WO2018051370A1

WO2018051370A1 - Broadband wavelength filter device using sidewall grating for filtering optical signals

Info

Publication number: WO2018051370A1
Application number: PCT/IN2017/050403
Authority: WO
Inventors: Parimal SAH; Bijoy KRISHNA DAS
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2016-09-16
Filing date: 2017-09-15
Publication date: 2018-03-22

Abstract

Embodiments herein provide a broadband wavelength filter device using sidewall grating for filtering optical signals. The broadband wavelength filter device includes a buried oxide layer and a silicon layer over the buried oxide layer. Further, the silicon layer includes an input waveguide and an output waveguide to operate in a fundamental mode of transverse electric (TE) field. Further, the silicon layer also includes an optical filtering portion with periodic perturbations on one side, positioned in between the input waveguide and the output wave guide to operate in the fundamental mode and a first order mode, wherein the optical filtering portion is tapered at the input waveguide and the output waveguide.

Description

"Broadband wavelength filter device using sidewall grating for filtering optical signals"

FIELD OF INVENTION

[0001] The embodiments herein relate to optical communications and waveguides. More particularly relates to a broadband wavelength filter device using sidewall grating for filtering optical signals. The present application is based on, and claims priority from an Indian Application Number 201641031641 filed on 16^th September, 2016 the disclosure of which is hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] Optical waves propagate through optical fibers as well as waveguide devices. Rapid advancement of complementary metal-oxide-semiconductor (CMOS) silicon photonics technology in silicon-on-insulator (SOI) has led to the integration of efficient light sources, modulators, photo-detectors, and many other active and passive waveguide devices. The waveguide devices like directional coupler (DC), multi-mode interference couplers (MMIC), micro ring resonator (MRR), Mach-Zehnder interferometer (MZI), arrayed waveguide grating (AWG), distributed Bragg reflector (DBR) grating are generally used in the CMOS silicon photonics technology. Further, the waveguide devices are polarization sensitive and have their own characteristic wavelength dependent transfer functions. The transfer functions of the waveguide devices can be reconfigured by availing excellent thermo-optic and/or plasma dispersion effects of silicon crystal.

[0003] The SOI waveguides has a compact design and prominent wavelength dependent characteristics with sub-wavelength grating which makes the SOI waveguides sought after in applications like optical cloaking, refractive index and dispersion engineering, broad-band grating coupler, narrow line-width DBR filters, high-QFabry-Perot resonator, DBR laser, compact modulator, etc.

[0004] Further, many SOI waveguides are used as filters like multilayer interference filters, fiber Bragg gratings, metallic gratings, and corrugated-waveguide gratings and so on. In conventional filters, the fundamental mode of the optical signal is passed along with the unwanted higher order modes, leading to loss of power and causing signal interference. [0005] The above information is presented as background information only to help the reader to understand the present invention. Applicants have made no determination and make no assertion as to whether any of the above might be applicable as prior art with regard to the present application.

OBJECT OF INVENTION

[0006] The principal object of the embodiments herein is to provide a broadband wavelength filter device using sidewall grating for filtering optical signals.

[0007] Another object of the embodiments herein is to provide an input waveguide and an output waveguide, which support only a fundamental mode of the transverse electric mode.

[0008] Another object of the embodiments herein is to select waveguide parameters such that an optical wave allows the required modes in a transverse electric mode.

[0009] Another object of the embodiments herein is to provide an optical filtering portion in the broadband wavelength filter device, with periodic perturbations on one side such that the optical filtering portion supports at least two modes i.e., a fundamental mode and a first order mode.

[0010] Another object of the embodiments herein is to filter higher order leaky modes.

SUMMARY

[0011] Accordingly the embodiments provide a broadband wavelength filter device using sidewall grating for filtering optical signals. The broadband wavelength filter device includes a buried oxide layer and a silicon layer over the buried oxide layer. Further, the silicon layer includes an input waveguide and an output waveguide to operate in a fundamental mode of transverse electric (TE) field. Further, the silicon layer also includes an optical filtering portion with periodic perturbations on one side, positioned in between the input waveguide and the output wave guide to operate in the fundamental mode and a first order mode, wherein the optical filtering portion is tapered at the input waveguide and the output waveguide.

[0012] Accordingly the embodiments herein provide a method for fabricating a broadband wavelength filter device using sidewall grating for filtering optical signals. The method includes providing a buried oxide layer and providing a silicon layer over the buried oxide layer. Further, the method includes etching the silicon layer to provide an input waveguide and an output waveguide to operate in a fundamental mode of transverse electric (TE) field. Further, the method also includes etching the silicon layer to provide an optical filtering portion with periodic perturbations on one side, positioned in between the input waveguide and the output wave guide to operate in the fundamental mode and a first order mode, wherein the optical filtering portion is tapered at the input wave guide and the output waveguide.

[0013] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF FIGURES

[0014] This invention is illustrated in the accompanying drawings, throughout which like reference letters indicate corresponding parts in the various figures. The embodiments herein will be better understood from the following description with reference to the drawings, in which:

[0015] FIG. 1A illustrates a broadband wavelength filter device using sidewall grating for filtering optical signals, according to the embodiments as disclosed herein; [0016] FIG. IB illustrates a cross sectional view of the broadband wavelength filter device for filtering optical signals, according to the embodiments as disclosed herein

[0017] FIG. 1C illustrates a top view of the broadband wavelength filter device for filtering optical signals, according to the embodiments as disclosed herein;

[0018] FIG. 2 is a flow chart illustrating a method for fabricating a broadband wavelength filter device for filtering optical signals, according to the embodiments as disclosed herein;

[0019] FIG. 3 A is a graph illustrating input power with respect to wavelength of the laser light which is being launched at the input waveguide, according to an embodiment as disclosed herein;

[0020] FIG. 3B is a graph illustrating a desired output power with respect to wavelength of the laser light which is being launched at the input waveguide, according to an embodiment as disclosed herein;

[0021] FIG. 4A is a graph illustrating variations of power coupling efficiency of the forward propagating fundamental mode, the first order mode and the sum of the forward propagating fundamental mode and the first order mode with respect to AW , according to an embodiment as disclosed herein;

[0022] FIG. 4B is a graph illustrating variations of power coupling efficiency of the forward propagating fundamental mode, the first order mode and the sum of the forward propagating fundamental mode and the first order mode with respect to taper length Lj, according to an embodiment as disclosed herein;

[0023] FIG. 5 is a graph illustrating variations of percentage power transfer with respect to the taper length, according to the embodiments as disclosed herein.

[0024] FIG. 6A is a graph illustrating variations of effective indices of guided modes with respect to waveguide width, according to an embodiment as disclosed herein;

[0025] FIG. 6B is a graph illustrating variations of Bragg wavelengths with respect to waveguide width, according to an embodiment as disclosed herein;

[0026] FIG. 7A is a graph illustrating variations of coupling constants κ¾ο ^and ¾¾i for a backward propagating fundamental mode and first order mode with respect to grating modulation width W, according to an embodiment as disclosed herein; [0027] FIG. 7B illustrates a distribution of electric field amplitude for a fundamental mode E_0y (x, y), according to an embodiment as disclosed herein;

[0028] FIG. 7C illustrates a distribution of electric field amplitude for a first order mode E_ly (x, y), according to an embodiment as disclosed herein;

[0029] FIG. 8A is a graph illustrating variations of FDTD simulation for four different grating lengths with respect to a transmission spectrum, according to an embodiment as disclosed herein;

[0030] FIG. 8B is a graph illustrating variations of FDTD simulation for four different grating lengths with respect to a reflection spectrum, according to an embodiment as disclosed herein;

[0031] FIG. 9 A is a graph illustrating transmission characteristics at two different temperatures , according to an embodiment as disclosed herein;

[0032] FIG. 9B is a graph illustrating transmission characteristics at two different cladding material of air and water, according to an embodiment as disclosed herein;

[0033] FIGS. 10A-10B illustrate SEM images for rib waveguide with side-wall grating modulation at two different grating modulations, according to an embodiment as disclosed herein;

[0034] FIG. 11 illustrates an experimental setup used for the characterizations of the broadband wavelength filter device for filtering optical signals, according to an embodiment as disclosed herein;

[0035] FIG. 12 is a graph illustrating transmission characteristics of the broadband wavelength filter device as a continuous function of wavelength corresponding to a reference waveguide, according to an embodiment as disclosed herein;

[0036] FIG. 13 is a graph illustrating normalized edge-filter characteristics in C-band of a plurality of broadband wavelength filter devices for a constant W, according to an embodiment as disclosed herein;

[0037] FIG. 14 is a graph illustrating normalized edge-filter characteristics in C-band of a plurality of broadband wavelength filter devices by varying W, according to an embodiment as disclosed herein; [0038] FIG. 15 is a graph illustrating normalized edge-filter characteristics of the broadband wavelength filter device with top cladding materials as air and water, according to an embodiment as disclosed herein;

[0039] FIG. 16A is a graph illustrating sharp edge filter characteristics of the broadband wavelength filter device with top cladding materials as air, according to an embodiment as disclosed herein; and

[0040] FIG. 16B is a graph illustrating sharp edge filter characteristics of the broadband wavelength filter device with top cladding materials as water, according to an embodiment as disclosed herein.

DETAILED DESCRIPTION OF INVENTION

[0041] Various embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. In the following description, specific details such as detailed configuration and components are merely provided to assist the overall understanding of these embodiments of the present disclosure. Therefore, it should be apparent to those skilled in the art that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

[0042] Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0043] Herein, the term "or" as used herein, refers to a non-exclusive or, unless otherwise indicated. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein can be practiced and to further enable those skilled in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

[0044] Accordingly, the embodiments provide a broadband wavelength filter device for filtering optical signals. The broadband wavelength filter device includes a buried oxide layer and a silicon layer over the buried oxide layer. Further, the silicon layer includes an input waveguide and an output waveguide to operate in a fundamental mode of transverse electric (TE) field. Further, the silicon layer also includes an optical filtering portion with periodic perturbations on one side, positioned in between the input waveguide and the output wave guide to operate in the fundamental mode and a first order mode, wherein the optical filtering portion is tapered at the input waveguide and the output waveguide.

[0045] In an embodiment, the buried oxide layer is made of silicon dioxide.

[0046] In an embodiment, the optical filtering portion with periodic perturbations on one side filters higher order leaky modes.

[0047] Accordingly, the embodiments provide a method for fabricating a broadband wavelength filter device for filtering optical signals. The method includes providing a buried oxide layer and providing a silicon layer over the buried oxide layer. Further, the silicon layer is etched to provide an input waveguide and an output waveguide to operate in a fundamental mode of transverse electric (TE) field and an optical filtering portion with periodic perturbations on one side, positioned in between the input waveguide and the output wave guide to operate in the fundamental mode and a first order mode. The optical filtering portion is tapered at the input wave guide and the output waveguide.

[0048] Unlike to the conventional methods and devices, the proposed broadband wavelength filter device exhibits a wavelength transfer function with a sharp-edge or rectangular edge filter (REF) response in C-band.

[0049] Unlike to the conventional methods and devices, the proposed broadband wavelength filter device has tapering which provides the advantage of obtaining all the power in the fundamental mode of the transverse electric mode and neglects the power in the first order mode.

[0050] In the conventional methods and device, both the fundamental mode and the first order modes are allowed to propagate through the waveguide, which leads to interference and signal cluttering. Unlike to the conventional methods and devices, with the proposed broadband wavelength filter, only the fundamental mode is allowed to propagate and the first order mode is suppressed by varying the tapering length between the input waveguide and optical filtering portion.

[0051] Unlike to the conventional methods and devices, the proposed broadband wavelength filter device filters the higher order leaky modes by passing the backward propagating higher order leaky modes to the slab.

[0052] Unlike to the conventional methods and devices, the proposed broadband wavelength filter device can be used as a refractive index sensor with higher sensitivity, in Raman spectroscopy, and also as a long pass wavelength filter.

[0053] Unlike to the conventional methods and devices, the proposed broadband wavelength filter attains a desired slope by varying the values of width 'W' of the broadband wavelength filter device.

[0054] The broadband wavelength filter device 100 can be used as a long pass wavelength filter in an on chip optical WDM networking system. The broadband wavelength filter device 100 can also be used in Raman spectroscopy as it can selectively filter Raman shifted signals from the pump. [0055] Referring now to the drawings, and more particularly to FIGS. 1 through 16, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments.

[0056] FIG. 1A illustrates the broadband wavelength filter device 100 for filtering optical signals, according to the embodiments as disclosed herein.

[0057] Referring to the FIG. 1A, the broadband wavelength filter device 100 for filtering optical signals has a buried oxide layer 102, a SOI silicon layer 104 (i.e., substrate layer) on top of the buried oxide layer 102, an input waveguide 106, an optical filtering portion 108 (i.e., periodic perturbations created by distributed Bragg reflector (DBR) gratings) and an output waveguide 110.

[0058] The input waveguide 106 and the output waveguide 110 are identical to each other and support only the fundamental mode (i.e., TEOO mode), whereas the optical filtering portion 108 has sub wavelength DBR grating on one of the sidewalls (i.e., periodic grating perturbations) and supports at least two modes i.e., the fundamental mode (i.e., TEOO mode) and a first order mode (i.e., TEOI mode). Further, the broadband wavelength filter device 100 supports only the TE like modes. An adiabatic taper section is provided to ensure smooth transitions of the guided modes at both input and output end of the broadband wavelength filter device 100.

[0059] FIG. IB illustrates a cross sectional view of the broadband wavelength filter device 100 for filtering optical signals, according to the embodiments as disclosed herein.

[0060] Referring to the FIG. IB, the cross sectional view provides a slab height 'h', the width W, the height H of the broadband wavelength filter device 10. Further, the value of W₂= AW+Wi, where AW is the grating overlap region.

[0061] The slab height 'h' of the input waveguide 106 and the output waveguide 110 are selected such that it allows only the fundamental mode of transverse electric field i.e., TEOO mode. Further, the slab height 'h' of the optical filtering portion 108 is selected such that it allows at least two modes of transverse electric field i.e., TEOO mode and TEOI mode.

[0062] The optical filtering portion 108 lies between two taper sections at input and output sides. The values for the width W, the height H and the slab height h are selected such that the optical filtering portion 108 allows both fundamental as well as first order mode of transverse electric field. The optical filtering portion 108 has propagation constants of βθ, βΐ and Psiab corresponding to the wavelength of λ. The TE modes with propagation constants βο and βι are guided modes in optical filtering portion 108 whereas p_sla is the slab mode which is oscillating in nature and is called leaky modes. The broadband wavelength filter device 100 is designed such that if any higher order leaky modes get excited in the optical filtering portion 108, the higher order leaky modes are radiated out to a slab. Further, the value of waveguide rib height (H) and slab height (h) are same for the input waveguide 106, the output waveguide 110 and the optical filtering portion 108.

[0063] FIG. 1C illustrates a top view of the broadband wavelength filter device 100 for filtering optical signals, according to the embodiments as disclosed herein.

[0064] Referring to the FIG.1C, the fundamental mode has a propagation constant of

corresponding to a wavelength of λ, which is excited from the input waveguide 106. The fundamental mode is adiabatically expanded into forward propagating fundamental mode with propagation constant ¾ in the optical filtering portion 108. The linear tapering in the waveguide width from Wi to W₂ in the input side and from W₂ to Wi in the output side provides adiabatic transition of the input and output fundamental modes into the forward propagating fundamental modes respectively. In the tapering section, the propagation constant changes from

at input side and from

at the output side.

[0065] Due to the grating perturbations with a designated periodicity of Λ, the forward propagating fundamental mode is phase matched to the backward propagating fundamental mode. Both the fundamental mode (with propagation constant ¾) and the first order mode (with propagation constant -β_\) have different operating wavelengths of

respectively. The periodic grating perturbations on one sidewall of the broadband wavelength filter device 100 ensure non-zero coupling coefficients for both the backward propagating modes. Since the input waveguide 106 and the output waveguide 110 support only the fundamental mode, the backward propagating first order mode in the optical filtering portion 108 will be radiated to the slab as it enters into the input waveguide 106. The propagation constant for the backward propagating first order mode which is radiated into the slab is Thus the

light signal with wavelength around will be missing from the reflected spectrum at the input

and from the transmitted spectrum at the output. However, the light signal around

will be appearing in the reflected spectrum but will be missing from the output spectrum. It is also possible that the light signal with wavelengths less than couples to the backward propagating

higher order leaky modes in the optical filtering portion 108 resulting in a high pass transmission characteristics with a sharp rectangular edge filtering (REF) response around

[0066] The condition for phase matching to occur is that

should be less than

which can be expressed as

wherein where is effective index (indices) of forward

(backward) propagating 0^th (m"¹) order mode(s).

[0067] If the broadband wavelength filter device 100 is designed to support two modes, then the integer values of m>lrepresent leaky or slab modes propagating in backward direction. The backward propagating phase matched wavelengths corresponding to m =0 and

1 are represented as:

[0068] Thus the transmitted spectral band between the two back reflected Bragg wavelengths is decided by the grating period Λ and difference in effective refractive indices of the guided modes. Therefore, the performance of the broadband wavelength filter device 100 depends on the phase matching conditions and the non-zero coupling constants to backward propagating fundamental and first order modes.

[0069] The possible excitations of forward propagating higher order modes and higher order Bragg diffractions have been ignored as the corresponding phase-matched wavelengths fall outside the optical C + L bands.

[0070] FIG. 2 is a flow chart illustrating a method for fabricating the broadband wavelength filter device 100 for filtering optical signals, according to the embodiments as disclosed herein.

[0071] Referring to FIG.2, at step 202, for fabrication of the broadband wavelength filter device 100 for filtering optical signals, the buried oxide layer is provided. The buried oxide layer used is silicon dioxide.

[0072] At step 204, the silicon layer is provided over the buried oxide layer.

[0073] At step 206, the silicon layer is etched to provide the input waveguide 106 and the output waveguide 110. The input waveguide 106 and the output waveguide 110 are identically etched single-mode access waveguides which operate in the fundamental mode of transverse electric (TE) field only. The taper section is provided to ensure smooth transitions of the guided modes at both input and output end of the broadband wavelength filter device 100.

[0074] At step 208, the optical filtering portion 108 with periodic perturbations on one side is positioned in between the input waveguide 106 and the output wave guide 110 to operate in the fundamental mode and the first order mode. The optical filtering portion 108 lies in between the tapering section provided at both input and output end of the broadband wavelength filter device 100.

[0075] FIG. 3A is a graph illustrating input power with respect to all wavelength of the laser light which is being launched at the input waveguide 106, according to an embodiment as disclosed herein.

[0076] Referring to the FIG. 3A, consider a multi wavelength laser light, the laser light is launched at the input waveguide 106. The input power Pin of the laser light is plotted against all the wavelengths of the laser light. It can be observed that the Pin value is constant for the laser light at unit value.

[0077] FIG. 3B is a graph illustrating a desired output power with respect to wavelength of the laser light which is being launched at the input waveguide 106, according to an embodiment as disclosed herein.

[0078] Referring to the FIG. 3B, the expected wavelength dependent transmission characteristics of the broadband wavelength filter device 100 is shown. The output power P₀ of the laser light is plotted against all the wavelengths of the laser light. Further,

are also plotted on the desired output power graph plot.

[0079] At wavelength Aedge the desired razor edge filter response is observed. The wavelength at which the desired razor edge filter response is obtained is a functions of:

[0080] The ratio of Po max to Po min defines an extinction ratio (ER) between a pass band and stop band of the REF. The extinction ratio is calculated as:

[0081] Further, a slope of the graph plot around Aed_ge is defined as an edge slope sedge, wherein the edge slope sedge can be calculated as below:

[0082] FIG. 4A is a graph illustrating variations of power coupling efficiency of the forward propagating fundamental mode, the first order mode and the sum of the forward propagating fundamental mode and the first order mode with respect to AW , according to an embodiment as disclosed herein.

[0083] FIG. 4B is a graph illustrating variations of power coupling efficiency of the forward propagating fundamental mode, the first order mode and the sum of the forward propagating fundamental mode and the first order mode with respect to taper length LT, according to an embodiment as disclosed herein.

[0084] Consider the waveguide parameters to be Wi = 560nm, AW =200nm, H =250nm and h=150nm.

[0085] A Finite difference time domain (FDTD) simulation is performed and the results for the same are provided. The effective waveguide width in the grating region (W₂) is assumed to be lower than Wi +AW = 760 nm due to the presence of the grating perturbations.

[0086] Referring to the FIG. 4A, the effective value of W2 is much greater than W ¹* = 665 nm (as shown in Table.2). Hence, the value of W₂ supports the first order mode (m = 1) and results in a higher value of k₀₁. Further, the 3D FDTD simulation is performed to estimate the forward modal power coupling efficiency (r|f) from the single mode to multi-mode waveguide region.

[0087] The value of corresponding to a percentage of power share for forward

propagating fundamental mode (TEOO), first order mode TEOI, and the sum of the forward propagating fundamental mode and the first order mode as a function of AW (for Wi = 560 nm) is provided for abrupt transition region (i.e., without linear taper). Further, more than 95% of a total incident power from the input waveguide 106 is coupled into the forward propagating fundamental mode if AW <200 nm. However, a linear taper section of length LT = 4μπι is sufficient to provide the adiabatic transition of incident fundamental mode (TEOO) into the forward propagating fundamental mode (TEOO) i-e., 100% power couples to the forward propagating fundamental mode, as shown in FIG. 4B.

[0088] FIG. 5 is a graph illustrating variation of percentage power transfer with respect to the taper length, according to the embodiments as disclosed herein.

[0089] Consider a broadband wavelength filter device 100, with waveguide parameters Wi=560nm, W₂=760nm, H=250nm, slab height h =140 nm and Ιντ=4μηι. The taper provides advantage of obtaining all the power in the fundamental mode and neglect the power in the first order mode of the broadband wavelength filter device 100 for filtering optical signals.

[0090] Referring to the FIG. 5, the graph plot includes η i.e., percentage of power transferred from one mode to the other with respect to the taper length Lj of the broadband wavelength filter device 100 for filtering optical signals.

[0091] represents the percentage of power transferred from the fundamental

mode to the fundamental mode of the broadband wavelength filter device 100 for filtering optical signals, represents the percentage of power transferred from the fundamental mode to

the first order mode of the broadband wavelength filter device 100 for filtering optical signals.

[0092] Further, it can be observed that for the power transferred from the fundamental mode to the fundamental mode (i.e., mO mode to mO mode), the percentage of power transferred approaches 100% for the given tapering length. Further, it can also be observed that for transferring power from the fundamental mode to the first order mode (i.e., mO mode to ml mode), the percentage of power transferred approaches 0 for the given tapering length. Therefore, the power is completely transferred to the fundamental mode and the first order mode is neglected. Thus, only the fundamental mode is allowed to propagate and the first order mode is suppressed by varying the tapering length of the broadband wavelength filter device 100 for filtering optical signals.

[0093] FIG. 6A is a graph illustrating variation of effective indices of guided modes with respect to waveguide width, according to an embodiment as disclosed herein.

[0094] Referring to the FIG. 6 A, a broadband wavelength filter device 100 for filtering optical signals, with waveguide parameters H=250nm at λ =1550 nm and with slab height h as 140 nm, 150 nm, and 160 nm the effective indices of the guided modes as a function of waveguide width W are plotted at different modes. The effective indices plotted include

,

for slab heights 'h' as 140 nm, 150 nm, and 160 nm each, with respect to width W ^m).Further, Table. 1 provide the ranges of waveguide width for different slab heights and the allowed mode of propagation for the waveguide parameters.

Table.1

[0095] The possible excitations of forward propagating higher order modes and higher order Bragg diffractions have been ignored, as the corresponding phase matched wavelengths fall outside the optical C + L bands.

[0096] The cutoff waveguide widths for the first order and second order guided

modes (i.e., m=l and 2) are given in Table.2.

Table.2

[0097] From FIG.6A, it is evident that the value of increases at a faster rate with the

increase of waveguide width but slows down for W>550 nm. Thus for the forward propagating fundamental mode in the input waveguide 106 having width Wi ~ 550 nm is transformed smoothly into the forward propagating fundamental mode in the broadband wavelength filter device 100 of width W₂ > 665 nm, for h = 150 nm and H = 250 nm. However, the value of W₂ must be kept below the value of

for not allowing the next higher order mode at m = 2.

[0098] FIG. 6B is a graph illustrating variation of Bragg wavelengths with respect to waveguide width, according to an embodiment as disclosed herein. [0099] Referring to the FIG. 6B, the variation of as a function of waveguide

width has been plotted at Λ=290 nm. The desired value of in C-band is

selected by using the waveguide parameters (W₂, H, and h) and grating period (Λ) such that

is kept in the L-band or beyond.

[00100] Further, the Bragg wavelengths are highly sensitive to the broadband wavelength filter device 100 dimensions. In an example, consider the waveguide parameters of W =760nm, H =250nm, and h=150nm for which the waveguide width and slab height tolerances are found to be respectively. Therefore, nanometer scale accuracy is required to be

maintained while fabricating the broadband wavelength filter device 100.

[00101] FIG. 7 A is a graph illustrating variations of coupling constants κ¾ο and κ¾ι for a backward propagating fundamental mode and first order mode with respect to grating modulation width W, according to an embodiment as disclosed herein.

[00102] The coupling constant for the backward propagating m"¹ mode is derived using a coupled mode theory

Wherein

is the side-wall modulation width,

are the normalized transverse field distributions of the fundamental forward propagating mode and backward coupled modes, respectively. Considering

and

the values of as a function of AW are computed and plotted,

as shown in FIG.7A.

[00103] The coupling constants for higher order backward propagating leaky modes are not calculated by the semi analytical method as it is not possible to obtain normalized field distributions for unbounded modes using Eigen mode solver. However, the coupling constants for higher order backward propagating leaky modes are non-zero values for wavelengths shorter than If such couplings are large and the broadband wavelength filter device 100 is long enough, a wider stop band associated with a steeper REF response around

IS expected. Further, the grating perturbation in both sidewalls or in the top surface of the broadband wavelength filter device 100 will result in k₀₁ = 0, because the integrand in Eq. 6 becomes odd function with respect to the y-coordinate. Also, the coupling constants for all anti symmetric backward propagating leaky modes (m 6 odd integers) would also be zero because the integrand in Eq. 6 becomes odd function with respect to the y-coordinate.

[00104] FIG. 7B illustrates a distribution of electric field amplitude for a fundamental mode s_0y (x, y), according to an embodiment as disclosed herein.

[00105] FIG. 7C illustrates a distribution of electric field amplitude for a first order mode £iy (^x _> y)_> according to an embodiment as disclosed herein.

[00106] In an embodiment, a Lumerical Mode Solutions technique is used to compute the electric field distributions and respective effective indices for the broadband wavelength filter device 100 with width W₂ and it is assumed that the mode field distributions are not affected by the grating perturbation.

[00107] Referring to the FIG. 7B, the dominant field component of e_0y (x, y) plotted for W2 = 760 nm and λ = 1550 nm is shown.

[00108] Referring to the FIG. 7C, the dominant field component of e_ly (x, y) plotted for W₂ = 760 nm and λ = 1550 nm is shown. Relatively stronger field overlap of the first order mode (m = 1) with the side-wall grating results into k₀₁ >k₀₀ as shown in FIG. 7A. Nevertheless, both k₀₀ and k₀₁ increases linearly with the increase of grating perturbations calculated for 110 nm

[00109] FIG. 8A is a graph illustrating variations of FDTD simulation for four different grating lengths with respect to a transmission spectrum, according to an embodiment as disclosed herein.

[00110] FIG. 8B is a graph illustrating variations of FDTD simulation for four different grating lengths with respect to a reflection spectrum, according to an embodiment as disclosed herein.

[00111] Consider four different device lengths Lg are 25 μm, 50 μm, 75 μm, and 100 μm.

[00112] Referring to the FIG. 8A, the transmission deepens at a centering point at

(within C-band) and (with in L-band). However, there are strong

reflection peaks observed at

The small peaks at

in reflection spectra, as shown in FIG.8B, are observed due to the backward propagating leaky modes towards the input waveguide 106. The backward propagating leaky modes would not have appeared if the backward reflected wave is allowed to propagate longer distance within the input waveguide 106 by availing a wider computation window. [00113] Further, using the Bragg wavelengths the values of are computed

using Eq. 2 and the value is found to be 2.76 and 2.56, respectively. Further, comparing the values of wifh the effective indices computed as shown in FIG.6A for the

broadband wavelength filter device 100 without periodic perturbation, it can be observed that the effective waveguide width (W₂) and dispersion characteristics are altered significantly due to the introduction of periodic perturbations.

[00114] Further, based on the semi-analytical calculations performed under FIG.7A where k₀₁ >k₀₀ , the transmission at

IS deeper than that at The lower transmission at

wavelengths λ < /l_B ⁰¹indicates the presence of significant coupling to the higher order backward propagating leaky modes. Moreover, the transmission deeps and the reflection peak become stronger and stronger for longer grating lengths which are also consistent with the coupled mode theory. Reflectivity at Bragg wavelength

is computed as R = tanh2 (k_0m L_g).Thus for longer device length L_g, the expected REF response is obtained around /lB⁰¹in the transmission spectrum.

[00115] FIG. 9A is a graph illustrating transmission characteristics at two different temperatures, according to an embodiment as disclosed herein.

[00116] FIG. 9B is a graph illustrating transmission characteristics at two different cladding material of air and water, according to an embodiment as disclosed herein.

[00117] The wavelength edge (λ^) of the broadband wavelength filter device 100 for filtering optical signals is associated with the fundamental modes around the Bragg wavelength /lB⁰¹which is tuned using a plasma dispersion effect by integrating p-i-n/p-n diode, thermo-optic effect by integrating a micro heater, cladding refractive index change, etc.

[00118] Referring to the FIG. 9A, consider the broadband wavelength filter device 100 with Lg = 100 μιη, Wl = 560 nm, AW = 200 nm, H = 250 nm, and h = 150 nm. The FDTD simulation of the broadband wavelength filter device 100 is performed at two different temperatures i.e., 300 K and 327 K and corresponding transmission characteristics (in dB scale) is plotted, as shown in FIG. 9A. For the FDTD simulation, the thermo-optic coefficient of Si 104 layer is taken as 1.86 10— 4/K and the thermo-optic coefficient of the BOX 102 layer (Si02) is taken as 1.2xlO-5/K. [00119] Further, based on the calculated results it can be observed that

Furthermore, it can also be observed that

[00120] Referring to the FIG. 9 A, the effect of cladding refractive index change is observed by simulating the transmission characteristics of the broadband wavelength filter device 100 at room temperature i.e., at T = 300 K with top cladding material as deionized water (DI water) having n = 1.318 and λ « 1550 nm. The transmission characteristics of the broadband wavelength filter device 100 with top cladding material as DI water is compared with an air cladding, as shown in FIG.9B.

[00121] Referring to the FIG. 9B, it can be observed that the transmissions at Bragg wavelengths

are reduced significantly, indicating an increased modal overlap due to DI water cladding. Further, a shift of

are observed in the graph. The corresponding changes in effective refractive indices are estimated as

[00122] Unlike to the conventional methods and devices, the broadband wavelength filter device 100 can be used as a refractive index sensor with an estimated sensitivity of

24.5 nm/RIU and 21 nm/RIU for the Bragg wavelengths at

respectively.

[00123] Unlike to the conventional methods and devices, the slope of the transmitted power with respect to the wavelength increases with the increase of device length Lg in case of the broadband wavelength filter device 100 for filtering optical signals. Further, the broadband wavelength filter device 100 for filtering optical signals can be used for various applications such as light intensity based sensing, modulation, switching, etc.

[00124] FIGS. 10A-10B illustrates SEM images for rib waveguide with side-wall grating modulation at two different grating modulations, according to an embodiment as disclosed herein.

[00125] In an embodiment, the broadband wavelength filter device 100 for filtering optical signals was fabricated with silicon layer having the device layer thickness of 250 nm, buried oxide layer thickness of 3 μm and a handle wafer thickness of 500 μm, 500 μm long sidewall gratings (Λ = 290 nm) are integrated into the middle of 5 mm long single mode input waveguide 106 and output waveguide 110 which have waveguide parameters of W = 560 nm, H = 250 nm, and h = 150 nm. The fabrication process of the optical filtering portion 108 includes using a single step e-beam lithography using negative tone resist (HSQ) and subsequent inductively coupled reactive ion etching (ICP RIE) to provide sidewall gratings and grating couplers.

[00126] The SEM image for optical filtering portion 108, having W is 560 nm, H is 250 nm, h is 150 nm and having the sidewall grating modulation AW isl50 nm is shown in FIG.10A.

[00127] The SEM image for optical filtering portion 108, having W is 560 nm, H is 250 nm, h is 150 nm and having the sidewall grating modulation W is200 nm is shown in FIG.10B.

[00128] FIG. 11 illustrates an experimental setup used for the characterizations of the broadband wavelength filter device 100 for filtering optical signals, according to an embodiment as disclosed herein.

[00129] Referring to the FIG. 11, a scheme of the experimental setup used for the broadband wavelength filter device 100 for filtering optical signals is provided. The experimental setup includes a GCi input grating coupler, a GCo output grating coupler, a tunable laser source(TLS) with an optical spectrum analyzer (OSA) and a distributed Bragg reflector (DBR). A high resolution OSA with inbuilt TLS e.g., Apex 2043B is used for characterizations. The fiber optic TLS output (1520 nm < XL < 1620 nm, Power - 250 μ\¥) is fed directly to the input grating coupler (GCi ) and the transmitted light through the output grating coupler (GCo ) is collected by another single mode fiber for measurement using OSA with a resolution bandwidth of 0.8 pm.

[00130] FIG. 12 is a graph illustrating transmission characteristics of the broadband wavelength filter device 100 as a continuous function of wavelength corresponding to a reference waveguide, according to an embodiment as disclosed herein.

[0002] Referring to the FIG.12, the transmission characteristics of the broadband wavelength filter device 100 for filtering optical signals operating in C Band with W = 200 nm and that of a reference waveguide is provided. The transmission spectrum of the broadband wavelength filter device 100 for filtering optical signals exhibits a sharp edge response at

within the C band and a DBR response at within L band.

[00131] Further, there is an insertion loss of REF of «1 dB for 1561 nm < λ < 1590 nm whereas there is almost no insertion loss for λ > 1605 nm. The band rejection of >20 dB is present for λ < Aedge = 1560 nm. It must be noted that the stop band extinction at

which is lower than the stop band extinction for which is almost 40dB. The wider

stop band observed at wavelengths is due to the couplings to the backward propagating

leaky modes. Thus the broadband wavelength filter device 100 can be used as a long pass filter applicable to a certain range of wavelength band.

[00132] FIG. 13 is a graph illustrating normalized edge-filter characteristics in C-band of a plurality of broadband wavelength filter devices for a constant W, according to an embodiment as disclosed herein.

[00133] Referring to the FIG.13, the characteristics of the broadband wavelength filter device 100 normalized with reference waveguide for 1530 nm < λ < 1590 nm is provided for seven different broadband wavelength filter devices (Dl to D7) where each broadband wavelength filter device is fabricated with W = 200 nm.

[00134] Further, all the broadband wavelength filter devices exhibit sharp edge filter response in the C-band (i.e., Aed_ge ~ 1561 ± 1 nm) with the stop band extinction greater than 40 dB. The variation in wavelength edge Aed_ge is due to the possible variations in the waveguide dimensions (i.e., Wi, H, h, and W₂) because of induced errors.

[00135] Further, Table.3, provides estimated figures of merit which are derived from transmission characteristics for the fabricated broadband wavelength filter devices.

Table.3

[00136] FIG. 14 is a graph illustrating normalized edge-filter characteristics in C-band of a plurality of broadband wavelength filter devices 100 by varying W, according to an embodiment as disclosed herein.

[00137] Referring to the FIG.14, the filter characteristic for six different devices for filtering optical signals (D8 to D13) with W = 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, and 200 nm, are provided. The value of Aed_ge (~ 1560 nm) differs for every device. However, the deviation in A^ge from one device to the other is limited to ± 2 nm variations only. [00138] Further, a monotonous of the edge slope is present in the

transmission characteristics as a function of W (as shown in the inset of FIG.14). For W > 170 nm the slope of the edge increases very fast. Thus various values of W can be used to obtain the desired slope.

[00139] FIG. 15 is a graph illustrating normalized edge-filter characteristics of the broadband wavelength filter device 100 with top cladding materials as air and water, according to an embodiment as disclosed herein.

[00140] The tunability of the REF characteristics of the broadband wavelength filter device 100 is verified with DI water cladding. Referring to the FIG.15, the comparison of the transmission characteristics of the broadband wavelength filter device 100 having waveguide parameters W = 200 nm, Lg = 500 μm for air and DI water claddings are plotted. A shift is observed in of the transmission characteristic of the DI water cladding of

(4.44 nm) for the broadband wavelength filter device 100 with Lg = 100 μm.

[00141] Further, the experimentally observed shift due to DI water cladding are substituted in Eq. 2 to estimate the change in effective refractive indices of

and

the results are consistent with theoretical predictions. Therefore, the fabricated devices exhibit 1-nm of tunability in

for an effective total change of

The change in effective refractive index can be obtained by integrating either a micro heater (to avail excellent thermo optic effect or a p-i-n/p-n phase shifter to avail high speed plasma dispersion effect.

[00142] FIG. 16A is a graph illustrating sharp edge filter characteristics of the broadband wavelength filter device 100 with top cladding materials as air, according to an embodiment as disclosed herein.

[00143] FIG. 16B is a graph illustrating sharp edge filter characteristics of the broadband wavelength filter device 100 with top cladding materials as water, according to an embodiment as disclosed herein.

[00144] The cladding refractive index provides the shift due to sensitivity of Si ~18 nm/RIU. In addition to the shift due to sensitivity a change in slope of the edge due to the cladding refractive index is also observed as it reduces the strength of grating modulation. The change in slope of the edge due to the cladding refractive index is plotted for two filtering devices with AW =200nm andl 80nm, as shown in FIG. 16A and FIG.16B respectively. [00145] Further, it is observed that the edge slope Sedge estimated at a transmission operating point of -20 dB near the filter edge is reduced from 118 dB/nm to 105 dB/nm for AW = 200 nm, when air cladding is replaced by DI water cladding, as shown in FIG.16A. The edge slope Sedge estimated at a transmission of -20 dB is reduced from 70 dB/nm to 54 dB/nm for AW =180 nm, when air cladding is replaced by DI water cladding, as shown in FIG.16B. Therefore, the broadband wavelength filter device 100 can be used as a refractive index sensor with high sensitivity by setting a laser wavelength fixed at

[00146] The transmitted power dependent refractive index sensitivity (S_p) can be evaluated as

By setting the laser wavelength at a transmission operating point of -20dB near the filter edge, the value of S_p is calculated as 1890 dB/RTU for a REF with

and at the cladding refractive index of n_c = 1.318 for DI water. The limit of detection (LOD) for a measurable differential power level of 1 dB is ~5.3 ×10-4 RIU.

[00147] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Claims

STATEMENT OF CLAIMS We claim:

1. A broadband wavelength filter device, the device comprising:

a buried oxide layer;

a silicon layer over the buried oxide layer; wherein the silicon layer comprises: an input waveguide and an output waveguide to operate in a fundamental mode of transverse electric (TE) field;

an optical filtering portion with periodic perturbations on one side, positioned in between the input waveguide and the output wave guide to operate in the fundamental mode and a first order mode, wherein the optical filtering portion is tapered at the input waveguide and the output waveguide.

2. The broadband wavelength filter device of claim 1, wherein the buried oxide layer is made of silicon dioxide.

3. The broadband wavelength filter device of claim 1, wherein the optical filtering portion with periodic perturbations on one side filters higher order leaky modes.

4. The broadband wavelength filter device of claim 1, wherein the first order mode is suppressed by varying a tapering length in between the input waveguide and the optical filtering portion.

5. A method for fabricating a broadband wavelength filter device for filtering optical signals, the method comprising:

providing a buried oxide layer;

providing a silicon layer over the buried oxide layer; wherein the silicon layer is etched to provide:

an input waveguide and an output waveguide to operate in a fundamental mode of transverse electric (TE) field;

an optical filtering portion with periodic perturbations on one side, positioned in between the input waveguide and the output wave guide to operate in the fundamental mode and a first order mode, wherein the optical filtering portion is tapered at the input wave guide and the output waveguide.

6. The method of claim 4, wherein the buried oxide layer is made of silicon dioxide.

7. The method of claim 4, wherein the optical filtering portion with periodic perturbations on one side filters higher order leaky modes.

8. The method of claim 4, wherein the first order mode is suppressed by varying a tapering length in between the input waveguide and the optical filtering portion.