Abstract:
A network device may include a receiver to receive optical pulses from an optical path, wherein the optical pulses include a plurality of intensities and represent data. The network device may also include a processor to determine a rate of bit errors introduced during propagation of the optical pulses through the optical path and to determine a parameter indicative of nonlinear effects of the optical path based on the rate of bit errors and the plurality of intensities.

Description:
BACKGROUND INFORMATION 
     Businesses and individuals increasingly rely on computer networks for communications. For example, home users expect to receive television programming on-demand over digital networks. Businesses may rely on applications (e.g., database applications, mail server applications, word processing applications, etc.) provided over a network, such as the public Internet or a leased private network. As time passes, communication networks are expected to carry more data through some of the same communication paths in a more reliable manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an exemplary measurement configuration for an optical path; 
         FIGS. 2A and 2B  are exemplary plots of the bit error rate with respect to the signal power through the optical path of  FIG. 1 ; 
         FIG. 3  is a block diagram of an exemplary deployed, in-service network; 
         FIG. 4  is a block diagram of exemplary components of a computing module; 
         FIG. 5  is a block diagram of exemplary components of a test transmitter; 
         FIGS. 6A and 6B  are plots of exemplary optical signals at different data rates; 
         FIG. 7  is a block diagram of exemplary components of a test receiver; 
         FIG. 8  is a flowchart of an exemplary process for determining the nonlinearity of an optical path; 
         FIG. 9  is a plot of signal intensity and optical power with respect to data rate; 
         FIG. 10  is a flowchart of an exemplary process for determining the nonlinearity of an optical path; 
         FIG. 11  is an exemplary plot of signal intensity versus bit error rate; and 
         FIG. 12  is a flowchart of an exemplary process for determining the nonlinearity of an unknown optical path. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following detailed description refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description does not limit the invention. 
     Nonlinear effects may impact the signal integrity of phase modulated optical signals because the nonlinearity may contribute to phase noise. Therefore, as optical networks move from amplitude modulation to phase modulation in high speed networks (e.g., 40 Gbps and 100 Gbps networks), the measurement of nonlinear effects may become more useful. In addition, because optical nonlinear effects, unlike linear effects, are nondeterministic, they are more difficult for a digital signal processor remove from an optical signal. Further, Dense Wavelength Division Multiplexing (DWDM) optical networks are increasingly moving to mesh-based architectures. In mesh-based architectures, as well as other architectures, measurement of optical nonlinearity effects of deployed, in-service optical fibers may become increasingly useful. That is, because the end-to-end path of a channel may not be known beforehand, the nonlinear cross-channel effects may become increasingly problematic. 
     Embodiments described herein allow for the measurement of nonlinear effects of optical paths. These embodiments may measure nonlinear effects in deployed, in-service optical paths that may include multiple sections or spans of optical fibers. These embodiments may also comply with the average power requirements of optical add-drop multiplexers currently deployed and, in some embodiments, measurements do not interfere with signals on other channels in an optical fiber. 
       FIG. 1  is a block diagram of an exemplary measurement configuration  100  for measuring nonlinear effects in an optical path. Configuration  100  may include an optical fiber  101 , a test transmitter  102 , and a test receiver  104 . Test transmitter  102  may transmit signals through fiber  101  (e.g. the signals propagate through fiber  101 ) to test receiver  104 . Analysis of the signal received by test receiver  104  may reveal a measurement of the nonlinear effects of the optical path of fiber  101 . 
     Measurement configuration  100  may include more, fewer, or different devices than shown. Further, although  FIG. 1  shows devices in a particular configuration, these devices and other devices may also be arranged in other configurations. 
     As discussed above, fiber  101  may include properties that effect the transmission (e.g., propagation) of signals through fiber  101  in a nonlinear fashion. These nonlinear effects may introduce errors in the data during propagation of signals through fiber  101 . The rate of the errors (e.g., the bit error rate (BER)) introduced during propagation by the nonlinear effects may increase as the optical power of the signal passing through fiber  101  increases. On the other hand, the BER introduced during propagation by other properties of fiber  101  (e.g., amplitude noise) may decrease as the optical power of the signal passing through fiber  101  increases (e.g., as the optical signal to noise ratio (OSNR) increases). These relationships are described with respect to  FIGS. 2A and 2B . 
       FIG. 2A  is an exemplary plot of the BER with respect to the signal power through a fiber. In  FIG. 2A , the abscissa represents optical power of a signal and the ordinate represents the corresponding BER. As shown, the BER may be relatively high in region  201  when the optical power of the signal is low. The relatively high BER in region  201  may be the result of a low OSNR. In this region, as the optical power of the signal increases, the BER decreases. As the optical power increases further, however, the curve enters region  202  where the BER is once again relatively high and increases with increased optical power. In region  202 , the nonlinear effects may be more influential than in region  201 , and these nonlinear effects may create a high penalty with respect to the BER. In the middle of region  201  and region  202 , however, the BER reaches a minimum where the nonlinear penalty is not significant enough to erase the benefits of the higher OSNR. 
       FIG. 2B  shows two exemplary plots of the BERs with respect to the signal power through two different optical paths (e.g., two different fibers). In  FIG. 2B , like in  FIG. 2A , the abscissa represents optical power of a signal and the ordinate represents the corresponding BER. A curve  210  shows the BER with respect to optical power for one fiber and a curve  212  shows the BER with respect to optical power for a second fiber. As shown in  FIG. 2B , the nonlinear penalty of the first fiber (represented by curve  210 ) is higher than the nonlinear penalty of the second fiber (represented by curve  212 ). As such, the nonlinear effects of the first fiber are greater than the nonlinear effects of the second fiber. In one example, the nonlinear effects of the second fiber may be “within budget” for the optical network, but the nonlinear effects of the first fiber may not be within budget and the first fiber may require diagnostics. 
     In some embodiments, test transmitter  102  may not be able to arbitrarily change the optical power of signals transmitted to test receiver  104 . Existing equipment in a network may regulate the optical power of signals to prevent cross-channel interference. For example, some network equipment, such as optical multiplexers, may regulate the average power of optical signals. The degree of nonlinearity, however, may depend on the intensity of an optical pulse. The intensity of the optical pulse may change with the data rate, even in a system where the average power of the optical signal is regulated (e.g., remains constant or is kept below a specified value).  FIG. 3  is an example of a network including network equipment that may regulate the average power of optical signals. 
       FIG. 3  is a block diagram of an exemplary network  300 . Network  300  may be a deployed (e.g., in-service, operational) network, e.g. a network that has already been installed in the field and may connect existing customer premises. Network  300  may include test transmitter  102 , test receiver  104 , a controller  306 , reconfigurable optical add-drop multiplexers (ROADMs)  308 - 1  and  308 - 2  (individually ROADM  308 - x , collectively ROADMs  308 ), optical fibers  310 - 1  through  310 - 7  (individually fiber  310 - x , collectively hereinafter fibers  310 ), an optical switch  312 , amplifiers  314 - 1  through  314 - 5  (individually amplifier  312 - x , collectively amplifiers  314 ), and network elements  316 - 1  through  316 - 3  (individually NE  316 - x , collectively NE  316 ). 
     Test transmitter  102  may transmit encoded data at different data rates and different signal intensities through an optical path (e.g., an optical fiber). Test receiver  104  may receive encoded data from test transmitter  102  and may determine the data rate of the received encoded data. Test receiver  104  may also determine the BER of the received encoded data. 
     Controller  306  may include one or more computing modules for hosting programs, databases, and/or applications, such as an application to analyze nonlinear effects in an optical path. Controller  306  may instruct test transmitter  102  to transmit encoded data, may instruct test transmitter  102  to transmit at a particular data rate and/or a particular signal intensity. In one embodiment, test transmitter  102  may include some of the functionality and components of controller  306 . 
     Controller  306  may also instruct test receiver  104  to receive the encoded data transmitted from test transmitter  102 . Controller  306  may also receive the data rate and BER determined by the test receiver  104 . Controller  306  may analyze test results, including the BER and the corresponding data rate and/or signal intensities. In one embodiment, test receiver  104  may include some of the functionality and components of controller  306 . 
     ROADM  308 - x  is a multiplexer that can add data to an optic fiber  310 - x  for transport to another network device, such as switch  312 . ROADM  308 - x  may include a group of ports  318  for receiving optical signals from network devices, such as NEs  316  or test transmitter  102 , for example, for adding to an optical fiber. Ports  318  may also be used for dropping signals from fiber  310 - x  to provide optical signals to network devices, such as test receiver  104 , for example. In one embodiment, each of ports  318  may correspond to a different channel and a different wavelength in a wavelength division multiplexing (WDM) network. ROADM  308 - x  may allow an optical signal to be added or dropped without converting the signal (or other signals on the fiber) to electronic (e.g., non-optical) signals. In one embodiment, ROADM  308 - x  may also include a switch, which may allow wavelength steering, e.g., passing wavelengths from one fiber span to another fiber span independently of other wavelengths without electrical conversion. 
     ROADM  308 - x  may automatically balance power among channels. In other words, ROADM  308 - x  may monitor channel power to achieve a pre-determined average power for each channel. In one embodiment, a gain equalization mechanism in ROADM  308 - x  may automatically adjust the power of a channel to a substantially constant level to avoid large power differences between channels. In this embodiment, power may be more evenly distributed across channels to reduce channel interference. The gain equalization, however, may concern only the average power of the channel, not that of each individual bit or pulse. 
     Optical fiber  310 - x  may include a single length of fiber or may include multiple spans of fibers. A single length of fiber may include, for example, a 1,000 km length of optical fiber. Multiple spans may include optical fibers strung together between optical amplifiers, ROADMs, and/or switches, such as amplifiers  314 , ROADMs  308 , and/or switch  312 . 
     Switch  312  may allow wavelength steering, e.g., passing wavelengths from one fiber span to another fiber span independently of other wavelengths without electrical conversion. Amplifier  314 - x  may amplify an optical signal in a fiber without converting the signal into an electrical signal. 
     NEs  316  may use network  300 , including ROADMs  308 , amplifiers  314 , and switch  310 , for communicating with other NEs  316 . For example, NE  316 - 1  may reside in a neighborhood for providing the neighborhood residents access to the Internet and to services that may be associated with, for example, to NE  316 - 3 . NE  316 - x  may receive optical signals on a channel from ROADM  308 - x  that are intended for the particular NE  316 - x . NE  316 - x  may also transmit an optical signal on a channel to ROADM  308 - x  that may be intended, for example, for a different NE  316 - x  in network  300 . NE  316 - x  may include, or may be coupled to, computers (e.g., servers, desktop computers, and/or laptop computers), televisions, telephones, personal digital assistants (PDAs), routers, switches, or any other computational device that may receive and transmit data. 
     Exemplary network  300  may include more, fewer, or different devices than shown. For example, network  300  may include hundreds or thousands of NEs, fibers, ROADMs, amplifiers, and/or switches. Further, although  FIG. 3  shows devices in a particular configuration, they may also be arranged in other configurations. For example, in one embodiment, one or more of ROADM  308 - x  may include test transmitter  102  and test receiver  104 . In this embodiment, controller  306  may remotely operate the test equipment (as a component of ROADM  308 - x ) and analyze the results. 
     Network  300  may include a mesh network, the Internet, an ad hoc network, a local area network (LAN), a wide area network (WAN), a metropolitan area network (MAN), a high-speed fiber optic network (e.g., FiOS™), or any other network or combinations of networks. 
     As mentioned above, controller  306  may include one or more computing modules for hosting programs, databases, and/or applications, such as a network test application. The other devices in network  300 , such as test transmitter  102  and/or test receiver  104 , may also include one or more computing modules. 
       FIG. 4  is a block diagram of exemplary components of a computing module  400 . Computing module  400  may include a bus  410 , processing logic  420 , an input device  430 , an output device  440 , a communication interface  450 , and a memory  460 . Computing module  400  may include other components (not shown) that aid in receiving, transmitting, and/or processing data. Moreover, other configurations of components in computing module  400  are possible. 
     Bus  410  may include a path that permits communication among the components of computing module  400 . Processing logic  420  may include any type of processor or microprocessor (or groups of processors or microprocessors) that interprets and executes instructions. In other embodiments, processing logic  420  may include an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or the like. 
     Input device  430  may include a device that permits a user to input information into computing module  400 , such as a keyboard, a mouse, a pen, a microphone, a remote control, a touch-screen display, etc. Output device  440  may include a device that outputs information to the user, such as a display, a printer, a speaker, etc. 
     Input device  430  and output device  440  may allow the user to activate a particular service or application, such as a network test application. Input device  430  and output device  440  may allow the user to receive and view a menu of options and select from the menu options. The menu may allow the user to select various functions or services associated with applications executed by computing module  400 . 
     Communication interface  450  may include any transceiver-like mechanism that enables computing module  400  to communicate with other devices and/or systems. Communication interface  450  may include an optical transmitter that may modulate a carrier signal based on encoded data and/or a receiver that may demodulate a received signal to recover the encoded data. Alternatively, communication interface  450  may include a transceiver to perform functions of both a transmitter and a receiver. 
     Communications interface  450  may include a network interface card, e.g., an optical line card or an Ethernet card, for communicating over an optical cable or an Ethernet cable. Communications interface  450  may include a wireless network interface card (e.g., a WiFi card) for wireless communications. Communication interface  450  may also include, for example, a universal serial bus (USB) port for communications over a cable. 
     Memory  460  may include a random access memory (RAM) or another type of dynamic storage device that may store information and instructions, e.g., an application and application data, for execution by processing logic  420 ; a read-only memory (ROM) device or another type of static storage device that may store static information and instructions for use by processing logic  420 ; and/or some other type of magnetic or optical recording medium and its corresponding drive, e.g., a hard disk drive (HDD), for storing information and/or instructions. 
     Memory  460  may include a network test application  462  to measure network  300  for nonlinear effects in fibers  310 . Memory  460  may also store measurement data  464 , which may reflect the results of tests performed by network test application  462 . Memory  460  may also store reference data  466 , which may reflect ideal or expected results for network measurements, or data for comparing to measurement data  464 , for example. 
     Computing module  400  may perform certain operations, as described herein. Computing module  400  may perform these operations in response to processing logic  420  executing software instructions contained in a computer-readable medium, such as memory  460 . A computer-readable medium may be defined as a physical or logical memory device. The software instructions may be read into memory  460  from another computer-readable medium or from another device via communication interface  450 . The software instructions contained in memory  460  may cause processing logic  420  to perform processes that are described herein. 
       FIG. 5  is a block diagram of exemplary components of test transmitter  102 . Test transmitter  102  may include an encoder  502 , a modulator  504 , a laser source  506 , a rate controller  508 , and a power controller  510 . Test transmitter  102  may include more, fewer, or different components than shown. Further, although  FIG. 5  shows components in a particular configuration, these components and other components may also be arranged in other configurations. 
     Encoder  502  may encode test data for test transmitter  102  to send to test receiver  104 . The test data may include pre-determined test patterns. Modulator  504  may input the test data and may modulate a laser from laser source  506  according to the test data. Laser source  506  may output a wavelength of light that may be tuned to match an open channel in an optical path, such as an open channel in network  300  between ROADM  308 - 1  and  308 - 2 . Modulator  504  may also modulate the encoded data according to a data rate output by rate controller  508 . Rate controller  508  may sweep through data rates (e.g., starting at a low data rate and increasing in steps to a high data rate). In this embodiment, modulator  504  may output modulated, encoded data at many different data rates. 
     Power controller  510  may ensure that the average power of the channel signal output is within the bounds expected by ROADM  508 - x , for example. If the average power of the channel signal were greater than expected by ROADM  508 - x , then a gain equalization mechanism in ROADM  508 - x  may adjust the power of the channel to avoid large average power differences between channels. Power controller  510  may allow test transmitter  102  to meet the power requirements of ROADM  508 - x  without disturbing other channels, and without having to adjust ROADM  508 - x  during testing. Power controller  510 , in conjunction with modulator  504 , encoder  502 , and rate controller  508 , may keep the transmitted test signal within a range (e.g., less than a value) to keep the channel “alive” in the ROADM-based DWDM system, such as that shown in  FIG. 3 . 
     One cause of the nonlinear effects in a fiber includes third-order nonlinear phase changes. These nonlinear phase changes may affect the integrity of self-phase modulation (SPM) signals, cross phase modulation (XPM) signals, and four-wave mixing (FWM) signals. Because SPM, XPM, and FWM may have the same root for their nonlinearity, the nonlinear effects of an optical path may be measured using any one of these modulation techniques. Therefore, in one embodiment, power controller  510  in conjunction with modulator  504 , encoder  502 , and rate controller  508 , may output signals that employ SPM. 
     In one embodiment, modulator  504  outputs a return-to-zero optical signal. In this embodiment, each ON bit in a transmitted signal may have a pulse that returns to zero even when followed by subsequent ON bits, for example. Thus, if the average power of a transmitted signal is to remain relatively constant, the power intensity of each bit at a high data rate may be lower than the power intensity of each bit at a lower rate. In other words, because there are more ON bits in a given period of time at a higher data rate than a lower data rate, each ON bit at a higher data rate may have a lower intensity in order to meet the average power requirement for the channel. Stated another way, because there are fewer ON bits in a given period of time at a lower data rate than a higher data rate, each ON bit at the lower data rate may have a higher intensity and still meet the average power requirement for the channel.  FIGS. 6A and 6B , discussed below, demonstrates this relationship between intensity and data rate. 
       FIGS. 6A and 6B  are plots showing power for two exemplary optical signals at different data rates. As shown in  FIGS. 6A and 6B , the abscissa represents time and the ordinate represents power. The data rate of signal  602  is defined as R a  and the intensity as I a . In  FIG. 6B , the data rate of signal  604  is R b  and is half the data rate of signal  602 , e.g, R a /2. Because the average power of signal  602  is the same as the average power of signal  602 , however, the intensity of an ON bit pulse of signal  604  (I b =2I a ) is twice that of an ON bit pulse of signal  602  (Ia). Intensity may measure, for example, the energy flux (e.g., radiant intensity, luminous intensity, or irradiance) during the pulse of a signal. In the embodiment shown, the pulse width corresponding to the data rate of  FIG. 6A  is the same as the pulse width corresponding to the data rate of  FIG. 6B , which may help avoid spectral width change to achieve consistent measurements of nonlinear effects over the different data rates. 
       FIG. 7  is a block diagram of exemplary components of test receiver  104 . Test receiver  104  may include an optical receiver  702 , a clock detector  704 , a demodulator  706 , and an error detector  708 . Test receiver  104  may include more, fewer, or different components than shown. Further, although  FIG. 7  shows components in a particular configuration, these components and other components may also be arranged in other configurations. 
     Optical receiver  702  may receive the test signal, transmitted form test transmitter  102 , that has been dropped from ROADM  308 - x , for example. The received test signal may include linear and nonlinear effects caused by propagation through the medium, such as fiber segments  310 - 3  and  310 - 6 . 
     Clock detector  704  may recover the clock of the test signal. Clock detector  707  may also inform network test application  462  (e.g., stored in controller  306 ) of the bit rate, and thus the intensity, of the received bits in the test signal. 
     Demodulator  706  may receive the signal from optical receiver  702  and may demodulate the signal to recover the encoded test data. Demodulator  706  may output the test data to error detector  708 . Error detector  708  may determine if any errors, which may occur during transport of the optical signal through fiber spans  310 - 3  and  310 - 4 , exist in the received test data. In one embodiment, error detector  708  is familiar with the pre-determined test patterns sent by test transmitter  102  and, therefore, can determine the number of errors in the received data. Alternatively, error detector  708  may use a cyclic redundancy check (CRC), error correction code (ECC) blocks, or another type of error detection mechanism or method. Error detector  708  may determine and output the BER of the test data. Error detector  704  may also inform network test application  462  (e.g., stored in controller  306 ) of the detected BER, which may be associated with corresponding data rate output from clock detector  704 . 
       FIG. 8  is a flowchart of an exemplary process  800  for measuring the nonlinear effects of an optical path. Process  800  may be performed in conjunction with a process  1000 , described with respect to  FIG. 10  below. Exemplary process  800  is described from the perspective of test transmitter  102 , whereas exemplary process  1000  is described from the perspective of test receiver  104 . 
     A data rate may be selected (block  802 ). For example, rate controller  508  of test transmitter  102  may select a data rate from a range of data rates (e.g., between start data rate Rs to end data rate Re) for testing. If process  800  is just starting, rate controller  508  may select start data rate Rs, such as 10 Mbps, for transmitting encoded test data. If process  800 A has already been running (e.g., process  800  just returned from block  810 ), then rate controller  508  may select an untested data rate, e.g., one that is incrementally higher than the previously tested data rate. For example, rate controller  508  may select 20 Mbps as the second data rate, e.g., one that is 10 Mbps higher than the first tested data rate (e.g., 10 Mbps). 
     Test data may be encoded (block  804 ). The encoded test data may include pre-determined test patterns or error correction information, such as CRCs, ECC blocks, or other error correction information. The encoded test data may be such that the resulting optical signal will include a known number of ON bits to control the intensity of each ON bit. 
     The encoded data may be modulated (block  806 ). Modulator  504  may receive the encoded data from encoder  502  and the data rate from rate controller  508  and may modulate the light received from light source  506  accordingly. The modulated encoded signal may include RZ modulation, as described above. 
     The modulated encoded data signal may be transmitted (block  808 ). Power controller  510  may received the modulated encoded test data from modulator  504  and may transmit an optical signal along an optical path. In the case of measuring the nonlinear effects of the optic path in network  300  (e.g., span  310 - 3  and span  310 - 6 ), power controller  510  may send the signal to ROADM  308 - 1 . In this example, the average power Pa of the channel signal may be within the bounds expected by ROADM  308 - 1 . As discussed above, however, even though the channel signal has average power Pa, the intensity of the signals during an ON bit pulse may be different at different data rates. In the case of measuring the nonlinear effects of the optic path of cable  101  in measurement configuration  100 , power controller  510  may send the signal through cable  101 . 
     If the test is not complete (block  810 : NO), then process  800  may continue to block  802  where a new data rate is selected. Process  800  may not be complete, for example, if the entire data rate range for testing (e.g., Rs to Re in 10 Mbps increments) has not yet been fully tested. If the test is complete (block  810 : YES), then process  800  may end. Process  800  may be complete, for example, if the entire data rate range has been fully tested. 
     The loop defined in process  800  may also be described in terms of  FIG. 9 .  FIG. 9  is a plot of signal intensity and optical power with respect to the data rate, e.g., the data rate chosen at block  802  by rate controller  508 . In  FIG. 9 , the abscissa represents the data rate of a signal and the ordinate represents the intensity (on the left side) and the corresponding average power (on the right side). As shown in  FIG. 9 , as the data rate increases (e.g., between start rate Rs and end rate Re), the signal intensity  902  decreases. Average optical power  904  of the channel signal is kept constant, however, as the data rate increases (e.g., between start rate Rs and end rate Re). 
       FIG. 10  is a flowchart of exemplary process  1000  for measuring the nonlinear effects of an optical path. As mentioned above, process  1000  may be performed in conjunction with process  800 . Exemplary process  1000  is described from the perspective of test receiver  104 , whereas exemplary process  800  is described from the perspective of test transmitter  102 . 
     A test signal may be received (block  1002 ). For example, optical receiver  702  of test receiver  104  may receive the test signal sent by test transmitter  102 . In the case of measuring the nonlinear effects of the optic path in network  300  (e.g., span  310 - 3  and span  310 - 6 ), optical receiver  702  may receive the signal from ROADM  308 - 2 . In the case of measuring the nonlinear effects of the optic path of cable  101  in measurement configuration  100 , optical receiver may receive the test signal from cable  101 . 
     The test signal may include clock information (e.g., as part of the modulation). The clock may be recovered (block  1004 ). Clock detector  704  may output the recovered clock to demodulator  706 . The recovered clock rate may also indicate the data rate of the received test signal and, as discussed above, the data rate may correspond to a signal intensity associated with the received test signal. 
     The received test signal may be demodulated (block  1006 ). Using the recovered clock, demodulator  706  may demodulate the received test signal and output the test data to error detector  708 . The bit error rate may be determined (block  1008 ). If the test data includes a pre-determined test pattern, for example, error detector  708  may determine the BER. Error detector  708  may also use CRCs or ECC blocks, or other error detection methods, to determine the BER. 
     The BER and the data rate may be stored (block  1010 ). Test receiver  104  may store the data rate and BER in its own memory (e.g., memory  460  in the case where test receiver  104  includes a computing module  400 ) and/or may send the BER and data rate to network test application  462  (e.g., stored in controller  306 ) for analysis. 
     If the test is not complete (block  1012 : NO), then process  1000  may continue to block  1002  where another test signal may be received, possibly at a different data rate. If the test is complete (block  1012 : NO), then process  1000  may end. 
       FIG. 11  shows exemplary plots of signal intensity versus BER. In  FIG. 11  the abscissa represents signal intensity (e.g., corresponding to the measured data rate) and the ordinate represents BER. In this example, curve  1102  represents the measured data collected by process  800  and  1000  of the optical path shown in network  300 . Further, curve  1104  represents the measured data collected by process  800  and  1000  of the fiber  101  shown in measurement configuration  100 . 
     With respect to curve  1102 , as the signal intensity increases (e.g., the data rate decreases), the BER first decreases (left half of curve  1102 ) and then the BER increases (right half of curve  1102 ). The minimum BER of curve  1102  occurs at measured intensity I is , which, in this embodiment, represents the point where the nonlinear penalty overcomes any advantages of an increased OSNR. 
     The characteristics of curve  1104  are similar to that of curve  1102 . As the signal intensity increases (e.g., the data rate decreases), the BER first decreases (left half of curve  1104 ) and then the BER increases (right half of curve  1104 ). The minimum BER of curve  1104 , however, occurs at measured intensity I r , which, in this example, is to the right of I is  of curve  1102 . Thus, the nonlinear penalty shown in curve  1102  (the optical path measured in network  300 ) is greater than the nonlinear penalty of the optical path of cable  101  of configuration  100  (curve  1104 ). As discussed below, measured intensity I r  of fiber  101  may be a reference intensity for comparing to intensities (e.g., I is ) of optic paths in deployed (e.g., installed, in-service, etc.) networks, such as network  300 . 
       FIG. 12  is a flowchart of an exemplary process  1200  for determining the nonlinear effects of an unknown fiber in a deployed network. A reference I r  intensity may be determined (block  1202 ). The reference intensity may be determined by employing processes  800  and  1000  on a known fiber, such as fiber  101  in measurement configuration  100 . The known fiber  101  may have nonlinear effects that are known to be acceptable and within budget of an optical communication network. The reference intensity I r  may be determined as shown in  FIG. 11 . 
     An intensity associated with an unknown optical path may be determined (block  1204 ). The intensity associated with the unknown, in-service fiber may be determined by employing processes  800  and  1000  on the in-service optical path, such as fiber span  310 - 3  and  310 - 6  between test transmitter  102  and test receiver  104  in network  300 . The intensity I is  may be determined, as shown in  FIG. 11 . 
     The intensity I is  of the unknown optical path may be compared to the reference intensity I r  (block  1206 ). This comparison may determine whether the penalty of the nonlinear effects of the unknown optical path are within budget, e.g., within the limits set by the network designers. End-to-end measurement for an optical path may reveal the overall nonlinear characteristics of the path, even if the path includes multiple fiber spans. 
     In one embodiment, after an optical fiber is installed in the field, it may be tested using test transmitter  102  and test receiver  104 . If the nonlinear penalty of the installed fiber is sufficiently small (e.g., 0.5 dB as compared to a reference), then the installed optical fiber may pass the test and be within the performance budget. If the nonlinear penalty of the installed fiber is too large (e.g., more than 2 dB as compared to a reference), then the installed optical fiber may fail the test, which may call for a diagnosis. In one embodiment, after the test, test transmitter  102  and test receiver  104  may be removed from the end-points of the tested optical fiber for testing a different optical fiber. 
     In the preceding specification, various preferred embodiments have been described with reference to the accompanying drawings. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the broader scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense. 
     For example, although network  300  includes optical fibers  310  and the embodiments disclosed above are described with respect to optical fibers, embodiments disclosed herein may be applied to any optical path, whether in an optical fiber or not. In addition, embodiments described herein may apply to transmission media other (other than optical media) in which nonlinear characteristics may be analyzed. For example, other transmission media may include wireless radio channels, Ethernet cables, twisted pairs, coaxial cables, etc. 
     While series of blocks have been described above with respect to different processes, the order of the blocks may differ in other implementations. Moreover, non-dependent acts may be performed in parallel. 
     It will be apparent that aspects of the embodiments, as described above, may be implemented in many different forms of software, firmware, and hardware in the embodiments illustrated in the figures. The actual software code or specialized control hardware used to implement these embodiments is not limiting of the invention. Thus, the operation and behavior of the embodiments of the invention were described without reference to the specific software code—it being understood that software and control hardware may be designed to the embodiments based on the description herein. 
     Further, certain portions of the invention may be implemented as “logic” that performs one or more functions. This logic may include hardware, such as an application specific integrated circuit, a field programmable gate array, a processor, or a microprocessor, or a combination of hardware and software. 
     No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the articles “a” and the term “one of” are intended to include one or more items. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.