Multi-unit wavelength selective switch

Abstract

A multi-unit wavelength selective switch (WSS) is disclosed. The WSS includes a plurality of WSS units, each WSS unit comprising a waveguide array for providing a light beam for propagation in the multi-unit WSS, and for receiving, in a wavelength-selective manner, the light beam propagated in the multi-unit WSS. A polarizing collimator splits each light beam into polarized collimated first and second sub-beams propagating in the multi-unit WSS along separate optical sub-paths. The polarized sub-beams of each light beam may be redirected by separate redirector arrays, e.g. liquid crystal on silicon (LCoS) arrays of a redirector array assembly specific to each light beam launched into the multi-unit WSS. Using separate redirector arrays to handle polarization sub-beams provides for a greater flexibility of optical configuration, allowing the use of inexpensive, off-the-shelf redirector arrays.

Description

MULTI-UNIT WAVELENGTH SELECTIVE SWITCHREFERENCE TO RELATED APPLICATION

[0001] This application claims priority from a U.S. Provisional Patent Application No. 63 / 554,267, filed on February 16, 2024, entitled “Multi-Unit Wavelength Selective Switch”, and incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure relates to optical switching devices, and in particular to wavelength selective optical switches.BACKGROUND

[0003] Wavelength selective switches are types of optical switches that can redirect light between input and output port(s) in a wavelength-selective manner. A light signal propagating in an optical network is independently modulated at a plurality of wavelengths, forming so-called wavelength channels. The wavelength channels are spaced apart from one another by optical frequency spacings configurable on a flexible grid, typically evenly spaced at 37.5GHz, 50GHz, 75GHz, 100GHz, 200GHz etc. in an infrared wavelength range of between approximately 1.3 micrometers and 1.6 micrometers.

[0004] Some wavelength selective switches are capable of independently switching individual wavelength channels or entire wavelength bands between different optical fibers in an optical network. The optical network may include multiple optical fibers linking different nodes in a same city or town (metro optical networks), in different cities of a same country, and even nodes disposed in different countries or on different continents (long-haul optical networks).

[0005] While being highly functional and versatile, wavelength selective switches often include a multitude of customized free-space and / or waveguiding optical and electro-optical components. Some of the components may need to be aligned to one another with sub-micrometer precision, which drives up the cost of these devices. The multiple wavelength selective switch units need to be inexpensive, compact, and environmentally stable.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Exemplary embodiments will now be described in conjunction with the drawings, in which:

[0007] FIG. 1 A is a schematic top view of a multi-unit wavelength selective switch (WSS) of this disclosure, each WSS unit of the multi -unit WSS using a separate deflector array assembly having two deflector arrays for redirecting two spatially separated polarization components of an input light beam of the WSS unit;

[0008] FIG. IB is a schematic side view of the multi-unit WSS of FIG. 1A;

[0009] FIG. 1C is a schematic three-dimensional view of optical paths of light beams propagating in the multi-unit WSS of FIGs. 1A and IB;

[0010] FIG. 2 is a side-view optical ray diagram of an embodiment of the multiunit WSS of FIGs. 1A and IB, illustrating light propagation in first and second WSS units of the multi -unit WSS;

[0011] FIG. 3A is a plan view of a Liquid Crystal on Silicon (LCoS) wafer or die having four LCoS deflector arrays sharing a common die substrate;

[0012] FIG. 3B is a plan view of two LCoS deflector array assemblies for the embodiment of the multi-unit WSS of FIG. 2, each assembly sharing a common die substrate;

[0013] FIG. 4 is a side-view optical ray diagram of a multi -unit WSS embodiment with crossed light beam paths;

[0014] FIG. 5 is a side-view optical ray diagram of a multi -unit WSS embodiment with diverging light beam paths;

[0015] FIG. 6A is a side view of a front end of a multi-unit WSS embodiment using an offset microlens array to divert the beams downwards;

[0016] FIG. 6B is a side view of a front end of a multi-unit WSS embodiment using an offset microlens array to divert the beams upwards;

[0017] FIG. 7 is a side-view optical ray diagram of a multi-unit WSS embodiment with crossed light beam paths, the multi-unit WSS embodiment having the front ends of FIGs. 6A and 6B;

[0018] FIG. 8 is a top view of an embodiment of a multi-unit WSS using a dual grism including an optically coupled pair of rhomboid grisms, showing light beam propagation in the multi-unit WSS;

[0019] FIG. 9 is a magnified top view of a back end of the multi -unit WSS of FIG. 8 showing directions of alignment of in- and out-coupling prisms of the dual grism; and

[0020] FIG. 10 is a flow chart of a method for independent and wavelength- selective switching of a plurality of light beams in accordance with this disclosure.DETAILED DESCRIPTION

[0021] While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. All statements herein reciting principles, aspects, and embodiments of this disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

[0022] As used herein, the terms "first", "second", and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element from another, unless explicitly stated. Similarly, sequential ordering of method steps does not imply a sequential order of their execution, unless explicitly stated. In FIGs. 1A-1C, FIG. 2, FIGs. 4-5, and FIGs. 7-9, similar reference numerals denote similar elements.

[0023] The scale of manufacturing of a product is one of most effective cost reduction factors for the product. Mass-produced optical and electro-optical components, however, are rarely manufactured to custom specifications. This hinders the use of the mass-produced components in wavelength selective switches, which often require custom specifications to accommodate a particular wavelength range, number of switching ports, etc.

[0024] This disclosure provides a configuration for a multi-unit wavelength selective switch (WSS) that may utilize mass-produced optical / electro-optical elements, components and sub-assemblies, in particular mass-produced deflector arrays usable as optical switching elements for switching individual wavelength channels and configurable wavelength bands between several optical ports or fibers. The multi-unit WSS may operate in a broad wavelength range beyond C band, e.g. in a 6THz-wide range. Herein and throughout the rest of the specification, the term “multi -unit” refers to two or more WSS devices sharing common optical elements and disposed in a same enclosure or body. The optical components sharing between different WSS units of a multi-unit WSS allows further cost reduction, as well as provides more efficient utilization of a network card and / or rack space at a telecommunications node.

[0025] In accordance with the present disclosure, there is provided a multi-unit wavelength selective switch (WSS) comprising a plurality of WSS units, each WSS unit comprising a waveguide array for providing a light beam for propagation in the multi-unit WSS, and for receiving, in a wavelength-selective manner, the light beam propagated in the multi-unit WSS. A polarizing collimator is provided for splitting each light beam into polarized collimated first and second sub-beams propagating in the multi-unit WSS along separate optical sub-paths.

[0026] In some embodiments, the polarizing collimator comprises a polarizing prism optically coupled to each waveguide of at least one waveguide array, for angularly separating the first and second sub-beams to propagate in the multi-unit WSS along the separate optical sub-paths. In such embodiments, polarizing collimator may further include a rotationally symmetric lens substantially one focal length downstream of the polarizing prism, for collimating the first and second sub-beams of the light beams to propagate parallel to each other. The first andsecond waveguide arrays may be configured to direct the light beams to impinge onto the rotationally symmetric lens.

[0027] The multi-unit WSS may include first and second WSS units comprising first and second waveguide arrays respectively. The polarizing collimator may further include first and second rotationally symmetric lenses substantially one focal length downstream of the polarizing prism, for collimating the sub-beams of the first and second light beams, respectively, to propagate parallel to one another along the separate optical sub-paths.

[0028] In some embodiments, the multi-unit WSS further includes a dispersive element downstream of the polarizing collimator, for angularly dispersing the first and second sub-beams of the light beams into wavelength components. The plurality of WSS units may include first and second WSS units comprising first and second waveguide arrays respectively for providing first and second light beams respectively to propagate proximate one another along first and second optical paths respectively. First and second angle-to-offset elements may be disposed downstream of the dispersive element for focusing the wavelength components of the first and second sub-beams respectively, of the first and second light beams. First and second deflector array assemblies may be disposed downstream of the first and second angle-to-offset elements, for redirecting the wavelength components of the first and second light beams respectively to propagate back through the multi-unit WSS along the first and second optical paths respectively. Each one of the first or second deflector array assemblies may include a first deflector array and a second, different deflector array for redirecting wavelength components of the first and second sub-beams, respectively, to propagate back through the multi-unit WSS along the separate optical sub-paths. In some embodiments, the first and second deflector arrays of the first deflector array assembly share a common die substrate, whereby the first and second deflector arrays the first deflector array assembly are parallel to one another. The first and second deflector arrays of the second deflector array assembly may also share a common die substrate, whereby the first and second deflector arrays the second deflector array assembly are parallel to one another.

[0029] In some embodiments, the dispersive element is configured to disperse the first and second sub-beams of the first and second light beams into the wavelength components in spaced apart parallel first planes. The first and second angle-to-offset elements may be configured to focus the wavelength components of the first and second sub-beams, respectively, in the first planes, and the first and second deflector array assemblies may be configured to redirect the wavelength components in spaced apart second planes perpendicular to the first planes. In such embodiments, the first and second angle-to-offset elements may each comprise an acylindrical lens having a non-zero optical power in the first planes, and a substantially zero optical power in the second planes. The waveguides of the first and second waveguide arrays may be disposed in a plane perpendicular to the first planes; the polarizing collimator may include a birefringent wedge optically coupled to each waveguide of the first and second waveguide arrays for angularly separating the first and second sub-beams in the respective first planes.

[0030] The rotationally symmetric lens may have a first focal length and may be disposed substantially one first focal length downstream of the birefringent wedge, for collimating the first and second sub-beams of the first and second light beams to propagate parallel to one another. The first and second waveguide arrays may be non-parallel to one another for directing the first and second light beams, respectively, to converge on the rotationally symmetric lens, or the first and second waveguide arrays may each include an offset microlens array for directing the first and second light beams, respectively, to converge on the rotationally symmetric lens. Alternatively, the first and second waveguide arrays may be non-parallel to one another for directing the first and second light beams, respectively, to diverge from one another before impinging on the rotationally symmetric lens, or the first and second waveguide arrays may each comprise an offset microlens array for directing the first and second light beams, respectively, to diverge from one another before impinging on the rotationally symmetric lens.

[0031] A polarization rotator may be disposed in an optical path of at least one of the first or second sub-beams upstream of the dispersive element, for converting a polarization state of at least one of the first or second sub-beams of the first and second light beams such that the first and second sub-beams of the first and secondlight beams have a substantially same polarization state. In some embodiments, the dispersive element includes first and second diffraction gratings for dispersing the first and second sub-beams, respectively, of the first and second light beams into the wavelength components, where the first and second diffraction gratings are disposed in different planes separated by a non-zero distance between them.

[0032] In some embodiments, the dispersive element further comprises first and second in-coupling prisms coupled to the first and second diffraction gratings respectively, for receiving the first and second sub-beams respectively, and for coupling the first and second sub-beams to the first and second diffraction gratings respectively. The first and second in-coupling prisms may be disposed parallel one another and may be optically joined by an interface layer between them extending along parallel paths of propagation of the first and second sub-beams in the first and second in-coupling prisms respectively, such that during alignment of the multi-unit WSS, a relative position of the first and second in-coupling prisms along the paths of propagation is adjustable by sliding at least one of the first or second in-coupling prism along the interface layer. An optical path of the wavelength components of the second sub-beam dispersed by the second diffraction grating may include in sequence the second in-coupling prism, the interface layer, and the first in-coupling prism.

[0033] In accordance with the present disclosure, there is further provided a method for independent and wavelength-selective switching of a plurality of light beams. The method includes, for each light beam of the plurality of light beams, using a waveguide array of a particular one of a plurality of WSS units of a multiunit WSS to launch the light beam for propagation in the multi-unit WSS, and to receive, in a wavelength-selective manner, the light beam propagated in the multiunit WSS. The method further includes using a polarizing collimator to split each light beam into polarized collimated first and second sub-beams for propagation in the multi-unit WSS along separate optical sub-paths.

[0034] The method may further include using a dispersive element downstream of the polarizing collimator to angularly disperse the first and second sub-beams of each light beam into wavelength components, such that: the plurality of light beams comprises first and second light beams; and the plurality of WSS unitscomprises first and second WSS units comprising first and second waveguide arrays respectively for providing the first and second light beams respectively to propagate proximate one another along first and second optical paths respectively.

[0035] The method may further include using first and second angle-to-offset elements downstream of the dispersive element to focus the wavelength components of the first and second sub-beams respectively, of the first and second light beams, and using first and second deflector array assemblies downstream of the first and second angle-to-offset elements to redirect the focused wavelength components of the first and second light beams respectively for propagation back through the multi-unit WSS along the first and second optical paths respectively.

[0036] Referring now to FIGs. 1 A and IB, a multi-unit WSS 100 of this disclosure includes first 101 and second 102 waveguide arrays, e.g. linear fiber arrays, for providing first 103 and second 104 light beams (FIG. IB), respectively, to the multi-unit WSS 100. The first light beam 103 may propagate in any of waveguides / fibers 101-1, 101-2, or 101-3 of the first waveguide array 101 disposed in XZ plane of FIG. IB, and the second light beam 104 may propagate in any of waveguides / fibers 102-1, 102-2, and / or 102-3 of the second waveguide array 102 disposed in the XZ plane. More waveguides and arrays of waveguides may be provided to accommodate more ports per WSS unit, and / or more WSS units in the multi-unit WSS 100.

[0037] A polarizing collimator 106 is coupled to each one of the first 101 and second 102 waveguide arrays for splitting each one of the first 103 and second 104 light beams into orthogonally polarized collimated first 111 and second 112 subbeams, which may be brought to a same polarization state at the output of the polarizing collimator 106 for subsequent propagation through the multi-unit WSS 100 along non-overlapping optical sub-paths. In other words, the collimated first 111 and second 112 sub-beams are orthogonally polarized components of the first 103 and second 104 light beams. The orthogonally polarized components may be brought to the same polarization state for the purpose of reduction of optical polarization-dependent loss (PDL). For example, the collimated first 111 and second 112 sub-beams may be brought to a linear polarization of a same angle of polarization (e.g. horizontal or vertical) for subsequent propagation through themulti-unit WSS 100 along the non-overlapping optical sub-paths, as illustrated in FIG. 1A.

[0038] A dispersive element 108, such as e.g. a diffraction grating, may be disposed downstream of the polarizing collimator 106. The dispersive element 108 may be optically coupled to the polarizing collimator 106 for angularly dispersing each one of the first 111 and second 112 sub-beams of the first 103 and second 104 light beams into wavelength components, e.g. individual wavelength channels. In FIG. 1A, the dispersive element 108 disperses the first sub-beam 111 into a plurality, or more generally a continuum, of wavelength components between a longest- wavelength component 191 A shown with solid lines, and a shortest- wavelength component 191B shown with dashed lines. Similarly, the dispersive element 108 disperses the second sub-beam 112 into a plurality / continuum of wavelength components between a longest- wav elength component 192A shown with solid lines, and a shortest- wav elength component 192B shown with dashed lines. The wavelength components 191 A-B and 192A-B of the first 111 and second 112 sub-beams, respectively, are angularly dispersed in spatially separated planes parallel to YZ plane of FIG. 1A.

[0039] Still referring to FIG. 1 A, first 121 and second 122 angle-to-offset elements may be disposed downstream of the dispersive element 108. The first 121 and second 122 angle-to-offset elements may be optically coupled to the dispersive element 108 and configured to focus the wavelength components 191 A-B and 192A-B of the first 111 and second 112 sub-beams, respectively, in YZ plane of FIG. 1A, while optionally having zero optical power, i.e. zero focusing / defocusing power, in XZ plane of FIG. IB. In other words, the first 121 and second 122 angle- to-offset elements have a non-zero optical power in YZ plane while having a substantially zero optical power in XZ plane.

[0040] By way of a non-limiting example, cylindrical and / or acylindrical lenses may be used as the first 121 and second 122 angle-to-offset elements. Herein, the term “acylindrical lens” means a lens that has an optical power only in one plane and has a surface profile deviating from a cylindrical surface profile. The first 121 and second 122 angle-to-offset elements may also be compound cylindrical and / or acylindrical lenses, i.e. they may each include multiple cylindrical and / oracylindrical elements. The purpose of the angle-to-offset elements 121, 122 is to convert the beam angle of wavelength-dispersed wavelength components into a beam coordinate at focal planes of the angle-to-offset elements 121, 122 in YZ plane i.e. the plane of FIG. 1A, while keeping the propagation of the angularly dispersed wavelength components in the XZ plane i.e. the plane of FIG. IB substantially unaffected. The dispersive element 108 may be disposed at front focal planes of the angle-to-offset elements 121, 122.

[0041] A first deflector array assembly 131 and a second, different deflector array assembly 132 may be disposed at a back focal plane of the first 121 and second 122 angle-to-offset elements, as best illustrated in FIG. 1A. The purpose of the first 131 and second 132 deflector array assemblies is to redirect the wavelength components at variable angles to propagate back through the first 121 and second 122 angle-to-offset elements, the dispersive element 108, and the polarizing collimator 106 for recombination and in-coupling into a desired waveguide of the respective waveguide arrays 101 and 102.

[0042] For example, the first light beam 103 coupled to the second waveguide 101-2 of the first waveguide array 101 is split into the first 111 and second 112 sub-beams, which are angularly dispersed into the wavelength components by the dispersive element 108. The wavelength components 191A, 191B originating from the first 111 and second 112 sub-beams of the first light beam 103 are focused by the first 121 angle-to-offset elements onto the first deflector array assembly 131, while the wavelength components 192A, 192B originating from the first 111 and second 112 sub-beams of the second light beam 104 are focused by the second 122 angle-to-offset elements onto the second deflector array assembly 132. The first 131 and second 132 deflector array assemblies redirect the wavelength components 191 A, 191B and 192A, 192B in a wavelength-selective manner, to propagate back through the multi-unit WSS 100 to be coupled into respective waveguides of the first 101 and second 102 waveguide arrays. In other words, each waveguide array emits a light beam for propagation in the multi-unit WSS 100, which distributes the wavelength components of the light beam between different waveguides of the waveguide array.

[0043] The multi-unit WSS 100 operates as a two independent WSS devices. More WSS devices may be added in a similar manner, as schematically depicted in FIG. IB by a series of vertical dots. It is to be understood that the direction of propagation of light in the multi-unit WSS 100 may be reversed, so that each IxN WSS unit of the multi-unit WSS 100 may also operate as an Nxl wavelength selective switch. More than two deflector array assemblies may be provided to accommodate more WSS units. Herein and throughout the rest of the application, the term “different deflector array assembly” means another copy of a deflector array assembly, although the two array assemblies may be of a same type, shape, and function, and may be coupled to a same mechanical supporting structure and / or disposed within a same body of the wavelength selective switch.

[0044] The first 131 and second 132 deflector array assemblies may be disposed at the back focal plane of the polarizing collimator 106 and may be configured to redirect the wavelength components in spaced apart planes parallel to XZ plane (i.e. up and down as illustrated in FIG. IB) and perpendicular to YZ plane. The first 131 and second 132 deflector array assemblies may include, for example, reflective liquid crystal arrays such as Liquid Crystal on Silicon (LCoS) arrays, microelectromechanical system (MEMS) tiltable reflector arrays, etc.

[0045] The independent and wavelength-selective switching of light beams in the multi-unit WSS 100 is further illustrated in FIG. 1C. The first light beam 103 emitted from the second waveguide 101-2 of the first waveguide array 101 propagates along a first optical path 171. The first light beam 103 is split by the polarizing collimator 106 (not shown in FIG. 1C for brevity) into polarized collimated sub-beams propagating in the multi-unit WSS 100 along separate optical sub-paths 171s and 17 Ip. The dispersive element 108 (not shown in FIG. 1C for brevity) angularly disperses each sub-beam into a fan of wavelength components. The wavelength components of each sub-beam are focused onto respective deflector arrays (of the first deflector array assembly 131, not shown) redirecting the wavelength components to propagate back through the multi-unit WSS 100 along the separate optical sub-paths 171s and 171p for coupling into a waveguide of the first waveguide array 101 in a wavelength-selective manner. Herein, the term “separate optical sub-paths” means physically separate,independent, individually adjustable optical sub-paths that may be controlled by individual, spaced apart deflector arrays.

[0046] For example, a wavelength component of the sub-beam propagating along the proximate sub-path 171s impinges onto a deflector 131 -Xs of a deflector array of the first deflector array assembly 131, which redirects the wavelength component to propagate upwards in FIG. 1C as shown with a dashed line. The deflected wavelength component eventually couples into the third waveguide 101- 3 of the first waveguide array 101. The same wavelength component propagating along the distal sub-path 17 Ip impinges onto a deflector 131-Xp of a deflector array of the first deflector array assembly 131, which redirects the wavelength component to propagate upwards in FIG. 1C (a dashed line). The deflected wavelength component couples into the third waveguide 101-3 as the first output beam 103’, which may retain some, but not all, wavelength components of the first light beam 103, depending on the state of corresponding deflector arrays. It is to be noted that the deflector arrays need to be operated in sync to reduce polarizationdependent loss (PDL) of the multi -unit WSS 100.

[0047] Similarly, the second light beam 104 emitted from the second waveguide 102-2 of the second waveguide array 102 propagates along a second optical path 172 proximate the first optical path 171. The second light beam 104 is split by the polarizing collimator into polarized collimated sub-beams propagating in the multiunit WSS 100 along separate optical sub-paths 172s and 172p. The dispersive element angularly disperses each sub-beam into a fan of wavelength components. The wavelength components of each sub-beam are focused onto respective deflector arrays (of the second deflector array assembly 132, not shown) redirecting the wavelength components to propagate back through the multi-unit WSS 100 along the separate optical sub-paths 172s and 172p for coupling into a waveguide of the first waveguide array 101.

[0048] Continuing with the above example, a wavelength component of the subbeam propagating along the proximate sub-path 172s impinges onto a deflector 132- s of a deflector array of the second deflector array assembly 132, which redirects the wavelength component to propagate downwards in FIG. 1C (a dashed line). The deflected wavelength component eventually couples into the firstwaveguide 102-1 of the second waveguide array 102. The same wavelength component propagating along the distal polarization sub-path 17 Ip impinges onto a deflector 131-Xp of a deflector array of the first deflector array assembly 131, which redirects the wavelength component to propagate downwards in FIG. 1C. The deflected wavelength component couples into the first waveguide 102-1 of the second waveguide array 102, recombining as the second output beam 104’ which, again, may retain an arbitrary sub-combination of the wavelength components of the second light beam 104. It is to be noted that the first 103 and second 104 light beams may be switched by the multi-unit WSS 100 independently and wavelength- selectively.

[0049] Referring to FIG. 2, an embodiment of the multi-unit WSS 100 of FIGs. 1A and IB is presented. The polarizing collimator 106 of a multi-unit WSS 200 (FIG. 2) may include a polarizing prism 116, e.g. a birefringent wedge, a Wollaston prism, etc. The polarizing prism 116 separates each light beam into two sub-beams in a plane perpendicular to the XZ plane of FIG. 2, i.e. YZ plane, such that in in the XZ view of FIG. 2 the paths of the first 111 and second 112 subbeams (FIG. 1A) appear overlapped. This is similar to FIG. IB discussed above.

[0050] The polarizing collimator 106 of FIG. 2 may further include first 271 and second 272 rotationally symmetric collimating lenses substantially one first focal length Fi of the first 271 and second 272 rotationally symmetric lenses downstream of the polarizing prism 116, for collimating both sub-beams of the first 103 and second 104 beams, respectively. The dispersive element 108 angularly disperses the first 111 and second 112 sub-beams (FIG. 1 A) of both light beams 103, 104 into wavelength components. The wavelength components of the first sub-beams 111 of the first 103 and second 104 light beams are focused by the first angle-to-offset element 121 onto first arrays 131-1 and 132-1 of the first 131 and second 132 deflector array assemblies respectively, and the wavelength components of the second sub-beams 112 of the first 103 and second 104 light beams are focused by the second angle-to-offset element 122 onto second arrays 131-2 and 132-2 of the first 131 and second 132 deflector array assemblies respectively.

[0051] The first 121 and second 122 angle-to-offset elements are disposed a second focal length F2 (FIG. 2) of the first 121 and second 122 angle-to-offset elements away from the dispersive element 108, and the first 131 and second 132 deflector array assemblies are disposed one second focal length F2 away from the first 121 and second 122 angle-to-offset elements, as illustrated. The first 271 and second 272 collimating lenses may also operate as angle-to-offset elements, converting an angle of redirection provided by the first 131 and second 132 deflector array assemblies into a linear offset for out-coupling into a corresponding waveguide.

[0052] Each one of the first 131 and second 132 deflector array assemblies may include a pair of deflector arrays, for deflecting the first 111 and second 112 polarization sub-beams, respectively (FIG. 1A). By way of example, the first deflector array assembly 131 (FIG. 2) includes the first deflector array 131-1 for redirecting the wavelength components of the first sub-beam 111 (FIG. 1 A), and a second deflector array 131-2 (FIG. 2) for redirecting the wavelength components of the second sub-beam 112 (FIG. 1 A). Similarly, the second deflector array assembly 132 also includes the deflector arrays 132-1 and 132-2 configured to deflect respective polarization sub-beams of the second light beam 104. The configuration presented in FIGs. 1A-1C and FIG. 2 allows two or more WSS units share many common elements of the multi-unit WSS 100 and 200, e.g. the polarizing prism 116, the dispersing element 108, and the first 121 and second 122 angle-to-offset elements.

[0053] The first 121 and second 122 angle-to-offset elements, also termed herein “Fourier” elements, may be cylindrical or acylindrical lenses, and may be singlet or compound lenses. The first 131 and second 132 deflector array assemblies may include, for example, reflective liquid crystal arrays such as LCoS arrays, MEMS tiltable mirror arrays, etc. Using physically separate (i.e. separate copies of) deflector arrays for different sub-beams provides a greater flexibility of selecting required wavelength dispersion and resolution, the number of wavelength channels, etc. for the wavelength selective switch 200, allowing one to use mass- produced inexpensive deflector arrays.

[0054] The configuration of the multi-unit WSS 100 of FIGs. 1A-1C and the multi-unit WSS 200 FIG. 2 is termed herein a “2f” configuration, since the collimating lenses 271, 272 are disposed one focal length Fi (“If’) of the collimating lenses 271, 272 away from the waveguide arrays 101, 102, and the first 131 and second 132 deflector array assemblies are disposed one focal length Fi (“If’) of the collimating lenses 271, 272 away from the collimating lenses 271, 272. Furthermore, the first 121 and second 122 angle-to-offset elements are disposed one focal length F2 of these elements away from the dispersing element 108, and the first 131 and second 132 deflector array assemblies are disposed one focal length F2 away from them. For this reason, the configuration of the multiunit WSS 100 may also be described as a “nested 2f ’ configuration. It is to be noted that the 2: 1 ratio between the focal lengths of the collimating lenses 271, 272 on one hand, and the angle-to-offset elements 121, 122 on the other, is not a necessary requirement; in some embodiments, the ratio may be as large as 4:1, or even larger, depending on the optical configuration.

[0055] Referring to FIGs. 3A and 3B with further reference to FIGs. 1A and IB, the first 131 and second 132 deflector array assemblies (FIGs. 1A and IB) may be manufactured from a die 300 (FIG. 3A) including first 301 to fourth 304 deflector arrays on a common substrate 310. Normally, the die 300 is diced along vertical 305 and horizontal 306 dashed lines to provide access to a contact area 307 of each deflector array 301-304 for wirebonding. For the multi-unit WSS 100 of FIGs. 1A- 1B and the multi-unit WSS 200 FIG. 2, the dicing may be performed along the vertical dashed line 305 only, and then the pair including the second 302 and fourth 304 deflector arrays is rotated by 180 degrees in plane of FIGs. 3A, 3B to have the contact area 307 on the outside for wirebonding to flex printed circuit boards (PCBs) 308. One advantage of such a configuration is that the first 301 and third 303 deflector arrays share a common die substrate and thus remain highly parallel to one another, and the second 302 and fourth 304 deflector arrays share a common die substrate and thus remain highly parallel to one another. For example, for LCoS deflector arrays, the first 301 and third 303 deflector arrays share the common silicon substrate, and the second 302 and fourth 304 deflector arrays share the common silicon substrate, which may have been the same substrate before dicing the die 300 along the vertical dashed line 305. Furthermore, the clocking(in-plane rotation) error between the two deflector arrays of a same deflector array assembly remains substantially non-existent. This may simplify their use in the multi-unit WSS 100 of FIGs. 1A and IB where, for example, the first deflector array assembly 131 would include the first 301 and third 303 deflector arrays, and the second deflector array assembly 132 would include the second 302 and fourth 304 deflector arrays. Each individual WSS unit may use two deflector arrays disposed vertically one over another, as illustrated in FIG. 3B.

[0056] FIG. 4 shows an alternative configuration of multiple WSS units in a multi-unit WSS. A multi-unit WSS 400 of FIG. 4 has not two but one collimating lens 470, and the first 101 and second 102 waveguide arrays are angled for the light beams to converge onto the common collimating lens 470 at an angle, i.e. at other than normal incidence. Deflector array assemblies 431 and 432 each include a pair of parallel deflector arrays; specifically, the first deflector array assembly 431 includes first 431-1 and second 431-2 deflector arrays parallel to one another, and the second deflector array assembly 432 includes first 432-1 and second 432-2 deflector arrays parallel to one another. The first 431 and second 432 deflector array assemblies are disposed at an angle to one another, to direct the light beams back to propagate through the common collimating lens 470. Such a configuration allows one to share optical components between different WSS units, i.e. the collimating lens 470, the dispersive element 108, and the first 121 and second 122 angle-to-offset elements. There can be more than two WSS units. The waveguide arrays of the extra WSS units, not shown, may be disposed at steeper angles and oriented to impinge onto the common collimating lens 470; accordingly, there may be more than the two deflector array assemblies 431, 432.

[0057] FIG. 5 illustrates another configuration of multiple WSS units in a multiunit WSS. A multi-unit WSS 500 of FIG. 5 has a single enlarged collimating lens 570, and the first 101 and second 102 waveguide arrays are angled for the light beams to diverge, impinging onto the common collimating lens 570 at an angle, i.e. at other than normal incidence. Deflector array assemblies 531 and 532 each include a pair of parallel deflector arrays; specifically, the first deflector array assembly 531 includes first 531-1 and second 531-2 parallel deflector arrays, and the second deflector array assembly 532 includes first 532-1 and second 532-2parallel deflector arrays. The first 531 and second 532 deflector array assemblies disposed at an angle to one another, to direct the light beams back to propagate through the common collimating lens 570. Such a configuration allows one to share optical components between different WSS units, i.e. the collimating lens 570, the dispersive element 108, and the first 121 and second 122 angle-to-offset elements. There can be more than two WSS units. The waveguide arrays of the extra WSS units, not shown, may be disposed at steeper angles and oriented to impinge onto the common collimating lens 570; accordingly, there may be more than the two deflector array assemblies 531, 532 in the multi-unit WSS 500.

[0058] Referring now to FIG. 6 A, a front end 600 A for a multi-unit WSS of this disclosure includes an array 601 of optical fibers 602, or more generally a waveguide array, optically coupled to an array 603 of lenslets 604 each having a focal length F. The fiber array 601 and the lenslet array 603 have a same pitch.Centers of the optical fibers 602 of the fiber array 601 are offset downwards from centers of lenslets 604 of the lenslet array 603 by a distance Y, causing output beams 605 to propagate at a downward angle 6*to optical axes 606 of the lenslets 604 given by

[0059] 0 = alan AX ) (1)

[0060] Turning to FIG. 6B, a front end 600B is similar to the front end 600A of FIG. 6A, but the centers of the optical fibers 602 of the fiber array 601 are offset upwards from centers of lenslets 604 of the lenslet array 603 by a distance zLY, causing the output beams 605 to propagate upwards, not downwards.

[0061] Referring to FIG. 7 with further reference to FIG. 4, a multi-unit WSS 700 (FIG. 7) is similar to the multi-unit WSS 400 of FIG. 4 and includes similar elements. The multi-unit WSS 700 of FIG. 7 includes the front end 600A of FIG. 6A and the front end 600B of FIG. 6B, which enable the fiber arrays 101 and 102 to remain parallel to each other and an axis of symmetry of the multi-unit WSS 700. A variant of the multi-unit WSS 500 of FIG. 5 may also include the front end 600A of FIG. 6A and the front end 600B of FIG. 6B to provide launching of the light beams in diverging directions as illustrated in FIG. 5. Such a variant is also considered as one of possible WSS embodiments of this disclosure.

[0062] Turning to FIG. 8, a multi -unit WSS 800 is a non-limiting example implementation of the multi-unit WSS 100 of FIGs. 1A-1B and FIG. 2, the multiunit WSS 400 of FIG. 4, the multi-unit WSS 500 of FIG. 5, and the multi-unit WSS 700 of FIG. 7, and includes similar elements. The multi-unit WSS 800 of FIG. 8 includes the first 101 and second 102 waveguide arrays disposed one under another (only the second waveguide array 102 is visible in the plan view of FIG. 8), for injecting the first 103 and second 104 light beams (only the second light beam 104 is drawn as propagating directly over the first light beam 103 in FIG. 8) into the multi -unit WSS 800, and for wavelength selective outputting the first 103 and second 104 light beams into any of the waveguide(s) of the respective waveguide arrays 101, 102. Optional lens arrays 809 may be coupled to the first 101 and second 102 waveguide arrays for providing required optical beam waist sizes, as well as divergence / beam direction in accordance with the explanation of FIGs. 6A and 6B presented above. It is to be understood that, just like in case of the multi-unit WSS 100 of FIGs. 1 A-1B and the multi-unit WSS 200 FIG. 2 and other multi-unit WSS devices considered herein, the direction of propagation of light may be reversed, so that IxN WSS unit(s) may operate as an Nxl WSS unit(s).

[0063] A polarizing collimator of the multi-unit WSS 800 includes the polarizing prism 116 optically coupled to the pair of collimating lenses 271, 272 (only the second collimated lens 272 is visible) by means of three folding mirrors 828, which are flat mirrors, but may be curved in some embodiments. A single collimating lens may also be used in combination with tilted / redirected light beams, as explained above with reference to FIGs. 4, 5, and 7. The polarizing prism 116 splits each one of the first 103 and second 104 light beams into first 811 and second 812 sub-beams. The optical path of the first sub-beam 811 is shown with dotted lines, and the optical path of the second sub-beam 812 is shown with dashed lines. Upstream of the collimating lenses 271, 272, the optical paths are represented by chief rays only for brevity, and downstream of the collimating lenses 271, 272, the optical paths are represented by boundary rays. The optical paths of the first 811 and second 812 sub-beams between the collimating lenses 271, 272 and respective diffraction gratings 841, 842 are parallel to one another and not overlapping with one another, i.e. separate from one another.

[0064] A polarization rotator 824 may be disposed in an optical path of the second sub-beams 812 to bring the polarization state of the second sub-beams 812 to that of the first sub-beams 811, making them nearly identically polarized. By way of a non-limiting illustrative example, the polarization rotator 824 may include a half-wave plate with an optic axis oriented at 45 degrees to a polarization direction of the linearly polarized second sub-beams 812. The polarization rotator 824 may be disposed in the optical path of the first sub-beams 811 and may be disposed downstream or upstream of the collimating lenses 271, 272.

[0065] A compensating element 860 may be provided in an optical path of the first sub-beam 811 downstream of the collimating lenses 271, 272, for further balancing the optical path lengths of the two different polarization components / sub-beams 811 and 812, with the purpose of reduction or elimination of polarization mode dispersion (PMD) of the multi-unit WSS 800. In some embodiments, the compensating element 860, or an additional compensating element, may be provided in an optical path of the second sub-beam 812. It is to be noted that the compensating element 860 may be placed anywhere in the optical path of the first 811 and / or second 812 sub-beams.

[0066] The multi-unit WSS 800 may further include a prismatic beam expander 826 having a set of several (in this case two) prisms configured to expand the first 811 and second 812 sub-beams in YZ plane for improvement of spectral resolution. The prisms of the prismatic beam expander 826, as well as other elements of the multi -unit WSS 800, may be anti-reflection (AR) coated to reduce optical losses and ghosting, i.e. ghost reflections. Tilted surfaces of both prisms of the prismatic beam expander 826 may be disposed at a Brewster angle for reduction of Fresnel reflections of the first 811 and second 812 sub-beams, which may be polarized in YZ plane to take advantage of Brewster angle reflection suppression. The prisms may be disposed in an optical path of the first 811 and second 812 sub-beams between the polarization rotator 824 of the polarizing collimator and a dual grism 808. The purpose of the dual grism 808 is to angularly disperse the first 811 and second 812 sub-beams into wavelength components / wavelength channels. Only one such wavelength component is shown for eachsub-beam for brevity: a first wavelength component 891 of the first sub-beam 811, and a second wavelength component 892 of the second sub-beam 812.

[0067] The multi-unit WSS 800 may further include the first 121 and second 122 cylindrical or acylindrical lenses for focusing first 891 and second 892 wavelength components of the first 811 and second 812 sub-beams onto the first 131 and second 132 deflector array assemblies in YZ plane, while having substantially zero optical power (i.e. focusing / defocusing power) in XZ plane. The first 821 and second 822 cylindrical or acylindrical lenses may be co-planar as illustrated. Only one of each of the first 891 and second 892 wavelength components is shown for brevity.

[0068] The first 131 and second 132 deflector array assemblies are configured to redirect the dispersed wavelength components of the first 103 and second 104 light beams, respectively (only the second light beam 104 is shown in FIG. 8 for brevity), to propagate back through the first 821 and second 822 lenses, the dual grism 808, and further retracing the optical path backward, towards the waveguide arrays 101, 102. The first 131 and second 132 deflector array assemblies may be separate units disposed on a common supporting plate, e.g. separate LCoS arrays on a common ceramic mount for ease of alignment and thermal control. The first 131 and second 132 deflector array assemblies may be manufactured as explained above with reference to FIGs. 3 A and 3B. The first 131 and second 132 deflector array assemblies redirect the optical components in planes perpendicular to the YZ plane, e.g. in planes parallel to XZ plane.

[0069] Configuration and operation of the dual grism 808 will now be described. The dual grism 808 may include first 861 and second 862 in-coupling prisms for receiving and propagating within the prisms the first 811 and second 812 spaced apart sub-beams, respectively, of the first 103 and second 104 light beams, respectively. The first 841 and second 842 diffraction gratings are coupled to the first 861 and second 862 in-coupling prisms respectively for dispersing the first 811 and second 812 sub-beams respectively into the first 891 and second 892 wavelength components respectively. The first 841 and second 842 diffraction gratings are disposed in different planes 851, 852 separated by anon-zero distance d between them, as illustrated in FIG. 8. The first 861 and second 862 in-couplingprisms are disposed parallel one another and optically joined by an interface layer 860 between them, e.g. by a layer of a transparent curable epoxy, allowing one to adjust the shape of the dual grism 808 by sliding the first 861 and / or second 862 in-coupling prisms relative to one another on a base 880 before curing the epoxy. The interface layer 860 extends along parallel paths of propagation of the first 811 and second 812 sub-beams in the first 861 and second 862 in-coupling prisms respectively.

[0070] The dual grism 808 may further include a first out-coupling prism 871 optically joined to the first in-coupling prism 861 via a first layer 881 between them, e.g. a layer of a transparent curable epoxy, for out-coupling the first wavelength components 891 from the first in-coupling prism 861. The optical path of the first wavelength components 891 includes in sequence the first in-coupling prism 861, the first diffraction grating 841, the first layer 881, and the first out- coupling prism 871, as illustrated. The optical path of the first wavelength components 891 may be adjusted without changing the optical path of the second wavelength components 892 by sliding the first out-coupling prism 871 along the first layer 881, if required.

[0071] A second out-coupling prism 872 may be optically joined to the first incoupling prism 861 via a second layer 882 between them, e.g. a layer of a transparent curable epoxy, for out-coupling the second wavelength components 892 from the first in-coupling prism 861. The optical path of the second wavelength components 892 includes in sequence the second in-coupling prism 862, the second diffraction grating 842, the interface layer 860, the first incoupling prism 861, the second layer 882, and the second out-coupling prism 872. The optical path length of the second wavelength components 892 may be adjusted without changing the optical path length of the first wavelength components 891 by sliding the second out-coupling prism 872 along the second layer 882.

[0072] The process of alignment of the dual grism 408 is further illustrated in a plan view of FIG. 9, which shows sliding directions 911-914 of all prisms of the dual grism 408 on the base 880. Alternatively, at least some of the prisms may be slid on a removable spacer layer placed onto the base 880. The spacer may be removed after alignment is complete. The sliding directions 911-914 are allparallel to the plane of FIG. 9. In the embodiment shown, the first 861 and second 862 in-coupling prisms of the dual grism 808 each have has first 901 to fourth 904 conterminous faces illustrated by straight lines. The first 901 and third 903 faces of both prisms may, but do not have to, be parallel to each other, and the second 902 and fourth 904 faces of both prisms may, but do not have to, be parallel to each other. The fourth face 904 of the second in-coupling prism 862 is coupled to the second face 902 of the first in-coupling prism 861 via the interface layer 860. The first 841 and second 842 diffraction gratings are coupled to the third faces 903 of the first 861 and second 862 in-coupling prisms respectively.

[0073] During alignment, the first 811 and second 812 sub-beams are received at the first faces 901 of the first 861 and second 862 in-coupling prisms respectively. A relative position of the first 861 and second 862 in-coupling prisms along the parallel paths of propagation of the first 811 and second 812 sub-beams is adjusted by sliding at least one of the first 861 or second 862 in-coupling prisms on the base 380 along the interface layer 860. Similarly, a relative position of the first 871 and second 872 out-coupling prisms may be adjusted by sliding at least one of the first 871 and second 872 out-coupling prisms along the first 881 and second 882 layers joining these prisms to the first in-coupling prism 861.

[0074] The dual grism 808 allows one to decouple the alignment of the optical paths of the first 811 and second 812 sub-beams. In other words, the alignment of the prisms in the light path of the first sub-beams 811 and their wavelength components does not impact the light path of the second sub-beams 812 and their wavelength components, and vice versa, the alignment of the prisms in the light path of the second sub-beams 812 and their wavelength components does not impact the light path of the first sub-beams 811 and their wavelength components. Such decoupling of the sub-beams 811, 812 alignment provides a greater degree of flexibility in selecting the order of alignment of the polarization sub-beam paths in the multi-unit WSS 800 of FIG. 8, as well as in selecting the specific optical elements, and / or groups of such elements, to shift / rotate / reposition at different steps of the alignment process of each WSS unit of the multi-unit WSS 800.

[0075] Turning now to FIG. 10 with further reference to FIGs. 1A to 1C, a method 1000 of this disclosure for independent and wavelength-selective switchingof a plurality of light beams includes using a waveguide array of a plurality of WSS units of a multi-unit WSS, e.g. the multi-unit WSS 100 of FIGs. 1A and IB, to launch (1002) one of the plurality of light beams for propagation in the multiunit WSS. By way of a non-limiting example, the first light beam 103 may be launched into the second waveguide 101-2 of the first waveguide array 101 (FIG. 1C). The first light beam may propagate along a first optical path 171 including non-overlapping optical sub-paths 171s and 171p for the two polarization subbeams. In this example, the first waveguide array 101 receives, in a wavelength- selective manner, the first light beam 103’ propagated in the multi-unit WSS 100 along the first optical path 171 into the third waveguide 101-3 of the first waveguide array 101, as explained above with reference to FIG. 1C. The step 1002 may be performed for may be performed for each light beam of the plurality of light beams, one light beam per one waveguide array; for example, the second light beam 104 may be inputted at the second waveguide 102-2 of the second waveguide array 102, propagate along the second optical path 172, and be coupled, in a wavelength-selective manner, into the first waveguide 102-1 of the second waveguide array 102.

[0076] A polarizing collimator is used (FIG. 10; 1004) to split each light beam into polarized first and second sub-beams for propagation in the multi-unit WSS along separate optical sub-paths. Continuing with the above example of the multiunit WSS 100, the polarizing collimator 106 (FIGs. 1 A and IB) may be used to split the first 103 and second 104 light beams into polarized collimated first 111 and secondl 12 sub-beams for propagation in the multi -unit WSS 100 along separate first 171 and second 172 optical paths (FIG. 1C).

[0077] The method 1000 may further include using a dispersive element downstream of the polarizing collimator, to angularly disperse (1006) the first and second sub-beams of each light beam into wavelength components. For example, referring to FIG. 1 A, the dispersive element 108 may be used to angularly disperse the first 111 and second 112 sub-beams of each light beam into wavelength components 191A, 191B; and 102A, 192B, respectively.

[0078] The method 1000 may further include using first and second angle-to- offset elements downstream of the dispersive element to focus (1008) thewavelength components of the first and second sub-beams respectively of the first and second light beams. For example, the first 121 and second 122 angle-to-offset elements of the multi-unit WSS 100 may be used to focus the wavelength components 191A-191B and 192A-192B of the first 111 and second 112 subbeams, respectively, of the first 103 and second 104 light beams (FIGs. 1A and IB). The focusing provides a required wavelength selectivity in switching different wavelength components and / or portions of the spectrum of the first 103 and second 104 light beams between output waveguides of the respective waveguide arrays. The first 131 and second 132 deflector array assemblies disposed downstream of the first 121 and second 122 angle-to-offset elements may be used to redirect (1010) the focused wavelength components of the first 103 and second 104 light beams respectively for propagation back through the multi-unit WSS 100 along the first 171 and second 172 optical paths respectively (FIG. 1C).

[0079] The present disclosure is not to be limited in scope by the specific embodiments described herein. Other various embodiments and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.

Claims

WHAT IS CLAIMED IS:

1. A multi-unit wavelength selective switch (WSS) comprising: a plurality of WSS units, each WSS unit comprising a waveguide array for providing a light beam for propagation in the multi-unit WSS, and for receiving, in a wavelength-selective manner, the light beam propagated in the multi-unit WSS; and a polarizing collimator for splitting each light beam into polarized collimated first and second sub-beams propagating in the multi-unit WSS along separate optical sub-paths.

2. The multi-unit WSS of claim 1, wherein the polarizing collimator comprises a polarizing prism optically coupled to each waveguide of at least one waveguide array, for angularly separating the first and second sub-beams to propagate in the multi-unit WSS along the separate optical sub-paths.

3. The multi-unit WSS of claim 2, wherein the polarizing collimator further comprises a rotationally symmetric lens substantially one focal length downstream of the polarizing prism, for collimating the first and second sub-beams of the light beams to propagate parallel to each other; wherein the first and second waveguide arrays are configured to direct the light beams to impinge onto the rotationally symmetric lens.

4. The multi-unit WSS of claim 2, comprising first and second WSS units comprising first and second waveguide arrays respectively, wherein the polarizing collimator further comprises first and second rotationally symmetric lenses substantially one focal length downstream of the polarizing prism, for collimating the sub-beams of the first and second light beams, respectively, to propagate parallel to one another along the separate optical sub-paths.

5. The multi-unit WSS of claim 1, further comprising a dispersive element downstream of the polarizing collimator, for angularly dispersing the first and second subbeams of the light beams into wavelength components; wherein the plurality of WSS units comprises first and second WSS units comprising first and second waveguide arrays respectively for providing first and secondlight beams respectively to propagate proximate one another along first and second optical paths respectively.

6. The multi-unit WSS of claim 5, further comprising: first and second angle-to-offset elements downstream of the dispersive element, for focusing the wavelength components of the first and second sub-beams respectively, of the first and second light beams; and first and second deflector array assemblies downstream of the first and second angle-to-offset elements, for redirecting the wavelength components of the first and second light beams respectively to propagate back through the multi-unit WSS along the first and second optical paths respectively.

7. The multi-unit WSS of claim 6, wherein each one of the first or second deflector array assemblies comprises a first deflector array and a second, different deflector array for redirecting wavelength components of the first and second sub-beams, respectively, to propagate back through the multi-unit WSS along the separate optical subpaths.

8. The multi-unit WSS of claim 7, wherein: the first and second deflector arrays of the first deflector array assembly share a common die substrate, whereby the first and second deflector arrays the first deflector array assembly are parallel to one another; and the first and second deflector arrays of the second deflector array assembly share a common die substrate, whereby the first and second deflector arrays the second deflector array assembly are parallel to one another.

9. The multi-unit WSS of claim 6, wherein: the dispersive element is configured to disperse the first and second sub-beams of the first and second light beams into the wavelength components in spaced apart parallel first planes;the first and second angle-to-offset elements are configured to focus the wavelength components of the first and second sub-beams, respectively, in the first planes; and the first and second deflector array assemblies are configured to redirect the wavelength components in spaced apart second planes perpendicular to the first planes.

10. The multi-unit WSS of claim 9, wherein the first and second angle-to-offset elements each comprise an acylindrical lens having a non-zero optical power in the first planes, and a substantially zero optical power in the second planes.

11. The multi -unit WSS of claim 9, wherein waveguides of the first and second waveguide arrays are disposed in a plane perpendicular to the first planes, wherein the polarizing collimator comprises a birefringent wedge optically coupled to each waveguide of the first and second waveguide arrays for angularly separating the first and second subbeams in the respective first planes.

12. The multi -unit WSS of claim 11, wherein the rotationally symmetric lens has a first focal length and is disposed substantially one first focal length downstream of the birefringent wedge, for collimating the first and second sub-beams of the first and second light beams to propagate parallel to one another.

13. The multi -unit WSS of claim 12, wherein at least one of: the first and second waveguide arrays are non-parallel to one another for directing the first and second light beams, respectively, to converge on the rotationally symmetric lens; or the first and second waveguide arrays each comprise an offset microlens array for directing the first and second light beams, respectively, to converge on the rotationally symmetric lens.

14. The multi-unit WSS of claim 12, wherein at least one of: the first and second waveguide arrays are non-parallel to one another for directing the first and second light beams, respectively, to diverge from one another before impinging on the rotationally symmetric lens; orthe first and second waveguide arrays each comprise an offset microlens array for directing the first and second light beams, respectively, to diverge from one another before impinging on the rotationally symmetric lens.

15. The multi -unit WSS of claim 9, further comprising a polarization rotator in an optical path of at least one of the first or second sub-beams upstream of the dispersive element, for converting a polarization state of at least one of the first or second sub-beams of the first and second light beams such that the first and second sub-beams of the first and second light beams have a substantially same polarization state.

16. The multi -unit WSS of claim 9, wherein the dispersive element comprises first and second diffraction gratings for dispersing the first and second sub-beams, respectively, of the first and second light beams into the wavelength components, wherein the first and second diffraction gratings are disposed in different planes separated by a non-zero distance therebetween.

17. The multi-unit WSS of claim 16, wherein the dispersive element further comprises first and second in-coupling prisms coupled to the first and second diffraction gratings respectively, for receiving the first and second sub-beams respectively, and for coupling the first and second sub-beams to the first and second diffraction gratings respectively, wherein the first and second in-coupling prisms are disposed parallel one another and optically joined by an interface layer therebetween extending along parallel paths of propagation of the first and second sub-beams in the first and second in-coupling prisms respectively, such that during alignment of the multiunit WSS, a relative position of the first and second in-coupling prisms along the paths of propagation is adjustable by sliding at least one of the first or second incoupling prism along the interface layer; wherein an optical path of the wavelength components of the second sub-beam dispersed by the second diffraction grating comprises in sequence the second in-coupling prism, the interface layer, and the first in-coupling prism.

18. A method for independent and wavelength-selective switching of a plurality of light beams, the method comprising:for each light beam of the plurality of light beams, using a waveguide array of a particular one of a plurality of wavelength selective switch (WSS) units of a multi -unit WSS to launch the light beam for propagation in the multi-unit WSS, and to receive, in a wavelength-selective manner, the light beam propagated in the multi-unit WSS; and using a polarizing collimator to split each light beam into polarized collimated first and second sub-beams for propagation in the multi-unit WSS along separate optical subpaths.

19. The method of claim 18, further comprising using a dispersive element downstream of the polarizing collimator, to angularly disperse the first and second subbeams of each light beam into wavelength components; wherein: the plurality of light beams comprises first and second light beams; and the plurality of WSS units comprises first and second WSS units comprising first and second waveguide arrays respectively for providing the first and second light beams respectively to propagate proximate one another along first and second optical paths respectively.

20. The method of claim 19, further comprising: using first and second angle-to-offset elements downstream of the dispersive element to focus the wavelength components of the first and second sub-beams respectively, of the first and second light beams; and using first and second deflector array assemblies downstream of the first and second angle-to-offset elements to redirect the focused wavelength components of the first and second light beams respectively for propagation back through the multi-unit WSS along the first and second optical paths respectively.

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