Wavelength selective switch with polarization balancing

Abstract

A wavelength-selective optical switch includes in sequence a waveguide array, a polarization element, a collimating lens, a dispersing element, a Fourier lens, and a redirector array. The polarization element splits a light beam from the waveguide array into diverging polarization sub-beams in a first plane for collimating by the collimating lens. The dispersing element separates the sub-beams into spectral components in the first plane for the Fourier lens to focus them into the redirector array. The redirector array redirects the spectral components in a second plane perpendicular to the first plane to propagate back to the waveguide array. Sub-beams of each spectral component may be focused by the Fourier lens onto a location specific to each spectral component and common for both polarization components, for improving polarization performance of the wavelength-selective optical switch.

Description

WAVELENGTH SELECTIVE SWITCH WITH POLARIZATIONBALANCINGREFERENCE TO RELATED APPLICATION

[0001] This application claims priority from a U.S. Provisional Patent Application No. 63 / 667,878 entitled “Wavelength Selective Switch with Polarization Balancing”, filed on July 5, 2024, 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 optical port(s) in a wavelength-selective manner. Light signals propagating in an optical network are independently modulated at a plurality of wavelengths, forming so-called wavelength channels. The wavelength channels are spaced apart in optical frequency from one another by fixed or flexible optical frequency spacings referenced to the ITU (International Telecommunications Union) grid, typically spaced at 37.5GHz, 50GHz, 75GHz, 100GHz, 200GHz etc. in an infrared wavelength band 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 free-space and / or waveguiding optical and electro-optical components for separating light beams into individual wavelength channels or sub-bands, and for optically switching or redirecting individualwavelength channels or sub-bands. The components of a wavelength selective switch may need to be aligned to one another with sub-micrometer precision to achieve target values of optical insertion loss (IL) and polarization-dependent loss (PDL). A tedious alignment for insertion loss and PDL may increase overall cost of the wavelength selective switch devices. Furthermore, high sensitivity of performance parameters to alignment may impact environmental stability and reliability of wavelength selective switches, causing their optical performance to depend on temperature and generally degrade with time.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 plan view of a wavelength selective switch (WSS) of this disclosure;

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

[0009] FIG. 2A is a plan view of an implementation of the WSS of FIGs. 1A and IB including a beam expanding prism, a pair of beam redirecting prisms, and a grism as a dispersive element;

[0010] FIG. 2B is a plan view of an implementation of the WSS of FIGs. 1A and IB including a pair of beam expanding prisms, a pair of beam redirecting prisms, and a grism as a dispersive element;

[0011] FIG. 3 A is a schematic view of back-end polarization diversity configuration of the WSS devices of FIG. 2A and FIG. 2B;

[0012] FIG. 3B is a schematic plan view of overall wavelength-selective polarization diversity configuration of a WSS device of this disclosure; and

[0013] FIG. 4 is a flow chart of a method for wavelength-selective switching in accordance with this disclosure.DETAILED DESCRIPTION

[0014] 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.

[0015] 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. 1 A- 1B, 2A-2B, and FIGs. 3A-3B, similar reference numerals denote similar elements.

[0016] This disclosure provides a simple yet efficient configuration for an inexpensive and scalable wavelength selective switch (WSS) featuring a compensated or balanced polarization dependent loss (PDL) and compensated or balanced polarization mode dispersion (PMD). The polarization balancing is achieved in the WSS of this disclosure by configuring the polarization sub-beams of an input optical signal to propagate along substantially same light paths but in opposite directions. The common bidirectional light path may be formed by causing wavelength-dispersed polarization sub-beams at a back end of the WSS to impinge onto a configurable deflector array at a same wavelength-specific location and at approximately at a same angle of incidence on opposite sides of a normal to the configurable deflector array. The latter condition may be fulfilled by placing prismatic elements in individual light paths of the polarization sub-beams. The prismatic elements may be translated or rotated on a flat supporting surface to conveniently, precisely, and independently adjust the angle and location of incidence of the spectral components of each polarization sub-beam onto the configurable deflector array.

[0017] In accordance with the present disclosure, there is provided a wavelength selective switch (WSS) comprising a waveguide array for providing a light beam, a polarization element for splitting the light beam into polarization subbeams in a first plane, a collimating lens for collimating the polarization sub-beams to propagate parallel to each other, a dispersing element for angularly separating each collimated polarization sub-beam into spectral components in the first plane, a Fourier lens for co-focusing a same spectral component of both polarization sub-beams to a common spectral component-specific location at a focal plane, and a redirector array at the focal plane for independently redirecting each focused spectral component in a second plane perpendicular to the first plane, to propagate back through the Fourier lens, the dispersing element, the collimating lens, and the polarization element, for coupling into an output waveguide of the waveguide array.

[0018] The collimating lens may have a same non-zero optical power in both the first and second planes, and the Fourier lens may have a non-zero optical power in the first plane but not the second plane. Same spectral components of the polarization sub-beams may impinge onto the respective common spectral component-specific locations at the focal plane at substantially same angles of incidence and at opposite sides of the normal to the surface of the deflector array, such that the two sub-beams of each spectral component repeat each other’s light path on the way back to the output waveguide.

[0019] In some embodiments, the dispersing element includes a grism. The grism may include an output facet slanted with respect to an optical axis of the Fourier lens to improve a spectral uniformity of optical insertion loss of the WSS which otherwise would exhibit a bell shape due to different spectral components being reflected at different distances from the Fourier lens. A slant angle of the output facet with respect to the optical axis of the Fourier lens may be e.g. at least 10 degrees, at least 20 degrees, etc. The WSS may further include a pair of beam redirecting prisms, one in an optical path of each polarization sub-beam.

[0020] In accordance with the present disclosure, there is provided a WSS comprising in sequence a waveguide array, a polarization element, a collimating lens, a dispersing element, a Fourier lens, and a redirector array. The collimating lens has optical power in two orthogonal planes e.g. it may be rotationally symmetric lens or acompound cylindrical or acylindrical lens with component lenses having optical power in orthogonal planes. The collimating lens may be disposed one focal length of the collimating lens away from the waveguide array and the redirector array. The Fourier lens is anamorphic and is disposed one focal length of the Fourier lens away from the dispersing element and the redirector array.

[0021] In operation, the polarization element splits a light beam from the waveguide array into diverging polarization sub-beams in a first plane for collimating by the collimating lens. The dispersing element separates the sub-beams into spectral components in the first plane for the Fourier lens to focus them into the redirector array. The redirector array redirects the spectral components in a second plane perpendicular to the first plane to propagate back to the waveguide array.

[0022] In some embodiments, the sub-beams of each spectral component are focused by the Fourier lens onto a common location specific to each spectral component and common for both polarization components. Same spectral components of the polarization sub-beams may impinge onto the respective common spectral component-specific locations at the focal plane at substantially same angles of incidence, to repeat each other’s light path on the way back to the waveguide array. The dispersing element may include a grism; the grism may have an output facet slanted with respect to an optical axis of the Fourier lens to improve a spectral uniformity of optical insertion loss of the WSS. The slant angle of the output facet with respect to the optical axis of the Fourier lens may be at least 10 degrees, at least 20 degrees, etc. A pair of beam redirecting prisms may be provided, one in an optical path of each polarization sub-beam, for individual adjustments of the polarization subbeams.

[0023] In accordance with the present disclosure, there is further provided a method for wavelength-selective switching of light. The method includes using a polarization element to split a light beam emitted from a waveguide array into polarization sub-beams diverging in a first plane for collimating by a collimating lens having optical power in two perpendicular planes, e.g. rotationally symmetric lens. A dispersing element is used to disperse the collimated polarization sub-beams into spectral components in the first plane for an anamorphic Fourier lens to focus the spectral components onto a redirector array. The redirector array is used to redirectthe spectral components in a second plane perpendicular to the first plane to propagate back to the waveguide array.

[0024] The sub-beams of each spectral component may be focused by the anamorphic Fourier lens onto a common location specific to each spectral component and common for both polarization components. Same spectral components of the polarization sub-beams may impinge onto the respective common spectral component-specific locations at the focal plane at substantially same angles of incidence, to repeat each other’s light path on the way back to the waveguide array. In embodiments where the dispersing element comprises a grism, the method may further include placing the grism such that an output facet of the grism is slanted with respect to an optical axis of the anamorphic Fourier lens to improve a spectral uniformity of the wavelength-selective switching. The slant angle of the output facet with respect to the optical axis of the Fourier lens is selected to be at least 10 degrees or at least 20 degrees. The method may further include using a pair of beam redirecting prisms, one in an optical path of each polarization sub-beam, to independently redirect the polarization sub-beams.

[0025] Referring now to FIGs. 1 A and IB, a WSS 100 of this disclosure includes a waveguide array 102 having first 102-1, second 102-2, and third 102-3 waveguides (FIG. IB), e.g. optical fibers of an optical fiber array. Only three waveguides are shown in FIG. IB for brevity. A suitable number of waveguides / optical fibers may be provided, e.g. four, eight, sixteen, etc. The number of waveguides may be even or odd. The waveguide array 102 is coupled to a polarization element 116, e.g. a birefringent wedge or a Wollaston prism.

[0026] In operation, an input light beam is coupled to one of the waveguides, e.g. to the second waveguide 102-2 (FIG. IB). The polarization element 116 splits the light beam into first 111 and second 112 polarization sub-beams diverging away from one another as shown in FIG. 1 A. At this point, the first 111 and second 112 polarization sub-beams are not collimated, i.e. each of them is a diverging light beam propagating towards a collimating lens 120. The first 111 and second 112 polarization sub-beams are collimated by the collimating lens 120 to propagate parallel to one another. The YZ plane of FIG. 1A is termed “first plane” or “wavelength dispersion plane”.

[0027] A polarization rotator 118 may be placed in the light path of the first polarization sub-beam 111 to rotate its polarization to match that of the second polarization sub-beam 112. Alternatively, the polarization rotator 118 may be placed into the light path of the second polarization sub-beam 112 to rotate its polarization to match that of the first polarization sub-beam 111. The polarization rotator 118 may be placed upstream or downstream of the collimating lens 120. One advantage of the downstream placement of the polarization rotator 118 is that operation in collimated light may provide a more spatially uniform state of polarization and minimize the impact on optical working distance.

[0028] A dispersing element 108, such as e.g. a prism, a diffraction grating, or a grism (a combination of a grating and a prism), angularly separates (angularly disperses) each one of the first 111 and second 112 polarization sub-beams into spectral components in the first plane. This is illustrated by solid lines to the right of the dispersing element 108 in FIG. 1A, corresponding to one (“first”) spectral component, and dashed lines to the right of the dispersing element 108, which correspond to another (“second”) spectral component. Only two spectral components are shown for brevity; in practice, there may be tens or hundreds of spectral components, or even a spectral continuum. In the context of wavelength switching, the angularly separated spectral components correspond to wavelength channels, which are also termed optical frequency channels.

[0029] A Fourier lens 122 focuses all spectral components of both polarization sub-beams 111, 112 at a focal plane 132. More specifically, the Fourier lens 122 cofocuses a same spectral component of both polarization sub-beams 111, 112 to a common, spectral component-specific, location at the focal plane 132. For example, the first spectral component, shown with the solid lines, and the second spectral component, shown with the dashed lines, are focused at locations shifted along Y-axis in FIG. 1 A, the same spectral components of the two polarization sub-beams 111, 112 being focused at the same locations.

[0030] Herein, the term “Fourier lens” is related to the Fourier lens 122 transforming a beam angle at a focal plane upstream of the Fourier lens 122, approximately corresponding to the location of the dispersing element 108, into a beam coordinate at the focal plane 132 downstream of the Fourier lens 122, and viceversa, which is characteristic of a Fourier transform between the linear and angular spaces. Thus, each wavelength channel, having a unique beam angle upon exiting the dispersive element 108, is focused onto a unique wavelength-specific location at the downstream focal plane 132.

[0031] A redirector array 130, e.g. a liquid crystal on silicon (LCoS) array or a microelectromechanical system (MEMS) array, is disposed at the focal plane 132. Since different wavelength channels are focused by the Fourier lens 122 on different locations and, therefore, at different redirector elements of the redirector array 130, independent redirection of individual wavelength channels is possible. The redirector array 130 independently redirects each focused spectral component in a second plane, or XZ plane, or “switching plane” (i.e. the plane FIG. IB), which is perpendicular to the first plane, or the wavelength dispersion plane, or YZ plane of FIG. 1A.

[0032] The reflected spectral components propagate back through the Fourier lens 122, get recombined by the dispersing element 108, get focused by the collimating lens 120, and get combined by the polarization element 116 for coupling into a waveguide 102-1, 102-2, and / or 102-3 of the waveguide array 102, depending on the switching angle in the XZ plane defined by the redirector array 130. Same spectral components of the polarization sub-beams may impinge onto the respective common spectral component-specific locations at the focal plane at substantially same angles of incidence, to repeat each other’s light path on the way back to the output waveguide.

[0033] In FIGs. 1A and IB, the collimating lens 120 has anon-zero optical power in both the first (YZ) and second (XZ) planes. For example, the collimating lens 120 may be a rotationally symmetrical lens, such as a spherical or aspheric lens. Alternatively, the collimating lens 120 may be a compound lens including a plurality of cylindrical or acylindrical lenses, e.g. at least one having optical power in YZ plane and at least one having optical power in XZ plane. The Fourier lens 122 has a nonzero optical power in the first (YZ) plane but not the second (XZ) plane, i.e. the Fourier lens 122 is an anamorphic lens. For example, the Fourier lens 122 may be a cylindrical or acylindrical lens. Herein, the term “acylindrical lens” means a lens that has an optical power only in one plane and has a surface profile deviating from acylindrical surface profile. More than one lens element may be provided in the cylindrical or acylindrical lens, i.e. the Fourier lens 122 may be a compound lens.

[0034] FIG. IB illustrates the switching operation of the WSS 100, where the redirector array 130 reflects the light beam collimated by the collimating lens 120 to propagate back through the collimating lens 120. The collimating lens 120 focuses the redirected light beam into a selected waveguide 102-1, 102-2, and / or 102-3 of the waveguide array 102, i.e. an output waveguide, depending on the reflection angle at the redirector array 130. In other words, the collimating lens 120 operates as a switching lens, converting the beam angle to the right of the collimating lens 120 into a beam offset to the left of the collimating lens 120. The reflected beam angle is defined by the redirector array 130, and the collimating lens 120 converts that beam angle back into beam offset to the left of the collimating lens 120, allowing the selection of the output waveguide of the waveguide array 102 by steering the light beam reflected from the redirector array 130.

[0035] FIG. 1 A illustrates the wavelength selective property of the abovedescribed wavelength-selective switching by the WSS 100. Each spectral component of the light beam may be redirected separately and independently from any other spectral component, because it is being redirected by a separate, independently controlled area of the redirector array 130, the spectral components being angularly dispersed by the dispersing element 108, and the angular dispersion is converted into a linear dispersion by the Fourier lens 122.

[0036] The polarization balancing property of the WSS 100 may be illustrated by considering the light path of the first polarization sub-beam 111, which is directed by the polarization element 116 to propagate from left to right and upwards in FIG. 1A, as indicated by atop solid arrow 161. The spectral components of the first polarization sub-beam 111 impinge onto the redirector array 130 at same locations as respective spectral components of the second polarization sub-beam 112, and at substantially same incidence angles. Due to that, and due to the principle of optical reciprocity or Helmholtz reciprocity, the first polarization sub-beam 111 will repeat the light path of the second polarization sub-beam 112 on its way from left to right, i.e. towards the redirector array 130. Specifically, the first polarization sub-beam 111 will propagate from the right to left and upwards, as indicated by a bottom solid arrow162 in FIG. 1A. Likewise, the second polarization sub-beam 112 will repeat the light path of the first polarization sub-beam 111 in opposite direction. As a result, the polarization dependent loss (PDL) and the polarization mode dispersion (PMD) of the WSS 100 will be balanced.

[0037] The configuration of the WSS 100 of FIGs. 1 A- IB is termed herein a “2f” configuration, since the collimating lens 120 is disposed one focal length Fi (“If’) of the collimating lens 120 away from the waveguide array 102, and the redirector array 130 is disposed one focal length Fi (“If’) of the collimating lens 120 away from the collimating lens 120. Furthermore, the Fourier lens 122 is disposed one focal length F2 of the Fourier lens 122 away from the dispersing element 108, and the redirector array 130 is disposed one focal length F2 away from the Fourier lens 122. For this reason, the configuration of the 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 lens 120 (Fy) and the Fourier lens 122 (F?) 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 specifics.

[0038] Referring to FIG. 2A, a WSS 200A is an implementation of the WSS 100 of FIGs. 1A and IB. The WSS 200A of FIG. 2A includes a waveguide array 202, e.g. a linear fiber array, for injecting an input light beam 204 into the WSS 200 A, and for wavelength selective outputting the light beam 204 into any of the waveguide(s) of the waveguide array 202. An optional lens array, or microlens array 209 may be coupled to the waveguide array 202 for providing required optical beam waist sizes / divergence. It is to be understood that, just like in the wavelength selective switches 100 of FIG. 1A-1B, the direction of propagation of light may be reversed, so that IxN WSS may operate as an Nxl WSS.

[0039] A birefringent element 216 is optically coupled to a collimating lens 220 by means of three folding mirrors 228, which are flat mirrors but may be curved in some embodiments. The birefringent element 216 splits the light beam 204 into first 211 and second 212 sub-beams in YZ plane i.e. in the plane of FIG. 2A. A forward optical path of the first sub-beam 211 is shown with dotted lines, and a forward optical path of the second sub-beam 212 is shown with dashed lines. The optical paths are represented by chief rays only, for brevity. Downstream of the collimating lens220, the first 211 and second 212 sub-beams are collimated and parallel to one another.

[0040] A polarization rotator 218 may be disposed in an optical path of the second sub-beam 212 to bring the polarization state of the second sub-beam 212 to that of the first sub-beam 211, making them nearly identically polarized. By way of a non-limiting illustrative example, the polarization rotator 218 may include a halfwave plate with an optic axis oriented at 45 degrees to a polarization direction of the linearly polarized second polarization sub-beam 212. In some implementations, the polarization rotator 218 may be placed in the optical path of the first polarization subbeam 111 and may be disposed downstream or upstream of the collimating lens 220.

[0041] A beam expanding prism 240 may be disposed in the optical path of the first 211 and second 212 sub-beams. Its purpose is to control the width of both polarization sub-beams 211 and 212 in YZ plane (the first plane). The WSS 200A may further include first 251 and second 252 beam redirecting prisms disposed in optical paths of the first 211 and second 212 polarization sub-beams, respectively. The first 251 and second 252 beam redirecting prisms are configured to shift and redirect the first 211 and second 212 polarization sub-beams, respectively, in YZ plane, for independent adjustment of locations and directions of the first 211 and second 212 polarization sub-beams during alignment of the WSS 200A.

[0042] The first 251 and second 252 beam redirecting prisms can control the gap between the first 211 and second 212 polarization sub-beams depending on their relative position and orientation. The gap control afforded by the separate placement of the first 251 and second 252 beam redirecting prisms allows one to reduce the length of the diffraction grating 288 without sacrificing the width of the individual polarization sub-beams 211, 212 required for wavelength selectivity. It is further noted that a beam expanding prism as defined herein may also redirect the light beam, and a beam redirecting prism may also expand the light beam. Such terms are used herein with the purpose of defining a main function of the prism.

[0043] The first 251 and second 252 beam redirecting prisms may include e.g. right-angle, acute-angle, or obtuse-angle prisms. The prisms 251, 252, as well as other elements of the WSS 200 A, may be anti -reflection (AR) coated to reduce opticallosses and ghosting, i.e. ghost reflections. Tilted surfaces of both beam redirecting prisms 251, 252, as well as the beam expanding prism 240, may be disposed at a Brewster angle for reduction of Fresnel reflections of the first 211 and second 212 sub-beams, which are substantially polarized after polarization rotator 218 in YZ plane to take advantage of Brewster angle reflection suppression.

[0044] The WSS 200A may further include a grism 208. The function of the grism 208 is to angularly disperse the first 211 and second 212 polarization sub-beams into spectral components / wavelength channels. Only one such spectral component is shown for each sub-beam for brevity: a first spectral component 291 of the first polarization sub-beam 211, and a second spectral component 292 of the second polarization sub-beam 212, at a same wavelength as the first spectral component 291. In the embodiment shown in FIG. 2A, the grism 208 includes a prism 286 optically coupled to a reflective diffraction grating 288. It is noted that the separation between the first 211 and second 212 polarization sub-beams may be further controlled by selecting a proper slant angle of an input facet 298 of the prism 286. For instance, reducing the slant angle of the input facet 298 reduces the distance between the polarization sub-beams 211, 212.

[0045] Still referring to FIG. 2A, the WSS 200A may include a Fourier lens 222, e.g. a cylindrical or acylindrical lens, for focusing the spectral components of the first 211 and second 212 sub-beams onto the deflector array 230, which is disposed at a rear focal plane 232 of the Fourier lens 222. The Fourier lens 222 is anamorphic e.g. it may only provide optical power (i.e. focusing / defocusing power) in YZ plane, while having a substantially zero optical power in XZ plane. By way of non-limiting examples, the Fourier lens 222 may include a single or multiple elements e.g. may be a singlet lens, a doublet lens, a triplet lens, or quadruplet lens. A shape of multiple lens elements of the Fourier lens 222 may be selected to optimize polychromatic and / or off-axis performance of the Fourier lens 222, e.g. to reduce chromatic aberration, spherical aberration, field curvature, coma, etc.

[0046] Same spectral components 291 and 292 of the first 211 and second 212 polarization sub-beams, respectively, i.e. spectral components at a same wavelength, are directed onto the deflector array 230 to a common location at substantially same angles of incidence onto the deflector array 230, such that optical paths of the first211 and second 212 polarization sub-beams are swapped during propagation through the WSS 200 A in backward direction. This causes the polarization sub-beams 211 and212 to counter-propagate along a same looped optical path. Configuring the polarization sub-beams 211, 212 to counter-propagate along a same light path balances the polarization performance of the WSS 200 A, significantly reducing its PDL and PMD, and improving environmental stability of these parameters.

[0047] At least some of the optical components of the WSS 200A may be supported by a base 280, which is shown only partially in FIG. 2A. The base 280 may be transparent or opaque, and / or may have openings for the spectral components 291 and 292 redirected by optional turning mirror or prism to propagate through the base 280. By way of non-limiting examples, the base 280 may made out of a material with a high thermal conductivity such as aluminum nitride (AIN), or an insulator such as fused silica (SiCh). The deflector array 230 may be mechanically coupled to the base 280 at the top or bottom of the base 280.

[0048] Referring now to FIG. 2B, a WSS 200B is an implementation of the WSS 100 of FIGs. 1A and IB. The WSS 200B is generally similar to the WSS 200A of FIG. 2A but includes not one but a pair of beam expanding prisms 241 and 242. Because the two beam expanding prisms 241 and 242 provide large enough beam sizes of the first 211 and second 212 polarization sub-beams, the input facet 298 of the prism 286 does not need to provide any further beam expansion so that it may be made more perpendicular to the impinging first 211 and second 212 polarization subbeams. A large beam size provided by the beam expanding prisms 241 and 242 comes with a large beam gap, which may be mitigated (reduced) by selecting the relative position of the beam redirecting prisms 251, 252, thus achieving wide beams required for wavelength selectivity with small beam gap desired to reduce the size of the grating 288 and the size and complexity of the Fourier lens 222.

[0049] Another feature of the WSS 200B of FIG. 2B, which may also be present in the WSS 200A of FIG. 2 A, is that an output facet 297 of the grism 208 is slanted such that a normal 285 of the output facet 297 forms a non-zero angle of e.g. at least 10 degrees, or in some embodiments at least 20 degrees, w.r.t. an optical axis 229 of the Fourier lens 222. The slanted facet 297 causes all spectral components of the first211 and second 212 polarization sub-beams to exit the prism 286 at a non-normal angle to the output facet 297 .

[0050] FIG. 3 A provides an example of how the light paths of the first 211 and second 212 polarization sub-beams may be overlapped onto one another at the backend to improve the polarization performance of a WSS. The first 211 and second 212 polarization sub-beams impinge onto different portions of the diffraction grating 288 (the prism 286 is omitted for brevity), which diffracts each of them into a plurality or continuum of spectral components. Out of all the spectral components, only first, second, and third spectral components are shown for brevity, at first Ai, second Z?, and third Z? wavelengths, respectively, where Zy < As < As.

[0051] The diffraction grating 288 is disposed at a frontal focal plane 231 of the Fourier lens 222, and the deflector array 230 is disposed at the rear focal plane 232 of the Fourier lens 222. In such a configuration, each spectral component is focused onto the deflector array at the rear focal plane 232. It is to be noted that the spectral components are only focused by the anamorphic Fourier lens 222 in the first plane (YZ plane, wavelength dispersion plane), while remaining collimated in the second plane (XZ plane, switching plane). Conveniently, the anamorphic focusing afforded by the anamorphic Fourier lens 222 causes the focused spots to be elongated in X- direction (the switching direction), which improves the directional selectivity in the switching plane, as the selectivity is inversely proportional to the spot size. Such improved performance is achieved by employing a rather simple optical configuration using only two lenses - rotationally symmetric and anamorphic - and generally not requiring curved mirrors, complex anamorphic mode converters, etc. The optimized wavelength and switching selectivity is achieved by merely using one non- anamorphic (i.e. rotationally symmetric) collimating lens 120 / 220 as a switching lens, and one anamorphic (e.g. cylindrical or acylindrical singlet or multi-component) Fourier lens 222 as the spectral components focusing lens.

[0052] In accordance with this disclosure, the sub-beams of each spectral component 211, 212 are focused by the Fourier lens 222 onto a common location specific to each spectral component and common for both polarization sub-beams 211, 212. For example, the first spectral components of both polarization sub-beams 211, 212 at the first wavelength Ai are focused at a first location 301; the secondspectral components of both polarization sub-beams 211, 212 at the second wavelength 12 are focused at a second location 302; and the third spectral components of both polarization sub-beams 211, 212 at the third wavelength As are focused at a third location 303. For a better insertion loss performance, same spectral components of the polarization sub-beams 211, 212 may impinge onto the respective common spectral component-specific locations 301-303 at the back focal plane 232 at substantially same angles of incidence.

[0053] For example, in FIG. 3A, the first components at the first wavelength Ai are focused at a same first angle of incidence 0i the second components at the second wavelength As are focused at a same second angle of incidence 6b; and the third components at the third wavelength As are focused at a same third angle of incidence 03. One can see that the angles of incidence are mirrored about a normal 310 to the rear focal plane 232. Such a configuration allows the polarization sub-beams 211 and 212 to repeat each other’s light path on the way back to an output waveguide of the waveguide array 202.

[0054] Even though magnitudes of the angles of incidence 0i, 02, and 0s are all different, nonetheless the conditions of angle of incidence equality for the polarization sub-beams 211 and 212 may be fulfilled simultaneously at each wavelength. It has been demonstrated experimentally that, even though the first 211 and second 212 polarization sub-beams impinge at the diffraction grating 288 at locations offset from the front focal plane 231 of the Fourier lens 222 as seen in FIG. 3A, a good insertion loss and polarization balancing of the WSS 200A and / or the WSS 200B may nonetheless be obtained. In accordance with this disclosure, the equality of angles of incidence of the first 211 and second 212 polarization sub-beams for each wavelength simultaneously may be achieved by slanting the output facet 297 w.r.t. the optical axis 229 of the Fourier lens 222 (FIG. 2B). The slanting the output facet 297 improves spectral uniformity of optical insertion loss of the WSS 100. It is to be noted that, although the angles of incidence of the first 211 and second 212 polarization subbeams are equal, these angle may vary from wavelength to wavelength as illustrated in FIG. 3A.

[0055] The slanting the output facet 297 w.r.t. the optical axis 229 of the Fourier lens 222 facilitates achieving the equality of angles of incidence, as well as good focusing, of polarization sub-beams of each spectral component. This reduces wavelength dependence of optical insertion loss. The slanted output facet 297 compensates for the first 211 and second 212 polarization sub-beams impinging onto the diffraction grating 288 at locations offset from the frontal focal plane 231 of the Fourier lens 222. For example, in FIG. 3A, the first polarization sub-beam 211 impinges onto the diffraction grating 288 ahead of the frontal focal plane 231 and below the optical axis 229 of the Fourier lens 222, and the second polarization subbeam 212 impinges onto the diffraction grating 288 behind the frontal focal plane 231 and above the optical axis 229. The slanted output facet 297 makes sure that all spectral components are properly focused at the rear focal plane 232, to provide a precise polarization swapping with low, spectrally uniform optical insertion loss.

[0056] The conditions of mirroring the angle of incidence and the overlap of the focal spot locations for the first 211 and second 212 polarization sub-beams may be met by actively aligning optical element(s) in the path of the first 211 and second 212 polarization sub-beams. By way of a non-limiting illustrative example, such balancing may be achieved by rotating and / or shifting the first 251 and second 252 redirecting prisms (FIGs. 2A and 2B), as well as by shifting the Fourier lens 222. It is to be kept in mind that, due to the geometry illustrated in FIG. 3A, the Fourier lens 222 operates as an angle-to-offset and offset-to-angle element converting a beam angle upstream of the Fourier lens 222 into a beam offset downstream of the Fourier lens 222, and vice versa, converting a beam offset upstream of the Fourier lens 222 into a beam angle downstream of the Fourier lens 222.

[0057] Overall wavelength-selective polarization diversity configuration of a WSS device such as the WSS 100 of FIGs. 1 A and IB, the WSS 200 A of FIG. 2A, and / or the WSS 200B of FIG. 2B, is further described with reference to FIG. 3B. A light beam 304 entering a WSS 300 is split by a polarization element 316, such as a birefringent wedge or a Wollaston prism, into first 311 and second 312 polarization sub-beams. The first 311 and second 312 polarization sub-beams diverge from one another and impinge onto a collimating lens 320, which collimates the first 311 and second 312 polarization sub-beams to propagate parallel to one another. Thecollimated first 311 and second 312 polarization sub-beams impinge onto a dispersing element 308, which spectrally disperses each one of the first 311 and second 312 polarization sub-beams into spectral components. For brevity, only two such spectral components are shown for each polarization sub-beam, specifically first 311-1 and second 311-2 spectral components of the first polarization sub-beam 311, and first 312-1 and second 312-2 spectral components of the second polarization sub-beam 312. The first polarization components 311-1 and 312-1 are shown with solid lines, and the second polarization components 311-2 and 312-2 are shown with dashed lines.

[0058] The first 311-1 and second 311-2 spectral components of the first polarization sub-beam 311 are focused by a Fourier lens 322 at first A,i and second h locations of a deflector array 330, respectively. The first 312-1 and second 312-2 spectral components of the second polarization sub-beam 312 are focused by the Fourier lens 322 at the same respective first 2 / and second fa locations of the deflector array 330. The angles of incidence of the first spectral components 311-1 and 312-1 are equal in magnitude and opposite in sign relative to the normal 310. The angles of incidence of the second spectral components 311-2 and 312-2 are also equal in magnitude and opposite in sign relative to the normal 310. Herein, “opposite in sign” means that the beams impinge at opposite sides to the normal 310.

[0059] The equality of angles of incidence of same spectral components ensures that the back propagation path (i.e. right to left in FIG. 3B) of the first spectral component 311-1 of the first polarization sub-beam 311 overlaps with the forward propagation path (i.e. left to right in FIG. 3B) of the first spectral component 312-1 of the second polarization sub-beam 312. Similarly, the back propagation path of the second spectral component 311-2 of the first polarization sub-beam 311 overlaps with the forward propagation path of the second spectral component 312-2 of the second polarization sub-beam 312. For each spectral component, the propagation paths of the first 311 and second 312 polarization sub-beams are equal to each other, which balances PDL and PMD for all wavelengths simultaneously.

[0060] Turning now to FIG. 4 with further reference to FIGs. 2A and 2B, a method 400 of wavelength selective switching of light may include emitting (402) a light beam from a waveguide array, e.g. the light beam 204 from an input waveguideof the waveguide array 202 (FIGs. 2A and 2B). A polarization element, e.g. the birefringent element 216 in FIG. 2, may be used (FIG. 4; 404) to split the light beam into two polarization sub-beams diverging in the YZ plane, or wavelength dispersion plane. A collimating lens such as e.g. the collimating lens 220 in the WSS 200A and / or the WSS 200B, may be used (FIG. 4; 406) to collimate the polarization subbeams.

[0061] A dispersing element may be used (408) to disperse the collimated subbeams into spectral components in the first plane, for an anamorphic Fourier lens, e.g. the Fourier lens 222, to focus (410) them onto a redirector array, such as the deflector array 230 in FIGs. 2A and 2B. As explained above, the sub-beams of each spectral component may be focused by the anamorphic Fourier lens 222 onto a common location specific to each spectral component and common for both polarization components. The redirector array may be used (FIG. 4; 412) to redirect the spectral components in the XZ plane to propagate back to the waveguide array through the Fourier lens, the dispersing element, the collimating lens, and the polarization element, for coupling into an output waveguide of the waveguide array.

[0062] A pair of redirecting prisms, one in an optical path of each polarization sub-beam such as, for example, the first 251 and second 252 beam redirecting prisms in the path of the first 211 and second 212 polarization sub-beams, respectively, may be used (FIG. 4; 422) to bring the focused sub-beams to the common location specific to each spectral component (424). The first 251 and second 252 beam redirecting prisms, as well as optionally the Fourier lens 222, may be adjusted to orient (426) the sub-beams of each spectral component to mirrored angles of incidence at the redirector array. To that end, the first 211 and second 212 polarization sub-beams may be brought to impinge onto the diffraction grating 288 at equal distances from the optical axis 229 of the Fourier lens 222, as illustrated in FIG. 3A. This allows the polarization sub-beams of an input light beam to counter-propagate along a looped light path for balancing polarization dependent loss and polarization mode dispersion of the WSS 200 A and / or the WSS 200B, or any other WSS using a non- anamorphic collimator lens as a switching lens and an anamorphic lens as a Fourier lens.

[0063] The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, 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 wavelength selective switch (WSS) comprising: a waveguide array for providing a light beam; a polarization element for splitting the light beam into polarization sub-beams in a first plane; a collimating lens for collimating the polarization sub-beams to propagate parallel to each other; a dispersing element for angularly separating each collimated polarization sub-beam into spectral components in the first plane; a Fourier lens for co-focusing a same spectral component of both polarization subbeams to a common spectral component-specific location at a focal plane; and a redirector array at the focal plane for independently redirecting each focused spectral component in a second plane perpendicular to the first plane, to propagate back through the Fourier lens, the dispersing element, the collimating lens, and the polarization element for coupling into an output waveguide of the waveguide array.

2. The WSS of claim 1, wherein the collimating lens has a same non-zero optical power in both the first and second planes, and the Fourier lens has a non-zero optical power in the first plane but not the second plane.

3. The WSS of claim 1, wherein same spectral components of the polarization subbeams impinge onto the respective common spectral component-specific locations at the focal plane at substantially same angles of incidence, to repeat each other’s light path on the way back to the output waveguide.

4. The WSS of claim 1, wherein the dispersing element comprises a grism.

5. The WSS of claim 4, wherein the grism comprises an output facet slanted with respect to an optical axis of the Fourier lens to improve a spectral uniformity of optical insertion loss of the WSS.

6. The WSS of claim 5, wherein a slant angle of the output facet with respect to the optical axis of the Fourier lens is at least 10 degrees.

7. The WSS of claim 1, further comprising a pair of beam redirecting prisms, one in an optical path of each polarization sub-beam.

8. A wavelength selective switch (WSS) comprising in sequence a waveguide array, a polarization element, a collimating lens, a dispersing element, a Fourier lens, and a redirector array, wherein: the collimating lens has optical power in two perpendicular planes and is one focal length of the collimating lens away from the waveguide array and the redirector array; the Fourier lens is anamorphic and is one focal length of the Fourier lens away from the dispersing element and the redirector array, wherein in operation, the polarization element splits a light beam from the waveguide array into diverging polarization sub-beams in a first plane for collimating by the collimating lens; the dispersing element separates the sub-beams into spectral components in the first plane for the Fourier lens to focus them into the redirector array; and the redirector array redirects the spectral components in a second plane perpendicular to the first plane to propagate back to the waveguide array.

9. The WSS of claim 8, wherein the sub-beams of each spectral component are focused by the Fourier lens onto a location specific to each spectral component and common for both polarization components.

10. The WSS of claim 9, wherein same spectral components of the polarization subbeams impinge onto the respective common spectral component-specific locations at the focal plane at substantially same angles of incidence, to repeat each other’s light path on the way back to the waveguide array.

11. The WSS of claim 8, wherein the dispersing element comprises a grism.

12. The WSS of claim 11, wherein the grism comprises an output facet slanted with respect to an optical axis of the Fourier lens to improve a spectral uniformity of optical insertion loss of the WSS.

13. The WSS of claim 12, wherein a slant angle of the output facet with respect to the optical axis of the Fourier lens is at least 10 degrees.

14. The WSS of claim 8, further comprising a pair of beam redirecting prisms, one in an optical path of each polarization sub-beam.

15. A method for wavelength-selective switching of light, the method comprising: using a polarization element to split a light beam emitted from a waveguide array into polarization sub-beams diverging in a first plane for collimating by a collimating lens having optical power in two perpendicular planes; using a dispersing element to disperse the collimated polarization sub-beams into spectral components in the first plane for an anamorphic Fourier lens to focus the spectral components onto a redirector array; and using the redirector array to redirect the spectral components in a second plane perpendicular to the first plane to propagate back to the waveguide array.

16. The method of claim 15, wherein the sub-beams of each spectral component are focused by the anamorphic Fourier lens onto a location specific to each spectral component and common for both polarization components.

17. The method of claim 16, wherein same spectral components of the polarization sub-beams impinge onto the respective common spectral component-specific locations at the focal plane at mirrored angles of incidence, to repeat each other’s light path on the way back to the waveguide array.

18. The method of claim 17, wherein the dispersing element comprises a grism, the method further comprising placing the grism such that an output facet of the grism is slanted with respect to an optical axis of the anamorphic Fourier lens to improve a spectral uniformity of the wavelength-selective switching.

19. The method of claim 18, wherein a slant angle of the output facet with respect to the optical axis of the Fourier lens is selected to be at least 10 degrees.

20. The method of claim 15, further comprising using a pair of beam redirecting prisms, one in an optical path of each polarization sub-beam, to independently redirect the polarization sub-beams.

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