Abstract:
The present invention provides an all optical system for correcting optical dispersions including at least one optical chopping device having an input terminal for receiving a first signal, which has been broadened by optical dispersions and corresponds to an optical information channel, and at least one output terminal, wherein the optical chopping device is arranged to produce in the at least one output a second signal that is narrower than the first signal. The second signal may be detectable more reliably than the first signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/467,563, filed May 5, 2003, entitled “All Optical Chromatic and Polarization Mode Dispersion Correctors”.  
         [0002]    In addition this application is a Continuation-In-Part of U.S. patent application Ser. No. 10/472,244, filed Sep. 22, 2003, entitled “Optical Pulse Chopper”, which is a National Phase of PCT International Application PCT/US02/09969, filed Mar. 28, 2002, entitled “Optical Pulse Chopper”.  
         [0003]    In addition this application is a Continuation-In-Part of U.S. patent application Ser. No. 10/826,363, filed Apr. 19, 2004, entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method”, which claims the benefit of U.S. Provisional Patent Application Serial No. 60/464,351, filed Apr. 22, 2003, entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method”.  
         [0004]    In addition this application is a Continuation-In-Part of U.S. patent application Ser. No. 10/827,314, filed Apr. 20, 2004, entitled “All Optical Chopping Using Logic Gates Apparatus And Method”, which claims the benefit of U.S. Provisional Patent Application Serial No. 60/465,237, filed Apr. 25, 2003, entitled “All Optical Chopping Using Logic Gates Apparatus And Method”.  
     
    
     
       FIELD OF THE INVENTION  
         [0005]    The invention relates to optical shaping, optical reshaping, optical communication devices and systems, in particularly to optical shapers and choppers for all-optical corrections and compensations of Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD).  
         BACKGROUND OF THE INVENTION  
         [0006]    In the field of optical communication there is a strong demand for optical shaping and reshaping of optical signals to achieve high speed transmission of optical information at a very high quality and very low Bit Error Rate (BER).  
           [0007]    The implementation of ultra fast optical communication network faces, among other challenges, a need to maintain high quality optical signals along significant distances for keeping very low BER. At high transmission rates, the pulse quality of the optical pulses degrades very rapidly in a relatively short distance due to pulse broadening caused by CD and PMD.  
           [0008]    The broadening phenomenon of optical pulses, in radiation guides, caused by CD is due to the dependency of the propagation speed on the wavelength. The longer is the wavelength the faster is the propagation speed. The optical pulses have a spectral width Δλ of wavelengths-around the central peak wavelength λ center . Each wavelength in the spectrum of the wavelengths, related to the optical pulses, propagates at different speed, resulting with pulses broadening. The amount of broadening depends linearly on the traveling length L, the relation between the wavelength and the refractive index described by the slope factor K, and the spectral width Δλ of the pulses.  
           [0009]    The broadening process of optical pulses in radiation guides caused by PMD is due to the dependency of the propagation speed on the polarization orientation. Due to production imperfections, the optical fibers have fast and slow propagation axes that are orthogonal. The polarization vector of the optical pulses may have components along the fast and the slow axes. Accordingly, each component of the polarization vector propagates at different velocity and results with broadening of the pulses. In addition, temporal environmental influences may affect the orientation of the fast and the slow axes and may cause changes in the pulses broadening and in the polarization orientation of the pulses.  
           [0010]    The PMD dispersions may include first and second orders dispersions corresponding to the broadening of the polarization modes by chromatic dispersions and depolarizations, respectively.  
           [0011]    The broadening of the pulses may cause adjacent pulses to overlap each other such that they cannot be resolved for the purpose of information reading. This process is known as Inter Symbol Interference (ISI). The broadening limit of the pulses is the maximum width of the pulses in which the BER exceeds a certain upper limit allowed. The broadening limit for CD and PMD is about 20% and 10%, respectively. In addition to the BER, there is another criterion of measuring the signal quality known as power penalty. The signal quality and the power penalty are determined according to the increasing power factor (measured in dB) needed to be used in order to restore the detection quality (BER) of signals corrupted by dispersions and to bring it into the detection quality of undistorted signals or signals with a certain detection level determined by a certain BER.  
           [0012]    Accordingly, if no correction is used to compensate for the pulse broadening caused by the CD and the PMD, many Optical-Electrical-Optical (OEO) regenerators should be distributed along the propagation path in order to regenerate new narrow pulses wherever the broadening of the pulses exceeds the limit that the system can tolerate. OEO regenerators are very expensive and complicated and thus dramatically increase the network cost in terms of infrastructure initial cost and maintenance cost. In addition, the OEO regenerators reduce the network reliability.  
           [0013]    There are several methods and techniques designed for CD compensation based on a fiber that produces a process that is opposite to the CD process, i.e., negative CD. According to these techniques, the compensation fiber creates a process in which the longer (and faster) wavelengths are delayed with respect to the shorter (and slower) wavelengths. The length of the compensation fiber is adjusted to produce the compensation needed for bringing the optical pulses back into their original width. However, when such CD compensation may be effective around a certain wavelength, it is very hard to produce dispersion management in which the CD compensation is effective for multiple wavelengths such as used with Dense Wavelength Division Multiplexing (DWDM). Thus, usually the CD compensation is not effective for all the wavelengths used. In addition, even where the CD compensation is effective, these methods provide local correction to the CD, but they do not contribute anything for decreasing the CD effect along further propagation post to the CD correction.  
           [0014]    The problem that PMD creates becomes dominant at bit rates above 10 Gbps. At such bit rates, the accepted amount of broadening is normally less than 10%.  
           [0015]    Compensating for PMD is more complex than compensating for CD due to the manifold of parameters (like temperature, small imperfections of the fiber, etc.), which may change over time and interact in an unpredictable way, resulting in an inherent randomness of this phenomenon. In addition, PMD does not scale linearly with the traveling length L, but with its square root.  
         SUMMARY OF THE INVENTION  
         [0016]    It is an object of the present invention to provide all optical pulse shapers and re-shapers to improve the quality of pulses broaden by CD and PMD that are wavelength insensitive.  
           [0017]    It is another object of the present invention to provide all optical pulse shapers and re-shapers to improve the quality of pulses broaden by PMD that may be phase insensitive.  
           [0018]    Yet another object of the present invention is to provide all optical systems including pulse shapers and re-shapers to improve the quality of pulses, broadened by CD and PMD, which have very fast response and operate on the fly.  
           [0019]    Still another object of the present invention is to provide all optical systems including pulse shapers and re-shapers to improve the quality of pulses, broadened by CD and PMD, by narrowing the broaden pulses back into their original width in a process that reduces the further broadening caused by CD (and/or PMD) post to their corrections.  
           [0020]    Exemplary embodiments of the present invention provide an all optical system for correcting optical dispersions include at least one optical chopping device having an input terminal for receiving a first signal, which has been broadened by optical dispersions and corresponds to an optical information channel, and at least one output terminal, wherein the optical chopping device is arranged to produce in the at least one output a second signal that is narrower than the first signal. The second signal may be detectable more reliably than the first signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The present invention will be understood and appreciated more fully from the following detailed description of exemplary embodiments of the invention, taken in conjunction with the accompanying drawings in which:  
         [0022]    [0022]FIGS. 1 a - 1   c  are schematic illustrations of CD, PMD, and both CD and PMD correctors including optical pulse choppers;  
         [0023]    [0023]FIG. 2 is a schematic illustration of a dispersion correcting system for a single information channel including a chopper that may be used with Return-to-Zero (RZ) modulation format;  
         [0024]    [0024]FIG. 3 is a schematic illustration of a system for correcting CD and PMD of multiple information channels including multiple choppers of FIG. 2 for multiple wavelengths in a DWDM configuration using multiplexing and demultiplexing;  
         [0025]    [0025]FIG. 4 is a schematic illustration of a dispersion correcting system for a single information channel including a chopper that may include variable chopping to be used with Non-Return-to-Zero (NRZ) modulation format and dynamic chopping control;  
         [0026]    [0026]FIG. 5 is a schematic illustration of a system for correcting CD and PMD of multiple information channels including multiple choppers of FIG. 4 for multiple wavelengths in a DWDM configuration using multiplexing and demultiplexing by the same device;  
         [0027]    [0027]FIGS. 6 a  and  6   b  illustrate systems for correcting CD and PMD of an information channel in a DWDM system using ADD and DROP devices;  
         [0028]    [0028]FIG. 7 a  schematically illustrates information pulses modulated by NRZ format and their respective time slots;  
         [0029]    [0029]FIG. 7 b  schematically illustrates the information pulses shown by FIG. 7 a  after being broadened by dispersions;  
         [0030]    [0030]FIGS. 7 c,    7   d,  and  7   e  are schematic illustrations of the pulses of FIG. 7 b  that are corrected by chopping of both head and tail, head, and tail, respectively. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    [0031]FIGS. 1 a,    1   b,  and  1   c  illustrate all-optical correctors (compensators)  10 ,  12 , and  14 , respectively, used for CD and PMD compensation. Choppers  30 ,  66 , and  106  of FIGS. 1 a,    1   b,  and  1   c,  respectively, represent any all-optical shaper, re-shaper, and chopper and particularly the all-optical shapers, re-shapers, and choppers disclosed in PCT Publication WO02079838 of International Application PCT/US02/09969, filed Mar. 28, 2002, entitled “Optical Pulse Chopper”, U.S. patent application Ser. No. 10/826,363, filed Apr. 19, 2004, entitled “All Optical Chopping For Shaping And Reshaping, Apparatus And Method”, and U.S. patent application Ser. No. 10/827,314, filed Apr. 20, 2004, entitled “All Optical Chopping Using Logic Gates Apparatus And Method”, the disclosures of all which applications are incorporated herein by references in their entirety. According to the disclosures of the above-referenced applications, choppers  30 ,  66 , and  106  may be all-optical choppers of any of the following types:  
         [0032]    1. Choppers that include interference devices;  
         [0033]    2. Choppers that include coincidence gates;  
         [0034]    3. Choppers that include logic gates;  
         [0035]    4. Choppers that include logic AND gates;  
         [0036]    5. Choppers that include optical thresholds;  
         [0037]    6. Choppers that include coincidence gates with delay-lines;  
         [0038]    7. Choppers that include loop-mirrors with non linear elements (NLE);  
         [0039]    8. Choppers that includes summing gates with optical threshold;  
         [0040]    9. Choppers that include Mach Zhender Interferometers (MZI);  
         [0041]    10. Choppers that operate with non-coherent light;  
         [0042]    11. Choppers that operate with coherent light;  
         [0043]    12. Choppers that are phase insensitive;  
         [0044]    13. Choppers that are wavelength insensitive  
         [0045]    14. Choppers that include closed loop phase control;  
         [0046]    15. Self all-optical choppers;  
         [0047]    16. External choppers;  
         [0048]    17. Choppers that produce head chopping;  
         [0049]    18. Choppers that produce tail chopping;  
         [0050]    19. Choppers that produce head and tail chopping;  
         [0051]    20. Choppers that produce symmetric head and tail chopping;  
         [0052]    21. Choppers that perform the optical chopping on the fly;  
         [0053]    22. Choppers with very fast time response;  
         [0054]    23. Choppers that the amount of their chopping is adjustable, and  
         [0055]    24. Choppers that the amount of their chopping is adjustable and is controlled according to the measured amount of the dispersions  
         [0056]    1. Single Channel CD and PMD Correctors  
         [0057]    [0057]FIGS. 1 a - 1   c  illustrate dispersion correctors (compensators) for a single optical channel. FIG. 1 a  illustrates a CD corrector  10  including an all-optical chopper  30  having input  28  and output  32  that may be connected to input  36  of optical amplifier  38  having output  40 . Radiation guide  20  carries input signal  22  along a relatively long propagation path, schematically illustrated by coil  24 . Pulse  22  is a high quality signal such as a signal produced by a generator or a regenerator (not shown). Signal  22  has a spectral width Δλ around the central peak wavelength λ center . During the propagation of pulse  22  along fiber  20  it is broaden by the CD and appears as broaden pulse  26  at input  28  of chopper  30 . As explained above, the amount of broadening between pulse  22  and pulse  26  is linearly dependent, among other parameters, on the length L of fiber  20  represented by coil  24 . The length L is chosen to be less than the maximum length allowed L max  in which the broadening of pulse  22  into pulse  26  exceeds the maximum limit allowed.  
         [0058]    Broaden pulse  26  is received, by chopper  30  at its input  28 . Chopper  30  chops pulse  26  and emits the narrower chopped pulse, from its output  32 , as pulse  34  which is narrower than pulse  26  and its spectral, Δλ, width is similar to the spectral width of original pulse  22 . Pulse  34  may be received by optical amplifier  38  at its input  36  for producing, at its output  40 , amplified signal  42 . Signal  42  may have a width similar to the width of original pulse  22 .  
         [0059]    Pulse  22  may suffers loss on its propagation along guide  20  till it arrives as pulse  26  to chopper  30 . In addition, chopper  30  chops out part of the energy of pulse  26  for converting it into narrower pulse  34 . Accordingly, amplifier  38  may be used for amplifying pulse  34  into pulse  42  to compensate for the loss in guide  20  or chopper  30  associated with the traveling of pulse  22  along guide  20  and the conversion of pulse  26 , by chopper  30 , into narrower pulse  34 . When using a chopper of the type illustrated and described in U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) where the chopper includes a Non Linear Element (NLE) which is an optical amplifier, the chopping is associated with intensity gain that compensates for the various loss such as the loss discussed above. In such a situation, there may be no need for amplifier  38 .  
         [0060]    The broadening of pulse  22 , by the CD, into broaden pulse  26  is symmetric. The shortest and the longest wavelengths in the spectrum of pulse  22  are the most delayed and the most advanced, respectively. Thus, the longest and the shortest wavelengths in the spectrum of pulse  26  are located in its head and tail, respectively. Due to the symmetric structure of pulse  26 , a head and tail symmetric chopping may be used by chopper  30 . The head and tail chopping that may be performed by chopper  30  removes the longest and the shortest wavelength in the spectrum of pulse  26 . Accordingly, chopper  30  acts not just as a chopper that narrows pulse  26  into pulse  34  but, it also acts as a filter that narrows the spectrum width Δλ of pulse  26  by removing the longest and the shortest wavelengths from the spectrum of pulse  26 . As mentioned above, the broadening of the optical pulse, caused by the CD, is linearly proportional to the spectral width of the pulse. Accordingly, the spectral width of pulse  34  or  42 , which is narrower than the spectral width of pulse  22 , enables pulse  34  or  42 , having time width similar to the time width of original pulse  22 , to propagate further with reduced broadening caused by the CD.  
         [0061]    Input  44  of chopper  30  may be used to operate chopper  30  as an external chopper that receives an output signal at input  44 . The signal at input  44  may be synchronized with pulse  26  at input  28 . For the simplicity of the illustrations, it should be clear that even when a chopper, such as choppers  66  and  106  of FIGS. 1 b  and  1   c,  is illustrated without an input, such as, input  44  of FIG. 1 a  used for receiving external signal, it still may represent both, self chopper and external chopper.  
         [0062]    [0062]FIG. 1 b  illustrates PMD corrector  12  including all-optical chopper  66  having input  64  and output  68  that may be connected to input  72  of an optical amplifier  74  having output  76 . Radiation guide  50  carries input signal  52  along a relatively long propagation path, schematically illustrated by coil  54 . Pulse  52  is a high quality signal such as a signal produced by a generator or a regenerator (not shown). Signal  52  has a polarization orientated along a certain direction that may have components along the fast and the slow polarization axes of guide  50 . Accordingly, each component propagates at a different propagation velocity resulting, after a distance L, with a broaden pulse  55 . Pulse  55  includes leading mode (fast mode)  62  that its polarization is oriented along the fast polarization axis  58  and delayed mode (slow mode)  60  that its polarization is oriented along the slow axis  56 . The existing of two displaced polarization modes  60  and  62  produces the broadening of pulse  55  which is known as PMD. The amount of broadening between pulse  52  and pulse  55  depends among other parameters, on the length L of fiber  50  represented by coil  54 . The length L is chosen to be less than the maximum length allowed L max  in which the broadening of pulse  52  into pulse  55  exceeds the maximum limit allowed by the designing rules of the networks.  
         [0063]    Broaden pulse  55  is received, by chopper  66  at its input  64 . Chopper  66  chops pulse  55  and emits chopped pulse  55 , from its output  68 , as pulse  70  which is narrower than pulse  55  and its width is similar to the width of original pulse  52 . Pulse  70  may be received by optical amplifier  74  at its input  72  for producing, at its output  76 , amplified signal  78 . Signal  78  may have a width similar to the width of original pulse  52 .  
         [0064]    Pulse  52  may suffers loss on its propagation along guide  50  till it arrives as pulse  55  to chopper  66 . In addition, chopper  66  chops out part of the energy of pulse  55  for converting it to pulse  70 . Accordingly, amplifier  74  may be used for amplifying pulse  70  into pulse  78  to compensate for the loss in guide  50  or chopper  66  associated with the traveling of pulse  52  along guide  50  and the conversion of pulse  55 , by chopper  66 , into narrower pulse  70 . When using a chopper of the type illustrated and described in U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) where the chopper includes a Non Linear Element (NLE) which is an optical amplifier, the chopping is associated with intensity gain that compensates for the various loss such as the loss discussed above. In such a situation, there may be no need for amplifier  74 .  
         [0065]    The broadening of pulse  52 , by the PMD, into broaden pulse  55  includes a displacement between two polarization modes (modes  60  and  62 ) and thus the broadening may be with asymmetric shape. Due to the possible asymmetric structure of pulse  55 , a head or tail chopping may be performed to remove part of the polarization mode having the smaller amplitude. The choice between the head or the tail chopping depends on the appearance of the small amplitude mode. If the small amplitude mode leads (such as mode  62  illustrated by FIG. 1 b ), head chopping may be used; if this small amplitude mode is the delayed mode, tail chopping may be used. In case that there is no way to predict if the polarization mode with the smaller amplitude is ahead or delayed, a symmetric or asymmetric chopping including head and tail chopping may be used by chopper  66 .  
         [0066]    [0066]FIG. 1 c  illustrates CD and PMD compensator  14  including chopper  102  and may include amplifier  114  connected in series to chopper  106 . Radiation guide  90  having length represented by coil  94  carries high quality signal  92  that degrades along its travel in guide  90  and appears as broaden pulse  95  at input  104  of chopper  106 . Pulse  95  includes two displaced polarization modes  102  and  100 . As explained above in the description of FIG. 1 b  in regards to PMD, modes  102  and  100  are the polarization components of pulse  95  propagating along the fast and the slow polarization axes  98  and  96 , respectively. In addition and as explained in the description to FIG. 1 a  in regard to CD broadening, there is also CD broadening of each of modes  102  and  100 .  
         [0067]    Broaden pulse  95  is received, by chopper  106  at its input  104 . Chopper  106  chops pulse  95  and emits chopped pulse  95 , from its output  108 , as pulse  110  which is narrower than pulse  95  and its width is similar to the width of original pulse  92 . Pulse  110  may be received by optical amplifier  114  at its input  112  for producing, at its output  116 , amplified signal  118 . Signal  118  may have a width similar to the width of original pulse  92 .  
         [0068]    Pulse  92  may suffers loss on its propagation along guide  90  till it arrives as pulse  95  to chopper  106 . In addition, chopper  106  chops out part of the energy of pulse  95  for converting it to pulse  110 . Accordingly, amplifier  114  may be used for amplifying pulse  110  into pulse  118  to compensate for the loss in guide  90  or chopper  106  associated with the traveling of pulse  92  along guide  90  and the conversion of pulse  95 , by chopper  116 , into narrower pulse  110 . When using a chopper of the type illustrated and described in U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) where the chopper includes a Non Linear Element (NLE) which is an optical amplifier, the chopping is associated with intensity gain that compensates for the various loss such as the loss discussed above. In such a situation, there may be no need for amplifier  114 .  
         [0069]    Chopper  106  may be head, tail, or head and tail chopper. Chopper  106  is unique in its ability to compensate, simultaneously, for both CD and PMD.  
         [0070]    Amplifiers  38 ,  74 , and  114  of FIGS. 1 a - 1   c  may be of the type of Solid state Optical Amplifier (SOA), Linear Optical Amplifier (LQA), Erbium Doped Fiber Amplifier (EDFA), and Raman amplifier.  
         [0071]    Choppers  30 ,  66 , and  106  of FIGS. 1 a - 1   c  may be of the type that is phase insensitive and where they are constructed from wavelength in sensitive components, such as wavelength in sensitive couplers, each of them may be used for CD and PMD compensation across a wide band of wavelengths, such as the band of wavelengths used in DWDM. When using SOA as the NLE of choppers  30 ,  66 , and  106 , they may have a very short time response and thus may chop pulses at a very high rate, on the fly, by self chopping or by external chopping.  
         [0072]    2. Dispersion Correction for Return-to-Zero Modulation Format  
         [0073]    2.1 Single Channel CD and PMD Correctors  
         [0074]    [0074]FIG. 2 illustrates CD and PMD corrector  501  including chopper  500 . Chopper  500  may be of any type, including any of the types mentioned above and in particular the type of choppers described in U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) and illustrated there by FIGS. 6 a,    6   b,    7   a,    7   b,    8 ,  13   a - 13   c,    14   a - 14   e,    15   a,    16 ,  17 , and  18 . Corrector  501  may include amplifier  517  connected in series to the input port of chopper  500 . Radiation guide  506  having length represented by coil  508  carries high quality signals  502  and  504  generated by high quality modulator (not shown). Signals  502  and  504  may be the pulses of an information channel generated by Return-to-Zero (RZ) modulation format. Signals  502  and  504  may degrade along their travel in guide  506  and appear as respective broaden pulses  514  and  510  at input  518  of chopper  500 . Pulses  510  and  514  include respective broaden portions  512  and  516  caused by any type of dispersions in guide  506 .  
         [0075]    Chopper  500  has two outputs  520  and  522 . As explained in U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) and illustrated there by FIGS. 6 a,    6   b,    7   a,    7   b,    8 ,  13   a - 13   c,    14   a - 14   e,    15   a,    16 ,  17 , and  18 , output  520  of chopper  500  receives the signal reflected back into input  518  of chopper  500  and is coupled from there, by a coupler or a circulator, into output  520 . Thus we refer to output  520  as the reflecting output. Reflecting output  520  performs chopping and the width of the signals at reflecting output  520 , illustrated by pulses  528  and  530 , is equal to the width of respective signals  514  and  510 , at input  518 , less a fixed amount of chopping determined by the setting of chopper  500 . As explained in U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) and illustrated there by FIGS. 17 and 18, chopper  500  is capable of performing variable chopping and the amount of chopping that chopper  500  produces may be selected by adjusting the positions (by variable optical delay lines) and or the gains of the NLE&#39;s that chopper  500  includes. Adjusting the gains of the NLE&#39;s of chopper  500  is performed by adjusting the current injected to these NLE&#39;s through injection-current terminals  524  and  526 .  
         [0076]    The amount of chopping that chopper  500  produces may be adjusted to be equal to broaden portions  512  and  516  of pulses  510  and  514 , respectively. It should be understood that broaden portions  512  and  516  of pulses  510  and  514 , respectively, are schematic illustration of the pulse broadening and indicate the width difference between the original width of pulses  504  and  502  and the width of pulses  510  and  514  broaden by dispersions, respectively. While the broaden portions  512  and  516  are illustrated as being only in the head parts of pulses  510  and  514 , respectively, they may appear in the tails, in the heads, and in both the tails and the heads of pulses  510  and  514  as well.  
         [0077]    Output  522  of chopper  500  receives signals  510  and  514  transmitted from input  518  and chopped by chopper  500 . Thus we refer to output  522  as the transmitting output. Transmitting output  522  performs a fixed amount of chopping and the width of the signals at transmitting output  522  illustrated by respective pulses  534  and  532  is fixed and independent on the width of signals  510  and  514  at input  518  of chopper  500 . However the width of signals (pulses)  532  and  534  is determined by the setting of chopper  500  as explained above with reference to U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference). Chopper  500  may be capable of performing variable chopping and the amount of chopping that chopper  500  produces may be adjusted to be equal to the original width of pulses  510  and  514  prior to their broadening by optical dispersions as illustrated by original pulses  502  band  504 , respectively.  
         [0078]    In such a case, once the width of pulses  532  and  534  is adjusted to be in the desired width, namely at the original width of respective pulses  502  and  504 , chopper  500  produces fixed width pulses  532  and  534  which is independent on the amount of broadening  512  and  516  caused by dispersions along guide  506 . This means that for correcting the broaden portions  512  and  516  caused by the dispersions, there is no need for a dynamic adjustment of the amount of chopping of chopper  500  to be according to the broaden portions  512  and  516  of pulses  510  and  514 . In this case and when RZ modulation format is used to carry the information in the information channel, transmitting output  522  produces dispersion correction by converting the width of the RZ pulses  510  and  514  back into a fixed width that may be equal to the original width of the RZ pulses  504  and  502 , respectively.  
         [0079]    It should be understood that while broaden portions  512  and  516  may be at the heads, tails and both heads and tails of pulses  510  and  514 , the chopping performed by chopper  500  for correcting the dispersions may be of the type of head, tail, and both head and tail chopping, regardless of the type of broadening of portions  512  and  516 .  
         [0080]    The ability of transmitting output  522  to accurately correct, even without dynamic adjustment of the chopping, broaden portions  512  and  516  caused by dispersion of pulses modulated by RZ modulation format, makes transmitting output port  522  a very attractive port for dispersion correction of RZ format modulated pulses and streams.  
         [0081]    2.2 CD and PMD Compensators for DWDM Systems  
         [0082]    [0082]FIG. 3 illustrates CD, PMD, or CD and PMD compensator system  201  for multiple channels, such as, DWDM system. System  201  may be designed especially for dispersion compensation of pulses modulated by RZ format. Radiation guide  200  may carry pulses broaden by CD and PMD of multiple information channels corresponding to multiple wavelengths which are divided, by DWDM demultiplexer  202 , into multiple ports  204 ,  206 ,  208 ,  210 , and  212 ; each of them carries a single wavelength corresponding to a single information channel. Radiation guide  200  may include optical amplifier  251  to compensate for the loss in guide  200 , chopping loss, and demultiplexing loss caused by demultiplexer  202 . Ports  204 ,  206 ,  208 ,  210 , and  212  include pulse choppers  216 ,  218 ,  220 ,  222 , and  224 , respectively, each of them may be of the type of chopper  500  illustrated by FIG. 2. In this case and for RZ format, each of ports  204 ,  206 ,  208 ,  210 , and  212  is analogue and similar to input  518  of chopper  500  in FIG. 2 and the broaden pulses chopped by choppers  216 ,  218 ,  220 ,  222 , and  224 , propagate, with their narrower form, in radiation guides  226 ,  228 ,  230 ,  232 , and  234 , each of them analogue and similar to transmitting output  522  of chopper  500  in FIG. 2. Radiation guides  226 ,  228 ,  230 ,  232 , and  234  may include delay-lines  236 ,  238 ,  240 ,  242 , and  244 , respectively. In a situation of dispersion compensation for pulses modulated by RZ format, the narrower chopped pulses arriving from guides  226 ,  228 ,  230 ,  232 , and  234  may have a fixed width and are recombined by DWDM multiplexer  246  that recombines the separated channels from guides  226 ,  228 ,  230 ,  232 , and  234  back into a single fiber  248 .  
         [0083]    In case that the multiplexing by multiplexer (combiner)  246  should be done in a certain time sequence between the different channels arriving from guides  226 ,  228 ,  230 ,  232 , and  234 , respective delay-lines  236 ,  238 ,  240 ,  242 , and  244  may be included in the corresponding guides to adjust the desired arrival time of each channel received from these guides.  
         [0084]    Each of choppers  216 ,  218 ,  220 ,  222 , and  224  at ports  204 ,  206 ,  208 ,  210 , and  212 , respectively, operates in the channel that it belong to as a compensator for CD and PMD for a single channel as illustrated and explained above for choppers  30 ,  66 , and  106  of FIGS. 1 a - 1   c.  In case that RZ modulation format is used and choppers  216 ,  218 ,  220 ,  222 , and  224  are of the type of chopper  500  of FIG. 2, only the inputs  204 ,  206 ,  208 ,  210 , and  212  that are the analogue of input  518  of chopper  500  of FIG. 2 and the transmitting ports  226 ,  228 ,  230 ,  232 , and  234  that are the analogue of transmitting port  522  are used. The reflecting outputs that are the analogue of reflecting output  520  are not in use and are not shown.  
         [0085]    It can be seen that CD, PMD, or CD and PMD compensation system  201  receives, at its input  200 , multiple information channels including broaden pulses at multiple wavelengths and system  201  emits, at its output  248  reshaped multiple information channels narrower pulses corresponding to the pulses that it receives at its input  200 .  
         [0086]    Output  248  may include optical amplifier  250  to amplify the chopped pulses that it receives in output  248  to compensate for propagation and chopping loss of these pulses.  
         [0087]    3. Dispersion Correction for Non-Return-to-Zero Modulation Format  
         [0088]    3.1 Single Channel CD and PMD Correctors  
         [0089]    [0089]FIG. 4 illustrates CD and PMD corrector  601  including chopper  600 . Chopper  600  may be of any type including any of the types mentioned above and in particular the type of chopper described in U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) and illustrated there by FIGS. 6 a,    6   b,    7   a,    7   b,    13   a - 13   c,    14   a - 14   e,    15   a,    16 ,  17 , and  18 . Corrector  601  may include amplifier  617  connected in series to the input port of chopper  600 . Radiation guide  606  having length represented by coil  608  carries high quality signals  602  and  604  generated by high quality modulator (not shown). Signals  602  and  604  may be the pulses of an information channel generated by Non-Return-to-Zero (NRZ) modulation format. Signals  602  and  604  may degrade along their travel in guide  606  and appear as respective broaden pulses  614  and  610  at input  618  of chopper  600  that may include tapping device  640 . Pulses  610  and  614  include broaden portions  612  and  616 , respectively, caused by any type of dispersions in guide  606 .  
         [0090]    Chopper  600  has two outputs  620  and  622 . As explained in U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) and illustrated there by FIGS. 6 a,    6   b,    7   a,    7   b,    13   a - 13   c,    14   a - 14   e,    15   a,    16 ,  17 , and  18 , output  620  of chopper  600  receives the signal reflected back into input  618  of chopper  600  and is coupled from there, by a coupler or a circulator, into output  620 . Thus we refer to output  620  as the reflecting output. Reflecting output  620  performs chopping and the width of the signals at reflecting output  620  illustrated by pulses  628  and  630  is equal to the width of respective signals  614  and  610  less a fixed amount of chopping determined by the setting of chopper  600 . As explained in U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) and illustrated there by FIGS. 17 and 18, chopper  600  is capable of performing variable chopping and the amount of chopping that chopper  600  produces may be selected by adjusting the positions (by variable optical delay lines) and or the gains of the NLE&#39;s that chopper  600  includes. Adjusting the gains of the NLE&#39;s of chopper  600  is performed by adjusting the current injected to these NLE&#39;s through injection-current terminals  624  and  626 .  
         [0091]    The amount of chopping that chopper  600  produces may be adjusted to be equal to broaden portions  612  and  616  of pulses  610  and  614 , respectively. It should be understood that broaden portions  612  and  616  of pulses  610  and  614 , respectively, are schematic illustration of the pulse broadening and indicate the width difference between the original width of respective pulses  604  and  602  and the width of pulses  610  and  614  broaden by dispersions, respectively. While the broaden portions  612  and  616  are illustrated as being only in the head parts of pulses  610  and  614 , respectively, they may appear in the tails, in the heads, and in both the tails and the heads of pulses  610  and  614  as well.  
         [0092]    Output  622  of chopper  600  receives signals  610  and  614  transmitted from input  618  and chopped by chopper  600 . Thus we refer to output  622  as the transmitting output. Transmitting output  622  performs a fixed amount of chopping and the width of the signals at transmitting output  622  illustrated by pulses  632  and  634  is fixed and independent on the width of signals  610  and  614  at input  618  of chopper  600 . However the width of signals (pulses)  632  and  634  is determined by the setting of chopper  600  as explained above with reference to U.S. Patent Application entitled “All Optical Chopping For Shaping and Reshaping Apparatus And Method” (see background cross-reference) and especially with reference to its FIGS. 17 and 18. Chopper  600  may be capable of performing variable chopping and the amount of chopping  632  and  634  that chopper  600  produces at port  622  may be adjusted to be equal to the width of broaden portions  616  and  612  caused by the optical dispersions, respectively. In NRZ modulation format, broaden portions  616  and  612  having the same width regardless of the width of original pulses  602  and  604  since the broadening, by optical dispersions, of original pulses  602  and  604  is the same no mater if the original signal is a single pulse or a train of multiple pulses forming a long block of logic ones.  
         [0093]    In such a case, when the width of NRZ pulses  632  and  634  (representing the amount of chopping) is adjusted to be in the desired width, namely at the width of broaden portions  616  and  612 , then chopper  600  produces, at port  622 , chopping at the amount that is equal to the amount of broadening caused by the optical dispersions. In this case pulses  628  and  630  at reflecting port  620  of chopper  600  having the width of pulses  614  and  610  less an amount of chopping that is equal to the width of broaden portions  616  and  612 , respectively. The PMD may vary statistically with time according to a Maxwellian distribution. This means that the width of broaden portions  612  and  616 , caused by the optical dispersions, may vary statistically as well. Thus an effective dispersion correction may includes a dynamic chopping where the amount of chopping performed at reflecting port  620  may be adjusted dynamically to be equal to the amount of broadening  612  and  616  caused by the optical dispersions along guide  606 . This means that for correcting effectively the broaden portions  612  and  616  caused by the dynamic dispersions, there may be a need for a dynamic adjustment of the amount of chopping of chopper  600  to be according to the broaden portions  612  and  616  of respective pulses  610  and  614  at input  618 . In this case and when NRZ modulation format is used to carry the information in the information channel, reflecting output  620  produces dispersion correction by converting the width of NRZ pulses  610  and  614  back into widths that may be equal to the original widths of repspective NRZ pulses  604  and  602 .  
         [0094]    It should be understood that while broaden portions  612  and  616  may be at the heads, tails and both heads and tails of pulses  610  and  614 , the chopping performed by chopper  600  for correcting the dispersions may be of the type of head, tail, and both head and tail chopping, regardless of the type of broadening of portions  612  and  616 .  
         [0095]    The ability of reflecting output  620  to accurately correct broaden portions  612  and  616  caused by dispersion of pulses modulated by NRZ modulation format, may depend on the efficiency of producing variable and dynamic chopping at reflecting output port  620 . Variable chopping according to the width of broaden portions  612  and  614  may be performed with the assistance of measurement and control system  644 .  
         [0096]    Part of the energy of broaden signals  610  and  614  is tapped out from radiation guide  606 , by coupler  640  and is directed into measurement and control system  644  by optical port  642 . System  644  measures the width of broaden signals  610  and  614  and compares their width to the closest integral number of the original width of a single pulse, such as pulse  602 . The width of the broaden portions  612  and  616  (broadening by the optical dispersions) is derived, by system  644 , from the difference between the actual measured width of pulses  610  and  620  and the above closest integral number. The amount of chopping that chopper  600  has to produce may be controlled by system  644  to be equal to the value of broadening derived by system  644 . System  644  may control the chopping either by controlling the location of the NLE in chopper  600  or by controlling the gains of the NLE&#39;s in copper  600 .  
         [0097]    System  644  may produce control signals to control an optical variable delay line in chopper  600  (not shown) to locate the NLE of chopper  600  (not shown) by adjusting the optical path of the variable optical delay line, such that, the chopping of chopper  600  may be equal to the amount of broadening caused by the optical dispersions.  
         [0098]    In an alternative options, System  644  may produce control signals at ports  646  and  648  that are canied by leads  650  and  652  into injection-current ports  624  and  626 , respectively, for controlling the gains of the NLE&#39;s (not shown) by adjusting the injection currents at ports  624  and  626  chopping of chopper  600  for producing an amount of chopping that may be equal to the amount of broadening caused by the optical dispersions.  
         [0099]    3.2 CD and PMD Compensators for DWDM Systems  
         [0100]    [0100]FIG. 5 illustrates CD, PMD, or CD and PMD compensator system  701  for multiple channels, such as, DWDM system. System  701  may be designed especially for dispersion compensation of pulses modulated by NRZ format. Radiation guide  700  may carry pulses broaden by CD and PMD of multiple information channels corresponding to multiple wavelengths which are divided, by DWDM demultiplexer  702 , into multiple ports  704 ,  706 ,  708 ,  710 , and  712 ; each of them carries a single wavelength corresponding to a single information channel. Radiation guide  700  may include circulator  754  and optical amplifier  750  to compensate for the loss in guide  700 , chopping loss, and demultiplexing loss caused by demultiplexer  702 . Ports  704 ,  706 ,  708 ,  710 , and  712  include pulse choppers  716 ,  718 ,  720 ,  722 , and  724 , respectively, each of them may be of the type of chopper  600  illustrated by FIG. 4. In this case and for NRZ format, each of ports  704 ,  706 ,  708 ,  710 , and  712  is analogue and similar to input  618  of chopper  600  in FIG. 4 and the broaden pulses chopped by choppers  716 ,  718 ,  720 ,  722 , and  724  are reflected, with their narrower form, back into radiation guides  704 ,  706 ,  708 ,  710 , and  712  that each of them is analogue and similar to reflecting output  620  and or input  618  of chopper  600  when the chopped signal is reflected back into input  618  of chopper  600  in FIG. 4. Radiation guides  704 ,  706 ,  708 ,  710 , and  712  may include delay-lines  736 ,  738 ,  740 ,  742 , and  744 , respectively. In a situation of dispersion compensation for pulses modulated by NRZ format, the narrower chopped pulses reflected back into guides  704 ,  706 ,  708 ,  710 , and  712  may have a width that is equal to the original width of the NRZ pulses prior to their broadening due to optical dispersions. These chopped signals reflected back along guides  704 ,  706 ,  708 ,  710 , and  712  toward DWDM demultiplexer  702  that operates, in the propagation direction of the reflected chopped pulses as multiplexer  702 . Accordingly, the chopped pulses reflected back into radiation guides  704 ,  706 ,  708 ,  710 , and  712  are recombined by DWDM multiplexer  702  (or demultiplexer  702  in the other direction) that recombines the separated channels from guides  704 ,  706 ,  708 ,  710 , and  712  back into a single fiber  700 .  
         [0101]    In case that the multiplexing by multiplexer (combiner)  702  should be done in a certain time sequence between the different channels arriving from guides  704 ,  706 ,  708 ,  710 , and  712 , respective delay-lines  736 ,  738 ,  740 ,  742 , and  744  may be included in the corresponding guides to adjust the desired arrival time of each channel received from these guides.  
         [0102]    Each of choppers  716 ,  718 ,  720 ,  722 , and  724  at ports  704 ,  706 ,  708 ,  710 , and  712 , respectively, operates in the channel that it belong to as a compensator for CD and PMD for a single channel as illustrated and explained above for choppers  30 ,  66 , and  106  of FIGS. 1 a - 1   c.  In case that NRZ modulation format is used and choppers  716 ,  718 ,  720 ,  722 , and  724  are of the type of chopper  600  of FIG. 4, only inputs  704 ,  706 ,  708 ,  710 , and  712  that are the analogue of input  618  (and, at the same time are the analogue of reflecting output  622  as well) of chopper  600  of FIG. 4 are used and shown. The reflecting outputs and the transmitting outputs that are the analogue of reflecting output  620  and transmitting output  622  are not in use and thus are not shown by the illustrations of choppers  716 ,  718 ,  720 ,  722 , and  724 .  
         [0103]    It can be seen that CD, PMD, or CD and PMD compensation system  701  receives, at its input  700 , multiple information channels including broaden pulses at multiple wavelengths and system  701  reflects, back into its input  700 , reshaped narrower pulses corresponding to the pulses that it receives at its input  700 .  
         [0104]    The reshaped narrow pulses corrected for the optical dispersions are directed out of radiation guide  700 , by circulator  754 , into radiation guide  752  for further propagation.  
         [0105]    4. CD and PMD Compensators at Splitting and Combinig Points  
         [0106]    [0106]FIGS. 6 a  and  6   b  illustrate CD and PMD compensators in the vicinity of ADD and DROP devices. The splitting points where ADD and DROP devices are used are very attractive for installing CD and PMD compensators  303  and  301  of FIGS. 6 b  and  6   a,  respectively. The ADD and DROP devices  346  and  306  of FIGS. 6 b  and  6   a,  respectively, already serve as channel combiner and channel separator and may save the need for a special combiner (multiplexer) or demultiplexer such as multiplexer  246  and demultiplexer  202  of FIG. 3 that are used for channel separation and combining, respectively.  
         [0107]    [0107]FIG. 6 a  illustrates CD and PMD compensator  301  including radiation guide  300  that its length is represented by coil  302 . Guide  300  carries multiple information channels at multiple wavelengths received by input  304  of DROP device  306 . Device  306  separates one of the information channels out into radiation guide  312  and from there into chopper  316 . The pulses of the single separated (dropped) wavelength channel, are received at input  314  of device  306  after being broadened by CD and PMD. Chopper  316  reshapes each of the pulses of the separated channel received from device  306  via guide  312  and converts them into narrower pulses at its output  318 . The pulses from output  318  may be received at input  320  of amplifier  322  to be amplified, by amplifier  322 , in order to produce amplified and narrow pulses at output  324 . The rest of the channels (all the channels except the separated channel in guide  312 ) continue to propagate in guide  308  that its length is illustrated by coil  310 .  
         [0108]    [0108]FIG. 3 b  illustrates CD and PMD compensator  303  including radiation guide  340  that its length is represented by coil  342 . Guide  340  carries multiple information channels at multiple wavelengths received by input  344  of ADD device  346 . Chopper  354  receives from radiation guide  352  pulses broaden by CD and PMD and reshapes each pulse by narrowing their width back into their original width as initially produced (before broadening). The reshaped pulses received from output  356  of chopper  354  may be received at input  358  of amplifier  359  to be emitted, from output  360  of amplifier  359 , as narrow and amplified pulses propagating in radiation guide  362 . Accordingly, the pulses of the channel received from guide  362  are compensated for the CD and the PMD. Device  346  adds the information channel from radiation guide  362  to combine this channel with the other channels received at input  344  and to emit all these channels together from ADD device  346  and into radiation guide  348  that its length is schematically illustrated by coil  350 .  
         [0109]    5. Improved Detection  
         [0110]    The detection of the signals in the optical information channel is synchronized, at the receiver side, by a synchronization system known as clock recovery system. In a synchronized situation the information pulses are sampled at each time slot of the clock, at the receiver side, for detecting the binary information carried by the optical channel. In a situation where the optical information pulses are broaden by the optical dispersions, they appear in the time slots where they should appear and in addition they may occupy time slots in which they should not appear which are adjacent to the time slots in which they should appear. In this case, the time slots that should not include any energy of the information pulses actually include such energy of the information pulses due to the broadening of the optical dispersions. This may result with poor detection having high Bit Error Rate (BER). When the broadening of the information pulses, caused by the optical dispersions is large, it may cause to some of the information pulses that should be separated by at least one vacant time slot to be joined together by occupying the vacant time slots with energy of the information pulses in a process known as Inter Symbol Interference (ISI) which results with a high Bit Error Rate (BER).  
         [0111]    Accordingly it is clear that the broadening of the information pulses, by the optical dispersions, causes to increase in the BER. The lower the BER the better is the detection.  
         [0112]    5.1. Improving Detection by Chopping  
         [0113]    Referring now to FIGS. 7 a - 7   e  that are illustrated with respect to reference time  800  and time axis  802 .  
         [0114]    [0114]FIG. 7 a  illustrates information pulses  804 ,  806 , and  808  modulated by NRZ modulation format that fit into time slots  810  (typ.).  
         [0115]    [0115]FIG. 7 b  illustrates pulses  804 A,  806 A, and  808 A that are pulses  804 ,  806 , and  808  of FIG. 7 a,  after being broadened by optical dispersions. Pulses  804 A,  806 A, and  808 A are shown with respect to same time slots  810  (typ.) shown in FIG. 7 a.  Each of pulses  804 A,  806 A, and  808 A has a typical head broadening  812  (typ.) and tail broadening  814  (typ.). The head and tail broadening  812  (typ.) and  814  (typ.), respectively, may be mistakenly detected as logic state “1” while in the time slots that they appear the logic state should be “0”. Accordingly, broadenings  812  (typ.) and  814  (typ.) may increase the BER and degrade the detection quality.  
         [0116]    5.1.1. Improving Detection by Head and Tail Chopping  
         [0117]    [0117]FIG. 7 c  shows pulses  804 B,  806 B, and  808 B, that are pulses  804 A,  806 A, and  808 A of FIG. 7 b,  after each of them was head and tail chopped by the amount of head and tail broadening  812  (typ.) and  814  (typ.) (shown in FIG. 7 b ), respectively. Pulses  804 B,  806 B, and  808 B are shown with respect to same time slots  810  (typ.) shown in FIG. 7 a.  It can be seen that same time slots  810  (typ.) fits the information pulses  804 ,  806 , and  808  of FIG. 7 a  prior to the dispersions and pulses  804 B,  806 B, and  808 B of FIG. 7 c  that are pulses  804 A,  806 A, and  808 A after head and tail broadening which were corrected by proper head and tail chopping.  
         [0118]    It can be seen that pulses  804 B,  806 B, and  808 B, and  804 ,  806 , and  808  are well fitted into their same respective time slots  810  (typ.). Accordingly, the detection of pulses  804 B,  806 B, and  808 B broaden by dispersions and corrected by head and tail chopping may be similar to that of original pulses  804 ,  806 , and  808  of FIG. 7 a  prior to the broadening by dispersions. This means that the head and tail chopping improves the detection quality.  
         [0119]    5.1.2. Improving Detection by Head Chopping  
         [0120]    [0120]FIG. 7 d  illustrates pulses  804 C,  806 C, and  808 C which are pulses  804 A,  806 A, and  808 A of FIG. 7 b,  after each of them was head chopped by the amount that is equal to the sum of head and tail broadening  812  (typ.) and  814  (typ.) (shown in FIG. 7 b ). Pulses  804 C,  806 C, and  808 C are shown with respect to time slots  816  (typ.).  
         [0121]    It can be seen that time slots  810  (typ.) that fit the information pulses  804 ,  806 , and  808  of FIG. 7 a  prior to the dispersions, and time slots  816  (typ.) of pulses  804 C,  806 C, and  808 C of FIG. 7 d  that are pulses  804 A,  806 A, and  808 A after head and tail broadening which were corrected only by proper head chopping, are shifted relative to each other by the amount marked  818 . The whole amount of broadening which is equal to the sum of head and tail broadening  812  (typ.) and  814  (typ.), respectively, is corrected only by head chopping that is equal to the total broadening.  
         [0122]    In this case the shifting  818  between time slots  810  (typ.) (aligned with reference  800 ) and  816  (typ.) is equal only to the typical tail broadening  814  (typ.) because the contribution of head broadening  812  (typ.) shown in FIG. 7 b  to the total broadening should be corrected by head chopping. Thus only the part of the chopping that is equal to tail broadening  814  (typ.) contributes to shifting  818 .  
         [0123]    Shifting  818  between time slots does not disturb the detection since the receiver includes a clock recovery system that synchronizes the clock of the receiver with the pulses that the receiver receives.  
         [0124]    It can be seen that time slots  816  (typ.) and  810  (typ,) have the same period and the information pulses that they contain  804 C,  806 C, and  808 C and  804 ,  806 , and  808 , respectively, are well fitted into their respective time slots. Thus the detection of information pulses  804 C,  806 C, and  808 C broaden by dispersions and corrected by proper head chopping may be similar to that of original pulses  804 ,  806 , and  808  of FIG. 7 a  prior to the broadening by dispersions. This means that pulses broaden by dispersions and corrected by chopping are detectable better.  
         [0125]    5.1.3. Improving Detection by Tail Chopping  
         [0126]    [0126]FIG. 7 e  illustrates pulses  804 D,  806 D, and  808 D which are pulses  804 A,  806 A, and  808 A of FIG. 7 b,  after each of them was tail chopped by the amount that is equal to the sum of head and tail broadening  812  (typ.) and  814  (typ.) (shown in FIG. 7 b ). Pulses  804 D,  806 D, and  808 D are shown with respect to time slots  820  (typ.).  
         [0127]    It can be seen that time slots  810  (typ.) that fit the information pulses  804 ,  806 , and  808  of FIG. 7 a  prior to the dispersions and time slots  820  (typ.) of pulses  804 D,  806 D, and  808 D of FIG. 7 e  that are pulses  804 A,  806 A, and  808 A of FIG. 7 b  broaden by head and tail broadening which were corrected only by proper tail chopping, are shifted relative to each other by the amount marked  822 . The whole amount of broadening which is equal to the sum of head and tail broadening  812  (typ.) and  814  (typ.), respectively, is corrected only by tail chopping that is equal to the total broadening.  
         [0128]    In this case the shifting  822  between time slots  810  (typ.) (aligned with reference  800 ) and  816  (typ.) is equal to the typical head broadening  812  (typ.) because the contribution of tail broadening  814  (typ.) shown in FIG. 7 b  to the total broadening should be corrected by tail chopping. Thus only the part of the chopping that is equal to head broadening  812  (typ.) contributes to shifting  822 .  
         [0129]    Shifting  822  between time slots  810  (typ.) and  820  (typ.) does not disturbed the detection since the receiver includes a clock recovery system that synchronizes the clock of the receiver with the pulses that the receiver receives.  
         [0130]    It can be seen that time slots  820  (typ.) and  810  (typ,) have the same period and the pulses that they contain  804 D,  806 D, and  808 D and  804 ,  806 , and  808 , respectively, are well fitted into their respective time slots. This means that the detection of pulses  804 D,  806 D, and  808 D broaden by dispersions and corrected only by tail chopping may be similar to that of pulses  804 ,  806 , and  808  of FIG. 7 a  prior to the broadening by dispersions. Accordingly, pulses broaden by dispersions that corrected by chopping are detectable better than broaden pulses.  
         [0131]    Thus BER may be reduced significantly by reducing the broadening effect caused by the optical dispersions in a process of reshaping the information pulses by chopping. The devices and the systems according to the present invention illustrated by FIGS. 1-6 b  are all of the type that reshapes the information pulses, broaden by the optical dispersions, by narrowing these pulses and chopping them back into a width that is similar to their original width prior to the broadening by dispersions. The optical choppers used to correct dispersions, according to the present invention, may include Non Linear Elements (NLE&#39;s) that are optical amplifiers. In this situation, these optical amplifiers and other optical amplifiers may compensate for the energy lost in the process of the chopping.  
         [0132]    These types of reshaping and chopping performed by the embodiments according to the present invention results with improving the detection and reducing the BER of the detected information pulses carried by the information channels.  
         [0133]    Any or all of the embodiments of the present invention, as described above, may include a continuous sequence of optical components connected by light guiding media such as, for example, optical fibers, planar waveguides, or planar circuits (PLC), which media may be fabricated using integrated optic techniques and/or on-chip manufacturing. Alternatively, All the embodiments according to the present may be constructed from discrete components, in which case the optical guiding media may be replaced by open space, e.g., vacuum, or by a non-solid, e.g., gaseous media, and the directional couplers may be replaced with beam splitters. It should be understood that all amplifiers and attenuators may include variable and/or adjustable components. It should be clear that all amplifiers may made of amplifying media and devices and in particular are made of SOA&#39;s, LOA&#39;s and EDFA&#39;s. It should be appreciated that all attenuators are made of attenuating media and devices and in particular are made of couplers and absorbing amplifiers.  
         [0134]    While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents may occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.