Abstract:
A wavelength division multiplexer includes a filter, a first lens, a second lens, a first holder, and a second holder. The pass-band of the filter is dependent on the incident angle of light incident upon the filter. The first lens is positioned on a first side of the filter. The second lens is positioned on a second side of the filter. The first holder is configured to hold at least first, second, third, and fourth fibers proximate to the first lens. The first holder is also configured to hold a center of the second and the fourth fibers on opposite sides of a line connecting a center of the first and the third fibers. The second holder is configured to hold at least fifth and sixth fibers proximate to the second lens.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to optical technology.  
           [0002]    Wavelength division multiplexers and wavelength division add-drop multiplexers are commonly used in optical communication systems and optical measurement systems. In optical communication systems, a composite signal that includes multiple individual signals each having an individual wavelength can be transmitted over a single fiber. A wavelength division multiplexer can be used to separate a composite signal into multiple individual signals each having an individual wavelength. A wavelength division add-drop multiplexer can be used to drop an individual signal having a first wavelength from a composite signal and add another individual signal having a second wavelength to that composite signal. The second wavelength can be the same as the first wavelength or different from the first wavelength. A wavelength division add-drop multiplexer can also be used to drop plural (e.g., two) individual signals and add plural (e.g., two) individual signals.  
         SUMMARY OF THE INVENTION  
         [0003]    In one aspect, the invention provides a wavelength division multiplexer. The wavelength division multiplexer includes a filter, a first lens, a second lens, a first holder, and a second holder. The filter is configured to have a pass-band thereof dependent on an incident angle of light incident thereon. The first lens is positioned on a first side of the filter. The second lens is positioned on a second side of the filter. The first holder is configured to hold at least first, second, third, and fourth fibers proximate to the first lens. The first holder is also configured to hold a center of the second and the fourth fibers on opposite sides of a line connecting a center of the first and the third fibers. The second holder is configured to hold at least fifth and sixth fibers proximate to the second lens.  
           [0004]    In another aspect, the invention provides a wavelength division add-drop multiplexer. The wavelength division add-drop multiplexer includes a filter, a first lens, a second lens, a first holder, and a second holder. The filter is configured to have a pass-band thereof dependent on an incident angle of light incident thereon. The first lens is positioned on a first side of the filter. The second lens is positioned on a second side of the filter. The first holder is configured to hold at least first, second, third, and fourth fibers proximate to the first lens. The first holder is also configured to hold a center of the second and the fourth fibers on opposite sides of a line connecting a center of the first and the third fibers. The second holder is configured to hold at least fifth, sixth, seventh, and eighth fibers proximate to the second lens. The second holder is also configured to hold a center of the sixth and eighth fibers on opposite sides of a line connecting a center of the fifth and the seventh fibers.  
           [0005]    In another aspect, the invention provides a method for manufacturing a wavelength division multiplexer. The method includes the step of configuring a filter to have a pass-band thereof depending on an incident angle of light incident thereon. The method includes the step of positioning a first lens on the first side of the filter. The method includes the step of positioning a second lens on the second side of the filter. The method includes the step of configuring a first holder to hold at least first, second, third, and fourth fibers proximate to the first lens. The method includes the step of configuring the first holder to hold a center of the second and the fourth fibers on opposite sides of a line connecting the centers of the first and the third fibers. The method includes the step of configuring a second holder to hold at least fifth and sixth fibers proximate to the second lens.  
           [0006]    In another aspect, the invention provides a method for manufacturing a wavelength division add-drop multiplexer. The method includes the step of configuring a filter to have a pass-band thereof depending on an incident angle of light incident thereon. The method includes the step of positioning a first lens on the first side of the filter. The method includes the step of positioning a second lens on the second side of the filter. The method includes the step of configuring a first holder to hold at least first, second, third, and fourth fibers proximate to the first lens. The method includes the step of configuring the first holder to hold a center of the second and the fourth fibers on opposite sides of a line connecting the centers of the first and the third fibers. The method includes the step of configuring a second holder to hold at least fifth, sixth, seventh, and eighth fibers proximate to the second lens. The method includes the step of configuring the second holder to hold a center of the sixth and eighth fibers on opposite sides of a line connecting the centers of the fifth and the seventh fibers.  
           [0007]    Advantages of the invention may include one or more of the following. Implementations of the invention provide wavelength division multiplexers and wavelength division add-drop multiplexers that can have small insertion loss, compact size, and reduced manufacturing cost. The wavelength division multiplexers and wavelength division add-drop multiplexers can add and drop plural individual signals each having an individual wavelength. The wavelength division multiplexers and wavelength division add-drop multiplexers are also designed to remove some limitations imposed on the wavelengths of the plural individual signals. Other advantages will be readily apparent from the attached figures and the description below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 a  shows an implementation of a wavelength division multiplexer.  
         [0009]    [0009]FIG. 1 b  shows the cross section A-A′ of the wavelength division multiplexer in FIG. 1 a  in the yx plane.  
         [0010]    [0010]FIG. 1 c  shows the cross section B-B′ of the wavelength division multiplexer in FIG. 1 a  in the yx plane.  
         [0011]    [0011]FIG. 1 d  shows that wavelengths λ 1  and λ 2  are related to, respectively, incident angles Φ 1  and Φ 2 .  
         [0012]    [0012]FIG. 1 e  shows that the difference between distances d1 and d2 is larger than a certain minimal distance in the implementation of the wavelength division multiplexer in FIG. 1 a.    
         [0013]    [0013]FIGS. 2 a  and  2   b  show an implementation of a wavelength division multiplexer, respectively, in the yz plane and the xz plane of a coordinate system.  
         [0014]    [0014]FIG. 2 c  shows the cross section A-A′ of the wavelength division multiplexer in FIGS. 2 a  and  2   b  in the yx plane.  
         [0015]    [0015]FIG. 2 d  shows the cross section B-B′ of the wavelength division multiplexer in FIGS. 2 a  and  2   b  in the yx plane.  
         [0016]    [0016]FIGS. 3 a  and  3   b  show an implementation of a wavelength division add-drop multiplexer, respectively, in the yz plane and the xz plane of a coordinate system.  
         [0017]    [0017]FIG. 3 c  shows the cross section A-A′ of the wavelength division add-drop multiplexer in FIGS. 3 a  and  3   b  in the yx plane.  
         [0018]    [0018]FIG. 3 d  shows the cross section B-B′ of the wavelength division add-drop multiplexer in FIGS. 3 a  and  3   b  in the yx plane.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The present invention relates to an improvement in optical technology. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the invention will be readily apparent to those skilled in the art and the generic principals herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principals and features described herein.  
         [0020]    The present invention will be described in terms of wavelength division multiplexers and wavelength division add-drop multiplexers each having specific components having specific configurations. Similarly, the present invention will be described in terms of components having specific relationships, such as distances or angles between components. However, one of ordinary skill in the art will readily recognize that the devices and systems described can include other components having similar properties, other configurations, and other relationships between components.  
         [0021]    [0021]FIG. 1 a  illustrates an implementation of a wavelength division multiplexer  200 . The detailed description of this implementation of wavelength division multiplexers can be found in U.S. Pat. No. 6,084,994, titled “Tunable, Low Back-reflection Wavelength Division Multiplexer”, which is incorporated herein by reference in its entirety.  
         [0022]    As shown in FIG. 1 a , wavelength division multiplexer  200  includes a first lens  220 , a filter  230 , and a second lens  240 . Filter  230  is a device designed in such a way such that the pass-band of filter  230  depends on the incident angle of light incident upon filter  230 . Four fibers  202 ,  204 ,  256  and  258  are positioned at one side of first lens  220 . First lens  220  is configured and positioned to collimate optical signals exiting from the end of fibers  202  and  204 . First lens  220  is also configured and positioned to focus optical signals to enter the end of fibers  256  and  258 . Two fibers  252  and  254  are positioned at a distal end of second lens  240 . Second lens  240  is configured and positioned to focus optical signals to enter the end of fibers  252  and  254 . Fibers  202 ,  204 ,  256 , and  258  can be fixed in position by a first holder  210 . Fibers  252  and  254  can be fixed in position by a second holder  250 . First holder  210  and second holder  250  each can be a capillary.  
         [0023]    As shown in FIG. 1 a , fibers  204  and  256  are separated by distance d1, and fibers  202  and  258  by distance d2. In FIG. 1 a , wavelength division multiplexer  200  is shown in the yz plane of a coordinate system. The cross sections A-A′ and B-B′ of wavelength division multiplexer  200  in FIG. 1 a  is shown, respectively, in FIG. 1 b  and FIG. 1 c , in the yx plane. FIG. 1 b  shows that the center of fibers  204  and  256  are separated by distance d1, and the center of fibers  202  and  258  by distance d2.  
         [0024]    In FIG.1 a , light signal  262  exiting from fiber  204  is collimated by first lens  220  and is incident upon filter  230  with an incident angle Φ 2 . Light signal  262  in general is a composite signal that includes multiple individual signals each having an individual wavelength. The individual signal with wavelength λ 1  is transmitted through filter  230  as light signal  264 , and the individual signals with wavelengths other than λ 1  are reflected by filter  230  as light signal  266 . Light signal  264  is focused by second lens  240  and enters fiber  252 . Light signal  266  is focused by first lens  220  and enters fiber  256 .  
         [0025]    In FIG. 1 a , light signal  272  exiting from fiber  202  is collimated by first lens  220  and is incident upon filter  230  with an incident angle Φ 2 . Light signal  272  in general is a composite signal that includes multiple individual signals each having an individual wavelength. The individual signal with wavelength λ 2  is transmitted through filter  230  as light signal  274 , and the individual signals with wavelengths other than λ 2  are reflected by filter  230  as light signal  276 . Light signal  274  is focused by second lens  240  and enters fiber  254 . Light signal  276  is focused by first lens  220  and enters fiber  258 .  
         [0026]    As described above, wavelength division multiplexer  200  can be designed to separate a first individual signal with wavelength λ 1  and a second individual signal with wavelength λ 2  into two fibers. As shown in FIG. 1 d , wavelengths λ 1  and λ 2  are related to, respectively, incident angles Φ 1  and Φ 2 . Incident angles Φ 1  and Φ 2  are further related to, respectively, distances d1 and d2. If the difference between distances d1 and d2 is limited to be larger than a certain minimal distance, the difference between wavelengths λ 1  and λ 2  in general will also be limited to be larger than a certain minimal wavelength-difference.  
         [0027]    [0027]FIG. 1 e  illustrates that, in some implementations of wavelength division multiplexer  200 , the difference between distances d1 and d2 is larger than a certain minimal distance. More specifically, fibers  202 ,  204 ,  256 , and  258  are generally in the form of fiber optic cables. A fiber optic cable can include a core, cladding, coating, strengthening fibers, and cable jacket. The core is for transmitting light. In general, the thickness of a fiber optic cable can be significantly larger than the thickness of the core in the fiber optic cable. FIG. 1 e  illustrates that, in the implementation of wavelength division multiplexer  200  in FIG. 1 a , the centers of fibers  202 ,  204 ,  256 , and  258  are approximately collinear. If fibers  202 ,  204 ,  256 , and  258  are generally in the form of fiber optic cables each has an outer diameter D, then, the minimal distance between the centers of fibers  204  and  256  is D, and the minimal distance between the centers of fibers  202  and  258  is 3D. That is, distance d1 has a minimal value of D, distance d2 has a minimal value of 3D. If the outer diameter D=125 μm, then, distance d1 has a minimal value d1=125 μm, and distance d2 has a minimal value d2=375 μm. Since the difference between distances d1 and d2 is limited to be larger than 2D, the difference between wavelengths λ 1  and λ 2  is limited to be larger than a certain minimal wavelength-difference.  
         [0028]    In certain applications, a preferred implementation of wavelength division multiplexer should not have minimal limitations imposed on the difference between wavelengths λ 1  and λ 2 . In the implementations of wavelength division multiplexer to be described below, no minimal limitations are imposed on the difference between wavelengths λ 1 , and λ 2 . In fact, in some of these implementations, the difference between wavelengths λ 1  and λ 2  can be zero.  
         [0029]    [0029]FIGS. 2 a  and  2   b  illustrate an implementation of a wavelength division multiplexer  500 , respectively, in the yz plane and the xz plane of a coordinate system. Wavelength division multiplexer  500  includes a first lens  220 , a filter  230 , and a second lens  240 . Filter  230  is a device designed in such a way that the pass-band of filter  230  depends on the incident angle of light incident thereon. Four fibers  202 ,  204 ,  256  and  258  are positioned at one side of first lens  220 . First lens  220  is configured and positioned to collimate optical signals exiting from the end of fibers  202  and  204 . First lens  220  is also configured and positioned to focus optical signals to enter the end of fibers  256  and  258 . Two fibers  252  and  254  are positioned at a distal end of second lens  240 . Second lens  240  is configured and positioned to focus optical signals to enter the end of fibers  252  and  254 . Fibers  202 ,  204 ,  256 , and  258  can be fixed in position by a first holder  210 . Fibers  252  and  254  can be fixed in position by a second holder  250 . First holder  210  and second holder  250  each can be a capillary.  
         [0030]    In FIG. 2 a , wavelength division multiplexer  500  is shown in the yz plane, and fibers  202  and  258  are separated by distance d2. In FIG. 2 b , wavelength division multiplexer  500  is shown in the xz plane, and fibers  204  and  256  are separated by distance d1. The cross section A-A′ of wavelength division multiplexer  500  in FIGS. 2 a  and  2   b  is shown in FIG. 2 c  in the yx plane. The cross section B-B′ of wavelength division multiplexer  500  in FIGS. 2 a  and  2   b  is shown in FIG. 2 d  in the yx plane. FIG. 2 c  shows that the center of fibers  204  and  256  are separated by distance d1, and the center of fibers  202  and  258  by distance d2. The center of fibers  204  and  256  are positioned on opposite sides of a line  101  connecting the center of fibers  202  and  258 .  
         [0031]    In FIGS. 2 a  and  2   b , light signal  272  exiting from fiber  202  is collimated by first lens  220  and is incident upon filter  230  with incident angle Φ 2 . Light signal  272  in general is a composite signal that includes multiple individual signals each having an individual wavelength. The individual signal with wavelength λ 2  is transmitted through filter  230  as light signal  274 , and the individual signals with wavelengths other than λ 2  are reflected by filter  230  as light signal  276 . Light signal  274  is focused by second lens  240  and enters fiber  254 . Light signal  276  is focused by first lens  220  and enters fiber  258 .  
         [0032]    In FIGS. 2 a  and  2   b , light signal  262  exiting from fiber  204  is collimated by first lens  220  and is incident upon filter  230  with incident angle Φ 1 . Light signal  262  in general is a composite signal that includes multiple individual signals each having an individual wavelength. The individual signal with wavelength λ 1  is transmitted through filter  230  as light signal  264 , and the individual signals with wavelengths other than λ 1  are reflected by filter  230  as light signal  266 . Light signal  264  is focused by second lens  240  and enters fiber  252 . Light signal  266  is focused by first lens  220  and enters fiber  256 .  
         [0033]    As described above, wavelength division multiplexer  500  can be designed to separate a first individual signal with wavelength λ 1  and a second individual signal with wavelength λ 2  into two fibers. The implementation of wavelength division multiplexer  500  in FIGS. 2 a  and  2   b  includes at least the advantage that the difference between distances d1 and d2 can be as small as zero. Consequently, the difference between wavelengths λ 1  and λ 2  can also be as small as zero.  
         [0034]    [0034]FIGS. 3 a - 3   d  illustrate that wavelength division multiplexer  500  in FIGS. 2 a - 2   d  can be modified to become a wavelength division add-drop multiplexer  600 . Wavelength division multiplexer  500  is modified by modifying second holder  250  in such a way that two additional fibers  251  and  253  can be fixed and positioned at the distal end of second lens  240 . Fibers  251  and  253  are positioned in such a way that light signals exiting from fibers  251  and  253  are collimated by second lens  240 .  
         [0035]    The cross section A-A′ of wavelength division add-drop multiplexer  600  in FIGS. 3 a  and  3   b  is shown in the yx plane in FIG. 3 c . The cross section B-B′ of wavelength division add-drop multiplexer  600  in FIGS. 3 a  and  3   b  is shown in the yx plane in FIG. 3 d . As shown in FIG. 3 d , the centers of fibers  251  and  252  are positioned on opposite sides of a line  102  connecting the center of fibers  253  and  254 .  
         [0036]    As shown in FIGS. 3 a - 3   b , light signal  273  with wavelength λ 2 ′ exiting from fibers  253  is collimated by second lens  240  and incident upon filter  230 . Light signal  273  with wavelength λ 2 ′ pass though filter  230  and is added to light signal  276 . Light signal  276  is focused by first lens  220  and enters fiber  258 .  
         [0037]    As shown in FIGS. 3 a - 3   b , light signal  263  with wavelength λ 1 ′ exiting from fibers  251  is collimated by second lens  240  and incident upon filter  230 . Light signal  263  with wavelength λ 1 ′ pass though filter  230  and is added to light signal  266 . Light signal  266  is focused by first lens  220  and enters fiber  256 .  
         [0038]    The implementation of wavelength division multiplexer  500  includes at least the advantage that the difference between distances d1 and d2 can be as small as zero. Consequently, the difference between wavelengths S 1  and S 2  can also be as small as zero.  
         [0039]    In FIGS. 2 a  and  2   b , wavelength division multiplexer  500  can be designed to separate a first individual signal with wavelength λ 1  and a second individual signal with wavelength λ 2  into two fibers. If the pass-band of filter  230  covers the wavelengths of a plurality of individual signals, wavelength division multiplexer  500  can be designed to separate a first group of individual signals and a second group of individual signals into two fibers. Similarly, if the pass-band of filter  230  covers the wavelengths of a plurality of individual signals, wavelength division add-drop multiplexer  600  in FIGS. 3 a  and  3   b  can be designed to add groups of individual signals and drop groups of individual signals.  
         [0040]    A method and system has been disclosed for providing wavelength division multiplexers and wavelength division add-drop multiplexers. Although the present invention has been described in accordance with the implementations shown, one of ordinary skill in the art will readily recognize that there could be variations to the implementations and those variations would be within the spirit and scope of the present invention. For example, an optical isolator can be placed between filter  230  and second lens  240 , for wavelength division multiplexer  500  in FIGS. 2 a - 2   d  or for wavelength division add-drop multiplexer  600  in FIGS. 3 a - 3   d . Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.