Abstract:
Techniques, devices and applications are provided for monitoring a polarization mode dispersion (PMD) effect in an optical signal.

Description:
PRIORITY CLAIM 
     This document claims the benefits of U.S. Provisional Patent Application Ser. No. 61/026,053 entitled “REAL TIME PMD &amp; SNR MONITORING BASED ON FAST POLARIZATION ANALYSIS” and filed on Feb. 4, 2008, the entire disclosure of which is incorporated by reference as part of the disclosure of this document. 
    
    
     BACKGROUND 
     This application relates to techniques, apparatus and systems for measuring signal properties of an optical signal, including optical polarization, polarization mode dispersion and signal-to-noise ratio. 
     Optical polarization is an important parameter of an optical signal in various optical systems. For example, in fiber optic communication systems, polarization-dependent effects in fibers and other devices, such as polarization-dependent loss (PDL) and polarization-mode dispersion (PMD), can have significant impacts on performance and proper operations of optical devices or systems. Hence, it may be desirable to measure and monitor the state of the polarization (SOP) and the degree of polarization (DOP) of an optical signal in these and other systems. 
     Optical signal-to-noise ratio (SNR) and differential group delay (DGD) of an optical signal are also important parameters for various optical devices and systems and hence monitoring of these parameters may be desirable under certain circumstances. For example, as the speed of optical wavelength division multiplexed (WDM) communication network increases to 40 Gb/s and beyond, the PMD effect of an optical transmission link need be measured and monitored to ensure transmission quality. 
     SUMMARY 
     Techniques, devices and applications are provided for monitoring a polarization mode dispersion (PMD) effect in an optical signal. In one implementation, a method for monitoring a PMD effect in an optical signal includes splitting an input optical signal with an input optical bandwidth into a first optical signal with a first optical bandwidth less than the input optical bandwidth and a second optical signal with a second optical bandwidth different from the first optical bandwidth; measuring a degree of polarization of the first optical signal and a degree of polarization of the second optical signal, respectively; and processing the measured degree of polarization of the first optical signal and the measured degree of polarization of the second optical signal to determine a depolarization factor of the input optical signal to reflect the PMD effect in the input optical signal and to determine an optical to signal noise ratio of the input optical signal. 
     In another implementation, a device for monitoring a PMD effect in an optical signal includes an optical coupler to split an input optical signal into a first optical signal in a first optical path and a second optical signal in a second, separate optical path; a first optical filter in the first optical path to filter the first optical signal to have a first optical bandwidth that is less than an input optical bandwidth of the input optical signal and is different from a second optical bandwidth of the second optical signal; a first optical detector coupled to the first optical path to measure a degree of polarization of the first optical signal after being filtered by the first optical filter; a second optical detector in the second optical path to measure a degree of polarization of the second optical signal; and a processing unit in communication with the first and second optical detectors to receive outputs of the first and second optical detectors. The processing unit processes the measured degree of polarization of the first optical signal and the measured degree of polarization of the second optical signal to determine a depolarization factor of the input optical signal to reflect the PMD effect in the input optical signal and to determine an optical to signal noise ratio of the input optical signal. 
     In yet another implementation, a device for monitoring a PMD effect in an optical signal includes a first optical path that transmits light; a second optical path that is separate from the first optical path and transmits light; a first optical switch coupled to a first end of the first optical path and a first end of the second optical path to switch an input optical signal received by the first optical switch into either the first optical path as a first optical signal or the second optical path as a second optical signal; a first optical filter in the first optical path to filter the first optical signal to have a first optical bandwidth that is less than an input optical bandwidth of the input optical signal received by the first optical switch and is different from a second optical bandwidth of the second optical signal; a second optical switch coupled to a second end of the first optical path and a second end of the second optical path to receive the first and second optical signals and to select either the first optical signal in the first optical path or the second optical signal in the second optical path as an optical output of the second optical switch; an optical detector coupled to receive the optical output of the second optical switch to measure a degree of polarization of the received optical output; and a processing unit in communication with the optical detector to receive output of the optical detector. The processing unit processes the measured degree of polarization of the first optical signal and the measured degree of polarization of the second optical signal to determine a depolarization factor of the input optical signal to reflect the PMD effect in the input optical signal and to determine an optical to signal noise ratio of the input optical signal. 
     These and other implementations of polarization stable lasers, the associated techniques and their application are described in greater detail in the drawings, the description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows one example of a PMD monitoring device based on measurements of the degree of polarization. 
         FIG. 2  shows spectra of optical signals in the device in  FIG. 1  to illustrate one aspect of the operations of the device in  FIG. 1 . 
         FIG. 3  shows another example of a PMD monitoring device based on measurements of the degree of polarization. 
     
    
    
     DETAILED DESCRIPTION 
     This application describes examples for implementing PMD monitoring based on measuring the degree of polarization of the optical signal in an optical system. 
       FIG. 1  shows one exemplary design for a PMD monitoring device. This PMD monitor includes an input tunable optical bandpass filter  110  to receive an input optical signal  101 , an optical splitter  120  (e.g., a 1×2 coupler) connected downstream from the input tunable filter  110  to split the received light in the input signal  101  into two optical signals along two optical paths  121  and  122 , respectively. A first optical bandpass filter  123 , e.g., a Fabry-Perot (F-P) filter, has a transmission bandwidth smaller than that of the input tunable filter  110  and is placed in the first optical path  121  connected to one of the coupler output port (port  1  in  FIG. 1 ) to filter the optical signal in the first optical path  121 . A polarization detector  131  is placed at the end of the first optical path  121  to measure the polarization and the degree of polarization (DOP) of the filtered optical signal in the first optical path  121 . Another polarization detector  132  is placed at the end of the second optical path  122  to measure the polarization and the degree of polarization (DOP) of the optical signal in the second optical path  122 . The optical signals in the two optical paths  121  and  122  that reach the two respective detectors  131  and  132  have different signal spectral bandwidths. This can be achieved by placing the first optical bandpass filter  123  in the first optical path  121  while having no optical filter in the second optical path  122  or a second optical bandpass filter that has a transmission bandwidth different from that of the first optical bandpass filter  123 .  FIG. 1  shows this optional second optical bandpass filter  124  (e.g., a F-P filter) of a different bandwidth from the first optical bandpass filter  123  connected downstream from the other port of the coupler  120  in the second optical path  122 . A processing module  140 , which may be a digital processor or a microprocessor, is used to receive and process the measurements from the two detectors  131  and  132  and to produce the PMD and SNR measurements. 
     In measuring optical communication signals, the input tunable filter  110  can be calibrated against the WDM ITU grid under the WDM standards by the International Telecommunications Union (ITU) to align its center transmission frequency at an ITU WDM channel frequency to select one optical WDM channel in the input signal  101  at a time. Hence, the input tunable filter  110  is used to sequentially direct different optical WDM channels in the input signal  101  into the device for monitoring their PMD, one channel at a time. The transmission peak of the filter  123  or  124  in each of the two optical paths  121  and  122  is set at the WDM frequency of the selected optical WDM channel to filter the selected optical WDM channel. In one implementation, a filter for the filter  123  or  124  in each of the two optical paths  121  and  122  can be a tunable filter that tunes in synchronization with the tuning of the input filter  110 . Alternatively, each of the first and second optical bandpass filters  123  and  124  can be a fixed filter that has multiple transmission peaks that respectively coincide with a set of adjacent WDM frequencies on the ITU grid of WDM systems and the free spectral range (FSR) or spacing between two adjacent transmission peaks each of the first and second optical bandpass filters  123  and  124  is set to be either the same as the channel spacing of the WDM system or less than the channel spacing by an integer fraction of the channel spacing so that the a WDM channel on the ITU grid can pass through each of the first and second optical bandpass filters  123  and  124 . For example, if the channel spacing is 100 GHz, the free spectral range can be either 100 GHz or 50 GHz. For example, such a fixed filter for implementing the first and second optical bandpass filters  123  and  124  can be a fixed Fabry-Perot filter that has resonance transmission peaks that respectively coincide with the WDM frequencies on the ITU grid. As the input optical filter  110  is tuned to select a WDM channel from the input signal  101 , the selected WDM channel coincides with one of the resonance transmission peaks of the filter  123  or  124 , without tuning the filter  123  or  124 , and thus transmits through the filter  123  or  124  after being filtered. This use of a fixed Fabry-Perot filter as the filter  123  or  124  can reduce the cost of the device in comparison with the design where a tunable filter is used to implement the filter  123  or  124 . 
     The device in  FIG. 1  is configured to make the transmission bandwidths of the optical signals in the two optical paths  121  and  122  sufficiently different from each other.  FIG. 2  shows an example of the spectra of the two optical signals in the two optical paths  121  and  122 . Hence, when both the first and second optical bandpass filters  123  and  124  are implemented, the transmission bandwidths of the optical bandpass filters  123  and  124  are set to be sufficiently different from each other and at least one of the optical bandpass filters  123  and  124  has a bandwidth less than that of the input tunable optical filter  110 . In a configuration where the optional second optical bandpass filter  124  is not connected in the second optical path  122 , the filter transmission function and the bandwidth in the second optical path  122  connected to the second port of the optical coupler  120  are effectively the same as those of the input tunable filter  110  and the first optical bandpass filter  123  is configured to have a bandwidth less than that of the input tunable filter  110 . The polarization detector in each optical path can be a polarimeter or another device capable of measuring the DOP of light and thus is a DOP meter. 
     The operation of the device in  FIG. 1  can be understood from the following analysis of processing for obtaining the PMD and SNR measurements from the output of the two detectors  131  and  132 . The DOP of a light beam can be expressed as: 
                     DOP   =       P   pol       P   T         ,           (   1   )               
where P pol  is the power of the polarized portion and P T  is the total optical power. The total power P T  received by each DOP meter  131  or  132  has two signal components: the signal power P s  and the amplified spontaneous emission (ASE) power P ASEi :
 
 P   Ti   =P   ASEi   +P   s   (2)
 
where i=1, 2 for the two detectors  131  and  132  in the optical paths  121  and  122 , respectively. Let ρ(ν) be the ASE power density of light after the input tunable filter  110 , the ASE powers after the two bandpass filters  123  and  124  are:
 
                       P   ASEi     =         ∫     -   ∞     ∞     ⁢       ρ   ⁡     (   v   )       ⁢       f   i     ⁡     (   v   )       ⁢     ⅆ   v         =       ρ   _     ⁢   Δ   ⁢           ⁢     v   i           ,           (   3   )               
where P ASEi  is the ASE power in port i and  ρ  is the average ASE power density, f i (ν) and Δν i  are the transmission function and bandwidth of the respective optical bandpass filter  123  or  124  in port i. The total power in port i is:
 
 P   Ti   =P   ASEi   +P   s =  ρ Δν i   (4)
 
     The total signal power of the input signal  101  has two portions based on the optical polarization state: the polarized portion (P pol ) and the depolarized portion (P dpol ):
 
 P   s   =P   pol   +P   dpol =(1−α) P   s   +αP   s   (5)
 
 P   pol =(1−α) P   s   (6)
 
 P   dpol   =αP   s   (7)
 
where α is the depolarization factor that characterizes the depolarization caused by PMD and other nonlinear effects. The depolarization caused by nonlinear effects tends to be much smaller than that caused by PMD in many practical systems and thus the PMD caused depolarization is dominant and is reflected by the depolarization factor α. Hence, the DOP of light is affected by the noise in the light and the depolarization of the light.
 
     The two optical paths  121  and  122  are designed to have different optical transmission bandwidths to obtain DOP parameters under two different signal transmission bandwidths. The DOP parameters of the two optical signals in the optical paths  121  and  122  into the two DOP meters  131  and  132  in  FIG. 1  are respectively given by: 
     
       
         
           
             
               
                 
                   
                     DOP 
                     1 
                   
                   = 
                   
                     
                       
                         P 
                         pol 
                       
                       
                         
                           P 
                           
                             ASE 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                         + 
                         
                           P 
                           s 
                         
                       
                     
                     = 
                     
                       
                         
                           ( 
                           
                             1 
                             - 
                             α 
                           
                           ) 
                         
                         ⁢ 
                         
                           P 
                           s 
                         
                       
                       
                         
                           
                             ρ 
                             _ 
                           
                           ⁢ 
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             v 
                             1 
                           
                         
                         + 
                         
                           P 
                           s 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     DOP 
                     2 
                   
                   = 
                   
                     
                       
                         P 
                         pol 
                       
                       
                         
                           P 
                           
                             ASE 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                         + 
                         
                           P 
                           s 
                         
                       
                     
                     = 
                     
                       
                         
                           ( 
                           
                             1 
                             - 
                             α 
                           
                           ) 
                         
                         ⁢ 
                         
                           P 
                           s 
                         
                       
                       
                         
                           
                             ρ 
                             _ 
                           
                           ⁢ 
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             v 
                             2 
                           
                         
                         + 
                         
                           P 
                           s 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Solving Eqs. (8) and (9) yields: 
     
       
         
           
             
               
                 
                   α 
                   = 
                   
                     
                       1 
                       - 
                       
                         
                           
                             ( 
                             
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   v 
                                   1 
                                 
                               
                               - 
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   v 
                                   2 
                                 
                               
                             
                             ) 
                           
                           ⁢ 
                           
                             DOP 
                             1 
                           
                           ⁢ 
                           
                             DOP 
                             2 
                           
                         
                         
                           
                             
                               DOP 
                               1 
                             
                             ⁢ 
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               v 
                               1 
                             
                           
                           - 
                           
                             
                               DOP 
                               2 
                             
                             ⁢ 
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               v 
                               2 
                             
                           
                         
                       
                     
                     = 
                     
                       1 
                       - 
                       
                         
                           
                             ( 
                             
                               1 
                               - 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     v 
                                     2 
                                   
                                 
                                 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     v 
                                     1 
                                   
                                 
                               
                             
                             ) 
                           
                           ⁢ 
                           
                             DOP 
                             2 
                           
                         
                         
                           1 
                           - 
                           
                             
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   v 
                                   2 
                                 
                               
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   v 
                                   1 
                                 
                               
                             
                             ⁢ 
                             
                               
                                 DOP 
                                 2 
                               
                               
                                 DOP 
                                 1 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Where Δν 1  is not equal to Δν 2 . When the device in  FIG. 1  does not use the optional optical bandpass filter  124  in the second optical path  122 , the signal bandwidth Δν 2  of the signal reaching the detector  132  is the signal bandwidth of the bandwidth Δν t  of the input tunable filter  110  and the Δν 1  is different from Δν t . This ensures that the signal transmission bandwidths of the two signal paths  121  and  122  are different. 
     From Eq. (8) and Eq. (9), the signal power of the input signal  101  is given by: 
     
       
         
           
             
               
                 
                   
                     P 
                     s 
                   
                   = 
                   
                     
                       
                         DOP 
                         i 
                       
                       ⁢ 
                       
                         P 
                         ASEi 
                       
                     
                     
                       ( 
                       
                         1 
                         - 
                         α 
                         - 
                         
                           DOP 
                           i 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     Based on the above, the signal to noise ratio (SNR) of the input signal  101  with respect to the bandwidth of the ith optical bandpass filter is: 
     
       
         
           
             
               
                 
                   
                     SNR 
                     i 
                   
                   = 
                   
                     
                       
                         P 
                         s 
                       
                       
                         P 
                         ASEi 
                       
                     
                     = 
                     
                       
                         DOP 
                         i 
                       
                       
                         1 
                         - 
                         α 
                         - 
                         
                           DOP 
                           i 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     In practice, the signal to noise ratio can be calculated from the noise in a spectral bandwidth of 0.1 nm. Therefore, the signal to noise ratio of the system is: 
     
       
         
           
             
               
                 
                   SNR 
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           v 
                           i 
                         
                       
                       0.1 
                     
                     ⁢ 
                     
                       SNR 
                       i 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     where Δν i  is expressed in nm. 
     As shown in Eq. (11)-Eq. (13), the apparatus in  FIG. 1  can be used to simultaneously monitor the SNR and depolarization caused by PMD effect. Because the polarimeter or DOP meter ( 131  or  132 ) can also measure the power after the tunable optical filter  110 , the optical power of each channel signal selected by the tunable optical filter  110  can also be determined. 
     Notably, the value of the depolarization factor α of the input signal  101  in  FIG. 1  depends the total PMD of the system and the relative orientation of the polarization with respect to the principle state of polarization (PSP) of the fiber link. The depolarization factor α, therefore, represents the total effect of PMD on the communication link. Because the polarization state can vary rapidly with time, e.g., faster than the PMD variation in general, in optical systems, a may vary accordingly. The difference between the maximum and minimum values of α is indicative of the total PMD. Therefore, the PMD value of the system can be estimated by obtaining the amplitude of the variation in α over a certain period of time, provided that the polarization variation during the period evenly covers the Poincare Sphere. 
     The device in  FIG. 1  implements parallel processing of two signals filtered out of the input signal  101  with different bandwidths by using two DOP meters  131  and  132 . Other  FIG. 3  shows an example of a “sequential” monitoring device based on the similar mechanism used in  FIG. 1 . In  FIG. 3 , the optical coupler  120  in  FIG. 1  is replaced by a first optical 1×2 switch  310  connected downstream from the tunable optical filter  110 , and a second 1×2 optical switch  320  is connected to the ends of the two optical paths  121  and  122  to combine the optical signals from the two optical paths  121  and  122  into a combined optical signal. A single DOP meter  131  is coupled downstream from the second optical switch  320  to perform DOP measurements. 
     In operation, the first and second optical switches  310  and  320  at the beginning and end of the two optical paths  121  and  122  are synchronized in two switching states where, in the first switching state, the switches  310  direct light into the first optical path  121  and then to the DOP meter  131  and the second optical path  122  is not used to route light; in the second switching state, the switches  310  and  320  direct light into the second optical path  122  and to the DOP meter  131  and the first optical path  121  is not used to route light. Hence, the DOP measurements on the two optical signals in two optical paths  121  and  122  are measured by the single DOP meter  131  in sequence at different times. This configuration works when the polarization change in the system is slower than the switching speed of the switches  310  and  320 . Other aspects of the processing and operations of the device in  FIG. 3  are similar to the device in  FIG. 1 . 
     Therefore, the above described techniques and devices in  FIGS. 1-3  can be used to simultaneously monitor SNR, PMD effect via the depolarization factor and the channel power. Such simultaneous measurements can be useful in optical communications. For example, when a system experiences an outage, the network operator can analyze the measurements of SNR, PMD and channel power obtained from a monitoring device based on the present techniques and identify the problem which can be caused by a severe PMD effect, a low channel power, or a unacceptably low SNR. 
     While this document contains many specifics, these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Only a few implementations are disclosed. Variations and enhancements of the disclosed implementations and other implementations can be made based on what is described and illustrated in this document.