Description:TECHNICAL FIELD
[0001] Embodiments of the present disclosure relates, generally, to a polarization converter device made of a sandwich waveguide layer, and more specifically to a wavelength selective polarization rotator/filter hybrid waveguide platform with a measured bandwidth.
BACKGROUND
[0002] Polarization sensitivity or birefringence of a waveguide is typically a critical parameter in a photonic integrated device. Quantitatively, it is measured as a difference between the effective index of transverse electric (TE) and transverse magnetic (TM) polarization. Ideally, it is known that zero birefringence is needed to prevent polarization dependent response in wavelength division multiplexing applications (WDM). For example, a square waveguide may show polarization insensitivity, but operating in a single mode is restrictive in dimension. Zero birefringence have been obtained along with single mode condition using rib waveguides. Thus, obtaining zero birefringence has a limitation on the design constraints of the waveguide geometry.
[0003] A polarization diversity scheme may be a better alternative to solving the issue of high birefringence. A polarization splitter and rotator normally are the primary components to realize polarization diversity. Normally, the arbitrary state of input polarization is first split, and then one of the orthogonal polarizations is rotated to obtain a single polarization. Polarization rotators are broadly classified as active and passive. An active rotator uses an electro-optic effect for polarization conversion. But, to use the electro-optic effect, the polarization rotators need to be made of anisotropic materials. Instead, passive rotators based on geometrical perturbations are universal and preferred.
[0004] Passive rotators may be based on mode evolution or mode coupling. Mode evolution-based rotators are both dimension and wavelength tolerant, but they require alignment of multiple layers. Mode-coupling rotators need the existence of hybrid mode through both horizontal and vertical asymmetry to facilitate the coupling of orthogonal polarizations. The asymmetry is achieved through slanted walls, asymmetric shallow etch of the waveguide edge or short waveguide bends. Further, symmetry can be broken by implementing asymmetrical directional couplers (ADC) which is easy to fabricate but suffers from poor dimensional tolerance. The use of inverse design may also be used for polarization rotation with a compact footprint.
SUMMARY
[0005] Embodiments of the present disclosure relate to a wavelength selective polarization rotator/filter in a SiN/amorphous-Si/SiN (where SiN refers to Silicon Nitride) hybrid waveguide platform. Further, experimentally it is demonstrate as a proof of concept for simultaneous coarse wavelength division multiplexing and polarization rotation in a passive configuration. Embodiments of the present disclosure relate to a polarization converter device (also referred to as a polarization rotation device or as a polarization decvice) comprising a sandwich waveguide layer formed on a base layer, which includes a first layer formed on the base layer, wherein a height H1 of the first layer is pre-determined. The device further includes a second layer formed on the first layer with a predetermined height H2. The device further includes a third layer formed on top of the second layer, wherein the height H3 of the third layer may be equivalent or higher or lesser in the height of the first layer H1. The refractive index of second layer is higher than the refractive index of the first layer and the refractive index of the third layer. Other embodiments are also disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The detailed description is described with reference to the accompanying figures. Features, aspects, and advantages of the present subject matter will be better understood with regard to the following description and the accompanying drawings. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference features and components. In order that the present disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages.
[0007] Figure 1A is an exemplary cross section of a sandwich hybrid waveguide platform used as a core to realize a polarization rotator with asymmetrical direction coupler (ADC) in accordance with the present disclosure.
[0008] Figure 1B is an exemplary electric field profile of TE0 and TM0 of the hybrid waveguide of Figure 1A in accordance with the present disclosure.
[0009] Figure 2A is an exemplary cross section of an asymmetrical direction coupler (ADC) based on a sandwich hybrid waveguide illustrating the rotator parameters in accordance with the present disclosure.
[0010] Figure 2B is an exemplary top view of the ADC based polarization rotator (converter) device of Figure 2A in accordance with the present disclosure.
[0011] Figure 3A illustrates an exemplary graph of the effective index of TE0 and TM0 modes as a function of width in accordance with the present disclosure.
[0012] Figure 3B illustrates an exemplary graph of the phase matching between TE0 and TM0 waveguides through a hybrid mode in accordance with the present disclosure.
[0013] Figure 3C illustrates an exemplary graph of the coupling length as a function of gap for several values of fraction in accordance with the present disclosure.
[0014] Figure 4A illustrates an exemplary graph of a spectral response of the polarization rotator considered for a few different gaps in accordance with the present disclosure.
[0015] Figure 4B illustrates an exemplary graph of a 3-dB bandwidth as a function of the gap in accordance with the present invention.
[0016] Figure 5A is an exemplary top view of a CWDM scheme showing the TE) bus waveguide and the TM0 branches.
[0017] Figure 5B is an exemplary spectral response of the proposed CDWM scheme spanning the S, C and L bands in accordance with the present disclosure.
DETAILED DESCRIPTION
[0018] The following describes technical solutions in exemplary embodiments of the present subject matter with reference to the accompanying drawings in the exemplary embodiments of the present subject matter. In this application as disclosed herein, "at least one" means one or more, and "a plurality of" means two or more. The term "and/or" describes an association relationship for describing associated objects and represents that three relationships may exist. For example, A and/or B may represent the following cases: Only A exists, both A and B exist, and only B exists, where A and B may be singular or plural. The character "/" usually indicates an "or" relationship between the associated objects. "At least one item (piece) of the following" or a similar expression thereof means any combination of the items, including any combination of singular items (piece) or plural items (pieces). For example, at least one item (piece) of a, b, or c may represent a, b, c, a and b, a and c, b and c, or a, b, and c, where a, b, and c each may be singular or plural.
[0019] It should be noted that in this application articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”. Throughout this specification defined above, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of elements or steps but not the exclusion of any other element or step or group of elements or steps. The term “including” is used to mean “including but not limited to”. “Including” and “including but not limited to” are used interchangeably. In the structural formulae given herein and throughout the present disclosure, the following terms have been indicated meaning, unless specifically stated otherwise.
[0020] Unless otherwise defined, all terms used in the disclosure, including technical and scientific terms, have meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included for better understanding of the present disclosure. The term ‘about’ as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of ±10% or less, preferably ±5% or less, more preferably ±1% or less and still more preferably ±0.1% or less of and from the specified value, insofar such variations are appropriate to perform the present disclosure. It is to be understood that the value to which the modifier ‘about’ refers is itself also specifically, and preferably disclosed.
[0021] It should be noted that in this application, the term such as "example" or "for example" or “exemplary” is used to represent giving an example, an illustration, or descriptions. Any embodiment or design scheme described as an "example" or "for example" in this application should not be explained as being more preferable or having more advantages than another embodiment or design scheme. Exactly, use of the word such as "example" or "for example" is intended to present a related concept in only a specific manner.
[0022] It should be understood that in the embodiments of the present disclosure that "B corresponding to A" indicates that B is associated with A, and B can be determined based on A. However, it should be further understood that determining B based on A does not mean that B is determined based on only A. B may alternatively be determined based on A and/or other information.
[0023] In embodiments of the present disclosure in this application, "a plurality of" means two or more than two. Descriptions such as "first", "second" in the embodiments of this application are merely used for indicating and distinguishing between described objects, do not show a sequence, do not indicate a specific limitation on a quantity of devices in the embodiments of this application, and do not constitute any limitation on the embodiments of this application.
[0024] In an embodiment, polarization birefringence of an optical waveguide may be amongst some critical parameters for photonic integrated circuits (PIC). In an embodiment, a high birefringence may cause polarization-dependent phase response, which may be detrimental to wavelength division multiplexing (WDM) applications. In an embodiment, a polarization insensitive waveguide with zero birefringence operating in a single mode region may be ideal for wavelength division multiplexing (WDM) and obtaining such a response may be challenging. In some embodiments, rib waveguides may have been used for single mode polarization independent response. In some other embodiments, wire waveguides with a square cross-section at a specific dimension operating at single mode may also be made polarization insensitive. However, such devices have a poor tolerance level. In some other embodiments, triplex waveguides may be used to obtain zero birefringence. However, all these designs are a constraint on waveguide geometry and dimensions, making such solutions highly restrictive.
[0025] In an embodiment, a polarization diversity scheme may be a more suitable choice to deal with polarization birefringence, where the entire PIC operates with a single polarization. In some embodiments, polarization beam splitters (PBS) and polarization rotators (PR) may be used as primary components to achieve polarization diversity. Normally, a polarization splitter may be used to split the arbitrary incoming polarization states, and the rotator converts it into a single polarization to be processed throughout the chip.
[0026] In an embodiment, polarization rotators (PR) may be configured to work in either active or passive configuration. In order to work in an active configuration, the PR requires an external field for polarization conversion, thereby placing inherent constraints on material systems. Alternatively, to work in a passive configuration, the PR may operate through geometrical perturbation thereby making its implementation universal and energy efficient. Polarization conversion in a passive PR can either be done through mode evolution or mode coupling. In mode evolution, the PR transforms a propagating mode adiabatically through geometrical evolution or twist of the waveguide axis. The PR based on mode evolution shows a broadband response but has long device lengths for adiabatic conversion and requires additional PBS for obtaining required extinction.
[0027] Polarization rotation may also be realized by transforming the input TM0 mode into an intermediate TE1 mode using a tapered waveguide and demultiplexing it into TE0 mode through a MMI, asymmetric Y-splitters, asymmetric directional coupler (ADC) or even using inverse design. In some embodiments, mode coupling based PR couples the orthogonal polarization by introducing asymmetry through geometrical perturbation that may excite the hybrid mode. The asymmetry may be introduced using overlay structures, etching a corner of the waveguide or through a double core, however the device length in these structures is small (< 50 µm) with a relatively high bandwidth, and the fabrication requires tight alignment tolerance.
[0028] An asymmetric directional coupler (ADC) based rotator is another structure that may be easy to design and fabricate as the concept is similar to a directional coupler discussed above. It may be modified with sub-wavelength structures with short coupling length and may also be used to implement polarization rotation in the ALIG platform due to simplicity. In some embodiments, phase-matched waveguides break the horizontal symmetry with a different width to couple between orthogonal polarization. The vertical asymmetry may be realized using different cladding material than the bottom clad or having an inherently asymmetric core with a uniform cladding.
[0029] All the passive PR inherently have a high bandwidth and active PR have a narrow bandwidth. The passive PR may show narrow bandwidth by use of Bragg reflection in the counter propagation direction. Embodiments of the present disclosure relate to a wavelength selective passive polarization rotator that has a potential for bandwidth engineering, which has not been found in the state of the art, wherein a “sandwiched hybrid waveguide” in accordance with the present disclosure may be used to realize the rotator with wavelength selective characteristics, and the scheme used in accordance with the embodiments of the present disclosure may be treated as an ADC due to its simple implementation.
[0030] Exemplary embodiments of the present disclosure relate to a wavelength selective polarization rotator/filter in a Silicon Nitride/amorphous-Si/Silicon Nitride hybrid waveguide platform with a measured 3dB bandwidth of 14.8 nm. Further, experimentally it may be demonstrated as a proof of concept for simultaneous coarse wavelength division multiplexing and polarization rotation in a passive configuration for the first time. Embodiments of the present disclosure relate to a polarization converter device (hereinafter also referred to as a polarization rotation device) including a sandwich waveguide layer formed on a base layer, which includes a first layer, the first layer is formed on the base layer, wherein a height H1 of the first layer is pre-determined or pre-defined. The device further includes a second layer formed on the first layer with a pre-determined or pre-defined height H2, wherein the height H2 of the second layer is smaller compared to the height H1 of the first layer. The device further includes a third layer formed on top of the second layer of amorphous-Si layer, wherein the height H3 of the third layer is equivalent or smaller or larger than the height H1 of the first layer.
[0031] In a further exemplary embodiment, the base layer may include an optically transparent material or an oxide of silicon, an oxide of germanium, an oxide of tin, oxide of titanium or an oxide of tantalum. In a further exemplary embodiment, the second layer may include an optically transparent material or a nitride of silicon or a nitride of germanium or a nitride of tin or a nitride of titanium or a nitride of tantalum.
[0032] In a further exemplary embodiment, the second layer may include at least one of amorphous material or crystalline material or polycrystalline material. In a further exemplary embodiment, the material may include at least one an optically transparent material or of silicon or germanium with a higher refractive index than a refractive index of the base layer and a refractive index of the second layer.
[0033] In a further exemplary embodiment, the total height H of the sandwich waveguide layer may include the height H1 of the first layer, the height H2 of the second layer and the height H3 of the third layer. In a further exemplary embodiment, a total width W of the sandwich waveguide layer may be pre-determined or pre-defined. In a further exemplary embodiment, when the height H1 of the first layer and the height H3 of the third layer are equivalent, the height H1 or H3 of the first and third layer may be determined by the formula H1 = [0.5 * (1-f) * H] or H3 H1 = [0.5 * (1-f) * H], and H1=H3. In a further exemplary embodiment, the height H2 of the second layer may be determined by the formula H2 = [f * H].
[0034] In an exemplary embodiment, “f” may be determined by as a ratio of the thickness of the higher refractive index to the total height of the sandwich waveguide layer. In a further exemplary embodiment, the second layer may be of higher refractive index and the first layer, and the third layer is of a medium refractive index.
[0035] A further exemplary embodiment includes a polarization converter (which may also be referred to as a polarization rotation) device which may include a sandwich waveguide layer formed on a base layer. In a further exemplary embodiment, the sandwich waveguide layer may include a common base layer. In a further embodiment, the device further includes a first sandwich waveguide layer separated by a pre-determined gap from a second sandwich layer. In a further exemplary embodiment, the first sandwich layer and the second sandwich waveguide layer may include a first layer of a predetermined height H1 formed on the base layer, a second layer of an amorphous-material formed on the first layer, the second layer having a predetermined height H2, wherein the height H2 of the second layer is smaller compared to the height H1 of the first layer; and a third layer of on top of the second layer, wherein the height H3 of the third layer is equivalent to the height H1 of the first layer.
[0036] In a further exemplary embodiment, the base layer may include an optically transparent material or an oxide of silicon, an oxide of germanium, an oxide of tin, oxide of titanium or an oxide of tantalum. In an exemplary embodiment, the second layer may include an optically transparent material or a nitride of silicon or a nitride of germanium or a nitride of tin or a nitride of titanium or a nitride of tantalum. In an exemplary embodiment, the second layer may include at least one of amorphous material or crystalline material or polycrystalline material.
[0037] In an exemplary embodiment, the material may include at least one an optically transparent material of Silicon or germanium with a higher refractive index than a refractive index of the base layer and a refractive index of the second layer. In an exemplary embodiment, the total height H of the sandwich waveguide layer is the height of the first layer H1, the height of the second layer H2 and the height of the third layer H3. In an exemplary embodiment, the total width “WTE” of the first sandwich waveguide layer and width “WTM” of the second sandwich waveguide layer is predetermined, and the width “WTM” of the second sandwich waveguide layer is wider than the width “WTE” of the first sandwich waveguide layer, wherein the first sandwich waveguide layer and the second sandwich waveguide layer is separated by a fixed distance "g”.
[0038] In an exemplary embodiment, the height H1 of the first layer and the height H3 of the third layer may be determined by the formula H1/H3 = [0.5 * (1-f) * H]. In an exemplary embodiment, the height H2 of the second layer may be determined by the formula H2 = [f * H]. In an example embodiment, “f” may be determined by as a ratio of the thickness of the higher refractive index to the total height. In an exemplary embodiment, the second layer may be of higher refractive index, and both the first layer and third layer may be of medium refractive index.
[0039] In an exemplary embodiment, the width of the first sandwich waveguide “WTE” and the width of the second sandwich waveguide “WTM” and the distance “g” between the first sandwich waveguide and the second sandwich waveguide is varied to achieve a waveguide selective polarization conversion.
[0040] In an exemplary embodiment, a spectral bandwidth of the polarization converter device can be tuned by a slide cladding with a lower refractive index material. In an exemplary embodiment, a maximum transmission wavelength of the polarization converter device may be tuned by varying the width of the first sandwich waveguide and the width of the second sandwich waveguide. In an exemplary embodiment, the first sandwich and the second sandwich may completely be covered with the base layer.
[0041] Reference is now made to Figure 1A, which is an exemplary cross section of a sandwich hybrid waveguide platform 100A, which may be used as a core to realize a polarization rotator with asymmetrical direction coupler (ADC) in accordance with the present disclosure. As illustrated in Figure 1A, the substrate 110 of the sandwich hybrid waveguide 110A includes a layer of a oxide, for example a metal oxide such as SiO2. It should be obvious to a person of ordinary skill in the art that other metal oxides may be used as a substrate and all such variation fall within the scope of the present disclosure. It should also be obvious to a person of ordinary skill in the art that any material, preferably a dielectric material or a semiconductor material may be used to form the substrate.
[0042] The sandwich hybrid waveguide 100A, include a first layer 120 of a dielectric nitride or oxide, for example a layer of SiN is formed on the base substrate 110, wherein the height H1 of the first layer 120 ( metal nitride layer) is approximately computed using the formula H1 = 0.5 * (1-f)*H. In the formula to compute the height H1, “f” is determined by a ratio of the thickness of the higher refractive index to the total height H of the sandwich waveguide layer, wherein the total height of the sandwich waveguide layer does not include the height of the base layer. The width of the first layer 120 is pre-determined or pre-defined, and may be determined based on various external parameters. A second layer 130 of the amorphous layer is formed on top of the first layer 120. The height H2 of the second layer 120 is computed using the formula H2 = f*H, where “f” is determined by as a ratio of the thickness of the higher refractive index to the total height H of the sandwich waveguide layer. A third layer 140 of the same nitride is as the first layer is formed on top of the second layer 130, wherein the height H3 of the third layer is the same as the height H1 of the first layer 120 or may be smaller than or greater than the height of the first layer H1, depending on the requirements of the device. Because of the structure of the metal nitride – amorphous metal – metal nitride layers, the waveguide is referred to as a sandwich hybrid waveguide platform. The materials used in this exemplary embodiment is Silicon, but it should be obvious to a person of ordinary skill in the art that any metal or non-metal or inorganic material or organic material which have properties similar to that of silicon may be used to create the sandwich hybrid waveguide 100A. The total height of the sandwich hybrid waveguide is H, where H = H1 + H2 + H3, and the width of the sandwich hybrid waveguide is W.
[0043] As illustrated Figure 1A illustrates an exemplary cross section of sandwiched hybrid waveguide (SHW) with geometrical parameters. In the exemplary embodiment, the width (W) and height (H) of the SHW may be varied to control various properties such as the effective index, the confinement, the birefringence, etc., which may be similar to the properties of the wire waveguide. However, additionally, the SHW as disclosed herein may have another parameter defined as ‘f,’ where ‘f’ is defined to be the thickness of high index divided by the Total height (‘H’). The parameter ‘f’ may be computed using the mathematically formula, f =THI/TH, where THI is the thickness of the high index and TH is the total height of the SWH. In accordance with the embodiments of the present disclosure, the ratio ‘f’ provides flexibility to control the waveguide properties.
[0044] Figure 1B is an exemplary field profile 100B, particularly the electric field profiles, of TE0 and TM0 of the hybrid sandwich waveguide of Figure 1A in accordance with the present disclosure. Figure 1B illustrates the electric field profile for TE mode 150 and TM mode 160, where the electric field confinement in TE is more in the high index (a-Si), and for TM it is more in the medium index (SiN). In accordance with the embodiments of the present disclosure, the value of ‘H’ is fixed as 500 nm and the value of ‘f’ is 20 % unless mentioned otherwise or determined. The resulting thickness of the top layer (third layer) and bottom layer (first layer) of SiN is around 200 nm, and the middle layers (a-Si layer) thickness is around 100 nm due to the values of ‘H’ and ‘f’. The refractive index of a-Si is considered to be about 3.48, refractive index of SiN to be about 2, and the refractive index of SiO2 is about 1.444 at a wavelength of about 1550 nm.
[0045] Reference is now made to Figure 2A, which is an exemplary cross section of an asymmetrical direction coupler (ADC) based on a sandwich hybrid waveguide illustrating the rotator parameters in accordance with the present disclosure. Figure 2A illustrates the top view of the ADC scheme for a polarization rotation. The cross section includes two sandwich stacks, a first stack 205A and a second stack 205B formed on a substrate 210, wherein the first stack 205A and the second stack 205B are separated by a gap ‘g’, wherein the gap ‘g’ may be pre-determined. The substrate 210 may be a dielectric or semiconductor, such as SiO2, as disclosed previously with respect to Figure 1A. The first stack 205A of the hybrid sandwich waveguide 200A is associated with an electric field TE and the second stack 205B of the hybrid sandwich waveguide 200A is associated with the electric field TM of the cross section of an asymmetrical direction coupler (ADC) based on a sandwich hybrid waveguide 200A. The first stack 205A includes a first layer 220A, which has been discussed with respect to Figure 1A. Above the first layer 220A, a layer of amorphous silicon 230 is formed and above the amorphous silicon layer a second layer 220B is formed, thereby completing the first stack 205A. The second stack 205B is associated with the electric field TM of the cross section of an asymmetrical direction coupler (ADC) based on a sandwich hybrid waveguide. The second stack 205B includes a first layer 240A, which has been discussed with respect to Figure 1A. Above the first layer 240A, a layer of amorphous silicon 250 is formed and above the amorphous silicon layer a second layer 240B is formed, thereby forming the second stack 205B. In an exemplary embodiment, the first layer 220A, 240A and the second layer 220B, 240B may preferably be a SiN layer as disclosed previously with respect to Figure 1A. The first stack 205A and the second stack 205B are separated by a predetermined distance “g” or a gap “g”, which may be varied depending on the properties required to be exhibited by the waveguide.
[0046] The height of each of the layers forming the first stack 205A and the second stack 205B may be computed as described with respect to Figure1A by using the formula described previously with respect to Figure 1A. The width (WTE) of the first stack 205A is pre-determined and is configured to be smaller than the width (WTM) of the second stack 205B, wherein the width (WTE)of the first stack and the width (WTM)of the second stack may be predetermined, and the width of the first stack and the width of the second stack may vary depending on the properties of the waveguide 200A. All other features of the oxide layer (base layer or substrate), nitride layer (first layer and third layer) and amorphous silicon layer (second layer or middle layer) used to form the first stack 205A and the second stack 205B remain the same as disclosed previously with respect to Figure 1A, except for the width (WTE)of the first stack 205A which is smaller than the width (WTM) of the second stack 205B. The first stack 205A and the second stack 205B may be separated by a gap “g,” which may be pre-determined depending of the properties of the waveguide 200A. The height of each of the first layer, the third layer and the amorphous layer of the first stack 205A and the second stack 205B is normally the same. In an exemplary case, the height of the first layer 220A in the first stack is the same as the height of the first layer 240A in the second stack 205B.
[0047] Reference is now made to Figure 2B, which is an exemplary cross-sectional view of the PR with various geometrical parameters. The value of the width (WTE)of the first stack 205A and the value of the width (WTM) of the second stack 205B WTM, i.e., the width of the pair is determined by matching the propagation constants through a simulation at about 1550 nm wavelength.
[0048] The coupling length ‘Lc’ indicated in Figure 2B is the minimum length at which maximum polarization conversion may occur. The coupling length is related to the propagation constants of the participating modes as ‘Lc = ??/???,’ where, ??? may be defined as the difference of propagation constants of the participating hybrid modes of the electric field (TE and TM). The spectral response of the PR can be analytically obtained from coupled mode theory as

where in the formular above, ‘?? = ??/2L’ may be defined as the coupling coefficient, ‘?? = ???/2’ may be defined as the wavelength dependent phase mismatch. The wavelength dependent phase mismatch ‘??’ is primarily responsible for the spectral response as other quantities are nearly constants with wavelength.
[0049] The PR in accordance with the present disclosure is designed by phase matching the TE and TM mode by selecting a proper choice of their widths. The phase matching width pair may be obtained by initially performing modal simulations on an isolated waveguide. The effective index (neff) may be found for various values of fraction ranging from 10 % to 30 % in steps of 5 % and widths followed by distinguishing the single mode region. For a given fraction, the maximum value of width within the single mode region may be considered and the neff of TM0 mode for this largest possible width is matched with neff of TE0 mode.
[0050] Reference is now made to Figure 3A, which illustrates an exemplary graph of the effective index of TE0 and TM0 modes as a function of width in accordance with the present disclosure. and the width is noted for ’f’ = 20 %. First, the effective index neff evolution with the width of an isolated waveguide is computed to find the waveguide width range that ensures single mode operation as shown in Figure 3A. In an exemplary case, the PR may be designed by phase matching the TE and TM mode through proper choice of their width. The phase matching width pair is obtained by initially performing modal simulations on an isolated waveguide. The effective index (neff) is found for various values of fraction ranging from 10 % to 30 % in steps of 5 % and width, followed by distinguishing the single mode region. For a given fraction, the maximum value of width within the single mode region is considered and the neff of TM0 mode for this largest possible width is matched with neff of TE0 mode and that width is noted for ’f’ = 20 %.
[0051] Figure 3B illustrates an exemplary graph of the phase matching between TE0 and TM0 waveguides through a hybrid mode in accordance with the present disclosure. The broad waveguide width (WTM) is fixed at 730 nm and keeping a gap ‘g’ of 250 nm, the narrow waveguide width (WTE) is varied illustrated in Figure 3B. At around a width of WTE = 415 nm, there is the existence of a hybrid mode necessary for mode conversion between orthogonal polarizations. A modal simulation for the directional coupler section for ’g’ ranging from 250 nm to 400 nm in steps of 25 nm may be performed by fixing WTM and varying W¬TE. For a given value of ‘g’, there is a distinct value of width WTE that results in the existence of hybrid mode, which may be required for orthogonal mode coupling. The phase matching and hybrid mode illustrated in Figure 3B is for a gap of 275 nm and for ‘f ’ = 20 %. The coupling length LC is calculated as LC = p/ ?ß from the propagation constants of the hybrid mode.
[0052] Figure 3C illustrates an exemplary graph of the coupling length as a function of gap for several values of fraction in accordance with the present disclosure. Figure 3C illustrates the variation of LC as a function of gap ‘g’ in nano meters for several values of ‘f’ in µm, for values of ‘f’ in the range of 10 µm to 30 µm, in intervals of 5 µm. The coupling length shows a minimum at ‘f’ = 20 %. Thus, the value of fraction is fixed at 20% in the design. The spectral response of the PR is obtained by performing propagation simulation of the structure illustrated in Figure 2B. The gap is varied by considering the appropriate waveguide widths and coupling length for this simulation. The full-width half maximum (FWHM) is found to be strongly dependent on the gap, and this property provides flexibility to engineer the bandwidth of the PR.
[0053] Figure 4A illustrates an exemplary graph of a spectral response of the polarization rotator considered for a few different gaps in accordance with the present disclosure. Figure 4A shows the spectral response for few values of gap. The effect of width and gap on the spectral response is simulated to see the tolerance to fabrication process variation. As illustrated the spectral response of the PR for gaps g = 250 nm, g= 300 nm, g = 350 nm and g = 400nm is illustrated. The spectral response peaks at a value of (?- ?0) nm of 0 and is like a gaussian distribution across 0. As mentioned previously, the full-width half maximum (FWHM) is found to be strongly dependent on the gap, and this property provides flexibility to engineer the bandwidth of the PR.
[0054] Reference is made to Figure 4B illustrates an exemplary graph of a 3-dB bandwidth as a function of the gap in accordance with the present invention. Figure 4B, which shows the FWHM with respect to gap along with a quadratic fit and the FWHM is 15 nm at a gap of 250 nm and close to 5 nm at a gap of 400 nm. First, WTE is fixed and gap is varied to find the spectral shift. Figure 4B shows the peak wavelength with gap. The data fitting is done using a two-term exponential function. The slope of the fitting function is -1.04 nm spectral shift per 1 nm increase in gap at g = 100 nm, where the negative sign indicates blue-shift. The magnitude of slope falls below 0.1 nm/nm at g = 270 nm and continues to decrease. Thus, it becomes robust to gap variations with increase in ’g’.
[0055] The simulation as illustrated in Figure 4A and Figure 4B was followed with varying WTE by ± 5 nm and keeping the gap fixed to find its effect on spectral response as shown in Figure 5B. The spectral shift of the peak wavelength is 4 nm/nm change in WTE. This is almost six times better than PR based on ADC using silicon on insulator (SOI). Thus, the sandwich waveguide is superior in terms of tolerance to fabrication imperfections compared to SOI waveguide.
[0056] Reference is now made to Figure 5A, which is an exemplary top view of a CWDM scheme showing the TE) bus waveguide and the TM0 branches. The schematic shown in Figure 5A is used for realizing CWDM. In an exemplary case, a 3-dB bandwidth of 20 nm is used with a channel spacing of 20 nm as per the standard CWDM grid and the peak wavelengths span the S, C, and L bands. The input gratings are designed to couple broadband light into TE0 mode. Light propagates through the broad patch waveguide and tapers down to match the narrow waveguide. The narrow TE waveguide width is phase matched with respective and relatively broader TM waveguides depending on the centre wavelength through tapering the bus waveguide. Only three TM branches are shown for illustration, but it should be obvious to a person of ordinary skill in the art that it can be extended to span the S, C and L bands.
[0057] Reference is now made to Figure 5B is an exemplary spectral response of the proposed CDWM scheme spanning the S, C and L bands in accordance with the present disclosure. Figure 5B illustrates the transmission as a function of wavelength (µm). The combined resultant response from the TM branches depicting the CWDM functionality is shown in Figure 5B. Thus, using a directional coupler, CWDM and polarization filter can be simultaneously realized.
[0058] Although the operations of the system and method according to the embodiments of the present disclosure are described in a specific order in the drawings, it does not require or imply that these operations have to be performed in that specific order, or a desired result can only be achieved by performing all of the illustrated operations. On the contrary, the steps illustrated in the flow diagrams may change their execution order. Additionally, or alternatively, some steps may be omitted, a plurality of steps may be combined into one step for execution, and/or one step may be decomposed into a plurality of steps for execution. It should also be noted that the features and functions of two or more modules according to the embodiments of the present disclosure may be embodied in one module. In turn, features and functions of one module described above may also be further divided into a plurality of modules for embodiment.
[0059] Although the present disclosure has been described with reference to several preferred embodiments, it should be understood that the present disclosure is not limited to the preferred embodiments disclosed here. Embodiments of the present disclosure are intended to cover various modifications and equivalent arrangements within the spirit and scope of the appended claims. Although the foregoing disclosure has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practised within the scope of the appended claims. Examples of the present disclosure have been described in language specific to structural features and/or methods. It should be noted that there are many alternative ways of implementing both the process and apparatus of the present invention. Accordingly, embodiments of the present disclosure are to be considered illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope and equivalents of the appended claims. It should be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed and explained as examples of the present disclosure.
, C , Claims:1. A polarization converter device comprising a sandwich waveguide layer formed on a base layer, wherein the sandwich waveguide layer comprises:
a first layer formed on the base layer, wherein a height H1 of the first layer is of a pre-determined;
a second layer formed on the first layer with a predetermined height H2, wherein the height of the second layer H2 is smaller compared to the height of the first layer H1; and
a third layer formed on top of the second layer, wherein the height H3 of the third layer equivalent to the height H1 of the first layer.

2. The device as claimed in claim 1, wherein the base layer comprises:
an optically transparent material or an oxide of silicon, an oxide of germanium, an oxide of tin, oxide of titanium or an oxide of tantalum.

3. The device as claimed in claim 1, wherein the second layer comprises:
an optically transparent material or a nitride of silicon or a nitride of germanium or a nitride of tin or a nitride of titanium or a nitride of tantalum.

4. The device as claimed in claim 1, wherein the second layer comprises:
at least one of amorphous material or crystalline material or polycrystalline material,

5. The device as claimed in claim 4, wherein the material includes at least one an optically transparent material of silicon or germanium, wherein the optically transparent material has a higher refractive index than a refractive index of the base layer.

6. The device as claimed in claim 1, wherein the total height H of the sandwich waveguide layer includes the height H1 of the first layer, the height H2 of the second layer and the height H3 of the third layer.

7. The device as claimed in claim 1, wherein a total width W of the sandwich waveguide layer is pre-determined.

8. The device as claimed in claim 1, wherein the height H1 of the first layer is determined by H1 = [0.5 * (1-f) * H] and the height H3 of third layer is determined by H3 = [0.5 * (1-f) * H].

9. The device as claimed in claim 1, wherein the height H2 of the second layer is determined by H2 = [f * H].

10. The device as claimed in claim 3 or 4, wherein “f” is determined as a ratio of the thickness of the higher refractive index to the total height of the sandwich waveguide layer.

11. The device as claimed in claim 1, wherein the second layer is of a higher refractive index and the first layer and the third layer is of a medium refractive index.

12. A polarization converter device comprising a sandwich waveguide layer formed on a base layer, wherein the sandwich waveguide layer comprises
a common base layer;
a first sandwich waveguide layer separated by a pre-determined gap from a second sandwich layer, wherein the first sandwich layer and the second sandwich waveguide layer comprises:
a first layer of a predetermined height H1 formed on the base layer;
a second layer of an amorphous-material formed on the first layer, the second layer having a predetermined height H2, wherein the height H2 of the second layer is smaller compared to the height H1 of the first layer; and
a third layer on top of the second layer, wherein the height H3 of the third layer is equivalent to the height H1 of the first layer.

13. The device as claimed in claim 12, wherein the base layer comprises:
an optically transparent material or an oxide of silicon, an oxide of germanium, an oxide of tin, oxide of titanium or an oxide of tantalum.

14. The device as claimed in claim 12, wherein the second layer comprises:
an optically transparent material or a nitride of silicon or a nitride of germanium or a nitride of tin or a nitride of titanium or a nitride of tantalum.

15. The device as claimed in claim 12, wherein the second layer comprises:
at least one of amorphous material or crystalline material or polycrystalline material,

16. The device as claimed in claim 15, wherein the material may include at least one an optically transparent material of Silicon or germanium with a higher refractive index than a refractive index of the base layer.

17. The device as claimed in claim 12, wherein the total height H of the sandwich waveguide layer is the height H1 of the first layer, the height H2 of the second layer and the height H3 of the third layer.

18. The device as claimed in claim 12, wherein the total width “WTE” of the first sandwich waveguide layer and width “WTM” of the second sandwich waveguide layer is predetermined, and the width “WTM” of the second sandwich waveguide layer is wider than the width “WTE” of the first sandwich waveguide layer, wherein the first sandwich waveguide layer and the second sandwich waveguide layer is separated by a fixed distance "g”.

19. The device as claimed in claim 12, wherein the height H1 of the first layer is determined by H1 = [0.5 * (1-f) * H] and the height H3 of the third layer is determined by H3 = [0.5 * (1-f) * H].

20. The device as claimed in claim 12, wherein the height H2 of the second layer is determined by H2 = [f * H].

21. The device as claimed in claim 19 or 20, wherein “f” is determined as a ratio of the thickness of the higher refractive index to the total height.

22. The device as claimed in claim 12, wherein the second layer is of a higher refractive index and the first layer and third layer is of a medium refractive index.

23. The device as claimed in claim 12, wherein the width of the first sandwich waveguide layer “WTE” and the second sandwich waveguide layer “WTM” and the distance “g” between the first sandwich waveguide layer and the second sandwich waveguide layer is varied to achieve waveguide selective polarization conversion.

24. The device as claimed in claim 12, wherein a spectral bandwidth of the polarization converter device can be tuned by a slide cladding with a lower refractive index material.

25. The device as claimed in claim 12, wherein a maximum transmission wavelength of the polarization converter device is be tuned by varying the width of the first sandwich waveguide layer and the width of the second sandwich waveguide layer.

26. The device as claimed in claim 12, wherein the first sandwich layer and the second sandwich layer is completely covered with the base layer.