Abstract:
A packaged stack of optical devices includes two or more WDM optical devices, the stack having a reduced per-channel manufacturing cost and an improved mean time between failure relative to individual optical devices. WDM optical devices, which may be contained in a packaged stack, include wavelength selective switches, optical add-drop multiplexers, and dynamic gain equalizers. The optical switching devices in the stack may be configured so that one or more optical elements are shared by multiple switching devices. Optical components that may be shared between the switching devices contained in the stack include cylindrical lenses, diffraction gratings, mirrors, and beam steering units.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention relate generally to optical communication systems and components and, more particularly, to a packaged stack of optical devices. 
         [0003]    2. Description of the Related Art 
         [0004]    In a wavelength division multiplexing (WDM) optical communication system, information is carried by multiple channels, each channel corresponding to a unique wavelength. WDM allows transmission of data from different sources over the same fiber optic link simultaneously, since each data source is assigned a dedicated channel. The result is an optical communication link with an aggregate bandwidth that increases with the number of wavelengths, or channels, incorporated into the WDM signal. In this way, WDM technology maximizes the use of an available fiber optic infrastructure; what would normally require multiple optic links or fibers instead requires only one. 
         [0005]    In WDM optical communication systems, it is often necessary to add, drop, and/or attenuate a light beam. This can be achieved by an optical switching device, which directs an input light beam to one of multiple output optical paths. For example, in a 1×2 optical switching device, an input light beam enters through an input fiber and is directed to one of two output fibers. There are also more complicated optical switching devices, such as 2×2, 1×N, and N by N switching devices, which are sometimes realized by combining several 1×2 devices. In some optical networks, the individual wavelength channels of a WDM input signal are directed to different output fibers by an optical switching device, such as a wavelength selective switch (WSS) or an optical add-drop multiplexer (OADM). In addition, individual wavelength channels of a WDM input signal may be attenuated by an optical switching device, such as a dynamic gain equalizer (DGE). 
         [0006]    The optical switching devices for WDM communication systems, such as WSSs, OADMs, and DGEs, are quite complex, and include active and passive optical elements that must be manufactured and aligned to high tolerances for proper operation of such switching devices. Because of this, the manufacturing costs for assembly, testing and quality assurance of WDM optical switching devices are substantial. As bandwidth requirements for optical communication networks increase, it is desirable to reduce per-channel manufacturing costs and improve per-channel mean time between failure (MTBF) of WDM optical switching devices. 
         [0007]    Accordingly, there is a need for optical switching devices used in communications networks, e.g., WSSs, OADMs, and DGEs, for which the per-channel cost is reduced and MTBF is improved without adversely affecting network bandwidth. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the present invention provide a packaged stack of optical devices that includes two or more WDM optical devices, the stack having a reduced per-channel manufacturing cost and an improved mean time between failure relative to individual optical devices. 
         [0009]    In one embodiment, a packaged optical device stack comprises a first WDM optical device, a second WDM optical device, and a common housing for the first and second optical devices. Light beams transmitted through the first WDM optical device travel along a first set of optical paths and light beams transmitted through the second WDM optical device travel along a second set of optical paths, and the optical paths in the first set do not intersect the optical paths in the second set. The first and second WDM optical devices may share one or more optical elements, including a diffraction grating and/or a beam polarization unit. 
         [0010]    In another embodiment, a packaged optical device stack comprises a first WDM optical device having N input ports and M output ports, a second WDM optical device having N input ports and M output ports, and a common housing for the first and second WDM optical devices, wherein the first WDM optical device and the second WDM optical device share one or more optical elements, and wherein N is any positive integer and M is any positive integer greater than or equal to 2. The shared optical element may include a diffraction grating and/or a beam polarization unit. 
         [0011]    In yet another embodiment, a wavelength selective switch comprises a first set of input and output ports, a second set of input and output ports, and a light dispersing element for dispersing a first input light beam received through the first set of input and output ports into a first set of multiple wavelength components and a second input light beam received through the second set of input and output ports into a second set of multiple wavelength components. The wavelength selective switch further comprises a first optical switch for receiving the first set of multiple wavelength components and directing them to one of multiple directions and a second optical switch for receiving the second set of multiple wavelength components and directing them to one of multiple directions. The light dispersing element may be configured to receive the first set of multiple wavelength components that passed through the optical switch and combine them into a single output light beam and to receive the second set of multiple wavelength components that passed through the optical switch and combine them into a single output light beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0013]      FIG. 1A  is a perspective view of a WSS that may be expanded to form a packaged stack of multiple switching devices according to an embodiment of the invention. 
           [0014]      FIG. 1B  illustrates a schematic side view of a beam polarization unit and inbound and outbound light beams. 
           [0015]      FIG. 2A  schematically illustrates a perspective view of an extended beam polarization unit that may act as a beam polarization unit for two optical switching devices contained in a packaged stack according to an embodiment of the invention. 
           [0016]      FIG. 2B  is a perspective view of a double-deck WSS according to an embodiment of the invention. 
       
    
    
       [0017]    For clarity, identical reference numerals have been used, where applicable, to designate identical elements that are common between figures. It is contemplated that features of one embodiment may be incorporated in other embodiments without further recitation. 
       DETAILED DESCRIPTION 
       [0018]    Embodiments of the present invention provide a packaged stack of two or more WDM optical switching devices having a reduced per-channel manufacturing cost and an improved MTBF over individual optical switching devices. The packaged stack is particularly advantageous when the switching devices are configured so that one or more optical elements are shared by the switching devices contained in the packaged stack. 
         [0019]      FIG. 1A  is a perspective view of a WSS that may be expanded to form a packaged stack of multiple switching devices according to an embodiment of the invention. WSS  100  is a 1×4 WSS and includes an optical input port  101 , an optical output port stack  102 , a first beam shaping/steering section  1   10 , a diffraction grating  117 , a second beam shaping/steering section  120 , and a switching optics assembly  130 . The components of WSS  100  are mounted on a planar surface  190  that is herein defined as the horizontal plane for purposes of description. In the example described herein, planar surface  190  is substantially parallel to the plane traveled by light beams interacting with WSS  100 . Also for purposes of description, the configuration of WSS  100  described herein performs wavelength separation of a WDM signal in the horizontal plane and switching selection, i.e., channel routing, in the vertical plane. A WSS configured to perform switching selection in the horizontal plane and wavelength separation in the vertical plane may also be expanded to form a packaged stack of multiple switching devices. 
         [0020]    As described below in conjunction with  FIG. 2B , WSS  100  is scalable in the vertical plane, i.e., in the plane perpendicular to the plane traveled by light beams interacting with WSS  100 , and therefore may be expanded to include 2 or more stacked WSS&#39;s packaged together as a single stack. For example, WSS  100 , which is a 1×4 WDM wavelength selective switch, may be expanded to an optical switch stack of two or more WSS&#39;s with minimal additional cost. One example of such an optical switch stack is a double-deck WSS  200 , described below in conjunction with  FIG. 2B . In this way, the optical switch stack serves as a 2×8 WDM switch, a 3×12 WDM switch, etc., depending on how many WSS “levels” WSS  100  has. For clarity, the operation and organization of WSS  100 , i.e., a single, unexpanded WDM optical switching device, is first described in conjunction with  FIGS. 1A and 1B . 
         [0021]    For illustrative purposes, inbound light beams  150 ,  152 A-C,  154 A-C, and outbound light beams  151 ,  153 A-C,  155 A-C are shown in  FIG. 1A  to more clearly indicate the optical coupling of various elements of WSS  100 . Because of the bi-directional nature of most components of WSS  100 , light beams are directed along parallel inbound and outbound paths simultaneously between optical components of WSS  100 . The inbound and outbound paths are displaced from each other vertically, and this vertical displacement is further described below. For clarity, a single light beam is used in  FIG. 1A  to schematically represent both an inbound and outbound light beam between two optical components of WSS  100  rather than two beams that are vertically displaced with respect to one another. For example, inbound light beam  150  and outbound light beam  151  are schematically represented by a single light beam between folding mirror  113  and diffraction grating  117 . 
         [0022]    Optical input port  101  optically couples a WDM optical input signal (not shown) to WSS  100 . Optical output port stack  102  is, in the configuration shown in  FIG. 1A , positioned proximate input port  101 . Optical output port stack  102  includes four vertically aligned optical output ports  102 A-D and four vertically aligned loss ports  102 E-H. Optical output ports  102 A-D act as the optical output interface between WSS  100  and other components of a WDM optical communication system. Loss ports  102 E-H serve as termini for light beams consisting of unwanted optical energy, for example wavelength channels blocked from a WDM output signal. 
         [0023]    First beam shaping/steering section  110  includes a folding mirror  113 , beam steering unit  114 , and cylindrical lenses  115  and  116 . First beam shaping/steering section  110  optically couples diffraction grating  117  with optical input port  101  and optical output port stack  102 , and shapes inbound beam  150  and outbound beam  151 . First beam shaping/steering section  110  is also configured to direct outbound beam  151  to either a loss port or an optical output port contained in optical output port stack  102 , depending on the polarization state of outbound beams  153 A-C. Inbound beam  150  and outbound beam  151  may each contain a plurality of wavelength channels that are multiplexed into a single, “white” beam. Beam steering unit  114  is configured to direct outbound beam  151  along two different optical paths depending on the polarization state of outbound beam  151 . The two paths may be separated in the horizontal plane by an angular or translational offset. Beam steering unit  114  may be a Wollaston prism, which angularly deflects light beams at different angles depending on their orthogonal polarization states, or a birefringent crystal, such as a YVO 4  crystal, which translationally deflects the light beams by different amounts depending on their orthogonal polarization states. Beam steering unit  114  has a vertical axis of symmetry. 
         [0024]    Cylindrical lens  115  vertically extends inbound beam  250 , and cylindrical lens  216 , which has a vertical axis of symmetry, horizontally extends inbound beam  150 . Together, cylindrical lenses  115 ,  116  shape inbound beam  150  so that the beam is elliptical in cross-section when incident on diffraction grating  117 , wherein the major axis of the ellipse is parallel with the horizontal plane. 
         [0025]    Diffraction grating  117  is a vertically aligned reflective diffraction grating configured to spatially separate, or demultiplex, each wavelength channel of inbound beam  150  by directing each wavelength along a unique optical path. In so doing, diffraction grating  117  forms a plurality of inbound beams, wherein the number of inbound beams corresponds to the number of optical wavelength channels contained in inbound beam  150 . In  FIG. 1A , diffraction grating  117  is shown to separate inbound beam  150  into three inbound beams  152 A-C. However, in practice, the number of optical channels contained in inbound beam  150  may be up to  50  or more. Because the separation of wavelength channels by diffraction grating  117  takes place horizontally in the configuration shown in  FIG. 1A , spectral resolution is enhanced by widening inbound beam  150  in the horizontal plane, as performed by cylindrical lens  116 . Diffraction grating  117  also performs wavelength combination, referred to as multiplexing, of outbound beams  153 A-C into outbound beam  151 . 
         [0026]    Second beam shaping/steering section  120  includes a folding mirror  122 , cylindrical lenses  116 ,  121 , and a focusing lens  123 . Second beam shaping/steering section  120  optically couples diffraction grating  117  with switching optics assembly  130 , shapes inbound beams  152 A-C and outbound beams  153 A-C, and focuses inbound beams  152 A-C on the first element of switching optics assembly  130 , i.e., beam polarization unit  131 . Focusing lens  123 , like cylindrical lens  116 , has a vertical axis of symmetry. 
         [0027]    Switching optics assembly  130  includes an LC-based beam polarization unit  131 , collimating lenses  132 ,  133 , a beam steering unit  134 , collimating lenses  135 ,  136 , and an LC-based beam polarization and steering unit  137 . The elements of switching optics assembly  130  are optically linked to enable the optical routing of a WDM optical input signal entering optical input port  101  to any one of the optical output ports  102 A-D or loss ports  102 E-H. The optical routing is performed by conditioning (via LC polarization) and vertically displacing inbound beams  152 A-C to produce outbound beams  153 A-C. Switching optics assembly  130  selectively determines the vertical displacement of outbound beams  153 A-C to correspond to the vertical position of the desired output port, i.e., optical output port  102 A,  102 B,  102 C, or  102 D, hence performing a 1×4 optical switching operation. In addition, switching optics assembly  130  may selectively condition each of inbound beams  152 A-C to allow independent attenuation or blocking thereof. Further, switching optics assembly  130  performs the 1×4 switching operation with a high extinction ratio. Lastly, switching optics assembly  130  allows switching of outbound beam  151  between optical output ports  102 A-D to be hitless,” i.e., without the transmission of a signal to unwanted output ports. 
         [0028]    Beam polarization unit  131  includes an LC switching array  160  (shown in  FIG. 1  B) and an array of transparent electrodes, which together are configured to condition the polarization of each of inbound beams  152 A-C and produce inbound beams  154 A-C. LC switching array  160  and the array of transparent electrodes are also configured to condition the polarization state of outbound beams  155 A-C so that each beam, and therefore each wavelength channel of outbound beam  151 , may be independently attenuated or directed to one of loss ports  102 E-H. The electrodes are arranged vertically and horizontally to define individual LC pixels, the pixels being optically coupled to inbound or outbound beams as described below in conjunction with  FIG. 1B . 
         [0029]      FIG. 1B  illustrates a schematic side view of beam polarization unit  131 , inbound beams  154 A-C, and outbound beams  155 A-C. Switching stack  160  includes three horizontal arrays  161 - 163  of LCs. Each horizontal array  161 - 163  contains a plurality of LC pixels, one corresponding to each wavelength channel demultiplexed from inbound beam  150  by diffraction grating  117 . Each of inbound beams  155 A-C are directed through a corresponding LC of horizontal array  162 . Each of outbound beams  155 A-C are directed through a corresponding LC of horizontal array  161  and/or horizontal array  163  via up to four vertically displaced optical paths, as shown. How outbound beams are directed along up to four possible optical paths is described below in regard to beam steering unit  134  and beam polarization and steering unit  137 . 
         [0030]    Referring back to  FIG. 1A , beam steering unit  134  is configured to direct inbound beams  154 A-C along two different optical paths, i.e., an upper and a lower path, depending on the polarization state of the beams. As noted above, the polarization state of inbound beams  154 A-C is determined by the polarization conditioning performed by beam polarization unit  131 . The two optical paths are separated angularly or by a translational offset in the vertical direction. In either case, the vertical offset between the two possible paths for inbound beams  154 A-C indicates that inbound beams  154 A-C may be directed to either an upper or lower region of beam polarization and steering unit  137 . Beam steering unit  134  is also configured to direct outbound beams  155 A-C back through beam polarization unit  131 . Similar to beam steering unit  114 , beam steering unit  134  may be a Wollaston prism or a birefringent crystal. In contrast to beam steering unit  114 , beam steering unit  134  is oriented to impart an angular or translational deflection to beams in the vertical direction rather than the horizontal direction. Further, beam steering unit  134  does not have an axis of symmetry in the vertical when the beam steering unit is a Wollaston prism. 
         [0031]    Similar to beam polarization unit  131 , beam polarization and steering unit  137  includes an LC array  137 A containing bistable LCs and a plurality of transparent control electrodes. Beam polarization and steering unit  137  further includes a birefringent crystal  137 B (e.g., a YVO 4  crystal) and a reflective element  137 C (e.g., a mirror). Beam polarization and steering unit  137  is configured to direct each incident beam, i.e., inbound beams  154 A-C, along two different parallel optical paths, separated by a vertical offset, depending on the polarization conditioning by LC array  137 A. Since each of inbound beams  154 A-C may be directed to beam polarization and steering unit  137  along two possible sets of optical paths from beam steering unit  134 , i.e., an upper path or lower path, outbound beams  155 A-C may be directed from beam polarization and steering unit  137  along any of four vertically displaced optical path sets. 
         [0032]    As noted above, WSS  100  is configured for expansion vertically, i.e., one or more WSS&#39;s may be positioned directly above or below WSS  100  to form a stack of 1×4 switching devices. The stack may then be packaged together as a single unit. To better protect the alignment and cleanliness of the optical elements that make up each optical switching device, the packaging of the stack may be hermetically sealed. Because the packaging, electronics, and one or more optical components for the stack are shared by multiple optical switching devices contained in the stack, the per-channel cost of the packaging, electronics, and optical components is lower than for an individually packaged and controlled optical switching device. 
         [0033]    The two or more WSS&#39;s (or other WDM optical switching devices) contained in a packaged stack may be optically and electrically isolated and functionally independent, thereby avoiding optical and electrical crosstalk between the optical switching devices. However, because a number of the optical elements of WSS  100  possess a vertically oriented axis of symmetry, i.e., the axis is perpendicular to the plane traveled by light beams in WSS  100 , these optical elements may be extended along the vertical axis to serve as optical elements in multiple optical switching devices contained in the stack. An example of an optical element being extended along a vertical axis of symmetry is described below in conjunction with  FIG. 2A . Each optical element that may be shared between optical switching devices substantially reduces the per-channel cost of manufacturing and alignment of the stack. For the configuration of WSS  100  illustrated in  FIG. 1A , optical elements that may be shared between multiple WSS&#39;s that are packaged in a single vertical stack include optical input port  101 , optical output port stack  102 , beam steering unit  114 , cylindrical lens  116 , diffraction grating  117 , folding mirrors  113  and  122 , focusing lens  123 , beam polarization unit  131 , beam polarization and steering unit  137 , and, if configured as a birefringent crystal, beam steering unit  134 . 
         [0034]      FIG. 2A  schematically illustrates a perspective view of an extended beam polarization unit  231  that may act as a beam polarization unit for two optical switching devices contained in a packaged stack according to an embodiment of the invention. Extended beam polarization unit  231  consists of beam polarization units  231 A,  231 B. In this example, each of beam polarization units  231 A,  231 B are substantially similar in organization and operation to beam polarization unit  131 , described above in conjunction with  FIGS. 1A and 1B , except that beam polarization units  231 A,  231 B are manufactured together as elements of extended beam polarization unit  231 . Hence, each of beam polarization units  231 A,  231 B includes an LC array and transparent electrodes positioned between glass plates and configured to modulate the polarity of inbound and outbound beams. Beam polarization unit  231 A modulates inbound beams  254 A and outbound beams  255 A as part of the wavelength channel switching process for the bottom level WSS of a double-deck WSS  200 , which is shown in  FIG. 2B  and described below. Similarly, beam polarization unit  231  B modulates inbound beams  254 B and outbound beams  255 B as part of the wavelength channel switching process for an upper level WSS of double-deck WSS  200 . 
         [0035]    The same manufacturing steps are required to make either an individual beam polarization unit, such as beam polarization unit  131 , or an extended beam polarization unit, such as extended beam polarization unit  231 . For example, the lithographic, deposition, etching, and assembly processes for forming the transparent electrodes and LC pixels of an extended beam polarization unit are identical to those for forming an individual beam polarization unit. The primary difference is that larger substrates are needed to produce an extended beam polarization unit compared to an individual beam polarization unit. Therefore, it is substantially more cost effective to manufacture and align a single extended beam polarization unit that is shared by multiple WSS&#39;s in a packaged stack than to manufacture and align an individual polarization unit for each WSS contained in the stack. Similarly, other optical elements of a WSS, such as folding mirrors, diffraction gratings, and cylindrical lenses, may also be extended vertically and shared by multiple WSS&#39;s contained in a packaged stack. As with extended beam polarization unit  231 , the use of vertically extended cylindrical lenses, vertically extended folding mirrors, etc., substantially reduces the number of optical components to be manufactured and aligned to produce a packaged stack of optical switching devices. In this way, the per-channel cost of such a packaged stack may be further reduced. 
         [0036]      FIG. 2B  is a perspective view of a double-deck WSS  200  according to an embodiment of the invention. Double-deck WSS  200  includes two functionally independent, 1×4 WSS&#39;s, WSS  200 A and WSS  200 B. WSS  200 A and WSS  200 B are each substantially similar in organization and operation to WSS  100 , described above, except that a number of optical elements are shared between WSS  200 A and  200 B. 
         [0037]    As illustrated in  FIG. 2B , WSS  200 A includes an input/output port assembly  201 A, a cylindrical lens  215 A, a cylindrical lens  221 A, collimating lenses  232 A,  233 A,  235 A, and  236 A, a beam steering unit  234 A, and a lower region of the following shared optical elements: folding mirrors  213   222 , beam steering unit  214 , cylindrical lens  216 , diffraction grating  217 , focusing lens  223 , beam polarization unit  231 , and beam polarization and steering unit  237 . Similarly, WSS  200 B includes an input/output port assembly  201 B, a cylindrical lens  215 B, a cylindrical lens  221 B, collimating lenses  232 B,  233 B,  235 B, and  236 A, a beam steering unit  234 B, and an upper region of the above named shared optical elements. 
         [0038]    In operation, WSS  200 A may act as an independent 1×4 WDM WSS, and directs a plurality of light beams  251 A along a lower horizontal plane contained in double-deck WSS  200  to perform the wavelength channel switching operation. Likewise, WSS  200 B may also act as an independent 1×4 WDM WSS, and directs a plurality light beams  251 B along an upper horizontal plane contained in double-deck WSS  200 . Hence, double-deck WSS  200  may serve as a 2×8 WSS for a WDM optical signal. 
         [0039]    The optical switching devices described in  FIGS. 1A and 2B  are configured with bidirectional optical elements, i.e., optical components positioned to interact with light beams travelling in two directions. It is noted that WDM optical switching devices that are not configured as optically bidirectional may also benefit from being incorporated into a packaged stack. Embodiments of the invention further contemplate that WDM optical switching devices other than WSS&#39;s may be beneficially incorporated into a packaged stack as described herein. Wavelength blockers, such as OADMs, DGEs, and others, may benefit from such a packaged stack configuration since per-channel packaging and electronics costs are reduced compared to individually packaged and controlled optical switching devices. In addition, an OADM or DGE stack may also have reduced component and alignment costs since such devices may include diffraction gratings, cylindrical lenses, mirrors, and beam steering units that may be shared between the switching devices of the stack, further reducing the per-channel cost of the stack. 
         [0040]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.