Abstract:
An apparatus for multiplexing and demultiplexing comprises: a substrate having a first surface and a second surface, where the second surface is opposite to the first surface; a first fiber array unit disposed on the first surface of the substrate and a second fiber array unit disposed on the second surface of the substrate, where a plurality of fibers in a fiber array unit are arranged in an array on a chip; a first microlens array disposed on the first surface of the substrate and a second microlens array disposed on the second surface of the substrate; a plurality of thin film filters disposed on the first and second surface of the substrate, where each thin film filter transmits light having a different wavelength band; a fiber collimator disposed on the first surface of the substrate; a turning prism disposed at an edge of the substrate for turning light from the first surface to the second surface of the substrate or also turning light from the second surface to the first surface of the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit to U.S. Provisional Application No. 61/995,692, filed Apr. 18, 2014. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to free-space multiplexer and demultiplexer, and more specifically relates to ultra compact free-space multiplexer and demultiplexer comprising fiber array unit (FAU) and microlens array (MLA). 
     BACKGROUND OF THE INVENTION 
     Wavelength division multiplexing (WDM) technology may multiplex more than one carrier signals in a single optical fiber. Each carrier signal has different wavelength. Carrier signals may be combined by a multiplexer (Mux). The combined carrier signals may travel in a fiber. The combined carrier signals form a multiplexed signal. After leaving the fiber, the combined carrier signals may be separated into the original carrier signals by a demultiplexer (Demux). 
     Mux and Demux may be constructed using various approaches, for example, Bragg grating approach, fused fiber coupler approach, and the like. An approach is based on a set of thin film filters (TFFs). A first TFF may transmit a first wavelength, e.g., λ1, and reflect all other wavelengths, e.g., λ2, λ3, and λ4. Thus, a first carrier signal having wavelength λ1 may be selected. A second TFF may transmit a second wavelength, e.g., λ2, and reflect the remaining wavelengths, e.g., λ3, and λ4. Thus, a second carrier signal having wavelength λ2 may be selected. A third TFF may transmit a third wavelength, e.g., λ3, and reflect the remaining wavelength, e.g., λ4. Thus, a third carrier signal having wavelength λ3 may be selected. Finally, the last carrier signal having wavelength λ4 remains and thus it may be selected as well. 
     A free-space WDM Demux using TFFs is disclosed in U.S. Pat. No. 5,583,683 to Scobey. Scobey teaches that the multiplexed carrier signals containing wavelength λ1-λ8 exit from a common fiber coupled with a fiber collimator. The multiplexed carrier signals are collimated by the fiber collimator and propagate in free space until impinge on a first TFF. The first TFF transmits the first carrier signal having wavelength λ1 and reflects the remaining carrier signals having wavelengths λ2-λ8. The transmitted collimated carrier signal having wavelength λ1 is coupled to a fiber channel 1 by a fiber collimator. 
     The reflected remaining carrier signals impinge on a second TFF. The second TFF transmits the second carrier signal having wavelength λ2 and reflects the remaining carrier signals having wavelengths λ 3 -λ8. The transmitted collimated carrier signal having wavelength λ2 is coupled to a fiber channel 2 by a fiber collimator. The process is repeated until the last TFF transmits the carrier signal having wavelength λ8. The device can also function as a Mux when the light path is reversed. 
     A compact free-space WDM Mux/Demux using TFFs is disclosed in U.S. Pat. No. 8,538,210 to Wang et al. Wang et al. teaches that the fibers and their corresponding fiber collimators are disposed on two opposite sides&#39; of a substrate. Both Scobey and Wang et al. use fiber collimators. A fiber collimator typically has a diameter larger than 1 mm. Thus the sizes of Muxs/Demuxs of Scobey and Wang et al. may not be reduced, since the sizes are limited by the size of fiber collimators used. Accordingly, new approaches to further reduce the size of free-space WDM Mux/Demux are required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views and embodiments unless otherwise specified. 
         FIG. 1  shows a Mux/Demux comprising a fiber collimator, a fiber array unit (FAU), and a microlens array (MLA). 
         FIG. 2A  shows a pair of mirrors replacing the prism in  FIG. 1 . 
         FIG. 2B  shows a pair of prisms replacing the prism in  FIG. 1 . 
         FIG. 3  shows a Mux/Demux comprising a FAU and a MLA without a fiber collimator. 
         FIG. 4  shows a Mux/Demux comprising a fiber collimator disposed on the opposite side from a FAU. 
         FIG. 5  shows an arrangement on a first surface of a substrate of a Mux/Demux. 
         FIG. 6  shows an arrangement on a second surface of the substrate of the Mux/Demux of  FIG. 5 . 
         FIG. 7A  shows a cross-section perpendicular to a turning prism. 
         FIG. 7B  shows a cross-section parallel to a first surface and a second surface of a substrate. 
         FIG. 8  shows a perspective view of the Mux/Demux of  FIG. 5  and  FIG. 6 . 
         FIG. 9  shows an arrangement on a first surface of a substrate of a Mux/Demux. 
         FIG. 10  shows an arrangement on a second surface of the substrate of the Mux/Demux of  FIG. 9 . 
         FIG. 11  shows a perspective view of the Mux/Demux of  FIG. 9  and  FIG. 10 . 
         FIG. 12A  shows a cross-section perpendicular to a turning prism for α=0. 
         FIG. 12B  shows a cross-section parallel to a first surface and a second surface of a substrate for α=0. 
         FIG. 12C  shows a cross-section similar to  FIG. 12A  for α≠0. 
         FIG. 12D  shows a cross-section similar to  FIG. 12B  for α≠0. 
         FIG. 13  shows an arrangement on a first surface of a substrate of a Mux/Demux. 
         FIG. 14  an arrangement on a second surface of the substrate of the Mux/Demux of  FIG. 13 . 
         FIG. 15A  shows a cross-section perpendicular to a turning prism. 
         FIG. 15B  shows a cross-section parallel to a first surface and a second surface of a substrate. 
         FIG. 16  shows a perspective view of the Mux/Demux of  FIG. 13  and  FIG. 14 . 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. 
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable combinations and/or sub-combinations in one or more embodiments. 
       FIG. 1  shows an ultra compact free-space Mux/Demux  100  using a fiber collimator  104 , a fiber array unit (FAU)  116 , and a microlens array (MLA)  114 , according to an embodiment of the invention. When Mux/Demux  100  operates as a Demux, an input fiber  102  coupled with fiber collimator  104  form a common (COM) port. For example, the multiplexed signal has four wavelength bands λ1-λ4. For simplicity, wavelength bands are also referred to as wavelengths. Light  106  exiting from fiber collimator  104  is reflected twice by a prism  108  and is directed toward a first TFF  110  in free space. First TFF  110  transmits light having wavelength λ1 and reflects the remaining light having other wavelengths toward a mirror  112 . Light transmitted by first TFF  110  is focused by a first microlens of MLA  114  onto a first fiber of FAU  116 . The first fiber of FAU  116  forms channel 1 (CH1) port. 
     Light reflected by first TFF  110  toward mirror  112  is further reflected by mirror  112  toward a second TFF  118 . Second TFF  118  transmits light having wavelength λ2 and reflects the remaining light having other wavelengths toward a mirror  120 . Light transmitted by second TFF  118  is focused by a second microlens of MLA  114  onto a second fiber of FAU  116 . The second fiber of FAU  116  forms channel 2 (CH2) port. 
     Light reflected by second TFF  118  toward mirror  120  is further reflected by mirror  120  toward a third TFF  122 . Third TFF  122  transmits light having wavelength λ3 and reflects the remaining light having other wavelengths toward a mirror  124 . Light transmitted by third TFF  122  is focused by a third microlens of MLA  114  onto a third fiber of FAU  116 . The third fiber of FAU  116  forms channel 3 (CH3) port. 
     Light reflected by third TFF  122  toward mirror  124  is further reflected by mirror  124  toward an optional fourth TFF  126 . Optionally, fourth TFF  126  transmits the remaining light having wavelength λ4. Light transmitted by optional fourth TFF  126  is focused by a fourth microlens of MLA  114  onto a fourth fiber of FAU  116 . The fourth fiber of FAU  116  forms channel 4 (CH4) port. All elements of Mux/Demux  100  may be disposed on a substrate  128 . 
     A FAU comprises at least one fiber. The fibers are arranged in an array on a chip. The chip may be V-grooved with a pitch 127 λm or 250 λm. The fibers may be disposed in the V-grooves. The chip may be made from glass, quartz, silicon, pyrex, polymeric materials, and the like. For example, a FAU may be available from AFOP, Fremont, Calif., the assignee of the present invention (www.afop.com). 
     A MLA may comprise 2D array or 1D array of microlenses. The lens pitch in the array may be 250 μm, which is the same as the fiber pitch of FAU. The MLA may be made from glass, quartz, silicon, pyrex, polymeric materials, and the like. For example, a MLA may be available from SUSS MicroOptics, Neuchatel, Switzerland (www.suss-microoptics.com). The fiber pitch and the microlens pitch, which are only 250 μm, make the ultra compact free-space Mux/Demux possible. 
     Prism  108  in  FIG. 1  may be replaced by a pair of mirrors  204  as shown in  FIG. 2A , according to an embodiment of the invention, or a pair of prisms  206  as shown in  FIG. 2B , according to an embodiment of the invention. 
       FIG. 3  shows an ultra compact free-space Mux/Demux  300  using a FAU and a MLA without a fiber collimator, according to an embodiment of the invention.  FIG. 3  is similar to  FIG. 1 . For brevity, detailed descriptions of identical portions are omitted. 
     When Mux/Demux  300  operates as a Demux, a first fiber  302  of a FAU  116  forms a common (COM) port. Light  304  exiting from first fiber  302  is collimated by a first microlens  306  of a MLA  114  and directed to a prism  108  in free space. The collimated light is reflected twice by prism  108  and is directed toward a first TFF  110 . Similar to  FIG. 1 , light having wavelength λ1 is outputted from CH1 port, light having wavelength λ2 is outputted from CH2 port, light having wavelength λ3 is outputted from CH3 port, and light having wavelength λ4 is outputted from CH4 port. 
     Similarly, prism  108  in  FIG. 3  may be replaced by a pair of mirrors  204  as shown in  FIG. 2A , according to an embodiment of the invention, or a pair of prisms  206  as shown in  FIG. 2B , according to an embodiment of the invention. 
     It is appreciated that 4 channel Mux/Demux  300  of  FIG. 3  and Mux/Demux  100  of  FIG. 1  may be extended to 8 channel or any channel Mux/Demux by adding fibers of FAU  116  and using a corresponding MLA  114 . 
       FIG. 4  shows an ultra compact free-space Mux/Demux  400  similar to Mux/Demux  100  of  FIG. 1 , according to an embodiment of the invention. For brevity, detailed descriptions of identical portions are omitted. Mux/Demux  400  comprises an input fiber  102  coupled with a fiber collimator  104  forming a common (COM) port. Fiber collimator  104  is disposed on the opposite side from FAU  116 . Light  106  exiting from fiber collimator  104  is directed toward a first TFF  110  in free space without using a prism as in Mux/Demux  100  of  FIG. 1 . Similar to  FIG. 1 , light having wavelength λ1 is outputted from CH1 port, light having wavelength λ2 is outputted from CH2 port, light having wavelength λ3 is outputted from CH3 port, and light having wavelength λ4 is outputted from CH4 port. 
     A Mux/Demux may be disposed on two surfaces of the substrate.  FIG. 5  shows arrangement of an ultra compact free-space Mux/Demux  500  on a first surface of a substrate  128  comprising four channels, according to an embodiment of the invention. When the Mux/Demux  500  operates as a Demux, an input fiber  102  coupled with a fiber collimator  104  forms a common (COM) port. Light  106  exiting from fiber collimator  104  is deflected by a prism  502  and is directed toward an edge pass filter  504  in free space. For example, Mux/Demux  500  has eight channels, four channels on each side of the substrate. Edge pass filter  504  may transmit light having wavelengths λ5-λ8 toward a turning prism  506  and reflect light having wavelengths λ1-λ4. 
     Light having wavelengths λ1-λ4 reflected by edge pass filter  504  is directed toward a first TFF  110 . First TFF  110  transmits light having wavelength λ1 and reflects the remaining light having other wavelengths toward a mirror  112 . Light transmitted by first TFF  110  is focused by a first microlens of a first MLA  114  onto a first fiber of a first FAU  116 . The first fiber of first FAU  116  forms channel 1 (CH1) port. 
     Light reflected by first TFF  110  toward mirror  112  is further reflected by mirror  112  toward a second TFF  118 . Second TFF  118  transmits light having wavelength λ2 and reflects the remaining light having other wavelengths toward a mirror  120 . Light transmitted by second TFF  118  is focused by a second microlens of first MLA  114  onto a second fiber of first FAU  116 . The second fiber of first FAU  116  forms channel 2 (CH2) port. 
     Light reflected by second TFF  118  toward mirror  120  is further reflected by mirror  120  toward a third TFF  122 . Third TFF  122  transmits light having wavelength λ3 and reflects the remaining light having wavelength λ4 toward a mirror  124 . Light transmitted by third TFF  122  is focused by a third microlens of first MLA  114  onto a third fiber of first FAU  116 . The third fiber of first FAU  116  forms channel 3 (CH3) port. 
     Light reflected by third TFF  122  toward mirror  124  is further reflected by mirror  124  toward an optional fourth TFF  126 . Fourth TFF  126  transmits the remaining light having wavelength λ4. Light transmitted by fourth TFF  126  is focused by a fourth microlens of first MLA  114  onto a fourth fiber of first FAU  116 . Optionally, there is no fourth TFF  126 , light reflected by mirror  124  is focused by a fourth microlens of first MLA  114  onto a fourth fiber of first FAU  116 . The fourth fiber of first FAU  116  forms channel 4 (CH4) port. 
       FIG. 6  shows arrangement of ultra compact free-space Mux/Demux  500  on a second surface of substrate  128  comprising four channels, according to an embodiment of the invention. The second surface of substrate  128  is opposite to the first surface of substrate  128 . In  FIG. 5 , edge pass filter  504  transmits light having wavelengths λ5-λ8 toward turning prism  506 . 
     For better understanding, turning prism  506  is illustrated in  FIG. 7A  and  FIG. 7B , according to an embodiment of the invention.  FIG. 7A  shows a cross-section  702  of turning prism  506 , cross-section  702  is perpendicular to turning prism  506 . Light  106  transmitted through edge pass filter  504  having wavelengths λ5-λ8 on the first surface of substrate  128  is reflected 90° by turning prism  506  becoming light  706 . Light  706  is once more reflected 90° by turning prism  506  becoming light  606  on the second surface of substrate  128 . 
       FIG. 7B  shows a cross-section  704  of turning prism  506 , cross-section  704  is parallel to the first surface and the second surface of substrate  128 . In  FIG. 7B , light  706  is not shown since it is perpendicular to the paper. Light  106  is incident obliquely to turning prism  506  and light  606  is reflected obliquely by turning prism  506 . Light  106  and light  606  do not overlap on cross-section  704 . Cross-section  702  ( FIG. 7A ) cuts along line BB′ in  FIG. 7B , and cross-section  704  ( FIG. 7B ) cuts along line AA′ in  FIG. 7A . 
     Referring back to  FIG. 6 , light  606  having wavelengths λ5-λ8 is reflected from turning prism  506  toward a fifth TFF  610  in free space. Fifth TFF  610  transmits light having wavelength λ5 and reflects the remaining light having other wavelengths toward a mirror  612 . Light transmitted by fifth TFF  610  is focused by a first microlens of a second MLA  614  onto a first fiber of a second FAU  616 . The first fiber of second FAU  616  forms channel 5 (CH5) port. 
     Light reflected by fifth TFF  610  toward mirror  612  is further reflected by mirror  612  toward a sixth TFF  618 . Sixth TFF  618  transmits light having wavelength λ6 and reflects the remaining light having other wavelengths toward a mirror  620 . Light transmitted by sixth TFF  618  is focused by a second microlens of second MLA  614  onto a second fiber of second FAU  616 . The second fiber of second FAU  616  forms channel 6 (CH6) port. 
     Light reflected by sixth TFF  618  toward mirror  620  is further reflected by mirror  620  toward a seventh TFF  622 . Seventh TFF  622  transmits light having wavelength λ7 and reflects the remaining light having wavelength λ8 toward a mirror  624 . Light transmitted by seventh TFF  622  is focused by a third microlens of second MLA  614  onto a third fiber of second FAU  616 . The third fiber of second FAU  616  forms channel 7 (CH7) port. 
     Light reflected by seventh TFF  622  toward mirror  624  is further reflected by mirror  624  toward an optional eighth TFF  626 . Eighth TFF  626  transmits the remaining light having wavelength λS. Light transmitted by eighth TFF  626  is focused by a fourth microlens of second MLA  614  onto a fourth fiber of second FAU  616 . Optionally, there is no eighth TFF  626 , light reflected by mirror  624  is focused by a fourth microlens of second MLA  614  onto a fourth fiber of second FAU  616 . The fourth fiber of second FAU  616  forms channel 8 (CH8) port. 
       FIG. 8  shows a perspective view of ultra compact free-space Mux/Demux  500  of  FIG. 5  and  FIG. 6 , according to an embodiment of the invention. Accordingly, elements shown in  FIG. 8  are the same as elements shown in  FIG. 5  and  FIG. 6 .  FIG. 8  especially shows the 3D position of turning prism  506  relative to substrate  128 . 
       FIG. 9  shows arrangement of an ultra compact free-space Mux/Demux  900  on a first surface of a substrate  128  comprising four channels, according to an embodiment of the invention. For example, Mux/Demux  900  has eight channels, four channels on each side. When the Mux/Demux  900  operates as a Demux, an input fiber  102  coupled with a fiber collimator  104  forms a common (COM) port. 
     Light  106  exiting from fiber collimator  104  having wavelengths λ1-λ8 is reflected by a first prism  108  and is directed toward a first TFF  110  in free space. First TFF  110  transmits light having wavelength λ1 and reflects the remaining light having other wavelengths toward a mirror  112 . Light transmitted by first TFF  110  is focused by a first microlens of a first MLA  114  onto a first fiber of a first FAU  116 . The first fiber of first FAU  116  forms channel 1 (CH1) port. 
     Light reflected by first TFF  110  toward mirror  112  is further reflected by mirror  112  toward a second TFF  118 . Second TFF  118  transmits light having wavelength λ2 and reflects the remaining light having other wavelengths toward a mirror  120 . Light transmitted by second TFF  118  is focused by a second microlens of first MLA  114  onto a second fiber of first FAU  116 . The second fiber of first FAU  116  forms channel 2 (CH2) port. 
     Light reflected by second TFF  118  toward mirror  120  is further reflected by mirror  120  toward a third TFF  122 . Third TFF  122  transmits light having wavelength λ3 and reflects the remaining light having other wavelengths toward a mirror  124 . Light transmitted by third TFF  122  is focused by a third microlens of first MLA  114  onto a third fiber of first FAU  116 . The third fiber of first FAU  116  forms channel 3 (CH3) port. 
     Light reflected by third TFF  122  toward mirror  124  is further reflected by mirror  124  toward a fourth TFF  126 . Fourth TFF  126  transmits light having wavelength λ4 and reflects the remaining light having other wavelengths toward a second prism  902 . Light transmitted by fourth TFF  126  is focused by a fourth microlens of first MLA  114  onto a fourth fiber of first FAU  116 . The fourth fiber of first FAU  116  forms channel 4 (CH4) port. 
     Light reflected by fourth TFF  126  toward second prism  902  is further deflected by second prism  902  toward a turning prism  904 . Light  906  deflected by second prism  902  has wavelengths λ5-λ8. Light  906  having wavelengths λ5-λ8 on the first surface of substrate  128  coming from second prism  902  is reflected 90° twice by turning prism  904  becoming light  1002  on a second surface of substrate  128  (see  FIG. 10 ). 
       FIG. 10  shows arrangement of ultra compact free-space Mux/Demux  900  on the second surface of substrate  128  comprising four channels, according to an embodiment of the invention. The second surface of substrate  128  is opposite to the first surface of substrate  128  shown in  FIG. 9 . 
     Light  1002  having wavelengths λ5-λ8 is coming from turning prism  904  toward a fifth TFF  1002  in free space. Fifth TFF  1004  transmits light having wavelength λ8 and reflects the remaining light having other wavelengths toward a mirror  1006 . Light transmitted by fifth TFF  1004  is focused by a first microlens of a second MLA  1008  onto a first fiber of a second FAU  1010 . The first fiber of second FAU  1010  forms channel 8 (CH8) port. Note that the first microlens of second MLA  1008  is positioned closest to fifth TFF  1004 , and the first fiber of second FAU  1010  is positioned next to the first microlens of second MLA  1008 . 
     Light reflected by fifth TFF  1004  toward mirror  1006  is further reflected by mirror  1006  toward a sixth TFF  1012 . Sixth TFF  1012  transmits light having wavelength λ7 and reflects the remaining light having other wavelengths toward a mirror  1014 . Light transmitted by sixth TFF  1012  is focused by a second microlens of second MLA  1008  onto a second fiber of second FAU  1010 . The second fiber of second FAU  1010  forms channel 7 (CH7) port. 
     Light reflected by sixth TFF  1012  toward mirror  1014  is further reflected by mirror  1014  toward a seventh TFF  1016 . Seventh TFF  1016  transmits light having wavelength λ6 and reflects the remaining light having wavelength λ5 toward a mirror  1018 . Light transmitted by seventh TFF  1016  is focused by a third microlens of second MLA  1008  onto a third fiber of second FAU  1010 . The third fiber of second FAU  1010  forms channel 6 (CH6) port. 
     Light reflected by seventh TFF  1016  toward mirror  1018  is further reflected by mirror  1018  toward an optional eighth TFF  1020 . Eighth TFF  1020  transmits the remaining light having wavelength λ5. Light transmitted by eighth TFF  1020  is focused by a fourth microlens of second MLA  1008  onto a fourth fiber of second FAU  1010 . Optionally, there is no eighth TFF  1020 , light reflected by mirror  1018  is focused by a fourth microlens of second MLA  1008  onto a fourth fiber of second FAU  1010 . The fourth fiber of second FAU  1008  forms channel 5 (CH5) port. 
       FIG. 11  shows a perspective view of ultra compact free-space Mux/Demux  900  of  FIG. 9  and  FIG. 10 , according to an embodiment of the invention. Accordingly, elements shown in  FIG. 11  are the same as elements shown in  FIG. 9  and  FIG. 10 .  FIG. 11  especially shows the 3D position of turning prism  904  relative to substrate  128 . Turning prism  904  is rotated by a from a vertical axis  1102  perpendicular to substrate  128 . 
     For better understanding, turning prism  904  is illustrated in  FIGS. 12A through 12D , according to an embodiment of the invention.  FIG. 12A  shows a cross-section  1202  of turning prism  904  for α=0. Cross-section  1202  is perpendicular to the first surface and the second surface of substrate  128  along line AA′ in  FIG. 9 . Light  906  coming from second prism  902  having wavelengths λ5-λ8 on the first surface of substrate  128  is reflected 90° by turning prism  904  becoming light  1206 . Light  1206  is once more reflected 90° by turning prism  904  becoming light  1002  on the second surface of substrate  128 .  FIG. 12B  shows a cross-section  1204  of turning prism  904  for α=0. Cross-section  1204  is parallel to the first surface and the second surface of substrate  128  along line BB′ in  FIG. 11 . In  FIG. 12B , light  1206  is not shown since it is perpendicular to the paper. 
       FIG. 12C  shows a cross-section  1202  of turning prism  904  for α≠0. Cross-section  1202  is perpendicular to the first surface and the second surface of substrate  128  along line AA′ in  FIG. 9 . Comparing to  FIG. 12A , light  1002  is closer to the second surface of substrate  128  because turning prism  904  is oriented at α≠0.  FIG. 12D  shows a cross-section  1204  of turning prism  904  for α≠0. Cross-section  1204  is parallel to the first surface and the second surface of substrate  128  along line BB′ in  FIG. 11 . Comparing to  FIG. 12B , light  1002  is shifted by a distance d  1208  from light  906  because turning prism  904  is oriented at α≠0. 
       FIG. 13  shows arrangement of an ultra compact free-space Mux/Demux  1300  on a first surface of a substrate  128  comprising four channels, according to an embodiment of the invention. For example, Mux/Demux  1300  has eight channels, four channels on each side. When the Mux/Demux  1300  operates as a Demux, an input fiber  102  coupled with a fiber collimator  104  forms a common (COM) port.  FIG. 14  shows arrangement of ultra compact free-space Mux/Demux  1300  on a second surface of substrate  128  comprising four channels, according to an embodiment of the invention. The second surface of substrate  128  is opposite to the first surface of substrate  128 . 
     Mux/Demux  1300  comprises a prism  1302  disposed on the first surface of substrate  128 , and a turning prism  1304  at a slating edge of substrate  128  for turning light from the first surface of substrate  128  to the second surface of substrate  128 , and from the second surface of substrate  128  to the first surface of substrate  128 . Prism  1302  deflects light  106  from fiber collimator  104  toward turning prism  1304  in free space. 
     The following description refers to both  FIG. 13  and  FIG. 14 . Ultra compact free-space Mux/Demux  1300  comprises a first MLA  1306  and a first FAU  1308  on the first surface of substrate  128 , and a second MLA  1310  and a second FAU  1312  on the second surface of substrate  128 . 
     Light  106  exiting from fiber collimator  104  having wavelengths λ1-λ8 is deflected by prism  1302  and is directed toward turning prism  1304  in free space. Turning prism  1304  reflects light from prism  1302 , turning it from the first surface of substrate  128  to the second surface of substrate  128 , and directs it toward a first TFF  1314  on the second surface of substrate  128  in free space. First TFF  1314  transmits light having wavelength λ1 and reflects the remaining light having other wavelengths toward turning prism  1304 . Light transmitted by first TFF  1314  is focused by a first microlens of second MLA  1310  onto a first fiber of second FAU  1312 . The first fiber of second FAU  1312  forms channel 1 (CH1) port on the second surface of substrate  128 . 
     Light reflected by first TFF  1314  toward turning prism  1304  is turned from the second surface of substrate  128  to the first surface of substrate  128  and reflected by turning prism  1304  toward a second TFF  1316  on the first surface of substrate  128 . Second TFF  1316  transmits light having wavelength λ2 and reflects the remaining light having other wavelengths toward turning prism  1304 . Light transmitted by second TFF  1316  is focused by a first microlens of first MLA  1306  onto a first fiber of first FAU  1308 . The first fiber of first FAU  1308  forms channel 2 (CH2) port on the first surface of substrate  128 . 
     Light reflected by second TFF  1316  toward turning prism  1204  is turned from the first surface of substrate  128  to the second surface of substrate  128  and reflected by turning prism  1304  toward a third TFF  1318  on the second surface of substrate  128 . Third TFF  1318  transmits light having wavelength λ3 and reflects the remaining light having other wavelengths toward turning prism  1304 . Light transmitted by third TFF  1318  is focused by a second microlens of second MLA  1310  onto a second fiber of second FAU  1312 . The second fiber of second FAU  1312  forms channel 3 (CH3) port on the second surface of substrate  128 . 
     Light reflected by third TFF  1318  toward turning prism  1304  is turned from the second surface of substrate  128  to the first surface of substrate  128  and reflected by turning prism  1304  toward a fourth TFF  1320  on the first surface of substrate  128 . Fourth TFF  1320  transmits light having wavelength λ4 and reflects the remaining light having other wavelengths toward turning prism  1304 . Light transmitted by fourth TFF  1320  is focused by a second microlens of first MLA  1306  onto a second fiber of first FAU  1308 . The second fiber of first FAU  1308  forms channel 4 (CH4) port on the first surface of substrate  128 . 
     Light reflected by fourth TFF  1320  toward turning prism  1204  is turned from the first surface of substrate  128  to the second surface of substrate  128  and reflected by turning prism  1204  toward a fifth TFF  1322  on the second surface of substrate  128 . Fifth TFF  1322  transmits light having wavelength λ5 and reflects the remaining light having other wavelengths toward turning prism  1304 . Light transmitted by fifth TFF  1322  is focused by a third microlens of second MLA  1310  onto a third fiber of second FAU  1312 . The third fiber of second FAU  1312  forms channel 5 (CH5) port on the second surface of substrate  128 . 
     Light reflected by fifth TFF  1322  toward turning prism  1304  is turned from the second surface of substrate  128  to the first surface of substrate  128  and reflected by turning prism  1304  toward a sixth TFF  1324  on the first surface of substrate  128 . Sixth TFF  1324  transmits light having wavelength λ6 and reflects the remaining light having other wavelengths toward turning prism  1304 . Light transmitted by sixth TFF  1324  is focused by a third microlens of first MLA  1306  onto a third fiber of first FAU  1308 . The third fiber of first FAU  1308  forms channel 6 (CH6) port on the first surface of substrate  128 . 
     Light reflected by sixth TFF  1324  toward turning prism  1304  is turned from the first surface of substrate  128  to the second surface of substrate  128  and reflected by turning prism  1304  toward a seventh TFF  1326  on the second surface of substrate  128 . Seventh TFF  1326  transmits light having wavelength λ7 and reflects the remaining light having wavelength λ8 toward turning prism  1304 . Light transmitted by seventh TFF  1326  is focused by a fourth microlens of second MLA  1310  onto a fourth fiber of second FAU  1312 . The fourth fiber of second FAU  1312  forms channel 7 (CH7) port on the second surface of substrate  128 . 
     Light reflected by seventh TFF  1326  toward turning prism  1304  is turned from the second surface of substrate  128  to the first surface of substrate  128  and reflected by turning prism  1304  toward an optional eighth TFF  1328  on the first surface of substrate  128 . Eighth TFF  1328  transmits the remaining light having wavelength λ8. Light transmitted by eighth TFF  1328  is focused by a fourth microlens of first MLA  1306  onto a fourth fiber of first FAU  1308 . Optionally, there is no eighth TFF  1328 , light reflected by turning prism  1304  is focused by a fourth microlens of first MLA  1306  onto a fourth fiber of first FAU  1308 . The fourth fiber of first FAU  1308  forms channel 8 (CH8) port on the first surface of substrate  128 . 
     For better understanding, turning prism  1304  is illustrated in  FIG. 15A  and  FIG. 15B , according to an embodiment of the invention.  FIG. 15A  shows a cross-section  1502  of turning prism  506 , cross-section  702  is perpendicular to turning prism  1304 . Light  1506  on the first surface of substrate  128  is reflected 90° by turning prism  1304  becoming light  1508 . Light  1508  is once more reflected 90° by turning prism  1304  becoming light  1510  on the second surface of substrate  128 . 
       FIG. 15B  shows a cross-section  1504  of turning prism  1304 , cross-section  1504  is parallel to the first surface and the second surface of substrate  128 . The incident light and the reflected light form oblique angles with turning prism  1302 . Cross-section  1502  ( FIG. 15A ) cuts along line BB′ in  FIG. 15B , and cross-section  1504  ( FIG. 15B ) cuts along line AA′ in  FIG. 15A . 
       FIG. 15B  shows schematically light paths: (1) from prism  1302  on the first surface of substrate  128  to first TFF  1314  on the second surface of substrate  128 , (2) from first TFF  1314  on the second surface of substrate  128  to second TFF  1316  on the first surface of substrate  128 , (3) from second TFF  1316  on the first surface of substrate  128  to third TFF  1318  on the second surface of substrate  128 , (4) from third TFF  1318  on the second surface of substrate  128  to fourth TFF  1320  on the first surface of substrate  128 , (5) from fourth TFF  1320  on the first surface of substrate  128  to fifth TFF  1322  on the second surface of substrate  128 , (6) from fifth TFF  1322  on the second surface of substrate  128  to sixth TFF  1324  on the first surface of substrate  128 , (7) from sixth TFF  1324  on the first surface of substrate  128  to seventh TFF  1326  on the second surface of substrate  128 , and (8) from seventh TFF  1326  on the second surface of substrate  128  to eighth TFF  1328  on the first surface of substrate  128 . Elements on the first surface of substrate are illustrated in solid lines, elements on the second surface of substrate  128  are illustrated in broken lines. 
     All light paths on the first surface of substrate  128  in  FIG. 15B  correspond to light  1506  in  FIG. 15A . All light paths on the second surface of substrate  128  in  FIG. 15B  correspond to light  1510  in  FIG. 15B . Light  1508  is not shown in  FIG. 15B  since it is perpendicular to the paper. 
       FIG. 16  shows a perspective view of ultra compact free-space Mux/Demux  1300  of  FIG. 13  and  FIG. 14 , according to an embodiment of the invention. Accordingly, elements shown in  FIG. 16  are the same as elements shown in  FIG. 12  and  FIG. 13 .  FIG. 16  especially shows the 3D position of turning prism  1304  relative to substrate  128 . 
     It is appreciated that ultra compact free-space Mux/Demux having eight channel ports is for example only. An ultra compact free-space Mux/Demux may have less or more than eight channel ports. One side of the Mux/Demux may have less or more than four channel ports. Mirrors  112 ,  120 , and  124  of  FIG. 5  may be replaced by a single mirror because the tilt angles of mirrors  112 ,  120  and  124  are the same. Similarly, mirrors  612 ,  620 , and  624  of  FIG. 6 , mirrors  112 ,  120 , and  124  of  FIG. 9 , mirrors  1004 ,  1012 , and  1016  of  FIG. 10 , may be replaced by respective single mirrors. 
     While the present invention has been described herein with respect to the exemplary embodiments and the best mode for practicing the invention, it will be apparent to one of ordinary skill in the art that many modifications, improvements and sub-combinations of the various embodiments, adaptations and variations can be made to the invention without departing from the spirit and scope thereof. 
     The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.