Abstract:
An optical multiplexer and demultiplexer for dense wavelength division multiplexed (“DWDM”) fiber optic communication systems is disclosed. As a multiplexer, the device functions to spatially combine the optical signals from several laser sources (each of which is a different wavelength) and launch the spatially combined laser beams into a single optical fiber. As a demultiplexer, the device functions to spatially separate the different wavelengths of a wavelength division multiplexed optical link and launch each of the different wavelengths into a different optical fiber. In either embodiment, the device includes both bulk optic and integrated optic components. The spatial separation or spatial combination of laser beams of different wavelength is achieved with the use of bulk diffraction gratings. Also, bulk optical components are used to collimate and shape (or steer) the free space propagating laser beams to enable efficient coupling of light into single mode optical fibers, or integrated optic waveguides, and to reduce optical cross talk. Polarizing beamsplitters orient the polarization direction of the light to enable maximum diffraction efficiency by the gratings and to reduce the polarization dependent loss. Further, the end faces of optical fibers and integrated optic waveguides are angle polished to reduce back reflection and thereby reduce noise caused by feedback to the laser source. Preferably, the diffraction grating and focusing optics are specified to permit multiplexing and demultiplexing of laser wavelengths separated by 0.4 nanometers (nm) in the 1550 nm wavelength band. The preferred field of view of the optics permit multiplexing and demultiplexing of up to 32-48 wavelength channels separated by 0.4 nanometers in the 1550 nm wavelength band. Although examples of performance are provided for the 1550 nm optical wavelength band, the device components can be designed for use at other wavelength bands, e.g., the optical fiber low absorption loss band at λ˜1310 nm.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending application Ser. No. 09/023,258, filed Feb. 13, 1998, titled “MULTIPLEXER AND DEMULTIPLEXER FOR SINGLE MODE OPTICAL FIBER COMMUNICATION LINKS”. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to an optic device and more particularly to an optical multiplexer and demultiplexer for dense wavelength division multiplexed (“DWDM”) fiber optic communication systems. 
     BACKGROUND 
     The impact of advances in photonics technology in the area of communication systems has been dramatic. By way of example, new communication system architectures have been proposed based on such photonics technology. These communication architectures take advantage of the ability of optical fibers to carry very large amounts of information—with very little marginal cost once the optical fiber is in place. 
     Photonics communication system architectures based on optical wavelength division multiplexing (WDM) or optical frequency division multiplexing (coherent techniques) to increase the information carrying potential of the optical fiber systems are being developed. For WDM systems, a plurality of lasers are used with each laser emitting a different wavelength. In these types of systems, devices for multiplexing and demultiplexing the optical signals into or out of a single optical fiber are required. Early WDM systems used a wide wavelength spacing between channels. For example, the bandwidth of a λ=1310 nm link was increased by adding a 1550 nm channel. Fiber optic directional coupler technology was used to multiplex such widely spaced wavelength channels. Since optical fiber system performance is best when optimized for use at a single wavelength window, optimum WDM systems use several closely spaced wavelengths within a particular wavelength window. Currently, the telecommunications industry is working towards the deployment of dense wavelength division multiplexed (DWDM) systems with up to 32 channels in the 1530 to 1565 nm wavelength window—with adjacent channels separated in wavelength by 8 angstroms (100 GHz optical frequency spacing). Future developments envision channel wavelength separations of 4 angstroms (50 GHz optical frequency spacing). 
     Several technologies are being developed to provide for DWDM. These include micro-optical devices, integrated optic devices, and fiber optic devices. Micro-optical devices use optical interference filters and diffraction gratings to combine and separate different wavelengths. Integrated optic devices utilize optical waveguides of different lengths to introduce phase differences so that optical interference effects can be used to spatially separate different wavelengths. Fiber optic devices utilize Bragg gratings fabricated within the light guiding regions of the fiber to reflect narrow wavelength bands. 
     Micro-optical devices utilizing diffraction devices have been proposed in the literature (See, e.g., W. J. Tomlinson, Applied Optics, vol. 16, no. 8, pp. 2180-2194, 1977; J. P. Laude,  Technical Digest of the Third Integrated Optics and Optical Fiber Communication Conference  San Francisco, 1981, pp. 66-67; R. Watanabe et. al., Electronics Letters, vol. 16, no. 3, pp. 106-107, 1980; Y. Fujii et. al., Applied Optics, vol. 22, no. 7, pp. 974-978, 1983). These references describe generally how diffraction gratings can be used for WDM. However, to meet the needs of DWDM fiber optic communication systems, high performance is required with respect to parameters such as polarization dependent loss, cross talk, return loss, and insertion loss. In order to meet the specifications for these DWDM performance parameters, the incorporation of additional optical elements to effectively use the wavelength multiplexing and demultiplexing capabilities possible with diffraction gratings is required. 
     Therefore, there arises a need for a high performance optical apparatus and method for use in a DWDM system. The present invention directly addresses and overcomes the shortcomings of the prior art by providing DWDM with low polarization dependent loss (&lt;0.5 dB), low insertion loss with single mode fiber optic systems (&lt;5 dB), low cross talk between wavelength channels (&lt;35 dB for 100 GHz channel separation and &lt;30 dB for 50 GHz channel separation), and low return loss (&lt;55 dB). 
     SUMMARY 
     The present invention provides for an optical multiplexer and demultiplexer for dense wavelength division multiplexed (“DWDM”) fiber optic communication systems. In one preferred embodiment of the present invention, a device may be constructed in accordance with the principles of the present invention as a multiplexer. This device functions to spatially combine the optical signals from several laser sources (each of which is a different wavelength) and launch the spatially combined laser beams into a single optical fiber. In a second preferred embodiment of the present invention, a device may be constructed in accordance with the principles of the present invention as a demultiplexer. Here the device functions to spatially separate the different wavelengths of a wavelength division multiplexed optical link and launch each of the different wavelengths into a different optical fiber. 
     In the preferred embodiments described herein, the device includes both bulk optic and integrated optic components. The spatial separation or spatial combination of laser beams of different wavelength is achieved with the use of bulk diffraction gratings. Also, bulk optical components are used to collimate and shape (or steer) the free space propagating laser beams to enable efficient coupling of light into single mode optical fibers, or integrated optic waveguides, and to reduce optical cross talk. Polarizing beamsplitters orient the polarization direction of the light to enable maximum diffraction efficiency by the gratings and to reduce the polarization dependent loss. 
     Another feature of the present invention is that the end faces of optical fibers and integrated optic waveguides are angle polished to reduce back reflection and thereby reduce noise caused by feedback to the laser source. Preferably, the diffraction grating and focusing optics are specified to permit multiplexing and demultiplexing of laser wavelengths separated by 0.4 nanometers (nm) in the 1550 nm wavelength band. The preferred field of view of the optics permit multiplexing and demultiplexing of up to 32-48 wavelength channels separated by 0.4 nanometers in the 1550 nm wavelength band. Although examples of performance are provided for the 1550 nm optical wavelength band, the device components can be designed for use at other wavelength bands, e.g., the optical fiber low absorption loss band at λ˜1310 nm. 
     Therefore, according to one aspect of the invention, there is provided a bi-directional optical apparatus, of the type which is used in connection with optical signals generated by a plurality of laser sources and which is carried by optical fibers, the apparatus comprising: an optical fiber; multiplexer means for spatially combining the optical signals from several laser sources, each of which is a different wavelength, and launching the spatially combined optical signals into a single optical fiber to form a wavelength division multiplexed optical signal; and demultiplexer means for spatially separating the different wavelengths from a single optical fiber carrying a wavelength division multiplexed optical signal and launching each of the different wavelengths into a separate optical fiber. 
     According to another aspect of the invention, there is provided a bi-directional optical apparatus, comprising: means for collimating the plurality of optical signals of different wavelength; means for splitting the plurality of optical wavelength signals into two parallel propagating beams which are polarized perpendicular to each other; means for rotating the polarization direction of one of the beams by 90° so that both beams at each wavelength are polarized in the same direction; means for expanding the diameter of the collimated beams in the direction parallel to the polarization direction; means for diffracting each of the different wavelengths into a different angular direction relative to a defined direction; means for reducing the expanded diameter of the collimated beams in the direction parallel to the polarization direction; means for recombining the two beams for each wavelength into a single beam for each wavelength, and wherein the recombined beams have two mutually perpendicular polarization components and each recombined beam is propagating in a different angular direction relative to an optic axis; means for focusing each beam of different wavelength to a different spatial location along a line in the focal plane of the focusing means; and means for receiving the focused optical signals at each wavelength and launching the individual signals into separate optical fibers. 
     According to another aspect of the invention, there is provided a bi-directional optical apparatus, comprising: means for collimating the plurality of optical signals of different wavelength; means for splitting the plurality of optical wavelength signals into two parallel propagating beams which are polarized perpendicular to each other; means for rotating the polarization direction of one of the beams by 90° so that both beams at each wavelength are polarized in the same direction; means for steering the propagation direction of the collimated beams; means for diffracting each of the different wavelengths into a different angular direction relative to a defined direction; means for recombining the two beams for each wavelength into a single beam for each wavelength, and wherein the recombined beams have two mutually perpendicular polarization components and each recombined beam is propagating in a different angular direction relative to an optic axis; means for focusing each beam of different wavelength to a different spatial location along a line in the focal plane of the focusing means; and means for receiving the focused optical signals at each wavelength and launching the individual signals into separate optical fibers. 
     One of the features of the present invention, is that it comprises a bi-directional device which can be used as both a multiplexer to spatially combine the optical signals from several laser sources, each of which is a different wavelength, and launch the spatially combined laser beams into a single optical fiber and as a demultiplexer to spatially separate the different wavelengths of a wavelength division multiplexed optical link and launch each of the different wavelengths into a different optical fiber. In either mode of operation, the device meets the DWDM requirements for low polarization dependent loss, low insertion loss with single mode fiber optic systems, low cross talk between wavelength channels, and low return loss. 
     While the invention will be described with respect to a preferred embodiment configuration and with respect to particular devices used therein, it will be understood that the invention is not to be construed as limited in any manner by either such configuration or components described herein. Also, while the particular types of lasers and optical components used in the preferred embodiment are described herein, it will be understood that such particular components are not to be construed in a limiting manner. Instead, the functionality of those devices should be appreciated. Further, while the preferred embodiment of the invention will be described in relation to transmitting and receiving information over an optical fiber, it will be understood that the scope of the invention is not to be so limited. The principles of the invention apply to the use of multiplexing and launching a plurality of different wavelength optical signals into a single transmission device and demultiplexing a plurality of different wavelength optical signals and launching the plurality of signals into separate transmission devices. These and other variations of the invention will become apparent to those skilled in the art upon a more detailed description of the invention. 
     The advantages and features which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. For a better understanding of the invention, however, reference should be had to the drawing which forms a part hereof and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Referring to the drawing, wherein like numerals represent like parts throughout the several views: 
     FIG. 1 is a functional block diagram of a demultiplexer constructed in accordance with the principles of the present invention. 
     FIGS. 2 a-   2   e  are diagrammatic figures illustrating the changes in beam diameter and the polarization state of the various wavelength optical signals as they progress through the apparatus  15  of FIG.  1 . 
     FIG. 3 is a functional block diagram of a multiplexer constructed in accordance with the principles of the present invention. 
     FIGS. 4 a - 4   e  are diagrammatic figures illustrating the changes in beam diameter and the polarization state of the various wavelength optical signals as they progress through the apparatus  16  of FIG.  3 . 
     FIG. 5 illustrates an environment in which the principles of the present invention multiplexer  16  and demultiplexer  15  (or alternative embodiment devices  115  and  116 ) may be employed. 
     FIG. 6 illustrates the polarizing beam splitter  23 ,  29 ,  23 ′, and  29 ′ in FIGS. 1 and 3 (and the polarizing beamsplitter  123  and  123 ′ of FIGS.  9  and  1 ). 
     FIG. 7 illustrates the light beams through prism  25  and  25 ′ (and  125  and  125 ′) in more detail. 
     FIGS. 8 a  and  8   b  illustrate two possible configurations of the polarizing beamsplitter  23 ,  29 ,  23 ′ and  29 ′ of FIGS. 1 and 3 (and devices  123  and  123 ′ of FIGS.  9  and  11 ). 
     FIG. 9 is a functional block diagram of an alternative embodiment demultiplexer constructed in accordance with the principles of the present invention. 
     FIGS. 10 a - 10   e  are diagrammatic figures illustrating the changes in beam diameter and the polarization state of the various wavelength optical signals as they progress through the apparatus  115  of FIG.  9 . 
     FIG. 11 is a functional block diagram of an alternative embodiment multiplexer constructed in accordance with the principles of the present invention. 
     FIGS. 12 a - 12   e  are diagrammatic figures illustrating the changes in beam diameter and the polarization state of the various wavelength optical signals as they progress through the apparatus  116  of FIG.  11 . 
    
    
     DETAILED DESCRIPTION 
     A device constructed in accordance with the principles of the present invention can preferably be used for either multiplexing or demultiplexing several closely spaced optical wavelengths. Therefore, the device operation and components will be described in detail for operation as a demultiplexer. The reverse operating mode, i.e., as a multiplexer, will be described more briefly below since those of skill in the art will appreciate that essentially only the direction of propagation of the light is changed. 
     Turning now to FIG. 1, there is illustrated in functional form the components and operation of an optical demultiplexer device constructed in accordance with the principles of the present invention. The demultiplexer device is shown generally by the designation  15 . Several wavelengths (e.g., λ 1 , λ 2 , λ 3 , through λ n ) are transmitted to the device  15  by a single optical fiber  20 . The light exiting the optical fiber  20  is collected and collimated by collimating lens assembly  21 . Light at each of the wavelengths exits the collimating lens assembly  21  as a collimated beam. It will be appreciated that the differing wavelengths exit the collimating lens assembly  21  as an equal number of collimated beams (i.e., there are a number of wavelength components of the beam equal to wavelengths λ n ) which propagate along parallel directions, along the same path, and are incident on beamsplitter component  23 . 
     Preferably the specifications for the collimating lens assembly  21  are that the numerical aperture (NA) of the lens assembly ( 21  and  21 ′) match that of the guided beam in the optical fiber  20  to minimize input and output coupling losses with the optical fiber. Also, the aperture of the lens assembly is preferably approximately twice the 1/e 2  beam diameter of the free space propagating collimated beams to reduce diffraction effects which can increase both insertion loss and polarization dependent loss. 
     Beamsplitter  23  splits the collimated beam into two collimated beams and also includes a half wave plate for rotating the polarization of one of the two beams (as defined by the beamsplitting interface) so that the polarization of both collimated beams is perpendicular to the grooves on the diffraction grating element  27 . By incorporating beamsplitter  23 , greater than 98% of the light exiting the optical fiber  20  is conditioned to have the proper polarization direction at the diffraction grating  27  so to achieve optimum diffraction efficiency, independent of the polarization state of the light exiting the optical fiber  20 . The polarization of the collimated beams at designation  22  is best seen in FIG. 2 a  and at designation  24  is best seen in FIG. 2 b.    
     Now referring to FIG. 6, the preferred specifications for the beamsplitter with half wave plate  23  are next described. Three components, a right angle prism  35 , a beam displacement prism  36 , and a half wave plate  37  are cemented together to form a monolithic structure  23 . The face F 2  of prism  36  (which forms an interface I 1  with prism  35 ) is coated with a multilayer dielectric polarizing beamsplitter coating. Component faces F 1 , F 6 , and F 8  are antireflective coated. Light incident on interface I 1  is split into two components, one polarized perpendicular to the plane of incidence (i.e., s component) and one polarized parallel to the plane of incidence (i.e., p component). The s component is reflected to face F 5  where it undergoes total internal reflection so as to exit face F 6  of prism  36 . The p component is transmitted to the half wave plate  37 . As the light propagates through the half wave plate, the polarization direction is rotated 90° so that when the light exits face F 8  of the half wave plate  37 , the polarization direction is parallel to that of the s component which exits face F 6  of prism  36 . 
     Polarizing beamsplitters  23 ,  23 ′,  29 , and  29 ′ of FIGS. 1 and 3 are shown oriented so that the two beams exiting (or entering) the polarizing beamsplitter propagate parallel to each other in a plane which is perpendicular to the plane of the DWDM device  15 . For this configuration, the polarizing beamsplitter is constructed as shown in FIG. 8 a . The polarizing beamsplitters could also be rotated 90° so that the two beams exiting (or entering) the polarizing beamsplitter propagate parallel to each other in a plane which is parallel the plane of DWDM device  15 . For this configuration, the polarizing beamsplitter is constructed as shown in FIG. 8 b . In this case, the p polarized component (as defined by the incident light direction and the interface I 1  of FIG. 6) is oriented perpendicular to the diffraction grating grooves. 
     Now returning to FIG. 1, the split, polarized, and collimated beams then pass through optically transparent prism  25  which expands the diameter of the beams in the direction of polarization, i.e., the direction perpendicular to the diffraction grating  27  grooves. FIG. 2 c  schematically illustrates the expansion of the diameter of the collimated beam shape along the path from the beam shaping prism  25  to the diffraction grating  27 , designated as  26 . Beam expansion in one direction is implemented because the beam undergoes an anamorphic demagnification upon diffraction at grating  27 . The diffracted beam then has a circular cross section which increases coupling efficiency to the circularly symmetric optical fibers ( 33  and  20 ) and integrated optic waveguides  32 . 
     The preferred prism  25  is described with reference to FIG.  7 . The prism is a right angle prism and fabricated using a high index (e.g., n=1.744) glass material. Angle Al of the right angle prism is in the range of 25° to 30°. The collimated light beam is incident on the hypotenuse (face F 9 ) of the right angle prism at an angle which is approximately equal to the Brewsters angle for the air to glass interface. The incident light which is s polarized relative to the beam splitting interface of the polarizing beamsplitter  23 , is p polarized relative to the plane of incidence at the anamorphic beam expanding prism  25 . Thus, the reflectance for the p polarized light incident on surface F 9  is less than one percent (&lt;1%). Light transmitted through prism  25  is incident on face F 10  at near normal incidence. Face F 10  is antireflective coated to reduce reflection losses. Refraction of the incident light beam at surface F 9  increases the diameter of the beam in the direction of the hypotenuse of the right angle prism  25 , and since the light is near normal incidence at face F 10 , the light exits prism face F 10  with an anamorphic magnification of the beam diameter as described in FIGS. 2 b  and  2   c.    
     At the diffraction grating  27 , the collimated beams of each of the different wavelengths (λ 1 , λ 2 , λ 3  through λ n ) is diffracted into a different angular direction relative to the grating normal (shown in phantom). Also, the collimated beam of each wavelength undergoes an anamorphic demagnification upon diffraction. That is, the beam diameter in the direction perpendicular to the grating grooves is reduced (as best seen at designation  28  in FIG. 2 d ). Accordingly, after diffraction, the collimated beam cross section is again nearly circular. The diffraction grating  27  is a holographic grating with ˜9000 grooves/cm for the 100 GHz channel spacing, and ˜11000 grooves/cm for the 50 GHz channel spacing. 
     The two collimated beams at each wavelength are then recombined into a single beam by the beamsplitting polarizer and half waveplate component  29 . Thus, there is a single beam for each wavelength exiting component  29 . The two beams are recombined into a single beam to improve the coupling efficiency to the integrated optic waveguides  32  (and to the optical fiber  20  in the reverse mode operation, i.e. as a multiplexer). Each beam at designation  30  again has two mutually perpendicular polarization components (best seen in FIG. 2 e ). Also, the collimated beam for each wavelength propagates in a different angular direction relative to the optic axis of the lens assembly component  31 . The beamsplitting polarizer and half waveplate component  29  is identical to component  23 . 
     Since the collimated beam for each wavelength is propagating in a different angular direction at designation  30 , the lens assembly  31  focuses each wavelength to a different spatial location along a line in the focal plane of the lens assembly  31 . In the preferred embodiment, the lens assembly  31  is identical to lens assembly  21 . 
     The integrated optic fan out circuit component  32  has an array of integrated optic waveguides with input coupling ports equally spaced at a distance of several tens of microns. The spacing of the waveguide input ports, along with the focal length of lens assembly  31  and the period of the diffraction grating  27  are specified so that the focused spot of each of the wavelengths aligns to a different waveguide coupling port. Also, the collimated beam diameters and the focal length of lens assembly  31  are specified to match the diameter of the focused spot with the mode diameter of the guided beam in the integrated optic waveguides. This ensures good optical coupling efficiency to the waveguides. 
     The integrated optic waveguides of component  32  fan out to a larger separation which permits butt coupling of the waveguides to a linear array of single mode optical fibers  33 . Thus, each wavelength is coupled to a different optical fiber  33  which can then be used to transmit each wavelength to different local terminals. The end faces of the waveguide coupling ports EF 2  and optical fiber end face EF 1  are angle polished to reduce back reflected light to &lt;60 dB. It will be appreciated that reducing feed back to the laser sources reduces optical intensity noise on the laser output beam. The waveguide device  32  is a silica-based integrated optical waveguide circuit. 
     Turning now to FIG. 3, there is illustrated a multiplexer device  16  which includes components similar to the demultiplexer described above in connection with FIG.  1 . It will be appreciated that the multiplexer device  16  is used in the reverse direction as a demultiplexer  15  and is used to combine several laser sources of different wavelengths. Accordingly, those components which are similar to components described above in connection with FIG. 1 are designated by the same number designation followed by a prime. It will be appreciated by those of skill in the art that the considerations for selection of the components are generally the same, although both overall and individually the components perform “reverse” functions in the two embodiments. 
     First, each of the wavelengths (λ 1 , λ 2 , λ 3  through λ n ) is coupled into the multiplexer device  16  from a different single mode optical fiber  33 ′. The wavelengths are launched into a fan-in circuit  32 ′, wherein the light in each fiber is coupled into a different integrated optic waveguide. These waveguides are arranged and configured to guide each of the wavelengths to a different output coupling port. The waveguide output coupling ports are equally spaced at a distance of several tens of microns. At the output coupling ports, each wavelength is launched into a free space propagating beam. 
     Lens assembly  31 ′ collects the light emitted at the linear array of waveguide output ports and collimates the light. Since each wavelength is launched from a port located at a different location along a line in the focal plane of lens assembly  31 ′, the light at each wavelength propagates in a different angular direction after collimation by lens assembly  31 ′. A schematic diagram of the light at designation  30 ′ is illustrated in FIG. 4 a.    
     Next, the beamsplitting polarizer and half wave plate assembly  29 ′ splits each of the collimated beams into two beams and rotates the polarization of the p component beam so that the polarization of each of the two beams for each of the wavelengths is perpendicular to the grating grooves of the diffraction grating  27 ′. A schematic diagram of the polarization state and the beam cross section shape at designation  28 ′ is shown in FIG. 4 b.    
     At the diffraction grating  27 ′, each of the collimated beams (for each of the wavelengths) is diffracted into the same angular direction. That is, the collimated beams for each of the diffracted wavelengths propagates in parallel directions along the same optical path. Upon diffraction by component  27 ′, the collimated beams undergo an anamorphic magnification so that the beam diameter in the direction perpendicular to the grating grooves is increased by approximately a factor of two. The beam cross sectional shape and the polarization direction of the beam at designation  26 ′ is shown schematically in FIG. 4 c.    
     Beam shaping prism  25 ′ then reduces the diameter of the collimated beams in the direction of polarization so that the collimated beams propagating from component  25 ′ to components  23 ′,  21 ′ and  20 ′ have a circular cross sectional shape. This circular cross section shape at designation  24 ′ is illustrated schematically in FIG. 4 d.    
     Polarizing beam splitter  23 ′ recombines the two collimated beams for each of the wavelengths and rotates the polarization of one of the two beams so that the collimated beam exiting component  23 ′ (e.g., at designation  22 ′) has two polarization states, as shown schematically in FIG. 4 e . Lens assembly  21 ′ focuses the collimated beams for each wavelength onto the end face of optical fiber  20 ′. Preferably, beam diameters and lens assembly focal lengths are specified to match the focused spot diameter to the diameter of the guided mode in the optical fiber. This ensures efficient input coupling of the optical beam. The end faces of the waveguide coupling ports  32 ′ and optical fiber end faces  33 ′, and  20 ′ are angle polished to reduce back reflected light to less than sixty dB (&lt;60 dB). It will be appreciated that reducing feed back to the laser sources reduces optical intensity noise on the laser output beam. 
     Alternative Embodiment 
     Turning now to FIG. 9, there is illustrated in functional form the components and operation of an alternative optical demultiplexer device constructed in accordance with the principles of the present invention. The demultiplexer device is shown generally by the designation  115 . Several wavelengths (e.g., λ 1 , λ 2 , λ 3 , through λ n ) are transmitted to the device  115  by a single optical fiber  120 . The light exiting the optical fiber  120  is collected and collimated by collimating lens assembly  121 . Light at each of the wavelengths exits the collimating lens assembly  121  as a collimated beam. It will be appreciated that the differing wavelengths exit the collimating lens assembly ′ 21  as an equal number of collimated beams (i.e., there are a number of wavelength components of the beam equal to wavelengths λ n ) which propagate along parallel directions, along the same path, and are incident on beamsplitter component  123 . 
     Preferably the specifications for the collimating lens assembly  121  are that the numerical aperture (NA) of the lens assembly ( 121  and  121 ′) match that of the guided beam and in the fan out integrated optic circuit waveguides  132  to minimize input and output coupling losses with the optical waveguide. Also, the aperture of the lens assembly is preferably approximately twice the 1/e 2  beam diameter of the free space propagating collimated beams to reduce diffraction effects which can increase both insertion loss and polarization dependent loss. 
     Beamsplitter  123  splits the collimated beam into two collimated beams and also includes a half wave plate for rotating the polarization of one of the two beams (as defined by the beamsplitting interface) so that the polarization of both collimated beams is perpendicular to the grooves on the diffraction grating element  127 . By incorporating beamsplitter  123 , greater than 98% of the light exiting the optical fiber  120  is conditioned to have the proper polarization direction at the diffraction grating  127  so to achieve optimum diffraction efficiency, independent of the polarization state of the light exiting the optical fiber  120 . The polarization of the collimated beams at designation  122  is best seen in FIG. 10 a  and at designation  124  is best seen in FIG. 10 b.    
     The preferred specifications for the beamsplitter with half wave plate  123  have been described above in connection with device  23  and FIG.  6 . 
     Polarizing beamsplitters  123  and  123 ′ of FIGS. 9 and 11 are shown oriented so that the two beams exiting (or entering) the polarizing beamsplitter propagate parallel to each other in a plane which is parallel to the plane of the DWDM device  115 . For this configuration, the polarizing beamsplitter is constructed as shown in FIG. 8 b . The polarizing beamsplitters could also be rotated 90° so that the two beams exiting (or entering) the polarizing beamsplitter propagate parallel to each other in a plane which is perpendicular to the plane of DWDM device  115 . For this configuration, the polarizing beamsplitter is constructed as shown in FIG. 8 a . In this case, the s polarized component (as defined by the incident light direction and the interface I 1  of FIG. 6) is oriented perpendicular to the diffraction grating grooves. 
     Now returning to FIG. 9, the split, polarized, and collimated beams then pass through optically transparent prism  125  which decreases the diameter of the beams in the direction of polarization, i.e., the direction perpendicular to the diffraction grating  127  grooves. FIG. 10 c  schematically illustrates the reduction of the diameter of the collimated beam shape along the path from the beam steering prism  125  to the diffraction grating  127 , designated as  126 . 
     The preferred prism  125  is described with reference to FIG.  7 . The prism is a right angle prism and fabricated using a high index (e.g., n=1.744) glass material. Angle A 1  of the right angle prism is in the range of 25° to 30°. The collimated light beam is incident on a leg (face F 10 ) of the right angle prism. The incident light which is p polarized relative to the beam splitting interface of the polarizing beamsplitter  123 , is p polarized relative to the plane of incidence at the beam steering prism  125 . Faces F 9  and F 10  are antireflective coated to reduce reflection losses. Refraction of the incident light beam at surface F 9  decreases the diameter of the beam in the direction of the hypotenuse of the right angle prism  125 . As prism  125  is rotated about an axis perpendicular to the plane of the drawing of FIG. 7, the angle of incidence at face F 10  is changed, resulting in a change in the propagation direction of the beam exiting face F 9 . The change in the angular direction of the light beam exiting face F 9  is less than the change in angle of incidence on face F 10 . The beam steering prism  125  therefore provides a fine tuning control of the angle of incidence on the diffraction grating  127 . 
     At the diffraction grating  127 , the collimated beams of each of the different wavelengths (λ 1 , λ 2 , through λ n ) are diffracted into a different angular direction relative to the grating normal (shown in phantom). The diffraction grating is used in the Littrow configuration, therefore the angular deviation between the incident beam and the diffracted beams is small. The diffraction grating  127  is a holographic grating with ˜11,000 grooves/cm for the 100 GHz and 50 GHz channel spacing, and ˜9,000 grooves/cm for the 200 GHz channel spacing. 
     The two collimated beams  128  at each wavelength are then recombined into a single beam by the beamsplitting polarizer and half waveplate component  123 . Thus, there is a single beam  130  for each wavelength exiting component  123 . The two beams are recombined into a single beam to improve the coupling efficiency to the integrated optic waveguides  132 . Each beam at designation  130  again has two mutually perpendicular polarization components (best seen in FIG. 10 e ). Also, the collimated beam for each wavelength propagates in a different angular direction relative to the optic axis of the lens assembly component  121 . 
     Since the collimated beam for each wavelength is propagating in a different angular direction at designation  130 , the lens assembly  121  focuses each wavelength to a different spatial location along a line in the focal plane of the lens assembly  121 . 
     The integrated optic fan out circuit component  132  has an array of integrated optic waveguides with input/output coupling ports spaced at distances of several tens of microns. The spacing of the waveguide input ports, along with the focal length of lens assembly  121  and the period of the diffraction grating  127  are specified so that the focused spot of each of the wavelengths aligns to a different waveguide coupling port. Also, the collimated beam diameters and the focal length of lens assembly  121  are specified to match the diameter of the focused spot with the mode diameter of the guided beam in the integrated optic waveguides. This ensures good optical coupling efficiency to the waveguides. 
     The integrated optic waveguides of component  132  fan out to a larger separation which permits butt coupling of the waveguides to a linear array of single mode optical fibers  133  and  120 . Thus, each wavelength is coupled to a different optical fiber  133  which can then be used to transmit each wavelength to different local terminals. The end faces of the waveguide coupling ports EF 2  are angle polished to reduce back reflected light to &lt;60 dB. It will be appreciated that reducing feed back to the laser sources reduces optical intensity noise on the laser output beam. The waveguide device  132  is an integrated optical waveguide circuit. 
     Turning now to FIG. 11, there is illustrated an alternative embodiment multiplexer device  116  which includes components similar to the demultiplexer described above in connection with FIG.  9 . It will be appreciated that the multiplexer device  116  is used in the reverse direction as a demultiplexer  115  and is used to combine several laser sources of different wavelengths. Accordingly, those components which are similar to components described above in connection with FIG. 9 are designated by the same number designation followed by a prime. It will be appreciated by those of skill in the art that the considerations for selection of the components are generally the same, although both overall and individually the components perform “reverse” functions in the two embodiments. 
     First, each of the wavelengths (λ 1 , λ 2 , λ 3  through λ n ) is coupled into the multiplexer device  116  from a different single mode optical fiber  133 ′. The wavelengths are launched into a fan-in circuit  132 ′, wherein the light in each fiber is coupled into a different integrated optic waveguide. These waveguides are arranged and configured to guide each of the wavelengths to a different output coupling port. The waveguide output coupling ports are spaced at a distance of several tens of microns. At the output coupling ports, each wavelength is launched into a free space propagating beam. 
     Lens assembly  121 ′ collects the light emitted at the linear array of waveguide output ports and collimates the light. Since each wavelength is launched from a port located at a different location along a line in the focal plane of lens assembly  121 ′, the light at each wavelength propagates in a different angular direction after collimation by lens assembly  121 ′. A schematic diagram of the light at designation  130 ′ is illustrated in FIG. 12 a.    
     Next, the beamsplitting polarizer and half wave plate assembly  123 ′ splits each of the collimated beams into two beams and rotates the polarization of the s component beam so that the polarization of each of the two beams for each of the wavelengths is perpendicular to the grating grooves of the diffraction grating  127 ′. A schematic diagram of the polarization state and the beam cross section shape at designation  129 ′ is shown in FIG. 12 b.    
     Beam steering prism  125 ′ refracts the two beams for each wavelength so that the beams are incident on the diffraction grating at an angle close to that required for the Littrow operating configuration. Large angular rotations of beam steering prism  125 ′ provides fine tuning control of the incident angle at the diffraction grating. 
     At the diffraction grating  127 ′, each of the collimated beams (for each of the wavelengths) is diffracted into the same angular direction when the incident angles are tuned properly. That is, the collimated beams for each of the diffracted wavelengths propagates in parallel directions along the same optical path. The beam cross sectional shape and the polarization direction of the beam at designation  126 ′ is shown schematically in FIG. 12 c.    
     Polarizing beam splitter  123 ′ recombines the two collimated beams for each of the wavelengths and rotates the polarization of one of the two beams so that the collimated beam exiting component  123 ′ (e.g., at designation  122 ′) has two polarization states, as shown schematically in FIG. 12 e . Lens assembly  121 ′ focuses the collimated beams for each wavelength onto the end face of the integrated optic waveguide  132 ′ which is coupled to optical fiber  120 ′. Preferably, beam diameters and lens assembly focal lengths are specified to match the focused spot diameter to the diameter of the guided mode in the integrated optic waveguide. This ensures efficient input coupling of the optical beam. The end faces of the waveguide coupling ports  132 ′ and optical fiber end faces  133 ′, and  120 ′ are angle polished to reduce back reflected light to less than sixty dB (&lt;60 dB). It will be appreciated that reducing feed back to the laser sources reduces optical intensity noise on the laser output beam. 
     In Operation 
     Turning now to FIG. 5, in use, the preferred multiplexer  16  and demultiplexer  15  may be used in a system  10  for transmitting information over optical fiber  20 . Devices which provide for multiplexing a plurality of wavelengths, including modulating the wavelengths to encode information therein are described in more detail in U.S. patent application Ser. No. 08/769,459, filed Dec. 18, 1996; U.S. patent application Ser. No. 08/482,642, filed Jun. 7, 1995; and U.S. patent application Ser. No. 08/257,083, filed Jun. 9, 1994. Each of the foregoing applications are owned by the Assignee of the present invention and are hereby incorporated herein and made a part hereof. It will be appreciated that alternative embodiment devices  115  and  116  may be used in a system as generally described in FIG. 5 in lieu of devices  15  and  16  respectively. 
     Still referring to FIG. 5, encoded information may be provided to multiplexer  16  by preprocessing block  11 . Providing control function(s) for block  11  is controller block  12  which may be comprised of a mini-computer, special purpose computer and/or personal computer as will be appreciated by those of skill in the art. The information provided to block  11  may include digitized data, voice, video, etc. However, it will be appreciated that amplitude modulation may be used in connection with multiplexer  16  and demultiplexer  15 . 
     The demultiplexer  15  provides the separated optical signals to post-processing block  14 . Providing control function(s) for block  14  is controller block  13  which may be comprised of a mini-computer, special purpose computer and/or personal computer. 
     In this manner, the multiplexer  16  and demultiplexer  15  help develop a building block on which new telecommunication system architectures can be developed. These new telecommunication system architectures are capable of distributing large amounts of information throughout the network. Wavelength division multiplexing and high speed external modulation of the laser light provide for the generation of the large bundles of information. 
     It will be appreciated that the principles of this invention apply not only to the circuitry used to implement the invention, but also to the method in general of automatically utilizing the plurality of wavelengths to transmit information over a single fiber optic device. While a particular embodiment of the invention has been described with respect to its application, it will be understood by those skilled in the art that the invention is not limited by such application or embodiment or the particular components disclosed and described herein. It will be appreciated by those skilled in the art that other components that embody the principles of this invention and other applications therefor other than as described herein can be configured within the spirit and intent of this invention. The arrangement described herein is provided as only one example of an embodiment that incorporates and practices the principles of this invention. Other modifications and alterations are well within the knowledge of those skilled in the art and are to be included within the broad scope of the appended claims.