Abstract:
Method and system for flattening tilt gain with a digital title gain equalizer (“DTGE”) constructed with a linear tilt optical filter (“LTOF”). In a first embodiment, a DTGE flattens tilt gain with a combination of LTOF and a rotative half-wave plate. In a second embodiment, a DTGE flattens tilt gain with a combination of LTOF and variable Faraday rotators.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of and claims the benefit of application Ser. No. 10/818,255, filed Apr. 5, 2004 entitled “Method and System for Flexible and Cost Effective Dynamic Tilt Gain Equalizer,” naming Shijie Gu and Zhanxiang Zhang as inventors. 
     
    
     BACKGROUND INFORMATION  
       [0002]     1. Field of Invention  
         [0003]     The present invention relates to fiber optics technology, and more particularly, to dynamic tilt gain equalizers.  
         [0004]     2. Description of Related Art  
         [0005]     In recent years, fiber optic communication systems have become increasingly popular for data transmission due to their high speed and high data capacity capabilities. Multiplexing the data transmitted via a fiber maximizes the transmittable data volume. Particularly, Wavelength Division Multiplexing (“WDM”) systems increase the transmission data rate through single-mode optical fiber by simultaneously propagating light from spectrally different but equally powered laser sources through the fiber.  
         [0006]     Moreover, in WDM optical links, it is important to keep the signals of all the channels in a fiber at the same power level in order to avoid signal-to-noise ratio degradation due to the gain characteristics in optical amplifiers. This is difficult to accomplish because the non-flat gain profiles over the desired spectral ranges in optical amplifiers cause variations in power levels for different channels.  
         [0007]     In a configuration of cascaded optical amplifiers in a WDM link, lower accumulated gain in certain wavelengths reduces signal-to-noise ratio, and this ratio limits the transmission distance. This problem may be resolved by installing fixed-gain filters in each amplifier to achieve a flattened gain. However, the gain profiles in the amplifiers vary in accordance to the number and power levels of the channels; and in a dynamically reconfigurable WDM network, the gain profiles of optical amplifiers will vary with network reconfiguration. Furthermore, even for simple point-to-point fixed add/drop WDM systems, there are design considerations relating to future addition of channels or reduction of WDM wavelength spacing. Thus, the gain profiles will vary as the number of channels varies.  
         [0008]     If the gain of an optical amplifier is linearly dependent on the wavelength of the amplified signal, this dependence is known as the “gain tilt” of the amplifier. Therefore, when a WDM signal light is amplified by an optical amplifier (e.g. erbium-doped fiber amplifier, etc.), each of the signals of the individual channel may be amplified with a different gain.  
         [0009]     The gain tilt effect occurs when the input power or channel numbers changes.  FIG. 1  illustrated a positively sloped gain tilt denoted S 1 , a flat gain tilt denoted S 3 , and a negatively sloped gain tilt denoted S 5 . Typically, positive sloped gain tilt S 1  occurs the most frequently, and for a WDM system, this gain tilt must be flattened. Therefore, with the fast-growing interest in dynamic reconfigurable WDM networks and scalability considerations, dynamically controlled optical gain equalizers become essential elements for the next generation optical networks.  
         [0010]     In an effort to equalize the gain tilt, several methods have been developed for optical power equalizers. Some approaches separate the WDM channels and adjust each individually. This can be done in a first method by using a multiplexer/demultiplexer pair such as a phased array grating with an array of liquid crystal variable optic attenuators (“VOA”). The use of such a dynamic gain tilt equalizer (“DTGE”) can flatten the gain tilt, but such equalizers are complex and costly.  FIG. 2  illustrates one method of flattening gain tilt by using this type of DTGE whereby the c-band is separated into four different windows. Subsequently, each window of channels goes through a corresponding WDM such that channels (“λ”) 3 to 9 go through WDM 1 , λ 13  to λ 19  go through WDM 2 , λ 23  to λ 29  go through WDM 3 , and λ 33  to λ 39  go through WDM 4 . After passing through its corresponding WDM, each window also goes through a corresponding VOA to adjust optical loss as shown in  FIG. 2 . Although the method shown in  FIG. 2  roughly flattens a gain tilt, it has many disadvantages: 1) it incorporates too many components such as the WDM&#39;s and VOA&#39;s shown in  FIG. 2 ;  2 ) the gaps between WDM&#39;s miss some of the channels; and 3) the gain tilt for channels in the same window is not eliminated.  
         [0011]     An alternative method for dynamic tilt gain equalizer uses all-fiber, acousto-optic tunable filter (“AOTF”) technology. An all-fiber AOTF system works by creating wavelength selective losses as signals travel through an optical fiber. The wavelength selective losses are induced by imposing a tunable small-amplitude acoustic wave on a short length of optic fiber. Each AOTF creates a “notch” or rejection band in the optical spectrum, whereby the notch position and depth is independently adjustable with software. Each tilt gain equalizer contains eight AOTF&#39;s in series to produce the desired attenuation profile over the c- or l-band. However, this method is also complex and costly due to the use of many super-sound generators.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention provides a method and system for flattening gain tilt with a DTGE constructed with a linear tilt optical filter (“LTOF”). In order to equalize the gain tilt, an input light beam is divided into two beams, one of which passes through the LTOF in the DTGE, and the two beams are subsequently combined into a single output beam. The DTGE flattens the tilt gain by controlling the dividing ratio between the two divided light beams.  
         [0013]     The present invention describes two embodiments for flattening gain tilt with a DTGE constructed with a LTOF. In a first embodiment, the DTGE flattens the tilt gain by using a LTOF in conjunction with a rotative half-wave plate. In a second embodiment, the DTGE flattens the tilt gain by using a LTOF in conjunction with variable Faraday rotators.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The accompanying drawings that are incorporated in and form a part of this specification illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention:  
         [0015]      FIG. 1  is a graph illustrating three gain tilt effects having a positive, a flat, and a negative slope respectively.  
         [0016]      FIG. 2  is a block diagram illustrating the use of a prior art DTGE in the process of flattening gain tilt by separating the c-band into multiple windows.  
         [0017]      FIG. 3  is an attenuation profile of a linear tilt optical filter (“LTOF”).  
         [0018]      FIG. 4  is a block diagram illustrating the top view of a first embodiment of a DTGE comprising a rotative half-wave plate.  
         [0019]      FIG. 5  is a block diagram illustrating the side view of the first embodiment of a DTGE comprising a rotative half-wave plate.  
         [0020]      FIG. 6  is a flow chart illustrating the method for flattening gain tilt with the DTGE shown in  FIG. 4  and  FIG. 5 .  
         [0021]      FIG. 7  is an illustration of rotate angles of the polarizations and the half-wave plate shown in  FIG. 4  and  FIG. 5 .  
         [0022]      FIG. 8  is a block diagram illustrating the top view of a second embodiment of a DTGE comprising variable Faraday rotators.  
         [0023]      FIG. 9  is a block diagram illustrating the side view of the second embodiment of a DTGE comprising variable Faraday rotators.  
         [0024]      FIG. 10  is a flow chart illustrating the method for flattening gain tilt with the DTGE shown in  FIG. 8  and  FIG. 9 .  
         [0025]      FIG. 11  is an attenuation profile of the DTGE with a rotative half-wave plate shown in  FIG. 4  and  FIG. 5 .  
         [0026]      FIG. 12  is an attenuation profile of the DTGE with a variable Faraday rotator shown in  FIG. 8  and  FIG. 9 .  
     
    
     DETAILED DESCRIPTION  
       [0027]     The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. In the following description, specific nomenclature is set forth to provide a thorough understanding of the present invention. It will be, apparent to one skilled in the art that the specific details may not be necessary to practice the present invention. Furthermore, various modifications to the embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein.  
         [0028]     The DTGE in accordance with the methods of the present invention are constructed with LTOF. The loss of light through a LTOF depends linearly on the wavelength of the light as illustrated by the attenuation profile in  FIG. 3 , wherein the horizontal axis  301  represents wavelength λ and the vertical axis  303  represents loss of light L(λ). Moreover, the loss of light may be calculated with the formula: 
 
 L (λ)= a (λ−λ min )+ b ( dB )  (Equation 1) 
 
 where λ is the wavelength of the light; λ min  is the shortest wavelength in the wavelength range of the LTOF; “a” (dB/nm) is the slope of the LTOF and “b” is the insertion loss. There are currently several manufacturers who can supply very cost effective LTOF&#39;s that have excellent performance specifications. For example, one such LTOF may have an “a” from −0.5 to 0.5 dB/nm; an insertion loss “b” less than 0.3 dB; a very low chromatic dispersion (&lt;0.2 ps/nm); and a polarization dependent loss less than 0.05 dB. 
 
         [0029]      FIG. 4  illustrates a block diagram  400  of the top view of a first embodiment of a DTGE using LTOF technology. Diagram  400  as shown in  FIG. 4  comprises: a first collimator denoted  101 , a first walk-off crystal denoted  103 , a first half-wave plate denoted  105 , a polarization beam splitter (“PBS”) denoted  107 , a Faraday rotator denoted  109 , a 22.5° cut half-wave plate denoted  111 , a rotative half-wave plate denoted  113 , a second walk-off crystal denoted  115 , a LTOF denoted  117 , a first mirror denoted  119 , a second mirror denoted  121 , a second half-wave denoted  122 , a third walk-off crystal denoted  123 , and a second collimator denoted  125 .  
         [0030]      FIG. 5  illustrates a block diagram  500  of the side view of the same embodiment of DTGE as shown in  FIG. 4 . Diagram  500  comprises: the collimator  101 , the first walk-off crystal  103 , the first half-wave plate  105 , the PBS  107 , the Faraday rotator  109 , the 22.5° cut half-wave plate  111 , the rotative half-wave plate  113 , the second walk-off crystal  115 , the LTOF  117 , and the first mirror  119 .  
         [0031]      FIG. 6  illustrates a flow chart  600  of the steps for dynamically equalizing gain tilt with the DTGE shown in  FIG. 4  and  FIG. 5 . The components and steps described in  FIG. 6  are illustrated in  FIG. 4  unless otherwise noted. In step  601 , collimator  101  collimates an input beam B 1 . The collimated B 1  then passes through the first walk-off crystal  103  that splits B 1  into two beams: an extraordinary beam denoted E 1  having a polarization parallel to the surface of the paper on which  FIG. 4  is drawn, and an ordinary beam denoted O 1  having a polarization vertical to the surface of the paper on which  FIG. 4  is drawn. In step  605 , beam O 1  passes through the first half-wave plate  105  that rotates the polarization of O 1  by 90° such that O 1  has a polarization parallel to the surface of the paper. Both O 1  and E 1  then pass through the PBS  107  in step  607 . Subsequently in step  609 , O 1  and E 1  pass through the Faraday rotator  109  that rotates the polarization of both O 1  and E 1  by 45°. The 22.5° cut half-wave plate  111  then rotates the polarization of both O 1  and E 1  by −45° such that O 1  and E 1  each has a polarization that is parallel to the surface of the paper after passing through the 22.5° cut half-wave plate  111  in step  611 .  
         [0032]     In step  613 , both E 1  and O 1  pass through the rotative half-wave plate  113 . When a linearly polarized beam passes through the rotative half-wave plate  113 , the polarization of the beam is rotated by an angle 2α wherein α is the angle between the polarization of the beam and the optic axis of the rotative half-wave plate  113 . After passing through the rotative half-wave plate  113 , the polarizations of both O 1  and E 1  are rotated by 2α from the polarization angles of O 1  and E 1  before they passed through the rotative half-wave plate  113 .  
         [0033]      FIG. 7  is an illustration of angle rotation through the rotative half-wave plate  113 .  FIG. 7  comprises: the horizontal direction (parallel to the surface of the paper on which  FIG. 4  is drawn) denoted  701 , the vertical direction (vertical to the surface of the paper on which  FIG. 4  is drawn) denoted  703 , the optic axis of the rotative half-wave plate  113  denoted  705  having an angle α from the horizontal direction  701 ; and the polarization of a beam after passing through the rotative half-wave plate  113  denoted  707  having an angle 2α from the horizontal direction  701 .  
         [0034]     In step  615 , the beam O 1  is split into an ordinary beam O 2  and an extraordinary beam E 2  after passing through the walk-off crystal  115 . Simultaneously in step  615 , the beam E 1  is split into an ordinary beam O 3  and an extraordinary beam E 3  after passing through the walk-off crystal  115 . Subsequently in step  617 , the extraordinary beams E 2  (shown in  FIG. 5 ) and E 3  (not shown in  FIG. 5  but operates as E 2  does in  FIG. 5 ) reflect off the mirror  119  back into the walk-off crystal  115  without passing through the LTOF  117  at all as shown in  FIG. 5 . Simultaneously in step  617 , the ordinary beams O 2  (shown in  FIG. 5 ) and O 3  (not shown in  FIG. 5  but operates as O 2  does in  FIG. 5 ) pass through the LTOF  117  before reflecting off the mirror  119 , pass through the LTOF  117  a second time after reflecting off the mirror  119  as shown in  FIG. 5 , and finally pass back through the walk-off crystal  115 . In step  619 , the beams E 2  and O 2  combine in the walk-off crystal  115  into a beam B 2 ; and the beams E 3  and O 3  combine in the walk-off crystal  115  into a beam B 3 .  
         [0035]     In step  621 , the beams B 2  and B 3  pass through the rotative half-wave plate  113  and the polarizations of the reflected beams B 2  and B 3  are returned to horizontal to the paper. In step  623 , the polarization of both beams B 2  and B 3  rotate by 45° after passing through the 22.5° cut half-wave plate  111 . Moreover, the polarization of both beams B 2  and B 3  rotate again by 45° in step  625  after passing through the Faraday rotator  109 . In step  627 , the beams B 2  and B 3  reflect off the PBS  107  onto the mirror  121  that reflects both beams. The polarization of the beam B 2  is then rotated by 90° after passing through the half-wave plate  122  while the polarization of the beam B 3  remains unaltered after step  629 . Both beams B 2  and B 3  then combine into beam B 4  after passing through the walk-off crystal  123  in step  631 . The beam B 4  is subsequently collimated after passing through the collimator  125 .  
         [0036]     The attenuation profile of the DTGE illustrated in  FIG. 4  and  FIG. 5  is defined as:  
                     H   ⁡     (     λ   ,   α     )       =       -   10     ⁢   log   ⁢         I   out     ⁡     (   λ   )         I   in                     =       -   20     ⁢     log   ⁡     [         (     sin   ⁢           ⁢   2   ⁢   α     )     2     +       (       E   ⁡     (   λ   )       ⁢   cos   ⁢           ⁢   2   ⁢   α     )     2       ]                       (     Equation   ⁢           ⁢   2     )                 E   ⁡     (   λ   )       =     10       -   0.05     ⁢           ⁢     L   ⁡     (   λ   )                   (     Equation   ⁢           ⁢   3     )             
 
 wherein λ is the wavelength of the light, L(λ) is the loss of the light through LTOF  117 , and a is the angle between the polarization of a light beam and the optic axis of the rotative half-wave plate  113 . As shown in  FIG. 11 , when α=0°, the polarization of the beams E 1  and O 1  are horizontal and parallel to the surface of the paper after they pass through the rotative half-wave plate  113 . In this case, all the power of the beams O 1  and E 1  are transferred to the beams O 2  and O 3  respectively before O 2  and O 3  pass through the LTOF  117  twice. As illustrated in  FIG. 11 , the slope of the α=0° line  1101  is double that of the slope (denoted  1105 ) of the light loss L(λ). 
 
         [0037]     Conversely, when α=45°, the polarization of the beams E 1  and O 1  are rotated by 90° after passing through the rotative half-wave plate  113  and become vertical to the surface of the paper. In this scenario, all the power of the beams O 1  and E 1  are transferred to the beams E 2  and E 3  respectively before E 2  and E 3  are reflected back by the mirror  119  without passing through the LTOF  117 . As illustrated in  FIG. 11 , the slope of the α=45° line  11 . 09  is zero.  
         [0038]     Alternatively, when ax is between 0° and 45°, the slope of the attenuation profile of the DTGE shown in  FIG. 4  and  FIG. 5  are in a range from 0 to 2×0.05 dB/nm (assuming LTOF  117  has a slope of 0.05 dB/nm). Therefore, the DTGE shown in  FIG. 4  and  FIG. 5  uses a LTOF  117  with a fixed slope in order to dynamically control the slopes of the attenuation profile within the range of 0° and double the constant slope of LTOF  117 .  
         [0039]      FIG. 8  illustrates a block diagram  800  of the top view of a second embodiment of a DTGE using LTOF technology. Diagram  800  as shown in  FIG. 8  comprises: a first collimator denoted  801 , a first walk-off crystal denoted  803 , a first half-wave plate denoted  804 , a first variable Faraday rotator (“VFR”) denoted  805 , a second walk-off crystal denoted  807 , a LTOF denoted  809 , a third walk-off crystal denoted  811 , a second VFR denoted  813 , a second half-wave plate denoted  815 , a fourth walk-off crystal denoted  817 , and a second collimator denoted  819 .  
         [0040]      FIG. 9  illustrates a block diagram  900  of the side view of the second embodiment of DTGE shown in  FIG. 8 . Diagram  900  as shown in  FIG. 9  comprises: the first collimator.  801 , the first walk-off crystal  803 , the first half-wave plate  804 , the first VFR  805 , the second walk-off crystal  807 , the LTOF  809 , the third walk-off crystal  811 , the second VFR  813 , the second half-wave plate  815 , the fourth walk-off crystal  817 , and the second collimator  819 .  
         [0041]      FIG. 10  illustrates a flow chart  1000  of the steps for dynamically equalizing gain tilt with the DTGE shown in  FIG. 8  and  FIG. 9 . The components and steps described in  FIG. 10  are illustrated in  FIG. 8  unless otherwise noted. In step  1001 , collimator  801  collimates an input beam B 1 . The collimated B 1  then passes through the first walk-off crystal  803  that splits B 1  into two beams: an extraordinary beam denoted E 1  having a polarization parallel to the surface of the paper on which  FIG. 8  is drawn, and an ordinary beam denoted O 1  having a polarization vertical to the surface of the paper on which  FIG. 8  is drawn. Subsequently in step  1005 , the polarization of E 1  rotates by 90° after passing through the half-wave plate  804 . In step  1007 , the polarization of both E 1  and O 1  rotate by θ after passing through VFR  805 , wherein  0  is controlled by the current passing through VFR  805 . The beam O 1  is then split into an extraordinary beam E 2  and an ordinary beam O 2 , and the beam E 1  is split into an extraordinary beam E 3  and an ordinary beam O 3 ; after both E 1  and O 1  pass through the walk-off crystal  807  in step  1009 . After E 1  and O 1  are split, the ordinary beams O 2  (shown in  FIG. 9 ) and O 3  (not shown in  FIG. 9  but operates as O 2  does in  FIG. 9 ) pass through LTOF  809  in step  1011  as shown in  FIG. 9 . In step  1013 , the extraordinary beam E 2  (shown in  FIG. 9 ) combines with ordinary beam O 2  to form B 2  (shown in  FIG. 9 ) and extraordinary beam E 3  (not shown in  FIG. 9  but operates as E 2  does in  FIG. 9 ) combines with ordinary beam O 3  to form B 3  (not shown) after passing through the walk-off crystal  811 . The polarization of the beams B 2  and B 3  rotate by −θ after passing through the VFR  813 , wherein the currents in VFR  805  and in VFR  813  are identical in amplitude but opposite in direction. The polarization of beam B 2  rotates by 90° after passing through the half-wave plate  815  in step  1017  while the polarization of B 3  remains constant as shown in  FIG. 8  and  FIG. 9 . In step  1019 , the beams B 2  and B 3  combine to form B 4  after passing through the walk-off crystal  817 . The beam B 4  is subsequently collimated after passing through the collimator  1021 .  
         [0042]     The attenuation profile of the DTGE shown in  FIG. 8  and  FIG. 9  is defined as:  
                     H   ⁡     (     λ   ,   θ     )       =       -   10     ⁢   log   ⁢         I   out     ⁡     (   λ   )         I   in                     =       -   20     ⁢     log   ⁡     [         cos   2     ⁢   θ     +       E   ⁡     (   λ   )       ⁢     sin   2     ⁢   θ       ]                       (     Equation   ⁢           ⁢   4     )             
 
 wherein λ is the wavelength of the light, E(λ) is as defined by Equation 3, and θ is the rotate angle of VFR  805 . 
 
         [0043]     When θ=0°, the polarization of the beams E 1  and O 1  are horizontal and parallel to the surface of the paper of  FIG. 9  after passing through VFR  805 . In this case, all the power of the beams E 1  and O 1  are transferred to E 3  and E 2  respectively before E 2  and E 3  are directed into the walk-off crystal  811  without passing through LTOF  809  shown in  FIG. 8 . Therefore, as shown in  FIG. 12 , the slope of the θ=0° line  1207  is zero.  
         [0044]     Conversely, when θ=90°, the polarizations of the beams E 1  and O 1  rotate by 90° and vertical to the surface of the paper of  FIG. 9  after passing through VFR  805 . In this scenario, all the power of the beams E 1  and O 1  are transferred to O 3  and O 2  respectively, and O 2  and O 3  pass through LTOF  809  before entering the walk-off crystal  811 . Therefore, the slope of the θ=90° line  1201  is the same as that of E(λ) of the LTOF  809 .  
         [0045]     Alternatively, when θ is between 0° and 90°, the slope of attenuation profile of the DTGE shown in  FIG. 8  and  FIG. 9  are in a range within 0 to 0.05 dB/nm (assuming LTOF  809  has a slope of 0.05 dB/nm). Therefore, the DTGE shown in  FIG. 8  and  FIG. 9  uses a LTOF  117  with a fixed slope in order to dynamically control the slopes of the attenuation profile within the range of 0° and the constant slope of LTOF  809 .  
         [0046]     Although the invention has been described in connection with several e mbodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications that would be apparent to a person skilled in the art.  
         [0047]     For example, although the invention as described above is configured to flatten a positively sloped gain, the system may be adjusted in order to flatten a negatively sloped gain.  
         [0048]     The foregoing descriptions of specific-embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the arts to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the (claims appended hereto and their equivalents.