Abstract:
A fibre optic transmission application, in particular, an optical device that can be incorporated into telecommunications equipment as well as into test and measurement equipment with reduced insertion loss, reduced crosstalk effects and reduced height, with increased versatility in the implementation of optical functions other than multiplexers and demultiplexers. Relates to components, modules, equipments and instruments such as multiplexers, demultiplexers, routers, channel monitors, and tunable filters that encompass such optical devices.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is based on and claims priority to European Patent Application No. 081001175.2 filed on Jan. 23, 2008, the content of which is hereby incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The optical devices are based on a dispersing system with a diffraction grating that is operated near Littrow. In such a system, the following relation is approximately valid 
         [0000]        Gm λ=2 sin(β)   (1) 
         [0000]    where G is the groove density of the grating, m is the order of diffraction (an integer), λ is the wavelength and β the Littrow angle. 
         [0003]    The dispersing system has to be compact in order to keep the size of the optical devices compatible with the requirements for telecommunications equipments as well as for test and measurement equipments. 
         [0004]    Wavelength multiplexers, demultiplexers and routers based on a compact dispersing system are available. These devices have been described and progressively developed, in particular in French patents FR-2.479.981, FR-2.496.260, FR-2.519.148, FR-2.543.768, FR-2.579.333, FR-2.731.573, FR-2.743.424, FR-2.761.485, FR-2.763.139, FR-2.764.393, FR-2.765.424, FR-2.765.972, FR-2.779.535, FR-2.803.046 and FR-2.832.882. A first generation of multiplexers-demultiplexers has been marketed under the brand &lt;&lt;STIMAX&gt;&gt;, and subsequently, a second generation has followed under the brand &lt;&lt;MINILAT&gt;&gt;. For a short description of the state of the art, we refer to chapter 3 of the book: Wavelength Filters in Fibre Optics, Herbert Venghaus (Ed.), Springer Verlag, Berlin, 2006. 
         [0005]    Wavelength multiplexers and demultiplexers are elementary devices in today&#39;s fibre optic long-haul and metro networks. The optical layer of these networks is evolving from static to more dynamic in order to reduce the number of O-E-O (Optical-Electrical-Optical) conversions as well as to remotely optimize transmission capacity for continuously changing traffic demands. This implies that devices are required for monitoring and routing of channels such as, for example, tunable lasers, tunable filters and optical channel monitors. It is noted that these devices also correspond to components and modules incorporated in test and measurement equipments. 
         [0006]      FIG. 1A  and  FIG. 1B  represent an optical device  10  of the prior art according to the French patent application FR-2.779.535. The optical device is composed of a fibre array  20  comprising an end face  25  and a compact dispersing system  30 .  FIG. 1A  is a top view of the optical device  10  and  FIG. 1B  is a side view of the same optical device  10 . The optical device  10  can be for example a wavelength multiplexer, demultiplexer, or router. When the optical device  10  is a multiplexer, it comprises N input fibres  21  and a single output fibre  22 . When the optical device is a demultiplexer, it comprises a single input fibre  22  and N output fibres  21 . When the optical device is a router, it comprises N input fibres  21  and M output fibres  22  or vice versa. 
         [0007]    The optical device  10  of  FIG. 1A  and of  FIG. 1B  will be described as a demultiplexer. A multiplexer and a demultiplexer are in principle the same device: a demultiplexer is a multiplexer operated in reverse direction. Hereafter, all multiplexers-demultiplexers will be described as demultiplexers. Operation of a multiplexer is obtained from a demultiplexer by reversing beam propagation in the device which implies that input fibre(s) become output fibre(s) and vice versa. It is noted that the optical device  10  is called an optical filter when it has a single input fibre  21  in combination with a single output fibre  22 . 
         [0008]    The compact dispersing system  30  is composed of a plane mirror  40  with a small aperture  41  in the centre, a concave spherical mirror  60  having a focus and a plane diffraction grating  50  having a dispersion plane. The system  30  ensures wavelength selective conjugation between the end faces of the input fibres  22  and the end faces of the output fibres  21  of the fibre array  20 . The optical axis of the system, comprising two parts referenced  31   a  and  31   b,  is folded for compactness of the device. The first part of the optical axis  31   a  runs from the end face  25  of the fibre array  20  through the aperture  41  in the plane mirror  40  up to the centre of the spherical mirror  60 , it is perpendicular to the end face  25  of the fibre array  20  as well as to the spherical mirror  60 , and it makes an angle a with respect to the plane mirror  40  ( FIG. 1A ). The diffraction grating  50  makes an angle α with respect to the plane mirror  40  and its dispersion plane coincides with the horizontal plane of the optical device  10 . The second part of the optical axis  31   b  runs from the aperture  41  in the plane mirror  40  to the diffraction grating  50 , it makes an angle β with the normal  33  of the grating  50  where β is the Littrow angle and it intersects the first part of the optical axis  31   a  in the plane of the plane mirror  40 , such that the angles α and β are related as follows: α=(β+90°)/2. 
         [0009]      FIGS. 1A and 1B  show an embodiment of the compact dispersing system  30  using three parts: a wedge prism  42 , a plano-concave lens  61 , and a substrate  51 . The wedge prism  42  with a small aperture in the centre is used to ensure the positioning of the plane mirror  40  with respect to the optical axis  31   a,    31   b.  The tilted face  43  of the wedge prism  42  serves as support for the plane mirror  40  whereas the opposite face  44  of the wedge prism  42  is parallel to the end face  25  of the fibre array  20 . The plano-concave lens  61  serves as support for the spherical mirror  60  that resides on its concave face  62 . The substrate  51  serves as support for the plane diffraction grating  50  which is formed on the surface of the substrate. The space  32  is filled with air, vacuum or a gas. 
         [0010]    The fibre array  20  enables accurate positioning of the end faces of the input fibres  22  and output fibres  21  with respect to the compact dispersing system  30 . The fibre end faces are located in the same plane as the end face  25  of the fibre array  20  which is perpendicular to the first part of the optical axis  31   a  of the dispersing system  30 . The fibre end faces are positioned with respect to the aperture  41  in the plane mirror  40  such that outgoing and incoming beams are not affected by the presence of the plane mirror  40 . 
         [0011]    The fibres  21 ,  22  are supported by the fibre array  20  which comprises a V-groove block  23  also called V-groove substrate and a V-groove lid  24 , between which the fibres  21 ,  22  are mounted. 
         [0012]      FIG. 2A  shows a first embodiment of a fibre array  20   a  for optical devices of the prior art.  FIG. 2B  shows the end faces of the fibres  21 ,  22  of the fibre array  20   a  depicted in  FIG. 2A . 
         [0013]    The fibre array  20   a  comprises a V-groove block  23   a  in which the fibres  21 ,  22  are placed and a lid  24   a  covering the fibres  21 ,  22 . The end faces of the M input fibres are referenced by  22   a1 , . . . ,  22   am , and the end faces of the N output fibres are referenced by  21   a1 , . . . ,  21   an . 
         [0014]    The fibre array  20   a  comprises an end face  25  which is polished such that the end faces of the fibres  21 ,  22  become part of the end face  25  of the fibre array  20   a.  In case there is a refractive index difference between the fibres  21 ,  22  and the adjacent medium, generally, an anti-reflection coating is applied on the end face  25  to eliminate the Fresnel reflection. 
         [0015]    The end faces of the fibres  21 ,  22  are positioned on a straight line as shown in  FIG. 2B . In the optical device  10 , the straight line is positioned parallel to the dispersion plane of the grating  50 . In case of demultiplexing of channels that are equidistantly spaced with respect to wavelength, the end faces of the output fibres  21   a1 , . . . ,  21   an  are equidistantly spaced at a distance d. The end face of the input fibre  22   a1  is separated from the last end face of the output fibre  21   an  by a minimum distance Δ, typically between  2   d  and  5   d  to keep the size of the aperture  41  in the plane mirror  40  limited while minimizing crosstalk effects. 
         [0016]      FIG. 3A  shows a second, more complex, embodiment of a fibre array  20   b  for optical devices of the prior art that enables further minimization of crosstalk effects, in particular return loss and directivity which is described in the French patent FR-2.731.573.  FIG. 3B  shows the end faces of the fibres  21 ,  22  of the fibre array  20   b  depicted in  FIG. 3A . 
         [0017]    The fibre array  20   b  comprises a first V-groove block  23   b  and a second V-groove block  24   b.  The first V-groove block  23   b  is the substrate in which the output fibres  21  are placed and it serves as the lid for covering the input fibres  22 . The second V-groove block  24   b  is the substrate in which the input fibres  22  are placed and it serves as the lid for covering the output fibres  21 . The end faces of the M input fibres are referenced by  22   b1 , . . . ,  22   bm , and the end faces of the N output fibres are referenced by  21   b1 , . . . ,  22   bn . It is noted that stacking of two V-groove blocks  23   b  and  24   b  requires accurate alignment to ensure parallelism between output fibres  21  of block  23   b  and input fibres  22  of block  24   b.    
         [0018]    Like in the fibre array  20   a,  the fibre array  20   b  comprises an end face  25  which is polished such that the end faces of the fibres  21 ,  22  become part of the end face  25  of the fibre array  20   b.  In case there is a refractive index difference between the fibres  21 ,  22  and the adjacent medium, generally, an anti-reflection coating is applied on the end face  25  to eliminate the Fresnel reflection. 
         [0019]    The end faces of the fibres  21 ,  22  are positioned on two parallel straight lines: the end faces of the input fibres  22   b1 , . . . ,  22   bm  on one line and the end faces of the output fibres  21   b1 , . . . ,  21   bn  on the other line as shown in  FIG. 3B . In the optical device  10 , the straight lines are positioned parallel to the dispersion plane of the grating  50 . In case of demultiplexing of channels that are equidistantly spaced with respect to wavelength, the end faces of the output fibres  21   b1 , . . . ,  21   bn  are equidistantly spaced at a distance d. The end face of the input fibre  22   b1  is separated from the straight line of the end faces of the output fibres  21   b1 , . . . ,  21   bn  by a minimum distance D, typically between d and  2   d  to keep the size of the aperture in the plane mirror  41  limited while minimizing crosstalk effects. 
         [0020]      FIG. 4A  and  FIG. 4B  show beam propagation in the optical device  10 , where  FIG. 4A  is a top view of the device  10  and  FIG. 4B  is a side view of the same device  10 . 
         [0021]    In case the optical device  10  operates as a demultiplexer, a signal containing a spectral multiplex of channels enters through the input fibre  22 , propagates up to its end face  22   a1 ,  22   b1  and continues its path by beam propagation in the homogeneous medium  32 , where the beam  70  propagates about parallel to the optical axis  31   a.  The beam  70  passes through the aperture  41  in the plane mirror  40  and diverges until it impinges on the concave spherical mirror  60 . 
         [0022]      FIG. 5  represents propagation in the single mode optical the fibre  22  up to its end face  22   a1 ,  22   b1  followed by beam propagation in the adjacent homogeneous medium  32 . Propagation inside the fibre  22  corresponds to a guided mode, having a constant Mode Field Diameter (relative field intensity level of 1/e 2 ), abbreviated as MFD. For example, in the commonly used fibre SMF-28 from Corning, the MFD is around 10.4 μm at a wavelength λ 0  of 1550 nm. Beam propagation in the adjacent homogeneous medium  32  starts from the fibre end face  22   a1 ,  22   b1  where the beam has its waist equal to the WD. In the adjacent medium  32 , the beam  70  diverges according to a cone with an angle θ for the relative field intensity level of 1/e 2 . The beam waist (MFD) and θ are related as follows: 
         [0000]      θ=arctan((2λ/(π MFD )).   (2) 
         [0023]    For the SMF-28 fibre, a wavelength λ=λ 0 /n with λ 0  (wavelength in vacuum) of 1550 nm, and an adjacent medium  32  with a refractive index n of 1, the angle θ is 5.4°. The cone intersects with the spherical mirror  60  at a propagation distance about equal to the focal length f of the mirror  60 . The reflection area of the beam  70  on the mirror  60  has a diameter t of approximately: 
         [0000]        t≈ 2 f  tan(θ)=4 f  λ/(π MFD ).   (3) 
         [0024]    For a focal length f of 65 mm in combination with the parameters of the preceding example, the diameter t is about 12.3 mm. 
         [0025]    The reflection of the beam  70  on the concave spherical mirror  60  collimates it and reverses its direction of propagation about parallel to the optical axis  31   a.  Subsequently, it impinges on the plane mirror  40  that reflects it towards the grating  50 . A portion of the beam  70  is not reflected due to the small aperture  41  in the plane mirror  40 ; therefore, increasing the insertion loss and crosstalk effects of the device  10 . The beam  70  incident on the grating  50  near Littrow is diffracted back towards the plane mirror  40 . The diffraction angularly separates the beam  70 , containing a spectral multiplex of channels, into beams as a function of wavelength and therefore separating the channels. Only the beams  71  and  72  corresponding to the first and the last channels are shown in  FIG. 4A  and  FIG. 4B . Subsequently, they impinge on the plane mirror  40  that reflects them towards the concave spherical mirror  60 . Again a portion of each beam  71 ,  72  is not reflected due to the small aperture  41  in the plane mirror  40 ; therefore, further increasing the insertion loss of the device  10 . The reflection of each beam  71 ,  72  on the concave spherical mirror  60  reverses the direction of propagation and focuses each beam  71 ,  72  about parallel to the optical axis  31  a through the small aperture  41  in the plane mirror  40  onto the end faces of their corresponding output fibres  21   a1 , . . . ,  21   an ,  21   b1 , . . . ,  21   bn . At these end faces, the size of the beams is about equal to the MFD of the guided mode of the output fibres  21  and propagation continues inside these single mode fibres by their guided mode. This implies that the signal present at the input fibre  22  is demultiplexed at the output fibres  21 : each output fibre contains one of the channels of the spectral multiplex, the signal that entered through the input fibre. 
         [0026]      FIG. 4B  shows the reflection area  45  of all impinging beams  70 ,  71  and  72  on the plane mirror  40  and  FIG. 4B  also shows the diffraction area  52  of the impinging beam  70  on the grating  50 . These areas, depending on the MFD of the input fibre  22  and the focal length of the dispersing system  30 , give an indication of the required size of the different parts. It is noted that the size of the optical device  10  increases when the spectral spacing between the channels decreases because an increase of the focal length of the dispersion system  30  is required. In telecommunications equipments, the height of the optical devices is limited by the distance between the printed circuit boards on which these devices are mounted; for multiplexers and demultiplexers, height of a packaged optical device is typically 14 mm and maximum 16 mm whereas more complex wavelength routing devices can have a height up to 50 mm. 
         [0027]    In the French patent application FR-2.779.535, it is indicated that laser diode arrays and photodiode arrays can be used in the optical devices  10 , because they have dimensions comparable to those of optical fibres. For example, an optical channel monitor is obtained by replacing the output fibres  21  of a demultiplexer with an array of photodiodes. The implementation is not obvious: a fibre array, similar to  FIG. 2  or  FIG. 3 , needs to be assembled in which the distance Δ or D between end face of the input fibre  22   a1 ,  22   b1  and the photodiodes must be kept small. It is feasible when the input fibre  22   a1 ,  22   b1  is incorporated into the mount of the photodiode array, but this is more difficult to manufacture. 
         [0028]    The optical devices of the prior art described above have a number of drawbacks concerning their insertion loss, their crosstalk effects, their height and their versatility. 
         [0029]    The presence of the small aperture  41  in the plane mirror  40  causes an increase in the insertion loss of the optical device  10 , because twice a portion of the beam incident on the mirror  40  enters into the aperture  41  instead of being reflected. Moreover, the portion of the beam coming from the spherical mirror  60  enters the aperture  41  about parallel to the optical axis  31   a.  Therefore, a small part of it couples into the input fibres  22  and output fibres  21  adding to the crosstalk effects. 
         [0030]    The aperture  41  in the plane mirror  40  must be kept small to limit the increase of the insertion loss which implies that the distance Δ or D between end faces of the input fibres  22   a1 , . . . ,  22   am ,  22   b1 , . . . ,  22   bn , and the output fibres  21   a1 , . . . ,  21   an , . . . ,  21   b1 , . . . ,  21   bn  must also be kept small,  FIG. 2B  and  FIG. 3B . Although, fibre array  20   b  enables further minimization of crosstalk effects compared to fibre array  20   a,  some crosstalk effects remain due to the fact that the end faces of the input fibres  22   a1 , . . . ,  22   am ,  22   b1 , . . . ,  22   bm , and the output fibres  21   a1 , . . . ,  21   an ,  21   b1 , . . . ,  21   bn  are very close. 
         [0031]    For optical devices  10  with a relatively great focal length, the beam diameter at the spherical mirror becomes bigger than the acceptable height for optical devices used in telecommunications equipments. In particular, multiplexers and demultiplexers have a tight limit with respect to height. 
         [0032]    As aforementioned, devices are required for monitoring and routing of channels such as, for example, tunable lasers, tunable filters and optical channel monitors. The optical devices  10  can be used for the implementation of these devices, but they are not very well suited from a manufacturing point of view since standard mounts for laser diode arrays and photodiode arrays cannot be directly used. For that reason, the optical devices  10  are not very versatile. 
       SUMMARY OF THE INVENTION 
       [0033]    The principal purpose of the present invention is to propose optical devices that are as least as compact as the optical devices of the prior art, that enable the implementation of the same functions but with reduced insertion loss, reduced crosstalk effects and reduced height, and that are more versatile with respect to the implementation of optical functions other than multiplexers and demultiplexers. 
         [0034]    To that end, the present invention concerns an optical device comprising:
       a fibre array comprising an end face and   a compact dispersing system,
 
the fibre array being a mounting assembly that comprises M input elements which beam emitting end faces are positioned on a first straight line said emission line, and N output elements which beam receiving end faces are positioned on a second straight line said reception line which is parallel to the first line and separated from the first line by a distance D chosen such that at least the input elements do not affect the output elements and vice versa, where the end faces of the input elements and the output elements are positioned such that they substantially coincide with the end face of the fibre array,
   the compact dispersing system ensuring wavelength selective conjugation of signals between the end faces of the input elements and the end faces of the output elements based on beam propagation and comprising:   a plane mirror,   a concave mirror having a focus, a focal plane and an axis that intersects the end face of the fibre array half way between the two said parallel straight lines while being perpendicular to the end face of the fibre array, and   a plane diffraction grating having a dispersion plane that is parallel with respect to the two said parallel straight lines while the diffraction grating makes an angle of ‘90°−φ’ with respect to the end face of the fibre array where the angle φ is chosen such that the position of the grating does not affect beam propagation in the compact dispersing system,   the end face of the fibre array being located in the vicinity of the focal plane of the concave mirror such that the diverging beams coming from the end faces of the input elements become collimated by reflection on the mirror while being directed towards the plane mirror,   the plane mirror reflecting the collimated beams coming from the concave mirror to the grating and, inversely, reflecting the diffracted collimated beams coming from the grating to the concave mirror, being firstly located between the end face of the fibre array and the concave mirror, being secondly perpendicular to the dispersion plane, and making thirdly an angle ‘a’ with respect to the axis of the concave mirror such that the diffraction grating is located in the vicinity of the focus of the concave mirror, the angle a being chosen such that the diffraction grating is operated near Littrow,   the concave mirror reflecting the dispersed collimated beams coming from the plane mirror such that the beams are focused on the reception line of the fibre array, about linearly distributed over the line with respect to wavelength, and entering the end faces of the output elements where they are present,   the optical device being characterised in that the size of the plane mirror is limited with respect to the two said parallel straight lines such that beams propagating from the end faces of the input elements to the concave mirror and beams propagating from the concave mirror to the end faces of the output elements are not affected by the presence of the plane mirror.       
 
         [0045]    Advantageously, the plane mirror has a rotation mechanism for tuning the angle α which enables wavelength tuning of the device. 
         [0046]    Advantageously, the diffraction grating has a rotation mechanism for tuning the angle φ which enables wavelength tuning of the device. 
         [0047]    Advantageously, the fibre array has a translation mechanism for simultaneously tuning the position of the end faces of the input elements and the output elements over the said parallel straight lines which enables wavelength tuning of the device. 
         [0048]    Advantageously, the concave mirror has a translation mechanism for tuning the position of the concave mirror parallel along the said parallel straight lines which enables wavelength tuning of the device. 
         [0049]    Advantageously, each input element is a single mode fibre. 
         [0050]    Advantageously, each single mode fibre is terminated with collimating means. 
         [0051]    Advantageously, each output element is a single mode fibre. 
         [0052]    Advantageously, each single mode fibre is terminated with collimating means. 
         [0053]    Advantageously, each input element is a multimode fibre terminated with collimating means. 
         [0054]    Advantageously, each output element is a multimode fibre terminated with collimating means. 
         [0055]    Advantageously, each output element is a photodiode. 
         [0056]    Advantageously, each photodiode is terminated with collimating means. 
         [0057]    Advantageously, each input element is a laser diode terminated with collimating means. 
         [0058]    Advantageously, each collimating mean consists of a microlens. 
         [0059]    Advantageously, each collimating mean consists of a graded-index lens spliced to the end of said single mode fibre. 
         [0060]    The present invention concerns also a single mode wavelength router characterised in that it comprises an optical device according to previous embodiments. 
         [0061]    The present invention concerns also a single mode wavelength multiplexer characterised in that it comprises an optical device according to previous embodiments, and in that there is only one output fibre. 
         [0062]    The present invention concerns also a single mode wavelength demultiplexer characterised in that it comprises an optical device according to previous embodiments, and in that there is only one input fibre. 
         [0063]    The present invention concerns also a single mode wavelength filter characterised in that it comprises an optical device according to previous embodiments, and in that there is only one input fibre and one output fibre. 
         [0064]    The present invention concerns also a single mode optical channel monitor characterised in that it comprises an optical device according to previous embodiments, and in that there is only one input fibre. 
         [0065]    The present invention concerns also a single mode router/multiplexer/demultiplexer/filter/channel monitor according to any of previous embodiments, characterised in that, when the optical device comprises single mode fibres terminated with collimating means, each said collimating mean consists of a graded-index lens spliced to the end of said single mode fibre. 
         [0066]    The present invention concerns also a single mode router/multiplexer/demultiplexer/filter/channel monitor according to any of previous embodiments, characterised in that, when the optical device comprises collimating means, each said collimating mean consists of a microlens. 
         [0067]    The present invention concerns also a multimode wavelength router characterised in that it comprises an optical device according to previous embodiments. 
         [0068]    The present invention concerns also a multimode wavelength multiplexer characterised in that it comprises an optical device according to previous embodiments, and in that there is only one output fibre. 
         [0069]    The present invention concerns also a multimode wavelength demultiplexer characterised in that it comprises an optical device according to previous embodiments, and in that there is only one input fibre. 
         [0070]    The present invention concerns also a multimode wavelength filter characterised in that it comprises an optical device according to previous embodiments, and in that there is only one input fibre and one output fibre. 
         [0071]    The present invention concerns also a multimode optical channel monitor characterised in that it comprises an optical device according to previous embodiments, and in that there is only one input fibre. 
         [0072]    The present invention concerns also a multimode router/multiplexer/demultiplexer/filter/channel monitor according to any of previous embodiments, characterised in that the optical device comprises collimating means where each said collimating mean consists of a microlens. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0073]    The characteristics of the invention will emerge more clearly from a reading of the following description of an example embodiment, the said description being produced with reference to the accompanying drawings, among which: 
           [0074]      FIG. 1A  and  FIG. 1B  represent an optical device of the prior art showing the optical axes of the dispersing system; 
           [0075]      FIG. 2A  and  FIG. 2B  represent a first embodiment of a fibre array for optical devices of the prior art; 
           [0076]      FIG. 3A  and  FIG. 3B  represent a second embodiment of a fibre array for optical devices of the prior art; 
           [0077]      FIG. 4A  and  FIG. 4B  represent the optical device of the prior art showing beam propagation in the dispersing system; 
           [0078]      FIG. 5  shows propagation in a single mode optical fibre up to its end face followed by beam propagation in the adjacent homogeneous medium; 
           [0079]      FIG. 6A  and  FIG. 6B  represent an optical device according to the invention, showing the optical axis of the dispersing system; 
           [0080]      FIG. 7A  and  FIG. 7B  represent a first embodiment of the fibre array used in an optical device according to the invention; 
           [0081]      FIG. 8  shows propagation in a single mode optical fibre, comprising a lensed end, up to its end face followed by beam propagation in the adjacent homogeneous medium; 
           [0082]      FIG. 9  shows propagation in a single mode optical fibre up to its end face followed by beam propagation in the adjacent homogeneous medium, a microlens, and another homogeneous medium; 
           [0083]      FIG. 10  represents a second embodiment of the fibre array used in an optical device according to the invention where the fibre array comprises microlenses; 
           [0084]      FIG. 11A  and  FIG. 11B  represent the optical device according to the invention, showing beam propagation in the dispersing system; 
           [0085]      FIG. 12  represents a third embodiment of the fibre array used in an optical device according to the invention where the fibre array comprises photodiodes; and 
           [0086]      FIG. 13  represents an optical device according to the invention, showing three wavelength timing mechanisms. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0087]      FIG. 6A  and  FIG. 6B  represent an optical device  100  according to the present invention which can be for example a wavelength router, multiplexer, demultiplexer, or filter. The optical device is composed of a fibre array  120  comprising an end face  125  and a compact dispersing system  130 .  FIG. 6A  is a top view of the optical device  100  and  FIG. 6B  is a side view of the same optical device  100 . 
         [0088]      FIG. 7A  and  FIG. 7B  represent a first embodiment of the fibre array  120  used in the optical device  100  according to the invention. The fibre array  120  is a mounting assembly comprising M input elements  122  and N output elements  121 . The beam emitting end faces  122   1 , . . . ,  122   m  of the input elements  122  are positioned on a first straight line said emission line  112 , whereas the beam receiving end faces  121   1 , . . . ,  121   n  of the output elements  121  are positioned on a second straight line  111  said reception line. The two straight lines  111  and  112  are parallel and are separated by a distance D that is chosen such that at least the input elements  122  do not affect the output elements  121  and vice versa. The end faces  122   1 , . . . ,  122   m  of the input elements  122  and the end faces  121   1 , . . . ,  121   n  of the output elements  121  are positioned such that they substantially coincide with the end face  125  of the fibre array  120 . 
         [0089]    In the embodiment depicted in  FIGS. 7A and 7B , the fibre array  120  comprises a double sided V-groove block  123 , an output lid  124   1  and an input lid  124   2 , M input elements  122  and N output elements  121  in which both input and output elements are optical fibres. These fibres are either single mode as shown in  FIG. 5 , single mode terminated with a graded-index lens as shown in  FIG. 8  or even multimode. The M input fibres  122  are mounted in the V-shaped grooves and covered by the input lid  124   2  on one side of the block  123  whereas the N output fibres  121  are mounted in the V-shaped grooves and covered by the output lid  124   1  on the other side of the block  123 . It is noted that the use of a double sided V-groove block ensures parallelism between input fibres  122  and output fibres  121  by construction opposed to stacking of V-groove blocks (fibre array  20   b  in  FIG. 3A  and  FIG. 3B ). 
         [0090]    The end face  125  of the fibre array  120  is polished such that the end faces  122   1 , . . . ,  122   m  of the input fibres  122  and the end faces  121   1 , . . . ,  121   n  of the output fibres  121  become part of the end face  125  of the fibre array  120 . In case there is a refractive index difference between the fibres  121 ,  122  and the adjacent medium  32 , generally, an anti-reflection coating is applied on the end face  125  to eliminate the Fresnel reflection. 
         [0091]    In case of demultiplexing of channels that are equidistantly spaced with respect to wavelength, the end faces  121   1 , . . . ,  121   n  of the output elements  121  are equidistantly spaced at a distance d as shown in  FIG. 7B . The distance d is typically comprised in the range from 40 μm to 250 μm which is related to the outer diameter of most common output elements  121 . 
         [0092]    The compact dispersing system  130  ensures wavelength selective conjugation between the end faces  122   1 , . . . ,  122   m  of the input elements  122  and the end faces  121   1 , . . . ,  121   n  of the output elements  121 . It comprises a plane mirror  140 , a concave mirror  60  having a focus and a focal plane, and a plane diffraction grating  50  having a dispersion plane. The end face  125  of the fibre array  120 , the end faces  122   1 , . . . ,  122   m  of the input elements  122  and the end faces  121   1 , . . . ,  121   n  of the output elements  121  are located in the vicinity of the focal plane of the mirror  60  such that the diverging beams coming from the end faces of the input elements  122  become collimated by reflection on the mirror  60  while being directed towards the plane mirror  140 . The concave mirror  60  has an axis  63  which intersects the end face  125  of the fibre array  120  half way between the two parallel straight lines  111  and  112  of the end faces  121   1 , . . . ,  121   n ,  122   1 , . . . ,  122   m  while being perpendicular to the end face  125  of the fibre array  120 . The diffraction grating  50  makes an angle of about 90°−φ with respect to the end face  125  of the fibre array  120  while its dispersion plane is parallel with respect to the two parallel straight lines  111  and  112  of the end faces  121   1 , . . . ,  121   n ,  122   1 , . . . ,  122   m . The angle φ is chosen such that the position of the grating  50  does not affect beam propagation in the compact dispersing system  130 , for example φ=0° ( FIG. 6A ). The plane mirror  140  reflects the collimated beams coming from the concave mirror  60  to the grating  50  and, inversely, reflects the diffracted collimated beams coming from the grating  50  to the concave mirror  60 . The plane mirror  140  located between the end face  125  of the fibre array  120  and the concave mirror  60  is perpendicular to the dispersion plane, it makes an angle α with respect to the axis  63  of the concave mirror  60  such that the diffraction grating  50  is located in the vicinity of the focus of the concave mirror  60 , and the angle α is chosen such that the diffraction grating  50  is operated near Littrow. The concave mirror  60  reflects the dispersed collimated beams coming from the plane mirror  140  such that the beams are focused on the reception line  111  of the fibre array  120 , about linearly distributed over the line with respect to wavelength, and entering end faces of the output elements  121  where they are present. The size of the plane mirror  140  is limited with respect to the two parallel straight lines  111  and  112  of the end faces such that beams propagating from the end faces  122   1 , . . . ,  122   m  of the input elements  122  to the concave mirror  60  and beams propagating from the concave mirror  60  to the end faces  121   1 , . . . ,  121   n  of the output elements  121  are not affected by the presence of the plane mirror  140 . 
         [0093]    The optical axis of the compact dispersing system  130  is folded which ensures compactness of the optical device  100 . The optical axis is divided into six segments referenced  131   a,    131   b,    131   c,    131   d,    131   e  and  131   f.    
         [0094]    The first segment  131   a  of the optical axis runs from the straight line  112  coinciding with the end faces  122   1 , . . . ,  122   m  of the input elements  122  up to the concave mirror  60  while passing the plane mirror  140 , it is perpendicular to the end face  125  of the fibre array  120 , it makes an angle α with respect to the plane mirror  140  ( FIG. 6A ) and its end point at the concave mirror  60  is located vertically at a distance ½ D from the axis  63  of the mirror  60  ( FIG. 6B ). 
         [0095]    The second segment  131   b  of the optical axis starts at the concave mirror  60  from the end point of the first segment  131   a  of the optical axis and runs up to the plane mirror  140 . The first segment  131   a  and the second segment  131   b  of the optical axis make an angle γ in the yz-plane of approximately: 
         [0000]      γ≈arctan(D/(2f)   (3) 
         [0000]    where f is the focal length of the concave mirror  60 . 
         [0096]    The third segment  131   c  of the optical axis starts at the plane mirror  140  from the end point of the second segment  131   b  of the optical axis and runs to the diffraction grating  50  where it intersects the normal of the grating  50 , referenced  33 . The projection in the xz-plane of the third segment  131   c  makes an angle β with the normal  33 , where is β the Littrow angle of the grating  50 , such that the angles α and β are related as follows: α=(β+90°)/2 when φ=0° ( FIG. 6A ). 
         [0097]    The three remaining segments  131   d,    131   c,    131   e  are related to the first three  131   a,    131   b,    131   c  by mirror symmetry with respect to the xz-plane that comprises the axis  63  of the mirror  60 . The fourth segment  131   d  of the optical axis is symmetrical with the third segment  131   c;  the angle between the two is about 2γ. The fifth segment  131   e  of the optical axis is symmetrical with second segment  131   b.  The sixth segment  131   f  of the optical axis runs up to the end faces  121   1 , . . . ,  121   n  of the output elements  121  and is symmetrical with the first segment  131   a;  both segments are separated by a distance D. 
         [0098]      FIG. 6A  and  FIG. 6B  show an embodiment of the compact dispersing system  130  using three parts: a wedge prism  142 , a plano-concave lens  61 , and a substrate  51 . The wedge prism  142  is used to ensure the positioning of the plane mirror  140  with respect to the end face  125  of the fibre array  120  and to the optical axis  131   a,    131   b,    131   c,    131   d,    131   e,    131   f.  The wedge prism  142  comprises a tilted face  143  serving as support for the plane mirror  140  whereas the opposite face  144  is parallel with the end face  125  of the fibre array  120 . In the embodiment of  FIG. 6A  and  FIG. 6B , the parallelism is ensured by mounting the wedge prism  140  onto the end face  125  of the fibre array  120  such that the said opposite face  144  and the end face  125  of the fibre array coincide. The plano-concave lens  61  serves as support for the mirror  60  that resides on its concave face  62 . It is noted that the concave mirror  60  can have different concave shapes: the most common shapes are spherical and parabolic. The substrate  51  serves as support for the plane diffraction grating  50  which is formed on the surface of the substrate. The space  32  is filled with air, vacuum or a gas. 
         [0099]    Beam propagation in the compact dispersing system  130  requires that the divergence angle θ of the input beam is compatible with the presence of the plane mirror  140  and a limited reflection area on the concave mirror  60 . It is noted that the limitation of this reflection area is due to the height restriction of optical devices used in telecommunications equipments, in particular for multiplexers and demultiplexers. 
         [0100]    The beam divergence coming from a single mode optical fibre has been explained in the description of the optical device  10  of the prior art (see  FIG. 5 ). As previously stated, for the commonly used SMF-28 fibre from Corning, a wavelength λ=λ 0 /n with λ 0  of 1550 nm, and an adjacent medium  32  with a refractive index n of 1, the divergence angle θ is 5.4°. For beam propagation in the compact dispersing system  130 , a smaller divergence angle is preferable and, in most cases, even mandatory. Special single mode optical fibres exist with an MFD of up to about 25 μm, which implies a divergence angle θ down to about 2.3° compared to the SMF-28 fibre. The use of special single mode fibres is a solution for some applications, but, for most applications, the use of SMF-28 or equivalent input and output fibres is required. 
         [0101]    Termination of an optical fibre with collimating means can significantly reduce the divergence angle θ of the outgoing beam. This implies that SMF-28 or equivalent input and output fibres can be used in an optical device  100  according to the present invention while adjusting the divergence angle θ with the collimating means to the requirements of the dispersing system  130 . Therefore, it is preferable and, in many cases, even mandatory to incorporate collimating means into the input elements  122  and the output elements  121 . Hereafter, different embodiments of the input elements  122  and the output elements  121  comprising collimating means are described. 
         [0102]      FIG. 8  represents a first embodiment of a single mode optical fibre terminated with collimating means. In this embodiment, the input element  122  comprises an input fibre  126 , a graded-index lens  128  and a coreless stub  129 ; the end face of the coreless stub  129  being the end face  122   j  of the input element  122 . The graded-index lens  128  of length L g  is spliced to the single mode fibre  126  and the core less stub  129  of length L s  is spliced to the graded-index lens  128 . It is noted that the coreless stub  129  is added to enable polishing of the end face  125  of the fibre array  120  without modifying the length L g  of the graded-index lens  128 . 
         [0103]      FIG. 8  shows propagation in a single mode optical fibre terminated with collimating means  122  up to the end face  122   1  of the coreless stub  129  followed by beam propagation in the adjacent homogeneous medium  32 . Propagation in the graded-index lens  128  spliced to the single mode optical fibre  126 , increases the mode field diameter of the fibre, MFD f , up to a mode field diameter, MFD g . The length L g  is preferably a ¼ pitch of the graded-index profile, which makes MFD g  coincide with the interface between the graded-index lens  128  and the coreless stub  129 . From this interface, the beam diverges over the length L s  in the coreless stub  129 , which is to be considered as a homogeneous medium, and subsequently the input beam  170  continues to diverge after the end face  122   1  in the adjacent homogeneous medium  32 . The increase of the MFD from MFD f  to MFD g  implies a reduction of the angle θ of the beam coming out of the end face  122   1  of the input element  122  (see relation (2)). 
         [0104]    For an SMF-28 input fibre  126  terminated with a graded-index lens  128  having a graded-index profile that increases the MFD to MFD g  of 75 μm, a wavelength λ=λ 0 /n with λ 0  (wavelength in vacuum) of 1550 nm, and an adjacent medium  32  with a refractive index n of 1, the angle θ of the beam is 0.75°. In the dispersing system  130 , this cone intersects with the spherical mirror  60  at a propagation distance about equal to the focal length f of the mirror  60 . For a focal length f of 65 mm, the reflection area of the beam  170  on the mirror  60  has then a diameter of about 1.7 mm (see relation (3)). 
         [0105]      FIG. 9  represents a second embodiment of a single mode optical fibre terminated with collimating means. In this embodiment, the input element  122  comprises an input fibre  126  having an end face  126   1 , a small gap  227  filled with a homogeneous medium, and a microlens  228 ; the end face of the microlens  228  being the end face  122   1  of the input element  122 . The microlens  228  is precisely aligned and mounted with respect to the end face  126   1  of the input fibre  126 . In case that the microlens is attached to the end face  126   1  of the input fibre  126  with an epoxy, the gap  227  is filled with the epoxy which is a homogeneous medium having a refractive index that is close to the refractive index of the materials used in the optical fibre  126  and the microlens  228 . Generally, an anti-reflection coating is applied on the end face of the microlens  228  to eliminate the Fresnel reflection. 
         [0106]      FIG. 9  shows propagation in a single mode optical fibre terminated with collimating means  122  up to the end face  122   1  of the microlens  228  followed by beam propagation in the adjacent homogeneous medium  32 . Propagation from the end face  126   1  of the input fibre  126  up to the end face  122   1  of the microlens  228  corresponds to a diverging beam in a cascade of 2 homogeneous media, subsequently, propagation through the curved surface (end face)  122   1  of the microlens  228  reduces the divergence angle of the beam, the resulting beam  170  continues to diverge after the end face  122   1  in the adjacent homogeneous medium  32  and has a divergence angle θ. The resulting beam  170  has a virtual beam waist MFD ma  that is located in the vicinity of the end face  126   1  of the input fibre  126 . So, the use of a microlens  228  as collimating means increases the MFD from MFD f  to MFD ma . 
         [0107]      FIG. 10  represents a second embodiment of a fibre array  220  used in the optical device  100  according to the invention where the fibre array  220  comprises microlenses. The embodiment of the fibre array  220  is composed of a fibre array  120  having single mode input and output fibres (see  FIG. 7A ,  FIG. 7B  and the corresponding description) and two microlens arrays  215 ,  216  which are precisely aligned and mounted with respect to the said emission line  112  and the said reception line  111  of the fibre array  120 . This way, each single mode optical fibre is terminated by a microlens (see  FIG. 9 ). Various microlens arrays are available on the market, for example, buried graded-index microlens arrays marketed by NSG (Nippon Sheet Glas) or plano-convex refractive microlens arrays marketed by SUSS MicroOptics. 
         [0108]    It is noted that microlenses are also well suited as collimating means for multimode optical fibres. Therefore, an optical device  100  according to the present invention can comprise multimode fibres instead of single mode fibres. 
         [0109]      FIG. 11A  and  FIG. 11B  represent the optical device  100  according to the present invention, showing beam propagation in the dispersing system  130 , where  FIG. 11A  is a top view of the device  100  and  FIG. 11B  is a side view of the same device  100 . 
         [0110]    In case the optical device  100  operates as a demultiplexer, a signal containing a spectral multiplex of channels enters through the input element  122 , propagates up to the end face  122   1  and continues its path by beam propagation in the homogeneous medium  32 , where the beam  170  propagates about parallel to the first segment of the optical axis  131   a.  The beam  170  diverges with an angle θ until it impinges on the concave mirror  60 , on its way, it passes the plane mirror  140  without being affected by its presence. 
         [0111]    The reflection of the beam  170  on the concave mirror  60  collimates it and directs it towards the plane mirror  140 . Subsequently, it impinges on the plane minor  140  that reflects it entirely towards the grating  50 . The beam  170  incident on the grating  50  near Littrow is diffracted back towards the plane mirror  140 . The diffraction grating angularly separates the beam  170 , containing a spectral multiplex of channels, into beams as a function of wavelength and therefore separating the channels. Only beams  171  and  172  corresponding to the first and the last channel are shown in  FIG. 11A  and  FIG. 11B . Subsequently, they impinge on the plane mirror  140  that reflects them entirely towards the concave mirror  60 . The reflection of each beam  171 ,  172  on the concave mirror  60  directs them about parallel to the sixth segment  131   e  of optical axis and focuses each beam  171 ,  172  onto the end faces  121   1 , . . . ,  121   n  of the corresponding output elements  121 ; on its way, each beam passes the plane mirror  140  without being affected by its presence. At the end faces  121   1 , . . . ,  121   n , the size of the beams is about equal to the MFD of the output elements  121  and propagation continues inside these elements (reverse direction in  FIG. 5 ,  FIG. 8  and  FIG. 9 ). This implies that the signal present at the input element  122  is demultiplexed at the output elements  121 : each output element contains one of the channels of the spectral multiplex, the signal that entered through the input element. 
         [0112]    Operation of the optical device  100  of the invention is similar to operation of the optical device  10  of the prior art, the main difference resides in the position of the end faces  122   1 , . . . ,  122   m  of the input elements  122  and of the end faces  121   1 , . . . ,  121   n  of the output elements  121  with respect to the plane mirror. It is therefore not necessary to realise an aperture in the plane mirror which reduces the insertion loss in comparison to the prior art. Moreover, the end faces  122   1 , . . . ,  122   m  of the input elements  122  and the end face  121   1 , . . . ,  121   n  of the output elements  121  are far apart which reduces crosstalk effects. Uniformity of the insertion loss is optimized when the end face  125  of the fibre array  120  is located in the vicinity of the focal plane of the concave mirror  60  while the diffraction grating  50  is located in the vicinity of its focus. 
         [0113]      FIG. 11B  shows the reflection area  145  of all impinging beams  170 ,  171  and  172  on the plane mirror  140  and  FIG. 11B  also shows the diffraction area  152  of the impinging beam  170  on the grating  50 . These areas, depending on the MFD of the input element  122  and the focal length of the dispersing system  130 , give an indication of the required size of the different parts. The size of the optical device  100  of the invention as well as the optical device  10  of the prior art ( FIG. 4A  and  FIG. 4B ) increase when the spectral spacing between the channels decreases, because this requires an increase of the focal length of the dispersion system  130  of the invention as well as of the dispersion system  30  of the prior art. The difference is that the height of the optical device  100  of the invention can be kept limited by terminating the input elements  122  and the output elements  121  with appropriate collimating means. 
         [0114]    A single mode demultiplexer comprising an optical device  100  according to the present invention has been implemented. The optical device  100  has a diffraction grating  50  with a groove density of 900 gr/mm that is optimized for use in the first order of diffraction over the spectral range from 1525 nm to 1575 nm, a concave spherical mirror  60  with a radius of 130 mm and a fibre array  120  with one single mode input fibre and 24 single mode output fibres, all terminated by a graded-index lens spliced to the fibre end which adapts the MFD to about 78 μm ( FIG. 8 ). In the fibre array  120 , the end faces  121   1 , . . . ,  121   n  of the terminated output fibres are equidistantly spaced at a distance d of 130 μm and the distance D between the emission line  112  and the reception line  111  is 6.5 mm ( FIG. 7B ). The resulting demultiplexer has 24 output channels that are equidistantly spaced at 1.6 nm. The filter function of each output channel has a Gaussian like shape with a full width half maximum of about 0.8 nm. 
         [0115]    As described above, the optical devices according to the present invention are adapted to the required optical functions by interchanging fibre arrays. The first two embodiments of fibre arrays  120 ,  220  comprise optical fibres but no optoelectronic components (laser diodes, photodiodes). The emission line  112  and the reception line  111  of the fibre arrays are well separated which makes it possible to combine optoelectronic components on one line with optical fibres on the other line while directly using the standard mounts of the optoelectronic components. It is noted that optoelectronic components can be terminated with collimating means, for example, microlenses. 
         [0116]      FIG. 12  represents a third embodiment of a fibre array  320  used in an optical device according to the invention that is comprised in an optical channel monitor. The fibre array  320  comprises an end face  125 , a double sided V-groove block  123 , an input lid  124   2 , a single input element  122  being an optical fibre and a photodiode array  321  comprising the N output elements  322  (photodiodes). The optical fibre is either single mode as shown in  FIG. 5  or single mode terminated with a graded-index lens as shown in  FIG. 8 . The input fibre  122  is mounted in the V-shaped groove and covered by the input lid  124   2  on one side of the block  123  whereas the photodiode array  321  is mounted on the other side of the block  123 . The end face  122   1  of the input element  122  and the end faces of the output elements  322  are positioned such that they substantially coincide with the end face  125  of the fibre array  320 . 
         [0117]    Operation of the optical device comprising fibre array  320  is understood from  FIG. 11A  and  FIG. 11B  by replacing fibre array  120  with fibre array  320 . Like in a demultiplexer, a signal containing a spectral multiplex of channels enters through the input fibre  122 , propagates up to its end face  122   1  and continues its path by beam propagation in the dispersing system which separates the beam  170  into beams  171 ,  172  implying separation of the channels at the output elements  322  of the fibre array  320  as previously described. Instead of coupling into output fibres like with fibre arrays  120 ,  220 , the beams  171 ,  172  couple into the photodiodes  322 . Then, each photodiode detects electrically the power level of its corresponding channel. 
         [0118]    A single mode optical channel monitor comprising an optical device according to the present invention has been implemented with a photodiode array  321  referenced by Hamamatsu under the number G8909-01. This photodiode array comprises 40 PIN photodiodes on a ceramic mount where the photodiodes are equally spaced on a straight line with a pitch d of 250 μm. The optical device has a diffraction grating  50  with a groove density of 600 gr/mm that is optimized for use in the second order of diffraction over the spectral range from 1525 nm to 1575 nm, a concave spherical mirror  60  with a radius of 190 mm and a fibre array  320  with the Hamamatsu photodiode array  321  as well as a single mode input fibre  122  terminated by a graded-index lens spliced to the fibre end adapting the MFD to about 67 μm ( FIG. 8 ). The end face  122   1  of the input fibre  122  is separated from the straight line of photodiodes  322  by a distance D of 6.5 mm in the y-direction while in the x-direction it is positioned with respect to the centre of the photodiode array  321 . The resulting channel monitor observes the power levels of 40 channels that are equally spaced at 0.8 nm. 
         [0119]    A multimode mode optical channel monitor comprising an optical device according to the present invention can also be implemented by using a multimode input fibre terminated with collimating means instead of the single mode input fibre. 
         [0120]    Similarly to fibre array  320 , a fibre array can be made that comprises one or more laser diodes as input elements in combination with an optical fibre as output element. It is noted that laser diodes require termination with collimating means due to the fact that their emitted beam is, in general, elliptic and strongly diverging. 
         [0121]    Up to this point, static optical devices according to the present invention have been described (devices without moving parts). Dynamic optical devices according to the present invention provide wavelength tuning (devices with moving parts). Such dynamic devices are more and more required in fibre optic long-haul and metro networks as well as in test and measurement equipments. 
         [0122]      FIG. 13  represents the top view of an optical device  500  according to the present invention comprising wavelength tuning. The depicted embodiment of the device  500  is for example comprised in a wavelength tunable filter.  FIG. 13  shows three different tuning mechanisms:
       1) The plane mirror  540  has a rotation mechanism for tuning the angle α;   2) The diffraction grating  50  has a rotation mechanism for tuning the angle φ;   3) The fibre array  520  has a translation mechanism for simultaneously tuning the position x of the end faces of the input elements  122  and the output elements  121  over the said parallel straight lines  111  and  112 .
 
It is noted that an optical device  500  requires only one of these three mechanisms in order to be tunable with respect to wavelength.
         
         [0126]    The references  531   a,    531   b,    531   c,    531   d,    531   e  and  531   f  represent the different segments of the optical axis of the optical device  500 . Each segment  531   a,    531   b,    531   c,    531   d,    531   e,    531   f  corresponds to the segment respectively referenced  131   a,    131   b,    131   c,    131   d,    131   e,    131   f  and described previously with  FIG. 6A  and  FIG. 6B . 
         [0127]    The two angular tuning mechanisms are understood from the fact that the angles α, β and φ are related as follows: α=(β+φ+90°)/2 in which β is the Littrow angle at a given wavelength λ. The angle β becomes a function of wavelength, i.e. β(λ), when at least one of the angles α and φ can be tuned. 
         [0128]    The angular tuning mechanism is, for example, provided by a mechanical rotation stage: tuning of the angle φ is then obtained by mounting the substrate  51  of the diffraction grating  50  on a rotation stage precisely ensuring the required movement whereas, equivalently, tuning of the angle α is obtained by mounting the substrate  542  of the plane mirror  540  on a rotation stage. The angular tuning mechanism for α can also be provided by a MEMS mirror which is a micro mechanical system comprising the plane mirror  540  that is rotated by an electrostatic mechanism. 
         [0129]    The position tuning mechanism is understood from the fact that position x of the end faces of the input elements  122  on the emission line  112  and the output elements  121  on the reception line  111  are approximately related as follows: Δλ/Δx. A translation of Δx of the end face of an input element  122  on the emission line  112  corresponds to a change in wavelength of about Δλ, whereas a translation of Ax of the end face of an output element  121  on the reception line  111  corresponds to a change in wavelength of about Δλ. This implies that a simultaneous translation of Δx of the end faces of the input elements  122  and the output elements  121  over the said parallel straight lines  111  and  112  corresponds to a change in wavelength of about 2Δλ. 
         [0130]    Simultaneous position tuning of the end faces of the input elements  122  and the output elements  121  of the fibre array  520  over the said parallel straight lines  111  and  112  is obtained by mounting the fibre array  520  on a translation stage precisely ensuring the required movement. Inversely, the dispersion system  530  can be mounted on a translation stage for precisely ensuring the equivalent movement, but this is in general less practical because of the size of the dispersion system  530 . 
         [0131]    It is noted that a translation with respect to the x-direction of the concave mirror  60  also provides wavelength tuning; the translation of the concave mirror  60  being similar to the translation of the fibre array  520  described above. To this end, the concave mirror  60  has a translation mechanism for tuning the position of the concave mirror  60  parallel along the said parallel straight lines. 
         [0132]    A single mode wavelength tunable filter comprising an optical device  500  according to the present invention has been implemented. The optical device  500  has a diffraction grating  50  with a groove density of 600 gr/mm that is optimized for use in the second order of diffraction over the spectral range from 1525 nm to 1575 nm, a concave spherical mirror  60  with a radius of 200 mm and a fibre array  520  with one single mode input fibre  122  and one single mode output fibre  121  both terminated by a graded-index lens spliced to the fibre end adapting the MFD to about 67 μm ( FIG. 8 ). The end face of the input fibre  122  has the same position as the end face of the output fibre  122  in the x-direction while being separated by a distance D of 6.5 mm in the y-direction. The filter is tuned over 50 nm from 1525 nm to 1575 nm. It has a Gaussian like shape with a fill width half maximum of about 0.17 nm over the tuning range. 
         [0133]    Table 1 shows λ, β, α, φ and x for wavelength tuning of the optical device  500  by tuning the angle φ where x=0 mm and a has been chosen such that φ=0° at the center wavelength of the tuning range. In this example, the entire wavelength range of the tunable filter is covered by an angular tuning range of Δφ=4.70°. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Wavelength tuning by tuning the angle φ 
               
             
          
           
               
                 λ 
                 β 
                 α 
                 φ 
                 x 
               
               
                   
               
               
                 1525 nm 
                 66.21° 
                 79.22° 
                 2.22° 
                 0.00 mm 
               
               
                 1550 nm 
                 68.43° 
                 79.22° 
                 0.00° 
                 0.00 mm 
               
               
                 1575 nm 
                 70.91° 
                 79.22° 
                 −2.48°  
                 0.00 mm 
               
               
                   
               
               
                 Angular tuning range: Δφ = 4.70° 
               
             
          
         
       
     
         [0134]    Table  2  shows λ, β, α, φ and x for wavelength tuning of the optical device  500  by tuning the angle α for φ=0° and x=0 mm. In this example, the entire wavelength range of the tunable filter is covered by an angular tuning range of Δα=2.35°. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Wavelength tuning by tuning the angle α 
               
             
          
           
               
                 λ 
                 β 
                 α 
                 φ 
                 x 
               
               
                   
               
               
                 1525 nm 
                 66.21° 
                 78.11° 
                 0.00° 
                 0.00 mm 
               
               
                 1550 nm 
                 68.43° 
                 79.22° 
                 0.00° 
                 0.00 mm 
               
               
                 1575 nm 
                 70.91° 
                 80.46° 
                 0.00° 
                 0.00 mm 
               
               
                   
               
               
                 Angular tuning range: Δα = 2.35° 
               
             
          
         
       
     
         [0135]    Table 3 shows λ, β, α, φ and x for wavelength tuning of the optical device  500  by tuning the position x for φ=0° in combination with a chosen such that x=0 mm at the center wavelength of the tuning range. In this example, the entire wavelength range of the tunable filter is covered by a position tuning range of Δx=8.16 mm. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Wavelength tuning by tuning the position x 
               
             
          
           
               
                 λ 
                 β 
                 α 
                 φ 
                 x 
               
               
                   
               
               
                 1525 nm 
                 68.43° 
                 79.22° 
                 0.00° 
                 −4.08 mm  
               
               
                 1550 nm 
                 68.43° 
                 79.22° 
                 0.00° 
                 0.00 mm 
               
               
                 1575 nm 
                 68.43° 
                 79.22° 
                 0.00° 
                 4.08 mm 
               
               
                   
               
               
                 Position tuning range: Δx = 8.16 mm 
               
             
          
         
       
     
         [0136]    Equivalently, a multimode wavelength filter can be implemented by replacing the input single mode fibre  122  and the output single mode fibre  121  of the optical device  500  by multimode fibres that are terminated with collimating means, for example microlenses ( FIG. 9 ). In fact, all previously described devices, routers, multiplexers, demultiplexers as well as optical channel monitors, can be made wavelength tunable with one of the previously described tuning mechanisms. It is further noted that the optical device of the present invention can also be used in tunable external cavity lasers and in optical spectrum analyzers. 
         [0137]    Although the present invention has been described in terms of illustrative embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those of ordinary skill in the art. It is therefore intended that the following claims are interpreted as covering all such alterations and modifications as falling within the true spirit and scope of the invention.