Abstract:
A method provides and/or controls an optical signal, wherein a control signal and at least one data signal are optically processed into a combined signal of substantially constant optical power. The level of the at least one data signal is substantially maintained within the combined signal. In addition, an according device is provided. Suitable for compensation of Raman tilt in WDM communication systems.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method and to a device for providing and/or controlling an optical signal. 
     Groups of optical signals with various wavelengths are transmitted via optical fibers utilizing in particular optical transmission amplifiers. 
     In a wavelength division multiplex (WDM) transmission system several data signals (channels) are combined to a channel group. In particular, these channels are amplified together. 
     In optical transmission systems, a particular nonlinearity is known as a Raman gain or Raman effect, which is a nonlinear scattering process and results in a power transfer from shorter wavelength channels to longer wavelength channels. The Raman gain becomes significant when a particular level of optical power distributed over a range of wavelengths is pumped into an optical fiber. In such case, the Raman gain is tilted in favor of the channels having the longer wavelengths. This undesired effect is referred to as Raman tilt. This tilt, described in dB/THz, is in a good approximation proportional to the total optical power fed into the fiber. Fast variation of total optical power results in fast variation of Raman induced tilt and constant total optical power results in constant Raman tilt, which can be simply compensated by tilted amplifiers or filters. 
     If a transmission is conveyed in a C-band at an overall power amounting to 100 mW per amplifier section with channel groups of 80 channels, the so-called Stimulated Raman Scattering (SRS) will result in an attenuation difference amounting of 1 dB. If the optical fiber extends to 10-20 amplifier sections, a signal difference in a wavelength range from 1528 nm to 1565 nm may amount to 10-20 dB. 
     In optical networks with optical add-drop-multiplexers (OADM), due to adding or dropping channels, the Stimulated Raman Scattering (SRS) has an impact on a level of the channels at the end of the optical fiber and hence at the input of a receiver or amplifier. For example, dropping “red” channels or a breach of fiber may result in the “blue” channels having a level beyond the input range of the amplifier. In addition, the transmission fiber may show an increased degree of nonlinear distortion. 
     BRIEF SUMMARY OF THE INVENTION 
     The problem to be solved is to overcome the disadvantages stated before and in particular to provide an efficient approach to compensate detrimental effects in an optical transmission system by enabling a pure optical solution without any electro-optical converter or electronic control circuitry. 
     This problem is solved according to the features of the independent claims. Further embodiments result from the depending claims. 
     In order to overcome this problem, a method for providing and/or controlling an optical signal is suggested,
         wherein a control signal and at least one data signal are optically processed into a combined signal of substantially constant optical power,   wherein the level of the at least one data signal is substantially maintained within the combined signal.       

     This approach allows maintaining a constant power level of the combined signal that may be supplied, e.g., at an output of a device or optical circuit arrangement. 
     Advantageously, said combined signal is processed by optical means, i.e. without any electro-optical converter or electronic control circuitry. 
     In an embodiment, an optical power of the control signal is at least partially supplied by a light source, in particular a laser. 
     In another embodiment, at least one wavelength of the light source is disjoint to at least one wavelength utilized for the at least one data signal. 
     In particular, the data signal may comprise several channels, wherein wavelengths utilized for the data signal are in particular disjoint from the wavelength used by the light source. 
     In a further embodiment, the light source comprises a Raman tilt control laser. 
     In a next embodiment, the optical signal of the light source and the at least one data signal are fed to an optical limiter in order to obtain the combined signal. 
     The optical limiter processes the signals provided in particular by limiting an output power to a predefined threshold “lim”. Below such threshold value the input power may substantially equal the output power of the limiter. 
     It is also an embodiment that the optical signal of the light source and the at least one data signal are fed to the optical limiter via a circulator or via a coupler. 
     Pursuant to another embodiment, the optical limiter comprises a semiconductor optical amplifier (SOA). 
     According to an embodiment, an output of the optical limiter is processed by an absorber and then combined with the data signal into said combined signal. Preferably, a filter may process the input signal of the absorber, wherein said filter may advantageously have a substantially inverse characteristics of the absorber. 
     The absorber may in particular be a saturable absorber. 
     According to another embodiment, the absorber comprises a semiconductor optical amplifier (SOA) or an erbium doped fiber amplifier (EDFA). 
     The EDFA in an absorber may in particular be used to adjust a threshold of its limiting power. 
     In yet another embodiment, the control signal compensates at least partially a Raman tilt and/or a Raman gain. 
     The problem stated above is also solved by a device for controlling an optical signal comprising
         a light source, in particular at least one laser, to at least partially provide an optical power of a control signal;   a limiter fed by the signal from the light source and by a data signal providing an combined signal of substantially constant optical power.       

     According to an embodiment, the data signal is coupled with the output of the limiter to provide said combined signal. 
     According to a next embodiment, the data signal is coupled via an attenuator, in particular via an adjustable attenuator, with the output of the limiter to provide said combined signal. 
     Pursuant to yet an embodiment, the output of the limiter is fed to an absorber, wherein the output of said absorber provides (an optical power of) a control signal portion of the combined signal. 
     As a next embodiment, a filter is provided before the absorber, wherein said filter in particular shows a substantially inverse characteristics of the absorber. 
     It is also an embodiment that a circulator is provided to combine the data signal and the signal from the light source. 
     As an alternative embodiment, a coupler may be provided to combine the data signal and the signal from the light source. 
     Pursuant to an embodiment, the device may be used in or part of an optical transmission system, said system comprising at least one semiconductor optical amplifier (SOA) for suppressing and/or reducing noise and/or interference by cross gain modulation. 
     Furthermore, such device may comprise at least one switch that is based on or realized by at least one semiconductor optical amplifier (SOA). 
     Embodiments of the invention are shown and illustrated in the following figures: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a transfer characteristics of an erbium  3 + doped glass fiber; 
         FIG. 2  shows a quadripole of a limiter comprising two ports; 
         FIG. 3  shows a block diagram of a power control device that allows adjustment of an optical power to provide a substantially constant optical power at its output; 
         FIG. 4  shows a block diagram of an optical power control device deployed within a two-stage optical amplifier; 
         FIG. 5  shows a graph of the power of the Raman tilt control laser as a function of an input signal power in view of an output signal; 
         FIG. 6  shows a block diagram of an optical power control device for two different subbands; 
         FIG. 7A  shows an optical total power control unit with a first path comprising an optical data signal and a control information and a second path comprising the optical data signal only, wherein a laser pumps external optical power to the first path to provide a control signal; 
         FIG. 7B  shows the optical total power control unit of  FIG. 7A  with an attenuator arranged in the second path. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     A semiconductor optical amplifier (SOA) is described in [1]. Such SOA may be utilized as a saturable absorber or as a limiter. Furthermore, an erbium doped fiber amplifier (EDFA) can be used in addition or instead. 
     The approach provided herein suggests an optical solution for a fast control of an optical signal without additional electro-optical converters or additional electronic control circuits. 
     A transparency T of an ideal optical limiter with a limiting power “lim” can be denoted as follows: 
             T   =       P   out       P   in                 with                   T   =   1             for   ⁢           ⁢     P   in       &lt;     lim   ⁢           ⁢   and                             T   =     lim     P   in                   for   ⁢           ⁢     P   in       ≥   lim     ,               
wherein P refers to a power of the optical signal.
 
 P   out   =P   in   ·T  
 
results to
 
                       P   out     =         P   in     ·     lim     P   in         =       lim   ⁢           ⁢   for   ⁢           ⁢     P   in       ≥   lim         ⁢     
     ⁢   and           (   1.1   )                 P   out     =       P   in     ⁢             ⁢             ⁢     otherwise   .               (   1.2   )               
In addition, a saturable absorber can be denoted as
 
 P   out =0 for  P   in &lt;lim abs  and  (2.1)
 
 P   out   =P   in −lim abs  for  P   in &gt;lim abs .  (2.2)
 
     Advantageously, the limiter and/or the absorber are used in order to provide an arrangement with a constant optical output power even if single data signals are dropped:
 
 P   data   +P   control =const.,  (3)
 
with P data  being a power of a data signal comprising several single data signals P λ     k    with k=1, . . . , n, n being a number of channels of the optical band to be controlled, and P control  being a power of a control signal.
 
     In addition, if at least one data signal is dropped, the remaining data signals sustain their respective power level. 
     The requirement set forth by equation (3) can be met by providing at least two signal paths:
     (a) A control path for adjusting the control signal and   (b) a data path to which the adjusted control signal is added.   

     The wavelength utilized by a control laser supplying the optical power to the control path is disjoint to the wavelengths of the data signals (channels). 
     The saturable absorber subtracts a constant amount of light power from the signal and may hence be utilized to remove a constant offset from the signal. This is in particular useful to remove an offset of a control laser that is set to full level. 
     Embodiments without such saturable absorber will be referred to in more detail below. 
     An absorber may in particular be realized by an erbium doped (Er 3+ ) glass fiber allowing for a time delay in a range of 100 s of milliseconds. A transfer characteristics of a typically doped glass fiber used as saturable absorber is shown in  FIG. 1 . 
     It is to be noted that such an erbium doped glass fiber may require a filter put in front of the absorber comprising an inverse characteristics of the saturable absorber. 
     The approach presented advantageously exploits the fact that the transmission characteristics of the optical limiter depends on the input power of the optical signal as follows: 
                     P     out   2       =         P     in   1       ·     lim       P     in   1       +     P     in   2             ⁢             ⁢             ⁢   and             (   4.1   )                 P     out   1       =       P     in   2       ·     lim       P     in   1       +     P     in   2                     (   4.2   )                 FIG. 2  shows the corresponding quadripole of the limiter. However, the indices in equations (4.1) and (4.2) may refer to different directions of a signal or to different wavelengths.
 
     It may in particular be extraneous whether the optical signals towards or from the limiter are co-directional or contra-directional. The limiter may be realized in order to work properly independent from particular wavelengths applied. 
     Equations (4.1) and (4.2) correspond to a term 
             1     1   +   x           
that can be expanded into a series as follows:
 
     
       
         
           
             
               1 
               
                 1 
                 + 
                 x 
               
             
             = 
             
               1 
               - 
               x 
               + 
               
                 x 
                 2 
               
               - 
               
                 x 
                 3 
               
               + 
               
                 
                   x 
                   4 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 … 
               
             
           
         
       
     
     For x&lt;0.3 an error for discontinuing the series after the linear term is below 10%. 
       FIG. 3  shows an block diagram of a device that allows for an adjustment of an optical power to provide a substantially constant optical power at its output. 
     A Raman tilt control laser (RTCL)  301  is attached to a Port  2  of an optical limiter  302 , which has a port Port  1  connected to a circulator  303 . An optical signal  306  is connected via a coupler  308  to the circulator  303  and to a static attenuator  305 . The circulator  303  is further connected to a saturable absorber  304 . The signal output of said saturable absorber  304  and the signal output by the static attenuator  305  are combined by a coupler  309  into an output signal  307 . 
     According to the circulator  303 , the optical signal  306  cannot directly reach the saturable absorber  304 . The optical signal  306  is directed via said circulator  303  to the Port  1  of the optical limiter  302  in order to control the control signal provided by the Raman tilt control laser  301  which is then fed back to the circulator  303  and hence to the saturable absorber  304 . 
     As an alternative for the circulator  303 , a 2×2-coupler may be used. 
     As an example, an optical signal P control     2    provided by the Raman tilt control laser  301  and input to the Port  2  of the optical limiter  302  may amount to
 
 P   control     2   &gt;1.5·max( P   306 ) via a 50:50 coupler;
 
 lim   abs =max( P   306 )
 
and
 
 lim&gt; 2·max( P   306 )
 
with P 306  being the power of the optical input signal  306 . The above exemplary values may result in an substantially constant output signal  307  with an error amounting to less than 10%. Hence, a Raman tilt determined in dB/THz can be reduced by a factor of more than 10.
 
     The arrangement described herein may be deployed directly after a photonic cross-connect either at the origin of an optical section or before a first optical amplifier (booster) a the origin of the optical section. 
     Advantageously, the approach provided herein allows a utilization of optical limiters for a nominal/actual value comparison and hence to optically control the optical power of an optical light source accordingly. 
     The arrangement according to  FIG. 3  may be deployed within a two-stage optical amplifier as depicted in  FIG. 4 . 
       FIG. 4  shows an arrangement  411  that is similar to  FIG. 3 , but without the saturable absorber. 
     Hence, a Raman tilt control laser  401  provides optical power to a fast optical limiter  402  which is further connected to a circulator  403 . The circulator  403  conveys an input signal  406  to the limiter  402  and an signal from the limiter  402  towards an output  407 . The input signal  406  is also fed to the output  407  via an attenuator  405 . 
     An amplifier  408  with constant gain provides the optical signal  406  and the output signal  408  is conveyed to adjustable slow optical components (ASOC)  409 . A signal  412  of constant power is fed to a booster  410 . In particular, a gain flattening filter may be provided as a part of the ASOC  409 . 
       FIG. 5  shows a graph  502  of the power of the Raman tilt control laser  401  as a function of an input signal power  503  in view of the output signal  501  (see signal  407  in  FIG. 4 ). 
     According to  FIG. 5 , the input signal power can be distributed at a ratio of 2:1, i.e. ⅔ of the total power is used for control power and ⅓ of the total power is used for data signals. The input signal  503  varies from a minimum value of (no signal at all) to a maximum value of 1.0. 
     In an example shown in  FIG. 6 , an output signal of an amplifier  601  is fed to a band splitter  604  that separates a red portion  602  and a blue portion  603  from the optical signal. Each portion  602  and  603  is fed to a separate arrangement  605  and  606  which correspond to the arrangement  411  of  FIG. 4 . The output of each arrangement  605  and  606  is fed to a band-combiner  607  and further conveyed to an adjustable slow optical component ASOC  608 . The output signal of the ASOC  608  is of constant power and is conveyed to a booster  610 . 
     Two Raman tilt control lasers are used, each for a subband of the optical transmission band. This allows to sustain not only the Raman tilt but also the Raman gain at a constant level. Furthermore, this approach may be adopted to more than two subbands. The embodiment of  FIG. 6  copes without a saturable absorber. As an option, saturable absorbers may be added according to the embodiment shown in  FIG. 3 . 
     It is in particular possible to provide a more complex arrangement comprising more parallel control paths and hence to obtain the same precision but without providing any saturable absorber. 
       FIG. 7A  shows a different arrangement that in particular utilizes couplers instead of the circulator shown and explained, e.g., in  FIG. 3  above. 
     In  FIG. 7A , a control and signal path is depicted comprising a laser  701 , a coupler  702 , a coupler  703 , a limiter  704 , a filter  705 , an absorber  706  and a coupler  707 . In addition, a signal only path  708  is depicted between the coupler  702  and the coupler  707 . 
     A signal  710  is input to the coupler  702  and conveyed via said signal path  708  to the coupler  707  that produces an output signal  711 . 
     In addition, the signal  710  is also branched off via said coupler  702  to the coupler  703  to which the laser  701  provides a portion of the optical power of the control signal. The coupler  703  conveys the combined signals to the limiter  704 . After said limiter  704  the signal is filtered by the filter  705  to reduce the portion of the original signal  710 . The absorber  706  removes the control offset and provides only the control signal to the coupler  707 . 
       FIG. 7B  is similar to the embodiment of  FIG. 7A . The only difference is that the signal path  708  comprises an attenuator  709  to provide a fine adjustment in order to meet or compensate tolerances of the optimizing limiter  704  and absorber  706 . 
     The output signal  711  comprises a signal portion and a control portion, wherein the power of the output signal  711  is maintained at a constant level. 
     REFERENCE(S) 
     
         
         [1] ROPP C, GOLDHAR J: “Nonlinear Mach-Zehnder Interferometer as a DPSK Signal Regenerator”, http://www.ece.umd.edu/merit/archives/merit2006/merit_fair06_papers/Paper — 11_Ropp.pdf