Abstract:
A miniature dense wavelength division multiplexer (DWDM) is disclosed. A plurality of multi-window wavelength multiplexers (MWDM) are cascaded and optically coupled to form a tree, and each of the MWMDMs forming the tree comprises a microbend coupler. The forming of the MWDM tree is characterized by the absence of the bending of optical fibers external to the microbend couplers. A finished DWDM assembly measures less than 50 mm in width.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to wavelength selective devices. In particular, the present invention relates to dense wavelength division multiplexers (DWDMs). 
     2. The Prior Art 
     Background 
     With the growing use of the Internet, users are accessing a wider variety of data, such as streaming voice and video, and as a result are placing greater demands on the existing Internet backbone. As a consequence, traditional coaxial cable, which forms the backbone of the Internet, can no longer support these increased demands. Thus, current information systems are continually being expanded to meet increasing bandwidth demands. 
     One viable alternative to the traditional coaxial backbone is optical fiber because of its potential for greatly increased bandwidth. Various methods have been proposed to maximize the bandwidth of optical systems. 
     One such system is disclosed in U.S. Pat. No. 5,809,190 (the &#39;190 patent) to the present inventor. Therein, a Dense Wavelength-Division Multiplexer (DWDM) is disclosed which utilizes a Fused-Biconical Taper (FBT) technique. 
     FIG. 1 shows a prior art diagram of a 1×N DWDM  100  according to the &#39;190 patent. As used herein, the symbol N indicates the number of channels that are used by a DWDM to multiplex or demultiplex a given input provided by an input fiber. The number N is equal to 2 m  wherein m represents the number of times a DWDM performs signal divisions for the given input signal prior to their being demultiplexed at a receiving end. 
     Accordingly, the prior art DWDM is known as a m-stage DWDM in which MWDM  111  is a first stage Wavelength Division Multiplexer (WDM) having a window spacing of Δλ. Likewise, MWDMs  121  and  122  are a pair of second stage WDMs, each having a window spacing 2Δλ. MWDMs  131 ,  133 , and  134  are a plurality of third stage WDMs, each having a window spacing of 4Δλ. 
     Each of the WDMs in FIG. 1 has a window with a center wavelength which varies with its sequence in the DWDM. Each stage in the DWDM  100  may be designated as  1 m 1 ,  1 m 2 , . . . , and  1 m(2 m−1 ), representing a m-th stage WDM of the DWDM. Regarding window spacing, the window spacing of a m-th stage MWDM is 2 m−1  Δλ, which is twice as large as a window spacing demonstrated by a m−1 stage MWDM, yet one half of the size of the window spacing of a m+1 stage MWDM. The number of stages m may be from be from 1 to n, where n=(logN/log2), forming a plurality of MWDMs,  1 n 1 ,  1 n 2 , . . . ,  1 n(N/2). 
     Each channel of the DWDM  100  has only one window with a characteristic central wavelength corresponding to a particular center wavelength originating from the first stage WDMs. For example, in FIG. 1, each of the windows included in channel pathways  111 - 131  and  111 - 132  has a center wavelength identical to a center wavelength in corresponding window of the channel  121 . Likewise, each of the windows in the channel pathways  111 - 133  and  111 - 134  has a center wavelength identical to a center wavelength in a corresponding window of the channel  122 . 
     Referring still to FIG. 1, the operation of the DWDM  100  as a demultiplexer may now be shown. A lightwave signal having wavelengths λ 1 -λ N  are provided by fiber  10  to MWDM  111 . Wavelength series λ 1 , λ 3 , . . . , λ N−1  is transmitted to WDM  121 , and wavelength series λ 2 , λ 4 , . . . , λ N  is transmitted to WDM  122 . FIGS. 2A and 2B show representative spectral distributions of the wavelength series where N=8. 
     Referring back to FIG. 1, after demultiplexing by subsequent stages, the light signals are demultiplexed into N individual channels and distributed to N individual fibers  11 ,  12 , . . . ,  1 N. 
     Referring now to FIGS. 3A-3E, detailed embodiments of the DWDM of the &#39;190 patent are shown. FIG. 3A is a logic diagram of a 1×4 DWDM according to the &#39;190 patent, also known as a 4-channel DWDM. The first stage MWDM  311  is cascadedly connected to two second stage MWDMs  321  and  322 . For demultiplexing purposes, a lightwave input having wavelengths λ 1 -λ 4  are input on fiber  30 , and outputs λ 1 , λ 2  , λ 3  , and λ 4  are provided on fibers  31 ,  32 ,  33 , and  34 , respectively. For multiplexing purposes, the inputs and outputs are reversed. FIGS. 3C,  3 D, and  3 E show the respective insertion loss of the MWDMs  311 ,  321 , and  322  wherein Δλ is the window spacing and δλ is the window bandwidth. The dash curve and the solid curve in FIG. 3C indicates respectively the insertion loss in channels  30 - 321  and  30 - 322 . The dash curve and the solid curve in FIG. 3D indicates respectively the insertion loss in channels  34 - 311  and  34 - 311 . The dash curve and the solid curve in FIG. 3E indicates respectively the insertion loss in channels  33 - 311  and  33 - 311 . 
     FIG. 3B shows an actual physical structure of the &#39;190 DWDM according to the &#39;190 patent. The first stage MWDM  311  is cascadedly connected to two second stage MWDMs  321  and  322 , and the DWDM of the &#39;190 patent in housed in a container  35  having a length L and a width W. 
     As is appreciated by those of ordinary skill in the art, the length and width of container  35  is dictated by the radius R about which the optical fibers of the DWDM of FIG. 3B may be bent. As a consequence, the DWDM of the &#39;190 patent suffers from certain disadvantages. While satisfactory for the purposes intended in terms of performance, the DWDM of the &#39;190 patent suffers from size disadvantages. Due to the fused-biconical technique used in the DWMs of the &#39;190 patent, the minimum radius about which fibers can be bent is approximately 35 mm. Thus, the minimum finished size of a DWDM according to the&#39;190 patent has a length L of approximately 100 mm and a width W of approximately 50 mm. 
     Given the need to upgrade communications system as discussed above, there is an apparent need to fabricate a DWDM which is smaller in size than DWDMs of the prior art. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention satisfies the above needs. The present invention relates to wavelength selective devices. In particular, the present invention relates to dense wavelength division multiplexers (DWDMs). 
     A miniature dense wavelength division multiplexer (DWDM) is disclosed. 
     In a first aspect of the present invention, a plurality of multi-window wavelength multiplexers (MWDMs) are cascaded and optically coupled to form a tree, and each of the MWDMs forming the tree comprises a microbend coupler. 
     In a second aspect of the present invention, the forming of the MWDM tree is characterized by the absence of the bending of optical fibers external to said microbend couplers. 
     A method for forming a DWDM is disclosed, which comprises providing a plurality of multi-window wavelength multiplexers (MWDMs) cascaded and optically coupled to form a tree, wherein each of the MWDMs of the tree comprises a microbend coupler. 
     Additional aspects of the present invention are disclosed wherein the DWDM formed by the present invention measures approximately 100 mm×50 mm, and as little as 50 mm×20 mm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a prior art diagram of a 1×N DWDM. 
     FIG. 2 is a spectral distributions of a prior art DWDM. 
     FIGS. 3A-3B are detailed embodiments of a prior art DWDM. 
     FIGS. 3C-3E are insertion loss plots of prior art MWDMs. 
     FIG. 4 is a diagram of a microbend coupler suitable for use with the present invention. 
     FIG. 5 is a miniature DWDM according to the present invention. 
     FIGS. 6A-6C are insertion loss plots representative of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Persons of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. 
     Fabrication 
     As discussed in the prior art section above, one limiting factor in the fabrication of DWDMs is the radius about which optical fibers may be bent. The DWDM of the present invention sidesteps the radius constraint suffered by the prior art devices by utilizing a different type of coupling technique, known as a microbend fused fiber technique. A Microbend Fused Fiber Coupler Method and Apparatus is disclosed in U.S. patent application Ser. No. 09/471,583 to the present inventor and assigned to a common assignor (“the microbend application”), and is incorporated herein by reference. 
     FIG. 4 is a cross-sectional diagram of a microbend coupler suitable for use in the present invention. The details of a method and apparatus for forming a microbend coupler are well documented in the microbend application and will not be detailed herein at the risk of obscuring the present invention. 
     From a general perspective, the microbend coupler  400  includes pigtail ends  404  and  406  and non-tapered  401 . Opposite ends  404  and  406 , the Y-juncture  405  tapers from the non-coupled taper region  401  into the coupled region  414 . Plastic coating  407  enshrouds the fibers from the ends  404 ,  406 ,  410 , and  412  up to , but not including tapered regions  410  and  403 . Fiber core  409  is of a standard dimension C, preferably 9 micrometers in diameter, and 125 micrometers with cladding. Likewise, the fibers with plastic coating are of a standard dimension D, preferably 250 micrometers in diameter. 
     As can be seen by inspection of FIG. 4, tapered regions  401  and  403  terminate in a radius  416  through coupling region  414 . It is this radius which is of interest to the present invention. As discussed in the microbend application, the radius  416  is preferably less than 20 mm, more preferably less than 10 mm, and most preferably in a range less than 5 mm. The actual radius will vary depending on the target application wavelength range, and the materials and processes used to fabricate the microbend coupler. This small radius is a great advantage to the present invention as will be shown below. 
     FIG. 5 is a diagram of a 1×N DWDM utilizing a microbend tree according to the present invention. In a preferred embodiment of the present invention, N=4, though it is contemplated that 1×16 DWDMs may be fabricated using the present invention. The manufacture of DWDMs of other configurations will be made possible by those of ordinary skill having been informed by this disclosure. Thus, the embodiments described herein are illustrative only and should not be used to limit the scope of the present invention. 
     FIG. 5 shows a miniature microbend tree  500  according to the present invention. Microbend tree  500  may be formed on a substrate  502 . In an exemplary non-limiting embodiment of the present invention, substrate  502  may be formed from an environmentally stable material such as metal, ceramic, or glass, and has a length L and a width W. 
     Miniature MWDM tree  500  has a common terminal  504 , which is optically coupled to a fiber of MWDM  5 - 11 . In a preferred embodiment, MWDM  5 - 11  comprises a microbend coupler, and includes a pigtail output pair  501  which is optically coupled to MWDM  5 - 11 . As can be seen by inspection of FIG. 5, when MWDM  5 - 11  is coupled to MWDM  5 - 11 , one leg of a pigtail pair may be left unused as is standard in the art. MDWM  5 - 11  may be affixed to substrate  502  through means standard in the art, such as epoxy. 
     MWDM  5 - 11  is optically coupled to MWDM  5 - 21  and MWDM  5 - 22  through fibers  506  and  508 , respectively, which form the output pigtail pairs of MWDM  5 - 11 . In a preferred embodiment of the present invention, MWDMs  5 - 21  and  5 - 22  each comprise microbend couplers. 
     MWDM  5 - 21  has an output pigtail pair comprising fibers  510  and  512 . Likewise, MWDM  5 - 22  has an output pigtail pair comprising fibers  514  and  516 . In an exemplary non-limiting embodiment of the present invention, fibers  510 ,  512 ,  514 , and  516  are disposed along an edge of substrate  502  opposite from common terminal  504 , and the fibers are configured with optical connectors standard in the art. 
     In a presently preferred embodiment of the present invention, the miniature MWDM tree  500  may be encapsulated in a hermetically-sealed container for use. 
     As mentioned in the prior art section above, there is a need for smaller DWDMs. In a presently preferred embodiment of the present invention, a DWDM utilizing a miniature MWDM tree according to the present invention has a length of approximately 100 mm and a width of approximately 50 mm. In a further preferred embodiment, the present invention has a length of approximately 50 mm and a width of approximately 20 mm. As will be appreciated by those skilled in the art, this is a significant size savings over devices of the prior art. For example, the size of the present invention may be as small as approximately one-fifth the size of the prior art device described above. 
     Furthermore, as can bee seen by inspection of FIG. 5, the fabrication of a MWDM tree according to the present invention does not require the bending of any optical fibers outside of the individual MWDMs themselves. In other words, all optical fiber bends are accomplished within a microbend coupler in the present invention. No optical fibers external to the microbend couplers need be bent. This feature is an important advantage of the present invention, and facilitates the miniature size of the present invention. 
     Operation 
     In operation, a lightwave signal having wavelengths λ 1 -λ 4  may be applied to common terminal  504  to accomplish a demultiplexing operation. The acts described herein may be reversed for a multiplexing operation. 
     FIG. 6A shows a plot of the transmission characteristics of one channel of MWDM  5 - 11 . As will be appreciated by those of ordinary skill in the art, the result of the demultiplexing process through MWDM  5 - 11  is that wavelengths λ 1  and λ 3  will be transmitted to MWDM  5 - 21  and wavelengths λ 2  and λ 4  will be transmitted to MWDM  5 - 22 . 
     FIG. 6B shows a plot of the representative transmission characteristics of one channel of a second stage MWDM, such as MWDM  5 - 21  and  5 - 22 . As will be appreciated by those of ordinary skill in the art, the result of the demultiplexing process through MWDM  5 - 21  is that wavelengths λ 1  and λ 3  will be transmitted to fibers  510  and  512 . Likewise, as will be appreciated by those of ordinary skill in the art, the result of the demultiplexing process through MWDM  5 - 22  is that wavelengths λ 2  and λ 4  will be transmitted to fibers  514  and  516 . 
     FIG. 6C shows a plot representing the response from the common port  504  to one of the other output fibers. As can be seen by inspection of FIG. 6C, only one wavelength will be transmitted from the common port to a particular output fiber. 
     While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.