Abstract:
The present invention relates to a device and a method for wavelength selective filtration of optical wavelength channels. The wavelength selective filter ( 1 ) includes one or more wavelength selective add-drop elements ( 10, 12, 14  and  16 ) and one or more on-off switches ( 20, 22, 24  and  26 ), wherein a first and a second side of the wavelength selective add-drop elements ( 10, 12, 14  and  16 ) includes an input and an output, wherein said on-off switches ( 20, 22, 24  and  26 ) are disposed between said input and output on the second side of the wavelength-selective add-drop elements ( 10, 12, 14  and  16 ), and wherein in the event of more than one wavelength selective add-drop element said elements are interconnected by means of a connecting waveguide between respective outputs and inputs on the first side of respective wavelength selective add-drop elements.

Description:
FIELD OF INVENTION 
     The present invention relates to a wavelength selective filter for tuneable filtration of one or more wavelength channels from a stream of Q-number of wavelength channels and to a method for wavelength selective filtering of a wavelength channel from a stream of Q-number of wavelength channels. 
     BACKGROUND OF THE INVENTION 
     There are known to the art a number of different methods by means of which the capacity of an existing optical network can be further increased. One method is to use so-called wavelength multiplexing technology (WDM) to improve the extent to which an available bandwidth can be exploited on an optical fibre in the optical network. 
     Wavelengths can also be used as an information address in an optical network, that is to say information can be multiplexed on a number of channels and these channels treated individually in the network. It may also be desirable to redirect traffic in the optical network. Filtering may be necessary in order to reduce crosstalk, for instance immediately upstream of an optical receiver (detector). A tuneable filter may be required to enable selection of an optical detection channel. 
     Tuneable filters according to the present standpoint of techniques generally have one or more of the following drawbacks. 
     Relatively high losses in respect of desired channels, and poor suppression of other channels. 
     Reflections in the device, which may be a problem to the transmission system as a whole. 
     An excessively pointed filter profile (not system-friendly). 
     Expensive devices. 
     Tuning is only possible over a narrow wavelength band. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to at least reduce these problems. 
     According to a first aspect of the invention, there is provided a wavelength selective filter for tuneable filtration of a wavelength channel from a stream of Q-number of wavelength channels. The filter includes one or more wavelength selective add-drop elements and one or more on-off switches wherein a first and a second side of the wavelength selective add-drop elements include an input and an output, wherein said on-off switches are disposed between said input and output on said second side of said wavelength selective add-drop elements. In the event of more than one wavelength selective add-drop element, said elements are interconnected by a connecting waveguide between respective outputs and inputs on the first side of the wavelength selective add-drop element. 
     According to another aspect of the present invention, there is provided a wavelength selective filter for tuneable filtration of one or more wavelength channels from a stream of Q-number of wavelength channels. The filter includes one or more devices for filtering out a specific wavelength channel, wherein each device includes an input and an output, a part through which solely said specific wavelength channel passes through and which includes at least one on-off switch which when in an off-mode prevents said specific wavelength channel from passing through said device for filtering out a specific wavelength channel. In the event of more than one device for filtering out a specific wavelength channel, said devices are interconnected by a connecting waveguide between the output of one device and the input of the other device, so as to enable several channels to be filtered out independently of each other. 
     According to another aspect of the present invention, there is provided a method for wavelength selective filtering of a wavelength channel from a stream of Q-number of wavelength channels. The method comprises inputting optical wavelength channels on an input provided on a first side of a first wavelength selective add-drop element and reflecting a specific wavelength channel and thereafter outputting said channel on an output provided on said first side of said wavelength selective add-drop element. The method also comprises outputting wavelength channels that are transmitted through said add-drop elements on an output provided on a second side of the add-drop element and blocking said wavelength channels or feeding said channels back to an input provided on said second side of the add-drop element. The method further comprises transmitting said fed-back wavelength channels through the add-drop element and outputting said channels on the output on the first side of said add-drop element and coupling the wavelength channels from the output on the first side of the add-drop element with an input on a first side of a second add-drop element and thereafter repeating the same procedure in respect of said add-drop element and the N-number of subsequent add-drop elements. 
     One advantage afforded by the present invention is that it improves performance with respect to crosstalk and the like, when seen in perspective with known techniques. 
     One advantage afforded by the preferred embodiments of the present invention resides in enabling tuning to be achieved over a relatively wide wavelength range. 
     So that these and other advantages will become more apparent, the invention will now be described with reference to preferred embodiments thereof and also with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an embodiment of an inventive tuneable filter. 
     FIG. 2 illustrates another embodiment of an inventive tuneable filter. 
     FIG. 3 illustrates still another embodiment of an inventive tuneable filter. 
     FIG. 4 illustrates yet another embodiment of an inventive tuneable filter. 
     FIG. 5 illustrates another embodiment of an inventive tuneable filter. 
     FIG. 6 illustrates a further embodiment of an inventive tuneable filter. 
     FIG. 7 illustrates a further embodiment of an inventive tuneable filter. 
     FIG. 8 illustrates a further embodiment of an inventive tuneable filter. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an embodiment of an inventive tuneable filter  1 A. The illustrated embodiment comprises the tuneable filter  1 A, four wavelength selective add-drop elements  10 ,  12 ,  14  and  16  and four on-off switches  20 ,  22 ,  24  and  26 . The add-drop elements of this embodiment are comprised of so-called MMIBg structures (Multi Mode Interference Bragg grating); see T. Augustsson, “Bragg Grating-Assisted MMI-Coupler for Add-Drop Multiplexing”, J. Lightwave Technol. Vol. 16(8), pp. 1517-1522, August 1998. Structures of this kind include a Bragg grating  310 ,  320 ,  340  and  360  disposed in an MMI-waveguide  210 ,  220 ,  240  and  260 . The Bragg gratings  310 ,  320 ,  340  and  360  may be arranged in respective MMI-waveguides  210 ,  220 ,  240  and  260  so that their centre lines coincide with the centre line of respective MMI-waveguides  210 ,  220 ,  240  and  260 . Each add-drop element drops a specific wavelength and transmits remaining wavelengths. The various add-drop elements  10 ,  12 ,  14  and  16  drop different wavelengths λi. The add-drop element  10  drops the wavelength λ 1 , the add-drop element  12  drops the wavelength λ 2 , the add-drop element  14  drops the wavelength λ 3  and the add-drop element  16  drops the wavelength λ 4 . 
     An input and an output is provided on both short sides of respective add-drop elements  10 ,  12 ,  14  and  16 . The on-off switches  20 ,  22 ,  24  and  26  are provided on a second such short side, between the input and output. Channel suppression is improved when several on-off switches are arranged in series. The on-off switch  20 ,  22 ,  24  and  26  in this embodiment is an MMIMZI-based on-off switch, although any optical on-off switch whatsoever can be used in principle. Each switch includes two MMI-waveguides  41 ,  42 ;  43 ,  44 ;  45 ,  46 ;  47  and  48  that are coupled to respective so-called Mach-Zehnder waveguides  51 ,  52 ;  53 ,  54 ;  55 ,  56 ;  57  and  58 . One of the Mach-Zehnder waveguides  51 ,  53 ,  55  and  57  includes a phase control element  30 ,  32 ,  34  and  36 . The phase control element may be controlled thermooptically. Phase control elements may also be provided in both Mach-Zehnder waveguides  51 ,  52 ;  53 ,  54 ;  55 ,  56 ;  57  and  58 . 
     Assume that Q number of wavelength channels are coupled to the input on the first side of the first add-drop element  10 . Also assume that wavelength channels λ 1 , λ 2 , λ 3  and λ 4  are represented among these wavelength channels, these wavelengths being the wavelengths that can be dropped by respective add-drop elements  10 ,  12 ,  14  and  16  via respective reflection gratings  310 ,  320 ,  340  and  360 . When these Q wavelength channels reach the Bragg grating in the add-drop element  10 , the wavelength λ 1  will be dropped and the remaining Q- 1  wavelength channels transmitted further through the add-drop element  10 . The dropped wavelength channel λ 1  is inputted to the connecting waveguide  62  disposed between the first add-drop element and second add-drop element. The transmitted wavelength channels are outputted on the output on the other side of the first add-drop element  10 . The second add-drop element and a third add-drop element are connected together via a connecting waveguide  63 , and the third add-drop element and a fourth add-drop element are interconnected via a connecting waveguide  64 . 
     The Q- 1  number of wavelength channels that are transmitted through the add-drop element  10  then pass through the on-off switch  20 . Wavelength channels that are inputted to the first MMI-waveguides  41  in the on-off switch  20  are split equally with respect to intensity and are outputted on the Mach-Zehnder waveguides  51  and  52 . The phase control element  30  provided in one of the Mach-Zehnder waveguides  51  is able to change the phase of the waveguide channels. When the waveguide channels reach the second MMI-waveguide  42 , the relative phase distribution at the interface between the Mach-Zehnder waveguides  51  and  52  and the MMI-waveguide  42  will determine whether or not the wavelength channels will be coupled to the connecting waveguide between the on-off switch and the add-drop element. 
     When the switch  20  is in an off-mode, the wavelength channels will not be coupled to the output of the MMI-waveguide  42 . If the on-off switch  20  is in an on-mode, the Q- 1  wavelength channel will pass through the on-off switch  20  relatively undisturbed and will be inputted via the input on the other side of the first add-drop element  10 . These wavelength channels are transmitted through the add-drop element  10  and focused on the output provided on the first side of said element. 
     The reflective wavelength channel λ 1 , possibly together with the remaining Q- 1  wavelength channels, is coupled from the output on the first side of the first add-drop element  10  to the input on a first side of a second add-drop element  12 , via a connecting waveguide. 
     In the illustrated embodiment, λ 1  can be filtered out when the on-off switch  20  is in an off-mode and the remaining on-off switches  22 ,  24  and  26  are in an on-mode. λ 2  can be filtered out when the on-off switch  22  is in an off-mode and the remaining on-off switches  20 ,  24  and  26  are in an on-mode. λ 3  can be filtered out when the on-off switch  24  is in an off-mode and the remaining on-off switches  20 ,  22  and  26  are in an on-mode. λ 4  is filtered out when the on-off switch  26  is in an off-mode and the remaining on-off switches  20 ,  22  and  24  are in an on-mode. 
     Fine adjustments can be made with the aid of a thermoelement provided on a respective Bragg grating structure  310 ,  320 ,  340  and  360 , such as to provide a strictly continuous tuneable filter. This approach can also be applied to achieve tuning over a wide range and thus to provide a filter that can be used in respect of a system in which a large number of channels are at work. 
     FIG. 2 illustrates another embodiment of an inventive tuneable filter, here referenced  1 B. In the illustrated embodiment, the tuneable filter  1 B comprises four wavelength selective add-drop elements  10 ,  12 ,  14  and  16  and four on-off switches  20 ,  22 ,  24  and  26 . In this embodiment, the add-drop elements  10 ,  12 ,  14  and  16  are comprised of so-called Bragg-grating assisted MMIMZI structures (Multi Mode Interference Mach-Zehnder Interferometer); see “Low-Loss Planar Lightwave Circuit OADM with High Isolation and No Polarization Dependence”, J. Albert et al, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 11, No. 3, March 1999, pp. 346-348. This embodiment differs from the FIG. 1 embodiment, in which the add-drop elements comprise MMIBg structures (Multi Mode Interference Bragg grating). In the case of Bragg-grating assisted MMIMZI structures there is included a Bragg grating structure  110 ,  120 ,  140  and  160  disposed in Mach-Zehnder waveguides  112 ,  114 ,  122 ,  124 ,  142 ,  144 ,  162  and  164  arranged between MMI-waveguides  210 ,  215 ;  220 ,  225 ;  240 ,  245  and between  260  and  265 . The Bragg grating structure  110 ,  120 ,  140  and  160  in respective Mach-Zehnder waveguides is preferably arranged so that the wavelength from one of the MMI-waveguides  210 ,  215 ,  220 ,  225 ,  240 ,  245 ,  260  or  265  of respective Bragg grating structures  110 ,  120 ,  140  or  160  will be equal to the Mach-Zehnder waveguides  112 ,  114 ;  122 ,  124 ;  142 ,  144  and  162  and  164  included in the Bragg-grating assisted MMIMZI-structure. The Bragg grating structures  110 ,  120 ,  140  and  160  are actually two separate gratings disposed in the two Mach-Zehnder waveguides, and they have been illustrated as a single Bragg grating structure  110 ,  120 ,  140  and  160  in order to show more clearly that the Bragg gratings will preferably be spaced equidistantly from one of the MMI-waveguides  210 ,  215 ,  220 ,  225 ,  240 ,  245 ,  260  or  265 . Hereinafter, the Bragg grating structures represented by reference numerals  110 ,  120 ,  140  and  160  in FIGS. 2,  3 ,  5  and  6 , and represented by reference numerals  110 A,  110 B,  110 C,  120 A,  120 B,  120 C,  140 A,  140 B,  140 C,  160 A,  160 B,  160 C in FIGS. 7 and 8 will be referred to simply as “Bragg gratings” for brevity and convenience. In this specification then, it will be readily apparent to the reader that the term “Bragg grating” can mean a single grating or more than one grating depending on context. Each add-drop element  10 ,  12 ,  14  and  16  drops a specific wavelength to the connecting waveguide  62 ,  63  and  64  and transmits remaining wavelengths. The various add-drop elements  10 ,  12 ,  14  and  16  drop different wavelengths λi. The add-drop element  10  drops the wavelength λ 1 , the add-drop element  12  drops the wavelength λ 2 , the add-drop element  14  drops the wavelength λ 3  and the add-drop element  16  drops the wavelength λ 4 . 
     An input and an output are provided on both, short sides of respective add-drop elements  10 ,  12 ,  14  and  16 . The on-off switches  20 ,  22 ,  24  and  26  are arranged between the input and the output on another such short side. In this embodiment, the on-off switch is an MMIMZI-based on-off switch. Several on-off switches may be arranged mutually sequentially. Any optical on-off switch whatsoever can be used in principle. This switch includes two MMI-waveguides  41 ,  42 ,  43 ,  44 ,  45 ,  46 ,  47  and  48  that are coupled together with so-called Mach-Zehnder waveguides  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57  and  58 . Respective Mach-Zehnder waveguides  51 ,  53 ,  55  and  57  include a phase control element  30 ,  32 ,  34  and  36 . 
     Assume that Q-number of wavelength channels are coupled to the input on the first side of the first add-drop element  10 . Further assume that wavelength channels λ 1 , λ 2 , λ 3  and λ 4  are represented among these wavelength channels and are the wavelengths that can be dropped by respective add-drop elements  10 ,  12 ,  14  and  16 . When these Q-number of wavelength channels reach the Bragg grating  110  in the add-drop element  10 , the wavelength λ 1  is dropped and the remaining Q- 1  wavelength channels are transmitted further through the add-drop element. The dropped wavelength channel λ 1  is coupled to the connecting waveguide  62  provided between the first add-drop element and a second add-drop element. The transmitted wavelength channels are coupled to the output on the other side of the first add-drop element  10 . These wavelength channels then pass through the on-off switch. The second add-drop element and a third add-drop element are interconnected via a connecting waveguide  63 , and the third add-drop element and a fourth add-drop element are interconnected via a connecting waveguide  64 . 
     Wavelength channels that are inputted in the first of the MMI-waveguides  41  in the on-off switch are split equally with respect to intensity and are outputted on two Mach-Zehnder waveguides  51  and  52 . The phase control element  30  provided in one of the Mach-Zehnder waveguides  51  is able to change the phase of the wavelength channels. The other Mach-Zehnder waveguide may also include a phase control element. When the wavelength channels reach the second MMI-waveguide  42 , the relative phase distribution in the interface between the Mach-Zehnder waveguides  51  and  52  and the MMI-waveguide  42  will determine whether or not the wavelength channel will be coupled to the add-drop element. 
     If the on-off switch  20  is in an off-mode, the wavelength channels cannot be coupled to the output of the MMI-waveguide  42 . If the on-off switch  20  is in an on-mode, the Q- 1  wavelength channels will pass through the on-off switch relatively undisturbed and be inputted via the input on the other side of the first add-drop element  10 . These wavelength channels are transmitted through the add-drop element and outputted on the output provided on the first side of said element. 
     The dropped wavelength channel λ 1  and possibly also the remaining Q- 1  wavelength channels is/are coupled from the output on the first side of the first add-drop element  10  to the input on a first side of a second add-drop element  20 , via a connecting waveguide. 
     In the illustrated embodiment, λ 1  can be filtered out when the on-off switch  20  is in an off-mode and the remaining on-off switches  22 ,  24  and  26  are in an on-mode. λ 2  can be filtered out when the on-off switch  22  is in an off-mode and the remaining on-off switches  20 ,  24  and  26  are in an on-mode. λ 3  can be filtered out when the on-off switch  24  is in an off-mode and the remaining on-off switches  20 ,  22  and  26  are in an on-mode. λ 4  is filtered out when the on-off switch  26  is in an off-mode and the remaining on-off switches  20 ,  22  and  24  are in an on-mode. 
     The MMIMZI-based add-drop elements  10 ,  12 ,  14  and  16  include trimming elements  116 ,  118 ,  126 ,  128 ,  146 ,  148 ,  166  and  168  that are able to adjust for process imperfections. One trimming element is provided in each of the Mach-Zehnder waveguides in the add-drop elements. 
     FIG. 3 illustrates another embodiment of an inventive tuneable filter, here referenced  1 C. The filter includes two devices for filtering out a specific wavelength channel. A first device includes two Bragg-grating assisted Mach-Zehnder Interferometers  10  and  12 . A first Bragg-grating assisted Mach-Zehnder Interferometer  10  includes a first MMI-waveguide  215 , a second MMI-waveguide  210 , a first Mach-Zehnder waveguide  112 , a second Mach-Zehnder waveguide  114 , a Bragg grating  110 , a first adjuster  116  and a second adjuster  118 . The first and the second MMI-waveguides  215  and  210  are interconnected via said first and second Mach-Zehnder waveguides  112  and  114 . The first and the second Mach-Zehnder waveguide  112  and  114  include said Bragg grating  110  and said adjuster elements  116  and  118 . The Bragg grating  110  actually comprises two separate gratings arranged in the two Mach-Zehnder waveguides, although they have been shown as a single Bragg grating  110  in order to make clear that they are preferably spaced from one of the MMI-waveguides. An input and an output are provided on a first side of the first MMI-waveguide  215 . The Mach-Zehnder waveguides  112  and  114  are provided on a second side of the first MMI-waveguide  215 . Mach-Zehnder waveguides  112  and  114  are also provided on a first side of the second MMI-waveguide. An output is provided on a second side of said MMI-waveguide. 
     There is connected to the first Mach-Zehnder interferometer  10  a second Mach-Zehnder interferometer  12  which includes a first MMI-waveguide  225 , a second MMI-waveguide  220 , a first Mach-Zehnder waveguide  122 , a second Mach-Zehnder waveguide  124 , a Bragg grating  120 , a first adjuster element  126  and a second adjuster element  128 , and which is of similar construction to the first Mach-Zehnder interferometer  10 . The output on the second side of the second MMI-waveguide  210  in the first Mach-Zehnder interferometer  10  is connected to a first input on a second side of the second MMI-waveguide  220  in the second Mach-Zehnder interferometer  12 , via a connecting waveguide  60 . The output on the first side of the first MMI-waveguide  215  belonging to the first Mach-Zehnder interferometer  10  is connected to a first input on the first side of the MMI-waveguide  225  belonging to the second Mach-Zehnder interferometer  12 , via a connecting waveguide  62 , which has an on-off switch  20  provided therein. 
     The Bragg gratings  110  and  120  in the first and the second Mach-Zehnder interferometers reflect the same wavelength channel and transmit remaining wavelength channels. The afore-described first device comprising the Mach-Zehnder interferometers  10  and  12  for filtering out a specific wavelength is connected to a second device for filtering out another specific wavelength. 
     This second device includes two Bragg-grating assisted Mach-Zehnder Interferometers  14  and  16 . A third Bragg-grating assisted Mach-Zehnder Interferometer  14  includes a first MMI-waveguide  245 , a second MMI-waveguide  240 , a first Mach-Zehnder waveguide  142 , a second Mach-Zehnder waveguide  144 , a Bragg grating  140 , a first adjuster element  146  and a second adjuster element  148 . The first and the second MMI-waveguides,  245  and  240  are interconnected via said first and second Mach-Zehnder waveguides  142  and  144 . The first and the second Mach-Zehnder waveguides  142  and  144  includes said Bragg grating  140  and said adjuster elements  146  and  148 . The Bragg grating  140  is actually two separate gratings arranged in the two Mach-Zehnder waveguides, although they have been illustrated as a single Bragg grating  140  in order to make clear that they are preferably arranged equidistantly from one of the MMI-waveguides  240  or  245 . An input and an output are provided on a first side of the first MMI-waveguide  245 . The Mach-Zehnder waveguides  142  and  144  are provided on a second side of said first MMI-waveguide  245 . The Mach-Zehnder waveguides  112  and  114  are arranged on a first side of the second MMI-waveguide. An output is provided on a second side of said MMI-waveguide. 
     There is connected to the third Mach-Zehnder interferometer  14  a fourth Mach-Zehnder interferometer  16  which includes a first MMI-waveguide  265 , a second MMI-waveguide  260 , a first Mach-Zehnder waveguide  162 , a second Mach-Zehnder waveguide  164 , a Bragg grating  160 , a first adjuster element  166  and a second adjuster element  168 , and is constructed in the same way as the third Mach-Zehnder interferometer  14 . The output on the second side of the second MMI-waveguide  240  in the third Mach-Zehnder interferometer  14  is connected to a first input on a second side of the second MMI-waveguide  260  in the fourth Mach-Zehnder interferometer  16 , via a connecting waveguide  61 . The output on the first side of the first MMI-waveguide  245  belonging to the third Mach-Zehnder interferometer  14  is connected to a first input on a first side of the MMI-waveguide  265  belonging to the fourth Mach-Zehnder interferometer  16 , via a connecting waveguide  64 . An on-off switch  22  is provided in said connecting waveguide. 
     The first device for filtering out a specific wavelength is connected to the second device for filtering out another specific wavelength via a connecting waveguide  63  provided between the output on the first side of the first MMI-waveguide in the second Mach-Zehnder interferometer  12  and the input provided on the first side of the first MMI-waveguide in the third Mach-Zehnder interferometer  14 . 
     Because two devices for filtering out a specific wavelength has been cascaded in the illustrated embodiment, it is possible to filter out two wavelength channels from Q-number of wavelength channels. It will be understood that when N-number of wavelength channels shall be filtered out from a Q-number of possible channels, the system will include N-number of devices in cascade for filtering out a specific wavelength channel. 
     Assume that Q-number of wavelength channels are coupled to the input of the wavelength selective filter  1 C. These wavelength channels will be transmitted through the first MMI-waveguide  215  in the first Mach-Zehnder interferometer  10  and then outputted on both Mach-Zehnder waveguides  112  and  114 . A wavelength channel λ 1  will be reflected by the Bragg grating  110 , whereas the remaining Q- 1  wavelength channels will be transmitted through the Mach-Zehnder waveguides  112  and  114 . These Q- 1  wavelength channels are then inputted into the second MMI-waveguide  210  in the first Mach-Zehnder interferometer  10 . The wavelength channels transmitted through said MMI-waveguide can be switched through the connecting waveguide  60  to the second MMI-waveguide in the second Mach-Zehnder interferometer  12 . Said wavelength channels will be transmitted through the entire second Mach-Zehnder interferometer  12  and inputted on the connecting waveguide  63  between the first and the second device for filtering out a specific wavelength channel. 
     After that said wavelength channel λ 1  has been reflected by the Bragg grating  110 , these are transmitted back through the Mach-Zehnder, waveguides  112  and  114  and through the MMI-waveguide  215 . Said wavelength channel λ 1  is then inputted on the connecting waveguide  62 . The connecting waveguide includes said on-off switch  20 , which may either be in an on-mode or an off-mode. If the on-off switch is in an on-mode, the wavelength channel will be transmitted therethrough relatively undisturbed. If the switch is, instead, in its off-mode, said wavelength channel λ 1  will be filtered out. 
     If the switch is an on-mode, it will pass the wavelength channel and be inputted into the second Mach-Zehnder interferometer  12 . The wavelength channel λ 1  will be reflected in said Mach-Zehnder interferometer by the Bragg grating  120 , said grating reflecting the same wavelength channel as the Bragg grating  110  disposed in the first Mach-Zehnder interferometer  10 . After having been reflected by said grating, the wavelength channel λ 1  is transmitted back through the Mach-Zehnder interferometer  12  and outputted on the connecting waveguide  63  that connects the first and the second devices for filtering out a specific wavelength channel. 
     The Q- 1  wavelength channels and possibly also the wavelength channel λ 1  are then transmitted into the MMI-waveguide  245  belonging to the second device for filtering out a specific wavelength channel. This device is able to filter out a second wavelength channel λ 2 . The procedure is the same as that described above with the difference that the Bragg gratings  140  and  160  reflect a wavelength λ 2  instead of a wavelength λ 1  as reflected by the Bragg grating  110  and  120 . Said wavelength can be filtered out in the on-off switch  22  arranged in the connecting waveguide  64  through which only the wavelength channel λ 2  passes. 
     At least Q- 2  number of wavelength channels will be outputted on the output of the tuneable filter  1 C. 
     FIG. 4 illustrates another embodiment of an inventive tuneable filter  1 D. The filter includes two devices, for filtering out a specific wavelength channel. 
     A first such device includes two MMIBg (Multi Mode Interference Bragg-grating)  10  and  12 . A second such device includes two second MMIBg  14  and  16 . An MMIBg  10 ,  12 ,  14  and  16  include an MMI-waveguide  210 ,  220 ,  240  and  260  in which a Bragg grating  310 ,  320 ,  340  and  360  is arranged. The Bragg gratings  310 ,  320 ,  340  and  360  illustrated in FIG. 4 are single Bragg gratings and are not Bragg grating structures made of two Bragg gratings, such as shown in FIGS. 2,  3  and  5 - 8 . 
     An input and an output are provided on a first side of a first MMIBg  10 . An output is provided on a second side of the first MMIBg  10 . An input and an output are provided on a first side of a second MMIBg  12 . An input is provided on a second side of the second MMIBg  12 . The output on the second side of the first MMIBg  10  is connected to the input on the second side of the second MMIBg  12  via a connecting waveguide  60 . The output on the first side of the first MMIBg  10  is connected to the input on the first side of the second MMIBg  12  via a connecting waveguide  62 . This connecting waveguide includes an on-off switch  20 . 
     An input and an output are provided on a first side of a third MMIBg  14 . An output is provided on a second side of the third MMIBg  14 . An input and an output are provided on a first side of a fourth MMIBg  16 . An input is provided on a second side of the fourth MMIBg  16 . The output on the second side of the third MMIBg  14  is connected to the input on the second side of the fourth MMIBg  16  via an intermediate waveguide  61 . The output on the first side of the third MMIBg  14  is connected to the input on the first side of the fourth MMIBg  16  via a connecting waveguide  64 . An on-off switch  22  is provided in the connecting waveguide. 
     The first and the second devices for filtering out a specific wavelength channel are interconnected via a connecting waveguide  63  provided between the output on the first side of the second MMIBg  12  and the input on the first side of the third MMIBg  14 . 
     Assume that Q-number of wavelength channels are inputted on the input of the wavelength selective filter  1 D. A wavelength channel λ 1  will then be reflected by the Bragg grating  310  arranged in the first MMIBg  10 , whereas the Q- 1  remaining wavelength channels will be transmitted through said MMIBg  10 . These Q- 1  wavelength channels are then inputted to the second MMIBg  12 , via the connecting waveguide  60 . These wavelength channels are transmitted through said MMIBg  12  and then forwarded through the connecting waveguide  63  to the second device for filtering out a specific wavelength channel. 
     Subsequent to having been reflected by the Bragg grating  110  in MMIBg  10 , said wavelength channel λ 1  will be transmitted back through MMIBg  10 . The wavelength channel λ 1  is then inputted on the connecting waveguide  62 . The connecting waveguide includes said on-off switch  20 , which may either be in an on-mode or in an off-mode. If the switch is in its on-mode said wavelength channel will be transmitted therethrough relatively undisturbed. On the other hand, if the switch is in its off-mode said wavelength channel λ 1  will be filtered out. 
     If the switch is in its on-mode, the wavelength channel will pass therethrough and be inputted to the second MMIBg  12 . The wavelength channel λ 1  will then be reflected in said MMIBg by the Bragg grating  320  which reflects the same wavelength channel as the Bragg grating  310  provided in the first MMIBg  10 . Subsequent to having been reflected by said grating, the wavelength channel λ 1  will be reflected back through MMIBg  12  and outputted on the connecting waveguide  63  that interconnects the first and the second devices for filtering out a specific wavelength channel. 
     The Q- 1  wavelength channels and possibly the wavelength channel λ 1  are then transmitted into MMIBg  14  belonging to the second specific wavelength channel filtering device. A second wavelength channel λ 2  can be filtered out in this device. The procedure is the same as that described above with the difference that the Bragg gratings  340  and  360  reflect a wavelength λ 2  instead of the wavelength λ 1  reflected by the Bragg gratings  310  and  320 . Said wavelength λ 2  is filtered out in the on-off switch  22  provided in the connecting waveguide  64 , in which only the wavelength channel λ 2  passes. At least Q- 2  number of wavelength channels will be transmitted on the output of the tuneable filter  1 D. 
     FIG. 5 illustrates another embodiment of an inventive tuneable filter, here referenced  1 E. The filter includes four devices each functioning to filter out a specific wavelength channel. 
     The devices include a Bragg-grating assisted Mach-Zehnder Interferometer  10 ,  12 ,  14  and  16 . A Bragg-grating assisted Mach-Zehnder Interferometer  10 ,  12 ,  14  and  16  includes a first MMI-waveguide  215 ,  225 ,  245  and  265 , a second MMI-waveguide  210 ,  220 ,  240  and  260 , a first Mach-Zehnder waveguide  112 ,  122 ,  142  and  162 , a second Mach-Zehnder waveguide  114 ,  124 ,  144  and  164 , a Bragg grating  110 ,  120 ,  140  and  160 , a first adjuster element  116 ,  126 ,  146  and  166  and a second adjuster element  118 ,  128 ,  148  and  168 . The first MMI-waveguide  215 ,  225 ,  245  and  265  and the second MMI-waveguide  210 ,  220 ,  240  and  260  are interconnected via said first Mach-Zehnder waveguides  112 ,  122 ,  142  and  162  and second Mach-Zehnder waveguides  114 ,  124 ,  144  and  164 . The first mach-Zehnder waveguide  112 ,  122 ,  142  and  162  and the second Mach-Zehnder waveguide  114 ,  124 ,  144  and  164  includes said Bragg grating  110 ,  120 ,  140  and  160  and said adjuster element  116 ,  126 ,  146 ,  166  and  118 ,  128 ,  148  and  168  respectively. The Bragg gratings  110 ,  120 ,  140  and  160  actually comprise two separate gratings arranged in the two Mach-Zehnder waveguides, although they have been shown as a single Bragg grating  110 ,  120 ,  140  and  160  in order to make clear that they are preferably arranged equidistantly from one of the MMI-waveguides. The Bragg gratings are of the phase-shifted Bragg grating kind; see “Phase-Shifted Fiber Bragg Gratings and Their Application for Wavelength Demultiplexing”, Govind P. Agraval and Stojan Radic, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 6, No. 8, August 1994, pp. 995-997. An input and an output are provided on a first side of the first MMI-waveguide  215 ,  225 ,  245  and  265 . Said Mach-Zehnder waveguides  112 ,  114 ,  122 ,  124 ,  142 ,  144 ,  162  and  164  are arranged on a second side of said first MMI-waveguide  215 ,  225 ,  245  and  265 . The Mach-Zehnder waveguides  112 ,  114 ,  122 ,  124 ,  142 ,  144 ,  162  and  164  are provided on a first side of the second MMI-waveguide  210 ,  220 ,  240  and  260 . An input and an output are provided on a second side of said MMI-waveguide  210 ,  220 ,  240  and  260 . 
     The output and the input on the second side of the second MMI-waveguide  210 ,  220 ,  240  and  260  are interconnected via an on-off switch  20 ,  22 ,  24  and  26 . 
     The first device for filtering out a specific wavelength is connected to the second device for filtering out a different specific wavelength, via a connecting waveguide  60  arranged between the output on the first side of the first MMI-waveguide  215  in the first Bragg-grating assisted Mach-Zehnder Interferometer  10  and the input on the first side of the first MMI-waveguide  225  in the second Bragg-grating assisted Mach-Zehnder waveguide  12 . Similarly, the third and the fourth devices for filtering out a specific wavelength channel are interconnected via respective connecting waveguides  61  and  62 . 
     In the illustrated embodiment four wavelength channels can be filtered out from Q-number of wavelength channels by virtue of the fact that four devices for filtering out, a specific wavelength have been cascade connected. It will be readily seen that when N-number of wavelength channels are to be filtered out from Q-possible wavelength channels, then N-number of devices for filtering out specific wavelength channels shall be arranged in cascade. 
     Assume that Q-number of wavelength channels are inputted on the input to the wavelength selective filter, here referenced  1 E. These wavelength channels will be transmitted through the first MMI-waveguide  215  in the first Bragg-grating assisted Mach-Zehnder Interferometer  10  and then outputted on the two Mach-Zehnder waveguides  112  and  114 . Q- 1  number of wavelength channels will be reflected by the Bragg grating  110 , while a wavelength channel λ 1  will be transmitted through said Mach-Zehnder waveguide  112  and  114  respectively. The wavelength channel λ 1  is then inputted to the second MMI-waveguide  210  in the first Mach-Zehnder interferometer. This wavelength channel is then transmitted through said MMI-waveguide and coupled or inputted to an on-off switch  20 . When the on-off switch is in an on-mode, said wavelength channel will be transmitted therethrough relatively undisturbed. On the other hand, if the switch is in its off-mode, said wavelength channel λ 1  will be filtered out. 
     When the switch is in its on-mode, the wavelength channel λ 1  is coupled to the input on the second side of the first Bragg-grating assisted Mach-Zehnder Interferometer. This wavelength channel is transmitted through the whole of the first Bragg-grating assisted Mach-Zehnder Interferometer and inputted on the connecting waveguide  60  between the first and the second device for filtering out a specific wavelength channel. 
     After having been reflected by the Bragg grating  110 , the wavelength channels Q- 1  are transmitted back through the Mach-Zehnder waveguides  112  and  114  and through the MMI-waveguide  215 . The wavelength channels are then inputted on the connecting waveguide  60 . 
     The, Q- 1  wavelength channels and possibly the wavelength channel λ 1  are then transmitted into the MMI-waveguide  225  belonging to the second device for filtering out a specific wavelength channel. A second wavelength channel λ 2  can be filtered out in this device. The procedure is the same as that described above but with the difference that the Bragg grating  120  transmits a wavelength λ 2  instead of a wavelength λ 1  as transmitted by the Bragg grating  110 . The wavelength can be filtered out in the on-off switch  22 , through which only the wavelength channel λ 2  passes. 
     A third and a fourth device for filtering out a specific wavelength channel are connected to the earlier devices and to each other via connecting waveguides  61  and  62 . 
     At least Q- 4  number of wavelength channels will be outputted on the output of the tuneable filter  1 E. 
     FIG. 6 illustrates another embodiment of an inventive tuneable filter, here referenced  1 F. The filter includes four devices for filtering out a specific wavelength channel. 
     These devices include an MMI-waveguide  215 ,  225 ,  245  and  265 , a first Michelson waveguide  112 ,  122 ,  142  and  162 , a second Michelson waveguide  114 ,  124 ,  144  and  164 , a phase-shifted Bragg grating  110 ,  120 ,  140  and  160 , a first adjuster element  116 ,  126 ,  146  and  166 , a second adjuster element  118 ,  128 ,  148  and  168 , a total reflection section  70 ,  72 ,  74  and  76 , a first on-off switch  20 A,  22 A,  24 A and  26 A, and a second on-off switch  20 B,  22 B,  24 B and  26 B. The MMI-waveguide  215 ,  225 ,  245  and  265  and the total reflection section  70 ,  72 ,  74  and  76  are interconnected via said first Michelson waveguide  112 ,  122 ,  142  and  162  and said second Michelson waveguide  114 ,  124 ,  144  and  164 . The first Michelson waveguide  112 ,  122 ,  142  and  162  and the second Michelson waveguide  114 ,  124 ,  144  and  164  include said Bragg gratings  110 ,  120 ,  140  and  160 . The Bragg gratings  110 ,  120 ,  140  and  160  are actually two separate gratings arranged in the two Michelson waveguides, although they have been illustrated as a single Bragg grating  110 ,  120 ,  140  and  160  in order to show clearly that they are preferably equidistant from respective MMI-waveguides  215 ,  225 ,  245  or  265 . An input and an output are provided on a first side of the MMI-waveguides  215 ,  225 ,  245  and  265 . The Michelson waveguides  112 ,  114 ,  122 ,  124 ,  142 ,  144 ,  162  and  164  are provided on respective second sides of the MMI-waveguide  215 ,  225 ,  245  and  265 . The Michelson waveguides  112 ,  114 ,  122 ,  124 ,  142 ,  144 ,  162  and  164  are arranged on a first side of the total reflection sections  70 ,  72 ,  74  and  76 . The first Michelson waveguides  112 ,  122 ,  142  and  162  include the second adjuster element  118 ,  128 ,  148  and  168  and the first on-off switch  20 A,  22 A,  24 A and  26 A. Respective second Michelson waveguides  114 ,  124 ,  144  and  164  include the first adjuster element  116 ,  126 ,  146 ,  166  and the second on-off switch  20 B,  22 B,  24 B and  26 B. The phase-shifted Bragg gratings  110 ,  120 ,  140  and  160  shown in FIG. 6 are arranged closer to the MMI waveguides  215 ,  225 ,  245  and  265  than to the on-off switches  20 A,  20 B,  22 A,  22 B,  24 A,  24 B,  26 A and  26 B. 
     The first device for filtering out a specific wavelength channel is connected to the second device for filtering out another specific wavelength channel via a connecting waveguide  60  connected between the output on the first side of the MMI-waveguide  215  in the first device and the input arranged on the first side of the MMI-waveguide  225  in the second device. Similarly, the third and the fourth devices for filtering out a specific wavelength channel are interconnected via connecting waveguides  61  and  62 . 
     The on-off switches  20 A and  20 B,  22 A and  22 B,  24 A and  24 B, and  26 A and  26 B preferably operate synchronously. 
     In the illustrated embodiment, four wavelength channels can be filtered out from Q-number of wavelength channels by virtue of connecting in cascade four devices that can each filter out a specific wavelength channel. It will readily be seen that if N-number of wavelength channels shall be filtered out from Q-possible channels, N-number of devices for filtering out a specific wavelength channel will be cascade connected. 
     Assume that Q-number of wavelength channels are inputted on the input of the wavelength selective filter  1 F. These wavelength channels will be transmitted through the first MMI-waveguide  215  in the first specific wavelength channel filtering device and then outputted on the two Michelson waveguides  112  and  114 . The Q- 1  number of wavelength channels will be reflected by the phase-shifted Bragg grating  110 , whereas a wavelength channel λ 1  will be transmitted through said phase-shifted Bragg grating  110  in the Michelson waveguide  112  and  114 . 
     The wavelength channel λ 1  is then inputted to the on-off switch  20 A and  20 B in respective Michelson waveguides  112  and  114 . If the on-off switches are in an on-mode, said wavelength channel will be transmitted through the switch relatively undisturbed. If, on the other hand, the switches are in an off-mode, the wavelength channel λ 1  will be filtered out. 
     When the switch is in its on-mode, the wavelength channel will be transmitted through the Michelson waveguides  112  and  114  downstream of said switches and will then be reflected by the total reflection section  70 . The wavelength channels will then pass the on-off switches  20 A and  20 B and the Bragg grating  110  for a second time and then pass through the MMI-waveguide  215  and be inputted on the connecting waveguide  60 . 
     After having been reflected by the Bragg grating  110 , the Q- 1  number of wavelength channels will be transmitted back through the Michelson waveguides  112  and  114  and through the MMI-waveguide  215 . The wavelength channels are then inputted on the connecting waveguide  60 . 
     The Q- 1  number of wavelength channels and possibly also the wavelength channel λ 1  are then transmitted into the MMI-waveguide  225  belonging to the second device for filtering out a specific wavelength channel. A second wavelength channel λ 2  can be filtered out in this device. The procedure is the same as that described above, but with the difference that the phase-shifted Bragg grating  120  transmits a wavelength λ 2  instead of a wavelength λ 1  as transmitted by the phase-shifted Bragg grating  110 . Said wavelength can be filtered out in the on-off switch  22 A and  22 B, in which solely the wavelength channel λ 2  passes. 
     A third and a fourth device for filtering out a specific wavelength channel are connected to the earlier filter devices and to each other via the connecting waveguides  61  and  62 . 
     At least Q- 4  number of wavelength channels will be transmitted on the output of the tuneable filter  1 F. 
     FIG. 7 illustrates still another embodiment of an inventive tuneable filter, here referenced  1 G. The filter includes four devices for filtering out a specific wavelength channel. 
     These devices includes respective MMI-waveguides  215 ,  225 ,  245  and  265 , a respective first Michelson waveguide  112 ,  122 ,  142  and  162 , a respective second Michelson waveguide  114 ,  124 ,  144  and  164 , a phase-shift Bragg grating  110 A,  120 A,  140 A and  160 A, a Bragg grating  110 B,  120 B,  140 B and  160 B, a first adjuster element  118 ,  128 ,  148  and  168 , a second adjuster element  116 ,  126 ,  146  and  166 , a first on-off switch  20 A,  22 A,  24 A and  26 A, and a second on-off switch  20 B,  22 B,  24 B and  26 B. The MMI-waveguides  215 ,  225 ,  245  and  265  are arranged on a first side of respective devices, and the Bragg grating  110 B,  120 B,  140 B and  160 B are arranged on a second side of respective devices. The first Michelson waveguide  112 ,  122 ,  142  and  162  includes said Bragg grating  110 A,  120 A,  140 A and  160 A and the second Michelson waveguide  114 ,  124 ,  144  and  164  includes said Bragg grating  110 B,  120 B,  140 B and  160 B. The Bragg grating  110 A,  110 B,  120 A,  120 B,  140 A,  140 B,  160 A and  160 B are actually two separate grating arranged in the two Michelson waveguides, although they have been shown as a single Bragg grating  110 A,  110 B,  120 A,  120 B,  140 A,  140 B,  160 A and  160 B in order to clearly show that they are preferably equidistant from the MMI-waveguide  2125 ,  225 ,  245  or  265 . An input and an output are provided on a first side of the MMI-waveguide  215 ,  225 ,  245  and  265 . The Michelson waveguide  112 ,  114 ,  122 ,  124 ,  142 ,  144 ,  162  and  164  is arranged on a second side of said MMI-waveguide  215 ,  225 ,  245  and  265 . The first Michelson waveguide  112 ,  122 ,  142  and  162  includes the adjuster element  118 ,  128 ,  148  and  168  and the first on-off switch  20 A,  22 A,  24 A and  26 A. The second Michelson waveguide  114 ,  124 ,  144  and  164  includes the second adjuster element  116 ,  126 ,  146  and  166  and the second on-off switch  20 B,  22 B,  24 B and  26 B. The phase-shifted Bragg gratings  110 A,  120 A,  140 A and  160 A shown in FIG. 7 are disposed nearest the MMI-waveguide  215 ,  225 ,  245  and  265 . 
     The first device for filtering out a specific wavelength is connected to the second device for filtering out another specific wavelength via a connecting waveguide  60  disposed between the output on the first side of the MMI-waveguide  215  in the first device and the input provided on the first side of the MMI-waveguide  225  in the second device. Similarly, the third and the fourth devices for filtering out a specific wavelength channel are interconnected via respective connecting waveguides  61  and  62 . 
     In the illustrated embodiment, four wavelength channels can be filtered out from Q-number of wavelength channels, by virtue of four devices for filtering out a specific wavelength channel have been cascade connected. It will readily be seen that if N-number of wavelength channels are to be filtered out from Q-number of possible channels, N-number of devices for filtering out a specific wavelength channel will be cascade connected. 
     Assume that Q-number of wavelength channels are inputted on the input of the wavelength selective filter  1 H. These wavelength channels will be transmitted through the first MMI-waveguide  215  in the first device for filtering out a specific wavelength channel and then outputted on the two Michelson waveguides  112  and  114 . Q- 1  number of wavelength channels will be reflected by the Bragg grating  110 , while one wavelength channel λ 1  will be transmitted through the Bragg grating  110  in said Michelson waveguides  112  and  114 . 
     The wavelength channel λ 1  is then inputted to the on-off switch  20 A and  20 B in respective Michelson waveguides  112  and  114 . If the on-off switches are in an on-mode, the wavelength channel will be transmitted through the switches relatively undisturbed. On the other hand, if said switches are in an off-mode, the wavelength channel λ 1  will be filtered out. 
     When the switches are in an on-mode, the wavelength channel will be transmitted through the Michelson waveguides  112  and  114  situated downstream of said switches, and will then be reflected by the Bragg grating  110 B. The wavelength channel will then pass the on-off switches  20 A and  20 B and the phase-shifted Bragg grating  110 A once and thereafter pass through the MMI-waveguide  215  and be inputted on the connecting waveguide  60 . 
     After having been reflected by the phase-shifted Bragg grating  110 A, said Q- 1  number of wavelength channels will be transmitted back through the Michelson waveguide  112  and  114  and through the MMI-waveguide  215 . The wavelength channels are then inputted on the connecting waveguide  60 . 
     The Q- 1  wavelength channels and possibly also the wavelength channel λ 1  will then be transmitted into the MMI-waveguide  225  belonging to the second device for filtering out a specific wavelength channel. A second wavelength channel λ 2  can be filtered out in this device. The procedure is the same as that described above, but with the difference that the Bragg grating  120 A transmits a wavelength λ 2  instead of a wavelength λ 1  as transmitted by the Bragg grating  110 A. 
     A third and a fourth device for filtering out a specific wavelength channel are connected to the earlier devices and to each other via respective connecting waveguides  61  and  62 . 
     At least Q- 4  number of wavelength-channels will be outputted on the output of the tuneable filter  1 H. 
     FIG. 8 illustrates yet another embodiment of an inventive tuneable filter  1 H. The filter includes four devices for filtering out a specific wavelength channel. 
     These devices include an MMI-waveguide  215 ,  225 ,  245  and  265 , a first Michelson waveguide  112 ,  122 ,  142  and  162 , a second Michelson waveguide  114 ,  124 ,  144  and  164 , a first Bragg grating  110 A,  120 A,  140 A and  160 A, a second Bragg grating  110 B,  120 B,  140 B and  160 B, a third Bragg grating  110 C,  120 C,  140 C and  160 C, a first adjuster element  118 ,  128 ,  148  and  168 , a second adjuster element  116 ,  126 ,  146  and  166 , a first on-off switch  20 A,  22 A,  24 A and  26 A and a second on-off switch  20 B,  22 B,  24 B and  26 B. The MMI-waveguides  215 ,  225 ,  245  and  265  are arranged on a first side of respective devices and the Bragg gratings  110 C,  120 C,  140 C and  160 C are arranged on a second side of respective devices. The first Michelson waveguide  112 ,  122 ,  142  and  162  and the second Michelson waveguide  114 ,  124 ,  144  and  164  includes said Bragg gratings  110 A,  110 B,  110 C,  120 A,  120 B,  120 C,  140 A,  140 B,  140 C,  160 A,  160 B,  160 C. The Bragg gratings  110 A,  110 B,  110 C,  120 A,  120 B,  120 C,  140 A,  140 B,  1400 C  160 A,  160 B and  160 C are actually two separate gratings disposed in the two Michelson waveguides, although in the illustrated case they have been shown as a single Bragg grating  110 A,  110 B,  110 C,  120 A,  120 B,  120 C,  140 A,  140 B,  140 C,  160 A,  160 B and  160 C in order to clearly show that they are preferably equidistant from the MMI-waveguide  215 ,  225 ,  245  or  265 . An input and an output are provided on a first side of respective MMI-waveguides  215 ,  225 ,  245  and  265 . The Michelson waveguides  112 ,  114 ,  122 ,  124 ,  142 ,  144 ,  162  and  164  are arranged on a second side of respective MMI-waveguides  215 ,  225 ,  245  and  265 . Respective first Michelson waveguides  112 ,  122 ,  142  and  162  include a respective adjuster element  118 ,  128 ,  148  and  168  and the first on-off switches  20 A,  22 A,  24 A and  26 A. The second Michelson waveguide  114 ,  124 ,  144  and  164  includes respective second adjuster elements  116 ,  126 ,  146  and  166  and second on-off switches  20 B,  22 B,  24 B and  26 B. 
     The first device for filtering out a specific wavelength is connected to the second device for filtering out another specific wavelength, via a connecting waveguide  60  arranged between the output on the first side of the MMI-waveguide  215  in the first device and the input on the first side of the MMI-waveguide  225  in the second device. Similarly, the third and the fourth devices for filtering out a specific wavelength channel are interconnected via connecting waveguides  61  and  62 . 
     In the illustrated embodiment, four wavelength channels can be filtered out from Q-number of wavelength channels, by virtue of four devices for filtering out a specific wavelength have been connected in cascade. It will readily be seen that if N-number of wavelength channels shall be filtered out from Q-number of possible channels, N-number of devices for filtering out a specific wavelength channel will be cascade connected. 
     Assume that Q-number of wavelength channels are inputted on the input of the wavelength selective filter, here referenced  1 I. Said wavelength channels will be transmitted through the first MMI-waveguide  215  in the first device for filtering out a specific wavelength channel and then outputted on the two Michelson waveguides  112  and  114 . Q- 1  number of wavelength channels will be reflected by the Bragg gratings  110 A and  110 B, whereas a wavelength channel λ 1  will be transmitted through the Bragg gratings  110 A and  110 B in said Michelson waveguides  112  and  114 . The Bragg grating  110 A reflects all wavelengths of shorter wavelength than a wavelength channel λ 1 , and the Bragg grating  110 B will reflect all wavelengths that have a longer wavelength than the wavelength channel λ 1 . Thus, only the wavelength channel λ 1  will be transmitted through both of the Bragg gratings  110 A and  110 B. 
     The wavelength channel λ 1  is then inputted to the on-off switch  20 A and  20 B in respective Michelson waveguides  112  and  114 . If the on-off switches are in an on-mode, said wavelength channel will be transmitted through the switches relatively undisturbed. On the other hand, if the switches are in an off-mode, said wavelength channel λ 1  will be filtered out. 
     When the switches are in an on-mode, the wavelength channel will be transmitted through the Michelson waveguides  112  and  114  disposed downstream of said switches, and will then be reflected by the Bragg grating  110 C. The wavelength channel will then pass the on-off switches  20 A and  20 B and the Bragg gratings  110 A and  110 B for a second time, and then pass through the MMI-waveguide  215  and inputted on the connecting waveguide  60 . 
     After having been reflected by the Bragg gratings  110 A and  110 B, said Q-l number of wavelength channels will be transmitted back through the Michelson waveguides  112  and  114  and through the MMI-waveguide  215 . Said wavelength channels are then inputted on the- connecting waveguide  60 . 
     The Q- 1  wavelength channels and possibly also the wavelength channel λ 1  are then transmitted into the MMI-waveguide  225  belonging to the second device for filtering out a specific wavelength channel. A second wavelength channel λ 2  can be filtered out in this device. The procedure is the same as that described above, although with the difference that the Bragg grating  120 A and  120 B transmit a wavelength λ 2  as opposed to a wavelength λ 1  as transmitted by the Bragg gratings  110 A and  110 B. In accordance with the a foregoing, the wavelength channel λ 2  can be filtered out in the on-off switches  22 A and  22 B, through which only the wavelength channel λ 2  passes. 
     A third and a fourth device for filtering out a specific wavelength channel are connected to the earlier devices and to each other via connecting waveguides  61  and  62 . The third device is able to function out a wavelength channel λ 3  and the fourth device is able to filter out a wavelength channel λ 4 . 
     At least Q- 4  number of wavelength channels will be outputted on the output of the tuneable filter  1 I. 
     A thermoelement can be provided on top of the Bragg grating structure of the various embodiments for achieving fine adjustments, in other words so that a strictly continuous tuneable filter can be obtained. This approach can also be used to provide tuneability over a wide range and consequently to provide a filter that can be used in a system that includes a large number of operative channels. 
     It lies within the concept of the invention to use a micromechanical on-off switch instead of an MMIMZI-based on-off switch or a digital on-off switch. A micromechanical on-off switch has the advantage that power need only be supplied when a change in wavelength channel shall take place. On the other hand, continuous power supply is required for continuous tuneability, which can be achieved, for instance, through the medium of said termoelement on top of the grating structure in the add-drop element. 
     The present embodiments of an optical filter  1  can be constructed in some monolithic semiconductor system or dielectric waveguide system of the type SiO 2 /Si or some other polymeric material. 
     It will be understood that the present invention is not limited to the aforedescribed and illustrated exemplifying embodiments thereof and that modifications can be made within the scope of the accompanying claims.