Abstract:
An optical device has a housing for receiving a plurality of optical fibers adapted to carry optical signals. A filter is disposed within the housing for transmitting specific optical signals having a predetermined wavelength range. A first ball lens is coupled to the housing and is positioned relative to the filter and the optical fibers to selectively collimate and focus the optical signals. A second ball lens is coupled to the housing and is also positioned relative to the filter and optical fibers to selectively collimate and focus the optical signals. Both ball lenses are optically coupled to the filter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to and the benefit of U.S. Provisional Patent Application No. 60/391,308, filed on Jun. 24, 2002, entitled Add/Drop Module Using Two Full-Ball Lenses, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. The Field of the Invention 
   The present invention relates to optical devices usable within an optical network. More particularly, the present invention relates to using full-ball lenses in an add/drop module of an optical device. 
   2. Background of the Invention 
   Optical add/drop modules are commonly used in existing optical communications networks. These add/drop modules may include add/drop multiplexers (OADM), add/drop de-multiplexers (OADDM), and other types of add/drop optical components. A multiplexer enables multiple carrier waves to be carried on a single transmission medium by combining the multiple carrier waves into a single carrier wave that propagates along the optical fiber. 
   The multiple carrier waves are at different frequencies and separated by some predetermined amount of frequency separation. This frequency separation is known as the channel spacing. Combining the multiple signals into a single carrier signal reduces the number of fiber cables that must be laid in order to transmit a required amount of data. Depending on the type of cable and the number of individual channels combined into the cable, a significant increase in overall data transmission rates may be achieved using OADMs. 
   At a location remote from the OADM is an optical add/drop de-multiplexer (OADDM) that separates the different wavelengths from the multiplexed carrier signals and transmits each of these wavelengths to their own individual receivers. Alternately, the OADDM might drop one or more single channels from the propagating signal. The data on these channels is received by a receiver specifically tuned to the frequency of that channel and which performs the optical to electrical conversions accordingly. The remaining channels are forwarded to the next node without being routed through a receiver. The use of OADMs and OADDMs in optical networks has increased significantly over the last few years, particularly as the channel spacing has been reduced from about 200 GHz for Dense Wavelength Division Multiplexing (DWDM) systems to about 25 GHz for Ultra-Dense Wavelength Division Multiplexing (UDWDM) systems. 
   Typically, one or more graded or gradient index (GRIN) lenses are used in optical communications equipment and imaging systems, such as optical add/drop multiplexers and de-multiplexers. Rods or optical fibers, and even a relatively flat piece of optical material, may function as a GRIN lens. Unfortunately, relatively precise and sometimes tedious alignment procedures are often required during fabrication and assembly of optical communications equipment using GRIN lenses. Also, the cost of optical communications equipment having one or more GRIN lenses may be high due to the requirement for the use of high precision mechanical parts to maintain desired optical alignment. 
   In addition to the cost aspects of using GRIN lenses, issues arise from the large beam diameter associated with the GRIN lens. Specifically, currently available GRIN lenses that have a relatively large beam diameter may conflict with aperture requirements and stability requirements of associated optical mechanical components. 
   In an attempt to alleviate the problems associated with GRIN lenses, optical add/drop modules have previously been manufactured using two half-ball lenses with a beam splitting filter disposed between the half-ball lenses. Unfortunately, this approach is as problematic as using a GRIN lens. For instance, cutting a full-ball lens and polishing the resulting half ball lenses can be expensive. Additionally, aligning the half ball lenses within an optical component can be both expensive and time consuming due to the precise alignment procedures required for packaging discrete optical components. 
   SUMMARY OF THE INVENTION 
   In accordance with teachings of the present invention, a pair of full-ball lenses and a filter may be used to manufacture an optical add/drop module and other optical communications devices and imaging systems. In this configuration, the present invention overcomes the limitations and problems described with respect to the use of GRIN lenses and half-ball lenses. 
   The present invention reduces the requirements for special-polishing of fiber interfaces in an optical module or other optical communications device through use of a pair of full-ball lenses and associated filter. Further, use of a pair of full-ball lenses reduces optical aberrations associated with GRIN lenses and half-ball lenses. Additionally, mechanical components associated with positioning and alignment of a pair of full-ball lenses formed in accordance with teachings of the present invention generally are less complicated and expensive than existing mechanical components associated with current optical communications devices. 
   An optical module formed in accordance with the teachings of the present invention may satisfy standard back reflection requirements for optical network components without adding costs to the associated system. An optical module formed with a pair of full-ball lenses in accordance with teachings of the present invention may include a relatively small beam diameter through the lenses, which is compatible with the aperture and numerical aperture requirements of other components. 
   Another aspect of the present invention further includes a significant reduction a of geometrical spot size and insertion loss. The present invention allows simplification of optical mechanical design and packaging of a resulting optical communications device. For some applications, an optical module may be produced using a pair of full-ball lenses in accordance with teachings of the present invention at a substantially reduced cost as compared to using GRIN lenses or two half ball lenses to produce an optical module with the same optical communications capabilities. For some applications, the cost of obtaining a ball lens which has been fully coated with antireflective material may be substantially less than the cost of a GRIN lens with the same optical characteristics. 
   These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete and thorough understanding of the present invention and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
       FIG. 1  is a schematic drawing showing an optical add/drop module according to one aspect of the present invention; 
       FIG. 2  is a schematic drawing showing the OADM of  FIG. 1  communicating a selected wavelength optical signal from the input port to a drop port; 
       FIG. 3  is a schematic drawing showing the OADM of  FIG. 1  communicating a selected wavelength optical signal from an add port to the output port; 
       FIG. 4  is a schematic drawing of an OADDM formed in accordance with another aspect of the present invention; 
       FIG. 5  is a sectional drawing of an OADM having a pair of full-ball lenses and a filter according to an alternate aspect of the present invention; 
       FIG. 6  is a cross sectional view of the OADM of  FIG. 5  along lines  6 — 6  of  FIG. 5 ; 
       FIG. 7  is an enlarged view a portion of  FIG. 5 ; 
       FIG. 8  is a sectional drawing of an OADM having a pair of full-ball lenses and a filter according to another alternate aspect of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made to  FIGS. 1–8  to describe exemplary embodiments and configurations of an add/drop module in accordance with the invention. It is to be understood that the drawings are diagrammatic and schematic representations of exemplary embodiments, and are not limiting of the present invention, nor are they necessarily drawn to scale. 
   The add/drop module generally includes a pair of ball lenses to direct the electromagnetic radiation propagating through the add/drop module. By using a pair of ball lenses, the present invention reduces the requirements for special-polishing of fiber interfaces in an optical module or other optical communications device, reduces optical aberrations associated with the optical module, and reduces geometrical spot size and insertion loss while reducing the complexity of the mechanical components used to position and align a pair of half ball lenses. As used herein, the term “optical signal” includes the full range of all electromagnetic waves which may be satisfactorily used to communicate information through waveguides and/or fiber optic cables. 
     FIGS. 1 ,  2  and  3  schematically depict one exemplary embodiment of an optical add/drop multiplexer (OADM)  20  of the present invention. With reference to  FIG. 1 , OADM  20  includes a first full-ball lens  30  separated from a second full-ball lens  32  by a thin-film filter  34 . The combination of ball lenses  30  and  32  with filter  34  may be termed an optical assembly. This optical assembly is disposed between an input port  22 , an output port  24 , an add port  26 , and a drop port  28 . Various types of optical a fibers and/or waveguides may be satisfactorily used as input port  22 , output port  24 , add port  26 , and drop port  28 . In one aspect of the present invention, standard single mode fibers such as Owens Corning SMF- 28  may be used to provide ports  22 ,  24 ,  26  and  28 . Although reference is made to a specific optical fiber, one skilled in the art will understand that any optical fiber or other waveguide may be used to function as a port in accordance with the teaching of the present invention. 
   Each full-ball lens  30  and  32  has a generally spherical configuration. Full-ball lenses are easier to handle than half ball lenses that are currently used in WDMs or WDDMs. The use of a full-ball lens overcomes many problems with existing optical systems that use a half-ball lens. Unlike a half ball lens, a full-ball lens does not have a planar surface. Therefore, a full-ball lens may be placed in any orientation with respect to an associated planar filter, or more appropriately, orientation is isotropic to ball lenses of a full-ball system formed in accordance with teachings of the present invention. A pair of full-ball lenses eliminates four angular tolerances without introducing other tolerances. For instance, the two surface tilt tolerances for each half-ball&#39;s planar surface are eliminated. Hence, the present invention saves the cost of making two high precision parts and reduces labor costs of manual alignment of the lenses. 
   The full-ball lenses  30  and  32  may collimate optical signals incident thereupon. For instance, lens  30  collimates optical signals received from port  22  and directs the optical signals on a central portion of optical filter  34 . The first ball lens  30  also focuses optical signals reflected from or passing through optical filter  34  into output port  24 , as shown in  FIG. 3 . Similarly, second ball lens  32  focuses optical signals passing through optical filter  34  into drop port  28 , while collimating optical signals exiting from add port  26  and directing the optical signals from add port  26  through the central portion of optical filter  34 . 
   Using full-ball lenses  30  and  32  aids with reducing the overall length needed for the optical assembly. Specifically, the paraxial object focal length of a full-ball lens is considerably smaller than that of a half-ball lens. Focal length (f) is generally defined as the distance from an on-axis object to the first spherical surface. The formulae for two cases are given below, the first for a half ball lens and the second for a full-ball lens.
 
 f =(1/( n− 1)) R   (1)
 
 f =[(2− n )/2]*[1/( n− 1)] R   (2)
 
where n is the index of refraction of the ball material and R is the radius. For BK7 glass (n=1.52), for instance, it gives a reduction of focal length by 4.1, for SF2 glass (n=1.65), a reduction by 5.7, regardless of the value of R.
 
   In addition to reducing the focal length of the lenses over a half-ball lens configuration, a reduction in the beam size on the lens surface is also achieved. Because the beam size on the lens surface is linearly dependent on the focal lengths for the same object numerical aperture (NA), there is a resultant reduction in the beam size on the first spherical surface of the full-ball lens. Since most problematic aberrations, such as third aberrations that are dominant in the half-ball or full-ball lenses, are proportional to the square of the beam size, using the configuration of the present invention results in a large reduction in the aberration. 
   Typical spot or beam sizes are around 0.87 μm for a point source with NA=0.14 and losses of less than 1.4 dB as predicted by computer simulation for an SF2 full-ball lens with a diameter of 3.4 mm. It will be understood that the beam sizes, NA, losses, and diameter of the full-lenses may be varied as desired depending upon the configuration of the present invention. Specific dimensions of the device will depend on the types of materials used and the wavelength of light for which the device is designed. Generally, it is understood that use of a pair of full-ball lenses instead of GRIN lenses or half-ball lenses solves the problems discussed herein. 
   Since the beam size influences the size of other associated opto-mechanical parts of the system, the smaller beam size associated with a full-ball lens also facilitates reducing the inner diameter of any associated opto-mechanical parts. Since the internal parts may be made smaller, this allows for an increase in the thickness of the walls of the housing surrounding the opto-mechanical parts so as to maintain an overall package of approximately the same size. This adds to the overall strength of the package and helps to decrease the potential breakage of mechanical parts that are relatively small as used in an optical module. 
   The diameter of ball lenses  30  and  32  is selected based on the type of material used to form each ball lens and associated design requirements for OADM  20 . For instance, each ball lens can be selected based upon the desired coupling efficiency, lens radius, lens refractive index, magnification in imaging the laser diode on the fiber, defocus to counterbalance spherical aberration of the lens, effective numerical apertures, wavelength, and other design requirements known to one skilled in the art. Regardless of the material used, a full ball lens can shorten the focal length by approximately half over the use of half ball lenses, thus reducing the overall size of the package containing the components. 
   Selected portions of the exterior surface of each ball lens  30  and  32  through a which light signals pass may be coated with any one of a variety of anti-reflective films or coatings known to those skilled in the art. For other applications, substantially all of the exterior surface of each ball lens  30 ,  32  may be coated with anti-reflective material. In either case, the anti-reflective film or coating may be single or multi-layer dielectrics, or other coatings that aid the transmission of electromagnetic radiation through the ball lens. Examples of particular coatings include, but are not limited to SiO 2 /Si 2 H 3 , and SiO 2 /TiO 2    
   Generally, full-ball lenses  30  and  32  may be formed from a wide variety of materials. For instance, full-ball lenses  30  and  32  may be formed from BK7 optical glass, SF2 optical glass, SF8 optical glass, or sapphires. In a more general sense, full-ball lenses  30  and  32  may be formed from glass, crystal, sapphires, LiNbO 3 , ZnO, semiconductors, polymers, natural materials, synthetic material, combinations thereof, or other materials that enable lenses  30  and  32  to perform the desired function of transmitting electromagnetic radiation. 
   As mentioned above, disposed between lenses  30  and  32  is filter  34 . For some applications, optical filter  34  may have the general configuration of a disc with a diameter approximately equal to the diameter of ball lenses  30  and  32 . Alternately, optical filter  34  may have a polygonal configuration. For the embodiment of the present invention as shown in  FIGS. 1 ,  2  and  3 , optical filter  34  includes a first surface  34   a  disposed proximate first ball lens  30  and a second surface  34   b  disposed proximate second ball lens  32 . A layer of wavelength selective film  36  may be disposed on either first surface  34   a  or second surface  34   b .  FIG. 1  shows a ray path that corresponds with placing the layer of wavelength selective film  36  on first surface  34   a.    
   Wavelength selective film  36  acts as a band-pass filter by allowing particular wavelengths of light to pass through, while reflecting all other wavelengths. Typical films may include a stack of thin layers of dielectrics, such as SiO 2 , Ta 2 O 5 , Si 3 N 4 , or other amalgamates. The change in the refractive index at the interfaces between these layers partially reflects the light. By properly choosing the thickness of the layers, the reflection builds up for certain wavelengths so that a nearly 100% reflection occurs. For narrow-band band pass filters, the dielectric layer is usually arranged as pairs of dielectric materials. Each pair or group of pairs can be regarded as a cavity. Typically, such films have one to four cavities. 
   The layer of wavelength selective film  36  is preferably formed from material which will reflect substantially all wavelengths of light signals communicated between input port  22  and output port  24  except at least one selected wavelength λ s . The layer of wavelength selective film  36  preferably has at least one passband centered at the selected wavelength λ s . For some applications the passband in the layer of wavelength selective film  36  may include a range of wavelengths centered at the selected wavelength λ s . 
   As shown in  FIG. 1 , optical signal  40 , having multiple wavelengths, exiting from input port  22  passes through first ball lens  30 . The optical signal  40  is incident upon first surface  34   a  that transmits the portion of the optical signal having wavelength λ s , as represented by numeral  42  in  FIG. 2 , while reflecting all other wavelengths of the optical signal, as represented in  FIG. 1 . The transmitted light passes through second ball lens  32  before entering drop port  28 , while the reflected light returns through first ball lens  30  to output port  24 . As shown in  FIG. 3 , the wavelength selective film  36  will allow optical signal  44  with wavelength λ s  to pass through optical filter  34  when optical signal  44  is input into the optical assembly from port  26  to pass through second ball lens  32  and be incident upon film  36 . 
   With continued reference to  FIG. 1 , optical signal  40  will typically diverge when it exits from end  22   a  of input port  22  adjacent to first ball lens  30 . The adjacent surface or first surface  30   a  of ball lens  30  will reduce the amount of divergence and deflect the center ray of optical signal  40  towards the optical axis of OADM  20 . When optical signal  40  exits from the opposite surface or second surface  30   b  of first ball lens  30 , optical signal  40  will preferably be collimated and directed toward the central portion of optical filter  34 . First surface  34   a  coated with wavelength selective film  36  of optical filter  34  will then reflect optical signal  40  towards the adjacent second surface of first ball lens  30 . The reflected optical signal  40  will converge within first ball lens  30  as it travels towards the first surface adjacent to the end  24   a  of output port  24 . When optical signal  40  exits from the first surface of first ball lens  30 , the signal will be focused into the end  24   a  of output port  24 . 
   For some applications, input port  22  and output port  24  may be disposed immediately adjacent to each other, although in other configurations input port  22  and output port  24  may be disposed some distance from each other. The extreme ends  22   a ,  24   a ,  26   a  and  28   a  of the ports, sometimes referred to as the fiber end faces, may be polished at a selected angle and may be coated with anti-reflective film to reduce back reflection. Additionally, ports  22 ,  24 ,  26  and  28  may be tilted with respect to the longitudinal axis of OADM  20  to compensate for the bending of the light rays by the angled end faces. In one embodiment, this angle may be approximately plus or minus 8° from normal relative to the longitudinal axis of the optical fiber. In an alternate embodiment, the angle of the end faces may be from about 2° to about 6°. The exact value of the optimal angle depends on the tolerance of the other mechanical parts included in OADM  20 . This angle may be calculated when performing an actual alignment of the fabricated parts. It is understood by those of skill in the art that the degree of tilt of the end faces depends on the angle of the polished end faces. 
   A typical signal path for dropping an optical signal having wavelength λ s  is shown in  FIG. 2 . For this embodiment of the present invention, optical signal  40  having wavelength λ s  is shown exiting from input port  22  and directed by first ball lens  30  to the central portion of optical filter  34 . The layer of wavelength selective film  36  allows drop signal  42  with wavelength λ s  to pass therethrough. Second full-ball lens  32  focuses drop signal  42  into drop port  28 . 
   A typical signal path for adding an optical signal having wavelength λ s  is shown in  FIG. 3 . For this embodiment of the present invention, optical signal or add signal  44  having wavelength λ s  is shown exiting from add port  26 . Second ball lens  32  directs add signal  44  to the central portion of optical filter  34 . The layer of wavelength selective film  36  disposed on first surface  34   a  allows add signal  44  to pass therethrough. First ball lens  30  focuses drop signal  44  into output port  24 . 
   For purposes of illustrating various features of the present invention, the communication of optical signal  40 , drop signal  42 , and add signal  44  are shown in separate drawings. For many applications, optical signals may be communicated continuously from input port  22  to output port  24 , while at the same time multiple add signals are communicated from add port  26  to output port  24  and multiple drop signals are communicated from input port  22  to drop port  28 . 
   In one embodiment, the separation between ball lenses  30  and endfaces  22   a ,  24   a , and the separation between the ball lens  30  and the filter surface  34   a , is approximately equal to f of Equation (2). The separation between ball lens  32  and endfaces  26   a ,  28   a  may likewise be approximately equal to f of Equation (2). The separation between filter surface  34   b  and ball lens  32  may generally be any value between 0 and 0.2 mm. However, the exact values can be determined only by a detailed computer simulation of aberrations in the device given the specific materials and optical wavelength. The optical wavelength determines n in Equation (2) provided the materials are known. Also in one embodiment, the distance between fibers  22 ,  24  and fibers  26 ,  28  may be from approximately 125 μm to approximately 180 μm. 
   In an exemplary configuration, ball lenses  30 ,  32  are made from SF2 glass and have a diameter of 3.4 mm. Filter  34  has a thickness of approximately 1 mm and is approximately 1.4 mm square. It is understood by those of skill in the art that other cross sectional shapes are possible including, but not limited to, polygonal, round, oval, etc. Filter  34  may be separated by approximately 0.52 mm from ball lens  30 , and by approximately 0.05 mm from ball lens  32 . Both ball lenses  30 ,  32  may be separated from their respective fiber endfaces by approximately 0.51 mm. 
   In another alternate configuration, ball lenses  30 ,  32  are made from BK7 glass with a diameter of 2.7 mm. Filter  34  has a thickness of approximately 1 mm and is approximately 1.4 mm square. It is understood by those of skill in the art that other cross sectional shapes are possible including, but not limited to, polygonal, round, oval, etc. Filter  34  may be separated by approximately 0.67 mm from ball lens  30 , and by approximately 0.12 mm from ball lens  32 . Both ball lenses  30 ,  32  may be separated from their respective fiber endfaces by approximately 0.65 mm. 
     FIG. 4  is a schematic drawing showing multiple wavelength division demultiplexer (WDDM)  60  that includes a plurality of optical drop modules formed in accordance with teachings of the present invention. WDDM  60  may be described as a four channel multiple wavelength division demultiplexer. Although reference is made to a four channel multiple wavelength division demultiplexer, one skilled in the art will appreciate that a multiple wavelength division demultiplexer with any number of channels may be used or obtain a benefit from the teachings of the present invention. 
   The optical drop modules associated with WDDM  60  have been designated  120   a ,  120   b  and  120   c . Optical drop modules  120   a ,  120   b  and  120   c  may have substantially the same configuration as OADM  20  except for the deletion or removal of add port  26 . For other applications optical drop modules  120   a ,  120   b  and  120   c  may be substantially identical with OADM  20  except add port  26  is blocked or disconnected. 
   For the embodiment of the present invention as shown in  FIG. 4 , a fiber optic cable  62  is preferably coupled with an amplifier  64 . An optical signal having at least four wavelengths (λ 1 , λ 2 , λ 3 , and λ 4  is communicated from amplifier  64  to drop module  120   a . The optical filter  34  associated with drop module  120   a  may include a passband corresponding with wavelength λ 1 . Therefore, an optical signal having wavelength λ 1  will exit through fiber optic output cable  71  connected with the drop port of drop module  120   a . The remaining optical signals (λ 2 , λ 3 , and λ 4 ) are then directed to drop module  120   b . The optical filter  34  associated with drop module  120   b  may include a wavelength selective film  36  with a passband corresponding with wavelength λ 2 . Therefore, optical signals having wavelength λ 2  will exit from drop module  120   b  through fiber optic output cable  72  connected with the associated drop port. 
   The remaining optical signals (λ 3  and λ 4  will be directed to drop module  120   c . The optical filter  34  associated with drop module  120   c  may include a wavelength selective film  36  having a passband corresponding with wavelength λ 3 . Therefore, optical signals having wavelength λ 3  will exit from drop module  120   c  through fiber optic output cable  73  connected with the associated drop port. The remaining optical signal λ 4  will exit through fiber optic cable  74  from the associated output port of drop module  120   c . Additional optical drop modules  120  may be added to WDDM  60  to accommodate demultiplexing any desired number of multiple wavelength optical signals. 
   In a similar manner, OADM  20  and WDDM  60  may be respectively modified to function as an optical add module (not expressly shown) for use in fabricating a multiple wavelength division multiplexer (WDM) (not expressly shown). For instance, WDDM  60  may be modified such that optical cables  71 – 74  act as input ports that deliver optical signals having wavelengths (λ 1 , λ 2 , λ 3 , and λ 4 ). Each optical cable  71 – 74  may include an amplifier, such as amplifier  64 , while output fiber  62  may be devoid of amplifier  64 . 
   In this illustrative configuration, module  120   c , modified as an add module rather than a drop module, combines optical signals having wavelengths ? 1 , and X 4  into an output signal that is input into modified module  120   b . For instance, the optical signal having wavelength λ 4  may be input to the modified OADM through port  26  and combined with the optical signal having wavelength λ 3  input through port  22  as both optical signals are directed to port  24  as the optical signal having wavelength λ 4  passes through a filter having a λ 4  passband and the optical signal having wavelength λ 3  is reflected from the filter. Similar functions occur at modified modules  120   b  and  120   a  so that (i) module  120   b  combines the optical signal having wavelengths λ 3 , and λ 4  with the optical signal having wavelength  2  to create an optical signal having wavelengths λ 2 , λ 3 , and λ 4 , and (ii) module  120   a  combines the optical signal having wavelengths λ 2 , λ 3  and λ 4  with the optical signal having wavelength λ 1  to create an optical signal having wavelengths λ 1 , λ 2 , λ 3 , and λ 4 . The final optical signal may be output along an optical fiber, such as optical fiber  62  acting as an output port. 
   It is understood that the OADM of the present invention may be incorporated into various other optical modules or components, whether or not such optical components are wavelength division multiplexers and wavelength division de-multiplexers. For instance, an OADM incorporating another aspect of the present invention is depicted in  FIGS. 5 ,  6  and  7 . The OADM, identified by reference numeral  120 , may be formed with some of the same optical components as previously described with respect to OADM  20 . 
   For this illustrative embodiment of the present invention, OADM  120  includes first full-ball lens  30  attached to one side of a spacer ring or support ring  131  disposed within an interior of a housing  150 . An optical filter  134  is attached to the opposite side of first support ring  131 . The optical filter  134  functions as a band-pass optical filter much the same way as previously described optical filter  34 . However, the configuration and dimensions associated with optical filter  134  may be substantially modified as compared to optical filter  34 . 
   Support ring  131  includes aperture or opening  133  extending therethrough. Aperture or opening  133  receives a portion of ball lens  30  as ball lens  30  is attached to support ring  131 . For instance, exterior portions of first ball lens  30  are attached to or coupled with first support ring  131  by a layer of UV cured epoxy  138  and an adjacent layer of thermally cured epoxy  140 . In a similar manner a layer of UV cured epoxy  138  and a layer of thermally cured epoxy  140  is used to attach optical filter  134  to the opposite side of support ring  131 . 
   Although reference is made to use of UV cured epoxy and thermally cured epoxy to connect or attached optical components to support ring  131 , one skilled in the art will understand that various other adhesives or bonding agents may be used to connect the optical component to support ring  131 . For instance, UV cured epoxy or thermally cured epoxy are not the only agents that may be used to connect the optical component to support ring  131 . In another configuration, mechanical components may be used to connect the optical component to support ring  131 . 
   OADM  120  includes an optical axis, illustrated in dotted lines, which extends through the central portion of aperture or opening  133 . The various components of OADM  120  such as first full-ball lens  30 , first support ring  131 , optical filter  134  and second full-ball lens  32  are generally aligned with and disposed generally concentric with this optical axis. By coupling first ball lens  30  with first support ring  131  and optical filter  134  in accordance with teachings of the present invention, the relative position between these components is fixed at predetermined values. It will be understood, that another support ring may couple optical filter  134  with second full-ball lens  32 . 
   Optical fibers  122  and  124 , which correspond to respective input port  22  and output port  24  of  FIG. 1 , are disposed within first ferrule  121 . Optical fibers  126  and  128 , which correspond to respective add port  26  and drop port  28  of  FIG. 1 , are disposed within second ferrule  127 . First ferrule  121  and second ferrule  127  may be fabricated from a wide variety of materials such as glass, crystal, metal, ceramics, semiconductors, molded plastics, synthetic materials, natural materials, combinations thereof, or other optical materials known to those skilled in the art. 
   Each ferrule  121  and  127  may have a single opening extending longitudinally therethrough, sized to accommodate their respective optical fibers  122  and  124  and optical fibers  126  and  128  with the desired spacing relative to each other. Alternatively, first ferrule  121  and second ferrule  127  may have two separate holes extending longitudinally therethrough and spaced a preselected distance from each other to receive respective optical fibers  122 ,  124 ,  126  and  128 . This configuration greatly eases the assembly of the fibers into the ferrules  121 ,  127 , and improves mechanical stability. The degradation of optical performance is negligible provided that the centers of the two holes in ferrules  121 ,  127  are separated a distance of 150 μm or less. The ferrules  121 ,  127  may be further enclosed in a holder  129 ,  129   a  respectively. Holders  129 ,  129   a  are attached to housing  150 . 
   Ends  122   a  and  124   a  of optical fibers  122  and  124  are preferably disposed a selected distance from end  129   b  of holder  129 . The portion of first ball lens  30  opposite from first support ring  131  may then be attached to end  129   b  of holder  129 . The longitudinal axis of first ferrule  121  and holder  129  is generally aligned with the center of first ball lens  30  and the center of aperture  133 . In one embodiment of the present invention, the outer diameter of ferrule  121  is smaller than the inner diameter of holder  129  by approximately 0.2 mm to 0.5 mm. This allows adjusting the alignment of optical fibers  122  and  124  relative to other optical components of OADM  120  to meet low-loss requirements in fiber-optics communications systems. 
   Ends  126   a  and  128   a  of optical fibers  126  and  128  are preferably disposed a selected distance from end  129   c  of holder  129   a . The portion of second ball lens  32  opposite from optical filter  134  may be attached to end  129   c  of holder  129   a  using a layer of UV cured epoxy  138  and a layer of thermally cured epoxy  140 , however other adhesives, bonding agents, or methods of attachment may be used to attach end  129   c  to holder  129   a . The longitudinal axis of second ferrule  127  is generally aligned with the longitudinal axis of OADM  120 . The focal plane after the ball lens is predetermined and the location for the filter can be determined using Oslo ray tracing software, geometric optics, or using the principles of symmetry. 
   First ferrule  121  with first full-ball lens  30 , holder  129  and optical fiber  134  attached thereto are preferably disposed within housing  150 . Housing  150  preferably has a generally cylindrical configuration with a longitudinal bore extending therethrough. The length of housing  150  is preferably selected to accommodate the length of first ferrule  121 , second ferrule  127  and required spacing for first ball lens  30 , second ball lens  32  and optical filter  134  for optimum optical performance of OADM  120 . An adhesive material  152  may be used to bond or couple the exterior of first ferrule  121  with housing  129 , and housing  129  with adjacent portions of the inside surface of housing  150 . Adhesive layer  152  may be used to bond or couple the exterior of second ferrule  127  with housing  129   a , and housing  129   a  with adjacent portions of the inside surface of housing  150 . 
   Another aspect of the present invention is shown in  FIG. 8 . OADM  220  may be formed with some of the same optical components as previously described with respect to OADM  20  and  120 . Therefore, the discussion relating to other OADMs of the present invention also applies to the discussion related to OADM  220 . 
   In this illustrative configuration, first full-ball lens  30  is attached to one side of a first support ring  231 . First support ring  231  preferably includes aperture or opening  233  extending therethrough. Aperture or opening  233  may be configured to accommodate a relatively small aperture size in associated components. Optical filter  234  attaches to and is partially disposed within the opposite side of first support ring  231 . Optical filter  234  may have a generally polygonal configuration which is compatible with a corresponding polygonal opening  233  formed in first support ring  231 . Alternately, optical filter  234  may be round, oval, or some other configuration having a diameter or dimension that is larger than opening  233 . Optical filter  234  may be disposed at least partially within first support ring  231  and bonded or coupled therewith using adhesives, bonding agents, materials, mechanical components, or combinations thereof, as previously described with respect to OADM  120 . 
   OADM  220  includes a second support ring  232 . This second support ring  232  couples with optical filter  234  opposite from first support ring  231 . Second full-ball lens  32  attaches to second support ring  232 . For this embodiment of the present invention, first full-ball lens  30 , first support ring  231 , optical filter  234 , second support ring  232 , and second full-ball lens  32  may be assembled having desired spacing and optical alignment with respect to each other and may be considered as an optical assembly. This optical assembly may then be disposed within first housing  250 . 
   First housing  250  has a generally cylindrical configuration with a longitudinal bore extending therethrough. First housing  250  includes a shoulder  252  formed on the interior of first housing  250  intermediate the ends thereof. The outside diameter of first support ring  231  is selected to engage shoulder  252 . One or more ports  254  and  256  are preferably formed in the exterior of first housing  250  adjacent to and spaced from shoulder  252 . When first full-ball lens  30 , optical filter  234 , second full-ball lens  32 , and support rings  231  and  232  are disposed at their desired location within first housing  250 , an appropriate adhesive or bonding compound may be injected through one or more ports  254  and  256  to securely fasten the optical assembly within first housing  250 . 
   Although reference is made to use of adhesives or bonding compounds to securely attach the optical assembly to first housing  250 , one skilled in the art will appreciate that mechanical components, whether alone or in combination with the adhesives or bonding compounds, may attach the optical assembly to first housing  250 . Similarly, portions of the optical assembly, such as, but not limited to, support rings  231  and  232  may slip-fit or friction fit with first housing  250  and may be secured in place through such frictional contact alone or in combination with one or more of the other techniques described herein and otherwise known to one skilled in the art in light of the teaching contained herein. 
   First ferrule  121  with optical fibers  122  and  124  is inserted into one end of first housing  250  and adjusted to obtain the desired optical position relative to first full-ball lens  30  and optical filter  234 . Housing  250  again has one or more ports  258  that facilitate bonding of first ferrule  121  in the optimum location. For some applications another ferrule (not expressly shown) may be disposed on the exterior of first ferrule  121  to provide additional assistance during the assembly of OADM  220 . It will be understood that various techniques can be used to attach first ferrule  121  to housing  250  and/or to attach another ferrule to first ferrule  121 . 
   A second ferrule  127  with optical fibers  126  and  128  attached therein may be installed through the opposite end of first housing  250 . The position of second ferrule  127  and associated optical fibers  126  and  128  may be adjusted for optimum optical alignment relative to second full-ball lens  32 . Again, one or more ports  260  provided in first housing  250  allow the injection of adhesive material to bond or couple second ferrule  127  with the adjacent inside diameter of first housing  250 . For the embodiment of the present invention as shown in  FIG. 8 , another housing  280  may be disposed on the exterior of second ferrule  127  for ease of assembly. For some applications, an additional housing  290  may be disposed over the exterior of first housing  250 . 
   If the end faces  122   a ,  124   a ,  126   a  and  128   a  are polished at an angle, the ferrules  121 ,  127  may be tilted to compensate for the bending of the light rays, so that the optical loss may be reduced to less than 0.6 dB at the output fiber  124 . In this case, housing  280  may be eliminated. The inner diameter of housing  250  may then be at least 0.8 mm larger than the outer diameter of ferrules  121 ,  127  to allow enough space for ferrules  121 , 127  to tilt at the required angle. 
   The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.