WO2000003277A1

WO2000003277A1 - Improved add/drop wdm multiplexer for fiberoptic networks

Info

Publication number: WO2000003277A1
Application number: PCT/US1999/015158
Authority: WO
Inventors: Jing-Jong Pan; Tian Zhu
Original assignee: E-Tek Dynamics, Inc.
Priority date: 1998-07-08
Filing date: 1999-06-30
Publication date: 2000-01-20

Abstract

The present invention provides for improved add/drop WDM multiplexers in fiberoptic networks. One embodiment has two cascade-connected WDM couplers (34, 35) with each coupler having a filter at a selected wavelength μi. Light received from one fiber section is passed to one or the other fiber sections according to wavelength by operation of the wavelength-dependent filter. The two WDM couplers act as an add/drop WDM multiplexer provide improved crosstalk isolation because light is effectively filtered twice by reflection. Likewise, a compact add/drop WDM multiplexer can be created with a WDM coupler structure having two sets of aligned optical fiber sections separated by a first collimating lens (72), a wavelength-dependent filter (73) at a selected wavelength μi, and a second collimating lens (74).

Description

IMPROVED ADD/DROP WDM MULTIPLEXER

FOR FIBEROPTIC NETWORKS

BACKGROUND OF THE INVENTION The present invention relates to fiberoptic network coupler devices and, more particularly, to add/drop WDM multiplexers for such WDM networks.

In WDM (Wavelength Division Multiplexed) fiberoptic networks, the wavelength of an optical signal is used to direct the signal through the network to its intended destination so that the optical signals of a particular wavelength can form a communication channel over a network. Ideally, couplers for such networks should have high performance, i.e., low insertion losses, and high isolation between channels. Additionally, the couplers should be manufactured at low cost, and easy to install and operate in a network .

One type of coupler is the add/drop WDM multiplexer by which optical signals at a particular wavelength are removed ( i.e., dropped) from a network fiber, and are inserted (i.e., added) into the network fiber. For example, in a ring network, such devices are often used with each user. As the optical signals circulate about the ring, the WDM multiplexer associated with a user removes signals of a preselected wavelength, λ_i; from the network fiber for the user and inserts signals from the user at the same wavelength, λj_ '

(the prime punctuation mark designates that the source is the user) , into the network fiber for transmission to other parts of the network.

Nonetheless, the devices presently used as add/drop WDM multiplexers suffer from various infirmities. For example, some add/drop WDM multiplexers require circulators, which are expensive, and fiber Bragg gratings, which require complicated and hence, expensive, packaging because the fiber Bragg gratings are very sensitive to temperature variations. The packaging implements temperature stabilization measures. Planar waveguide add/drop WDM multiplexers also have high manufacturing costs and high temperature sensitivity.

On the other hand, the present invention provides for add/drop WDM multiplexers which are relatively inexpensive to manufacture and more robust to operate, with high performance .

SUMMARY OF THE INVENTION The present invention provides for an improved add/drop WDM multiplexer. In one embodiment, two WDM couplers are connected in a cascade. The first WDM coupler has the ends of three optical fibers and a wavelength-dependent filter. Entering from a first optical fiber, light which is reflected by the wavelength-dependent filter passes to a second optical fiber, while light which is transmitted by the wavelength-dependent filter passes to a third optical fiber. The second WDM coupler has the ends of three more optical fibers and a second wavelength-dependent filter having the same wavelength-dependent characteristics as the first filter arranged so that light entering the second coupler from its first optical fiber and reflected by the second wavelength- dependent filter is passed to the second optical fiber of the second WDM coupler, and light entering the third optical fiber of the second WDM coupler and transmitted by the second wavelength-dependent filter is passed to the second optical fiber also. The first optical fiber of the second WDM coupler is connected by a splice to the second optical fiber of the first WDM coupler so that light which is reflected by the first wavelength-dependent filter is also reflected by the second wavelength-dependent filter and passed to the second optical fiber of the second WDM coupler. The first and second WDM couplers form an add/drop WDM multiplexer by connecting the first optical fiber of the first WDM coupler as an input fiber from a network fiber, the second optical fiber of the second WDM coupler as the output fiber to the network fiber, the third optical fiber of the first WDM coupler as a drop fiber, and the third optical fiber of the second WDM coupler as an add fiber.

In another embodiment, the present invention provides for an improved add/drop WDM multiplexer which has the ends of six optical fibers, a first collimating lens, a second collimating lens and a wavelength-dependent filter arranged such that the ends of the first, second, third, and fourth optical fibers are in close proximity with each other and one end face of the first collimating lens and the ends of the fifth and sixth optical fibers in close proximity with a first end face of the second collimating lens. The wavelength-dependent filter is between the second end faces of the first and second collimating lenses. The wavelength- dependent filter and the ends of said first, second, third and fourth optical fibers are arranged with respect to each other so that light from the first optical fiber end and reflected by the wavelength-dependent filter passes into the second optical fiber end and light from the third optical fiber end and reflected by the wavelength-dependent filter passes into the fourth optical fiber end. The second and third optical fibers are connected together so that light passing into the second fiber end passes out from the third optical fiber end. The ends of the fifth and sixth optical fibers are arranged so that light from the first optical fiber transmitted by the wavelength-dependent filter passes into the fifth optical fiber and light from the sixth optical fiber and transmitted by the wavelength-dependent filter passes into the fourth optical filter. The WDM multiplexer operates as an add/drop WDM multiplexer by connecting the first optical fiber as a network input fiber, the fourth optical fiber as a network output fiber, the fifth optical fiber as a drop fiber and the sixth optical fiber as an add fiber.

The present invention also provides for an embodiment in which optical fiber sections are connected to form two loops so that the wavelength-dependent filter twice reflects light from the section acting as the network input fiber before passing the light to the section acting as the network output fiber, for higher performance.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a diagram of a prior art add/drop WDM multiplexer using fiber Bragg gratings and optical circulators; Fig. IB is a diagram of a prior art add/drop WDM multiplexer with an optical waveguide and fiber Bragg gratings; Fig.^' 2A is a diagram of an add/drop WDM multiplexer formed from cascaded WDM couplers, according to one embodiment of the present invention; Fig. 2B is a diagram of light paths through the WDM couplers of Fig. 2A;

Fig. 3A illustrates the general organization of an add/drop WDM multiplexer according to another embodiment of the present invention; Fig. 3B is a diagram of light paths through the WDM multiplexer of Fig. 3A;

Fig. 4A illustrates one variation of the add/drop WDM multiplexer of Fig. 3A in which the input, output and loop optical fiber sections are arranged in a bundle; Fig. 4B is an end cross-sectional view of the input, output and loop optical fiber sections in Fig. 4A with representational light paths between the optical fiber sections; Fig. 4C is an end cross- sectional view of the input, output and loop optical fiber sections arranged in a sleeve having a square cross-sectional aperture ;

Fig. 5A illustrates another variation of the add/drop WDM multiplexer of Fig. 3A in which the input, output and loop optical fiber sections are arranged linearly; Fig. 5B is an end cross-sectional view of the input, output and loop optical fiber sections in Fig. 5A with representational light paths between the optical fiber sections; Fig. 5C illustrates the light traces through the GRIN lenses, which are transmitted by the wavelength-dependent filter in the WDM coupler of Fig. 5A; Fig. 5D illustrates the light traces through a GRIN lens, which are reflected by the wavelength- dependent filter in the WDM multiplexer of Fig. 5A; Fig. 6A is a cross-sectional diagram of one package assembly for the add/drop WDM multiplexer of Fig. 3A; and Fig. 6B is a cross-sectional diagram of another package assembly for the add/drop WDM multiplexer of Fig. 3A; Fig. 7A illustrates an add/drop WDM multiplexer according to another embodiment of the present invention; Fig. 7B is a cross-sectional side diagram of light paths through the WDM multiplexer of Fig. 7A; and Fig. 7C is an cross- sectional end view of the input, output and loop-connected optical fiber sections in Fig. 7A with representational light paths between the optical fiber sections.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT (S) Fig. 1A is a diagram of a prior art add/drop WDM multiplexer and illustrates the basic operation of such devices in WDM fiberoptic networks. The multiplexer is connected to an network optical fiber, which carries light signals for the network, by an input fiber 10 and an output fiber 11. The coupler has two optical circulators 14 and 15, which each route light signals received at a circulator's port 1 to the circulator's port 2, and route light signals received at the circulator's port 2 to the circulator's port 3. Port 1 of the first circulator 14 is connected to the input fiber 10, port 3 is connected to a drop fiber 12, and port 2 is connected to port 2 of the second circulator 15 with an optical fiber section containing a fiber Bragg grating 16. The remaining ports 1 and 3 of the second circulator 15 are connected to an add fiber 13 and the output fiber 11.

The WDM multiplexer is connected to the network optical fiber by the input and output fibers 10 and 11. The multiplexer receives network signals at the different wavelengths, λ_l λ₂, ...λ_n, and removes signals at a wavelength, λ_χ selected by the fiber Bragg grating 16. These λ_x signals are sent to the drop fiber 12. Conversely, optical signals at the same wavelength λ_x ' (the prime symbol is used to indicate that the added λ_x ' signals may be different from the dropped ones, λ_j., although their wavelengths are the same) received on the add fiber 13 can be transmitted to the network optical fiber by the output fiber 11. Hence a user can receive from, and transmit optical signals at a selected wavelength λ to, the network optical fiber. Another prior art add/drop WDM multiplexer is shown in Fig. IB. A planar dielectric substrate 24, typically lithium niobate, has a waveguide structure with gratings 25 and 26 in split legs 27 and 28 of the waveguide. The net result is that one path 20 to the planar waveguide substrate 24 can operate as an input fiber from a network fiber, a second path 21 can operate as an output fiber to the network fiber, a third path 22 as a drop fiber, and a fourth path 23 as an add fiber.

The disadvantages of these devices are their complexities and manufacturing expense, which tend to discourage the adoption of WDM networks. In particular, the optical circulators make the Fig. 1A WDM multiplexer expensive. The planar waveguide WDM multiplexer of Fig. IB is difficult to manufacture; its operation is very sensitive to mismatches between the two Bragg gratings 25 and 26. Even with properly matched gratings, the device is very sensitive to changes in temperature. Like the WDM coupler of Fig. 1A, the planar waveguide WDM multiplexer requires packaging with excellent temperature control . The present invention provides for add/drop WDM multiplexers which are relatively inexpensive to manufacture and insensitive to changes in operating conditions. Furthermore, with the use of wavelength-dependent filters, i.e., not only bandpass filters, but also long-pass and short- pass filters, the add/drop WDM multiplexers of the present invention can flexibly "add" and "drop" a single or multiple wavelength channels to and from a network fiber.

One aspect of the present invention is illustrated in Fig. 2A. U.S. Patent No. 5,642,448, entitled "INTEGRABLE FIBEROPTIC COUPLER AND RESULTING DEVICES AND SYSTEMS," filed March 28, 1996 by J.J. Pan et al . , and U.S. Appln. No. 08/614,864, entitled "INTEGRATED WDM COUPLER DEVICES FOR FIBEROPTIC NETWORKS," filed March 13, 1996, both assigned to the present assignee and incorporated herein by reference, describe WDM couplers with first and second optical fiber sections at a first end and a third optical fiber section at a second end of the coupler. For example, see the description with respect to Fig. 11 of U.S. Patent No. 5,642,448. Depending upon its wavelength, light which enters the first optical fiber section is either transmitted to the third optical fiber section or reflected to the second optical fiber end section by a wavelength-dependent filter. In the reverse direction, light from the second or third optical fiber sections is transmitted to the first optical fiber section.

To achieve sufficient isolation between the wavelength channels to operate effectively as an add/drop WDM multiplexer, two of these types of WDM couplers 34 and 35 are connected together in a cascade, as illustrated in Fig. 2A. The first WDM coupler 34 has a first sleeve 71, a first collimating GRIN lens 72, a wavelength-dependent filter 73, a second collimating GRIN lens 74 and a second glass sleeve 75. Likewise, the second WDM coupler 35 has a first sleeve 81, a first collimating GRIN lens 82, a wavelength-dependent filter 83, a second collimating GRIN lens 84 and a second glass sleeve 85. Both the wavelength-dependent filter 73 of the WDM coupler 34 and the wavelength-dependent filter 83 of the WDM coupler 35 are selective for the same wavelength λj_.

In a central aperture 76 in its first sleeve 71, the first WDM coupler 34 holds optical fiber sections 30 and 36. A central aperture 77 of the second sleeve 75 of the first WDM coupler 34 holds the optical fiber section 32. Likewise, the second WDM coupler 35 holds optical fiber sections 31 and 37 in a central aperture 86 in the first sleeve 81, and holds the fiber section 33 in the central aperture 87 in the second sleeve 85. The first optical fiber section 30 is connected to the network optic fiber and operates as an input fiber; the optical fiber section 32 of the coupler 34 transmits the outgoing optical signals from the WDM coupler 34 and acts as the drop fiber. The first WDM coupler 34 also has its optical fiber section 36 connected to the optical fiber section 37 of the second WDM coupler 35 by a splice 38. The optical fiber end section 33 of the coupler 35 operates as an add optical fiber, while the fiber end section 31 operates as the output fiber connected to the network optical fiber.

Fig. 2B illustrates the operation of the cascade- connected WDM couplers 34 and 35 and how the two couplers operate as an add/drop WDM multiplexer, as explained with respect to the coupler of Fig. 2A. The first end section 30 of the first WDM coupler 34 operates as an input fiber; the third optical fiber end section 32 is the drop fiber. The first end section 31 of the second WDM coupler 35 is the output fiber, while the third optical fiber section 33 is the add fiber. In the WDM coupler 34, the arrangement of the fiber sections 30 and 36 in the aperture 76, the filter 73 and the fiber section 32 in the aperture 77 is such that incoming light signals (from the fiber section 30) which is reflected by the filter 73 is sent to the fiber section 36 and light which is transmitted by the filter 73 is sent to the fiber section 32. The WDM coupler 35 has the fiber sections 37 and 31 in the aperture 86, the filter 83 and the fiber section 33 in the aperture 87 arranged such that incoming light signals (from the fiber section 37) which is reflected by the filter 83 is sent to the fiber section 31. Likewise, incoming light from the fiber section 33 transmitted by the filter 83 is sent to the fiber section 31.

The cascade-connected WDM couplers 34 and 35 have improved wavelength channel isolation because the light is reflected twice off the wavelength-dependent filters 73 and 83. This operation effectively removes light at λ received from the input fiber section 30 from being passed to the output fiber section 31. Furthermore, it should be understood that though the diagram of Fig. 2B illustrates the operation of the filters 73 and 83 as bandpass filters, the described add/drop WDM multiplexer works effectively if the filters are high-pass or low-pass filters also. The WDM multiplexer drops and adds wavelength channels equal to, or less than, a selected wavelength λ. if the filters 73 and 83 are high-pass filters, or drops and adds wavelength channels greater than λ, if the filters 73 and 83 are low-pass filters, where "high- pass" and "low-pass" refer to frequency. More descriptions of the operation and assembly of

WDM couplers having input optical fibers and output optical fibers are found in the assignee ' s patent and patent application referenced above.

In accordance to the present invention, the general organization of a more compact add/drop WDM multiplexer 50 is illustrated in Figs. 3A and 3B. The WDM coupler has a glass sleeve 45 with a central aperture 38 which receives the optical fiber sections 40, 41 and 44, a collimating GRIN lens 46, a wavelength-dependent filter 47, a second GRIN lens 48, and a second sleeve 49 with a central aperture 39 which receives the optical fiber sections 42 and 43.

Fig. 3B illustrates the light paths between the different optical fiber sections 40-44 through the coupler 50. The optical fiber section 40 operates as the input fiber carrying network signals at different wavelengths, λ_1# λ₂, ...λ_n, into the coupler 50. Light transmitted through the filter 47 are received by the optical fiber section 42, which acts as the drop fiber. Light reflected by the selective filter 47, i.e., light at wavelength λi, is passed to optical fiber section 44. The optical fiber section 44, shown here as a loop, passes the light back into the coupler 50 and the filter 47 for a second time. The λi light is reflected by the filter 47 into the optical fiber section 41 acting as the output fiber. The optical fiber section 43, which carries incoming light signals at the same wavelength transmitted by the filter 47 into the coupler 50, operates as an add fiber. The transmitted light denoted by λi¹ passes into the optical fiber section 41, which now carries light at wavelengths, λ_1;

Λ.2 i . . . , Λ , . . . Λ_n . Fig. 4A illustrates one arrangement of the optical fiber sections 40, 41 and 44 in the central aperture 38 in the sleeve 45 of the add/drop WDM coupler of Fig. 3A. The Fig. 4A WDM coupler is shown without the sleeves 45 and 49 and the loop optical fiber section 44 is separated into two optical fiber sections 44A and 44B. In fact, it has been found that the most effective method of forming the loop section 44 is by splicing the two sections 44A and 44B together after alignment of the various optical fiber sections, as explained below.

The four sections 40, 41, 44A and 44B are arranged as a bundle in the central aperture 38 which has a circular cross-section, as shown in the cross-sectional view of Fig. 4B. As illustrated by Fig. 4A, the collimating GRIN lens 46 between the sleeve 45 and the wavelength-dependent filter 47 is opposed by the second collimating GRIN lens 48 and the sleeve 49 having its central aperture 39 into which the ends of the optical fiber sections 42 and 43 are inserted. The filter 47 may be a bandpass filter, or dichroic mirror filter, such as long-pass filter or short-pass filter. In any case, the ends of the optical fibers sections 40, 41, 44A and 44B are arranged with respect to the collimating GRIN lens 46 and the filter 47 such that light from optical fiber 40 is reflected back and refocused by the GRIN lens 46 at the end of the fiber section 44A, as illustrated by Fig. 4B . Similarly, light from the optical fiber section 44B (as received from the optical fiber section 44A as indicated by the splice connection 51) is reflected back by the filter 47 again and refocused by the GRIN lens 46 on the end of the optical fiber section 41.

Light which is transmitted through the filter 47 is refocused by the second GRIN lens 48. The second GRIN lens 48 and the ends of the fiber sections 42 and 43 are arranged with respect to the filter 47, the first collimating GRIN lens 46 and the ends of the optical fiber sections 40, 41, 44A and 44B such that light from the optical fiber section 40 is refocused by the second collimating GRIN lens 48 on the end of the optical fiber 42 and, conversely, collimated light from the optical fiber 43 is refocused by the first GRIN lens 46 at the end of the optical fiber section 41. The optical fiber sections 40-43, 44A and 44B are unjacketed so that only the core and cladding of the fibers are inserted into the respective apertures 38 and 39 which run longitudinally through the sleeves 45 and 49. The core and cladding of the fiber sections 40-43, 44A and 44B may, or may not, be tapered but, in any case, are inserted into the apertures 38 and 39 without being fused together. The size of the central aperture 38 is such that the four optical fiber sections 40, 41, 44A and 44B fit snugly in the aperture 38. Likewise, the aperture 39 is sized to fit the optical fiber sections 42 and 43 snugly. The aperture 38 may be circular in cross-section or the aperture may have a square cross-section as shown in Fig. 4C. Glass sleeves with such central apertures with square cross-sections may be obtained from Vitro Dynamics, Inc. of Rockaway, New Jersey. Both cross- sections are effective in ensuring a snug fit for the end sections 40, 41, 44A and 44B.

The face of the sleeve 45 (and the ends of the fiber sections 40, 41, 44A and 44B) toward the collimating GRIN lens 46 is polished at an angle from 8-12° from a plane perpendicular to the longitudinal axis of the sleeve 45. The face of the lens 46 toward the sleeve 45 is also angle- polished, but at a reciprocal angle. The faces of the sleeve 45 (and the ends of the fiber sections 40, 41, 44A and 44B) and GRIN lens 46 are covered with anti-reflection coatings.

Likewise, the face of the sleeve 49 (and the ends of the fiber sections 42 and 43) and the second GRIN lens 48 are angle- polished and covered with anti-reflection coatings. While conventional lenses could be used in place of the collimating GRIN lenses 46 and 48, quarter-pitch GRIN (GRaded INdex) lenses has been found to be superior in terms of ease of assembly and reliability in the completed coupler.

Fig. 5A illustrates another arrangement of the optical fiber sections 40, 41 and 44 in a modified central aperture 38 in the sleeve 45 of the add/drop WDM coupler of Fig. 3A. In this arrangement, the optical fiber sections 40, 41 and 44 are arranged in a linear alignment. As described with respect to the Fig. 4A WDM coupler, the ends of the optical fibers sections 40, 41, 44A and 44B are arranged with respect to the collimating GRIN lens 46 and the filter 47 such that light from optical fiber 40 is reflected back and refocused by the GRIN lens 46 at the end of the fiber section 44A. Similarly, light from the optical fiber section 44B (as received from the optical fiber section 44A as indicated by the splice connection 51) is reflected back by the filter 47 again and refocused by the GRIN lens 46 on the end of the optical fiber section 41. Fig. 5B traces these light paths with the cross-sectional end faces of the linearly arranged optical fiber sections 40, 41, 44A and 44B. Fig. 5D traces these light paths in a cross-sectional side view of the optical fiber sections 40, 41, 44A and 44B and the first GRIN lens 46. The dotted line 52 is the longitudinal axis of the GRIN lens 46 and the modified aperture 38 in the sleeve 45.

Light which is transmitted through the filter 47 is refocused by the second GRIN lens 48. The second GRIN lens 48 and the ends of the fiber sections 42 and 43 are arranged with respect to the filter 47, the first collimating GRIN lens 46 and the ends of the optical fiber sections 40, 41, 44A and 44B such that light from the optical fiber section 40 is refocused by the second collimating GRIN lens 48 on the end of the optical fiber 42 and, conversely, collimated light from the optical fiber 43 is refocused by the first GRIN lens 46 at the end of the optical fiber section 41. These light path traces are shown in Fig. 5C.

The add/drop WDM multiplexer of Fig. 3A can be packaged very compactly. A packaged multiplexer assembly illustrated in Fig. 6A is particularly suitable for the Fig. 5A multiplexer arrangement. The cylindrically shaped sleeve 45 with a rectangular aperture to accommodate the linearly aligned optical fibers is mounted in a holder 55 which, in turn, is fixed within a package 60 by supports 56. Likewise, the GRIN lens 46 is mounted within the package 60. The GRIN lens 48 and sleeve 49 are mounted in one holder 57 which is fixed within the package 60 by supports 58. A protection tube 53 extends from the sleeve 45 through the wall of the package 60 to protect the fiber sections 40, 41, and 44, while another protection tube 54 extends from the sleeve 49 through the wall of the package 60 to protect the fiber sections 42 and 43. The resulting rectangular package is 40mm long, 8mm wide and 8mm high. An even more compact package assembly is shown in Fig. 6B in which supports, such as those of Fig. 6A, are not used. This cylindrical package is 40mm long and 5.5mm in diameter. This packaged assembly is highly suitable for the multiplexer arrangement shown in Fig. 4A in which all parts are cylindrically shaped.

Finally, it should be noted that the wavelength- dependent filters in the package assemblies of Figs. 6A and 6B are slightly different for purposes of illustration. The filter 47A of Fig. 6A represents a filter plate which is attached to a face of the GRIN lens 46. The filter plate is a substrate upon which multiple optical coatings are deposited so that a dichroic filter, i.e., a low-pass or high-pass filter, or a bandpass filter is created. Alternatively, the coatings may be deposited directly upon the face of the GRIN lens 46, as represented by the filter 47B in Fig. 6B. The two types of wavelength-dependent filters may be used in either types of package assemblies.

In passing, it should be noted that it is difficult to make dichroic filters with sharp cut-off wavelengths.

Rather, it is easier to build wide bandpass filters with very sharp shirts. If the width of the bandpass filter is large enough, it is often more effective to use the wide bandpass filter in place of the low-pass or high-pass filter. Depending upon the function desired, a skirt of the bandpass filter can be aligned with the cut-off wavelength λ_j so that all wavelengths above λ_j are transmitted and the wavelengths below λ_j are reflected, or all wavelengths below λ_j are transmitted and the wavelengths above λ_j are reflected. The operation of a dichroic filter is created.

The performance of these compact add/drop WDM couplers have been found to be excellent. Insertion loss has been found to be less than 2.0dB; drop channel isolation greater than 25dB; add channel isolation greater than 25dB; return loss greater than 55dB; and polarization dependent loss less than O.ldB. This performance is good enough so that these add/drop WDM couplers may be used in DWDM (Dense WDM) networks in which the channels are separated by only 100GHz. Nonetheless, an add/drop WDM multiplexer with even better crosstalk isolation according to the present invention is illustrated in Fig. 7A. The WDM multiplexer 100 has a glass sleeve (not shown) which holds optical fiber sections 90, 91, 94A, 94B, 95A and 95B, a collimating GRIN lens 96, a wavelength-dependent filter 97, a second GRIN lens 98, and a second sleeve (not shown) which receives optical fiber sections 92 and 93. The sections 94A and 94B, and 95A and 95B are spliced together so that the multiplexer 100 operates like the previously described multiplexer 50. Construction techniques of this WDM multiplexer is similar to those of the multiplexer 50. However, in the present multiplexer 100, light which is received from the section 90 (connected to a network input fiber) is reflected three times before passing to the section 91 (connected to a network output fiber) .

Fig. 7B illustrates the light paths between the different optical fiber sections 90, 91, 92, 93, 94A, 94B, 95A and 95B, through the multiplexer 100. The optical fiber section 90 operates as the input fiber carrying network signals at different wavelengths, λ_x, λ₂, ...λ_n, into the multiplexer 100. Light transmitted through the filter 97 is received by the optical fiber section 92, which acts as the drop fiber. Light reflected by the selective filter 97, i.e., light at wavelength λ_± , is passed to the optical fiber section 94A which is connected to the section 94B. The light loops back into the multiplexer 80 and to the filter 97 for a second time. The λ_t light is reflected by the filter 97 into the section 95A which is connected to the section 95B. The light loops back into the multiplexer 100 and is reflected a third time by the wavelength-dependent filter 97 into the optical fiber section 91 acting as the output fiber. Fig. 7C illustrates the path of the light reflected by the wavelength-dependent filter 97 between the optical fiber sections 90, 91, 94A, 94B, 95A and 95B in a cross- sectional end view of the bundled fiber sections. The connections between the optical fiber sections 94A-94B and 95A-95B are symbolically indicated by curved lines 99 running between the cores of the sections 94A-94B and 95A-95B. Note a central spacer fiber section 89 which is used to help position the ends of the optical fiber sections 90, 91, 94A, 94B, 95A and 95B properly.

Light which is transmitted through the filter 97 is refocused by the second GRIN lens 98. The second GRIN lens 98 and the ends of the fiber sections 92 and 93 are arranged with respect to the filter 97, the first collimating GRIN lens 96 and the ends of the optical fiber sections 90, 91, 94A, 94B, 95A and 95B such that light from the optical fiber section 90 is refocused by the second collimating GRIN lens 98 on the end of the optical fiber 92 and, conversely, collimated light from the optical fiber 93 is refocused by the first GRIN lens 96 at the end of the optical fiber section 91. The optical fiber section 93, which carries incoming light signals at the same wavelength transmitted by the filter 97 into the multiplexer 100, operates as an add fiber. The transmitted light denoted by X₁ ' passes into the optical fiber section 91, which now carries light at wavelengths, λ_x , λ₂, ... , λ_A ' , ...λ_n.

While the above is a complete description of the preferred embodiments of the present invention, various alternatives, modifications and equivalents may be used. It should be evident that the present invention is equally applicable by making appropriate modifications to the embodiment described above. For example, different combinations of long/short pass filters and band-pass filters may adapted into the WDM couplers described above. Therefore, the above description should not be taken as limiting the scope of invention which is defined by the metes and bounds of the appended claims .

Claims

WHAT IS CLAIMED IS:

1. A WDM multiplexer comprising a first WDM coupler having an end of a first optical fiber; an end of a second optical fiber; an end of a third optical fiber; a first collimating lens having first and second end faces, said first end face of said first collimating lens and said ends of said first and second optical fibers proximate each other and said first end face; a first wavelength-dependent filter proximate said second end face of said first collimating lens; and a second collimating lens having first and second end faces, said first end face of said second collimating lens proximate said end of said third optical fiber, said second end face of said second collimating lens proximate said first wavelength-dependent filter opposite said first collimating lens; said first wavelength-dependent filter, said ends of said first, second and third optical fibers arranged with respect to each other so that light from said first optical fiber reflected by said first wavelength-dependent filter passes into said second optical fiber, and light from said first optical fiber transmitted by said first wavelength-dependent filter passes into said third optical fiber; and a second WDM coupler comprising an end of a fourth optical fiber, said fourth optical fiber connected to said second optical fiber; an end of a fifth optical fiber; an end of a sixth optical fiber; a third collimating lens having first and second end faces, said first end face of said third collimating lens and said ends of said fourth and fifth optical fibers proximate each other and said first end face; a second wavelength-dependent filter proximate said second end face of said third collimating lens; and a fourth collimating lens having first and second end faces, said first end face of said fourth collimating lens proximate said end of said sixth optical fiber, said second end face of said fourth collimating lens proximate said second wavelength-dependent filter opposite said third collimating lens; said second wavelength-dependent filter, said ends of said fourth, fifth and sixth optical fibers arranged with respect to each other so that light from said fourth optical fiber reflected by said second wavelength-dependent filter passes into said fifth optical fiber, and light from said sixth optical fiber transmitted by said second wavelength-dependent filter passes into said fifth optical fiber; whereby said WDM multiplexer may be operate as an add/drop WDM multiplexer by connecting said first optical fiber as a network input fiber, said fifth optical fiber as a network output fiber, said third optical fiber as a drop fiber and said sixth optical fiber as an add fiber.

2. The WDM multiplexer of claim 1 wherein said second and fourth optical fibers are spliced together.

3. An WDM multiplexer comprising an end of a first optical fiber; an end of a second optical fiber; an end of a third optical fiber; an end of a fourth optical fiber; an end of a fifth optical fiber; an end of a sixth optical fiber; a first collimating lens having first and second end faces, said first end face of said collimating lens and said ends of said first, second, third, and fourth optical fibers in close proximity with each other and said first end face; a wavelength-dependent filter proximate said second end face of said first collimating lens, said wavelength- dependent filter reflecting or passing light responsive to wavelength of light incident thereupon, said wavelength- dependent filter, said ends of said first, second, third and fourth optical fibers arranged with respect to each other so that light from said first optical fiber end and reflected by said wavelength-dependent filter passes into said second optical fiber end and so that light from said third optical fiber end and reflected by said wavelength-dependent filter passes into said fourth optical fiber end, said second and third optical fibers connected so that light passing into said second fiber end passes out from said third optical fiber end; and a second collimating lens in the path of said collimated light from said wavelength-dependent filter, said ends of said fifth and sixth optical fibers in close proximity with a first end face of said second collimating lens and arranged so that light from said first optical fiber passed by said wavelength-dependent filter passes into said fifth optical fiber and light from said sixth optical fiber and passed by said wavelength-dependent filter passes into said fourth optical filter; whereby said WDM multiplexer may be operate as an add/drop WDM multiplexer by connecting said first optical fiber as a network input fiber, said fourth optical fiber as a network output fiber, said fifth optical fiber as a drop fiber and said sixth optical fiber as an add fiber.

4. The WDM coupler of claim 3 wherein said wavelength- dependent filter comprises a bandpass filter.

5. The WDM coupler of claim 3 wherein said wavelength- dependent filter comprises a dichroic filter.

6. The WDM coupler of claim 3 wherein said first and second collimating lenses each comprises a quarter-pitch GRIN lens .

7. The WDM coupler of claim 3 further comprising a sleeve having a longitudinal aperture holding said ends of said first, second, third, and fourth optical fibers, said aperture having a square cross-section.

8. The WDM coupler of claim 3 further comprising a sleeve having a longitudinal aperture holding said ends of said first, second, third, and fourth optical fibers, said aperture having a circular cross-section.

9. The WDM coupler of claim 3 wherein said ends of first, second, third, and fourth optical fibers are aligned linearly.

10. The WDM coupler of claim 3 wherein said second and third optical fibers are spliced together.

11. An WDM multiplexer comprising an end of a first optical fiber; an end of a second optical fiber; an end of a third optical fiber; an end of a fourth optical fiber; an end of a fifth optical fiber; an end of a sixth optical fiber; an end of a seventh optical fiber; an end of a eighth optical fiber; a first collimating lens having first and second end faces, said first end face of said collimating lens and said ends of said first, second, third, fourth, fifth and sixth optical fibers in close proximity with each other and said first end face; a wavelength-dependent filter proximate said second end face of said first collimating lens, said wavelength- dependent filter reflecting or passing light responsive to wavelength of light incident thereupon, said wavelength- dependent filter, said ends of said first, second, third, fourth, fifth and sixth optical fibers arranged with respect to each other so that light from said first optical fiber end and reflected by said wavelength-dependent filter passes into said second optical fiber end and so that light from said third optical fiber end and reflected by said wavelength- dependent filter passes into said fourth optical fiber end and so that light from said fifth optical fiber end and reflected by said wavelength-dependent filter passes into said sixth optical fiber end, said second and third optical fibers connected so that light passing into said second fiber end passes out from said third optical fiber end, said fourth and fifth optical fibers connected so that light passing into said fourth fiber end passes out from said fifth optical fiber end; and a second collimating lens in the path of said collimated light from said wavelength-dependent filter, said ends of said seventh and eighth optical fibers in close proximity with a first end face of said second collimating lens and arranged so that light from said first optical fiber passed by said wavelength-dependent filter passes into said seventh optical fiber and light from said eighth optical fiber and passed by said wavelength-dependent filter passes into said sixth optical filter; whereby said WDM multiplexer may be operate as an add/drop WDM multiplexer by connecting said first optical fiber as a network input fiber, said sixth optical fiber as a network output fiber, said seventh optical fiber as a drop fiber and said eighth optical fiber as an add fiber.

12. The WDM coupler of claim 11 wherein said wavelength- dependent filter comprises a bandpass filter.

13. The WDM coupler of claim 11 wherein said wavelength-dependent filter comprises a dichroic filter.

14. The WDM coupler of claim 11 wherein said second and third optical fibers are spliced together and said fourth and fifth optical fibers are spliced together.