Abstract:
A method may include generating first and second optical signals and modulating the first and second optical signals simultaneously to synchronize the signals. The method may include varying the polarization of the second optical signal and transmitting the first and second optical signals. The method may include receiving the first and second optical signals, wherein the signals traveled along a same optical path. The method may include determining a plurality of differential travel delays between the first and second optical signals over a period of varying polarizations and determining a differential group delay based on the maximum and the minimum differential travel delays.

Description:
BACKGROUND INFORMATION 
       [0001]    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 over some of the same communication paths in a more reliable manner. Increasingly the data paths are optical data paths, and the providers of the optical networks need to test these paths. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  is a diagram of an overview of an exemplary embodiment described herein; 
           [0003]      FIGS. 2A and 2B  are block diagrams of exemplary components of a test transmitter; 
           [0004]      FIGS. 3A ,  3 B, and  3 C are frequency plots of exemplary optical signals generated in the test transmitter of  FIGS. 2A and 2B ; 
           [0005]      FIGS. 4A and 4B  are block diagrams of exemplary components of a test receiver; 
           [0006]      FIGS. 5A and 5B  are plots of exemplary optical signals received in the test receiver of  FIGS. 4A and 4B ; 
           [0007]      FIG. 6  is a block diagram of exemplary components of a computing module; 
           [0008]      FIG. 7  is a flowchart of an exemplary process for determining the differential group delay and/or the polarization mode dispersion of an optical path; and 
           [0009]      FIGS. 8 through 12  are block diagrams of exemplary networks in which embodiments disclosed herein may be employed. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0010]    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. 
         [0011]    Polarization mode dispersion (PMD) may impact the integrity of optical signals. PMD is a form of modal dispersion where two different polarizations of light in an optical path, which normally travel at the same speed, travel at different speeds causing the spreading of the optical signal. PMD effects may cause an optical signal to have different travel speeds based on polarization and frequency. PMD can be characterized by a series of parameters: a first-order PMD parameter or “differential group delay” (“DGD”); a second-order PMD parameter (“SOPMD”); a third-order PMD parameter (“TOPMD”); etc. 
         [0012]    The measurement of PMD effects may become increasingly useful in Dense Wavelength Division Multiplexing (DWDM) optical networks (including networks employing mesh-based architectures). Further, the measurement of PMD effects may become increasingly useful for optical paths in deployed, in-service networks. One or more embodiments disclosed herein may measure PMD and/or DGD of an optical path in a deployed network, e.g., an in-service, operational network that may have already been installed in the field and may connect existing customer premises. Some embodiments described herein allow for the measurement of PMD of an optical fiber or path. In one embodiment, the first-order PMD parameter, or DGD, may be measured to indicate the PMD, because the DGD parameter may be more indicative of PMD than the other parameters. 
         [0013]      FIG. 1  is a diagram of an overview of an exemplary embodiment in a network  100  for measuring PMD and/or DGD. Network  100  may include a deployed, in-service network. Network  100  may include test transmitter  102 , test receiver  104 , controllers  106  (individually, controller  106 - 1  and controller  106 - 2 ), reconfigurable optical add-drop multiplexers (ROADMs)  108 , optical fibers  110 , optical amplifiers  114 , and network elements (NE)  116 . Amplifiers  114 , ROADMs  108 , optic fibers  110 , and NEs  116  are referred to individually as amplifier  114 - x , ROADM  108 - x , optic fiber  110 - x , and NE  116 - x , respectively. 
         [0014]    Test transmitter  102  may transmit signals through fibers  110  along an optical path  120  to test receiver  104 . As the signals travel along optical path  120 , the signals may pass through multiple fiber lengths  110 , multiple amplifiers  114 , multiple ROADMs  108 , etc., and may experience PMD and/or DGD effects along path  120 . Test transmitter  102  is described in more detail with respect to  FIGS. 2A and 2B . 
         [0015]    Test receiver  104  may receive the signals transmitted from test transmitter  102  after the signals have passed through optical path  120 . In one embodiment, analysis of the signal received by test receiver  104  may reveal a measurement of PMD and/or DGD of optical path  120 . Test receiver  104  is described in more detail below with respect to  FIGS. 4A and 4B . 
         [0016]    Controllers  106  may include one or more computing modules for hosting programs, databases, and/or applications, such as an application to control test transmitter  102  and/or test receiver  104  for the measurement of PMD and/or DGD in an optical path. 
         [0017]    Optical fiber  110 - 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 kilometer length of optical fiber. Multiple spans may include optical fibers strung together between optical amplifiers, ROADMs, and/or switches, such as amplifiers  114  and ROADMs  108 . 
         [0018]    Amplifier  114 - x  may amplify an optical signal in an optical path, such as optical path  120 , without converting the signal into an electrical signal. ROADM  108 - x  may include a multiplexer that can add data to an optic fiber  110 - x  for transport to another network device. ROADM  108 - x  may include a group of ports  118  for receiving optical signals from network devices for adding to an optical fiber. Ports  118  may also be used for dropping signals from fiber  110 - x  to provide optical signals to network devices, such as NEs  116  or test receiver  104 , for example. In one embodiment, each of ports  118  may correspond to a different channel and a different wavelength in a wavelength division multiplexing (WDM) network. ROADM  108 - 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. 
         [0019]    NEs  116  may use network  100 , including ROADMs  108 , and amplifiers  114  for communicating with other NEs  116 . For example, NE  116 - x  may reside in a neighborhood for providing residents&#39; access to the Internet. NE  116 - x  may receive optical signals on a channel from ROADM  108 - x  that are intended for the particular NE  116 - x . NE  116 - x  may also transmit an optical signal on a channel to ROADM  108 - x  that may be intended, for example, for a different NE  116 - x  in network  100 . NE  116 - 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. 
         [0020]    Exemplary network  100  may include more, fewer, or different devices than shown. For example, network  100  may include hundreds or thousands of NEs, fibers, ROADMs, amplifiers, and/or switches. Further, although  FIG. 1  shows devices in a particular configuration, the devices may also be arranged in other configurations. For example, in one embodiment, ROADM  108 - x  may include test transmitter  102  and/or test receiver  104 . In this embodiment, controllers  106  may remotely operate the test equipment (as a component of ROADM  308 - x ) and analyze the results. Further, network  100  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. 
         [0021]      FIG. 2A  is a block diagram of exemplary components of one embodiment of test transmitter  102  (hereinafter, “test transmitter  102 A”). Test transmitter  102 A may include a first light source  202 - 1  and a second light source  202 - 2  (collectively, “light sources  202 ”), a signal combiner  204 , a modulator  206 , a three-port filter  208 , and a polarization scrambler  210 . 
         [0022]    First light source  202 - 1  may output a wavelength of light tuned to match an open channel in an optical path, such as an open channel in network  100  in optical path  120 . Likewise, second light source  202 - 2  may also output a wavelength of light tuned to match an open channel in an optical path, such as an open channel in network  100  in optical path  120 . In one embodiment, the wavelength and frequency of light output by second light source  202 - 2  (the “second signal” output at a “second wavelength” and a “second frequency”) may be chosen to be different than the wavelength and frequency of light output by first light source  202 - 1  (the “first signal” output at a “first wavelength” and a “first frequency”). In one embodiment, light sources  202  may include a laser or a filtered ASE (amplified stimulated emission). 
         [0023]    Combiner  204  may receive the light from the first light source  202 - 1  and the light from the second light source  202 - 2  and may combine the light into a single fiber, for example. Combiner  204  may output the combined signal (including both the first and second signal at the first and second wavelength, respectively) on a single optical fiber. 
         [0024]    Modulator  206  may input the combined signal and may modulate the combined signal to generate two synchronized signals (e.g., two optical clock signals, one at the first wavelength and one at the second wavelength). In this embodiment, because modulator  206  operates on both the first and second signals simultaneously, the first and second signals are synchronized (e.g., the two optical clocks are synchronized). In one embodiment, the modulation frequency may be between 1 to 10 GHz (e.g., 2.5 GHz). In one embodiment, modulator  206  may employ amplitude and/or phase modulation, for example. 
         [0025]    Three-port filter  208  may input the two synchronized signals (e.g., the first and second signals on a single fiber) and may output the first signal (e.g., at the first wavelength) to a port of ROADM  108 - x  corresponding to the appropriate wavelength and coupled to the optical path under test (e.g., path  120 ). Three-port filter  208  may also output the second signal (e.g., the second wavelength) to polarization scrambler  210 . 
         [0026]    Polarization scrambler  210  may change or vary the polarization of the received optical signal. Polarization scrambler  210  may cycle through many different polarizations. In one embodiment polarization scrambler  210  may cycle through 8,000 polarizations described by three coordinates (e.g., a horizontal component, a vertical component, and an axial component relative to the fiber receiving the output of polarization scrambler  210 ). In one embodiment, polarization scrambler  210  changes polarization every time increment. Polarization scrambler  210  may output the second signal (e.g., the second wavelength) to a port of ROADM  108 - x  corresponding to the appropriate wavelength and coupled to the optical path under test (e.g., path  120 ). 
         [0027]    Thus, test transmitter  102 A may output two signals at two wavelengths. These two signals may include a well synchronized and may include two well synchronized clocks. One of the signals may include varying polarizations. 
         [0028]      FIG. 2B  is a block diagram of exemplary components of another embodiment of test transmitter  102  (hereinafter, “test transmitter  102 B”). Test transmitter  102 B may include a single-sided filter  212  in addition to light sources  202 , signal combiner  204 , modulator  206 , three-port filter  208 , and a polarization scrambler  210 . 
         [0029]    In this embodiment, single-sided filter  212  may be added to the output of polarization scrambler  210 .  FIGS. 3A through 3C  are referred to for describing single-sided filter  212 . As shown in  FIG. 3A , second light source  202 - 2  may include a carrier at the second frequency. After the carrier is modulated by modulator  206 , the modulated signal includes two sidebands centered about the second frequency. Polarization scrambler  210  acts on the modulated signal shown in  FIG. 3B  and, thus, without single-sided filter  212 , the output signal would have the frequency characteristics shown in  FIG. 3B . Because the optical path (e.g., optical path  120 ) may exhibit chromatic dispersion, the portion of the signal in one of the side bands may be delayed differently than the portion of the signal in the other side band even though both may have the same polarization. Thus, since the delay being measured includes the delay based on the different polarization, measuring accuracy may be increased by removing one of the side bands and/or the residual signal at the carrier frequency. Thus, in one embodiment, single-sided filter  212  passes only one side band of the modulated signal and may help remove the effects of chromatic dispersion when recovering the second signal.  FIG. 3C  illustrates an exemplary pass bandwidth (e.g., shape) of single-sided filter  212  and the frequency spectrum of the filtered modulated signal. 
         [0030]    In one embodiment, controller  106 - 1  may control test transmitter  102 . Controller  106 - 1  may turn on test transmitter  102 , may program the polarizations to which polarization scrambler  210  can change a signal, may instruct the tuning of light sources  202 , may vary three-port filter, etc. In another embodiment, the components of test transmitter  102  (e.g., polarization scrambler  210 , three-port filter  208 , etc.) may include a controller. 
         [0031]      FIG. 4A  is a block diagram of exemplary components of one embodiment of test receiver  104  (hereinafter, “test receiver  104 A”). Test receiver  104 A may include a clock detector  402 , a relative group delay detector  404  (hereinafter, “RGD detector  404 ”), and a DGD calculator  406 . 
         [0032]    Clock detector  402  may detect the clock signal carried by the first signal at the first wavelength (e.g., the signal without the scrambled polarization). Clock detector  402  may output the detected clock as a clock signal  410  to RGD detector  404 . 
         [0033]    RGD detector  404  may measure the delay between clock signal  410  output from clock detector  402  (e.g., without scrambled polarization) and the recovered second signal (e.g., with scrambled polarization). RGD detector  404  is described with respect to  FIG. 5A , which includes plots of exemplary signals received by test receiver  104 A. As shown, recovered second signal  502  is delayed by delay time T 1  relative to recovered clock signal  410 . In this example, RGD detector  404  may output delay time T 1  to DGD calculator  406 . RGD detector  404  may measure the relative delay once every polarization time increment, for example. RGD detector  404  may measure the relative delay over a period of time such that polarization scrambler  210  cycles once through all polarizations for measurement. RGD detector  404  may output the measured delays as they are measured or as a group of delays. 
         [0034]    In one embodiment, RGD detector  404  includes a clock detector to detect the clock signal carried by the second wavelength (e.g., the second signal). In this embodiment, a phase detector circuit may detect the phase difference between the first signal (e.g., first wavelength) and the second signal (e.g., second wavelength). In another embodiment, the relative delay may be detected indirectly without recovering the clock signal superimposed on (e.g., carried by) either the first signal or the second signal. 
         [0035]    DGD calculator  406  receives the measured delays from RGD detector  404 . DGD calculator  406  may determine the minimum and maximum delay values output from RGD detector  404  and may calculate the DGD value based on these minimum and maximum delay values. RGD detector  404  is described with respect to  FIG. 5B , which includes plots of exemplary signals received by test receiver  104 A. As shown in  FIG. 5B , recovered second signal  516  is delayed by minimum delay time T 2  relative to recovered clock signal  410 . Further, at a different time, recovered second signal  518  is delayed by maximum delay time T 3  relative to recovered clock signal  410 . In this example, RGD detector  404  outputs delay time T 3  and delay time T 2 , along with other delay values, to DGD calculator  406 . DGD calculator  406  determines that maximum delay time T 3  is the maximum value and that minimum delay T 2  is the minimum delay. In one embodiment, DGD calculator  406  calculates the DGD value by subtracting the maximum delay from the minimum delay. In the current example, DGD calculator  406  may calculate the DGD value by subtracting minimum delay time T 2  from maximum delay value T 3 . 
         [0036]    In one embodiment, an indication or measurement of the PMD is given by the measurement of DGD. In this embodiment, DGD (e.g., the first-order PMD parameter) is indicative of the PMD because this first-order parameter may contribute more to PMD than the other, higher-order parameters. 
         [0037]    If there is PMD and/or DGD in the optical path, then the arrival time of the second signal (e.g., with scrambled polarization) may exhibit a different delay relative to the first signal (e.g., without scrambled polarization) when the polarization of the second signal changes. In this case, DGD calculator  406  may determine the maximum delay time and the minimum delay time and output the difference between the two. 
         [0038]    If there is no PMD and/or DGD in the measured optical path, then the arrival time of the second signal (e.g., with scrambled polarization) relative to the first signal (e.g., without scrambled polarization) does not change when the polarization of the second signal changes. In this case, DGD calculator  406  may determine that the PMD and/or DGD are zero (e.g., that the minimum delay and the maximum delay are the same, and the difference is zero). 
         [0039]    In one embodiment, the accuracy of RGD detector  404  (assuming a duty cycle of 0.1%) may be 1 ps (picoseconds) for a modulation frequency of 1 GHz, 0.1 ps for a modulation frequency of 10 GHz, and 0.4 ps for a modulation frequency of 2.5 GHz. In this embodiment, the measurement range of DGD calculator  406  may be 1,000 ps for a modulation frequency of 10 GHz, 100 ps for a modulation frequency of 10 GHz, and 400 ps for a modulation frequency of 2.5 GHz. 
         [0040]      FIG. 4B  is a block diagram of exemplary components of another embodiment of test receiver  104  (hereinafter, “test receiver  104 B”). Test transmitter  104 B may include a single-sided filter  408  in addition to clock detector  402 , relative group delay detector  404 , and DGD calculator  406 . 
         [0041]    In this embodiment, single-sided filter  408  may be added to the input of test receiver  104 B. As described above with respect to  FIGS. 2B , and  3 A through  3 C, and single-sided filter  212 , the modulated second signal may include two sidebands centered about the second frequency. As an alternative to single-sided filter  212 , a single-sided filter  408  may be placed in test receiver  104 B rather than in test transmitter  102 B. In this embodiment, single-sided filter  408  may reduce chromatic dispersion by passing only one sideband for the same reasons given above for single-sided filter  212 . 
         [0042]    In one embodiment, controller  106 - 2  may control test receiver  104 . Controller  106 - 2  may turn on test receiver  104 , may program DGD calculator  406 , may change single-sided filter  408 , etc. In another embodiment, the components of test receiver  104  (e.g., DGD calculator  406 , etc.) may include a controller. 
         [0043]      FIG. 6  is a block diagram of exemplary components of a computing module  600 . Devices in network  100  may each include one or more computing modules  600 . For example, DGD calculator  406  of test receiver  104 A may include a computing module. Controllers  106  may also each include a computing module. Computing module  600  may include a bus  610 , processing logic  620 , an input device  630 , an output device  640 , a communication interface  650 , and a memory  660 . Computing module  600  may include other components (not shown) that aid in receiving, transmitting, and/or processing data. Moreover, other configurations of components in computing module  600  are possible. 
         [0044]    Bus  610  may include a path that permits communication among the components of computing module  600 . Processing logic  620  may include any type of processor or microprocessor (or families of processors or microprocessors) that interprets and executes instructions. In other embodiments, processing logic  620  may include an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or the like. 
         [0045]    Input device  630  may allow a user to input information into computing module  600 . Input device  630  may include a keyboard, a mouse, a pen, a microphone, a touch-screen display, etc. Output device  640  may output information to the user. Output device  640  may include a display, a printer, a speaker, etc. For example, controllers  106  may each include a display and ROADMS  108  may include light-emitting diodes (LEDs). Some devices may be managed remotely (e.g., “headless” devices) and may not include input device  630  or output device  640 . 
         [0046]    Input device  630  and output device  640  may allow the user to activate and interact with a particular service or application, such as an application to test an optical path, by test transmitter  102  and test receiver  104 . Input device  630  and output device  640  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  600 . 
         [0047]    Communication interface  650  may include a transceiver that enables computing module  600  to communicate with other devices and/or systems. Communication interface  650  may include a transmitter that may convert baseband signals to radio frequency (RF) signals and/or a receiver that may convert RF signals to baseband signals. Communication interface  650  may be coupled to an antenna for transmitting and receiving RF signals. Communication interface  650  may include a network interface card, e.g., Ethernet card, for wired communications or a wireless network interface (e.g., a WiFi) card for wireless communications. Communication interface  650  may also include, for example, a universal serial bus (USB) port for communications over a cable, a Bluetooth™ wireless interface, etc. 
         [0048]    Memory  660  may store, among other things, instructions (e.g., applications  664  and operating system (OS)  662 ) and data (e.g., application data  666 ). Memory  660  may include a random access memory (RAM) or another type of dynamic storage device that may store information and instructions; 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  620 ; 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. 
         [0049]    OS  662  may include software instructions for managing hardware and software resources of computing module  600 . For example, OS  662  may include Linux, Windows, OS X, an embedded operating system, etc. Applications  664  and application data  666  may provide network services or include applications, depending on the device in which the particular computing module  600  is found. 
         [0050]    Computing module  600  may perform the operations described herein in response to processing logic  620  executing software instructions contained in a computer-readable medium, such as memory  660 . A computer-readable medium may include a physical or logical memory device. The software instructions may be read into memory  660  from another computer-readable medium or from another device via communication interface  650 . The software instructions contained in memory  660  may cause processing logic  620  to perform processes that are described herein. 
         [0051]      FIG. 7  is a flowchart of an exemplary process  700  for determining PMD and/or DGD of an optical path. Process  700  is described with respect to  FIG. 8 , which is a block diagram of an exemplary network  800 , which may include a deployed, in-service network. 
         [0052]    Network  800  includes ROADMs  808 - 1  through  808 - 4  (collectively, “ROADMs  808 ”), amplifiers  814 - 1  through  814 - 6  (collectively “amplifiers  814 ”), optical fibers  810 - 1  through  810 - 6  (collectively, “optical fibers  810 ”), test transmitter  102 A (shown in  FIG. 2A ), and test receiver  104 A (shown in  FIG. 4A ). ROADMs  808 , amplifiers  814 , and optical fibers  810  may operate similarly to ROADMs  108 , amplifiers  114 , and optical fibers  110  described above with respect to  FIG. 1 . Exemplary network  800  may include more, fewer, or different devices than shown. For example, network  800  may include hundreds or thousands of NEs, fibers, ROADMs, amplifiers, and/or switches. Further, although  FIG. 8  shows devices in a particular configuration, they may also be arranged in other configurations. 
         [0053]    Process  700  may test the optical path between test transmitter  102 A and test receiver  104 A, which includes ROADMs  808 - 1  and  808 - 2 , optical fibers  810 - 1  and  810 - 2 , and amplifier  814 - 1 . Process  700  may begin with the selection of a channel pair for measurement (block  702 ). For example, two input channels to ROADM  808 - 1 , each associated with a different wavelength, may be selected. 
         [0054]    The light sources may be tuned (block  704 ). For example, referring to  FIG. 2A , first light source  202 - 1  may be tuned to the wavelength associated with the first channel of the selected channel pair. Second light source  202 - 2  may be tuned to the wavelength associated with the second channel of the selected channel pair. First light source  202 - 1  and second light source  202 - 1  may output the first and second signal on a first and second fiber, respectively. 
         [0055]    The light sources may be combined (block  706 ). In the current example, combiner  204  may receive the light from first light source  202 - 1  and the light from second light source  202 - 2  and may combine the light into a single fiber, for example. Combiner  204  may output the combined signal on a single optical fiber. 
         [0056]    The combined signal may be modulated (block  708 ). Modulator  206  may input the combined signal and may modulate the combined signal to generate two synchronized signals (e.g., two optical clock signals, one at the first wavelength and one at the second wavelength). In this embodiment, because modulator  206  operates on both the first and second wavelength simultaneously, the two signals are synchronized. 
         [0057]    The signals may be separated (block  710 ). For example, three-port filter  208  may receive the two synchronized signals (e.g., the first and second wavelength on a single fiber) and may output the first signal (e.g., the first wavelength) to a port of ROADM  808 - 1  corresponding to the appropriate wavelength. Three-port filter  208  may also output the second signal (e.g., the second wavelength) to polarization scrambler  210 . 
         [0058]    The polarization of one of the signals (e.g., the second signal) may be scrambled (block  712 ). For example, polarization scrambler  210  may change or vary the polarization of the second signal. In one embodiment, polarization scrambler  210  may cycle through up to 8,000 polarizations over a period of time. Other quantities of polarizations are possible. For example, polarization scrambler  210  may cycle through up to 1,000, 3,000, 5,000, 7,000, 9,000, 11,000, etc., polarizations. 
         [0059]    The first signal may be fed into the first port (block  714 ). As discussed above, three-port filter  208  may output the first signal (e.g., the first wavelength without scrambled polarization) to a port of ROADM  808 - 1  that corresponds to the appropriate wavelength. The second signal may be fed into the second port (block  716 ). Polarization scrambler  210  may output the second signal (e.g., the second wavelength with scrambled polarization) to a port of ROADM  808 - 1  corresponding to the appropriate wavelength. 
         [0060]    The first and second signals (at the first and second wavelengths) traverse the optical path. The first signal may be received on a third port and a clock may be recovered (block  718 ). Referring to  FIG. 4A , clock detector  402  may detect the clock superimposed on (e.g., carried by) the first signal at the first wavelength (e.g., without scrambled polarization). Clock detector  402  may output the detected clock as a clock signal  410  to RGD detector  404 . The second signal may be received on a fourth port (block  718 ) and fed into, for example, RGD detector  404 . 
         [0061]    The relative group delay may be measured (block  722 ). RGD detector  404  measures the delay between clock signal  410  output from clock detector  402  and the recovered second signal (e.g., the signal with the scrambled polarization). Relative group delay detector  404  may measure the relative delay once every polarization time increment, for example. As shown in  FIG. 5A , for example, recovered second signal  502  is delayed by delay time T 1  relative to recovered clock signal  410 . In this example, RGD detector  404  may output delay time T 1  to DGD calculator  406 . RGD detector  404  may measure the relative delay over a period of time such that polarization scrambler  210  cycles once through all polarizations for measurement. RGD detector  404  may output the measured delays as they are measured or as a group of delays. 
         [0062]    The differential group delay (DGD) may be measured (block  724 ). For example, DGD calculator  406  may receive the measured delays from RGD detector  404 . DGD calculator  406  may determine the minimum and maximum delay times output from RGD detector  404 . As discussed above with respect to  FIG. 5B , DGD calculator  406  may determine the maximum delay value and the maximum delay value. In one embodiment, DGD calculator  406  calculates DGD by subtracting the maximum delay time from the minimum delay time. In the example of  FIG. 5B , discussed above, DGD calculator  406  may calculate DGD by subtracting minimum delay time T 2  from maximum delay time T 3 . 
         [0063]    As discussed above, if there is PMD and/or DGD in the optical path, then the arrival time of the second signal (e.g., with scrambled polarization) may exhibit a different delay relative to the first signal (e.g., without scrambled polarization) when the polarization of the second signal changes. In this case, DGD calculator  406  may output the difference between the minimum and maximum delay time as the calculated DGD. If there is no PMD and/or DGD in the measured optical path, then the arrival time of the second signal (e.g., with scrambled polarization) relative to the first signal (e.g., without scrambled polarization) does not change when the polarization of the second signal changes. In this case, the minimum and maximum delay time are the same, and the difference is zero (e.g., the DGD value is calculated to be zero). 
         [0064]    If another channel pair is to be measured (block  726 : YES), then another channel pair may be selected (block  702 ). For example, test transmitter  102 A in  FIG. 8  may choose to test the optical path from test transmitter  102 A to second test receiver  822 , including ROADMs  808 - 1  through  808 - 3 , amplifiers  814 - 1  and  814 - 3 , and optical fibers  810 - 1 ,  810 - 2 ,  810 - 5 , and  810 - 6 . In this case, process  700  may begin again at block  702 . If there are no other channel pairs to measure (block  726 : NO), then process  700  may end. 
         [0065]      FIG. 9  is a block diagram of an exemplary network  900  including optical switches. Like network  800 , network  900  may include a deployed, in-service network. Network  900  includes some of the same components of network  800 , including ROADMs  808 , fibers  810 , amplifiers  814 , test transmitter  102 A, and test receiver  104 A. Network  900  also includes switches  902 - 1  and  902 - 2  (collectively, “switches  902 ”). Switches  902  may allow wavelength steering, e.g., passing one wavelength from one fiber span to a different wavelength on another fiber span, independently of other wavelengths and without electrical conversion, for example. Switches  902  may allow for more flexibility in testing optical paths. In one embodiment, switches  902  may be considered part of test transmitter  102  and test receiver  104 . 
         [0066]    For example, returning to process  700 , tuning light sources (block  704 ) may include tuning first light source  202 - 1  and second light source  202 - 2  to two pre-set wavelengths. In this embodiment, feeding the first signal into the first port (block  714 ) may include switch  902 - 1  steering the first signal (at the first wavelength) to the correct port (with the corresponding wavelength) in ROADM  808 - 1 . Likewise, in this embodiment, feeding the second signal into the second port (block  716 ) may include switch  902 - 1  steering the second signal (at the second wavelength) to the correct port (with the corresponding wavelength) in ROADM  808 - 1 . This embodiment may allow switching between ports (block  726 ) in ROADM  808 - 1  (e.g., to test different optical paths) without having to retune light sources  202 . 
         [0067]      FIG. 10  is a block diagram of an exemplary network  1000  including a test transmitter and a test receiver at the same ROADM  808 . Like network  900 , network  1000  may include a deployed, in-service network. Also, like network  900 , network  1000  may include some of the same components of network  800 , with the addition of switches  902 . In  FIG. 10 , however, switch  902 - 2  is configured to send the two signals at the two wavelengths back to ROADM  808 - 1  (passing a second time through ROADM  808 - 2 ). That is, network  1000  is in a “loop-back” configuration while networks  900  and  800  are in a “linear” configuration. Thus, test transmitter  102 A and test receiver  104 A, in this configuration, may be located at ROADM  808 - 1 . The configuration of network  1000  may be more convenient than the test configuration in network  900  if test transmitter  102 A and  104 A are already collocated, if they are physically attached to one another, of if they form part of ROADM  808 - 1  and/or switch  902 - 1 . 
         [0068]    Returning to process  700 , in the configuration of network  1000 , feeding the first signal into the first port (block  714 ) may include switching the first signal to a port in ROADM  808 - 2  to return the first signal to originating ROADM  808 - 1 . Likewise, feeding the second signal into the second port (block  716 ) may include switching the second signal to a port in ROADM  808 - 2  to return the second signal to originating ROADM  808 - 1 . In the configuration of network  1000 , the tested optical path includes fibers  810 - 3  and  810 - 4  and amplifiers  814 - 1  and  814 - 2 . 
         [0069]    As mentioned, network  1000  allows for test transmitter  102 A and test receiver  104 A to be at the same physical location. In one embodiment, however, in networks  800  and  900 , test transmitter  102 A and test receiver  104 A, while located separately, may function without communicating with each other if configured properly. 
         [0070]      FIG. 11  is a block diagram of an exemplary network  1100  including single-sided filter  212  in test transmitter  102 B. Like network  800 , network  1100  may include a deployed, in-service network. Also, like network  800 , network  1100  may include some of the same components of network  800 , including ROADMs  808 , amplifiers  814 , fibers  810 , and test receiver  104 A. Network  1100 , however, includes test transmitter  102 B rather than test transmitter  102 A. 
         [0071]    As discussed above with respect to  FIG. 2B , test transmitter  102 B may include single-sided filter  212  at the output of polarization scrambler  210 . In this embodiment, as shown in  FIG. 3C , single-sided filter  212  passes only one sideband of the modulated signal, which may help remove the effects of chromatic dispersion when recovering the second signal. 
         [0072]    Returning to process  700 , in the configuration of network  1100 , feeding the second signal into the second port (block  716 ) may include first filtering the second signal with single-sided filter  212 . In this embodiment, test receiver  104 A does not include a single sided filter because frequencies other than the single sideband were already filtered out by single-sided filter  212 . 
         [0073]      FIG. 12  is a block diagram of an exemplary network  1200  including a single-sided filter in test receiver  104 B. Like network  800 , network  1200  may include a deployed, in-service network. Also, like network  800 , network  1100  may include some of the same components of network  800 , including ROADMs  808 , amplifiers  814 , fibers  810 , and test transmitter  102 A. Network  1100 , however, includes test receiver  104 B rather than test receiver  104 A. 
         [0074]    In this embodiment, single-sided filter  408  may be added to the input of test receiver  104 B. In this embodiment, single-sided filter  408  may pass only one sideband of the modulated signal, which may help reduce the effects of chromatic dispersion when recovering the second signal. 
         [0075]    Returning to process  700 , in the configuration of network  1200 , receiving the second signal on the fourth port (block  720 ) may include filtering the second signal with single-sided filter  408 . In this embodiment, test transmitter  102 A does not include a single sided filter because frequencies other than the single sideband are filtered out by single-sided filter  408 . 
         [0076]    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. 
         [0077]    Although embodiments described above include deployed, in-service networks, embodiments may also allow for the measurement of PMD and/or DGD in “dark fiber” or out-of-service networks. Embodiments described herein may allow for the accurate measurement of PMD and/or DGD in a timely manner. 
         [0078]    In addition,  FIGS. 2A and 2B  disclose modulator  206  acting on two wavelengths simultaneously in a single fiber after combiner  204  combines signals from first light source  202 - 1  and second light source  202 - 2 . In another embodiment, two modulators may act on the two wavelengths separately (e.g., in separate fibers) without combiner  204  combining the two wavelengths. In this embodiment, the modulators are synchronized themselves so as to create two well synchronized signals. 
         [0079]    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. 
         [0080]    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. 
         [0081]    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. 
         [0082]    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.