Abstract:
The invention relates to wavelength division multiplexing (WDM) and demultiplexing of optical signals using optical filters in free space. To minimize dispersion, the present invention uses relatively small angles of incidence. To avoid the need for a large package, the present invention utilizes reflective prisms to steer the reflected beams to provide a lateral shift in the beam path that ensures that the necessary space is provided between the input/output ports. Devices including a single adjustable filter are disclosed, along with devices having a plurality of filters.

Description:
TECHNICAL FIELD  
         [0001]    The present application relates to a wavelength division multiplexer/demultiplexer, and in particular to a compact wavelength division multiplexer/demultiplexer utilizing optical filters in free space.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional wavelength division multiplexing (WDM) couplers, such as the one disclosed in FIG. 1, include a double bore ferrule  21  encasing the ends of optical fibers  11  and  12 , which are optically coupled to a lens  31 . The lens  31  collimates a beam of light from the fiber  11  onto an optical filter  41 , which passes light centered around a single wavelength. This wavelength corresponds to one or more channels. The remainder of the light is reflected back through the lens  31  and out the fiber  12 . The light that is passed through the filter  41  is focused by a lens  32  onto a fiber  13 , which is encased in a ferrule  22 . Alternatively, when used as a multiplexer, light from the fiber  12  is reflected by the filter  41  and combined with light passing therethrough from fiber  13 . The combined signal is output the fiber  11 .  
           [0003]    There are certain drawbacks to these conventional couplers, such as a certain amount of cross talk between the fibers  11  and  12  due to their close proximity. The center wavelength (λ p ) passed by the optical filter  41  is dependent upon the angle of incidence of the light hitting the filter, i.e. λ p =λ o (1−c·sin 2 θ) {fraction (1/2)} in which c is a constant dependent upon the particular filter. However, due to the fact that the position of filter 41 is fixed, the center wavelength of the passband of the filter cannot easily be tuned. As a result, selecting the appropriate filter for the desired passband before manufacturing the coupler becomes critical.    
           [0004]    One type of optical device provided as a solution for overcoming these shortcomings is disclosed in U.S. Pat. No. 4,244,045 issued Jan. 6, 1981 to Nippon Telegraph and Telephone. During the manufacture of these devices, each optical filter can be individually angle tuned to maximize optical coupling. However, as is well known in the art, the bandwidth of the passband decreases as the angle of incidence increases, resulting in an increase in the insertion loss of the filter. Furthermore, polarization dependent loss (PDL), i.e. the difference in the transmission of the S and P components of the signal, greatly increases with an increase in the angle of incidence.  
           [0005]    Accordingly, what is required is a device that enables the filters to be angle tuned, while minimizing the angle at which the light is incident on the filter. However, this raises another problem related to the physical restraints of positioning two ports, each with their own lens and ferrule, beside each other. With reference to FIG. 2, we will assume that for a beam of diameter Φ=0.46 mm the distance D between the ports must be at least 6.5 mm. If we also assume that the angle of incidence between the beam of light and the filter normal must be 1.8° or less, we can calculate the distances L 1  and L 2  to be approximately 100 mm each for a total beam path L 1 +L 2  of 200 mm. This is a great distance for the beams to travel, which would require an unnecessarily large package to support. FIG. 3 schematically represents a multi-filter WDM platform that would be required assuming the aforementioned calculations. This device includes a plurality of filters  41 , between which a WDM signal bounces while dropping channels at the ports.  
           [0006]    An object of the present invention is to overcome the shortcomings of the prior art by providing a compact wavelength divisional multiplexer with tunable optical filters and without requiring relatively large angles of incidence.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, the present invention relates to an optical multiplexer/demultiplexer device comprising:  
           [0008]    a first input/output port through which a wavelength division multiplexed (WDM) signal comprising a plurality of wavelength channels is input or output along an first path in free space;  
           [0009]    a first optical filter disposed in the first path for transmitting at least a first predetermined wavelength channel of the WDM signal along the first path, and for reflecting other wavelength channels of the WDM signal at a reflection angle between the first path and a second path in free space;  
           [0010]    a first add/drop port for outputting or inputting the first predetermined wavelength channel;  
           [0011]    a second input/output port adjacent to the first input/output port for outputting or inputting the other wavelength channels of the WDM signal traveling along a third path in free space; and  
           [0012]    a first prism for redirecting the other wavelength channels of the WDM signal traveling between the second path and the third path, whereby the position of the second input/output port is independent of the reflection angle and the required distance between the first drop port and the second input/output port is reduced.  
           [0013]    Another aspect of the present invention relates to an optical demultiplexer device comprising:  
           [0014]    a first input/output port through which a wavelength division multiplexed (WDM) signal comprising a plurality of wavelength channels is input along an first path in free space;  
           [0015]    a first optical filter disposed in the first path for transmitting at least a first predetermined wavelength channel of the WDM signal, and for reflecting a first sub-signal of the WDM signal along a second path in free space at an angle to the first path;  
           [0016]    a first drop port for outputting at least the first predetermined wavelength channel transmitted by the first optical filter;  
           [0017]    a first prism for redirecting the first sub-signal of the WDM signal traveling along the second path to a third path;  
           [0018]    a second optical filter disposed in the third path for transmitting at least a second predetermined channel of the WDM signal, and for reflecting a second sub-signal of the WDM signal along a fourth path in free space at an angle to the third path; and  
           [0019]    a second drop port adjacent to the first input/output port for outputting at least the second predetermined channel transmitted by the first optical filter;  
           [0020]    whereby the position of the second drop port is independent of the angle between the second and third paths, and the required distance between the first drop port and the second drop port is reduced.  
           [0021]    Another feature of the present invention provides an optical demultiplexer device comprising:  
           [0022]    a housing for supporting a plurality of ports at the periphery, and defining a free space therein;  
           [0023]    an input port through which a wavelength division multiplexed (WDM) signal comprising a plurality of wavelength channels is input into the free space;  
           [0024]    a plurality of optical filters sequentially disposed in the path of said WDM signal, each filter for transmitting at least one predetermined channel of the WDM signal, and for reflecting the remainder of the WDM signal at an angle of reflection to be incident upon the next optical filter;  
           [0025]    a plurality of drop ports, one drop port corresponding to each optical filter, for outputting the predetermined channels transmitted by the corresponding optical filter; and  
           [0026]    a plurality of prisms for redirecting the WDM signal traveling between the plurality of optical filters, whereby the position of each drop port is dependent upon the prisms and independent of the angles of reflection. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:  
         [0028]    [0028]FIG. 1 is an exploded schematic view of a conventional WDM coupler;  
         [0029]    [0029]FIG. 2 is a schematic illustration of a conventional WDM filter arrangement in free space;  
         [0030]    [0030]FIG. 3 is a schematic illustration of a conventional WDM multi-filter platform arrangement in free space;  
         [0031]    [0031]FIG. 4 is a schematic illustration of a WDM filter arrangement according to the present invention;  
         [0032]    [0032]FIGS. 5 a  and  5   b  are a cross-sectional views of a WDM filter device according to the present invention;  
         [0033]    [0033]FIG. 6 is a cross-sectional view of a WDM multi-filter platform device according to the present invention;  
         [0034]    [0034]FIG. 7 is a schematic illustration of a second embodiment of a WDM multi-filter platform device according to the present invention;  
         [0035]    [0035]FIG. 8 is a schematic illustration of a third embodiment of a WDM multi-filter platform device according to the present invention; and  
         [0036]    FIGS.  9  to  14  are schematic illustrations of other embodiments of WDM filter devices according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0037]    In contrast to FIG. 2, the WDM filter arrangement according to an embodiment of the present invention is illustrated in FIG. 4 and includes an input/output port  101 , an optical filter  102 , an add/drop port  103 , a prism  104 , and an output/input port  105 . The ports  101 ,  103  and  105  include a single bore ferrule encasing an end of an optical fiber, which is optically coupled to a lens. The lens is typically a ¼ pitch graded index (GRIN) lens, such as those available under the trademark SELFOC® from NSG Co. Ltd. of Osaka, Japan.  
         [0038]    The addition of the prism  104  enables the beam to be reflected at a relatively small angle of reflection, e.g. θ i =θ r =1.8°, without being a factor in the positioning of the output/input port  105 . In the illustrated embodiment, the opposite faces of the prism  104  are parallel, which ensures that the beam exits the prism  104  along a path parallel to the path along which the beam entered the prism  104 . The opposite faces of the prism  104  can be arranged at an angle causing total internal reflection of the beam or coated with a reflective coating. Using the aforementioned assumptions, the total beam path L 1 +L 2  can be cut down to approximately 20 to 30 mm.  
         [0039]    [0039]FIGS. 5 a  and  5   b  illustrate the elements of FIG. 4 mounted in a housing  109 . The ports  101 ,  103  and  105  are attached to the housing  109  using either an adhesive or a weld  108 . The optical filter  102  is mounted on a rotatable base  110 , which enables the optical filter  102  to be angle tuned prior to attachment. A recess  112  is provided on the base  110  to facilitate rotation thereof using a separate implement (not shown). The prism  104  is also independent of the other elements, and is therefore also adjustable prior to permanent assembly to allow for tuning. An adhesive layer  111  is provided to fix the prism  104  to the housing  109 .  
         [0040]    The multi-filter version of the present invention is illustrated in FIG. 6, and includes a housing  301  defining a free space  302 . An input/output port  303 , along with a plurality of add/drop ports  304   a  to  304   g , are mounted on the sides of the housing  301 . Each port includes a ferrule  306  surrounding an end of an optical fiber  307 , which is optically coupled to a lens  308 . Each add/drop port  304   a  to  304   g  has a corresponding optical filter  309   a  to  309   g , each one adapted to pass a different passband, i.e. channel or channels, at substantially the same angle of incidence. Prisms  310   a  to  310   c  are disposed in the path of the signal traveling from one filter to the next.  
         [0041]    When demultiplexing, a WDM signal with a plurality of channels (λ 1  to λ n ) is launched via the input/output port along a first path  311 . The first optical filter  309   a  passes a first channel λ 1  to the first add/drop port  304   a . The remainder of the WDM signal is reflected at a relatively small angle towards the other side of the housing  301  containing the next add/drop port  304   b  along a second path  312 . In order to minimize the footprint of the device, the prism  310   a  redirects the remainder of the WDM signal from the second path  312  to a third path  313 . The lateral shift in the beam path provides the space necessary to be able to mount the second add/drop port  304   b  adjacent thereto, while keeping the desired angle of incidence. The second optical filter  309   b  passes a second channel λ 2  to the second add/drop port  304   b , and reflects the remaining signal towards the next optical filter  309   c  along a fourth path  314 . The WDM signal continues to zig-zag through the housing  301  dropping the appropriate channel or channels at the desired add/drop ports. What is left of the signal can be captured in the final add/drop port  304   g  or simply reflected off the final optical filter  309   g  and lost. To facilitate reproduction, the second path  312  is substantially parallel to the third path  313 , while the first path  311  is substantially parallel to the fourth path  314 . A rotatable mount  316  is provided for each filter  309  to facilitate the tuning of the optical filters during assembly.  
         [0042]    Alternative layouts for the multi-filter assembly are illustrated in FIGS. 7 and 8. With reference to FIG. 7, the input/output port  303  is positioned on one side of the housing, while all of the add/drop ports  304   a  to  304   d  are positioned on the opposite side. In this embodiment, the prisms  315   a  to  315   b  redirect WDM signals from the second path  312  to a third path  323 , which is substantially parallel to the first path  311 . If all of the ports are required to be on one side, as in FIG. 8, an additional prism  325  is used to redirect the initial WDM signal back along the first path  311 .  
         [0043]    The above discussion is directed towards using the device as a demultiplexer; however, it would be obvious to one skilled in the art to use the device of the present invention as a multiplexer or a demultiplexer.  
         [0044]    FIGS.  9  to  14  illustrate additional embodiments of the WDM filter arrangement including a variety of different kinds of prisms. In FIG. 9, the prism  404  has opposite sides cut with two different angles, thereby causing the beam to exit the prism  404  along a path that is not parallel to the entry path. Total internal reflection on one face of prism  504  is used in the device illustrated in FIG. 10 to redirect the beam towards the port  105 . Alternatively, the device in FIG. 11 uses a reflective surface  604  to redirect the beam to the port  105 .  
         [0045]    In FIG. 12, the reflective surface  604  is used in combination with a prism  704  to direct the reflected beam to one of the fibers of a double bore tube. The other fiber is used to transmit the light passed by the filter  102 . In FIGS. 13 and 14 prism  804  and  904 , respectively, are used to direct the reflected beam back to a double bore tube  101 , which inputs the initial WDM signal via fiber  107  and outputs the reflected signal via fiber  106 .