Abstract:
The present invention relates to an arrangement and a method relating to optical channel equalizers. The channel equalizer includes at least one Q-port direction-dependent router ( 40 ), where Q≧3, one N-channel WDM (de)multiplexer ( 30 ), where N≧2, N-number of amplifying waveguides ( 31, 32, 33, 34, 35, 36, 37  and  38 ), at least N-number of fiber amplifiers ( 51, 52, 53, 54, 55, 56, 57  and  58 ), at least N-number of Bragg gratings ( 61, 62, 63, 64, 65, 66, 67  and  68 ), at least N-number of variable optical pump laser attenuators ( 71, 72, 73, 74, 75, 76, 77  and  78 ), at least one optical splitter ( 20  and  22 ) and at least one pump laser ( 10, 12, 14  and  16 ) per optical splitter ( 20  and  22 ). At least one of the ports ( 42, 44  and  46 ) on the direction-dependent router ( 40 ) is disposed on a first side of the WDM (de)multiplexer ( 30 ). Each amplifying waveguide ( 31, 32, 33, 34, 35, 36, 37  and  38 ) includes at least one fiber amplifier ( 51, 52, 53, 54, 55, 56, 57  and  58 ) and at least one Bragg grating ( 61, 62, 63, 64, 65, 66, 67  and  68 ). At least one fiber amplifier ( 51, 52, 53, 54, 55, 56, 57  and  58 ) is disposed between a Bragg grating ( 61, 62, 63, 64, 65, 66, 67  and  68 ) and the WDM (de)multiplexer ( 30 ). At least one variable optical pump laser attenuator ( 71, 72, 73, 74, 75, 76, 77  and  78 ) is disposed between each last Bragg grating ( 61, 62, 63, 64, 65, 66, 67  and  68 ) and a first side of said optical splitter ( 20  and  22 ). The pump laser ( 10, 12, 14  and  16 ) is disposed on a second side of the optical splitter ( 20  and  22 ).

Description:
FIELD OF INVENTION 
     The present invention relates to an optical device for achieving channel equalizing amplification of the power level of optical wavelength channels. 
     BACKGROUND ART 
     Several different methods of increasing the capacity of existing optical networks are known. One way is to use so-called wavelength multiplexing (WDM) techniques to enhance the extent to which available bandwidths can be utilised on an optical fibre in the optical Network. In an optical network, the wavelength can also be used as an information address, that is to say the information can be multiplexed on a number of channels which can then be processed individually in the network. This can result in different channels being subjected to losses of different magnitudes, among other things because the various channels are attenuated to differing degrees in filter structures and in switch structures, because said channels take paths of different lengths through the network, or because said channels are amplified to different extents in optical amplifiers. This imbalance can impair the quality of the transmitted information, since a channel that has a low power level can be easily disturbed by a channel that has a high power level, which is normally referred to as cross-talk. 
     One known device that achieves channel equalization of optical channels is an equalizer based on multiplexing/demultiplexing elements and variable optical attenuators. The problem with this solution is that the optical channels are equalized by attenuating high power levels. Another problem with this solution is that performance impairing interference powers can occur. 
     SUMMARY OF THE INVENTION 
     Any one of a number of known methods can be used to increase the capacity of an optical transmission system. In the case of wavelength multiplexing for instance, transmission channels are multiplexed and demultiplexed on different carrier wave lengths to and from an information stream respectively, This multiplexing and demultiplexing requires the presence of optical wavelength selective devices. Different transmission channels are subjected to losses of different high magnitudes, among other things because the various transmission channels are attenuated to different extents in filter and switch structures, because said channels pass through the network in paths of mutually different lengths, or because the channels are amplified to different extents in optical amplifiers. 
     One problem with known channel equalizers is that they attenuate the highest channel-power levels, which is a waste of power and can considerably impair performance. 
     Another problem with known channel equalizers is that they are sensitive to interference powers, which can result in further impairment of performance. 
     The present invention addresses these problems with an optical channel equalizer that includes at least one direction-dependent router that has Q-number of ports, where Q≧3, one WDM (de)multiplexer that has N-number of channels, where N≧2, N-number of amplifying waveguides, where each amplifying waveguide includes at least one fibre amplifier and at least one Bragg grating, at least N number of variable optical pump laser attenuators, at least one optical splitter, and at least one pump laser per optical splitter. At least one of the ports On the direction-dependent router is disposed on a first side of the N-channel WDM (de)multiplexer. At least one fibre amplifier is disposed between a Bragg grating and the WDM (de)multiplexer. At least one variable optical pump laser attenuator is disposed between each last Bragg grating and a first side of said optical splitter. The pump laser is disposed on the other side of the optical splitter. 
     In a preferred embodiment of the inventive channel equalizer, the Q-port direction-dependent router is a Q-port optical circulator. 
     The N-channel WDM (de)multiplexer may, for instance, be an AWG (Arrayed Waveguide Grating) or an MMIMZI (Multi Mode Interference Mach-Zehnder Interferometer). 
     In another embodiment of the inventive channel equalizer, said equalizer includes at least one Q-port direction-dependent router, where Q≧3, an N-channel WDM (de)multiplexer, where N≧2, N-number of amplifying waveguides, where each amplifying waveguide includes at least one fibre amplifier and at least one Bragg grating, and at least one pump laser per amplifying waveguide. At least one of the ports on the direction-dependent router is disposed on a first side of said N-channel WDM (de)multiplexer. At least one fibre amplifier is disposed between a Bragg grating and the WDM (de)multiplexer. The pump laser is disposed at the end of each amplifying waveguide. 
     In one method according to the present invention for equalizing the power level of optical channels, optical wavelength channels are first transmitted into a first port on a Q-port direction-dependent router. The wavelength channels are then transmitted out through a second port on said router, which is disposed on a first side of an N-channel WDM (de)multiplexer. The wavelength channels are then transmitted through said WDM (de)multiplexer. Different wavelength channels are then transmitted through different amplifying waveguides, For each amplifying waveguide, a wavelength channel passes at least one optical amplifier before being reflected by a Bragg grating. Laser light is pumped into each amplifying waveguide in a direction towards the WDM (de)multiplexer. The reflected optical wavelength channels are transmitted through said WDM (de)multiplexer. These reflected wavelength channels are transmitted in through said second port on said Q-port direction-dependent router, so as to be finally transmitted out through a third port on said router. 
     The object of the present invention is to provide an arrangement for channel equalizing amplification of the power level of WDM channels, with which channels that have a low power level are amplified to a greater extent that channels that have high power levels. One advantage afforded by the present invention is that dispersion compensation can be made for each channel when the period in the grating structures varies. 
     Another advantage afforded by the invention is that its performance in other respects can be improved relative to known techniques, for instance with respect to cross-talk and the like. 
     Another advantage afforded by the present invention is that a high level of reliability can be achieved by using a solution in which at least two pump lasers pump laser light to all fibre amplifiers disposed in the amplifying waveguides, and in which at least one of these pump lasers can be driven harder when replacing a malfunctioning pump laser. 
     The present invention will now be described in more detail with reference to preferred exemplifying embodiments thereof and also with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an embodiment of an inventive optical channel equalizer. 
     FIG. 2 illustrates another embodiment of an inventive optical channel equalizer. 
     FIG. 3 illustrates yet another embodiment of an inventive optical channel equalizer. 
     FIG. 4 illustrates still another embodiment of an inventive optical channel equalizer. 
     FIG. 5 illustrates an example of a variable attenuator that can be used in conjunction with the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an embodiment of a channel equalizer according to the invention. The channel equalizer includes a pump laser  10 , an MMI-based splitter  20 , four amplifying waveguides  32 ,  34 ,  36  and  38 , four fibre amplifiers  52 ,  54 ,  56  and  58 , four Bragg gratings  62 ,  64 ,  66  and  68 , four variable optical pump laser attenuators  72 ,  74 ,  76  and  78 , one four-channel multiplexer/demultiplexer  30  and one three-port optical circulator  40 . 
     One of the ports,  46 , of the optical circulator is located on a first side of the four-channel (de)multiplexer  30 . Four amplifying waveguides  32 ,  34 ,  36  and  38  are provided on a second side of the (de)multiplexer  30 . Each amplifying waveguide  32 ,  34 ,  36  and  38  includes a fibre amplifier  52 ,  54 ,  56  and  58  and a Bragg grating  62 ,  64 ,  66  and  68 . The fibre amplifiers  52 ,  54 ,  56  and  58  are disposed between the (de)multiplexer  30  and respective Bragg gratings  62 ,  64 ,  66  and  68 . Variable optical pump laser attenuators  72 ,  74 ,  76  and  78  are disposed between a second side of the splitter  20  and the Bragg gratings  62 ,  64 ,  66  and  68 . A pump laser is disposed on a first side of the splitter  20 . 
     Optical wavelength channels are transmitted in through a first port  42  on the optical circulator  40 . These wavelength channels pass through the circulator and are transmitted out through a second port  46  on the circulator. The wavelength channels are transmitted into the (de)multiplexer  30  and demultiplexed out on four amplifying waveguides  32 ,  34 ,  36  and  38 . 
     At least one wavelength channel is transmitted from the (de)multiplexer  30  to the amplifying waveguide  32 , for instance. The wavelength channel passes through and is amplified in the fibre amplifying section  52  a first time, and is then reflected by the Bragg grating  62 . This reflected wavelength channel then passes through and is amplified in the fibre amplifier a second time. The variable optical pump laser attenuator  72  disposed between the second side of the splitter  20  and the Bragg grating  62  controls the extent to which the fibre amplifying section shall amplify, i.e. it regulates the effective energy of the pump laser  10  to the fibre amplifier  52 . Each of the optical pump laser attenuators  72 ,  74 ,  76  and  78  can be handled individually therewith enabling respective signal strengths of the various wavelengths that are demultiplexed out to the various amplifying waveguides  32 ,  34 ,  36  and  38  to be regulated separately and independently of each other. The wavelength channels are mutliplexed in the (de)multiplexer  30  after having been reflected by the Bragg gratings  62 ,  64 ,  66  and  68 . The wavelength channels are transmitted to the second port on the circulator and pass out through a third port on said circulator. 
     FIG. 2 illustrates another embodiment of an inventive channel equalizer. The channel equalizer includes two pump lasers  10  and  12 , one MMI-based splitter  20 , four amplifying waveguides  32 ,  34 ,  36  and  38 , four fibre amplifiers  52 ,  54 ,  56  and  58 , four Bragg gratings  62 ,  64 ,  66  and  68 , four variable optical pump laser attenuators  72 ,  74 ,  76  and  78 , one four-channel (de)multiplexer  30  and one three-port optical circulator  40 . 
     One of the ports,  46 , of the optical circulator is connected to a first side of said four-channel (de)multiplexer  30 . Connecting with the second side of the (de)multiplexer  30  are four amplifying waveguides  32 ,  34 ,  36  and  38 . Each amplifying waveguide  32 ,  34 ,  36  and  38  includes a respective fibre amplifier  52 ,  54 ,  56  and  58  and a respective Bragg grating  62 ,  64 ,  66  and  68 . The fibre amplifiers  52 ,  54 ,  56  and  58  are disposed between the (de)multiplexer  30  and respective Bragg gratings  62 ,  64 ,  66  and  68 . Variable optical pump laser attenuators  72 ,  74 ,  76  and  78  are disposed between a second side of the splitter  20  and the Bragg gratings  62 ,  64 ,  66  and  68 . Pump lasers  10  and  12  are disposed on a first side of the splitter  20 . 
     Optical wavelength channels are transmitted in through a first port  42  on the optical circulator  40 . These wavelength channels pass through the circulator and are transmitted out through a second port  46  on said circulator. The wavelength channels are transmitted into the (de)multiplexer  30  and are demultiplexed out on four amplifying waveguides  32 ,  34 ,  36  and  38 . 
     At least one wavelength channel is transmitted from the (de)multiplexer  30  to the amplifying waveguide  32 , for instance. This wavelength channel passes through and is amplified in the fibre amplifying section  52  for a first time and is then reflected by the Bragg grating  62 . The reflected wavelength channel then passes through and is amplified by the fibre amplifier for a second time. The variable attenuator  72  disposed between the second side of the splitter  20  and the Bragg grating  62  controls the extent to which the fibre amplifying section amplifies, i.e. it regulates the effective energy of the pump lasers  10  and  12  to the fibre amplifier  52 . Each of the optical attenuators  72 ,  74 ,  76  and  78  can be handled individually, therewith enabling respective signal strengths of the various wavelengths that are demultiplexed out to the various amplifying waveguides  32 ,  34 ,  36  and  38  to be regulated separately and independently of each other. The wavelength channels are multiplexed in the (de)multiplexer  30  after having been reflected by the Bragg gratings  62 ,  64 ,  66  and  68 . The wavelength channels are transmitted to the second port on the circulator and pass out through a third port thereon. 
     FIG. 3 shows another embodiment of an inventive channel equalizer. The channel equalizer includes four pump lasers  10 ,  12 ,  14  and  16 , four amplifying waveguides  32 ,  34 ,  36  and  36 , four fibre amplifiers  52 ,  54 ,  56  and  58 , four Bragg gratings  62 ,  64 ,  66  and  68 , one four-channel multiplexer/demultiplexer  30 , and one three-port optical circulator  40 . 
     The optical circulator is disposed with one of its ports  46  on a first side of the four-channel (de)multiplexer  30 . Disposed on the other side, or second side, of the (de)multiplexer  30  are four amplifying waveguides  32 ,  34 ,  36  and  38 . Each amplifying waveguide  32 ,  34 ,  36  and  38  includes a fibre amplifier  52 ,  54 ,  56  and  58  and a Bragg grating  62 ,  64 ,  66  and  68 . The fibre amplifiers  52 ,  54 ,  56  and  58  are disposed between the (de)multiplexer  30  and respective Bragg gratings  62 ,  64 ,  66  and  68 . A respective pump laser  10 ,  12 ,  14  and  16  is disposed at the end of each amplifying waveguide  32 ,  34 ,  36  and  38 . 
     Optical wavelength channels are transmitted in through a first port  42  on the optical circulator  40 . These wavelength channels pass through the circulator and are transmitted out through a second port  46  on said circulator. The wavelength channels are transmitted into the (de)multiplexer  30  and demultiplexed out on four Mach-Zehnder waveguides  32 ,  34 ,  36  and  38 . 
     At least one wavelength channel is transmitted from the (de)multiplexer  30  to the amplifying waveguide  32 , for instance. This wavelength channel passes through and is amplified in the fibre amplifying section  52  for a first time and is then reflected by the Bragg grating  62 . Said wavelength channel then passes through and is amplified by the fibre amplifier for a second time. Each of the pump lasers can transmit at different powers independently of one another, i.e. respective pump lasers  10 ,  12 ,  14  and  16  control the extent to which the fibre amplifiers  52 ,  54 ,  56  and  58  shall amplify, therewith enabling the signal strengths of the different wavelengths that are demultiplexed out to the various amplifying waveguides  32 ,  34 ,  36  and  38  to be regulated separately and independently of each other. The wavelength channels are multiplexed in the (de)multiplexer  30  after having been reflected by the Bragg gratings  62 ,  64 ,  66  and  68 . The wavelength channels are transmitted to the second port on the circulator and pass out through a third port thereon. 
     FIG. 4 illustrates yet another embodiment of an inventive channel equalizer. The channel equalizer includes four pump laser  10 ,  12 ,  14  and  16 , two MMI-based splitters  20  and  22 , eight amplifying waveguides  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  and  38 , eight fibre amplifiers  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57  and  58 , eight Bragg gratings  61 ,  62 ,  63 ,  64 ,  65 ,  66 ,  67  and  68 , eight variable attenuators  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77  and  78 , one four-channel (de)multiplexer  30  and one three-port optical circulator  40 . 
     One of the ports  46  of the optical circulator is disposed on a first side of the eight-channel (de)multiplexer  30 . Disposed on the other side, or second side, of the (de)multiplexer  30  are eight amplifying waveguides  31 ,  32 , is  33 ,  34 ,  35 ,  36 ,  37  and  38 . Each amplifying waveguide  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  and  38  includes a fibre amplifier  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57  and  58  and a Bragg grating  61 ,  62 ,  63 ,  64 ,  65 ,  66 ,  67  and  69 . The fibre amplifiers  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57  and  58  are disposed between the (de)multiplexer  30  and respective Bragg gratings  6 i,  62 ,  63 ,  64 ,  65 ,  66 ,  67  and  68 . Variable optical pump laser attenuators  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77  and  78  are disposed between a second side of the splitter  20  and  22  and the Bragg gratings  61 ,  62 ,  63 ,  64 ,  64 ,  65 ,  66 ,  67  and  68 , The pump lasers  10  and  12  are disposed on a first side of the splitter  20  while pump lasers  14  and  16  are disposed on a first side of the splitter  22 . The pump lasers  10  and  12  can, advantageously, transmit on different wavelengths. The pump lasers  14  and  16  can also, advantageously, be transmitted on different wavelengths, either the same wavelengths as the pump lasers  10  and  12  or on wavelengths different therefrom. 
     Optical wavelength channels are transmitted in through a first port  42  on the optical circulator  40 . These wavelength channels pass through the circulator and are transmitted out through a second port  46  on said circulator. The wavelength channels are transmitted into the (de)multiplexer  30  and demultiplexed out on eight amplifying waveguides  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  and  28 . 
     For instance, at least one wavelength channel is transmitted from the (de)multiplexer  30  to the amplifying waveguide  31 . This wavelength channel passes through and is amplified in the fibre amplifying second  51  for a first time, and is then reflected by the Bragg grating  61 . Said wavelength channel then passes through and is amplified in the fibre amplifier for a second time. The variable optical pump laser attenuator  71  disposed between the second side of the splitter  20  and the Bragg grating  61  controls the extent to which the fibre amplifying section shall amplify, in other words it regulates the effective energy delivered by the pump lasers  10  and  12  to the fibre amplifier  52 . Each of the optical pump laser attenuators  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77  and  78  can be handled individually, therewith enabling respective signal strengths of the various wavelengths that are demultiplexed out to the various amplifying waveguides  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  and  38  to be regulated separately and independently of each other. The wavelength channels are multiplexed in the (de)multiplexer  30  after having been reflected by the Bragg gratings  61 ,  62 ,  63 ,  64 ,  65 ,  66 ,  67  and  68 . The wavelength channels are transmitted to the second port on the circulator and pass out through a third port thereon. 
     FIG. 5 illustrates a variable optical pump laser attenuator  72  that can be used advantageously in the present invention. The variable attenuator  72  includes two 1×2 MMI waveguides  110  and  120 , two Mach-Zehnder waveguides so and  90 , one phase control element  132  and one trimming section  134 . The MMI waveguides  110  and  120  are interconnected via said two Mach-Zehnder waveguides  80  and  90 . A first Mach-Zehnder waveguide  80  includes said phase control element  132 , while a second Mach-Zehnder waveguide  90  includes said trimming section  134 . 
     It will be understood that the invention is not restricted to the aforedescribed and illustrated exemplifying embodiments thereof, and that modifications can be made within the scope of the following claims.