Abstract:
The present invention relates to a device and to a method for the wavelength selective filtration of optical wavelength channels. The device includes at least one 3 dB-coupler or at least one Q port circulator ( 40 ), where Q≧3, a 1×N WDM-(de)multiplexer ( 30 ), where N≧2, N number of waveguides ( 31, 32, 33, 34, 35, 36, 37  and  38 ), at least N number of reflection sections ( 61, 62, 63, 64, 65, 66, 67  and  68 ) and at least N number of variable optical attenuators ( 71, 72, 73, 74, 75, 76, 77  and  78 ). One of the ports on the circulator ( 40 ) or on the 3 dB-coupler is connected to a first side of the WDM-(de)multiplexer ( 30 ). Each waveguide ( 31, 32, 33, 34, 35, 36, 37  and  38 ) includes at least one variable optical attenuator ( 71, 72, 73, 74, 75, 76, 77  and  78 ) and at least one reflection section ( 61, 62, 63, 64, 65, 66, 67  and  68 ), such that at least one variable optical attenuator ( 71, 72, 73, 74, 75, 76, 77  and  78 ) will be located between a reflection section ( 61, 62, 63, 64, 65, 66, 67  and  68 ) and the WDN-(de)multiplexer ( 30 ).

Description:
FIELD OF INVENTION 
     The present invention relates to a device and to a method for optical filtering. 
     BACKGROUND OF THE INVENTION 
     Various methods are known for improving the capacity of existing optical networks. One method is to use so-called wavelength multiplexing technology (WDM) to improve the extent to which an optical fibre in the optical network can utilise available bandwidths. The wavelength can also be used as an information address in an optical network, in other words the information can be multiplexed on a number of channels which can then be processed individually in the network. This can cause different channels to be subjected to losses of different magnitudes, among other things because the different channels are attenuated to different extents in the filter and switching structures, pass through the network along paths of mutually different lengths, or are amplified to different extents in optical amplifiers. This imbalance can impair the quality of the transmitted information, due to the fact that a channel that has a low power level is easily disturbed by a channel that has a high power level, this phenomenon normally being referred to as crosstalk. 
     Consequently, it is desirable to incorporate in an optical network tuneable filters which will enable undesirable channels to be suppressed while amplifying desired channels. 
     Devices constructed in accordance with the present standpoint of techniques for tuneable filtering of optical channels are generally encumbered with one or more of the following defects: 
     Relatively high losses with respect to desired channels and poor suppression of remaining channels. 
     Other defects include reflections in the device which impair performance and cause disturbances in the transmission system as a whole. 
     Another drawback is that wavelength channels are filtered only over a narrow wavelength band. 
     Another drawback is that these known devices have an over-sharp filter profile (not system-friendly). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome the aforesaid problems and defects, at least partially. 
     This object is achieved in accordance with a first aspect of the invention by means of a device and a method for selective optical wavelength filtering. A filter includes a coupler or a circulator, a multiplexer/demultiplexer and a plurality of waveguides. Each of the plurality of waveguides includes a reflection section and a variable optical attenuator wherein the variable optical attenuator is disposed between the reflection section and the multiplexer/demultiplexer. One of the ports on the circulator or on the coupler is connected to a first side of the multiplexer/demultiplexer. 
     One advantage afforded by the present invention is that dispersion compensation can be achieved for each channel when the period in the grating structures is varied. 
     Another advantage afforded by the invention is that undesirable channels can be strongly suppressed. 
     One preferred embodiment affords the additional advantage of enabling desired channels to be amplified to great extent. 
    
    
     The invention will now be described in more detail with reference to preferred exemplifying embodiments thereof and with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates one embodiment of an inventive optical filter. 
     FIG. 2 illustrates another embodiment of an inventive optical filter. 
     FIG. 3 illustrates a further embodiment of an inventive optical filter. 
     FIG. 4 illustrates still another embodiment of an inventive optical filter. 
     FIG. 5 illustrates yet another embodiment of an inventive optical filter. 
     FIG. 6 illustrates an example of a variable attenuator that can be used with the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an embodiment of an inventive optical filter that includes four waveguides  32 ,  34 ,  36  and  38 , four reflection sections  62 ,  64 ,  66  and  68 , four variable optical attenuators  72 ,  74 ,  76  and  78 , a four-channel multiplexer/demultiplexer  30 , and a three-port optical circulator  40 . The circulator  40  may include more than three ports and those ports that are not used actively in the device will preferably be plugged. 
     One port  46  of the optical circulator is connected to a first side of the four-channel (de)multiplexer  30 . Four waveguides  32 ,  34 ,  36  and  38  are connected to the other side of the (de)multiplexer  30 . Each waveguide  32 ,  34 ,  36  and  38  includes a reflection section  62 ,  64 ,  66  and  68  and a variable optical amplifier  72 ,  74 ,  76  and  78 . The variable optical attenuators  72 ,  74 ,  76  and  78  are disposed between the (de)multiplexer  30  and respective reflection sections  62 ,  64 ,  66  and  68 . The variable optical attenuators will preferably operate in the manner of an on/off switch. 
     The (de)multiplexer may be constructed in accordance with the MMIMZI (Multi Mode Interference Mach Zehnder Interferometer) principle. The reflection sections may be Bragg gratings. A 3 dB-switch or coupler may be used instead of an optical circulator, although use of the switch may result in additional losses which can be considered a disadvantage. Furthermore, there may occur a reflection which can give rise to problems in the transmission system as a whole. 
     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  thereon. The wavelength channels are transmitted into the (de)multiplexer  30  and demultiplexed out on four waveguides  32 ,  34 ,  36  and  38 . 
     At least one wavelength channel is transmitted from the (de)multiplexer  30  to the waveguide  36 , for instance. If it is assumed that this wavelength channel is undesirable, the channel is attenuated one time by the variable optical attenuator  72  prior to being reflected by the reflection section  62 , and a second time after having been reflected by said reflection section. The wavelength channel then passes through the (de)multiplexer and is transmitted out on a third port  44  on the optical circulator  40 . 
     Assume that a desired wavelength channel is transmitted to the waveguide  34  via the optical circulator and the (de)multiplexer. The wavelength channel passes through the variable optical attenuator practically unnoticed, both prior to being reflected by the reflection section and subsequent to being reflected thereby. The wavelength channel then passes through the (de)multiplexer and is transmitted out on a third circulator port  44 . 
     FIG. 2 illustrates a second embodiment of an optical filter constructed in accordance with the invention. The filter includes four waveguides  32 ,  34 ,  36  and  38 , four reflection sections  62 ,  64 ,  66  and  68 , four variable optical attenuators  72 ,  74 ,  76  and  78 , a four-channel multiplexer/demultiplexer  30 , a switch  20 , a pump laser  10 , four amplifier sections  52 ,  54 ,  56  and  58  and a three-port optical circulator  40 . The optical circulator can also be replaced with a 3 dB-switch or coupler in this case. 
     One of the ports,  46 , of the optical circulator is connected to a first side of said four-channel (de)multiplexer  30 . Four waveguides  32 ,  34 ,  36  and  38  are connected to the other side of the (de)multiplexer  30 . Each waveguide  32 ,  34 ,  36  and  38  includes a reflection section  62 ,  64 ,  66  and  68 , an amplifier section  52 ,  54 ,  56  and  58  and a variable optical attenuator  72 ,  74 ,  76  and  78 . The variable optical attenuators  72 ,  74 ,  76  and  78  and the amplifier sections  52 ,  54 ,  56  and  58  are arranged between the (de)multiplexer  30  and respective reflection sections  62 ,  64 ,  66  and  68 . The variable optical attenuator is placed nearest the (de)multiplexer  30  in the FIG. 2 illustration. The positions of the variable optical attenuator and the amplifier section can be reversed. The amplifier section may be a plain wavelength amplifier or a fibre amplifier. The variable optical attenuator may be doubled and be seated both upstream and downstream of the amplifier section. In order for the position of the variable optical attenuator (preferably with an on/off switch function) between the reflection section and the amplifier section to be meaningful, the attenuator will be made so insensitive to wavelength that both the power of the pump wavelength and the power of the signal wavelength can be influenced in the same way by the variable optical attenuator. 
     The (de)multiplexer may, for instance, be constructed in accordance with the MMIMZI (Multi Mode Interference Mach Zehnder Interferometer) principle. The reflection sections may be Bragg gratings, for instance. The amplifier sections may be fibre amplifiers, for instance. A 3 dB-coupler may be used instead of an optical circulator. The switch may be constructed in accordance with the MMIMZI (Multi Mode Interference Mach Zehnder Interferometer) principle. 
     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 circulator port  46 . The wavelength channels are transmitted into the (de)multiplexer  30  and demultiplexed out on four waveguides  32 ,  34 ,  36  and  38 . 
     Assume that the channel desired is the channel that is coupled to waveguide  34 . The switch  20  is then set so as to connect the pump laser  10  to the waveguide  34  and passes through the amplifier section ( 54 ) and activates said section. The power of the signal reaching this amplifier section will then be amplified. 
     At least one wavelength channel is transmitted from the (de)multiplexer  30  to, for instance, the waveguide  32 . If it is assumed that this wavelength channel is undesirable, the wavelength channel is attenuated once by the variable optical attenuator  72  prior to being reflected by the reflection section  62 , and a second time after having been reflected by said reflection section. The wavelength channel passes through the (de)multiplexer and is transmitted out on a third port  44  of the optical circulator  40 . This wavelength channel can be influenced to a greater or lesser extent by the amplifier section. 
     Assume that a desired wavelength channel is transmitted to the waveguide  34  via the optical circulator and the (de)multiplexer. This wavelength channel passes through the variable optical attenuator  74  practically unnoticed and can then be amplified via the amplifier section  54  before being reflected by the reflection section  64 . Laser light is pumped from a pump laser  10 , via a switch  20 , into the waveguide in which it is desired to amplify a given wavelength. In the illustrated case, when the desired channel is located in waveguide  34 , the switch  20  is set so that laser light will be pumped into said waveguide. After the wavelength channel has been reflected by the reflection section  64 , the wavelength channel is amplified one more time via said amplifier section  54  and then passes practically unnoticed through the attenuator  74 , which in the present case attenuates said wavelength minimally. The wavelength channel then passes into the (de)multiplexer and is transmitted out through a third port  44  on the optical circulator  40 . 
     Each of the optical attenuators  72 ,  74 ,  76  and  78  can be handled individually, therewith enabling the signal strengths of the various wavelengths demultiplexed out to the different waveguides  32 ,  34 ,  36  and  38  can be controlled separately and independently of each other. 
     FIG. 3 illustrates another embodiment of an optic filter constructed in accordance with the invention. The filter includes two pump lasers  10  and  12 , a switch  20  (which may be an MMIMZI-based switch), four waveguides  32 ,  34 ,  36  and  38 , four amplifier sections  52 ,  54 ,  56  and  58 , four reflections sections  62 ,  64 ,  66  and  68 , four variable optical attenuators  72 ,  74 ,  76  and  78 , a four-channel (de)multiplexer  30  and a 3 dB-coupler  40 . As in the earlier mentioned cases, the 3 dB-coupler may be replaced with an optical circulator. 
     The 3 dB-coupler, or switch, is connected through one of its ports  46  to a first side of said four-channel (de)multiplexer  30 . Four waveguides  32 ,  34 ,  36  and  38  are connected to a second side of the (de)multiplexer  30 . Each waveguide  32 ,  34 ,  36  and  38  includes an amplifier section  52 ,  54 ,  56  and  58 , a variable optical attenuator  72 ,  74 ,  76  and  78 , and a reflection section  62 ,  64 ,  66  and  68 . The amplifier section  52 ,  54 ,  56  and  58  and the variable optical attenuator  72 ,  74 ,  76  and  78  are disposed between the (de)multiplexer  30  and respective reflection sections  62 ,  64 ,  66  and  68 . The pump lasers  10  and  12  are connected to a first side of the switch  20 . In the illustrated case, the variable optical attenuators are arranged nearest the (de)multiplexer  30 . 
     The positions of the amplifier sections and the variable optical attenuators can be reversed. One prerequisite for positioning the variable optical attenuator (preferably with an on/off switch function) between the reflection section and the amplifier section to be meaningful is that it can be given a wavelength insensitivity such that the power of the signal wavelength and the power of the pump wavelength can be influenced in the same way by the variable optical attenuator. The optical circulator can be replaced with a 3 dB-coupler. 
     Optical wavelength channels are transmitted in through a first port  42  of the 3 dB-coupler  40 . These wavelength channels pass through the coupler and are transmitted out through a second coupler port  46 . The wavelength channels are transmitted into the (de)multiplexer  30  and are demultiplexed out on four waveguides  32 ,  34 ,  36  and  38 . 
     At least one wavelength channel is transmitted, e.g., to the waveguide  32  from the (de)multiplexer  30 . If this wavelength channel is undesirable, the channel is attenuated for a first time by the variable optical attenuator  72  and then passes through the amplifier station and can be influenced by said section to a greater or lesser extent and thereafter reflected by the reflection section  62 . 
     The wavelength channel then passes a second time through the amplifier section  52  and can be influenced thereby to a greater or lesser extent and thereafter attenuated in the attenuator  72  for a second time. 
     A desirable wavelength channel can be transmitted, e.g., to waveguide  34 . This wavelength channel passes practically unnoticed through the attenuator  74  for a first time. The wavelength channel is then amplified by the amplifier section  54  for a first time prior to said wavelength channel being reflected by the reflection section  64 . Amplification is controlled by pumping laser light into the waveguide in which amplification of a certain wavelength channel is desired. In the case of the illustrated embodiment, this laser light is pumped into the waveguide by means of two pump lasers  10  and  12 , via a switch  20 . The switch is set so that laser light will be excited into the correct waveguide. The pump lasers preferably operate with mutually the same amplification wavelength, although these wavelengths can, of course, differ from one another. Preferably, only one laser is switched-on while the other functions as a backup. 
     Subsequent to said wavelength channel having been reflected by the reflection section  64 , the channel is amplified once more via the amplifier section  54 , and then passes through the attenuator  74  practically unnoticed, said attenuator  74  attenuating said wavelength minimally in the illustrated case. The wavelength channel then passes into the (de)multiplexer and is transmitted out through a third port  44  on the 3 dB-coupler  40 . 
     Each of the optical attenuators  72 ,  74 ,  76  and  78  can be handled individually, therewith enabling respective signal strengths of the different wavelengths that are demultiplexed out to the various waveguides  32 ,  34 ,  36  and  38  can be regulated separately and independent of each other. 
     FIG. 4 illustrates a further embodiment of an inventive optic filter that can also be used as an amplifying channel equaliser. The channel equaliser includes four pump lasers  10 ,  12 ,  14  and  16 , four waveguides  32 ,  34 ,  36  and  38 , four amplifier sections  52 ,  54 ,  56  and  58 , four reflection sections  62 ,  64 ,  66  and  68 , four variable optical attenuators  72 ,  74 ,  76  and  78 , a four-channel multiplexer/demultiplexer  30  and a three-port optical circulator  40 . 
     One of the ports,  46 , of the optical circulator  40  is connected to a first side of said four-channel (de)multiplexer  30 . Four waveguides  32 ,  34 ,  36  and  38  are connected to the second side of the (de)multiplexer  30 . Each waveguide  32 ,  34 ,  36  and  38  includes an amplifier section  52 ,  54 ,  56  and  58  and a reflection section  62 ,  64 ,  66  and  68 , a variable optical attenuator  72 ,  74 ,  76  and  78  and a pump laser  10 ,  12 ,  14  and  16 . The amplifier sections  52 ,  54 ,  56  and  58  and the variable optical attenuators  72 ,  74 ,  76  and  78  are disposed between the (de)multiplexer  30  and respective reflection sections  62 ,  64 ,  66  and  68 . Respective pump lasers  10 ,  12 ,  14  and  16  are arranged at the end of each waveguide  32 ,  34 ,  36  and  38 . 
     Optical wavelength channels are transmitted in through a first port  42  of the optical circulator  40 . These wavelength channels pass through the circulator and are transmitted out through a second port  46  thereon. The wavelength channels are transmitted into the (de)multiplexer  30  and demultiplexed out on four waveguides  32 ,  34 ,  36  and  38 . 
     At least one wavelength channel is transmitted from the (de)multiplexer  30  to the waveguide  32 , for instance. If this wavelength channel is undesirable, it is attenuated a first time by the variable optical attenuator  72  and then passed through the amplifier section  52  in which it can be influenced to a greater or lesser extent, and thereafter reflected by the reflection section  62 . 
     The wavelength channel then passes a second time through the amplifier section  52  a second time, in which it is influenced to a greater or lesser extent, and thereafter attenuated a second time by the variable optical attenuator  72 . The wavelength channel then passes through the (de)multiplexer and is transmitted out through a third port on the optical circulator. 
     A desired wavelength channel can be coupled to wave conductor  34 , for instance. This wavelength channel passes practically unnoticed through the attenuator  74  a first time. The wavelength channel is then amplified in the amplifier section  54  for a first time prior to said wavelength channel being reflected by the reflection section  64 . Amplification is controlled by pumping laser light into the waveguide in which amplification of a certain wavelength channel is desired. In the case of the illustrated embodiment, the laser light is pumped by a pump laser  12  provided at the end of the waveguide  34 . After the wavelength channel has been reflected by the reflection section  64 , the wavelength channel is amplified for a second time by the amplifier section  54  and then passes practically unnoticed through the variable optical attenuator  74 . The wavelength channel then passes through the (de)multiplexer  30  and is transmitted out through a third port  44  on the circulator  40 . 
     Each of the pump lasers is able to transmit with different powers independently of one another, i.e. respective pump lasers  10 ,  12 ,  14  and  16  regulate the extent to which the amplifier sections  52 ,  54 ,  56  and  58  shall amplify, therewith enabling respective signal strengths of the various wavelengths that are demultiplexed out to the different waveguides  32 ,  34 ,  36  and  38  can be regulated separately and independently of each other. 
     FIG. 5 illustrates yet another embodiment of an inventive filter. The filter includes pump lasers  10 ,  12 ,  14  and  16 , two switches  20  and  22 , eight waveguides  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  and  38 , eight amplifier sections  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57  and  58 , eight reflection sections  61 ,  62 ,  63 ,  64 ,  65 ,  66 ,  67  and  68 , eight variable optical attenuators  71 ,  72 ,  73 ,  74 ,  76 ,  76 ,  77  and  78 , one eight-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 eight-channel (de)multiplexer  30 . Eight waveguides  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  and  38  are connected to the second side of the (de)multiplexer  30 . Each waveguide  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  and  38  includes an amplifier section  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57  and  58 , a variable optical attenuator  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77  and  78 , and a reflection section  61 ,  62 ,  63 ,  64 ,  65 ,  66 ,  67  and  68 . The amplifier sections  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57  and  58  and the variable optical attenuators  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77  and  78  are arranged between the (de)multiplexer  30  and respective reflection sections  61 ,  62 ,  63 ,  64 ,  65 ,  66 ,  67  and  68 . The pump lasers  10  and  12  are connected to a first side of the switch  20 , while pump lasers  14  and  16  are connected to a first side of the switch  22 . The pump lasers  10  and  12  preferably operate at mutually the same wavelengths. The pump lasers  14  and  16  also preferably operate at mutually the same wavelengths, these wavelengths either being the same as those at which the pump lasers  10  and  12  operate or differ therefrom. The waveguides  31 ,  32 ,  33  and  34  are connected to a second side of the switch  20 , while the waveguides  35 ,  36 ,  37  and  38  are connected to a second side of the switch  22 . 
     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  thereon. The wavelength channels are transmitted into the (de)multiplexer  30  and demultiplexed out on eight waveguides  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  and  38 . 
     At least one wavelength channel is transmitted from the (de)multiplexer  30  to the waveguide  31 , for instance. If this wavelength channel is undesired, the channel is attenuated for a first time by the variable optical attenuator  71 , and thereafter passes through the amplifier section  51  in which it can be influenced to a greater or lesser extent thereby, and is thereafter reflected by the reflection section  61 . 
     The wavelength channel then passes for a second time through the amplifier section  51  and can again be influenced to a greater or lesser extent thereby, and thereafter attenuated for a second time by the variable optical attenuator  71 . The wavelength channel then passes into the (de)multiplexer and in transmitted out through a third port  44  on the optical circulator  40 . 
     A desired wavelength channel can be transmitted to waveguide  35 , for instance. This wavelength channel passes through the attenuator  75  for a first time, practically unnoticed. The wavelength channel is thereafter amplified by the amplifier section  55  for a first time, prior to said channel being reflected by the reflection section  65 . Amplification is controlled by pumping laser light into the waveguide in which it is desired to amplify a certain wavelength channel. In the case of this embodiment, the laser light is pumped by two pump lasers  14  and  16  connected via a switch  22 . The two lasers will preferably operate one at a time. 
     After the wavelength channel has been reflected by the reflection section  65 , the channel is amplified for a second time by the amplifier section  55  and then passes through the variable optical attenuator  75  practically unnoticed. 
     The wavelength channels then pass through the (de)multiplexer  30  and are transmitted out through a third port  44  on the circulator  40 . 
     Each of the variable optical attenuators  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77  and  78  can be handled individually, thereby enabling the respective signal strengths of the various wavelengths that are demultiplexed out to the different waveguides  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37  and  38  can be regulated separately and independently of each other. As earlier mentioned, the variable optical attenuators operate in a manner similar to an on/off switch. 
     FIG. 6 illustrates a variable optical attenuator that can be used to advantage in the invention. The variable optical attenuator includes two 1×2 MMI-waveguides  110  and  120 , two Mach Zehnder waveguides  80  and  90 , a phase control element  132  and a trim section  134 . The MMI-waveguides  110  and  120  are connected together via said two Mach Zehnder waveguides  80  and  90 . A first Mach Zehnder waveguide  80  includes said phase control element  132 , and a second Mach Zehnder waveguide  90  includes said trim 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 accompanying claims.