Abstract:
An optical transceiver module includes N light sources, N light detectors, a bidirectional fiber port, and an optical network having 2N−1 wavelength-selective elements. The number 2N represents the total number of transmit and receive channels in a bidirectional system in which transmit and receive signals corresponding to the transmit and receive channels. Each light source corresponds to one transmit channel and emits an optical transmit signal having a unique transmit wavelength. Each light detector corresponds to one receive channel and detects an optical receive signal having a unique receive wavelength. The optical network couples each light source to the bidirectional fiber port via a corresponding transmit path through the optical network. The optical network further couples each light detector to the bidirectional fiber port via a corresponding receive path through the optical network. Each transmit and receive path includes some of the wavelength-selective elements.

Description:
BACKGROUND 
     In data communications systems, it is often useful to modularize interface electronics and other interface elements in a data communication module. For example, in an optical data communication system, an optical data transceiver module may include a light source such as a laser, and a light detector such as a photodiode, and may also include driver and receiver circuitry associated with the laser and photodiode. The laser and associated circuitry convert electrical signals that the module receives via electrical contacts into optical signals that the module outputs via one or more optical fibers. The photodiode and associated circuitry convert optical signals received via the one or more optical fibers into electrical signals that the module outputs via the electrical contacts. 
     Generally, there are two types of semiconductor laser devices: edge-emitting lasers and Vertical Cavity Surface Emitting Lasers (VCSELs). An advantage of VCSELs is that they can be tested economically at wafer level rather than chip level. Another advantage of VCSELs is their well-defined spot size, which promotes high coupling efficiency to optical fibers without the need to provide beam shape correction, thus facilitating economical packaging. Edge-emitting lasers also have advantages, such as high output optical power. Edge-emitting lasers remain the most commonly used laser in long-distance high-speed optical data transceivers. 
     An optical data transceiver module may be of a bidirectional type that transmits a modulated optical transmit signal having a first wavelength via an optical fiber and receives a modulated optical receive signal having a second wavelength via the same optical fiber. Such a module generally includes a wavelength-selective filter (also referred to as a beam splitter) to separate the transmit signal and the receive signal. 
     Coarse wavelength division multiplexing (CWDM) is a technique by which a single optical fiber can simultaneously carry two or more communication channels, each characterized by a unique wavelength. A CWDM optical transceiver module commonly interfaces with at least one fiber that carries two or more outgoing or transmit channels and at least one other fiber that carries two or more incoming or receive channels. The CWDM optical transceiver modules that are currently commercially available generally have either four or eight channels. One type of optical multiplexer that has been suggested for use in a CWDM optical transceiver module includes four edge-emitting lasers, four corresponding narrowband optical filters, and three reflectors that redirect optical signals from one optical filter to another in a daisy-chain fashion. The four edge-emitting lasers must be precisely aligned so that their emitted signals that bounce among the reflectors and filters are ultimately coupled into the end of the fiber. The multiple bounces that some of the optical signals experience results in significant insertion loss. The difficulty in achieving sufficiently precise laser alignment and filter passbands can result in low manufacturing yield. 
     It would be desirable to provide a wavelength-multiplexed optical transceiver module that has low insertion loss and high manufacturing yield. 
     SUMMARY 
     Embodiments of the present invention relate to an optical transceiver module and method by which it operates. In an exemplary embodiment, an optical transceiver module includes N light sources, N light detectors, a bidirectional optical fiber port connectable to an optical fiber, and an optical network that includes 2N−1 wavelength-selective optical elements, where N is an integer power of two that is greater than or equal to two. The number 2N (which is thus likewise an integer power of two that is greater than or equal to four) represents the total number of transmit and receive channels in a bidirectional system in which transmit and receive signals corresponding to the transmit and receive channels are communicated via the optical fiber. Each light source corresponds to one transmit channel and is configured to emit an optical transmit signal having a unique transmit wavelength, i.e., a wavelength that is different from the transmit wavelengths of all others of the light sources. Each light detector corresponds to one receive channel and is configured to detect an optical receive signal having a unique receive wavelength, i.e., a wavelength that is different from the receive wavelengths of all others of the light detectors. 
     The optical network couples each light source to the bidirectional optical fiber port via a corresponding transmit path through the optical network. The optical network further couples each light detector to the bidirectional optical fiber port via a corresponding receive path through the optical network. Each transmit and receive path includes some of the wavelength-selective elements. 
     In the exemplary embodiment, a method for optical communication in the above-described optical transceiver module includes: each light source emitting an optical transmit signal that corresponds to one transmit channel and that has a transmit wavelength different from the transmit wavelengths of all others of the light sources; each light detector detecting an optical receive signal that corresponds to one receive channel and that has a receive wavelength different from the receive wavelengths of all others of the light detectors; each optical transmit signal propagating through the first optical network from one light source to the first bidirectional optical fiber port via a corresponding transmit path through the optical network; transmitting each optical transmit signal from the bidirectional optical fiber port via the optical fiber coupled to the bidirectional optical fiber port; receiving each optical receive signal at the bidirectional optical fiber port via the optical fiber; and each optical receive signal propagating through the optical network from the bidirectional optical fiber port to one of the light detectors via a corresponding receive path through the first optical network. 
     Other systems, methods, features, and advantages will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the specification, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an optical communication system, in accordance with an exemplary embodiment of the invention. 
         FIG. 2  is a schematic diagram of a transmitter and receiver of another optical communication system. 
         FIG. 3  is a schematic diagram of a transmitter and receiver of the exemplary optical communication system of  FIG. 1 . 
         FIG. 4  is a schematic diagram of another transmitter and receiver of the exemplary optical communication system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIG. 1 , in an illustrative or exemplary embodiment of the invention, an optical communication system  10  includes a first portion  12  located remotely from a second portion  14 . First portion  12  includes a first portion first transceiver  16  and a first portion second transceiver  18 , which can be located together with each other (e.g., within a first transceiver housing). Second portion  14  includes a second portion first transceiver  18 ′ and a second portion second transceiver  16 ′, which can be located together with each other (e.g., within a second transceiver housing). First portion first transceiver  16  and second portion first transceiver  18 ′ are capable of communicating bidirectionally with each other via a single optical fiber  24 . The ends of optical fiber  24  are connected to respective optical fiber ports  21  and  23  of first transceivers  16  and  18 ′. First portion second transceiver  18  and second portion second transceiver  16 ′ are capable of communicating bidirectionally with each other via a single optical fiber  26 . The ends of optical fiber  26  are connected to respective optical fiber ports  25  and  27  of second transceivers  18  and  16 ′. 
     As described in further detail below, in the exemplary embodiment optical communication system  10  has four channels. Each channel is characterized by a unique wavelength, and optical fibers  24  and  26  carry the channels in a wavelength-multiplexed manner. First portion  12  can transmit optical data signals on any of the four channels and receive optical data signals on any of the four channels. Likewise, second portion  14  can transmit optical data signals on any of the four channels and receive optical data signals on any of the four channels. Accordingly, optical communication system  10  can be characterized as a full-duplex data communication system. 
     First portion first transceiver  16  includes two first light sources  28  and  30  and corresponding driver circuits  32  and  34 . Light sources  28  and  30  can be, for example, vertical cavity surface-emitting lasers (VCSELs). Driver circuits  32  and  34  drive light sources  28  and  30 , respectively, thereby causing them to produce optical data signals, in response to electrical data signals. First portion first transceiver  16  also includes two first light detectors  36  and  38  and corresponding receiver circuits  42  and  44 . Light detectors  36  and  38  can be, for example, positive-intrinsic-negative (PIN) diode photodetectors. Receiver circuits  42  and  44  convert and amplify the outputs of light detectors  36  and  38 , respectively, thereby producing electrical data signals, in response to optical data signals. Driver circuits  32  and  34  receive the electrical data signals from an electrical contact array  40 . Receiver circuits  42  and  44  output other electrical data signals to electrical contact array  40 . Optical elements  46  couple the optical transmit signals produced by light sources  28  and  30  into optical fiber  24 . Similarly, optical elements  46  couple optical receive signals from optical fiber  24  into light detectors  36  and  38 . Thus, first light sources  28  and  30  and corresponding driver circuits  32  and  34  provide electrical-to-optical signal conversion, while first light detectors  36  and  38  and corresponding receiver circuits  42  and  44  provide optical-to-electrical signal conversion. 
     First portion second transceiver  18  includes two second light sources  48  and  50  and corresponding driver circuits  52  and  54 . Light sources  48  and  50  can be, for example, vertical cavity surface-emitting lasers (VCSELs). Driver circuits  52  and  54  drive light sources  48  and  50 , respectively, thereby causing them to produce optical data signals, in response to electrical data signals. First portion second transceiver  18  also includes two second light detectors  56  and  58  and corresponding receiver circuits  62  and  64 . Light detectors  56  and  58  can be, for example, PIN diode photodetectors. Receiver circuits  62  and  64  convert and amplify the outputs of light detectors  56  and  58 , respectively, thereby producing electrical data signals, in response to optical data signals. Driver circuits  52  and  54  receive the electrical data signals from electrical contact array  60 . Receiver circuits  62  and  64  output other electrical data signals to electrical contact array  60 . Optical elements  66  couple the optical transmit signals produced by light sources  48  and  50  into optical fiber  26 . Similarly, optical elements  66  couple optical receive signals from optical fiber  26  into light detectors  56  and  58 . Thus, second light sources  48  and  50  and corresponding driver circuits  52  and  54  provide electrical-to-optical signal conversion, while second light detectors  56  and  58  and corresponding receiver circuits  62  and  64  provide optical-to-electrical signal conversion. 
     Each of light sources  28 ,  30 ,  48  and  50  corresponds to one of the four channels and is configured to emit an optical transmit signal having a unique transmit wavelength corresponding to that channel. That is, the transmit wavelength emitted by each of light sources  28 ,  30 ,  48  and  50  is different from the other transmit wavelengths emitted by the others of light sources  28 ,  30 ,  48  and  50 . The wavelengths emitted by light sources  28 ,  30 ,  48  and  50  can be referred to for illustrative purposes as a first wavelength (λ1), a second wavelength (λ2), a third wavelength (λ3), and a fourth wavelength (λ4). Although in the exemplary embodiment first portion  12  of optical communication system  10  has four channels, embodiments can more generally have 2N channels, where N is a number indicating the number of light sources (e.g., two in the exemplary embodiment) in each of first and second transceivers  16  and  18 . Note that 2N thus indicates the total number of channels (i.e., the number of transmit channels plus the number of receive channels) accommodated by each of first and second transceivers  16  and  18 . Preferably, 2N is a power of two greater than or equal to four. Thus, although in the exemplary embodiment first portion  12  of optical communication system  10  has four channels, in other embodiments such an optical communication system alternatively can have, for example, eight, 16 or 32, etc., channels. 
     Each of light detectors  36 ,  38 ,  56  and  58  also corresponds to one of the four channels and is configured to be capable of detecting an optical receive signal having a unique receive wavelength corresponding to that channel. The wavelengths detectable by light detectors  56 ,  58 ,  36  and  38  are, respectively, the first wavelength (λ1), the second wavelength (λ2), the third wavelength (λ3), and the fourth wavelength (λ4). 
     Significantly, note that in first portion first transceiver  16 , light sources  28  and  30  are configured to emit the first and second wavelengths, while light detectors  36  and  38  are configured to detect the third and fourth wavelengths. Likewise, note that in first portion second transceiver  18 , light sources  48  and  50  are configured to emit the third and fourth wavelength, while light detectors  56  and  58  are configured to detect the first and second wavelengths. Stated another way, the transmit and receive wavelengths of first transceiver  16  and second transceiver  18  are complementary. 
     Second portion  14  of optical communication system  10  is identical to first portion  12  of optical communication system  10 . Accordingly, second portion first transceiver  18 ′ is identical to first portion second transceiver  18 , and second portion second transceiver  16 ′ is identical to first portion first transceiver  16 . Note that the transmit and receive wavelengths of first transceiver  18 ′ and second transceiver  16 ′ are complementary. 
     As second portion first transceiver  18 ′ is identical to first portion second transceiver  18 , and second portion second transceiver  16 ′ is identical to first portion first transceiver  16 , their elements are not described in similar detail. Rather, it is sufficient to note that: light sources  28 ′,  30 ′,  48 ′ and  50 ′ are identical to light sources  28 ,  30 ,  48  and  50 , respectively; driver circuits  32 ′,  34 ′,  52 ′ and  54 ′ are identical to driver circuits  32 ,  34 ,  52  and  54 , respectively; light detectors  36 ′,  38 ′,  56 ′ and  58 ′ are identical to light detectors  36 ,  38 ,  56  and  58 , respectively; receiver circuits  42 ′,  44 ′,  62 ′ and  64 ′ are identical to receiver circuits  42 ,  44 ,  62  and  64 , respectively; optical elements  46 ′ and  66 ′ are identical to optical elements  46  and  66 , respectively; and electrical contact arrays  40 ′ and  60 ′ are identical to electrical contact arrays  40  and  60 , respectively. 
     First portion first transceiver  16  is coupled via optical fiber  24  to second portion first transceiver  18 ′, and first portion second transceiver  18  is coupled via optical fiber  26  to second portion second transceiver  16 ′. As described above, first portion first transceiver  16  can transmit optical data signals modulated on the first and second wavelengths and receive optical data signals modulated on the third and fourth wavelengths, while first portion second transceiver  18  can transmit optical data signals modulated on the third and fourth wavelengths and receive optical data signals modulated on the first and second wavelengths. Due to this complementary configuration of transmit and receive wavelengths, first portion first transceiver  16  can receive optical data signals modulated on the third and fourth wavelengths from second portion first transceiver  18 ′, while second portion first transceiver  18 ′ can receive optical data signals modulated on the first and second wavelengths from first portion first transceiver  16 . Note that this arrangement of first portion first transceiver  16  and second portion first transceiver  18 ′ defines half of a full-duplex optical data communication system. Conversely, first portion second transceiver  18  can receive optical data signals modulated on the first and second wavelengths from second portion second transceiver  16 ′, while second portion second transceiver  16 ′ can receive optical data signals modulated on the third and fourth wavelengths from first portion second transceiver  18 . Note that this arrangement of first portion second transceiver  18  and second portion second transceiver  16 ′ defines another half of a full-duplex optical data communication system, and that the entire optical data communication system  10  is accordingly full duplex. The significance of the above-described system in which each communicating transceiver pair uses complementary transmit and receive wavelengths will become apparent from the following. 
     Consider the four-channel CWDM optical data communication system  68  shown in  FIG. 2 , which includes a transmitter  70  coupled to a receiver  72  by an optical fiber  74 . Although not shown for purposes of clarity, such an optical data communication system can further include another receiver similar to receiver  72  that is co-located with transmitter  70 , and another transmitter similar to transmitter  70  that is co-located with receiver  72 , so that data can be transmitted and received on four channels between two locations. 
     Transmitter  70  includes four light sources  76 ,  78 ,  80  and  82 , such as VCSELs, which can be mounted on a suitable substrate (not shown) such as a printed circuit board. Driver circuitry of the type commonly included in optical transmitters is included in transmitter  70  but not shown for purposes of clarity. Light sources  76 ,  78 ,  80  and  82  are configured to respectively emit a first wavelength (λ1), a second wavelength (λ2), a third wavelength (λ3), and a fourth wavelength (λ4). A first wavelength-selective filter  84 , a second wavelength-selective filter  86 , and a third wavelength-selective filter  88  direct the optical signals emitted by light sources  76 ,  78 ,  80  and  82  along respective optical transmit paths terminating at the entrance to optical fiber  74 . The optical transmit and receive paths also include lenses  90 ,  92 ,  94 ,  96  and  98 . 
     A first optical transmit path exists between light source  76  and optical fiber  74  via first wavelength-selective filter  84  and third wavelength-selective filter  88 . First wavelength-selective filter  84  is substantially reflective to the first wavelength (λ1), and third wavelength-selective filter  88  is substantially transparent to the first wavelength. Accordingly, optical signals emitted by light source  76  are transmitted through lens  90  and are reflected by first wavelength-selective filter  84  toward third wavelength-selective filter  88 . These optical signals that are reflected by first wavelength-selective filter  84  are transmitted through third wavelength-selective filter  88  and through lens  98  into the end of optical fiber  74 . 
     A second optical transmit path exists between light source  78  and optical fiber  74  via first wavelength-selective filter  84  and third wavelength-selective filter  88 . First wavelength-selective filter  84  is substantially transparent to the second wavelength (λ2), and third wavelength-selective filter  88  is substantially transparent to the second wavelength. Accordingly, optical signals emitted by light source  78  are transmitted through lens  92  and through first wavelength-selective filter  84  toward third wavelength-selective filter  88 . These optical signals that are transmitted through first wavelength-selective filter  84  are further transmitted through third wavelength-selective filter  88  and through lens  98  into the end of optical fiber  74 . 
     A third optical transmit path exists between light source  80  and optical fiber  74  via second wavelength-selective filter  86  and third wavelength-selective filter  88 . Second wavelength-selective filter  86  is substantially reflective to the third wavelength (λ3), and third wavelength-selective filter  88  is substantially reflective to the third wavelength. Accordingly, optical signals emitted by light source  80  are transmitted through lens  94  and then reflected by second wavelength-selective filter  86  toward third wavelength-selective filter  88 . These optical signals that are reflected by second wavelength-selective filter  86  are further reflected by third wavelength-selective filter  88  and through lens  98  into the end of optical fiber  74 . 
     A fourth optical transmit path exists between light source  82  and optical fiber  74  via second wavelength-selective filter  86  and third wavelength-selective filter  88 . Second wavelength-selective filter  86  is substantially transparent to the fourth wavelength (λ4), and third wavelength-selective filter  88  is substantially reflective to the fourth wavelength. Accordingly, optical signals emitted by light source  82  are transmitted through lens  94  and then transmitted through second wavelength-selective filter  86  toward third wavelength-selective filter  88 . These optical signals that are transmitted through second wavelength-selective filter  86  are then reflected by third wavelength-selective filter  88  and through lens  98  into the end of optical fiber  74 . 
     Transmitter  70  poses a potential manufacturing challenge. In a repetitive manufacturing process that uses known opto-electronic device manufacturing techniques, it would be difficult to consistently produce physical embodiments of transmitter  70  in which all four light sources  76 ,  78 ,  80  and  82  are optically aligned in their optical transmit paths with sufficient accuracy to allow transmitter  70  to operate properly, i.e., to reliably transmit data on each of the four channels. This consistency is commonly referred to in a manufacturing context as “yield.” Stated another way, in a manufacturing process in which a certain number of devices are produced in the exact same manner as each other, it is desirable to maximize the proportion of those devices that operate properly (the remainder that do not operate properly being deemed defective or unusable for their intended purpose). Achieving sufficiently accurate optical alignment is challenging for a manufacturing process because VCSELs (i.e., light sources  76 ,  78 ,  80  and  82 ) have a small spot size and a large numerical aperture, and optical fiber  74  has a small acceptance angle. Deviation from alignment of any one of light sources  76 ,  78 ,  80  and  82  beyond a very small tolerance range (e.g., only a few microns) results in coupling loss that adversely impacts communications quality. If even only one out of the four light sources  76 ,  78 ,  80  and  82  is not aligned within the tolerance range, the entire transmitter  70  device is deemed defective and unusable, thereby impacting yield. For a manufacturing process to achieve alignment of all four light sources  76 ,  78 ,  80  and  82  within the tolerance range is roughly four times less likely than achieving alignment of only one such light source. Thus, the yield of a process for manufacturing the four-channel CWDM transmitter  70  could reasonably be expected to be only one-fourth the yield of a similar process used for manufacturing a single-channel optical transmitter. Embodiments of the present invention address this potential manufacturing challenge. Receiver  72  does not pose a similarly great challenge for the manufacturing process because PIN photodiodes have large active areas and wide optical acceptance angles and thus have wide tolerance to deviation of the optical signals from the optical path. Nevertheless, for the sake of completeness, receiver  72  is now described in similar detail to transmitter  70 . 
     Receiver  72  includes four light detectors  100 ,  102 ,  104  and  106 , such as PIN photodiodes, which can be mounted on a suitable substrate (not shown) such as a printed circuit board. Receiver circuitry of the type commonly included in optical receivers is included in receiver  72  but not shown for purposes of clarity. Light detectors  100 ,  102 ,  104  and  106  are configured to respectively detect the first wavelength (λ1), the second wavelength (λ2), the third wavelength (λ3), and the fourth wavelength (λ4). A first wavelength-selective filter  108 , a second wavelength-selective filter  110 , and a third wavelength-selective filter  112  direct the optical signals received through optical fiber  74  through respective optical receive paths to each of light detectors  100 ,  102 ,  104  and  106 . The optical receive paths also include lenses  114 ,  116 ,  118 ,  120  and  122 . 
     A first optical receive path exists between light detector  100  and optical fiber  74  via third wavelength-selective filter  112  and first wavelength-selective filter  108 . Third wavelength-selective filter  112  is substantially transparent to the first wavelength (λ1), and first wavelength-selective filter  108  is substantially reflective to the first wavelength. Accordingly, optical signals of the first wavelength emitted from the end of optical fiber  74  are transmitted through lens  122  and through third wavelength-selective filter  112  toward first wavelength-selective filter  108 . These optical signals are then reflected by first wavelength-selective filter  108  and through lens  114  onto light detector  100 . 
     A second optical receive path exists between light detector  102  and optical fiber  74  via third wavelength-selective filter  112  and first wavelength-selective filter  108 . Third wavelength-selective filter  112  is substantially transparent to the second wavelength (λ2), and first wavelength-selective filter  108  is substantially transparent to the second wavelength. Accordingly, optical signals of the second wavelength emitted from the end of optical fiber  74  are transmitted through lens  122  and through third wavelength-selective filter  112  toward first wavelength-selective filter  108 . These optical signals are further transmitted through first wavelength-selective filter  108  and through lens  116  onto light detector  102 . 
     A third optical receive path exists between light detector  104  and optical fiber  74  via third wavelength-selective filter  112  and second wavelength-selective filter  110 . Third wavelength-selective filter  112  is substantially reflective to the third wavelength (λ3), and second wavelength-selective filter  110  is substantially reflective to the third wavelength. Accordingly, optical signals of the third wavelength emitted from the end of optical fiber  74  are transmitted through lens  122  and then reflected by third wavelength-selective filter  112  toward second wavelength-selective filter  110 . These optical signals are further reflected by second wavelength-selective filter  110  and through lens  118  onto light detector  104 . 
     A fourth optical receive path exists between light detector  106  and optical fiber  74  via third wavelength-selective filter  112  and second wavelength-selective filter  110 . Third wavelength-selective filter  112  is substantially reflective to the fourth wavelength (λ4), and second wavelength-selective filter  110  is substantially transparent to the fourth wavelength. Accordingly, optical signals of the fourth wavelength emitted from the end of optical fiber  74  are transmitted through lens  122  and then reflected by third wavelength-selective filter  112  toward second wavelength-selective filter  110 . These optical signals are then transmitted through second wavelength-selective filter  110  and through lens  120  onto light detector  106 . 
     Optical communication system  10 , described above with regard to  FIG. 1 , addresses the above-described manufacturing yield problem. First portion first transceiver  16  and second portion first transceiver  18 ′ are shown in further detail in  FIG. 3 , while first portion second transceiver  18  and second portion second transceiver  16 ′ are shown in further detail in  FIG. 4 . Electrical connector arrays  40 ,  60 ,  40 ′ and  60 ′, driver circuits  32 ,  34 ,  52 ,  54 ,  32 ′,  34 ′,  52 ′ and  54 ′, and receiver circuits  42 ,  44 ,  62 ,  64 ,  42 ′,  44 ′,  62 ′ and  64 ′ ( FIG. 1 ) are not shown in  FIGS. 3-4  for purposes of clarity. 
     As illustrated in  FIG. 3 , optical elements  46  ( FIG. 1 ) of first portion first transceiver  16  includes, in addition to light sources  28  and  30  and light detectors  36  and  38 , a first wavelength-selective filter  124 , a second wavelength-selective filter  126 , and a third wavelength-selective filter  128 . First, second and third wavelength-selective filters  124 - 126  direct the optical signals emitted by light sources  28  and  30  along respective optical transmit paths terminating at a first end of optical fiber  24  and direct optical signals emitted from the first end of optical fiber  24  along respective optical receive paths terminating at respective ones of light detectors  36  and  38 . Wavelength-selective filters  124 ,  126  and  128  can comprise, for example, thin-film dielectric coatings on a glass or similarly optically transparent substrate. Wavelength-selective filters  124 ,  126  and  128  can be of a high-pass filter or low-pass filter type. The optical paths also include lenses  130 ,  132 ,  134 ,  136  and  138 . Light sources  28  and  30 , which can be VCSELs, and light detectors  36  and  38 , which can be PIN photodiodes, can be mounted on a suitable substrate (not shown) such as a printed circuit board. 
     In first portion first transceiver  16 , a first optical transmit path exists between light source  28  and optical fiber  24  via first wavelength-selective filter  124  and third wavelength-selective filter  128 . First wavelength-selective filter  124  is substantially reflective to the first wavelength (λ1), and third wavelength-selective filter  128  is substantially transparent to the first wavelength (λ1). Accordingly, optical signals emitted by light source  28  are transmitted through lens  130  and are reflected by first wavelength-selective filter  124  toward third wavelength-selective filter  128 . These optical signals that are reflected by first wavelength-selective filter  124  are transmitted through third wavelength-selective filter  128  and through lens  138  into the end of optical fiber  74 . 
     In first portion first transceiver  16 , a second optical transmit path exists between light source  30  and optical fiber  24  via first wavelength-selective filter  124  and third wavelength-selective filter  128 . First wavelength-selective filter  124  is substantially transparent to the second wavelength (λ2), and third wavelength-selective filter  128  is substantially transparent to the second wavelength (λ2). Accordingly, optical signals emitted by light source  30  are transmitted through lens  132  and through first wavelength-selective filter  124  toward third wavelength-selective filter  128 . These optical signals that are transmitted through first wavelength-selective filter  124  are further transmitted through third wavelength-selective filter  128  and through lens  138  into the end of optical fiber  24 . 
     In first portion first transceiver  16 , a first optical receive path exists between light detector  36  and optical fiber  24  via third wavelength-selective filter  128  and second wavelength-selective filter  126 . Third wavelength-selective filter  128  is substantially reflective to the third wavelength (λ3), and second wavelength-selective filter  126  is substantially reflective to the third wavelength (λ3). Accordingly, optical signals of the third wavelength emitted from the end of optical fiber  24  are transmitted through lens  138  and then reflected by third wavelength-selective filter  128  toward second wavelength-selective filter  126 . These optical signals are further reflected by second wavelength-selective filter  126  and through lens  134  onto light detector  36 . 
     In first portion first transceiver  16 , a second optical receive path exists between light detector  38  and optical fiber  24  via third wavelength-selective filter  128  and second wavelength-selective filter  126 . Third wavelength-selective filter  128  is substantially reflective to the fourth wavelength (λ4), and second wavelength-selective filter  126  is substantially transparent to the fourth wavelength (λ4). Accordingly, optical signals of the fourth wavelength emitted from the end of optical fiber  24  are transmitted through lens  138  and then reflected by third wavelength-selective filter  128  toward second wavelength-selective filter  126 . These optical signals are then transmitted through second wavelength-selective filter  126  and through lens  136  onto light detector  38 . 
     Although in the exemplary embodiment wavelength-selective filters  124 ,  126  and  128  are aligned at 45-degree angles with respect to the optical paths, in other embodiments such wavelength-selective filters can be aligned at any other suitable angle with respect to one or more optical paths. Also, in other embodiments the optical paths in such a first transceiver can include more or fewer optical elements than in the exemplary first portion first transceiver  16  shown in  FIG. 3 , such as additional lenses, reflectors, etc. The optical paths in such other embodiments thus can have configurations other than those shown in  FIG. 3 , such as additional turns, zig-zags, etc. 
     As further illustrated in  FIG. 3 , optical elements  66 ′ ( FIG. 1 ) of second portion first transceiver  18 ′ includes, in addition to light sources  48 ′ and  50 ′ and light detectors  56 ′ and  58 ′, a first wavelength-selective filter  140 , a second wavelength-selective filter  142 , and a third wavelength-selective filter  144 . First through third wavelength-selective filters  140 - 144  direct the optical signals emitted by light sources  48 ′ and  50 ′ along respective optical transmit paths terminating at the second end of optical fiber  24  and direct optical signals emitted from the second end of optical fiber  24  along respective optical receive paths terminating at respective ones of light detectors  56 ′ and  58 ′. Wavelength-selective filters  140 ,  142  and  144  can comprise, for example, thin-film dielectric coatings on a glass or similarly optically transparent substrate. Wavelength-selective filters  140 ,  142  and  144  can be of a high-pass filter or low-pass filter type. The optical paths also include lenses  146 ,  148 ,  150 ,  152  and  154 . Light sources  48 ′ and  50 ′, which can be VCSELs, and light detectors  56 ′ and  58 ′, which can be PIN photodiodes, can be mounted on a suitable substrate (not shown) such as a printed circuit board. 
     In second portion first transceiver  18 ′, a first optical transmit path exists between light source  48 ′ and optical fiber  24  via second wavelength-selective filter  142  and third wavelength-selective filter  144 . Second wavelength-selective filter  142  is substantially reflective to the third wavelength (λ3), and third wavelength-selective filter  144  is substantially reflective to the third wavelength. Accordingly, optical signals emitted by light source  48 ′ are transmitted through lens  150  and are reflected by second wavelength-selective filter  142  toward third wavelength-selective filter  144 . These optical signals that are reflected by second wavelength-selective filter  142  are further reflected by third wavelength-selective filter  144  and through lens  154  into the end of optical fiber  24 . 
     In second portion first transceiver  18 ′, a second optical transmit path exists between light source  50 ′ and optical fiber  24  via second wavelength-selective filter  142  and third wavelength-selective filter  144 . Second wavelength-selective filter  142  is substantially transparent to the fourth wavelength (λ4), and third wavelength-selective filter  144  is substantially reflective to the fourth wavelength. Accordingly, optical signals emitted by light source  50 ′ are transmitted through lens  152  and through second wavelength-selective filter  142  toward third wavelength-selective filter  144 . These optical signals that are transmitted through second wavelength-selective filter  142  are reflected by third wavelength-selective filter  144  and through lens  154  into the end of optical fiber  24 . 
     In second portion first transceiver  18 ′, a first optical receive path exists between light detector  56 ′ and optical fiber  24  via third wavelength-selective filter  144  and first wavelength-selective filter  140 . Third wavelength-selective filter  144  is substantially transparent to the first wavelength (λ1), and first wavelength-selective filter  140  is substantially reflective to the first wavelength. Accordingly, optical signals of the first wavelength emitted from the end of optical fiber  24  are transmitted through lens  154  and through third wavelength-selective filter  144  toward first wavelength-selective filter  140 . These optical signals are further reflected by first wavelength-selective filter  140  and through lens  146  onto light detector  56 ′. 
     In second portion first transceiver  18 ′, a second optical receive path exists between light detector  58 ′ and optical fiber  24  via third wavelength-selective filter  144  and first wavelength-selective filter  140 . Third wavelength-selective filter  144  is substantially transparent to the second wavelength (λ2), and first wavelength-selective filter  140  is substantially transparent to the second wavelength. Accordingly, optical signals of the second wavelength emitted from the end of optical fiber  24  are transmitted through lens  154  and through third wavelength-selective filter  144  toward first wavelength-selective filter  140 . These optical signals are then transmitted through first wavelength-selective filter  140  and through lens  148  onto light detector  58 ′. 
     Although in the exemplary embodiment wavelength-selective filters  140 ,  142  and  144  are aligned at 45-degree angles to the optical paths, in other embodiments such wavelength-selective filters can be aligned at any other angle to one or more optical paths. Also, in other embodiments the optical paths in such a first transceiver can include more or fewer optical elements than in the exemplary second portion first transceiver  18 ′ shown in  FIG. 3 , such as additional lenses, reflectors, etc. The optical paths in such other embodiments thus can have configurations other than those shown in  FIG. 3 , such as additional turns, zig-zags, etc. 
     First portion second transceiver  18  and second portion second transceiver  16 ′ are illustrated in further detail in  FIG. 4 . As first portion second transceiver  18  is identical to above-described second portion first transceiver  18 ′, its elements and operation are not described in similar detail. Rather, it is sufficient to note that: first through third wavelength-selective filters  140 ′,  142 ′ and  144 ′ are identical to above-described first through third wavelength-selective filters  140 ,  142  and  144 , respectively, and lenses  146 ′,  148 ′,  150 ′,  152 ′ and  154 ′ are identical to above-described lenses  146 ,  148 ,  150 ,  152  and  154 . The optical transmit paths and optical receive paths through first portion second transceiver  18  are identical to the above-described optical transmit and receive paths through second portion first transceiver  18 ′. Likewise, as second portion second transceiver  16 ′ is identical to above-described first portion first transceiver  16 , its elements and operation are not described in similar detail. Rather, it is sufficient to note that: first through third wavelength-selective filters  124 ′,  126 ′ and  128 ′ are identical to above-described first through third wavelength-selective filters  124 ,  126  and  128 , respectively, and lenses  130 ′,  132 ′,  134 ′,  136 ′ and  138 ′ are identical to above-described lenses  130 ,  132 ,  134 ,  136  and  138 . The optical transmit paths and optical receive paths through second portion second transceiver  16 ′ are identical to the above-described optical transmit and receive paths through first portion first transceiver  16 . 
     A number of characteristics of the exemplary optical communication system  10  can be noted. First, it can be noted in  FIG. 3  that first portion first transceiver  16  includes only two light sources  28  and  30 , and second portion first transceiver  18 ′ includes only two light sources  48 ′ and  50 ′, yet first portion first transceiver  16  and second portion first transceiver  18 ′ communicate a total of four channels of optical signals bidirectionally via optical fiber  24 . That is, from the perspective of first portion first transceiver  16  there are two transmit channels, one corresponding to the first wavelength (λ1) and the other corresponding to the second wavelength (λ2), plus two receive channels, one corresponding to the third wavelength (λ3) and the other corresponding to the fourth wavelength (λ4), thus totaling four (transit and receive) channels. Similarly, from the perspective of second portion first transceiver  18 ′ there are two transmit channels, one corresponding to the third wavelength (λ3) and the other corresponding to the fourth wavelength (λ4), plus two receive channels, one corresponding to the first wavelength (λ1) and the other corresponding to the second wavelength (λ2), thus totaling four (transit and receive) channels. First portion first transceiver  16  and second portion first transceiver  18 ′ configured in the above-described manner to communicate optical signals bidirectionally with each other via optical fiber  24  defines on half of a full-duplex communication link. A full-duplex communication link is defined by first portion first transceiver  16  and second portion first transceiver  18 ′ configured in the above-describe manner to communicate optical signals bidirectionally with each other via optical fiber  24  in combination with first portion second transceiver  18  and second portion second transceiver  16 ′ configured in the above-describe manner to communicate optical signals bidirectionally with each other via optical fiber  26 . 
     Second, but significantly, it can be noted that although optical communication system  10  ( FIGS. 1 ,  3  and  4 ) has four channels, each of transceivers  16  and  18  (and thus also each of transceivers  16 ′ and  18 ′) potentially can be manufactured with higher yield than transmitter  70  of the four-channel optical communication system  68  described above with regard to  FIG. 2 . As described above, current manufacturing processes cannot consistently (i.e., with high yield) produce physical embodiments of transmitter  70  in which all four light sources  76 ,  78 ,  80  and  82  are aligned within the tolerance required to enable transmitter  70  to operate properly. In contrast, current manufacturing processes have the potential to consistently (i.e., with high yield) produce physical embodiments of transceivers  16  and  18  (and thus transceivers  16 ′ and  18 ′) because there are only half as many light sources in each of transceivers  16  and  18  as in transmitter  70 . Again, note that although there are only half as many light sources in each of transceivers  16  and  18  as in transmitter  70 , each of transceivers  16  and  18  handles just as many communication channels as transmitter  70  handles (i.e., four). 
     Third, it can be noted that the above-described configuration of transceivers  16  and  18  helps minimize insertion loss. As each of the wavelength-selective filters is either a high-pass filter or a low-pass filter, each wavelength-selective filter can readily either split the optical signals that are incident upon it from a single direction into two directions or, alternatively, combine optical signals that are incident upon it from two directions into a single direction. Accordingly, in a transceiver having N light sources and N light detectors, an optical transmit signal following a transmit path from a light source to the optical fiber or an optical receive signal following a receive path from the optical fiber to a light detector interacts with (i.e., is either transmitted through or reflected by) no more than log 2 (N)+1 wavelength-selective high-pass or low-pass filters. Minimizing the number of optical elements in a transmit path or receive path helps minimize insertion loss. Furthermore, high-pass and low-pass wavelength-selective filters are more economical and have wider alignment tolerance than narrow bandpass filters. 
     It should be noted that the invention has been described with respect to illustrative embodiments for the purpose of describing the principles and concepts of the invention. The invention is not limited to these embodiments. As will be understood by those skilled in the art in view of the description being provided herein, many modifications may be made to the embodiments described herein without deviating from the goals of the invention, and all such modifications are within the scope of the invention.