Abstract:
A four-port optical filter is provided including two tapered optical fibers, each having a loop in the area of their tapered portions, which are located relative to one another to form a ring resonator. The optical filter has a low Q with a broadband filter response which is useful as a channel interleaver or de-interleaver for dense wavelength division multiplexing and other applications.

Description:
FIELD OF THE INVENTION 
   This invention is directed to a four-port optical filter, and, more particularly, to an optical filter which employs two tapered optical fibers, each having a loop, which collectively form a ring resonator. The filter is particularly useful in applications requiring a low Q, broadband filter response such as channel interleavers or de-interleavers for dense wavelength division multiplexing. 
   BACKGROUND OF THE INVENTION 
   Fiber-optic communications systems employ techniques known as wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) to transit large volumes of information at high data rates over a single optical fiber. Many optical wavelengths of light or “optical channels” are employed in both WDM and DWDM systems, with each optical channel individually transmitting a substantial amount of data. The optical power of one optical channel co-propagates with a number of the other optical channels within a single optical fiber. 
   It is often necessary to remove or add an optical channel to an optical fiber depending on routing requirements of the communications systems and for other reasons. This function is accomplished by a filter, typically referred to as a drop/add filter or an interleaver/de-interleaver. There are different types of drop/add filters disclosed in the prior art, but the general principal of operation involves the introduction of a wavelength division multiplexed or dense wavelength division multiplexed signal into the input port of the filter. The signal includes a number of different optical channels, as noted above. In performing the “drop” function, the filter is operative to remove at least one of the optical channels from the signal and transfer it to a drop port while allowing the remainder of the signal to pass through the filter to an output port. The “add” function of the filter is realized by introducing a signal with one or more optical channels into the filter through an add port where it is combined within the filter with another signal from the input port. The combined signal is then transmitted to the output port. 
   One type of optical filtering device currently in use is an in-fiber Bragg grating placed in series with an optical circulator. A wavelength division multiplexed signal or dense wavelength division multiplexed signal is input to the circulator and the Bragg grating is effective to allow certain optical channels of the signal to pass through while one or more others are reflected back to the circulator and output through a drop port. These types of filtering systems have excellent spectral performance, but are large and have significant throughput loss. 
   Another type of filtering system is shown, for example, in U.S. Pat. No. 6,580,851 to Vahala et al which describes a four-port optical filter employing one or more tapered optical fibers coupled to a spherical resonator. A wavelength division multiplexed signal is input through one optical fiber to the spherical resonator by evanescent field coupling. The resonator is effective to drop one or more optical channels from the signal, or add an optical channel, which is then output from the resonator to the same optical fiber or a second optical fiber. Filtering systems of this type require precise alignment between the resonator and tapered optical fiber(s), and spherical resonators are difficult to manufacture. Additionally, this system has a high optical Q. The term “optical Q” refers to the quality factor of the resonator in the system, and high Q resonators exhibit a much narrower filter response as compared to low Q resonators. A broadband filter response is preferable in channel interleavers and de-interleavers for dense wavelength division multiplexing. 
   SUMMARY OF THE INVENTION 
   This invention is directed to a four-port optical filter comprising two tapered optical fibers, each having a loop, which are located relative to one another to form a ring resonator. The filter has a low Q with a broadband filter response which is useful as a channel interleaver or de-interleaver for dense wavelength division multiplexing, and other applications. 
   In one presently preferred embodiment, the tapered portion of each of two tapered optical fibers is wrapped around a cylindrical rod to form a first loop in a first optical fiber and a second loop in the second optical fiber. These loops are located relative to one another along the rod to form a ring resonator. The resonator is capable of interleaving or adding one or more optical channels to an input signal, and dropping or de-interleaving one or more optical channels from such signal, while outputting other optical channels. 
   One important advantage of the filter of this invention is that the ring resonator is formed by the optical fibers themselves, thus eliminating the difficulties of coupling tapered optical fibers to discrete spherical or other types of resonators such as taught in the prior art discussed above. Additionally, tapered optical fibers are easily integrated with conventional single mode fiber because essentially zero loss transitions may be made between the two at essentially any tapered fiber diameter. Further, evanescent field coupling may be employed with tapered optical fibers since the light propagating through the optical fiber is guided by the boundary between the taper in the optical fiber and the external environment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structure, operation and advantages of the presently preferred embodiment of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of the filter of this invention; 
       FIG. 2  is a plan view of the filter depicted in  FIG. 1 ; 
       FIG. 3  is an enlarged view of the encircled portion of  FIG. 2 ; 
       FIG. 4  is a side elevational view of the filter illustrated in  FIG. 1 ; 
       FIG. 5  is an enlarged view of the encircled portion of  FIG. 4 ; 
       FIG. 6  is a schematic representation of a drop or de-interleaver function of the filter of this invention; 
       FIG. 7  is a view similar to  FIG. 6 , except of an add or interleaver function of the filter herein; 
       FIG. 8  is a cross sectional view of the cylindrical ring and the optical fibers shown in  FIG. 1 , illustrating a meniscus disposed between the fibers; 
       FIG. 9  is a view similar to  FIG. 8  depicting the fibers fused together without the rod; 
       FIG. 10  is a graphical depiction of the transmission of optical power from the input port of the filter to its through port for several coupling coefficients; and 
       FIG. 11  is a graphical depiction of the transmission of optical power from the input port of the filter to its drop port for several coupling coefficients. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the Figures, the four-port optical filter  10  of this invention comprises a first tapered optical fiber  12 , a second tapered optical fiber  14 , and, in one embodiment, a cylindrical rod  16 . Each of the optical fibers  12  and  14  is formed of silica, and at least a portion of the rod  16  is formed of a material having an index of refraction below that of the optical fibers  12 ,  14  so that coupling between the optical fibers  12 ,  14  and rod  16  is suppressed, for purposes to become apparent below. The optical fibers  12  and  14  are characterized as being “tapered” because they were drawn while heated by a flame to form a portion along their length having a reduced, uniform waist diameter. For purposes of the present discussion, the “tapered portion”  18  of optical fiber  12  is considered to be the area of uniform waist diameter along its length as best seen in  FIG. 1 , and the tapered portion  20  is the same area along the optical fiber  14 . 
   Referring specifically to  FIGS. 1-5 , the tapered portion  18  of the first optical fiber  12  is wrapped around the rod  16  to form a first loop  22  along the rod  16 . The optical fiber  12  is folded back on itself so that its opposed ends are generally parallel to one another. One of these ends forms the input port  24  of the filter  10 , and the opposite end forms the drop port  26 , as discussed in more detail below. Similarly, the tapered portion  20  of the second optical fiber  14  is wrapped around the rod  16 , from the opposite direction to that of the first optical fiber  14 , to form a second loop  28  along the rod  16 . The optical fiber  14  is folded back on itself in the same manner as optical fiber  12 , and its opposite ends form the thru port  30  of the filter  10  and the add port  32 . The term “loop” refers to that part of the tapered portion  18  and  20  of respective optical fibers  12  and  14  which extends around the rod  16 . Each of the first and second loops  22  and  28  extend approximately 180° around the rod  16 , from opposite directions, and therefore collectively form a ring which functions as a ring resonator  34  in the filter  10  of this invention. 
   In one presently preferred embodiment of this invention, each of the optical fibers  12  and  14  are held in tension at their respective, opposed ends to maintain the position of loops  22  and  28  relative to the rod  16 . Such tensioning means (not shown) is therefore applied to both the input port  24  and drop port  26  of first optical fiber  12 , and to the thru port  30  and add port  32  of the second optical fiber  14 . Alternatively, the first and second optical fibers  12 ,  14  may be annealed in the area of loops  22  and  28  so that they permanently assume the radius of curvature of the rod  16  and need not be held in tension. In that instance, other means (not shown) are provided to maintain the loops  22  and  28  in position along the rod  16 . Regardless of whether the loops  22  and  28  are held in tension or annealed, they are maintained in close proximity to one another along the rod  16 . See  FIGS. 1 ,  4  and  5 . 
   With further reference to  FIGS. 8 and 9 , it is contemplated that a small amount of a fluid, preferably water, oil or a polymer, may be interposed between the loops  22  and  28  to form a meniscus  36  and thus increase the modal overlap between the two, as discussed below. It is also contemplated that the fluid may be applied as a liquid and subsequently cured to form a solid. In the embodiment illustrated in  FIG. 9 , the loops  22  and  28  may be fused to one another at one or more locations, as schematically shown at  37 . Where the loops  22  and  28  are fixed relative to one another, it is contemplated that the rod  16  may be eliminated and no meniscus  36  would be employed. 
   Referring now to  FIGS. 2 ,  3 ,  6  and  7 , the operation of the filter  10  is schematically depicted. A dense wavelength division multiplexed signal  38  is shown by the arrows in such Figs., which, for purposes of discussion, is assumed to include the optical wavelengths or optical channels λ 1 , λ 2  and λ 3 . The signal  38  propagates through the first optical fiber  12  where it is confined within the boundaries of such fiber  12  until reaching the tapered portion  18 . Because of the reduction in diameter of the optical fiber  12  at that location, a fraction of the optical energy of the signal  38  escapes the interior of the optical fiber  12  and travels along its boundary forming an evanescent field, as represented by the number  40  in  FIGS. 2 and 3 . The external environment at the boundary of the tapered portion  18  of first optical fiber  12  is chosen to determine the number of modes supported by the taper waist, i.e. the diameter of the tapered portion  18 . Upon reaching the resonator  34 , the optical energy of the signal  38  within the evanescent field  40  created along the first optical fiber  12  couples to the second loop  28  of the second optical fiber  14 , i.e. a fraction of the optical energy from the first optical fiber  12  is transferred to the second optical fiber  14  in the area of their respective loops  22  and  28 . These loops  22  and  28  provide a recirculating path for the optical energy around rod  16 , thus forming the resonator  34  with resonance frequencies spaced by the free spectral range according to the following relationship: 
                 F   =     C       n   eff     ⁢   L               (   1   )               
Where:
 
   F=Free spectral range 
   c=speed of light 
   n eff =effective index of the fundamental mode of the taper waist 
   L=circumference of the resonator 
   The taper waist of the optical fiber  12  is not single mode in air, but the bend radius of the loops  22  and  28  is large enough and the modal dispersion sufficiently large that coupling to higher order modes does not occur within the resonator  34 . Additionally, at least a portion of the rod  16  has an index of refraction which is below that of the optical fibers  12  and  14  to suppress coupling between the optical power circulating within the resonator  34  and the rod  16 . In particular, at least that portion of the surface of the rod  16  which is exposed to the evanescent field, including the depth of penetration of such field into the rod  16 , need have an index of refraction below that of the optical fibers  12  and  14 . 
   As noted above, the filter  10  of this invention is a four-port filter including an input port  24  and drop port  26  formed by the first optical fiber  12 , and a thru port  30  and add port  32  formed by the second optical fiber  14 .  FIG. 6  is a schematic depiction of the filter  10  performing a drop or de-interleaver function in which one of the optical channels, λ 2 , is to be removed from the remainder of the signal  38 . A fraction of the optical energy of the signal  38  is transmitted to the resonator  34  formed by the loops  22  and  28  by evanescent field coupling, as discussed above. The resonator  34  has one resonance frequency, among potentially a number of resonance frequencies, corresponding to the optical channel λ 2  of interest. The optical channel λ 2  is therefore transmitted to the drop port  26  of the first optical fiber  12  by the resonator  34  while the other optical channels λ 1  and λ 3  are allowed to pass through the resonator  34  into the thru port  30  of the second optical fiber  14 . 
   The “add” function of the filter  10  is shown in  FIG. 7 . In this mode of operation, it is assumed that a signal  42  is input through the input port  24  of the first optical fiber having optical channels λ 1  and λ 3 , and it is desired to add or interleave a third optical channel λ 2 . The signal  42  is coupled to the resonator  34 , in the same manner discussed above. Another signal  44  carrying the optical channel λ 2  is input to the resonator  34  through the add port  32  of the second optical fiber  14 , also by evanescent field coupling, as denoted by the number  41  in  FIGS. 2 and 3 . The signals  42  and  44  are added or interleaved within the resonator  34  which then outputs a signal  38  having all three optical channels λ 1 , λ 2  and λ 3 . It should be understood that the depictions shown in  FIGS. 6 and 7  are for purposes of illustration only, and the various dense wavelength division multiplexed signals may contain many more optical channels, as desired. 
   Referring now to  FIGS. 2 ,  3 ,  10  and  11 , the transmission coefficient from the input port  24  of first optical fiber  12  to the thru port  30  of the second optical fiber  14 , represented by τ 1 , is considered equal to the coupling coefficient between the contact point of the loops  22  and  28 , identified by the number  46 . Likewise, coupling occurs between the loops  22  and  28  on the opposite side of the rod  16 , as at  48 , designated by coupling coefficient τ 2  in  FIGS. 2 and 3 . The strength of the coupling is determined by the overlap of the mode fields in each tapered portion  18 ,  20  of respective optical fibers  12  and  14 , and the length of the interaction region, e.g. the length of the overlapping portions of the loops  22  and  28 . Factors influencing the magnitude of the coupling coefficient include the diameter of the tapered portions  18 ,  20 , the radius of the rod  16  and the index of refraction of the surroundings. Coupling can be enhanced by the introduction of a meniscus  36  between the loops  22  and  28 , as described above, and shown in  FIG. 8 , which increases the modal overlap. 
   The transmission of the filter  10  from the input port  24  to the through port  30  is given by the following relationship: 
                     T   thru     =             τ   1   2     ⁢     e   aL       +       τ   2   2     ⁢     e     -   aL         -     2   ⁢           ⁢     τ   1     ⁢     τ   2     ⁢     cos   ⁡     (     2   ⁢           ⁢   π   ⁢           ⁢     n   eff     ⁢     L   /   λ       )           ,         e   aL     +       τ   1   2     ⁢     τ   2   2     ⁢     e     -   aL         -     2   ⁢           ⁢     τ   1     ⁢     τ   2     ⁢     cos   ⁡     (     2   ⁢           ⁢   π   ⁢           ⁢     n   eff     ⁢     L   /   λ       )               ,           (   2   )               
The transmission of the filter from the input port  24  to the drop port  26  is given by the following relationship:
 
                     T   drop     =         (     1   -     τ   1   2       )     ⁢     (     1   -     τ   2   2       )           e   aL     +       τ   1   2     ⁢     τ   2   2     ⁢     e     -   aL         -     2   ⁢           ⁢     τ   1     ⁢     τ   2     ⁢     cos   ⁡     (     2   ⁢           ⁢   π   ⁢           ⁢     n   eff     ⁢     L   /   λ       )               ,           (   3   )               
Where:
 
   τ 1 =transmission coefficient from input port to thru port 
   τ 2 =transmission coefficient from input port to drop port 
   e −aL =round trip transmission of the resonator accounting for propagation losses and excess loss in the coupling regions 
   L=circumference of the resonator 
   n eff =effective index of the fundamental mode of the taper waist 
   λ=wavelength 
     FIGS. 10 and 11  are graphical depictions of a plot of transmission versus frequency for a broad range of coupling coefficient values, e.g. 0.3 to 0.9, neglecting round trip loss (α=0). The transmission from the input port  24  to the thru port  30  is shown in  FIG. 10 , and the transmission from the input port  24  to the drop port  26  is shown in  FIG. 11 . The diameter of the resonator  34  modeled in  FIGS. 10 and 11  is 230 um. As is evident from such Figs., a coupling coefficient of near unity is required to achieve narrow band filter features. 
   While the invention has been described with reference to a preferred embodiment, it should be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.