Abstract:
A method and apparatus for a filterless parallel WDM multiplexer is disclosed. The filterless multiplexer comprises two or more planes of lenses to allow light to be directed into a number of waveguides. The filterless multiplexer may be have either refractive or diffractive lenses.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to the patent application Ser. No. 10/427,671 entitled “VCSEL ARRAY CONFIGURATION FOR A PARALLEL WDM TRANSMITTER”, filed on the same day and assigned to the same assignee. 
     BACKGROUND OF THE INVENTION 
     Parallel optics and wavelength division multiplexing (WDM) are two optical communication techniques that permit increased bandwidth density in optical communications systems. In parallel optics, multiple optical signals are transmitted in parallel along a multi-optical fiber ribbon, with a single optical signal being transmitted on each optical fiber. In WDM, multiple optical data signals are combined and transmitted on a single optical fiber, with each optical signal being carried on a different wavelength. In parallel WDM, the two techniques are combined by transmitting multiple optical wavelengths through each optical fiber of a parallel optical fiber ribbon. A key component of a parallel WDM system is a parallel WDM multiplexer, an optical device that combines multiple optical beams into a single optical fiber. 
     In some implementations of parallel WDM, the optical transmitter includes an array of vertical cavity surface emitting lasers (VCSELs). The number of VCSELs in the array is typically equal to the number of optical fibers in the optical fiber ribbon multiplied by the number of wavelengths in each optical fiber. The optical multiplexer serves to couple light from one VCSEL of each wavelength into each optical fiber in the optical fiber ribbon. Typical multiplexers use wavelength selective means such as dielectric interference filters or diffraction gratings to accomplish this. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a filterless parallel WDM multiplexer using simple wavelength insensitive sets of lenses to combine light beams is described. A general array of beams of arbitrary configuration is mapped into a grouping of lenses in an arbitrary, tightly spaced configuration such as micro-lens arrays and then mapped into an array of optical fibers. The optical fibers may be arranged into a linear array such as a ribbon or a two dimensional array such as a fiber bundle. 
     In embodiments where a single mode fiber is used, a wavelength-insensitive combiner introduces an inherent insertion loss equal to dividing the incident power at each wavelength by the number of wavelengths. If multimode fiber is used this inherent loss is not present if the phase space volume of the optical fiber exceeds the sum of the phase space volumes of all the incident beams. Hence, embodiments in accordance with the invention provide simple and low-cost optical devices for multiplexing multiple wavelengths into a fiber with minimal insertion loss. 
     Typical embodiments in accordance with the invention have at least two planes containing lenses. Two planes may be on two sides of an optically transparent wafer made of glass or semiconductor material or on two separate wafers that are attached or separated by a gap such as an air gap. Lenses may be either refractive or diffractive, spherical or aspherical. Lenses in the various planes may be the same or different and the lens material may be glass, plastic, silicon, GaAs, InP, GaP or other material that is optically transparent in the wavelength range of interest. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conceptual view of an embodiment in accordance with the invention. 
         FIG. 2   a  shows a VCSEL die configuration in an embodiment in accordance with the invention. 
         FIG. 2   b  shows a first plane of lenses in an embodiment in accordance with the invention. 
         FIG. 2   c  shows a second plane of lenses in an embodiment in accordance with the invention. 
         FIG. 3  shows a side view of an embodiment in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a conceptual view of filterless parallel WDM multiplexer element  176  in accordance with the invention. Using two or more filterless parallel WDM multiplexer elements such as filterless parallel WDM multiplexer element  176  together, one filterless parallel WDM multiplexer element for each optical fiber, results in a filterless parallel WDM multiplexer in accordance with the invention. Filterless parallel WDM multiplexer element  176  directs light from VCSELs  130 ,  135 ,  140 ,  145  to optical fiber  150 . Each one of VCSELs  130 ,  135 ,  140 ,  145  operates at a different wavelength. A typical configuration for VCSELs  130 ,  135 ,  140 ,  145  may be a linear array having a 250 micron pitch. Optical sources other than VCSELs such as edge emitting lasers may also be used. 
     Filterless parallel WDM multiplexer element  176  includes plane  110  containing lenses  160 ,  165 ,  170 ,  175  to provide a lens for each incident beam. Lenses  160 ,  165 ,  170 ,  180  map the light from each VCSEL  130 ,  135 ,  140 ,  145  to spots on lenses  180 ,  185 ,  190 ,  195 , respectively, in plane  120 . Lenses  180 ,  185 ,  190 ,  195  are configured in a dense pattern to allow focusing the light beams onto the aperture of optical fiber  150 . The number of optical light sources and lenses may be increased along with the number of optical fibers. For example, filterless parallel WDM multiplexer element  176  shown in  FIG. 1  may be repeated 12 times, resulting in a filterless parallel WDM multiplexer to image a 4 by 12 array of VCSELs into a 1 by 12 optical fiber ribbon. 
     Typically, an embodiment in accordance with the invention such as filterless parallel WDM multiplexer element  176  has at least two planes of lenses as seen in  FIG. 1 . The planes of lenses may be implemented on two sides of an optically transparent wafer made from, for example, glass, plastic, silicon, GaAs, InP, GaP or any other material that is optically transparent in the wavelength range of interest. In another embodiment, the planes may be implemented on separate wafers, each wafer having a plane of lenses. The wafers may be in contact with each other or have an intervening material such as air between them. 
     General design considerations for embodiments in accordance with the invention such as filterless parallel WDM multiplexer element  176  follow and are discussed with respect to but not limited to  FIG. 1 . Because a VCSEL, such as VCSEL  130 , typically emits a vertical cone of light, the center of the aperture of lens  160  in plane  110 , should be aligned with the aperture of VCSEL  130  to capture the VCSEL light. In the case that the sources are edge emitting lasers, the emitted beam is astigmatic and the lenses of filterless parallel WDM multiplexer element  176  and other embodiments in accordance with the invention need to be biconic lenses. 
     In order to direct light from lens  160  in plane  110  to the appropriate lens in plane  120 , the vertex of lens  160  must lie on the line connecting the aperture of VCSEL  130  to the center of the aperture of lens  180  in plane  120 . The vertex for lens  130  is taken to be the point on the extended lens surface that is closest to the plane in which VCSELs  130 ,  135 ,  140 ,  145  or other suitable optical light sources reside. This results in an offset between the center of the aperture of lens  160  and the vertex of lens  160 . Therefore, lens  160  is an off-axis section of a lens. Lens  180  in plane  120  needs to be large enough to capture most of the light incident on it and focus this light into optical fiber  150 . Lens  180  in plane  120  focuses the incident light into optical fiber  150  that is positioned to minimize the overall range of angles of the incident light going into optical fiber  150 . Because lens  180  in plane  120  needs to focus the incident light into optical fiber  150 , the line connecting the center of optical fiber  150  with the vertex of lens  180  needs to be parallel to the incident light. By design, the incident light is parallel to the line connecting the aperture of VCSEL  130  to the center of lens  180  in plane  120 . The vertex of lens  180  is taken to be the point on the extended lens surface that is closest to the plane of the aperture of optical fiber  150 . This requires that there be an offset between the center of the aperture of lens  180  in plane  120  and the vertex of lens  180 . Hence, lens  180  in plane  120  is also off-axis. Lenses  165 ,  170 ,  175 ,  185 ,  190 ,  195  of filterless parallel WDM multiplexer element  176  and any additional optical fibers and lenses associated with additional filterless parallel multiplexer elements are similarly positioned. Optical fiber  150  and other optical fibers used in accordance with the invention may be single-mode or multimode optical fiber. Optical fiber  150  may be replaced by any known waveguide. If a single-mode optical fiber is used, embodiments in accordance with the invention such as filterless parallel WDM multiplexer element  176  introduces an insertion loss equal to a factor at least as large as the number of wavelengths being combined into a single optical fiber. Hence, for the example of filterless parallel WDM multiplexer element  176  that combines four wavelengths into optical fiber  150 , if optical fiber  150  is a single-mode optical fiber there is an inherent minimum 6-db insertion loss. The inherent loss is not present if optical fiber  150  is a multimode fiber. If a phase space volume is defined as the beam area multiplied by the solid angle, then a low loss filterless parallel WDM multiplexer can be implemented provided that the phase space volume of each optical fiber, such as, for example, optical fiber  150 , is greater than the sum of the phase space volumes of the incident optical beams. 
     The general design considerations discussed above assume that VCSELs  130 ,  135 ,  140 ,  145  are point sources. This assumption is an approximation. Additional assumptions have neglected diffraction and lens aberrations. The design implementation of filterless parallel WDM multiplexer element  176  in accordance with the invention corrects for these factors and embodiments typically will differ from the above description which, however, results in a baseline design that is qualitatively similar to the typical embodiments. In practice, the qualitative description provides a starting configuration that may be iteratively modified using ray tracing software packages such as ZEMAX® or CODE V® until the amount of VCSEL light reaching the optical fiber has been optimized. 
     In accordance with an embodiment of the invention,  FIG. 2   a  shows VCSELs  250   a – 250   d ,  260   a – 260   d ,  270   a – 270   d ,  280   a – 280   d  on VCSEL die  250 ,  260 ,  270 ,  280 , respectively, labeled to illustrate how light is optically directed from individual VCSELs into optical fibers  352 ,  362 ,  372 ,  382  by filterless parallel WDM multiplexer  200  (see  FIG. 3 ). Each VCSEL on each die emits one of four different wavelengths so that there are four VCSELs emitting at each wavelength. 
     In accordance with an embodiment of the invention  FIG. 2   b  shows first lens plane  210  of filterless parallel WDM multiplexer  200  (see  FIG. 3 ). Lenses  251   a–d ,  261   a–d ,  271   a–d ,  281   a–d  belong to lens groups  201 ,  202 ,  203 ,  204 , respectively. Each of lenses  251   a–d ,  261   a–d ,  271   a–d ,  281   a–d  in first lens plane  210  is offset in the horizontal plane with respect to VCSELs  250   a–d ,  260   a–d ,  270   a–d ,  280   a–d , respectively, according to the requirements discussed above. This allows light coming from the VCSELs  250   a–d ,  260   a–d ,  270   a–d ,  280   a–d  through lenses  251   a–d ,  261   a–d ,  271   a–d ,  281   a–d  to be directed at an angle to intersect corresponding lenses  252   a–d ,  262   a–d ,  272   a–d ,  282   a–d  in second lens plane  220  of filterless parallel WDM multiplexer  200 (see  FIG. 3 ). 
       FIG. 2   c  shows how light from first lens plane  210  of filterless parallel WDM multiplexer  200  (see  FIG. 3 ) is mapped into lenses  252   a–d ,  262   a–d ,  272   a–d ,  282   a–d  of second lens plane  220  of filterless parallel WDM multiplexer  200  (see  FIG. 3 ) as viewed from the optical fiber side. Lenses  252   a–d ,  262   a–d ,  272   a–d ,  282   a–d  in second lens plane  220  of filterless parallel WDM multiplexer  200  are positioned so that light from lens groups  301 ,  302 ,  303 ,  304  is focused into optical fibers  352 ,  362 ,  372 ,  382  (see  FIG. 3 ), respectively. Starting from the nine o&#39;clock position in each group and going clockwise, lens group  301  has lenses  252   a ,  262   c ,  282   a ,  272   c ; lens group  302  has lenses  252   c ,  262   a ,  282   c ,  272   c , lens group  303  has lenses  252   b ,  262   d ,  282   b ,  272   d ; lens group  304  has lenses  252   d ,  262   b ,  282   d ,  272   b . The axis of each optical fiber  352 ,  362 ,  372 ,  382  (see  FIG. 3 ) is aligned with the center of lens groups  304 ,  303 ,  302 ,  301 , respectively. Lenses in each lens group  304 ,  303 ,  302 ,  301  are positioned such that the four lenses in each group focus the light into optical fibers  352 ,  362 ,  372 ,  382 , respectively. Light from lens  251   a  is directed to lens  252   a ; light from lens  251   b  is directed to lens  252   b ; light from lens  251   c  is directed to lens  252   c ; light from lens  251   d  is directed to lens  252   d ; light from lens  261   a  is directed to lens  262   a ; light from lens  261   b  is directed to lens  262   b , light from lens  261   c  is directed to lens  262   c , light from lens  261   d  is directed to lens  262   d ; light from lens  271   a  is directed to lens  272   a ; light from lens  271   b  is directed to lens  272   b ; light from lens  271   c  is directed to lens  272   c ; light from lens  271   d  is directed to lens  272   d ; light from lens  281   a  is directed to lens  282   a ; light from lens  271   b  is directed to lens  282   b ; light from lens  281   c  is directed to lens  282   c ; light from lens  281   d  is directed to lens  282   d.    
     The mapping of the light beams between first lens plane  210  and second lens plane  220  is designed to minimize the largest required angular bending of the light within the configuration constraints.  FIG. 3  shows a side view of the configuration shown in top view in  FIGS. 2   a – 2   c  with VCSEL die  250  and  260  and the position of optical fibers  352 ,  362 ,  372 ,  382 . VCSEL die  270  and  280  with the associated light beams and lenses are suppressed in  FIG. 3  to aid clarity. Dashed lines in  FIG. 3  relate to the hidden VCSELs  260   d ,  260   c ,  250   d ,  250   c  and corresponding hidden lenses  261   d ,  261   c ,  251   d ,  251   c.    
     As shown in  FIG. 3 , filterless parallel WDM multiplexer  200  directs light from VCSELs  260   a ,  260   b ,  260   c ,  260   d  on VCSEL die  122  and from VCSELs  250   a ,  250   b ,  250   c ,  250   d  on VCSEL die  121  into optical fibers  352 ,  363 ,  372 ,  382 . Filterless parallel WDM multiplexer  200  maps light beams  310 ,  311 ,  312 ,  313 ,  314 ,  315 ,  316 ,  317  from lens plane  210  to lens plane  220  and into optical fibers  352 ,  362 ,  372 ,  382 . Light beam  310  originates from VCSEL  260   b  passing through lens  261   b  to lens  262   b  and into optical fiber  352 . Light beam  311  originates from VCSEL  260   d  passing through lens  261   d  to lens  262   d  and into optical fiber  362 . Light beam  314  originates from VCSEL  260   a  passing through lens  261   a  to lens  262   a  and into optical fiber  372 . Light beam  316  originates from VCSEL  260   c  passing through lens  261   c  to lens  262   c  and into optical fiber  382 . Light beam  312  originates from VCSEL  250   d  passing through lens  251   d  to lens  252   d  and into optical fiber  252 . Light beam  313  originates from VCSEL  250   b  passing through lens  251   b  to lens  252   b  and into optical fiber  362 . Light beam  315  originates from VCSEL  250   c  passing through lens  251   c  to lens  252   c  and into optical fiber  372 . Light beam  317  originates from VCSEL  250   a  passing through lens  251   a  to lens  252   a  and into optical fiber  382 . 
     For filterless parallel WDM multiplexers in accordance with the invention such as filterless parallel WDM multiplexer element  176  or filterless parallel WDM multiplexer  200 , micro-lens arrays are typically used in each plane and may be made up of either refractive or diffractive lenses. Refractive lenses offer the highest possible efficiency because with application of the appropriate anti-reflective coating most incident light is transmitted and refracted according to Snell&#39;s law. Typically, the less expensive methods used for fabricating refractive micro-lens arrays are restricted to spherical surface profiles while more expensive methods are typically needed for the fabrication of aspherical surface profiles. However, spherical profiles typically suffer from higher levels of aberration. Aspherical surface profiles typically have lower levels of aberration and result in lower overall optical losses. Further, most refractive micro-lens technologies are limited in the overall sagitta of the lenses that can be fabricated. Limitations on the height of the lenses limits focusing power and the degree of angular deflection that can be applied to the laser beam. 
     Diffractive lenses are typically less expensive to fabricate than refractive lenses. Diffractive lenses that approximate refractive lenses having complex aspherical surface profiles and large sagitta can be fabricated without extra expense. A typical disadvantage of diffractive optics lies in the area of efficiency. Only light that is diffracted into the correct diffraction order is properly focused with the remaining light being lost. Insertion losses per diffractive lens are typically in the range from about 1 dB to more than 3 dB. Specifically, diffraction efficiency typically declines as focusing power and angular deflection increase. When large angular deflections are needed, the insertion loss due to the diffraction efficiency may become substantial. Choices for using either refractive or diffractive lenses will depend on the particular embodiment in accordance with the invention given the issues of insertion loss and cost. 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.