DENSE WAVELENGTH DIVISION MULIPLEXER (DWDM)

Abstract: An optical multiplexer/demultiplexer for fiber optical communication is disclosed. The device includes a collimator configured to collimate a beam of wavelength-division-multiplexed optical signal received from an optical fiber; a splitter for dividing the collimated beam from the collimator into two parallel propagating beams which are polarized 90 DEG from each other; a retarder, such as a half-wave plate, for rotating the polarization direction of one of the beams by 90 DEG so that both beams are polarized in the same direction; a beam steerer, such as a prism, for modifying the diameter of the split and rotated beams in the direction parallel to the polarization direction and for modifying the propagation direction of the beams; a wavelength dispersing element, such as a diffraction grating, for separating each of the split, rotated and steered beams into a plurality of beams, each having a different center wavelength and propagating along a different angular direction relative to a predetermined direction. The optical elements in the multiplexer/demultiplexer are arranged in such a way to direct the diffracted beams through the optical elements, except the wavelength dispersing element itself, in reverse order, resulting in a plurality of beams of different center wavelengths, which may be launched into a corresponding plurality of optical fibers. The parameters of the optical elements may be chosen to be suitable for multimode dense wavelength multiplexing/demultiplexing applications. The device is also suitable for demultiplexing signals from a single-mode fiber.

Description: 2 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., WJ. 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. Recently, a need for DWDM capabilities for multi-mode optical fiber systems has developed. Many older fiber optic installations in local area networks (LAN) utilize multi-mode optical fiber. Replacing the multi-mode optical fiber in these LANs with single mode optical fiber is too costly in many situations. So as DWDM fiber optic links are being extended from the main long distance fiber optic transmission line further into the metropolitan and local area networks, components must be developed to realize some of the benefits of DWDM in multi-mode optical fiber LANs. The present invention addresses the need for a multiplexer and demultiplexer for multi-mode optical fiber communication links. Summary The present invention provides for an optical multiplexer and demultiplexer for dense wavelength division multiplexed ("DWDM") fiber optic 3 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 bulk 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 multi-mode optical fibers and to reduce optical cross talk. Polarizing beam splitters orient the polarization direction of the light to enable maximum diffraction efficiency by the gratings and to reduce the polarization dependent loss. 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 multi-mode optical fibers, the apparatus comprising: a multi-mode 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 the single multi-mode optical fiber to form a wavelength division multiplexed optical signal; and demultiplexer means for spatially separating the different wavelengths from the single multi-mode 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 bidirectional optical apparatus, comprising: means for collimating a plurality of 4 optical signals of different wavelength received from a single multi-mode fiber in a multi-mode fiber array; 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 modifying 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 changing the angular divergence between the propagation directions of the plurality of optical signal wavelengths; 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 multi-mode optical fibers in the multi-mode fiber array. According to another aspect of the invention, there is provided a bidirectional optical apparatus, comprising: means for collimating a plurality of optical signals of different wavelength received from a plurality of multi-mode fibers in a multi-mode fiber array; 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 5 optical signals at each wavelength and launching the individual signals into a single multi-mode optical fiber in the fiber array. One of the features of the present invention, is that it comprises a bidirectional 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. In another aspect of the invention, a device may be constructed in accordance with the principles of the present invention as a demultiplexer for receiving from a single-mode optical fiber an optical signal containing a plurality of components of different center wavelengths. The demultiplexer is capable of separating the signal into a plurality of optical signals, each of a single center wavelength, and launching each of the the plurality of signals into a separate optical device. 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 multi-mode optical fiber and demultiplexing a plurality of different wavelength optical signals from an optical 6 fiber, of either multi-mode or single-mode type, and launching the plurality of signals into separate optical 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. 2a - 2e 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. 4a - 4e 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 may be employed. Fig. 6 illustrates the polarizing beam splitter 23 and 23' in Figs 1 and 3. Fig. 7 illustrates the light beams through prism 25 and 25'. Figs. 8a and 8b illustrate two possible configurations of the polarizing beam splitter 23 and 23' of Figs. 1 and 3. Fig. 9 schematically illustrates the cross-sectional view of the optical fibers in relation to the demultiplexed input beams in a demultiplexer in accordance with the principles of the invention. Fig. 10 illustrates a transmission spectra for single-mode input for a demultiplexer in accordance with the principles of the invention. Fig. 11 illustrates an expanded view of the transmission spectra shown in Fig. 10. Fig. 12 shows the bit eπor rate performance of a demultiplexer constructed in accordance with the principles of the invention. 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 only the direction of propagation of the light is changed. Prior to a discussion of a detailed description of a preferred embodiment, a discussion of the unique features of multi-mode optical fibers which affect the design of a DWDM will be presented. Some of these features include the large diameter of the guided mode and the existence of many guided modes — each of which has a different propagation velocity in the optical fiber. The larger diameter of the guided mode impacts the size and performance of any optical device which requires coupling the guided light into a collimated free space propagating beam. To achieve a collimated beam of low divergence, long focal length optical lenses are required. Reducing insertion losses when using long focal length lenses requires larger diameter optics. Larger optical components such as narrow passband optical filters would display degraded performance due to film thickness variations over the surface area. To implement an integrated optic phased array multiplexer or a fiber grating multiplexer would require coupling the light from the multi-mode optical fiber into a large diameter free space propagating collimated beam, and then end fire coupling the collimated beam into the integrated optic or single mode fiber optic device. The existence of several guided modes of different propagation constant decreases the efficiency of interference based fiber optic or integrated optic devices. Features of the present invention constructed according to the principles of the present invention provide for a multi-mode fiber optic systems which enables a relatively smaller device package, low insertion loss, low cross talk, and low polarization dependent loss (PDL). 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 present invention. The demultiplexer device is shown generally by the designation 15. Several wavelengths (e.g., λ„ λ 2 , λ 3 , through λ n ) are transmitted to the device 15 by a single multi-mode 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 beam splitter component 23. There are several important specifications for the collimating lens assembly 21. 1. The numerical aperture (NA) of the lens assembly (21 and 21 ') must equal or exceed that of the guided beam in the optical fiber 20 to minimize optical power loss. Loss can occur both in collimating the light beam and in coupling the diffracted light back into the output fibers 33 and 33'. 2. The focal length of the lens assembly must be sufficiently long to produce a collimated beam (22, 22', 30, and 30') with a divergence angle less than the difference between the angular direction of propagation of two adjacent channel wavelengths (λ, and λ 1+1 ) diffracted by the planar holographic grating (27 and 27'). 3. Finally, the focal length of the lens assembly must provide the linear dispersion required to locate two adjacent channel wavelengths (λ, and λ, +1 ) diffracted by the planar holographic grating (27 and 27') at the input faces of two adjacent output fibers (33 and 33'). For example, a 100 GHz DWDM for a 50 micron core diameter multi-mode optical fiber could use a 5.08 cm focal length lens (21 and 21 '). To efficiently collect the light exiting the 0.2 numerical aperture fiber, the lens assembly should have an aperture of 2 cm or greater. With a 11000 grooves / cm diffraction grating (27 and 27'), the lens assembly would focus two channel wavelengths at 0.8 nm spacing (i.e., 100 GHz channel spacing) to two spots separated in space by 127 microns. Optical fiber holding device (32 and 32') provides for fixing the linear arrays of optical fibers with a predetermined fiber spacing. For example, multi- mode optical fibers may be held in linear arrays with a fiber spacing of 127 microns. Such devices are commercially available under various designations and are commonly referred to as fiber array blocks. The blocks may be constructed of silicon or glass and include either mechanically or lithographically fabricated fiber grooves. Beam splitter 23 splits the collimated beam into two collimated beams and also includes a half wave plate for rotating the polarization of the s component (as defined by the beam splitting interface) so that the polarization of both collimated beams is perpendicular to the grooves on the diffraction grating element 27. By incorporating beam splitter 23, greater than ninety eight percent (98%) of the light exiting the optical fiber is conditioned to have the proper polarization direction at the diffraction grating 27 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. 2a and at designation 24 is best seen in Fig. 2b. The preferred specifications for the beam splitter with half wave plate 23 are described with reference to Figure 6. Three components, a right angle prism 35, a beam displacement prism 36, and a retarder such as a half wave plate 37 are 10 cemented together to form a monolithic structure 38. The face F2 of prism 36 which forms an interface II with prism 35 is coated with a multilayer dielectric polarizing beam splitter coating. Component faces FI, F6, and F8 are antireflective coated. Light incident on interface II 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 F5 where it undergoes total internal reflection so as to exit face F6 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 F8 of the half wave plate 37, the polarization direction is parallel to that of the s component which exits face F6 of prism 36. Polarizing beam splitters 23 and 23' of Figures 1 and 3 are shown oriented so that the two beams exiting (or entering) the polarizing beam splitter propagate parallel to each other in a plane which is parallel to the plane of the DWDM device 15. For this configuration, the polarizing beam splitter is constructed as shown in Figure 8b. The polarizing beam splitters could also be rotated 90° so that the two beams exiting (or entering) the polarizing beam splitter propagate parallel to each other in a plane which is perpendicular to the plane of the DWDM device 15. For this configuration, the polarizing beam splitter is constructed as shown in Figure 8a. In this case, the s polarized component (as defined by the incident light direction and the interface II of Figure 6) is oriented perpendicular to the diffraction grating grooves. The split, polarized, and collimated beams then pass through optically transparent prism 25 which alters the diameter of the beams in the direction of polarization, i.e., the direction perpendicular to the diffraction grating 27 grooves. Fig. 2c schematically illustrates the reduction of the diameter of the collimated beam shape along the path from the beam steering prism 25 to the diffraction grating 27, designated as 26. This reduction in beam diameter reduces the size of the holographic grating, enabling a more compact size DWDM. The beam steering prism also serves the function of either magnifying or demagnifying the change in angle at which the demultiplexed light beams 29 exit face F10 with changes in angle of incidence at face F9. This function permits adjustment of the angular dispersion between the demultiplexed light beams 30 as they propagate to and through the lens assembly 23. By adjusting the angular dispersion between the demultiplexed light beams 30, the spacing of the focused spots at the linear optical fiber array 32 can be adjusted. The preferred prism 25 is described with reference to Figure 7. The prism is a right angle prism, fabricated using a high index (e.g., n=l .744) glass material. Angle Al of the right angle prism is in the range of 25° to 30° (best seen in Fig. 7). The multiplexed collimated light beam is incident on face F10 of the right angle prism, and the demultiplexed collimated light beams are incident on face F9. The incident light which is p polarized relative to the beam splitting interface of the polarizing beam splitter 23, is p polarized relative to the plane of incidence at the beam steering prism 25. Faces F9 and F10 are antireflective coated to reduce reflection losses. At the diffraction grating 27, the collimated beam of each of the different wavelengths (λ l5 λ 2 , λ 3 , through λ n ) is 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 multiplexed incident beam and the demultiplexed diffracted beams is small. The diffraction grating 27 is a holographic grating with ~11000 grooves / cm for the 100 GHz channel spacing. The use of the high frequency diffraction grating 27 (i.e., ~1 lOOOgrooves / cm) and the polarizing beam splitter / half waveplate optical component 23 are the important components which enable the compact size and low PDL of the DWDM. The two collimated beams 28 at each wavelength are then recombined into a single beam by the beam splitting polarizer and half waveplate component 23. Thus, there is a single beam 30 for each wavelength exiting component 23. The two beams are recombined into a single beam to improve the coupling efficiency to the output optical fibers 32 (and to the optical fiber 20 in the reverse mode operation, i.e., as a multilplexer). Each beam at designation 30 again has two mutually perpendicular polarization components (best seen in Fig. 2e). Also, the collimated beam for each wavelength propagates in a different angular direction relative to the optic axis of the lens assembly component 21. Since the collimated beam for each wavelength is propagating in a different angular direction at designation 30, the lens assembly 21 focuses each wavelength to a different spatial location along a line in the focal plane of the lens assembly 21. The multi-mode optical fiber array component 32, is a linear array of fibers with cleaved and polished end faces, equally spaced at a distance of 127 microns. The spacing of optical fibers, along with the focal length of lens assembly 21 and the period of the diffraction grating 27 are specified so that the focused spot of each of the wavelengths aligns to a different optical fiber end face. Since the same lens assembly is used to collimate the multiplexed optical beam as is used to focus the demultiplexed optical beams, the diameter of the focused spots match the mode diameter of the guided beam in the output optical fibers. This ensures good optical coupling efficiency to the optical fibers. The end faces of the optical fiber end faces EF1 (20 and 33) are angle polished to reduce back reflected light to less than 60 dB. It will be appreciated that reducing feed back to the laser sources reduces optical intensity noise on the laser output beam. The optical fiber array 32 can be fabricated by sandwiching the optical fibers between a silicon V-groove bottom plate and a flat or silicon V-groove top plate. 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 wavelength. Accordingly, those components which are similar to components described above in connection with Fig. 1 are designated by the same number designation followed by an asterisk or 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 l5 λ 2 , λ 3 , through λ n ) is coupled into the multiplexer device 16 from a different multi-mode optical fiber 33'. The optical fiber output coupling ports are equally spaced at a distance of 127 microns. At the output coupling ports, each wavelength is launched into a free space propagating beam. Lens assembly 21 ' collects the light emitted at the linear array of optical fiber 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 21 ', the light at each wavelength propagates in a different angular direction after collimation by lens assembly 21 '. A schematic diagram of the light at designation 30'is illustrated in Fig. 4a. Next, the beam splitting polarizer and half wave plate assembly 23' 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 27'. A schematic diagram of the polarization state and the beam cross section shape at designation 28' is shown in Fig. 4b. At the diffraction grating 27', 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 26' is shown schematically in Fig. 4c. Beam steering prism 25' refracts the two beams for each wavelength so that the angular deviation between the demultiplexed beams 28' is such that the diffracted beams 26' all propagate parallel to each other after diffraction at the grating 27'. This ensures that the diffracted beams are multiplexed into the output optical fiber 20'. 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. 4e. 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 optical fiber end faces 33' and 20' are angle polished to reduce back reflected light to < 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 December 18, 1996; U.S. Patent Application Ser. No. 08/482,642, filed June 7, 1995; and U.S. Patent Application Ser. No. 08/257,083, filed June 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. 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 postprocessing 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 will distribute 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. Typically, multi-mode multiplexers/demultiplexers have not been used in optical systems using single-mode optical fibers for long-distance signal transmission because of the perceived problems associated with the differences in optical properties of the multi-mode and single-mode devices. However, in certain applications, the use of a multi -mode DWDM constructed in accordance of the principles of the invention can offer some performance advantages over using a single-mode DWDM. Examples include application in which the demultiplexed wavelength channel does not need to be transmitted long distances beyond the demultiplexer using single-mode optical fiber, The advantages include (1) lower insertion losses, (2) lower crosstalk, and (3) a flatter passband. These advantages arise due to several reasons. First, the large guided beam diameter and large numeric aperture of multi -mode fibers results in a more efficient coupling of a free-space propagating light beam into a multi-mode fiber than can be achieved with a single-mode fiber, which has a smaller guided mode diameter and numerical aperture. The insertion loss is therefore reduced. Second, the air gap between the adjacent optical fibers at the output ports of a multi-mode DWDM enables higher levels of optical isolation between adjacent channels than what is achieved with a single-mode DWDM. For example, in a illustrative embodiment of the present invention, two adjacent fibers are spaced about 127 microns apart, with a small air gap in between. In contrast, a typical single-mode DWDM employs an integrated optic chip with a linear array of thin film waveguides spaced about 24 microns apart, with no air gap between adjacent guides. The air gap reduces the evanescent wave coupling between adjacent optical fibers in the multi-mode DWDM, thereby reducing crosstalk. Third, as illustrated in Fig. 9, because the diameter 92 of the guided beam launched into the multi-mode fiber from a single-mode fiber is smaller than that 94 which the multi-mode fiber can support when all the guided modes are excited, the demultiplexed output channels have a broader and flatter passband. This is because the smaller diameter guided beam which is launched in the input multi mode fiber remains small through the short link of multi mode fiber which directs the light to the multi-mode DWDM. In the DWDM, the optical fiber guided beam is launched as a collimated, free space propagating beam which is diffracted by the grating into an array of collimated beams, each propagating in a different angular direction and each of a different wavelength. At the DWDM output ports, each of the free space propagating beams is focused to a spot of smaller diameter than the core of the output multi mode optical fibers. Since there is a near linear relationship between the wavelength and the position along the line running through the centers of the linear array of output multi mode fibers, the focused output beam overlaps the multi- mode fiber core for a larger range of wavelengths, resulting in a broader and flatter passband. As an example, a 16-channel demultiplexer of the present invention with 200 GHz channel separation was used to demultiplex a single-mode input into channels 1-16. As shown by the performance data listed in Table I and transmission spectra in Figs. 10 and 11, the demultiplexer has an insertion loss of 2dB or less, crosstalk of less than 42 dB and transmission bandwidth of about 0.4 nm at -0.5 dB. The use of the multi -mode demultiplexer of the invention in the example above has also produced surprisingly good bit-error rate ("BER") performance. For example, the BER data were obtained at 2.5 Gb/sec, for transmission from a single-mode fiber to an avalanche photodiode ("APD") with a multi -mode fiber pigtail. As shown in Fig. 12, the BER is virtually unchanged whether the a demultiplexer of the invention is placed between the single-mode fiber and the detector. 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.

Claims: WE CLAIM: 1. An optical device capable of receiving from an optical fiber a beam carrying wavelength-division-multiplexed optical signal and demultiplexing the signal into a plurality of beams of optical signals, each having a different center wavelength, the device comprising: a) a collimator configured to collimate the multiplexed optical beam; b) a splitter for dividing the collimated beam from the collimator into two parallel propagating beams which are polarized 90° from each other; c) a retarder for rotating the polarization direction of one of the beams by 90° so that both beams are polarized in the same direction; d) a beam steerer for modifying the diameter of the split and rotated beams in the direction parallel to the polarization direction and for modifying the propagation direction of the beams; Θ) a wavelength dispersing element for separating each of the split, rotated and steered beams into a plurality of beams, each having a different center wavelength and propagating along a different angular direction relative to a predetermined direction; 2. The optical device of claim 1, wherein the retarder comprises a half-wave plate, the beam steerer comprises a prism, and the wavelength dispersion element comprises a diffraction grating. 3. The optical device of claim 1, further comprising: f) means for changing the angular divergence between the propagation directions of the plurality of beams from the wavelength dispersing element; 19 g) 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 path; h) 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 i) means for receiving the focused optical signals at each wavelength and launching the individual signals into separate multi-mode optical fibers in the multi-mode fiber array. 4. The optical device of claim 3, wherein the angular divergence changing means comprises the beam steerer, the recombining means comprises the retarder and the splitter, and the focusing means comprises the collimator, wherein the optical elements a)-e) are configured and arranged to direct the rotated beams emanating from the wavelength dispersing element back through elements d), c), b) and a), and the unrotated beams back through elements d), b) and a). 5. 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 multi-mode optical fibers, the apparatus comprising: a) a multi-mode optical fiber; b) 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 the single multi-mode optical fiber to form a wavelength division multiplexed optical signal; and c) demultiplexer means for spatially separating the different wavelengths from the single multi-mode optical fiber carrying a wavelength division multiplexed optical signal and launching each of the different wavelengths into a separate optical fiber. 20 6. A bi-directional optical apparatus, comprising: a) means for collimating a plurality of optical signals of different wavelength received from a single multi-mode fiber in a multi- mode fiber array; b) means for splitting the plurality of optical wavelength signals into two parallel propagating beams which are polarized perpendicular to each other; c) 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; d) means for modifying the diameter of the collimated beams in the direction parallel to the polarization direction; e) means for diffracting each of the different wavelengths into a different angular direction relative to a defined direction; f) means for changing the angular divergence between the propagation directions of the plurality of optical signal wavelengths; g) 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; h) 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 i) means for receiving the focused optical signals at each wavelength and launching the individual signals into separate multi-mode optical fibers in the multi-mode fiber array. 7. A bi-directional optical apparatus, comprising: a) means for collimating a plurality of optical signals of different wavelength received from a plurality of multi-mode fibers in a multi-mode fiber array; b) means for splitting the plurality of optical wavelength signals into two parallel propagating beams which are polarized perpendicular to each other; c) 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; d) means for steering the propagation direction of the collimated beams; e) means for diffracting each of the different wavelengths into a different angular direction relative to a defined direction; f) 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; g) 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 h) means for receiving the focused optical signals at each wavelength and launching the individual signals into a single multi-mode optical fiber in the fiber array. 8. An optical demultiplexer for spatially separating a wavelength- division-multiplexed optical signal from a single-mode optical fiber into a plurality of optical signals, each having a different center wavelength, and launching each of the separated signals into a separate optical device, the demultiplexer comprising: 22 (a) a collimator for collecting and collimating the optical signal from the input fiber; (b) an array of plurality of multi-mode optical fibers; (c) a wavelength dispersing element for separating the signal collimated by the collimator into the plurality of optical signals, each having a different center wavelength; and (d) a lens for directing each one of the plurality of optical signals into one of the plurality of multi-mode optical fibers. 9. The demultiplexer of claim 1, wherein the wavelength dispersing element comprises a diffraction grating. 10. An optical apparatus for separating a wavelength-division-multiplexed optical signal into a plurality of optical signals, each having a different center wavelength, and launching each of the separated signals into a separate optical device, the apparatus comprising: (a) a single-mode optical fiber for carrying the wavelength-division- multiplexed optical signal; (b) a demultiplexer optically connected to the single-mode fiber, the demultiplexer including an array of multi-mode optical fibers connected to the separate optical devices and including a diffraction grating for spatially separating the wavelength-division-multiplexed optical signal received from the single-mode fiber into the plurality of optical signals and launching each of the plurality of optical signals into one of the separate optical devices through one of the plurality of multi -mode optical fibers. 11. A method of separating a wavelength-division-multiplexed optical signal into a plurality of optical signals, each having a different center wavelength, and launching each of the separated signals into a separate optical device, the method comprising: 23 (a) transmitting the wavelength-division-multiplexed optical signal in a single-mode optical fiber; (b) separating the wavelength-division-multiplexed optical signal from the single-mode optical fiber into a plurality of optical signals, each having a center wavelength different from any other one of the plurality of optical signals; and (c) launching each of the plurality of optical signals into one of the separate optical devices through one of a plurality of multi-mode optical fibers.

Inventors: BOORD WARREN TIMOTHY, JAIN ANIL K, VANDERWERT TERRY, BOORD, WARREN, TIMOTHY, JAIN, ANIL, K., VANDERWERT, TERRY
Applicants: APA OPTICS INC, APA OPTICS, INC.
Application year: 2000
Main classification: H04J14
IPC codes: G02B6