A polarization-independent phase modulator

Abstract

The disclosure proposes a polarization-independent phase modulator, a wavelength selective switch comprising the phase modulator, and a corresponding method for operating said devices. The phase modulator comprises: a LC layer; a retarder layer provided on the LC layer, wherein the LC layer and the retarder layer are divided into an array of pixels, each pixel being arranged to receive a wavelength channel of one or more wavelength channels, having a respective effective refractive index at the wavelength channel in the LC layer, and comprising a respective one or more retardance properties in the retarder layer that are adapted to the wavelength channel; a reflective layer provided on the retarder layer; an array of electrodes provided on or in the reflective layer; and a controller configured to individually control each electrode of the array of electrodes to regulate the respective effective refractive indices of at least some pixels.

Description

[0001] A POLARIZATION-INDEPENDENT PHASE MODULATOR

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a phase modulator. The disclosure proposes a phase modulator, a wavelength selective switch comprising the phase modulator, and corresponding methods for operating the phase modulator and the wavelength selective switch. The phase modulator comprises new kind of layers.

[0004] BACKGROUND

[0005] Liquid crystal (LC)-based spatial light modulators (SLMs) are inherently polarizationdependent. Most SLMs are based on Liquid Crystal on Silicon (LCOS) technology, which is similar to reflective Liquid Crystal Displays (LCDs). Conventionally, an incident optical wave is linearly polarized, wherein a LCOS device is configured to change the phase delay between two orthogonal components of the optical wave, which results in a change of the polarization state of the optical wave. In the phase-only regime (which is used in almost all beam steering and beam deflection applications), the linearly polarized light is required to be in the same plane as the optic axis of the LC (parallel to the “alignment direction”). Any deviation from this condition results in a steep drop of the diffraction efficiency and device performance in overall. Thus, wavelength selective switches (WSS), where the optical input can be of arbitrary polarization, comprise polarization diversity optics. The polarization diversity optics split the input polarization into two perpendicular components and converts them into a single linear polarization to generate a suitable optical input for LCOS devices.

[0006] SUMMARY

[0007] In view of the above, an objective of this disclosure is to provide a phase modulator that enables the use of a simplified optical system. For example, and objective is to use an unpolarised optical input, for example, one or more wavelength channels that are unpolarised. Another objective is to improve the efficiency, for example, manufacturing cost, size, and / or polarization-dependent loss, of the phase modulator. These and other objectives are achieved by this disclosure as described in the enclosed independent claims. Advantageous implementations are further defined in the dependent claims.

[0008] This disclosure is based on the following considerations.

[0009] LC-based devices are based on polarization transformation, which can be reduced to phase- only modulation if the incident polarization is in-plane with the optic axis of the LC.

[0010] For example, conventional WSS modules are using polarization-dependent LCOS technology, whereas an input signal from a fiber is arbitrary polarized. The input signal needs to be split in two polarizations S and P, wherein one of those polarizations is rotated in order to match the polarization that is suitable for a LCOS panel. However, removing polarization diversity optics may provide cost and efficiency advantages.

[0011] The performance of a retarder in a phase modulator may be wavelength dependent. For example, the polarization conversion ratio (PCR) may be wavelength dependent. Some wavelengths of an optical wave may be associated with a lower PCR values and other wavelengths of the optical wave may be associated with a higher PCR.

[0012] The wavelength spectrum may be defined spatially across an LCOS area. A retarder layer in the phase modulator may be patterned in spectral bands to maximize its performance. The retarder may be combined with additional layers, which allow to reduce the thickness of the retarder, increase the efficiency, and reduce parasite diffraction orders by increasing, for example, the PCR.

[0013] A first aspect of this disclosure provides a polarization-independent phase modulator comprising: a LC layer configured to receive an optical wave comprising one or more wavelength channels; a retarder layer provided on, for example, provided below, the LC layer, wherein the LC layer and the retarder layer are divided into an array of pixels, each pixel being arranged to receive a wavelength channel of the one or more wavelength channels, having a respective effective refractive index at the wavelength channel in the LC layer, and comprising a respective one or more retardance properties in the retarder layer that are adapted, for example, individually adapted, to the wavelength channel; a reflective layer provided on, for example, provided below, the retarder layer; an array of electrodes provided on, for example, provided below, or in the reflective layer; and a controller configured to individually control each electrode of the array of electrodes to regulate the respective effective refractive indices of at least some pixels, for example, all pixels, of the array of pixels; wherein the phase modulator is configured to provide the one or more wavelength channels through the LC layer and through the retarder layer to the reflective layer, and wherein the reflective layer is configured to reflect the one or more wavelength channels back through the retarder layer to the LC layer.

[0014] In this disclosure, the term "regulate" may refer to at least one of: control, adjust, or modulate.

[0015] The one or more wavelength channels may be two or more wavelength channels.

[0016] The array of pixels in the retarder layer may comprise a set of respective one or more retardance properties. Each respective one or more retardance properties may be associated with one, for example, only one, pixel of the array of pixels.

[0017] Each respective one or more retardance properties may be individually adapted to the associated wavelength channel.

[0018] The term “re tardance properties’" may refer to the fact that the retardance of the one or more wavelength channels and / or the reflections of the one or more wavelength channels is influenced by the physical properties of the retarder layer in the associated pixel. The respective one or more retardance properties may be the physical properties that influence the retardance of the one or more wavelength channels and / or the retardance of the reflections of the one or more wavelength channels.

[0019] The phase modulator may be a LCOS phase modulator.

[0020] Each electrode of the array of electrodes may be associated with one respective pixel of the array of pixels. For example, each electrode may regulate, for example, only, the respective effective refractive index of the respective pixel that is associated with the electrode.

[0021] The retarder layer may be sandwiched between the LC layer and the reflective layer. The reflective layer may comprise the array of pixel electrodes.

[0022] The phase modulator may comprise a circuitry layer provided on, for example, below, the array of electrodes and / or the reflective layer. The circuitry layer may comprise CMOS circuitry.

[0023] The retarder layer may be a quarter-wave retarder layer and / or may be an effectively half-wave retarder layer, as there is a double pass through the quarter-wave retarder layer.

[0024] The quarter-wave retarder layer may be configured to provide a half-wave retardance on the one or more wavelength channels as the one or more wavelength channels pass twice through the quarter-wave retarder layer. Thus, the quarter-wave retarder layer may be considered to be an effectively half-wave retarder layer in the phase modulator. The reflective layer and the quarter-wave retarder layer may be combined into one layer that may be referred to as an effectively half-wave retarder layer.

[0025] In this disclosure the term “optical input” may refer to the one or more wavelength channels that are received by the phase modulator and / or the one or more input optical signals of the wavelength selective switch. For example, “optical input” may refer to any optical signal in an optical path that propagates towards the reflective layer in the optical path.

[0026] In this disclosure the term “optical output” may refer to the one or more output optical signals of the wavelength selective switch and / or the reflections of the one or more wavelength channels from the reflective layer. For example, “optical output” may refer to any optical signal in an optical path that propagates away from the reflective layer in the optical path.

[0027] The phase modulator may not depend on the polarization of an optical input. The phase modulator may function with any polarization of the optical input, wherein no additional optics for adjusting the polarization of the optical input or an optical output of the phase modulator that is based on the optical input are required.

[0028] For example, the optical input may be one of unpolarised, linearly polarized, circular polarized, and elliptically polarized. Thus, the phase modulator may be flexible regarding the optical input. Each wavelength channel may comprise, for example, consist of, a respective first component and a respective second component.

[0029] In an implementation form of the first aspect, the retarder layer comprises a metasurface and / or a subwavelength grating. The metasurface may be or comprise a subwavelength grating. The metasurface or metasurface retarder may be a subwavelength structure comprising, for example, a network of tiny, antenna-like elements (meta-atoms) that can manipulate the phase, amplitude, and polarization of light, effectively changing its polarization state.

[0030] The subwavelength grating may be a subwavelength structure comprising, for example, a dense array of thin dielectric or metal wires that functions as a polarizer, selectively reflecting or transmitting different polarizations of light.

[0031] In a further implementation form of the first aspect, the metasurface is a subwavelength grating, and wherein the subwavelength grating is a one-dimensional grating or a two-dimensional grating.

[0032] In a further implementation form of the first aspect, the respective one or more retardance properties of each pixel of the array of pixels comprise at least one of a material of the metasurface in the pixel, a period of the metasurface in the pixel, a ridge width of the metasurface in the pixel, a height of the metasurface in the pixel, and an optic axis of the retarder layer in the pixel.

[0033] Each retardance property of the respective one or more retardance properties of each pixel may be individually adapted to the wavelength channel of the pixel. For example, the material of the metasurface may be different between pixels.

[0034] In a further implementation form of the first aspect, the LC layer has a LC optic axis, and wherein the retarder layer has a retarder optic axis that is oriented at an angle of substantially 45° to the LC optic axis.

[0035] In a further implementation form of the first aspect, the respective one or more retardance properties of at least one pixel of the array of pixels are different from the respective one or more retardance properties of at least one other pixel of the array of pixels. Thus, the pixels may be individually adapted in the retarder layer to the respective wavelength channels.

[0036] In a further implementation form of the first aspect, further comprising a glass-based layer, wherein the LC layer is provided on, for example, below, the glass-based layer, and wherein the glass-based layer is configured to: receive the optical wave and transmit the optical wave to the LC layer, and / or receive the reflections of the one or more wavelength channels, and transmit said reflections. For example, the LC layer may be sandwiched between the glassbased layer and the retarder layer. The glass-based layer may be for protecting the LC layer.

[0037] In a further implementation form of the first aspect, the retarder layer comprises one or more capping layers, and / or wherein the metasurface is provided on the one or more capping layers and the one or more capping layers are configured to isolate the metasurface from the LC layer.

[0038] The one or more capping layers may protect the LC layer from the retarder layer. For example, molecules of the LC layer may be prevented from entering the gaps of a subwavelength grating in the retarder layer or from aligning along the topography of the subwavelength grating.

[0039] In a further implementation form of the first aspect, the one or more capping layers form an antireflection coating for suppressing reflections, for example, spurious reflections, from the metasurface of the one or more wavelength channels.

[0040] The reflections that are generated at the surface of the metasurface may be suppressed. The antireflection coating may not suppress the reflections of the one or more wavelength channels that are formed in the reflective layer. The reflections of the one or more wavelength channels that are formed in the reflective layer are intended to be provided out of the phase modulator and may form the modified one or more wavelength channels, and / or, after further modification, the one or more output optical signals.

[0041] The one or more capping layers may be configured to supress reflections of the one or more wavelength channels that are formed when the one or more wavelength channels propagate from the LC layer to the retarder layer. In a further implementation form of the first aspect, the metasurface forms air gaps in the retarder layer. Thus, a low refractive index medium can be provided in the retarder layer to improve the PCR.

[0042] In a further implementation form of the first aspect, the reflective layer comprises one or more reflectivity enhancement layers provided on the retarder layer. The reflective layer may comprise a reflective sub-layer. The one or more one or more reflectivity enhancement layers may be sandwiched between a reflective sub-layer of the reflective layer and the metasurface.

[0043] In an implementation form of the first aspect, the metasurface is configured to reflect a first polarization component of at least some wavelength channels of the one or more wavelength channels back through the retarder layer to the LC layer, wherein the reflective layer is configured to reflect a second polarization component of the at least some wavelength channels of the one or more wavelength channels back through the retarder layer to the LC layer, and wherein the one or more reflectivity enhancement layers are configured to provide a phase delay between the first polarization component and the second polarization component of the at least some wavelength channels. Thus, the retarder layer may be thinner, as the one or more reflectivity enhancement layers provide an additional phase delay. Thus, manufacturing of the phase modulator can be improved.

[0044] In a further implementation form of the first aspect, the glass-based layer comprises a first alignment layer for aligning at least some LC molecules of the LC layer in the vicinity of the glass-based layer, and / or wherein the retarder layer comprises a second alignment layer for aligning at least some other LC molecules of the LC layer in the vicinity of the retarder layer.

[0045] In a further implementation form of the first aspect, wherein the reflective layer comprises a passivation layer provided on, for example, provided below, the retarder layer, and a reflective sub-layer provided on, for example, below, the passivation layer, wherein the reflective sublayer comprises the array of electrodes. The passivation layer may be sandwiched between the metasurface and the reflective sub-layer. The one or more reflectivity enhancement layers may comprise the passivation layer. Alternatively or additionally, the reflectivity enhancement layer may be sandwiched between the passivation layer and the reflective sub-layer. In a further implementation form of the first aspect, the phase modulator comprises at least one of the first alignment layer that forms an interface between the glass-based layer and the LC layer; the second alignment layer that forms an interface between the LC layer and the retarder layer; and the passivation layer that forms an interface between the retarder layer and the reflective layer.

[0046] In a further implementation form of the first aspect, the glass-based layer is coated with an index-matched indium tin oxide (IMITO) layer. Alternatively, the glass-based layer may be coated with other transparent conductive (TCO) layers.

[0047] In a further implementation form of the first aspect, each electrode of the array of electrodes comprises aluminium.

[0048] In a further implementation form of the first aspect, the array of electrodes is provided on a circuitry layer that is electrically connected to the controller.

[0049] In a further implementation form of the first aspect, the circuitry layer is provided on, for example, above, a substrate of the phase modulator.

[0050] In a further implementation form of the first aspect, the retarder layer is configured to provide half-wave retardance of the one or more wavelength channels that are passing twice through the retarder layer, and / or wherein the retarder layer and the one or more reflectivity enhancement layers are configured to provide half-wave retardance of the one or more wavelength channels that are passing twice through the retarder layer.

[0051] In a further implementation form of the first aspect, the LC optic axis is parallel to an orientation of the array of pixels in the LC layer.

[0052] In a further implementation form of the first aspect, the controller is configured to control at least one electrode in the array of electrodes to be in a first state and at least one other electrode to be in a second state.

[0053] In a further implementation form of the first aspect, for each electrode in the array of electrodes, the electrode is configured to: apply a voltage across the LC layer in the respective pixel associated with the electrode in the first state, and apply zero voltage across the LC layer in the respective pixel associated with the electrode in the second state, or wherein, for each electrode in the array of electrodes, the electrode is configured to: apply a voltage across the LC layer in the respective pixel associated with the electrode in the second state, and apply zero voltage across the LC layer in the respective pixel associated with the electrode in the first state.

[0054] A second aspect of this disclosure provides a wavelength selective switch comprising: one or more input ports comprising one or more first beamforming optics, the one or more input ports being configured to receive one or more input optical signals; a wavelength separating element configured to receive the one or more input optical signals from the one or more input ports and separate the one or more input optical signals into one or more spatially separated wavelength channels; one or more Fourier lenses configured to receive the one or more wavelength channels from the wavelength separating element; the phase modulator according to the first aspect or any of its implementation forms, configured to receive the one or more wavelength channels from the one or more Fourier lenses and individually spatially control respective phase shifts of the one or more wavelength channels to deflect, attenuate, or split the one or more wavelength channels and form one or more modified wavelength channels, wherein the one or more Fourier lenses are further configured to receive the one or more modified wavelength channels from the phase modulator; a wavelength combining element configured to receive the one or more modified wavelength channels from the one or more Fourier lenses and combine the one or more modified wavelength channels into one or more output optical signals; and one or more output ports comprising one or more second beamforming optics, wherein the one or more output ports are configured to receive the one or more output optical signals from the wavelength combining element. The one or more wavelength channels may be two or more wavelength channels.

[0055] The phase modulator may comprise, be optically connectable, or be optically connected to an additional optical system.

[0056] The additional optical system and the phase modulator may form the wavelength selective switch of the second aspect or any if its implementation forms. The wavelength selective switch and / or the additional optical system may not comprise optics to modify the polarization of the optical output or the optical input except in the phase modulator.

[0057] A third aspect of this disclosure provides a method of operating a polarization-independent phase modulator, the phase modulator comprising: a LC layer; a retarder layer provided on the LC layer, wherein the LC layer and the retarder layer are divided into an array of pixels, each pixel being arranged to receive a wavelength channel of one or more wavelength channels, having a respective effective refractive index at the wavelength channel in the LC layer, and comprising a respective one or more retardance properties in the retarder layer that are adapted to the wavelength channel; a reflective layer provided on the retarder layer; an array of electrodes provided on or in the reflective layer; and a controller; wherein the method comprises: individually controlling, with the controller, each electrode of the array of electrodes to regulate the respective effective refractive indices of at least some pixels of the array of pixels; receiving, with the LC layer, an optical wave comprising the one or more wavelength channels; providing, with the phase modulator, the one or more wavelength channels through the LC layer and through the retarder layer to the reflective layer; and reflecting, with the reflective layer, the one or more wavelength channels back through the retarder layer to the LC layer.

[0058] The method of the third aspect may have implementation forms that correspond to the implementation forms of the phase modulator of the first aspect. The method of the third aspect and its implementation forms achieve the advantages and effects described above for the phase modulator of the first aspect and its respective implementation forms.

[0059] A fourth aspect of this disclosure provides a method of operating a wavelength selective switch, wherein the wavelength selective switch comprises: one or more input ports comprising one or more first beamforming optics; a wavelength separating element; one or more Fourier lenses; the phase modulator according to the first aspect or any of its implementation forms; a wavelength combining element; and one or more output ports comprising one or more second beamforming optics; wherein the method comprises: receiving, with the one or more input ports, one or more input optical signals; receiving, with the wavelength separating element, the one or more input optical signals from the one or more input ports; separating, with the wavelength separating element, the one or more input optical signals into one or more spatially separated wavelength channels; receiving, with the one or more Fourier lenses, the one or more wavelength channels from the wavelength separating element; receiving, with the phase modulator, the one or more wavelength channels from the one or more Fourier lenses; individually spatially controlling, with the phase modulator, respective phase shifts of the one or more wavelength channels to deflect, attenuate, or split the one or more wavelength channels and form one or more modified wavelength channels; receiving, with the one or more Fourier lenses, the one or more modified wavelength channels from the phase modulator; receiving, with the wavelength combining element, the one or more modified wavelength channels from the one or more Fourier lenses; combining, with the wavelength combining element, the one or more modified wavelength channels into one or more output optical signals; and receiving, with the one or more output ports, the one or more output optical signals from the wavelength combining element.

[0060] The method of the fourth aspect may have implementation forms that correspond to the implementation forms of the phase modulator of the first aspect. The method of the fourth aspect and its implementation forms achieve the advantages and effects described above for the phase modulator of the first aspect and its respective implementation forms.

[0061] The method of the fourth aspect may have implementation forms that correspond to the implementation forms of the wavelength selective switch of the second aspect. The method of the fourth aspect and its implementation forms achieve the advantages and effects described above for the wavelength selective switch of the second aspect and its respective implementation forms.

[0062] In this disclosure, providing or forming a layer "on" another layer may mean growing / depositing these layers one, for example, directly, upon the other. A layer “provided on” another layer may mean these layers are provided directly on each other. Thus, surfaces of these layers may be in contact.

[0063] “Below" and “above", as used herein, are defined with respect to a „body-fixed' reference frame / coordinate system, i.e., a reference frame that is fixed to the phase modulator 100 and rotates with the latter in case the latter is rotated. Further, in this disclosure the term “substantially” and / or “approximately” can mean „within a certain range comprising the nominal value“. That range may be +- 10 %, +-5% or +-2 % of the nominal value.

[0064] For example, the retarder optic axis being oriented at an angle of substantially 45° to the LC optic axis may refer to the fact that an angle of 45° may be a preferred value to achieve an improved effect or efficiency. The phase modulator may still function if there is a deviation from 45°.

[0065] It has to be noted that all devices, elements, units and means described in the disclosure could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the disclosure as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities.

[0066] Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

[0067] BRIEF DESCRIPTION OF DRAWINGS

[0068] The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

[0069] FIG. 1 shows a phase modulator according to this disclosure.

[0070] FIG. 2 shows an exemplary phase modulator according to this disclosure.

[0071] FIG. 3 shows an exemplary subwavelength grating according to this disclosure.

[0072] FIG. 4 shows an exemplary metasurface element according to this disclosure. FIG. 5 shows the PCR over a wavelength range for a structure that is optimized over the entire wavelength range according to this disclosure.

[0073] FIG. 6 shows a uniform metasurface and a metasurface with each pixel of the array of pixels in the metasurface being individually modified according to this disclosure.

[0074] FIG. 7 shows an exemplary retarder layer and an exemplary LC layer divided into an array of pixels according to this disclosure.

[0075] FIG. 8 shows exemplary orientations of the LC optic axis and the retarder layer optic axis according to this disclosure.

[0076] FIG. 9 shows a representation of port direction vs wavelength direction according to this disclosure.

[0077] FIG. 10 shows PCR values for different wavelength channels according to this disclosure.

[0078] FIG. 11 shows the dependence of the phase shift of S and P polarizations versus the thickness of a single SiCh layer under the grating according to this disclosure.

[0079] FIG. 12 shows the propagation for S and P polarizations through a phase modulator according to this disclosure.

[0080] FIG. 13 shows the propagation for S and P polarizations through a phase modulator according to this disclosure.

[0081] FIG. 14 shows the reflectivity for the two polarizations, depending on the thickness of the SiCh layer according to this disclosure.

[0082] FIG. 15 shows a phase retardance distribution dependent on the thickness of the one or more capping layers according to this disclosure. FIG. 16 shows a reflectivity for the spurious reflection of the metasurface according to this disclosure.

[0083] FIG. 17 shows exemplary orientations of the LC optic axis and the retarder layer optic axis according to this disclosure.

[0084] FIG. 18 shows a representation of the retarder axis orientation in relation to the port direction vs wavelength direction according to this disclosure.

[0085] FIG. 19 shows two exemplary 2D metasurfaces according to this disclosure.

[0086] FIG. 20 shows a wavelength selective switch according to this disclosure.

[0087] FIG. 21 shows an exemplary wavelength selective switch according to this disclosure.

[0088] FIG. 22 shows a method according to this disclosure.

[0089] FIG. 23 shows a method according to this disclosure.

[0090] DETAILED DESCRIPTION OF EMBODIMENTS

[0091] FIG. 1 shows a phase modulator 100 according to this disclosure.

[0092] The phase modulator 100 comprises: a LC layer 101; a retarder layer 102 provided on the LC layer 101; a reflective layer 103 provided on the retarder layer 102; an array of electrodes 104 provided on or in the reflective layer 103; and a controller 105.

[0093] The phase modulator 100 is a polarization-independent phase modulator 100. The LC layer 101 configured to receive an optical wave comprising one or more wavelength channels 108. The LC layer 101 and the retarder layer 102 are divided into an array of pixels 106, which is exemplified by the brace illustrated in FIG. 1. Each pixel of the array of pixels 106 is arranged to receive a wavelength channel of the one or more wavelength channels 108. For example, each pixel may be associated with only one respective wavelength channel of the one or more wavelength channels 108. Each pixel has a respective effective refractive index at the wavelength channel in the LC layer 101. Each pixel comprises a respective one or more retardance properties 107 in the retarder layer 102 that are adapted to the wavelength channel, which is illustrated with the dashed line in FIG. 1.

[0094] The controller 105 is configured to individually control each electrode of the array of electrodes 104 to regulate, for example, control, the respective effective refractive indices of at least some pixels of the array of pixels 106.

[0095] The phase modulator 100 is configured to provide the one or more wavelength channels 108 through the LC layer 101 through the retarder layer 102 to the reflective layer 103, and the reflective layer 103 is configured to reflect the one or more wavelength channels 108 back through the retarder layer 102 to the LC layer 101.

[0096] The one or more wavelength channels 108 may be two or more wavelength channels 108.

[0097] The polarization independent phase modulator 100 may be a polarization independent LCOS phase modulator 100.

[0098] A phase modulator 100 may comprise a subwavelength grating retarder with all parameters constant over an entire LCOS area. However, the performance of the retarder may be wavelength dependent. For example, the PCR may be wavelength dependent. Some wavelengths of an optical wave may be associated with a lower PCR values and other wavelengths of the optical wave may be associated with a higher PCR.

[0099] PCR may be defined as the intensity of the rotated (converted) polarization divided over the intensity of the initial (unconverted) polarization in dB.

[0100] FIG. 2 shows an exemplary phase modulator 100 according to this disclosure.

[0101] The phase modulator 100 may comprise a glass based layer, the LC layer 101; LC alignment layers; one or more capping layers; the retarder layer 102, for example, in form of subwavelength grating or metasurface; one or more reflectivity enhancement layers; and the reflective layer 103, which may be a backplane layer and / or may comprise pixel electrodes and underlying circuitry.

[0102] This disclosure may provide a polarization-independent LCOS phase modulator 100. The polarization independence can be achieved by using a polarization flip based on the retarder layer 102.

[0103] This disclosure may be based on combining the retarder layer 102, for example, a quarter- wave retarder layer 102 and / or comprising the metasurface, a reflectivity enhancement layer stack below the retarder layer 102, and the one or more capping layers.

[0104] The retarder layer 102, for example, the metasurface comprised in the retarder layer 102, may be patterned through the dispersion axis of the LCOS (see FIG. 9).

[0105] The reflective layer 103 may comprise the array of pixel electrodes, for example, aluminium electrodes.

[0106] The phase modulator may comprise a circuitry layer provided on, for example, below, the array of electrodes 104 and / or the reflective layer 103. The circuitry layer may comprise CMOS circuitry.

[0107] The glass-based layer 109 may be a coverglass. The glass-based layer 109 may comprise an IMITO glass, which may contain antireflection (AR) layers on the front surface and / or on the inner surface to reduce respective reflections, and IMITO electrodes, and an LC alignment layer on an inner side to improve alignment of the LC molecules in the LC layer 101.

[0108] The retarder layer 102 may be a quarter- wave retarder layer 102.

[0109] The quarter- wave retarder layer 102 may comprise, for example, a metasurface with quarterwave retardance (half- wave retardance in a double pass). The respective optic axis orientation may be as described above.

[0110] The retarder layer 102 may be configured to provide quarter- wave retardance of the one or more wavelength channels 108 that pass through the quarter- wave retarder layer 102. FIG. 1 exemplary shows that the optical path based on the one or more wavelength channels 108 passes twice through the retarder layer 102, which may be a quarter- wave retarder layer 102. Thus, an effective half-wave retardance can be achieved, as the quarter-wave retardance effect of each of the two passes can be combined to a half-wave retardance effect.

[0111] Thus, the quarter- wave retarder layer 102 may be acting as a half-wave retarder layer 102, as any wavelength channel that is reflected at the reflective layer 103 passes two times through the quarter- wave retarder layer 103. In this disclosure, the quarter- wave retarder layer 102 may be referred to as an effectively half-wave retarder layer 102.

[0112] The metasurface retarder may be a subwavelength wiregrid grating as shown in FIG. 2. For example, if the period of the grating is smaller than a wavelength of a wavelength channel, only the Oth order of diffraction of the wavelength channel is propagating and the grating acts as birefringent medium, i.e., retarder.

[0113] The retarder layer 102 may comprise passivation / embedding layers and an alignment layer to have LC alignment from the quarter-wave retarder layer 102 facing side. For example, the reflective backplane may comprise one or more passivation layers above two or more reflective aluminium electrodes comprised in the array of electrodes 104 to improve reflectivity and surface conditions. n0may be an ordinary refractive index of the LC layer 101. nemay be the extraordinary refractive index of the LC layer 101.

[0114] The optical input of arbitrary polarization (e.g. unpolarized) may be going through the cover glass towards the reflective backplane. The optical input may propagate in the LC layer 101 as an initially ordinary wave (perpendicular to the optic axis of the LC) and an initially extraordinary wave (parallel to the optic axis). The initially ordinary wave may experience the refractive index n0. The initially extraordinary wave may experience the refractive index ne.

[0115] The initially ordinary wave may be a second component of the optical input. The initially extraordinary wave may be a first component of the optical input. The initially ordinary wave may be reflected from the reflective layer 104 and may pass twice through the retarder layer 102. Since the optic axis of the retarder layer 102, for example, a quarter- wave retarder layer 102, may be making 45° with the polarization of the initially ordinary wave, the polarization may be flipped after its reflection or in other words rotated by 90° and may match the polarization of the initially extraordinary wave. The initially ordinary wave may then be referred to as a concluding extraordinary wave. During the propagation of the concluding extraordinary wave back through the LC layer 101 it may thus experience the refractive index neas an extraordinary wave.

[0116] The initially extraordinary wave may be reflected from the reflective layer 103 and may pass twice through the retarder layer 102. Since the optic axis of the retarder layer 102, for example, a quarter- wave retarder layer 102, may be making 45° with the polarization of the initially extraordinary wave, the polarization may be flipped after its reflection or in other words rotated by 90° and may match the polarization of the initially ordinary wave. The initially extraordinary wave may then be referred to as a concluding ordinary wave. During the propagation of the concluding ordinary wave back through the LC layer 101 it may thus experience the refractive index n0as an ordinary wave.

[0117] A voltage can be applied through the LC layer 101. The LC layer 101 may comprise a plurality of LC molecules.

[0118] As an example, without applying voltage the reactive indices may be: ne= 1.8 and n0= 1.5.

[0119] By increasing a voltage across the LC layer 101, the LC molecules may increase their tilt towards the surface normal (aligning along the electric field). The initial extraordinary wave and the concluding extraordinary wave may thus experience not the neof 1.8, but a decreasing neff< 1.8. For example, the nemay match the n0= 1.5.

[0120] / / may be defined as: where 0(z) is the voltage-dependent tilt angle of the optic axis of LC 101.

[0121] The examples above are implying a “positive” LC 101 (positive dielectric anisotropy, the molecules are tending to align along the electrical field). In the case of a “negative” LC 101 (negative dielectric anisotropy), the situation will inverse: by zero voltage (E-field off) the orientation of the LC optic axis would be 90° to a substrate (homeotropic or vertical alignment conditions). By increasing a voltage across the LC layer 101, the LC molecules may tilt towards the substrates.

[0122] The above can be controlled pixel wise by applying respective voltages.

[0123] The phase modulator 100 according to this disclosure may provide at least one of the following advantages:

[0124] 1) Air in the gaps in a metasurface may achieve a high refractive index contrast

[0125] 2) A stack of capping layers above the metasurface may improve phase retardance

[0126] 3) A stack of buffer layers below the metasurface may improve reflectivity

[0127] 4) Modulation of the respective metasurface parameters over the pixel array may optimize matching to the wavelength spectrum

[0128] FIG. 3 shows an exemplary subwavelength grating according to this disclosure.

[0129] The retarder may be based on metasurface, e.g., a subwavelength grating.

[0130] For example, the retarder layer 102 may be based on a Si subwavelength grating with air between its ridges, which may provide the highest index contrast and allow to reduce the grating thickness.

[0131] The one or more capping layers may have the function to isolate the subwavelength grating from the LC layer 101, but also to enable alignment (planarization), and for adjusting the retardance.

[0132] The key performance parameters, for example, PCR or 2nd order XT, may oscillate vs. wavelength. In WSS applications however, the wavelength spectrum is spatially defined over the LCOS area. This can be used to further improve performance of polarization independent LCOS. For example, the duty cycle of the subwavelength grating may be varied to improve the performance at a particular wavelength.

[0133] FIG. 4 shows an exemplary metasurface element according to this disclosure.

[0134] The metasurface may comprise one or more elements, each associated with a respective wavelength channel of the one or more wavelength channels 108.

[0135] A metasurface, for example, a subwavelength grating as shown in FIG. 3, may be associated with one or more retardance parameters that can be adapted to a respective wavelength channel. For example:

[0136] The grating period, for example, a width of a unit cell, Lambda

[0137] The width of the silicon ridge grating

[0138] The height of the grating tgrating

[0139] The one or more retardance properties can be optimized to yield the highest PCR.

[0140] Gnconv. where iconv. is the fraction of the optical intensity which has been polarization converted and iunconv. is the unconverted fraction. A typical target for the PCR may be 25 dB.

[0141] A polarization independent phase modulator 100 may be intended to cover the C- and L-band from 1530 nm to 1625 nm and / or approximately 100 nm of bandwidth. Such a structure may be optimized for PCR over this entire range. However, the effect of the metasurface or subwavelength grating may be based on resonances which are wavelength dependent.

[0142] FIG. 5 shows the PCR over a wavelength range for a structure that is optimized over the entire wavelength range according to this disclosure. The darker line represents the off-state and the brighter line represents the on state at 3.3V applied. While at some wavelengths, the condition of PCR >25dB is met, a large part does not reach this threshold. FIG. 6 shows a uniform metasurface and a metasurface with each pixel of the array of pixels 106 in the metasurface being individually modified according to this disclosure.

[0143] Thus, wavelength subsets { Ji = ...N1,{ }2= jv1+1... AW2, ... may be defined within the wavelength band whereas for each subset the geometry and / or the respective one or more retardance parameters 107 are optimized. This may require the knowledge of how the wavelength space, for example, via dispersive optics, is mapped onto the geometric space, for example, the LCOS chip.

[0144] The optimization of the geometry and / or the respective one or more retardance properties 107 for the different subsets may be restricted by processing. For example, all height parameters txxxmay have to be identical over the complete structure. However, other parameters, for example, the width WN and the period Lambda Awmay not be restricted for individual optimization.

[0145] FIG. 7 shows an exemplary retarder layer 102 and an exemplary LC layer 101 divided into an array of pixels 106 according to this disclosure.

[0146] FIG. 8 shows exemplary orientations of the LC optic axis and the retarder layer 102 optic axis according to this disclosure. The quarter- wave retarder layer 102 optic axis may be substantially 45° to the orientation of the LC optic axis.

[0147] FIG. 9 shows a representation of port direction vs wavelength direction according to this disclosure.

[0148] In this disclosure the LCOS area may be split into several regions, each of which may be corresponding to a specific respective wavelength band (see FIG. 9).

[0149] The term “LCOS area'' may refer to an addressable and / or active are of the array of pixels 106. The WDM channels in a WSS may be dispersed (demultiplexed) along a wavelength direction. The elliptical spots shown in FIG. 9 are elongated along port direction. In this example, the spots are corresponding to the individual wavelength channels (1 to n). The retarder may not be the same for the whole LCOS area, but may be individual for each wavelength band. Thus, gratings can be individually optimized for the narrower bands and achieve much higher PCR values (see FIG. 10).

[0150] FIG. 10 shows PCR values for different wavelength channels according to this disclosure. FIG. 10 shows PCR values of 32 to 50 dB, which is much higher, than for other phase modulators 100 as shown in FIG. 5, for example, 13 to 20 dB.

[0151] A PCR > 25 for C- and L-band can be achieved. In general, the higher the number of subsets the higher the PCR. This example shows the optimization for N=10 subset wavelength bands for the retarder layer over the C+L band spectral range with a patterned retarder layer with unique wNand Awfor each subset.

[0152] An additional thin film layer stack may be inserted below the retarder layer 102. The thin film layer stack may provide reflectivity enhancement and / or may be referred to as one or more reflectivity enhancement layers. The thin film layer stack may enhance the phase delay between the P and S components, which may allow to reduce the thickness of the retarder layer 102.

[0153] In the following examples the one or more reflectivity enhancement layers comprise a single SiCh layer. Additionally or alternatively, the one or more reflectivity enhancement layers may comprise other layers.

[0154] FIG. 11 shows the dependence of the phase shift of S and P polarizations versus the thickness of a single SiCh layer under the grating according to this disclosure. In this example, one of the polarizations (P) has a phase delay almost linear dependent on the SiCh thickness, whereas another polarization (S) has very weak dependence. Further, around 100 nm SiCh layer may provide half of the wavelength (Ipi) phase delay between P and S.

[0155] FIG. 12 shows the propagation for S and P polarizations through a phase modulator according to this disclosure. FIG. 12 shows a propagation of the S and P polarizations from the retarder layer 102 to the reflective layer 103.

[0156] FIG. 13 shows the propagation for S and P polarizations through a phase modulator according to this disclosure. A phase delay of half of the wavelength between the orthogonal polarizations is shown, as the maximum for the one polarization is corresponding to the minimum for other polarization.

[0157] FIGs. 12 and 13 show light propagation with two polarizations: P-pol (left) and S-pol (right). The thickness of the retarder grating is small enough, that P-pol comes through it and is reflected from the layers beneath the grating. The S-pol however is exciting a mode in the grating and irradiated back. Multiple designs of the layers beneath the grating may be used, for example, a single layer, for achieving a desired retardance, for example, a half-wave retardance. Said layers may comprise, for example, SiCL layer.

[0158] FIG. 14 shows the reflectivity for the two polarizations, depending on the thickness of the SiCL layer according to this disclosure. The average reflectivity for the parameters mentioned above is over 90%.

[0159] The one or more capping layers may be used for at least one of: creating a smooth interface for the LC orientation, tuning the phase retardance distribution, which is dependent on the thickness of said layer(s), and forming an antireflection coating for suppressing reflections from the metasurface of the one or more wavelength channels.

[0160] FIG. 15 shows a phase retardance distribution dependent on the thickness of the one or more capping layers according to this disclosure.

[0161] The one or more capping layer may have at least one of the following functions: isolate the metasurface, for example, the subwavelength grating, from the LC layer 101; enable alignment (planarization); and adjusting the retardance.

[0162] The metasurface may be filled with a material comprised in the one or more capping layers or with other low refractive index materials, for example, air.

[0163] The one or more capping layers may form a stack. The one or more capping layers (e.g. 2 layers) may allow to minimize an unconverted part of the optical wave by minimizing a reflection from a surface of the metasurface facing towards the LC layer 101. FIG. 16 shows a reflectivity for the spurious reflection of the metasurface according to this disclosure. FIG. 16 shows the influence of an exemplary 2-layer stack of a high and a low index material to the reflectivity from the underlying layer, wherein the reflection is largely suppressed in the C and L bands (1520-1620 nm). In this example, the 2-layer stack comprises silicon and silicon oxide.

[0164] Thus, the PCR values can be considerably improved over a broad spectral range.

[0165] FIG. 17 shows exemplary orientations of the LC optic axis and the retarder layer 102 optic axis according to this disclosure. The retarder layer 102 optic axis may be substantially 45° to the orientation of the LC optic axis.

[0166] FIG. 18 shows a representation of the retarder axis orientation in relation to the port direction vs wavelength direction according to this disclosure.

[0167] An embodiment of this disclosure according to FIGs. 8 and / or 9 may provide higher PCR, higher reflectivity, and / or may be implemented with a thinner subwavelength retarder grating compared to conventional devices.

[0168] An alternative embodiment may be according to FIGs. 17 and / or 18. The subwavelength grating may be oriented along the pixels, whereas the alignment may be at 45°.

[0169] If, the grating lines are parallel to the port direction (light steering direction, typically is perpendicular to the wavelength direction in WSS), continuous modulation of the duty cycle is possible to continuously fit for all of the spectrum across the panel (the grating changes only across this direction, see FIG. 17).

[0170] FIG. 19 shows two exemplary 2D metasurfaces according to this disclosure.

[0171] The metasurface, for example, the subwavelength grating, may be implemented as a ID structure, for example, a linear grating. Alternatively, the metasurface, for example, the subwavelength grating, may be implemented as a 2D structure or grating, where instead of stripes, more complex forms may be used for example: elliptical or rectangular elements or other forms (see FIG. 19). A ID binary grating may be more simplistic and easier to manufacture, but may have less retardance parameters to optimize the performance of the phase modulator 100.

[0172] The introduction of layers above and below the wiregrid can improve the performance of the polarization independent LCOS.

[0173] The phase modulator 100 may be used in a WSS optical switch. It may be used in WSS blocker (no functional steering, just a blocking function) or other devices, which are spectrometer-like (for example for laser pulse shaping).

[0174] The phase modulator 100 according to this disclosure enables the use of unpolarised light, which may allow to drastically simplify optical systems comprising the phase modulator 100.

[0175] FIG. 20 shows a wavelength selective switch 200 according to this disclosure.

[0176] The wavelength selective switch 200 comprises: one or more input ports 201 comprising one or more first beamforming optics; a wavelength separating element 202; one or more Fourier lenses 203; a phase modulator 100, for example the phase modulator shown in FIG. 1; a wavelength combining element 204; and one or more output ports 205 comprising one or more second beamforming optics.

[0177] The one or more first beamforming optics may be or comprise the one or more second beamforming optics.

[0178] The one or more input ports 201 are configured to receive one or more input optical signals 206. The wavelength separating element 202 is configured to receive the one or more input optical signals 206 from the one or more input ports 201 and separate the one or more input optical signals 206 into one or more spatially separated wavelength channels 108. The one or more Fourier lenses 203 are configured to receive the one or more wavelength channels 108 from the wavelength separating element 202. The phase modulator 100 is configured to receive the one or more wavelength channels 108 from the one or more Fourier lenses 203 and individually spatially control respective phase shifts of the one or more wavelength channels 108 to deflect, attenuate, or split the one or more wavelength channels and form one or more modified wavelength channels 108. The one or more Fourier lenses 203 are further configured to receive the one or more modified wavelength channels 108 from the phase modulator 100. The wavelength combining element 204 is configured to receive the one or more modified wavelength channels 108 from the one or more Fourier lenses 203 and combine the one or more modified wavelength channels 108 into one or more output optical signals 207. The one or more output ports 205 are configured to receive the one or more output optical signals 207 from the wavelength combining element 204.

[0179] FIG. 21 shows an exemplary wavelength selective switch 200 according to this disclosure.

[0180] The exemplary wavelength selective switch 200 shown in FIG. 21 comprises one Fourier lens 203, one diffraction grating 202, 204, and one anamorphic magnification optics 201, 205.

[0181] The wavelength separating element 202 and the wavelength combining element 204 may be combined into one element, for example, a diffraction grating 202, 204. The one or more input ports 201 comprising the one or more first beamforming optics and the one or more output ports 205 comprising the one or more second beamforming optics may be combined into one element, for example, an anamorphic magnification element 201, 205. The one or more Fourier lenses 203 may be only one Fourier lens 203. Thus, the efficiency in terms of space, manufacturing costs, and / or alignment, for example, alignment accuracy, may be improved.

[0182] The controller 105 may be a processor 105.

[0183] Generally, the processor 105 may be configured to perform, conduct or initiate the various operations of the phase modulator 100 described herein. The processor 105 may comprise hardware and / or may be controlled by software. The hardware may comprise analog circuitry or digital circuitry, or both analog and digital circuitry. The digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or multi-purpose processors. The phase modulator 100 may further comprise memory circuitry, which stores one or more instruction(s) that can be executed by the processor 105, in particular under control of the software. For instance, the memory circuitry may comprise a non-transitory storage medium storing executable software code which, when executed by the processor 105, causes the various operations of the phase modulator 100 to be performed. In one embodiment, the phase modulator 100 may comprises one or more processors 105 and a non-transitory memory connected to the one or more processors 105. The non-transitory memory may carry executable program code which, when executed by the one or more processors 105, causes the phase modulator 100 to perform, conduct or initiate the operations or methods described herein.

[0184] FIG. 22 shows a method 300 according to this disclosure. The method 300 is a method of operating a polarization-independent phase modulator 100, for example, the phase modulator shown in FIG. 1. The phase modulator 100 comprises: a LC layer; a retarder layer 102 provided on the LC layer 101, wherein the LC layer 101 and the retarder layer 102 are divided into an array of pixels 106, each pixel being arranged to receive a wavelength channel of one or more wavelength channels 108, a reflective layer 103 provided on the retarder layer 102; an array of electrodes 104 provided on or in the reflective layer 103; and a controller 105. Each pixel has a respective effective refractive index at the wavelength channel in the LC layer 101, and comprises a respective one or more retardance properties 107 in the retarder layer 102 that are adapted to the wavelength channel.

[0185] The method 300 comprises a step 301 of individually controlling, with the controller 105, each electrode of the array of electrodes 104 to regulate the respective effective refractive indices of at least some pixels of the array of pixels 106. Further, the method 300 comprises a step 302 of, receiving with the LC layer 101, an optical wave comprising the one or more wavelength channels 108. Further, the method 300 comprises a step 303 of providing, with the phase modulator 100, the one or more wavelength channels 108 through the LC layer 101 and through the retarder layer 102 to the reflective layer 103. Further, the method 300 comprises a step 304 of reflecting, with the reflective layer 103, the one or more wavelength channels 108 back through the retarder layer 102 to the LC layer 101.

[0186] FIG. 23 shows a method 400 according to this disclosure. The method 400 is a method of operating a wavelength selective switch 200, for example, the wavelength selective switch 200 shown in FIG. 20. The wavelength selective switch 200 comprises: one or more input ports 201 comprising one or more first beamforming optics; a wavelength separating element 202; one or more Fourier lenses 203; a phase modulator 100, for example, the phase modulator shown in FIG. 1; a wavelength combining element 204; and one or more output ports 205 comprising one or more second beamforming optics. The method 400 comprises a step 401 of receiving, with the one or more input ports 201, one or more input optical signals 206. Further, the method 400 comprises a step 402 of receiving, with the wavelength separating element 202, the one or more input optical signals 206 from the one or more input ports 201. Further, the method 400 comprises a step 403 of separating, with the wavelength separating element 202, the one or more input optical signals 206 into one or more spatially separated wavelength channels 108. Further, the method 400 comprises a step 404 of receiving, with the one or more Fourier lenses 203, the one or more wavelength channels 108 from the wavelength separating element 202. Further, the method 400 comprises a step 405 of receiving, with the phase modulator, the one or more wavelength channels 108 from the one or more Fourier lenses 203. Further, the method 400 comprises a step 406 of individually spatially controlling, with the phase modulator, respective phase shifts of the one or more wavelength channels 108 to deflect, attenuate, or split the one or more wavelength channels 108 and form one or more modified wavelength channels 108. Further, the method 400 comprises a step 407 of receiving, with the one or more Fourier lenses 203, the one or more modified wavelength channels 108 from the phase modulator. Further, the method 400 comprises a step 408 of receiving, with the wavelength combining element 204, the one or more modified wavelength channels 108 from the one or more Fourier lenses 203. Further, the method 400 comprises a step 409 of combining, with the wavelength combining element 204, the one or more modified wavelength channels 108 into one or more output optical signals 207. Further, the method 400 comprises a step 410 of receiving, with the one or more output ports 205, the one or more output optical signals 207 from the wavelength combining element 204.

[0187] The disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

CLAIMS1. A polarization-independent phase modulator (100) comprising: a liquid crystal, LC, layer (101) configured to receive an optical wave comprising one or more wavelength channels (108); a retarder layer (102) provided on the LC layer (101), wherein the LC layer (101) and the retarder layer (102) are divided into an array of pixels 106, each pixel being arranged to receive a wavelength channel of the one or more wavelength channels 108, having a respective effective refractive index at the wavelength channel in the LC layer (101), and comprising a respective one or more retardance properties (107) in the retarder layer (102) that are adapted to the wavelength channel; a reflective layer (103) provided on the retarder layer (102); an array of electrodes (104) provided on or in the reflective layer (103); and a controller (105) configured to individually control each electrode of the array of electrodes (104) to regulate the respective effective refractive indices of at least some pixels of the array of pixels (106); wherein the phase modulator (100) is configured to provide the one or more wavelength channels (108) through the LC layer (101) through the retarder layer (102) to the reflective layer (103), and wherein the reflective layer (103) is configured to reflect the one or more wavelength channels 108 back through the retarder layer (102) to the LC layer (101).

2. The phase modulator (100) according to claim 1, wherein the retarder layer (102) comprises a metasurface and / or a subwavelength grating.

3. The phase modulator (100) according to claim 2, wherein the metasurface is a subwavelength grating, and wherein the subwavelength grating is a one-dimensional grating or a two-dimensional grating.

4. The phase modulator (100) according to claim 2 or 3, wherein the respective one or more retardance properties (107) of each pixel of the array of pixels (106) comprise at least one of: a material of the metasurface in the pixel, a period of the metasurface in the pixel, a ridge width of the metasurface in the pixel, a height of the metasurface in the pixel, and an optic axis of the retarder layer (102) in the pixel.

5. The phase modulator (100) according to any one of the preceding claims, wherein the LC layer (101) has a LC optic axis, and wherein the retarder layer (102) has a retarder optic axis that is oriented at an angle of substantially 45° to the LC optic axis.

6. The phase modulator (100) according to any one of the preceding claims, wherein the respective one or more retardance properties (107) of at least one pixel of the array of pixels (106) are different from the respective one or more retardance properties (107) of at least one other pixel of the array of pixels (106).

7. The phase modulator (100) according to any one of the preceding claims, further comprising a glass-based layer (109), wherein the LC layer (101) is provided on the glass-based layer (109), and wherein the glass-based layer (109) is configured to: receive the optical wave and transmit the optical wave to the LC layer (101), and / or receive the reflections of the one or more wavelength channels (108), and transmit said reflections.

8. The phase modulator (100) according to any one of the claims 2 to 7, wherein the retarder layer (102) comprises one or more capping layers, and / orwherein the metasurface is provided on the one or more capping layers and the one or more capping layers are configured to isolate the metasurface from the LC layer (101).

9. The phase modulator (100) according to claim 8, wherein the one or more capping layers form an antireflection coating for suppressing reflections from the metasurface of the one or more wavelength channels (108).

10. The phase modulator (100) according to any one of the claims 2 to 9, wherein the metasurface forms air gaps in the retarder layer (102).

11. The phase modulator (100) according to any one of the preceding claims, wherein the reflective layer (103) comprises one or more reflectivity enhancement layers provided on the retarder layer (102).

12. The phase modulator (100) according to claim 11, wherein the metasurface is configured to reflect a first polarization component of at least some wavelength channels of the one or more wavelength channels (108) back through the retarder layer (102) to the LC layer (101), wherein the reflective layer (103) is configured to reflect a second polarization component of the at least some wavelength channels of the one or more wavelength channels (108) back through the retarder layer (102) to the LC layer (101), and wherein the one or more reflectivity enhancement layers are configured to provide a phase delay between the first polarization component and the second polarization component of the at least some wavelength channels.

13. The phase modulator (100) according to any one of the preceding claims, wherein the glass-based layer (109) comprises a first alignment layer for aligning at least some LC molecules of the LC layer (101) in the vicinity of the glass-based layer (109), and / orwherein the retarder layer (102) comprises a second alignment layer for aligning at least some other LC molecules of the LC layer (101) in the vicinity of the retarder layer (102).

14. The phase modulator (100) according to any one of the preceding claims, wherein the reflective layer (103) comprises a passivation layer provided on the retarder layer (102), and a reflective sub-layer provided on the passivation layer, wherein the reflective sub-layer comprises the array of electrodes (104).

15. The phase modulator (100) according to any one of the preceding claims, wherein the retarder layer (102) is configured to provide half-wave retardance of the one or more wavelength channels (108) that are passing twice through the retarder layer (102), and / or wherein the retarder layer (102) and the one or more reflectivity enhancement layers are configured to provide half-wave retardance of the one or more wavelength channels (108) that are passing twice through the retarder layer (102).

16. The phase modulator (100) according to any one of the preceding claims, wherein the LC optic axis is parallel to an orientation of the array of pixels (106) in the LC layer (101).

17. A method of operating a polarization-independent phase modulator (100), the phase modulator (100) comprising: a liquid crystal, LC, layer; a retarder layer (102) provided on the LC layer (101), wherein the LC layer (101) and the retarder layer (102) are divided into an array of pixels (106), each pixel being arranged to receive a wavelength channel of one or more wavelength channels (108), having a respective effective refractive index at the wavelength channel in the LC layer (101), and comprising a respective one or more retardance properties (107) in the retarder layer (102) that are adapted to the wavelength channel;a reflective layer (103) provided on the retarder layer (102); an array of electrodes (104) provided on or in the reflective layer (103); and a controller (105); wherein the method comprises: individually controlling, with the controller (105), each electrode of the array of electrodes (104) to regulate the respective effective refractive indices of at least some pixels of the array of pixels (106); receiving, with the LC layer (101), an optical wave comprising the one or more wavelength channels (108); providing, with the phase modulator (100), the one or more wavelength channels (108) through the LC layer (101) and through the retarder layer (102) to the reflective layer (103); and reflecting, with the reflective layer (103), the one or more wavelength channels (108) back through the retarder layer (102) to the LC layer (101).

18. A wavelength selective switch (200) comprising: one or more input ports (201) comprising one or more first beamforming optics, the one or more input ports (201) being configured to receive one or more input optical signals (206); a wavelength separating element (202) configured to receive the one or more input optical signals (206) from the one or more input ports (201) and separate the one or more input optical signals (206) into one or more spatially separated wavelength channels (108); one or more Fourier lenses (203) configured to receive the one or more wavelength channels (108) from the wavelength separating element (202); the phase modulator (100) according to any one of the claims 1 to 16, configured to receive the one or more wavelength channels (108) from the one or more Fourier lenses (203) and individually spatially control respective phase shifts of the one or more wavelength channels (108) to deflect, attenuate, or split the one or more wavelength channels and form one or more modified wavelength channels (108), wherein the one or more Fourier lenses (203) are further configured to receive the one or more modified wavelength channels (108) from the phase modulator (100);a wavelength combining element (204) configured to receive the one or more modified wavelength channels (108) from the one or more Fourier lenses (203) and combine the one or more modified wavelength channels (108) into one or more output optical signals (207); and one or more output ports (205) comprising one or more second beamforming optics, wherein the one or more output ports (205) are configured to receive the one or more output optical signals (207) from the wavelength combining element (204).

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