Dynamic spectral filter

Abstract

A multi-unit dynamic spectral filter (MDSF) is disclosed. The MDSF includes a free space optical assembly comprising in sequence a collimating element, a dispersing element, a Fourier element, and a redirector array. Pairs of waveguides are optically coupled to the free space optical assembly, each pair corresponding to a unit of the MDSF and comprising an input waveguide for providing an input light beam at a unit-specific angle, and an output waveguide for receiving an output light beam parallel to the input light beam. In operation, the collimating element collimates each input light beam, the dispersing element angularly disperses each collimated light beam into spectral components in a first plane, and the Fourier element focuses the spectral components onto the redirector array at unit-specific locations.

Description

DYNAMIC SPECTRAL FILTERREFERENCE TO RELATED APPLICATION

[0001] The present application claims priority from U.S. Provisional Patent Application No. 63 / 715,803 filed on November 4, 2024, entitled “Dynamic Spectral Filter”, and incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure relates to optical devices, and in particular to dynamic spectral filters.BACKGROUND

[0003] Light signals propagating in an optical network are independently modulated and transmitted at a plurality of wavelengths, forming so-called wavelength channels. The wavelength channels are spaced apart from one another by fixed or flexible optical frequency spacings known as ITU (International Telecommunications Union) 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. All light signals propagating in a particular fiber of the network may be amplified together by an optical amplifier whose spectral gain depends on the load and the properties of the active gain medium.

[0004] Dynamic spectral filters provide a dynamically controllable wavelength- selective optical attenuation or insertion loss. A dynamic spectral filter may be used to equalize gain for different spectral components of an optical network. Such a dynamic spectral filter is termed dynamic gain equalizer or DGE. A dynamic spectral filter capable of independently attenuating or blocking individual wavelength channels or entire wavelength bands is called a wavelength blocker or WB.

[0005] While being highly functional and versatile, dynamic spectral filters often include a multitude of free-space and / or waveguiding optical and electro-opticalcomponents which drives up manufacturing costs. As dynamic spectral fdters are needed in ever-increasing quantity for a variety of optical networks, in particular for datacenter networks, cost reduction per individual dynamic spectral fdter becomes important.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 plan schematic view of a dynamic spectral fdter (DSF) embodiment of this disclosure;

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

[0009] FIG. 2 is a plan view of an implementation of the DSF 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. 3 is a side schematic view of a multi-unit DSF (MDSF) embodiment of this disclosure, which may be based on the DSF of FIGs. 1A-1B and FIG. 2;

[0011] FIG. 4A is a simplified raytrace diagram of a single unit of the DSF of FIG. 3 with straight input / output light beams;

[0012] FIG. 4B is a simplified raytrace diagram of a single unit of the DSF of FIG. 3 with angled input / output light beams;

[0013] FIG. 5 is a schematic diagram illustrating the angle-to-offset and offset- to-angle operation of a collimating lens of a DSF of this disclosure;

[0014] FIG. 6 is a side schematic view of a multi-unit dual-band DSF of this disclosure;

[0015] FIG. 7 is a side schematic view of a multi-unit DSF of this disclosure utilizing a redirector comprising an array of tiltable reflectors;

[0016] FIG. 8 is a schematic view of a DSF light input / output module including a microlens array coupled to an array of off-axis waveguides; and

[0017] FIG. 9 is a set of spectral diagrams illustrating the operation of a DSF of this disclosure as a dynamic gain equalizer.DETAILED DESCRIPTION

[0018] 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.

[0019] 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- 1B, 2-3, FIGs. 4A-4B, and FIGs. 6-7, similar reference numerals denote similar elements.

[0020] This disclosure provides a simple yet efficient configuration for inexpensive scalable multi-unit dynamic spectral filter (MDSF). The scalability is achieved by tilting input and output light beams to allow spectrally dispersed light beams of different wavelength channels to impinge onto a redirector array at offset locations, thus providing a plurality of dynamic spectral filter (DSF) units in a singlemodule, each DSF unit providing fully independent spectrally selective dynamic attenuation of wavelength channels.

[0021] A DSF of this disclosure may use redirector arrays and spectral dispersing elements having low polarization-dependent optical loss (PDL) and low polarization mode dispersion (PMD) to provide polarization-insensitive operation of the entire DSF module. Some types of redirector arrays, however, are intrinsically polarization selective. Such polarization-selective elements may also be used in DSF embodiments of this disclosure. A compensated or balanced PDL and PMD may be provided by using polarization diversity. An incoming optical signal is split into polarization sub-beams to propagate along a substantially same light path in opposite directions. The bidirectional light path may be formed by causing wavelength- dispersed polarization sub-beams at a back end of the DSF to impinge onto a configurable redirector array at a same wavelength-specific location and at a same angle of incidence at opposite sides of a normal to the configurable redirector array.

[0022] In accordance with the present disclosure, there is provided a multi-unit dynamic spectral filter (MDSF) comprising a free space optical assembly and pairs of waveguides optically coupled to the free space optical assembly. The free space optical assembly includes in sequence a collimating element, a dispersing element, a Fourier element, and a redirector array. Each pair of waveguides corresponds a single unit of the MDSF and includes an input waveguide for providing an input light beam at a unit-specific angle, and an output waveguide for receiving an output light beam parallel to the input light beam. In operation, the collimating element collimates each input light beam, the dispersing element angularly disperses each collimated light beam into spectral components in a first plane, and the Fourier element focuses the spectral components onto the redirector array at unit-specific locations. In some embodiments, the input and output waveguides of each pair may be disposed adjacent one another, on opposite sides of an optical axis of the collimating element, and / or interspersed with one another.

[0023] In some embodiments, the MDSF may include a microlens array, each microlens of the microlens array being coupled to a particular one of the input or output waveguides. The input and output waveguides of each pair are offset from anoptical axis of a corresponding microlens by a same unit-specific amount. An angular dispersion of the dispersing element and a beam size of the focused spectral components on the redirector array may be selected so as to provide a smoothly varying optical loss spectrum in each MDSF unit for functioning as a dynamic gain equalizer (DGE) and / or a wavelength blocker (WB).

[0024] In some embodiments, the pairs of waveguides are disposed in a second plane perpendicular to the first plane, the redirector array independently redirects the focused spectral components in the second plane, the collimating element includes a lens having non-zero optical power in the first and second planes, and the Fourier element includes an anamorphic lens having non-zero optical power in the first plane and substantially zero optical power in the second plane. In such embodiments, the free space optical assembly may further include a beam expanding prism in the optical path of each input light beam to control a width of each input light beam in the first plane. The dispersing element may include a grism i.e. a grating coupled to a prism. The redirector array may include an array of tiltable micromirrors and / or a liquid crystal on silicon (LCoS) array.

[0025] The MDSF may further include a polarization element in an optical path between the pairs of waveguides and the free space optical assembly for splitting each input light beam into first and second polarization sub-beams in the first plane. For each unit of the MDSF, an optical path of the first polarization sub-beam from the corresponding input waveguide to the redirector array may overlap an optical path of the second polarization sub-beam from the redirector array back to the corresponding output waveguide, and vice versa. In such embodiments, the Fourier element may be configured to co-focus a same spectral component of both polarization sub-beams corresponding to each input light beam to a common spectral component-specific and MDSF unit-specific location at the redirector array.

[0026] In accordance with the present disclosure, there is further provided a dynamic spectral filter (DSF) comprising an input waveguide for providing a light beam, and an output waveguide for receiving the light beam propagated through the DSF. A polarization element splits the light beam into polarization sub-beams in a first plane. A collimating lens collimates the polarization sub-beams, and thecollimated sub-beams propagate parallel to one another downstream of the collimating lens. A dispersing element angularly separates each collimated polarization sub-beam into spectral components in the first plane. A Fourier lens cofocuses a same spectral component of both polarization sub-beams to a common spectral component-specific location at a focal plane. A redirector array at the focal plane independently redirects 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 the output waveguide.

[0027] The redirector array may include e.g. a liquid crystal on silicon (LCoS) array. The dispersing element may include a grism. An angular dispersion of the dispersing element and a beam size of the focused spectral components on the redirector array may be selected so as to provide a sharply varying optical loss spectrum for functioning as a dynamic gain equalizer (DGE) and / or a wavelength blocker (WB).

[0028] Referring now to FIGs. 1 A and IB, a DSF 100 of this disclosure includes an optical coupling unit, e.g. a waveguide array 102, having input 102-1 and output 102-2 waveguides (FIG. IB), e.g. optical fibers of an optical fiber array. The waveguide array 102 may be coupled to a polarization element 116, e.g. a birefringent wedge or a Wollaston prism. An input light beam is coupled to the input waveguide 102-1. The polarization element 116 splits the input light beam into first 111 and second 112 polarization sub-beams diverging away from one another in FIG. 1 A. The YZ plane of FIG. 1A is termed “wavelength dispersion plane” or “first plane”, and the XZ plane of FIG. IB is termed “attenuation plane” or “second plane”.

[0029] The first 111 and second 112 polarization sub-beams are collimated by a collimating lens 120 to propagate parallel to one another. 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 may be placed upstream or downstream of thecollimating lens 120. One advantage of the downstream placement of the polarization rotator 118 is that operation in collimated light may minimize the impact on optical working distance.

[0030] A dispersing element 108, such as e.g. a diffraction grating or a grism i.e. a diffraction grating coupled to 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 an actual device, there may be hundreds of spectral components, or even a spectral continuum.

[0031] A Fourier lens 122 focuses all spectral components of both polarization sub-beams 111 and 112 at a focal plane 132. More specifically, the Fourier lens 122 co-focuses 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.

[0032] A redirector array 130, e.g. a liquid crystal on silicon (LCoS) array or a microelectromechanical system (MEMS) reflector array, is disposed at the focal plane 132. In operation, the redirector array 130 independently redirects each focused spectral component in the second plane, or “attenuating plane” of FIG. IB, which is perpendicular to the first plane, or wavelength dispersion plane of FIG. 1A. 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 an output waveguide 102-2 of the waveguide array 102 at an attenuation level depending on the beam redirection angle in the XZ plane defined by the redirector array 130. Same spectral components of the polarization sub-beams may impinge onto the respectivecommon spectral component-specific locations at the focal plane at opposite angles of incidence of substantially equal magnitude, 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 (i.e. focusing or defocusing 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. The Fourier lens 122 has a non-zero 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 a purely cylindrical surface profile. More than one lens element may be provided in the cylindrical or acylindrical lens.

[0034] FIG. IB illustrates the attenuation function of the DSF 100. The redirector array 130 reflects the light beam collimated by the collimating lens 120 back through the collimating lens 120. The collimating lens 120 focuses the redirected light beam into the output waveguide 102-2 of the waveguide array 102 at an offset causing attenuation. The magnitude of the offset and the corresponding attenuation level depend on the reflection angle at the redirector array 130. In other words, the collimating lens 120 operates as a Fourier lens or an angle-to-offset 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, providing adjustable level of attenuation by steering the light beam reflected from the redirector array 130 along X- axis, and / or by applying difference holograms to control individual pixels of the redirector.

[0035] FIG. 1A illustrates the wavelength selective property of the abovedescribed attenuation by the DSF 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 theredirector 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 DSF 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 a top 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 opposite incidence angles of substantially equal magnitude. Due to that, and due to the principle of 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 arrow 162 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, thereby automatically balancing the PDL and PMD of the DSF 100.

[0037] The configuration of the DSF 100 of FIGs. 1A-1B 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 DSF 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 (fi) and the Fourier lens 122 ( )) is not a necessary requirement; in some embodiments, the ratio may be as large as 4:1, or even larger, depending on the specific optical configuration.

[0038] Referring to FIG. 2, a DSF 200 is an example implementation of the DSF 100 of FIGs. 1A and IB. The DSF 200 of FIG. 2 includes a waveguide array202, e.g. a linear fiber array, for injecting an input light beam 204 into a free space optical assembly of the DSF 200 for wavelength selective attenuation of the light beam 204 coupled into an output waveguide 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 / beam directions. It is to be understood that, just like in case of the DSF 100 of FIG. 1A-1B, the directions of propagation of light may be reversed.

[0039] A birefringent element 216 may be optically coupled to a collimating lens 220 by means of three folding mirrors 228, which are flat mirrors but may be curved i.e. convex or concave 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. 2. 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 lens 220, 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 DSF 200 may further include first 251 and second 252 beam redirecting prisms 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 independentadjustment of locations and directions of the first 211 and second 212 polarization sub-beams during alignment of the DSF 200. In addition, 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 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, and to reduce the aperture of the Fourier lens 222. 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, with an understanding that it is not necessarily the only function.

[0042] The first 251 and second 252 beam redirecting prisms may include e.g. right-angle, acute-angle, or obtuse-angle prisms. The prisms 251 and 252, as well as other elements of the DSF 200, may be anti-reflection (AR) coated to reduce optical losses 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.

[0043] The DSF 200 may further include a grism 208. Its purpose 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 subbeam 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. 2, 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.

[0044] Still referring to FIG. 2, the DSF 200 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 redirector array 230, which is disposed at a rear focal plane 232 of the Fourier lens 222. The Fourier lens 222 is anamorphic i.e. 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.

[0045] 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, may be directed onto the redirector array 230 to a common location at opposite angles of incidence of substantially equal magnitude onto the redirector array 230, such that optical paths of the first 211 and second 212 polarization sub-beams are swapped during propagation through the DSF 200 in backward direction. This causes the polarization sub-beams 211 and 212 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 DSF 200, significantly reducing its PDL and PMD, and improving environmental stability of these parameters.

[0046] At least some of the free space optical components of the DSF 200 may be supported by a base 280, which is shown only partially in FIG. 2, for brevity. 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 redirector array 230 may be mechanically coupled to the base 280 at the top or bottom of the base 280.

[0047] Referring now to FIG. 3, a multi-unit DSF (MDSF) 300 is an embodiment of the DSF 200 of FIG. 2 and may include similar elements. The redirector 300 is shown in XZ or attenuating plane, the YZ plane view being similar to that of FIGs. 1 A and 2. The MDSF 300 includes a waveguide array 302 with four pairs of interspersed optical fibers or waveguides, each pair corresponding to a particular DSF device DI, D2, D3, and D4. Input (“in”) and output (“out”) fibers of the first device DI are shown with solid lines; the fibers of the second device D2 are shown with thin solid lines; the fibers of the third device D3 are shown with double lines; and the fibers of the fourth device D4 are shown with thick solid lines. One can see that the waveguides of each input-output pair have a same tilt angle in XZ plane. The waveguide array 302 is coupled to a free-space optical assembly including in sequence a collimating element 320, a dispersing element (omitted for brevity), a Fourier element (omitted for brevity), and a redirector array 330.

[0048] The collimating element 320, which corresponds to the collimating lens 120 of the DSF 100 of FIGs. 1A-1B and the collimating lens 220 of the DSF 200 of FIG. 2, is disposed one focal length fi of the collimating element 320 from the waveguide array 302 and from a redirector array 330, which is shown turned by 90 degrees towards the viewer for convenience. In such a configuration, the collimating lens 220 operates as an angle-to-offset element converting an impinging beam angle into an output beam displacement, and vice versa, an impinging beam displacement into an output beam angle. This is illustrated in FIGs. 4 A and 4B: in FIG. 4A, the impinging diverging beam is parallel to the optical axis of the collimating element 320, causing it to impinge onto the redirector array 330 at its center, while in FIG. 4B, the impinging diverging beam is non-parallel to the optical axis of the collimating element320, causing it to impinge onto the redirector array 330 at a location displaced by a distance 8 from its center, the distance 8 being calculated as

[0049] 8=f *d (1)

[0050] for small angles 6.

[0051] Referring back to FIG. 3, each unique tilt angle of the waveguide pair of the individual DSF devices D1-D4 in XZ plane is converted into unique locations ofwavelength-dispersed spots SI, S2, S3, and S4 at the redirector array 330. Thus, each wavelength channel of each DSF device or unit may be steered separately. For example, a focal point 333 corresponds to the third device D3 having a third wavelength-dispersed spot S3. The wavelength-dispersed spots S1-S4 are shown with the same line styles as the corresponding input / output waveguides. The redirector array 330 may be controlled to manipulate the reflected light beams in XZ plane, e.g. by the holograms applied to the redirector array 330, to return to the waveguide array 302 with a corresponding offset for in-coupling, with a pre-determined optical loss, into a corresponding output waveguide.

[0052] The angle-to-offset and offset-to-angle operation of the collimating element 320 is further illustrated in FIG. 5. The collimating element 320 converts an upstream beam’s offset into a downstream beam’s angle, and an upstream beam angle into a downstream beam offset. The redirector array 330 modifies the beam angle, and the modified angle (marked with an asterisk *) is converted to a modified beam offset (also marked with an asterisk *). This allows one to control the offset of the output light beam, thereby providing a freedom to select the output waveguide’s location along X axis (FIG. 3) and the attenuation of the output light beam.

[0053] The additional degree of freedom afforded by the redirector array 330 modifying the reflected beam angle in XZ plane, as explained above with reference to FIG. 5, allows one to customize light paths of individual DSF devices or units. This opens some interesting possibilities. For example, it allows different DSF units of a multi-unit DSF device to operate in different wavelength bands. Optical communications may use a C-band of ITU grid spanning between 1530nm and 1565nm, or an L-band of the ITU grid spanning between 1575nm and 1625nm, and a multi-unit DSF device may operate in both of these bands. Referring for a nonlimiting illustrative example of this to FIG. 6, a dual-band multi-unit DSF 600 is similar to the DSF 200 of FIG. 2, includes similar elements, and disperses and attenuates individual wavelength channels in a similar manner. The dual-band multiunit DSF 600 includes first DI and second D2 DSF devices or units operating in L- band of ITU grid, and third D3 and fourth D4 DSF devices or units operating in C- band of the ITU grid. This is achieved by grouping the DI and D2 input / output waveguide pairs of a waveguide array 6-2 to be above the optical axis of a collimatinglens 620, and the D3 and D4 devices operating in C-band of the ITU grid to be below the optical axis of the collimating lens 620.

[0054] Additional beam turning prisms or other beam-turning elements 650 may be added to the free space optical assembly and disposed above the optical axis, to modify the impinging angle of the light beams onto the dispersive element (such as the grism 208 in FIG. 2) for the diffractive beams of the L- and C-bands to have similar angle of diffraction, allowing them to share a same Fourier lens 622 and a same redirector array 630. It is noted that the locations of wavelength-dispersed spots S1-S4 of different devices D1-D4 respectively are defined by tilt angles of corresponding input / output waveguide pairs.

[0055] The DSF device examples considered above with reference to FIGs. 1A- 1B, FIG. 2 - FIG. 3, FIG. 6, and FIG. 7 use polarization diversity to achieve polarization-independent performance while using polarization-selective redirector arrays. A variety of polarization diversity configurations may be used according to this disclosure. By way of a non-limiting illustrative example, the polarization subbeams may be separated and / or combined not angularly as shown in FIG. 1 A but linearly i.e. parallel and offset from one another. In the latter case, the collimating lens may be a cylindrical / acylindrical lens having optical power in XZ plane but not in YZ plane. Additional optical elements, such as prisms, telescope cylindrical lens pair in YZ plane, reflectors, etc. may be provided to properly route and / or expand the propagating light beam(s).

[0056] In embodiments where the redirector arrays are not polarization- selective, the polarization-selective optics may be dealt away with, and the overall optical configuration becomes much simpler, with the optical path corresponding to that of a single polarization, which may backpropagate along the same path, or along a different path. Some MEMS arrays of tiltable micromirrors, e.g. a digital light processor (DLP™) micromirror array manufactured by Texas Instruments, Texas, USA, has a low polarization dependence of reflection. For such a polarizationinsensitive redirector array, the input / output waveguides corresponding to different DSF units may be disposed symmetrically in a waveguide array 702 w.r.t. the optical axis of the collimating lens 620, as shown in FIG. 7.

[0057] Tilted waveguide arrays may be replaced with a functionally equivalent array 802 depicted in FIG. 8. The waveguide array 802 of FIG. 8 includes an array of microlenses 860 with a constant microlens pitch coupled to a waveguide / optical fiber array 862 with different offsets w.r.t. optical axes of respective microlenses 860. For example, an offset Ax along X-axis will result in an angle -0 of approximately Ax / F, where F is focal lens of the microlenses 860. Similarly, an offset -Ax along X-axis will result in an angle 0, an offset -2 Ax along X-axis will result in an angle 20, an offset of 2 Ax along X-axis will result in an angle -20, and so on. The off-centered waveguides 862 may remain parallel to one another, which may be convenient from the manufacturing standpoint.

[0058] Turning to FIG. 9, a DSF of this disclosure may be configured to operate as a dynamic gain equalizer (DGE) for flattening spectral gain profile of an optical amplifier such as erbium-doped fiber amplifier (EDFA). An EDGA gain spectrum 901 has a peak that causes wavelength channels near the peak to be amplified stronger than wavelength channels near shoulders of the gain spectrum 901, which increases inter-channel crosstalk and may cause instability of the optical network. To offset the non-uniformity of the gain spectrum, an equalizing filter may have a transmission spectrum 902 of an inverse shape to the gain spectrum 901, such that when the EDFA and the equalizing filter are coupled in series, the resulting gain spectrum 903 is flat. Because the shape of the gain spectrum 901 depends on the EDFA load, the equalizing filter needs to have a dynamically adjustable transmission spectrum 902. The multi-unit DSF of this disclosure may be used to dynamically flatten gain of several EDFAs independently of one another. To that end, an angular dispersion of the dispersing element and a beam size of the focused spectral components on the redirector array may be selected so as to provide a smoothly varying optical loss spectrum in MDSF channels.

[0059] A DSF of this disclosure may also operate as a wavelength blocker (WB) which may selectively and independently block individual wavelength channels of a wavelength-division multiplexed (WDM) optical signal propagating in an optical fiber. Spectral selectivity of the WB embodiment of the DSF of this disclosure needs to be high enough, higher than that of the DGE, to be able to attenuate neighboringchannels independently of one another. To that end, the angular dispersion of the dispersing element and the beam size of the focused spectral components on the redirector array may be selected so as to provide a sharply varying optical loss spectrum in MDSF units.

[0060] Providing several DSF units sharing a same free space optical assembly as described herein allows an optical network operator to significantly reduce costs and / or improve reliability by redundancy of dynamic gain equalization and / or wavelength channel equalization or attenuation. Furthermore, having some DGE / WB units reserved for future expansion allows for a greater degree of flexibility when adapting the optical communication system to quickly evolving requirements in such technology areas as artificial intelligence (Al) datacenters, for example.

[0061] 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. For example, collimating and / or Fourier lenses may be replaced by functionally equivalent elements operating by reflection and / or diffraction.

[0062] The number of units may be any practical number greater than one. The redirector arrays may be based on liquid crystals, MEMS, or other technologies. The DSF and MDSF of this disclosure may use polarization splitting elements to provide polarization diversity or may be based on polarization insensitive elements without having to split the polarization of the input light beam. Such other embodiments and modifications are intended to fall within the scope of the present disclosure.

[0063] 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 dynamic spectral filter (MDSF) comprising: a free space optical assembly comprising in sequence a collimating element, a dispersing element, a Fourier element, and a redirector array; and pairs of waveguides optically coupled to the free space optical assembly, each pair corresponding to a unit of the MDSF and comprising an input waveguide for providing an input light beam at a unit-specific angle, and an output waveguide for receiving an output light beam parallel to the input light beam, wherein in operation: the collimating element collimates each input light beam; the dispersing element angularly disperses each collimated light beam into spectral components in a first plane; and the Fourier element focuses the spectral components onto the redirector array at unitspecific locations.

2. The MDSF of claim 1, wherein the input and output waveguides of each pair are disposed adjacent one another.

3. The MDSF of claim 1, wherein input and output waveguides of each pair are disposed on opposite sides of an optical axis of the collimating element.

4. The MDSF of claim 1, wherein the input and output waveguides of the pairs of waveguides are interspersed with one another.

5. The MDSF of claim 1, further comprising a microlens array, each microlens coupled to a particular one of the input or output waveguides, wherein input and output waveguides of each pair are offset from an optical axis of a corresponding microlens by a same unit-specific amount.

6. The MDSF of claim 1, wherein the free-space optical assembly comprises a beamturning element in an optical path of a unit of the MDSF whereby different units of the MDSF are configured to operate in different wavelength bands.

7. The MDSF of claim 1, wherein an angular dispersion of the dispersing element and a beam size of the focused spectral components on the redirector array are selected so as to provide a smoothly varying optical loss spectrum of each MDSF unit for functioning as a dynamic gain equalizer (DGE).

8. The MDSF of claim 1, wherein an angular dispersion of the dispersing element and a beam size of the focused spectral components on the redirector array are selected so as to provide a sharply varying optical loss spectrum of each MDSF unit for functioning as a wavelength blocker (WB).

9. The MDSF of claim 1, wherein: the pairs of waveguides are disposed in a second plane perpendicular to the first plane; the redirector array independently redirects the focused spectral components in the second plane; the collimating element comprises a lens having non-zero optical power in the first and second planes; and the Fourier element comprises an anamorphic lens having non-zero optical power in the first plane and substantially zero optical power in the second plane.

10. The MDSF of claim 9, wherein the free space optical assembly further comprises a beam expanding prism in the optical path of each input light beam to control a width of each input light beam in the first plane.

11. The MDSF of claim 9, wherein the dispersing element comprises a grism.

12. The MDSF of claim 9, wherein the redirector array comprises an array of tiltable micromirrors.

13. The MDSF of claim 9, wherein the redirector array comprises a liquid crystal on silicon (LCoS) array.

14. The MDSF of claim 9, further comprising a polarization element in an optical path between the pairs of waveguides and the free space optical assembly for splitting each input light beam into first and second polarization sub-beams in the first plane; wherein for each unit of the MDSF, an optical path of the first polarization sub-beam from the corresponding input waveguide to the redirector array overlaps an optical path of the second polarization sub-beam from the redirector array back to the corresponding output waveguide, and vice versa.

15. The MDSF of claim 14, wherein the Fourier element is configured to co-focus a same spectral component of both polarization sub-beams corresponding to each input light beam to a common spectral component-specific and MDSF unit-specific location at the redirector array.

16. A dynamic spectral filter (DSF) comprising: an input waveguide for providing a light beam, and an output waveguide for receiving the light beam propagated through the DSF; 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, wherein the collimated sub-beams propagate parallel to one another downstream of the collimating lens; 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 the output waveguide.

17. The DSF of claim 16, wherein the redirector array comprises a liquid crystal on silicon (LCoS) array.

18. The DSF of claim 16, wherein the dispersing element comprises a grism.

19. The DSF of claim 16, wherein an angular dispersion of the dispersing element and a beam size of the focused spectral components on the redirector array are selected so as to provide a smoothly varying optical loss spectrum for functioning as a dynamic gain equalizer (DGE).

20. The DSF of claim 16, wherein an angular dispersion of the dispersing element and a beam size of the focused spectral components on the redirector array are selected so as to provide a sharply varying optical loss spectrum for functioning as a wavelength blocker (WB).

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