all known standards. Alternatively, the standard comparison procedure 400 may be selectively automated, whereby the technician may input the identification for one or more standards against which the sensitivity should be compared to and only those selected standards are compared to the measured sensitivity.

[0039] The standard comparison procedure 400 begins by the technician initiating the procedure (step 402). This can be as simple as the technician pushing a "start button" on the graphical user interface 160, or by the technician setting forth all of the standards against which the sensitivity will be tested and pushing a "start test" button. The control unit 158 retrieves the standard success criteria from memory 214 (step 404). The control unit 158 then compares the success criteria for a standard to the measured sensitivity and desired BER (step 406). The system then determines whether the sensitivity and desired BER meet or exceed the success criteria (step 408) for that standard. If so, the control unit 158 stores the positive test result in memory 214 (step 410) and/or optionally output the result to the GUI (step 412). [0040] Alternatively, if the sensitivity and the desired BER do not meet or exceed the success criteria for that standard, the control unit 158 stores test failure result in memory 214 (step 414) and optionally outputs the test failure result to the GUI (step 416). The control unit 158 then determines whether there are more standard success criteria against which the DUT's sensitivity should be compared (step 418). If so, the control unit 158 advances the comparison to the next set of success criteria (step 420) and repeats steps 408-418. If all of the comparisons are complete, the standard test procedure 400 is then terminated. [0041 ] The standard test procedure 400 provides a tremendously powerful tool for the technician by specifically outputting which standards the optical DUT has passed and which it has failed. Via the output control interface (301) the system can also provide an output such that a "sticker" or success report may be printed out and kept with each optical component under tests. Ths further enhances the ease of use of the system.

[0042] It should be noted that the steps set forth in the sensitivity test procedure 300 and the standard comparison procedure 400 need not necessarily occur in the exact order set forth in Figures 4 and 5 respectively. For example, step 304 may occur after step 306. Those of skill in the art would clearly recognize that there is flexibility in the ordering of some of these steps. Additionally, some steps, (such as steps 414 and 410) may be eliminated, whereby the results of the standard test procedure 400 are not stored.

[0043] Referring to Figure 6, the graphical user interface 160 is shown in greater detail. Preferably, the graphical user interface 160 comprises a touch- sensitive screen 130 which will change depending upon the graphical buttons 132- 140 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. [0044] In order to make the system as user friendly as possible, the bottom portion of the screen 130 preferably includes a discrete selection option (i.e., hereinafter "button") for each of the transmitter 132, the receiver 134, the attenuator 136, the power monitor 138, and a separate button for the calibration procedure 140 and the test procedure 142. Of course, those of skill in the art should realize that more or fewer buttons 132-142, or hardwired buttons, may be provided as desired by the user in order to implement or control certain functions that are commonly used.

[0045] In operation, one of the buttons, 132-142 is selected, for example, the test button as shown Figure 6, to initiate the desired function. This will implement the test procedure 400 as hereinbefore described. The test results as output in steps 412, 416 may also be displayed on the screen 130.

[0046] While the present invention has been described in terms of the preferred embodiment, other variations which are within the scope of the invention as outlined in the claims below will be apparent to those skilled in the art.

Claims

What is claimed is:

1. A system for measuring the sensitivity of an optical device under test (DUT) comprising: an optical transmitter, for transmitting a test signal; an attenuator, for selectively attenuating said test signal to provide an output signal to the DUT at an initial output power level; a power monitor, for monitoring said attenuated signal; an optical receiver, for receiving an input signal from said DUT and determining whether there are errors in said input signal; a graphical user interface, for providing an interface with a user; and a controller, coupled to said transmitter, attenuator, power monitor, receiver and graphical user interface, to provide central control of the transmitter, attenuator, power monitor, receiver and graphical user interface to determine the bit error rate (BER) at said initial output power level; whereby the controller selectively adjusts the output power level in response to said determined BER, until said determined BER equals a predetermined BER.

2. The system of claim 1 , whereby said controller controls said transmitter, attenuator and power monitor to transmit said test signal at an output power level, and controls said receiver to detect the number of bit errors in said input signal.

3. The system of claim 2, whereby said controller controls the output power level of said transmitter, controls the attenuation level of said attenuator and monitors the output of said power monitor in a recursive loop to provide said test signal at an output power level.

4. The system of claim 2 whereby the controller counts the number of errors in the input signal to determine the BER and adjusts the power level of said output signal in response thereto.

5. The system of claim 3 whereby the controller counts the number of errors in the input signal to determine the BER and adjusts the power level of said output signal in response thereto.

6. The system of claim 1 , whereby said transmitter, attenuator and power monitor comprise a single optical component.

7. A method for measuring the sensitivity of an optical device under test (DUT) comprising: a) generating and transmitting a test signal at an initial power level as an output signal; b) receiving an input signal; c) determining if there are bit errors in said input signal; d) determining the bit error rate .(BER) at said initial power level; e) comparing the determined BER with a predetermined BER; f) adjusting the power level of said output signal in response to said comparison; g) repeating steps b-f until said determined BER is equal to said predetermined BER within a predetermined tolerance, and then outputting the current power level; whereby all steps a-g are centrally controlled.

8. The method of claim 8, whereby said generating step generates a test signal, monitors the power level of the test signal and adjusts the power level of the test signal in a recursive loop to provide said output signal.

9. The method of claim 8, whereby said determining step further comprises counting the number of bit errors in said input signal.

10. A system for measuring the bit error rate of an optical device under test (DUT) comprising: a controller for providing central control of the system; an optical generator, coupled to said controller, for generating and transmitting an output signal to said DUT at an initial power level; an optical receiver, coupled to said controller, for receiving an input signal from said DUT and determining whether said input signal includes errors; and a graphical user interface, coupled to said controller, for providing an interface with a user; whereby said controller determines the bit error rate (BER) at said initial power level and selectively adjusts the power level until said determined BER equals a predetermined BER.

11. A system for measuring the sensitivity of an optical device under test (DUT) comprising: a controller for providing central control of the system; a optical transmitter, coupled to said controller, for transmitting an output signal at an initial power level to said DUT; an optical receiver, coupled to said controller, for receiving an input signal from said DUT and determining if there are errors in said input signal; a graphical user interface, coupled to said controller, for providing an interface with a user; whereby said controller determines the bit error rate (BER) at said initial power level and selectively adjusts the power level in response to said determined BER until said determined BER equals a predetermined BER.