WO2002086466A1

WO2002086466A1 - Automated system and method for performing bit error rate measurements on optical components

Info

Publication number: WO2002086466A1
Application number: PCT/US2002/012816
Authority: WO
Inventors: John Sargent French; William Joseph Thompson
Original assignee: Circadiant Systems, Inc.
Priority date: 2001-04-23
Filing date: 2002-04-23
Publication date: 2002-10-31

Abstract

A system and method for testing optical components (205) comprises an optical testing unit (200) 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). All routine calibration and testing is automated by the control unit (158). A common user interface permits user friendly control of the system and provides clear system readouts.

Description

AUTOMATED SYSTEM AND METHOD

FOR PERFORMING BIT ERROR RATE

MEASUREMENTS ON OPTICAL COMPONENTS

[0002] BACKGROUND

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

More particularly, the invention pertains to a system for testing optical communication systems and components to determine the sensitivity of the optical communication system or component.

[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. At a minimum, such tests are useful in determining the reliability of any communication channel. However, depending on the nature of the system, measuring the BER is often useful in understanding potential problems prior to deploying a system, or resolving them once in the field. 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:

( PmΨλ ^P _άβm ≡ 10 log — — Equation (1)

V lrnWV

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 much higher than the sensitivity power level the device can be operated at specifically 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. And even more surprising, there exists a best case upper limit, represented by a "perfect" receiver, on how quickly the error rate can improve as a function of input power. Verifying that a real receiver exhibits the appropriate functional form demonstrates good front end design and reduces the likelihood of surprises for the network designer. The true figure of merit is how closely the real world receiver approximates the perfect receiver.

[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 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, optical coupler 30 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 performance of the DUT 25, the technician energizes the optical transmitter 12 and starts transmitting at a level of optical power which is sufficient for the DUT 25 to process without errors. In order to arrive at the correct optical power for measurement, the technician must turn the optical transmitter 12 to a desired output optical power, measure the output power to determine if the measured power is the same as the desired optical power, and then adjust the output optical power to compensate for any power overshoot or undershoot. If there is power overshoot or undershoot, the technician adjusts the output optical power accordingly and then remeasures. Once the desired optical power is achieved, the technician begins measuring the number of errors at the optical receiver 18.

[0012] The optical signal is transmitted from the optical transmitter 12, through the optical attenuator 14, through the DUT 25 and is received by the optical receiver 18. At the optical receiver 18, the technician measures the number of errors received during the transmission between the optical transmitter 12 and the optical receiver 18. This could take from several minutes to many hours or more. During a typical testing regiment, once the technician measures the number of errors at a certain optical power, the attenuator 14 is subsequently used to attenuate the test signal to a lower optical power as seen by the DUT 25. The technician must then again implement the procedure for determining the correct optical output power, and measure the number of errors at this lower optical power. This process is repeated over a range of optical powers that is suitable for the DUT 25. [0013] 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, it is generally required to test an optical device over an entire range of optical powers. As shown in Table 1, the number of errors exhibited by an optical component changes over a range of optical powers.

Table 1

It is important to understand how a particular optical component will behave over a wide range of optical powers. As aforementioned, this information is used to define the technical specifications for that component, which are later used by system designers to select components based upon those technical specifications. [0014] One of the most important aspects of an optical component's test is whether or not its behavior is predictable over the entire operating range of optical powers. If an optical component behaves in a predictable manner, system designers can rely on this predictability in designing their systems. Unpredictable behavior is extremely undesirable for an optical component. If a component exhibits unpredictable behavior, an optical component designer will typically redesign aspects of the component until it behaves in a predictable manner. [0015] Due to the current state of technology for optical testing equipment, testing an optical component at many small increments of optical power over the full operating range is not realistic. As aforementioned, in order to perform these measurements, a technician must first set the optical power of the test equipment to the desired optical power (which is an iterative process), and then must separately measure the number of errors at that optical power. Since the number of errors exhibited by optical equipment is extremely low, (i.e., lxlO^"9 or less), it would necessitate a technician to continually attend to the testing equipment over a series of hours or days.

[0016] Most current testing regimens avoid this problem by requiring a technician to measure the BER over a few discrete levels of optical power, (as shown in Table 1 ). These results are then extrapolated throughout the entire operating range of the optical component to arrive at the behavior of the component over the entire operating range of the component. Extrapolating the results in such a manner increases the risk that the true behavior exhibited by the optical component at levels of power between the measured discrete levels will be missed. This can lead to later errors in the technical specifications for the particular component. [0017] What is needed is a simple and effective system and method for testing the sensitivity of optical components.

[0018] SUMMARY

[0019] The present invention is a system and method for testing the bit error rate of an optical component over an operating range. 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. All routine calibration and testing is automated by the control unit. A common user interface permits user-friendly control of the system and provides clear system readouts.

[0020] 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.

[0021 ] BRIEF DESCRIPTION OF THE DRAWING(S)

[0022] Figure 1 is a prior art testing scheme.

[0023] Figure 2 is a block diagram of the preferred embodiment of the present invention.

[0024] Figure 3 is a block diagram of a control unit.

[0025] Figure 4 is a flow chart of the calibration procedure.

[0026] Figure 5 is a flow chart of the test procedure.

[0027] Figure 6 shows the front view of a graphical user interface.

[0028]DET AILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) [0029] The preferred embodiment of the present invention will be described with reference to the drawing figures where like numerals represent like elements throughout.

[0030] 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.

[0031] 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. This permits the elimination of redundant power supplies and power feeds to each separate component, permits a single control bus to control all of the components 150-160, and eliminates all redundant user interfaces with each optical component. As those of skill in the art would realize, having a single control unit 158 providing selective control of each optical component 150-156 greatly simplifies the testing procedure as will be explained in further detail hereinafter. Having a single control unit 158 also permits calibration of the entire system from a common point of control. [0032] Preferably, all of the optical components 150-156 are fixed in a rigid spatial relationship. Also the optical cables 186, 188, 190, 194, 196 are also rigidly fixed to prevent an inadvertent degradation or complete separation of an interconnection between optical components.

[0033] 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. [0034] 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 a plurality of individual software modules, predetermined component test parameters and any other information which is required to be stored by the control unit 158. For example, the memory 214 includes a calibration module 216 and a test module 218. The memory 214 may also include spare capacity or may be expanded by adding additional memory capacity 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. 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 the control bus 184. Alternatively, the data bus 228 may be detected and the I/O buffer 212 be coupled directly to the control bus 18. 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 may be monitored and/or controlled by the microprocessor 210.

[0035] The process implemented by the individual modules will now be described in greater detail with reference to Figures 4 and 5. Referring to Figure 4, the calibration procedure 300 in accordance with the present invention is shown. It should be noted that in order to calibrate the system, an external optical jumper must be placed between the optical output port 198 and the optical input port 199. This completes the optical path between the optical transmitter 150 and the optical receiver 156. Alternatively, an internal optical switch 197 may be provided to switch the output of the optical attenuator 152 to the input of the optical receiver 156. This optical switch 197 is preferably automatically controlled by the control unit 158 when the calibration procedure 300 is implemented. Alternatively, it could be manually operated. Use of the optical switch 197 is advantageous since it would eliminate the need for a technician to install an external optical jumper. [0036] The calibration procedure 300 begins with the control unit 158 retrieving prestored optical transmitter calibration parameters from memory (step 302). The calibration parameters may be either predefined values stored in memory by a technician, (or factory settings), or may be the results of prior calibration tests. The control unit 158 then energizes all of the optical components and, after a predetermined duration which permits the electronic components therein to reach steady state, the control unit 158 measures the operating parameters, which comprise the optical output power and the internal BER (step 306).

[0037] The control unit 158 then compares the calibration parameters to the measured operating parameters (step 308). If the operating parameters are within a predefined range of the calibration parameters, the control unit 158 stores the positive test results (step 310). If the operating parameters are not within the predetermined range of the calibration parameters, the system has failed the calibration procedure 300 and the control unit 158 stores this failure 312. Preferably, as the control unit 158 receives the test results, whether pass or fail, the control unit 158 provides an output to the graphical user interface 160 to keep the technician apprised of the results of the procedure (steps 314, 316).

[0038] It should be noted that the steps 302-316 set forth in the calibration procedure 300 need not necessarily occur in the order set forth in Figure 4. For example, step 302 may occur between steps 306 and 308. Those of skill in the art would clearly recognize that there is flexibility in the ordering of some of these steps. Additionally, some of the steps may not be part of this procedure. For example, step 304 relating to energizing the optical components may be performed by the technician upon powering up the equipment. Additionally, steps 310 and 312 may be eliminated, whereby the results of the calibration procedure 300 are not stored. [0039] Once the calibration procedure 300 has been completed, the system is ready to test an optical component. Referring back to Figure 3, in order to implement the test procedure 400, the microprocessor 210 accesses the test module 218. The test procedure 400 is shown in greater detail in the flow diagram of Figure 5. It should be noted that the test procedure 400 may be fully automated, whereby the technician initiates the process (i.e., pushes a button) and permits the control unit 158 to fully carry out the test procedure 400. Alternatively, the test procedure 400 may be selectively automated, whereby the technician may set certain parameters for the system to test. For example, the technician may set specific power levels for the system to test and may also set a specific number of errors, or range of uncertainty, at each power level. In any event, the preferred embodiment of the present invention is implemented via the test procedure 400 shown in Figure 5. [0040] The test procedure 400 begins by the technician initiating the procedure (step 402). This can be as simple as the technician pushing a "start test" 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 optical transmitter 150 is energized to transmit generally at a desired power level (step 404). Since the optical transmitter 150 is unable to fine-tune its output power level, additional steps 406-410 using the optical power monitor 154 and the optical attenuator 152 to iteratively fine-tune the output power level are required. The optical power monitor 154 measures the output of the optical transmitter 150 (step 406) and determines whether the output power level is at the desired power level (step 408). If the output power level is not at the desired power level, the optical attenuator 152 is tuned to attenuate the output of the optical transmitter 150 as appropriate in order to more closely achieve the desired power level (step 410). The system then uses the optical power monitor 150 to again measure the output of the optical transmitter 150 (step 406). Steps 406-410 are repeated until the output power level is at the desired power level.

[0041] Once the desired power level has been achieved, the optical receiver

156 measures the number of bit errors at that specific power level 412. This step is performed until the "completion criteria" is met. The completion criteria may be a particular duration, a particular number of bit errors received, or attaining a particular uncertainty. Regardless of which criteria is used to determine whether the testing is complete at that power level (step 414), the system continues steps 412 and 414 until the testing is complete for that particular power level. Once the testing is complete at the particular power level, the system determines whether the testing for all power levels has been completed (step 416). If not, the system adjusts the power level to the new desired power level (step 418) and the entire process (steps 406-416) is repeated until the testing at all power levels has been completed. The results are then output to the graphical user interface 160 (step 420).

[0042] 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.

[0043] 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. [0044] 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 step 420 may also be displayed on the screen 130.

[0045] 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

1. A system for measuring the bit error rate of an optical device under test (DUT) over an operating range of the 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; a power monitor, for monitoring said attenuated signal; an optical receiver, for receiving an input signal from said DUT; 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.

2. The system of claim 1 , whereby said controller controls said transmitter, attenuator and power monitor to transmit said test signal at a desired 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 a desired power level.

4. The system of claim 2 whereby the controller counts the number of errors in the input signal received by said receiver 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 received by said receiver and adjusts the power of said output signal in response thereto.

6. The system of claim 2 , whereby said controller controls the transmitter, attenuator and power monitor such that said output signal is output at a plurality of power levels and the controller counts the number of errors received by said receiver at each of said plurality of power levels.

7. The system of claim 6, whereby said plurality of power levels are located throughout the operating range of the DUT.

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

9. A method for measuring the bit error rate of an optical device under test (DUT) over the operating range of the DUT comprising: a) generating transmitting a test signal at a desired power level as an output signal; b) receiving an input signal; c) determining if there are bit errors in said input signal; d) providing an interface with a user; and e) centrally controlling steps a-d.

10. The method of claim 9, whereby said controlling step selectively controls said generation step to transmit said test signal at a desired power level, and said receiving step to detect the number of bit errors in said input signal.

11. The method of claim 9, whereby said controlling step implements steps a by generating a test signal, monitoring the power level of the test signal and adjusting the power level of the test signal in a recursive loop to provide said output signal at a desired power level.

12. The method of claim 9, further comprising counting the number of bit errors received by said receiver and adjusting the power of said output signal in response thereto.

13. The method of claim 12, further comprising calculating the bit error rate of the received signal for the desired power level.

14. The method of claim 10, whereby said output signal is output at a plurality of power levels, and further comprising counting the number of errors received by said receiving step at each of said plurality of power levels.

15. The method of claim 13, whereby said plurality of power levels are located throughout the operating range of the DUT.

16. 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 a desired power level; an optical receiver, coupled to said controller, for receiving an input signal from said DUT; and a graphical user interface, coupled to said controller, for providing an interface with a user.

17. The system of claim 16 whereby the controller counts the number of errors in the received input signal and adjusts the power level of said output signal in response thereto.

18. 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; a optical transmitter, coupled to said controller, for transmitting an output signal at a desired power level to said DUT at an output port; a power monitor, coupled to said controller, for monitoring said output signal; an optical receiver, coupled to said controller, for receiving an input signal from said DUT at an input port; a graphical user interface, coupled to said controller, for providing an interface with a user; and a bypass switch for optically coupling said output port to said input port.