Patent Publication Number: US-7715092-B2

Title: Dynamic raman tilt compensation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention claims priority from U.S. Patent Application No. 60/827,957 filed Oct. 3, 2006, which is incorporated herein by reference for all purposes. 

   TECHNICAL FIELD 
   The present invention relates to the detection and compensation of dynamic tilt created in optical fiber due to the Raman effect during and after a transient event i.e. when the channel load is variable in time, and in particular to Raman tilt compensation performed by an EDFA. 
   BACKGROUND OF THE INVENTION 
   In a wavelength division multiplexing (WDM) transmission system, various different information channels are encoded, i.e. modulated, into light at different frequencies, i.e. different wavelength channels. Typically continuous wavelength light is generated at a particular frequency, modulated with some kind of modulator, which encodes the information into the light, and then combined with other optical channels at different light frequencies using a multiplexer. The combined light is transmitted through an optical fiber and/or an optical fiber network to a receiver end of the optical fiber. At the receiver end, the signal is separated, i.e. demultiplexed, back into the individual optical channels through a de-multiplexer, whereby each optical channel can be detected by some optical detector, e.g. photo-diode, and the information can reconstructed on a per-channel basis. 
   While propagating through the optical fiber, light tends to loose intensity due to the losses related to the physics of how the light interacts with the optical fiber. Yet some minimal level of optical channel intensity is required at the receiver end in order to decode information encoded on the optical channel. In order to boost the optical signal while propagating in the optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, along the transmission link. The amplifiers extend the maximum possible length of the link, e.g. from a few hundred kilometers to several thousand kilometers, whereby after each fiber span, the optical signal is amplified to power levels close to the original levels at the transmitter. During the amplification process some amount of noise is introduced which prevents links from being of unlimited length. 
   The amplifiers at amplification nodes should similarly amplify all optical wavelength channels, which are propagated in the link; otherwise, some channels will not have sufficient intensity and signal-to-noise level at the receiver end, resulting in information being lost. A typical communication link  5 , schematically illustrated in  FIG. 1 , includes a transmitter  10  for generating the optical wavelength channels and multiplexing the channels into a single WDM signal, and a plurality of spaced apart optical amplifiers  11 , e.g. Erbium Doped Fiber Amplifiers (EDFAs) separated by fiber spans  13 . The number of spans  12  and amplifiers  11  will vary from link to link. The WDM signal is demultiplexed back into wavelength channels and then separately detected at the end of the link  5  at a receiver  13 . 
   Optical fibers in communication links introduce optical dispersion, which has undesirable effects on the performance of the link. Typically, Dispersion Compensation Modules (DCMs) are inserted at amplifier nodes of the link, between stages of EDFAs, in order to compensate the link dispersion and thus to improve the link performance. Moreover, additional optical components, such as add/drops, cross-connects and DGEs (Dynamic Gain Equalizers) may also be inserted in the middle of an amplifier, requiring multiple controlled gain stages in the amplifier to compensate for the loss due to the additional optical components. 
   A multi-stage EDFA  20 , illustrated in  FIG. 2 , comprises first and second controlled gain stages  21  and  22 , and first and second optical amplification sub-stages  23  and  24 . In the first control stage  21   a  first variable optical attenuator (VOA)  31  is situated after the first optical amplification stage  31 , and a second VOA  32  is embedded within the second optical amplification stage  32 . Accordingly, the second VOA  32  is located between EDF (Erbium Doped Fiber) coils that produce amplification. A DCM  38  or other optical device is positioned in between the first and second controlled gain stages  21  and  22 . 
   Portions of the light are deviated from the main optical link by taps  41 ,  42 ,  43  and  44  into photo-detectors  51 ,  52 ,  53  and  54 , respectively, for measuring the light&#39;s power before and after the first and second optical amplification sub-stages  23  and  24 . The information needed for gain control is passed by electrical signals  61 ,  62 ,  63  and  64  from detectors  51 ,  52 ,  53  and  54 , respectively, into a master controller  75 . The detectors  51 ,  52 ,  53  and  54  are calibrated in such a way that an accurate representation of the power at various parts of the amplifier gain stages  21  and  22  can be determined by the measurements performed thereby. 
   The amplifiers  11 , i.e. the first and second controlled gain stages  23  and  24  can be Raman optical amplifiers, distributed or discrete, or a combination of EDFA and Raman amplifiers. During Raman amplification, pump light is launched into the optical fiber via the first and/or second controlled amplifier stages  23  and  24 , and signal amplification occurs in the fiber spans  12 . The pump light can be launched either co-propagating with the WDM signal or counter-propagating therewith. The pump light can consist of multiple wavelengths to achieve desired signal amplification characteristics. The internal portions of each amplifier  11 , such as dispersion compensation module containing long portions of the fiber can also be pumped for Raman amplification. 
   One effect of light propagation through the communication fiber is inter-channel Raman interaction, which manifests as tilt in the transmitted spectra, i.e. the wavelength channels with shorter wavelengths have lower power than the wavelength channels with longer wavelengths, after propagation through the fiber. The spectral tilt depends on both total optical power and wavelength channel distribution. Conventional optical amplifiers  11  have tried to compensate for the Raman spectral tilt effect by introducing a gain tilt of the opposite sign. 
   The communication links described above are so called point-to-point links, in which all information is transmitted from one point only to another point. However, in a realistic transmission system there are multiple points that need to transmit information and multiple points that need to receive information. Different optical channels, which originated at the same transmitter  10 , are required to go to different receivers situated at different locations. Instead of simple point-to-point optical communication links, more complex, network-type or web-type topology is used, in which optical channels are switched from one path to another path at multiple network nodes, which are referred as cross-connect nodes and add/drop nodes. 
   The process of switching the channels at multiple network nodes results in the number of channels passing through each optical amplifiers  11  to vary with time. In order to keep the channel power at the output of each amplifier  11  constant over time, regardless of the number of wavelength channels passing through, the pump power of the first and second controlled amplifier stages  23  and  24  needs to be adjusted to compensate for the changes in the wavelength channel load, which is called “transient control”. Amplifiers  11  with transient control are called either transient controlled amplifiers or gain controlled amplifier, i.e. the control is achieved by monitoring and keeping the average gain of the amplifier constant. Failing to do transient control results in the signal power significantly varying at the receiver  13  over time and over wavelength, which could result in some of the transmitted information being lost. 
   During a transient event, conventional transient controlled amplifiers adjust the pump power in the first and second controlled amplifier stages  23  and  24  to compensate for variations in input signal power by keeping the average amplifier gain constant; however, conventional transient controlled amplifiers do not compensate for Raman tilt variations with time when channel loading and total power is changing. 
   While some Raman tilt compensation techniques have been developed to compensate for different steady state loads, most require measurement of the per channel power by an optical channel monitor (OCM) or other similar device. Due to the relatively long time for OCM devices to perform measurements, accurate tilt compensation is not possible during fast transient events. 
   An object of the present invention is to overcome the shortcomings of the prior art by providing an optical amplifier which compensates for Raman tilt during and after a transient event. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention relates to a method for compensating for Raman tilt in an optical signal, defined by a center wavelength, transmitted in an optical fiber link, which includes an optical amplifier, comprising the steps of: 
   a) tapping off a portion of the optical signal; 
   b) determining a measure of optical power from the tapped off portion; 
   c) determining the Raman tilt from the measure of optical power; and 
   d) adjusting gain provided by the optical amplifier to compensate for the Raman tilt. 
   Another aspect of the present invention relates to an optical amplifier device for compensating for Raman tilt in an optical signal, defined by a center wavelength, transmitted in an optical fiber link, which includes an optical amplifier, comprising: 
   a tap for separating a portion of the optical signal; 
   a photo-detector for determining a measure of optical power from the separated portion; 
   a controller for determining the Raman tilt based on the measure of optical power; and 
   an adjustable optical amplifier controlled by the controller for amplifying the optical signals to compensate for the Raman tilt. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
       FIG. 1 , illustrates a conventional point-to-point optical communication link; 
       FIG. 2  illustrates a conventional optical amplifier with mid-stage access; 
       FIG. 3  illustrates a optical amplifier in accordance with the present invention with mid-stage access; 
       FIG. 4  illustrates a Raman gain curve created by a single channel in C-band; 
       FIG. 5  illustrates a simulation of Raman tilt with different channel loads; 
       FIGS. 6   a  to  6   c  illustrate examples of signal measurement systems for the estimation of the necessary Raman tilt correction; and 
       FIGS. 7   a  to  7   d  illustrate filter transmission functions used for extraction of spectral information for Raman tilt compensation calculation. 
   

   DETAILED DESCRIPTION 
   With reference to  FIG. 3 , a first embodiment of the present invention is based on a two stage amplifier  80  comprising first and second controlled gain stages  81  and  82 , and first and second optical amplification sub-stages  83  and  84 , as in the conventional amplifier  11 . In the first control stage  81   a  first variable optical attenuator (VOA)  85   a  is situated after the first optical amplification stage  81 , and a second VOA  85   b  is embedded within the second optical amplification stage  82 . Accordingly, the second VOA  82  is located between EDF (Erbium Doped Fiber) coils that produce amplification. A dispersion compensation module DCM  86  or other optical device is positioned in between the first and second controlled gain stages  81  and  82 , as hereinbefore discussed. 
   Portions of the light are deviated from the main optical link by taps  91 ,  92 ,  93  and  94  into photo-detectors  101 ,  102 ,  103  and  104 , respectively, for measuring the light&#39;s power before and after the first and second optical amplification sub-stages  83  and  84 . The information needed for gain control is passed by electrical signals  111 ,  112 ,  113  and  114  from detectors  101 ,  102 ,  103  and  104 , respectively, into a master controller  125 . The detectors  101 ,  102 ,  103  and  104  are calibrated in such a way that an accurate representation of the power at various parts of the amplifier gain stages  81  and  82  can be determined by the measurements performed thereby. 
   The first and second controlled gain stages  83  and  84  can be Raman optical amplifiers, distributed or discrete, or a combination of EDFA and Raman amplifiers. During Raman amplification, pump light is launched into the optical fiber via the first and/or second controlled amplifier stages  83  and  84 , and signal amplification occurs in the fiber spans  12 . The pump light can be launched either co-propagating with the WDM signal or counter-propagating therewith. The pump light can consist of multiple wavelengths to achieve desired signal amplification characteristics. The internal portions of each amplifier  80 , such as the dispersion compensation module  86  containing long portions of the fiber can also be pumped for Raman amplification. Alternatively, the first and second controlled gain stages  83  and  84  are EDFA&#39;s and the DCM  86  is replaced by a Raman optical amplifier (ROA). 
   In a steady state situation, i.e. when the input channel load is constant, the first and second controlled amplifier stages  83  and  84 , (and the ROA  86 , if provided) of the amplifier  80  are set into a particular gain level via pump control through communication lines  130   a ,  130   b  and  130   c , respectively, and by control of the first and second VOAs  85   a  and  85   b  via communication lines  135   a  and  135   b , respectively. The gain of each amplifier stage is obtained from the powers measured by detectors  101 ,  102 ,  103  and  104 . 
   A typical goal of the transient control of the amplifier  80  is to hold the average signal gain constant, by adjusting the pump powers in such a way that the measured gain of each stage  81  and  82  (and  86 , if an ROA) is also constant, which is done through some kind of feed-back control by controller  125 . Some other additional types of control, such as feed-forward control, can be employed in combination with feed-back control. 
   Understanding how tilt is formed is important in order to understand what parameters of the signal need to be measured for the fast compensation of Raman tilt. When multiple wavelength channels propagate through the communication fiber, each channel creates wavelength dependent gain or loss to other channels. An example of the gain shape created by a single channel is shown in  FIG. 4 . Each wavelength channel amplifies the channels with longer wavelengths and de-amplifies or is amplified itself by the channels with shorter wavelengths, resulting in the channel spectrum being tilted after propagation through the fiber. 
     FIG. 4  illustrates that the Raman gain G R  can be approximated either by a linear function, or by a polynomial, e.g. cubic, function. The cubic function provides an excellent approximation of the Raman gain when the wavelengths differ by less than or about 35 nm, which is the width of C-band. The approximation of the Raman gain by the cubic approximation is as follows:
   G   R (λ)= a·λ+b·λ   3   (1) 
   The total gain G of all channels acting upon themselves is: 
   
     
       
         
             
           
             
               
                 
                   
                     
                       
                         
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   λ 0  is an arbitrary wavelength that is introduced for convenience; which can be put into the center of the band. The linear tilt is defined by the last term 
             (     λ   -     λ   0       )     ·     [       a   ⁢       ∑     λ   ′       ⁢     P   ⁡     (     λ   ′     )           +     3   ⁢   b   ⁢       ∑     λ   ′       ⁢       P   ⁡     (     λ   ′     )       ·       (       λ   ′     -     λ   0       )     2             ]           
which includes multiplication by the linear term (λ−λ 0 ). The rest of the terms do not have linear dependence on (λ−λ 0 ). The value of the linear tilt T is proportional to the expression in the square brackets:
 
   
     
       
         
           
             
               
                 
                   
                     
                       
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   Thus the Raman tilt is the sum of two terms, the first term is proportional to the total power P total  and the second term is proportional to the second moment P 2 . 
   The total power P total  term is a numerically larger term than the second moment term P 2 , whereby the total power can be used for a rough approximation of the Raman induced tilt.  FIG. 5  illustrates the accuracy of such approximation. In the illustrated simulations the total power at the input of the optical fiber is the same, but the channel load, i.e. the number of channels, has changed. Namely, the channels were gradually removed starting from the edges of the spectrum in one case, and from the center of the spectrum in another case. The channel power is adjusted in order to have total power the same for all cases. Due to the fact that total power is the same in all simulated cases, all differences in the tilt value for  FIG. 5  are coming from the terms other than the total power related term, i.e. the second moment term P 2 . 
   There are a plurality of means and methods for estimating the required tilt correction. Preferably, the tap  94  and the detector  104  (hereinafter referred to as tap/detector module) shown in  FIG. 3 , are replaced by one of the tap/detector modules shown in  FIG. 6   a  to  6   c . However, any tap/detector module in the amplifier  80  can be substituted and used for the tilt compensation estimation. Some additional calculations may be needed if other output tap/detector modules are used for tilt compensation estimation. The tap/detector module, illustrated in  FIG. 6   a , includes the tap  94  and the detector  104 , i.e. identical to any tap/detector module in  FIG. 3 , and relies on the total power value only. 
   A general topology of a tap/detector module in accordance with a preferred embodiment of the present invention is shown in  FIG. 6   b . A portion of the light, deviated by the tap  94 , is split into N optical paths by 1×N splitter  201 , wherein N is equal to one, two or more. Each optical path has a respective optical filter  202   a  to  202   n , each of which filters some portion of the light, before the light comes to the detectors  203   a  to  203   n . One of the filters may be eliminated to provide the corresponding detector with an optical power measurement proportional to the total optical power of the optical signal. 
   A practical topology, in which both P total  and P 2  values are estimated by the tap/detector module, is illustrated in  FIG. 6   c , in which a 1×2 splitter  211  is used to split the portion of the light tapped off by tap  94  into two paths. The total signal power P total  is estimated by a first detector  212 , while the second moment term P 2  is estimated using a second detector  213 , which receives the light after the light has been filtered by an optical filter  214 . 
   Various possible examples of the transmission function for the filters  202   a  to  202   n  and  214  are shown in  FIGS. 7   a  to  7   d .  FIG. 7   a  illustrates the transmission function of the filter  214 , which enables the detector  213  to measure light power directly proportional to the second moment P 2  of equation 3.  FIG. 7   b  illustrates an alternative transmission function for the filter  214 , which is the inverse to filter  214  shown in  FIG. 7   a , but can be used to calculate the value of P 2  for use in measurement of the light power by the detector  213 . 
   Generally, the filter  214  blocks all of center wavelength&#39;s λ 0  optical power and the optical power of wavelength&#39;s on either side thereof, and gradually decreasingly less amounts of the optical power of the wavelengths on either side of the center wavelength, as the wavelengths get farther away from the center wavelength λ 0 . With reference to  FIGS. 7   a  and  7   b , the transmission function of the filter is T(λ)=c 1 +c 2 ·(λ−λ 0 ) 2  in which c 1 , c 2  and λ 0  are constants based on the filter. When c 1 =0 and c 2 &gt;0, the transmission function is parabola shaped, illustrated in  FIG. 7   a . When c 1 &gt;0 and c 2 &lt;0, the transmission function is inverted parabola shaped, illustrated in  FIG. 7   b.    
     FIGS. 7   c  and  7   d  illustrate alternative transmission functions for the filters  202  and  214 , which are only linear approximations, e.g. trapezoidal-shaped, of the above transmission functions illustrated in  FIGS. 7   a  and  7   b , respectively, whereby the value of P 2  obtained with these filters is less precise than the values obtained with the filters detailed above. The possible reason for the use of filters  7   c  and  7   d  is the ease of their manufacturing. 
   Once the values corresponding to P total  alone, or corresponding to P total  and P 2  are obtained, the calculation of tilt T becomes possible with known constants a and b according to the equation 3. The constants a and b can be provided to the amplifier  80  by an external control system of the service provider or alternatively, a set of a and b coefficients can be measured in advance for all possible fibers that are used in optical communication systems and the external system and memorized in the amplifier controller  125 . The external system then provides only the type of the fiber for which amplifier controller selects the appropriate a and b coefficients. The coefficients a and b are on the order of a=0.3 dB/(W*nm) and b=0.0001 dB/(W*nm 3 ). 
   After the tilt value T is calculated, the appropriate correction to the gains of the first and second controlled amplifier stages  83  and  84  (and the ROA  86 , if provided) and the first and second VOA&#39;s  85   a  and  85   b  should be applied from the controller  125  via communication lines  130   a ,  130   b  and  130   c . Typically the best results in terms of amplifier performance are obtained when all gains and VOA values are modified. But for practical reasons some of the gains and VOA values can be kept constant and others are modified. After the gain and VOA values corrections are calculated, the controller  125  adjusts the gain targets and the loss targets in the EDFA control algorithms. Due to the fact that the Raman tilt change occurs at the same time when the channels are added and dropped, the amplifier  80  reacts on the change of the input power at the same time as it reacts on the command to change VOA values and stage gain values for Raman tilt compensation. The control loop and the reaction time for the VOA&#39;s  85   a  and  85   b , should be sufficiently fast, so that by the time the transient adjustments of the pumps in the EDFA&#39;s  83  and  84  are done, the VOA adjustment is also finished. 
   
     
       
         
             
             
             
             
             
             
           
             
                 
                 
             
             
                 
               Tilt T 
               ΔG1 
               ΔG2 
               ΔVOA1 
               ΔVOA2 
             
             
                 
                 
             
           
          
             
                 
               Tilt 
                 
                 
                 
                 
             
             
                 
               Value 1 
             
             
                 
               Tilt 
             
             
                 
               Value 2 
             
             
                 
               . . . 
             
             
                 
               Tilt 
             
             
                 
               Value N 
             
             
                 
                 
             
          
         
       
     
   
   Table 1 defines stage gains (ΔG 1  and ΔG 2 ) for the first and second controlled amplifier stages  83  and  84 , and VOA loss values (ΔVOA 1  and ΔVOA 2 ) for the first and second VOAs  84  and  85  for various Raman tilt values T for compensation of the Raman tilt for the preferred embodiment of a two stage amplifier, e.g. EDFA. During the amplifier&#39;s design process, Table 1 is filled with the data, which is later stored in the amplifier controller memory  125 . During a transient event when the tilt value is calculated, the amplifier control  125  finds the appropriate line in Table 1, and adjusts the stage gains and VOA loss values accordingly. If particular tilt values are not in the table, then the amplifier controller  125  can calculate the adjustments by interpolating values from neighboring lines. 
   In an alternate embodiment of the present invention, the Raman tilt is compensated by Raman amplifiers present in the system, not by EDFA. Compensation is achieved by adjusting the pump powers in the Raman amplifiers at different pump wavelengths in such a manner that the Raman gain tilt produced by the pump lights changes. A feedback loop can be provided to ensure complete compensation or a table of known Raman tilt and Raman amplifier settings can be accessed by the controller  125 . If at the same time EDFAs are present in the system, i.e. a hybrid Raman/EDFA system as in  FIG. 3  in which DCM  86  is replaced by a ROA or if a ROA is placed before the first EDFA stage, then EDFA and ROA gain may also need to be adjusted by the controller  125 , via communication lines  135   a ,  135   b  and  135   c  to work in combination with the Raman tilt adjustment. In all cases the measurement of the necessary adjustment is obtained by one of the tap/detector modules show in  FIG. 6   a  to  6   c , which can be positioned anywhere in the link. Raman amplifiers can adjust tilt relatively fast, whereby the adjustments can be done during a transient event. Again, a table of Raman tilt values and predetermined gain and attenuation settings, as disclosed above, can be utilized by the controller  125  to control any one or more of the amplifier and attenuation stages. 
   Special tilt compensation devices, which canc  have variable tilt in the loss spectrum thereof, can also be deployed in the link. The special device, which are fast enough, also can be used for dynamic tilt compensation to change tilt values during the transient event.