WO2017214366A1

WO2017214366A1 - Photonic methods and apparatus for controlling polarization

Info

Publication number: WO2017214366A1
Application number: PCT/US2017/036505
Authority: WO
Inventors: Sasan FATHPOUR; Jeffrey CHILES
Original assignee: University Of Central Florida Research Foundation, Inc.
Priority date: 2016-06-08
Filing date: 2017-06-08
Publication date: 2017-12-14
Also published as: US20200209467A9; US10877208B2; US10732348B2; US20190204504A1; US20200150338A1; US10877209B2; US20200150337A1

Abstract

A photonic device having a polarization-dependent region. The device having a device layer comprising a first cladding film and a second cladding film, and a core film. The core film comprises one of (1) a material having an index n_M and (2) alternating layers of a first material having a first index and second material having a second index. The alternating layers have an effective index for TE polarized light n_TE and an effective index for TM polarized light n_TΜ. Each of the first cladding film and the second cladding film comprise the other of (1) the material having the index of refraction n_M and (2) the alternating layers n_TM< n_M < n_TE, and the indices of the upper cladding and the lower cladding are less than n_TM, n_M and n_TE. A polarizer, polarizing beam splitter and coupler using clipped coupling, employ the material and the alternating layers.

Description

PHOTONIC METHODS AND APPARATUS FOR CONTROLLING POLARIZATION

CROSS REFERENCE

[0001] This application claims the benefit of the filing date of U.S. Provisional Patent Application Serial No. 62/347,212, filed June 8, 2016, titled PHOTONIC APPARATUS, METHOD, AND APPLICATIONS which is hereby incorporated by reference in its entirety.

[0002] This invention was made with government support under grant number Career Award # ECCS1150672 awarded by National Science Foundation. The government has certain rights in this invention.

FIELD

[0003] Photonic methods and apparatus for controlling polarization, and in particular integrated photonic methods and apparatus for controlling polarization.

BACKGROUND

[0004] Polarization management is a critical requirement for state-of-the-art integrated photonic systems. Conventional structures exhibit a high degree of asymmetry in the vertical direction, either by design or by limitations of the particular fabrication method employed. Also, the achievable index contrast (which affects the achievable compactness of the photonic structures desired) is typically very small, which has the particular disadvantage of poor temperature stability, and cannot be brought to very small dimensions as a restriction of methods employed. Key building blocks such as polarizers and polarization beam-splitters (PBS) to date have achieved operation over only limited optical band widths, limiting their uses.

[0005] Polarization management in modern integrated photonics is conducted in many ways, depending on the processes available or the platforms considered. Most commonly, a polarization-filtering effect is achieved using a metal cladding or grating on the surface of a waveguide, which introduces large losses for one polarization but not the other. Alternatively, shallow etching is applied to one area of a waveguide such that the transverse- magnetic (TM) light will leak out. These methods generally require significant additional processing on a wafer in order to achieve polarization. Furthermore, if both TM and transvers-electric (TE) polarization are desired, the amount of additional processing increases further since separate designs are needed to process TE polarized light and TM polarized light. Additionally, large losses for the "pass" polarization can result from their interaction with metal claddings, or from transitions between shallow and deep etched regions. The demonstrated bandwidths of conventional integrated polarizes and PBS are fairly limited, generally not exceeding 100 nm in the telecom band.

[0006] Concerning integrated PBS devices, state-of-the-art approaches are typically precision-engineered directional couplers that selectively couple one polarization into a specific output channel but not the other polarization. Although they can be quite compact, they generally either require difficult-to-fabricate geometries (e.g., two waveguides of different height next to each other), or complicated additional processing steps. Still, the bandwidths are typically limited to < 300 nm even in simulated designs.

[0007] In the telecommunications market, polarization diversity functions are often implemented in fiberized components that are bulky and expensive.

[0008] In another field, remote optical sensing, it is often desirable to extract information on the polarization dependence of a received signal from a target. Additionally, spectroscopic analysis may be required simultaneously with the information extraction. Such processing can be achieved with free-space optics, but such systems are alignment sensitive and expensive to implement.

[0009] There remains a need for improved integrated photonic devices to control polarization.

SUMMARY

[0010] Aspects of the present invention utilize a fundamentally different technique for performing polarization-selective operations on integrated photonic channels, enabling greater optical bandwidths (e.g., more than an octave in frequency) to be achieved in devices such as polarizers and polarization beam-splitters. Furthermore, techniques according to aspects of the present invention enable optical functionality that is not possible with conventional photonic devices. For example, this platform enables the co-existence of "transverse-electric-only" and "transverse-magnetic-only" single-polarization waveguides on a same layer of a single device, as well as conventional waveguides supporting both polarizations. Such configurations lead to other novel devices such as photonic resonators that are nearly invisible to light of one polarization, but which interact normally with light polarized in the orthogonal direction.

[0011] Some embodiments according to aspects of the present invention enable both TE and TM polarizers to be implemented together, and with no additional processing compared to the normal fabrication flow. Furthermore, in some embodiments, because of the high degree of symmetry in the structure and its wavelength-independent operation, bandwidths spanning an octave are achievable, representing a landmark improvement over conventional devices.

[0012] Aspects of the present invention comprise an arrangement of optical materials on a substrate (a photonic device) that enables precise and spatially variable control of the refractive index experienced by light of different polarizations interacting with the photonic device. As such, optically anisotropic features can be exploited for useful functions on an integrated photonic platform.

[0013] Aspects and embodiments of the invention include novel photonic apparatus, associated methods, and applications thereof.

[0014] In accordance with one aspect of the present invention, there is provided a photonic device to guide light in a first direction, the light having a wavelength λ, the device having a polarization-dependent region comprising a lower cladding layer, a device layer disposed on the lower cladding layer. The device layer comprises a first cladding film and a second cladding film, and a core film extending in the first direction between the first cladding film and the second cladding film. The device further comprises an upper cladding layer disposed on the device layer. The core film comprises one of (1) a material having an index of refraction n_M and (2) alternating layers of a first material having a first index of refraction and second material having a second index of refraction different than the first material, the alternating layers having an effective index of refraction for TE polarized light ηχε and an effective index of refraction for TM polarized light ητΜ. Each of the first cladding film and the second cladding film comprises the other of (1) the material having the index of refraction n_M and (2) the alternating layers. nj_M is greater than n_M which is greater than ηχε at the wavelength λ. The index of refraction of the upper cladding and the index of refraction of lower cladding are both less than nj_M,

and nj_E at wavelength λ [0015] In some embodiments, the difference between n™ and ΠΜ is substantially equal to the difference between ΠΜ and ητε, at the wavelength λ. The difference between ητΜ and ητε may be in the range 0.01 to 0.8.

[0016] In some embodiments, the core film comprises the material having an index of refraction n_M. In some embodiments, the core film comprises the alternating layers.

[0017] In some embodiments, the device further comprises an input E/M waveguide having an input core optically coupled to the core film.

[0018] The input E/M waveguide may comprise an input E/M waveguide lower cladding layer, an input E/M waveguide device layer disposed on the input E/M waveguide lower cladding layer, the input E/M waveguide device layer comprising an input E/M waveguide first cladding film and an input E/M waveguide second cladding film, and the input core extending in the first direction between the input E/M waveguide first cladding film and the input E/M waveguide second cladding film, and an input E/M waveguide upper cladding layer disposed on the input E/M waveguide device layer. The input core comprises a material having index of refraction greater than the index of refraction of the input E/M waveguide lower cladding layer, the input E/M waveguide first cladding film, the input E/M waveguide second cladding film and the upper cladding layer at wavelength λ.

[0019] In some embodiments, the device is a polarizer, the device further comprising: a first transition region disposed between the input waveguide and the polarization-dependent region where a width of the first cladding film is disposed between the core film and a width of the input E/M waveguide first cladding film, and a width of the second cladding film is disposed between the core film and a width of the input E/M waveguide second cladding film. In the first transition region, (1) the width of the first cladding film increases and the width of the input E/M waveguide first cladding film decreases along the first direction and (2) the width of the second cladding film increases and the width of the input E/M waveguide second cladding film decreases along the first direction.

[0020] In some embodiments, the device further comprising an output E/M waveguide having an output core optically coupled to the core film, at an opposite end from the input E/M waveguide.

[0021] In some embodiments, the output E/M waveguide comprises an output E/M waveguide lower cladding layer, an output E/M waveguide device layer disposed on the output E/M waveguide lower cladding layer, the output E/M waveguide device layer comprising an output E/M waveguide first cladding film and an output E/M waveguide second cladding film, and the input core extending in the first direction between the output E/M waveguide first cladding film and the output E/M waveguide second cladding film, and the E/M waveguide comprises an output E/M waveguide upper cladding layer disposed on the output E/M

waveguide device layer. The output core comprises a material having an index of refraction greater than the index of refraction of the output E/M waveguide lower cladding layer, the output E/M waveguide first cladding film, the output E/M waveguide second cladding film and the upper cladding layer at wavelength λ.

[0022] In some embodiments, the device further comprise a second transition region disposed between the output waveguide and the polarization-dependent region where a width of the first cladding film is disposed between the core film and a width of the output E/M waveguide first cladding film, and a width of the second cladding film is disposed between the core film and a width of the output E/M waveguide second cladding film. In the second transition region, (1) the width of the first cladding film decreases and the width of the output E/M waveguide first cladding film increases along the first direction and (2) the width of the second cladding film decreases and the width of the E/M waveguide second cladding film increases along the first direction.

[0023] In some embodiments, the polarizer is a TE-pass polarizer. In some embodiments, the polarizer is a TM-pass polarizer.

[0024] Another aspect of the invention is directed to a photonic device to guide light in a first direction, the light having a wavelength λ, comprising: a bus waveguide; and a second waveguide having a core characterized by a width equal to W transverse to the core and the bus waveguide, the second waveguide having a first tapered region proximate the bus waveguide in which the width is reduced along the first direction, and a second tapered region proximate the bus waveguide in which the width is increased along the first direction back to W, the second waveguide being evanescently coupled to the bus waveguide between the first tapered region and the second tapered region. The device further comprises a cladding material disposed between the bus waveguide and the second waveguide, and the second waveguide comprising one of (1) a material having an index of refraction n_M and (2) alternating layers of a first material having a first index of refraction and second material having a second index of refraction different than the first material, the alternating layers having an effective index of refraction for TE polarized light ητΕ and an effective index of refraction for TM polarized light ητΜ. The bus waveguide comprises the other of (1 ) the material having an index of refraction n_M and (2) the alternating layers, and ηχΜ< ΠΜ < ηχε at the wavelength λ, and the index of refraction of the upper cladding and the lower cladding is less than ητΜ, ΠΜ and ητε at wavelength λ.

[0025] In some embodiments, the second waveguide is a ring waveguide.

[0026] In some embodiments, the bus waveguide comprises the material having an index of refraction n_M. In some embodiments, the bus waveguide comprises the alternating layers.

[0027] Still further aspects of the invention are directed to a photonic device to guide light in a first direction and to divide the light into a first output having only TE-polarized light and a second output having only TM-polarized light, the light having a wavelength λ, the device comprising a lower cladding layer, a device layer disposed on the lower cladding layer, an upper cladding layer disposed on the device layer, the device layer comprising a first cladding film and a second cladding film, and a core film extending in the first direction between the first cladding film and the second cladding film, the core film comprising a transition region and a separation region. In the transition region, the core film comprises an input core having a first width transverse to the first direction and a transition core contacting the input core and the transition core having a second width that increases along the first direction until core film has a width equal to the 1.3 to 3.0 times the input core width; and in the separation region, the input core is separated from the transition core by a separation distance that increases along the first direction to a size that prevents coupling of the light of wavelength λ between the input core and the transition core. The input core comprises one of (1 ) a material having an index of refraction n_M and (2) alternating layers of a first material having a first index of refraction and second material having a second index of refraction different than the first material, the alternating layers having an effective index of refraction for TE polarized light ητε and an effective index of refraction for TM polarized light ηχΜ. The transition core comprises the other of (1) the material having an index of refraction n_M and (2) the alternating layers. ηχΜ is less than nM which is less than njE t the wavelength λ. Each of the upper cladding layer, the lower cladding layer, the first cladding film, the second cladding film and the separation cladding film have an index of refraction less than each of ηχΜ, nM , njE at wavelength λ Accordingly, the input core forms the output of only a first of TE-polarized light and the TM-polarized light, and the transition core forms an output of only a second of the TE-polarized light and the TM-polarized light.

he input core and the transition core are separated from one another in the separation region at an angle from 0.1 to 10 degrees.

[0028] In some embodiments, the first width equals the second width where the transition region and the separation region meet.

[0029] In some embodiments, the device further comprises a second transition region where the transition core has a constant width and the input core has a width that increases along the first direction

[0030] These and other aspects of the present invention will become apparent upon a review of the following detailed description and the claims appended thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 is a cross-sectional view of an example of a photonic device for propagating only light having a TM-polarization, according to aspects of the present invention;

FIG. 2 is an expanded cross-sectional view of Film C of the photonic device of FIG. i ;

FIG. 3 is a cross-sectional view of an example of a photonic device for propagating only light having a TE-polarization, according to aspects of the present invention;

FIGs. 4A and 4B are cross-sectional views of examples of photonic devices for propagating both TE-polarized and TM-polarized light, according to aspects of the present invention;

FIGs. 5A-5C are cross-sectional views of examples of transition regions of photonic device for controlling polarization, according to aspects of the present invention;

FIG. 6 is a top-view of an example of an arrangement of materials of a TM-pass polarizer (having an upper cladding omitted to facilitate viewing) according to aspects of the present invention;

FIGs. 7A and 7B are top view snapshots of the propagation of TM and TE light, respectively, of a two-dimensional simulation of a polarizer as described with reference to FIG.

6; FIG. 8 is a top-view of an arrangement of materials of an example of a TE-pass polarizer according to aspects of the present invention;

FIG. 9 is a top-view of an example of an arrangement of materials of a polarization beam splitter (PBS) according to aspects of the present invention

FIGs. 10A and 10B are 2D top view simulations of a PBS having the arrangement of FIG. 9 showing the TM-polarized and TE-polarized light, respectively;

FIG. 1 1 is a top view of another embodiment of a PBS according to aspects of the present invention;

FIG. 12 is a top view schematic of a TE-cloaked resonator having a clipped resonator waveguide;

FIGs. 13A and 13B are top views of two-dimensional simulations of the TE- clocked resonator of FIG. 12 showing a portion of the TM-polarized light being directed into the resonator, and negligible TE-polarized light being directed into the resonator, respectively;

FIG. 14 is a top view schematic of a TM-cloaked resonator achieved by reversing the core materials of the bus and resonator of FIG. 12;

FIG. 15 is a top view of a direction coupler employing "clipped" coupling for coupling-out a fraction of energy of TE-polarized light from bus waveguide to second waveguide;

FIGs. 16-26 illustrate selected steps of methods of fabricating devices according to some aspects of the present invention;

FIG. 27 illustrates an example of an embodiment of a system according to aspects of the present invention including a wave guide device added to augment a pre-processed substrate;

FIGs. 28A and 28B show low-loss propagation as observed from the top surface of a chip having a TM-only waveguide and a TE-only waveguide, respectively; and

FIGs. 29A and 29B include (on the right side) an image of TE-polarized light and TM-polarized light exiting the ports of a PBS, respectively, and (on the left side) corresponding reference paths. DETAILED DESCRIPTION

[0032] Aspects of the invention will be further illustrated with reference to the following specific examples. It is understood that these examples are given by way of illustration and are not meant to define the full scope of the disclosure or the claims to follow.

[0033] FIG. 1 is a cross-sectional view of an example of a photonic device 100 according to aspects of the present invention to guide light in a first direction Z, the light having a wavelength λ. Photonic device 100 comprises optical materials deposited onto a thick handle substrate 1 10 to form features as enumerated below. Photonic device 100 comprises a lower cladding layer 120, a device layer 130 comprising a first side cladding film 132a and a second side cladding film 132b, and a core film 134, and an upper cladding layer 140. Examples of methods of fabricating these structures are provided below with reference to FIGs. 16 - 26. The cross- sectional structure of the device as illustrated in FIG. 1 remains the same in direction Z over a selected length; however, as discussed below with reference to figures described below, a device having a cross section as illustrated in FIG. 1 can be combined with one or more structures to form devices having a different cross section, to selectively process light propagating in direction Z.

[0034] Substrate 1 10 is constructed of any suitable material currently known or later developed for maintaining devices described herein. For example, substrate 110 may have a thickness between 300 - 1000 um thick.

[0035] Lower-cladding film (Film A) 120 is formed on the top surface of substrate 110 and has a refractive index nA. For example, Film A may have a thickness between 0.5 to 10 um.

[0036] Device layer 130 is formed on top of the lower cladding film 120. Device layer 130 comprises first side cladding film (Film B) 132a and second side cladding film (Film C) 132b, and core film 134 that extends in the first direction Z between the first cladding film 132a and second cladding film 132b. In the illustrated embodiment, Film B has a refractive index n_m and Film C is anisotropic. It is typically desirable that the films of the device layer (e.g., Film B and Film C) have thicknesses equal to one another (ideally within a difference of less than 10% between the height of Film B and the height of Film C). As illustrated in FIG. 2, Film C is constructed of alternating layers of two materials 210 and 220 with refractive indices nn and ¾, respectively. The index of refraction njj of material 210 is higher than the index of refraction of material 220, at working wavelength λ. Any suitable light source (not shown) may be present to provide the light of wavelength λ. For example the source may generate the light such a laser or the source may direct the light to the device such as an optical fiber or another waveguide (e.g., an E/M waveguide as described below).

[0037] The alternating layers have thicknesses that are much smaller than the shortest optical wavelength λ that is to be processed by device 100 such that the effective medium approximation holds. For example, the thicknesses may be less than 1/10th the effective wavelength of the light inside the material. As a result of the relatively small thicknesses, Film C can be considered to have an effective refractive index that depends on the relative thickness and index of each particular layer, and which also depends on the polarization of light under consideration due to the different continuity relations for the electric and magnetic fields.

Mathematical expressions of the effective medium approximation are shown below in Equations 1 (a) and 1 (b) for TE and TM light, respectively. Layers 210 may be identical to one another and layers 220 may be identical to one another; however, some variation is possible.

[0038] The effective refractive indices for the transverse-electric (TE) and transverse- magnetic (TM) polarizations, ηχε and ηχ_Μ, are chosen to be greater than and lower than the refractive index ΠΜ of Film B, (n™ < ΠΜ < ^ηχε), respectively. It is typically desirable that indices nj_E and n™ are separated from refractive index ΠΜ by an approximately equal value. A relatively larger separation between n™ to nj_E is generally desirable; for example a separation of 0.01 to 0.8 at operating wavelengths λ is advantageous. In some embodiments the separation is in the range 0.1 to 0.8 at an operating wavelength.

[0039] Upper-cladding 140 (Film D) is disposed on top of device layer 130, and typically has a thickness between 0.5 to 10 um, consisting of a material with a refractive index n∑> that is approximately equal to that of Film A, n_A. Refractive index ri advantageously is within a difference of < 0.05 refractive index units of n_A. Also, the index of refraction of the upper cladding and the lower cladding is less than ΠΜ (i.e., the index of the core) at wavelength λ, and the index of refraction of the upper cladding and the lower cladding is typically less than nx_M and nj_E at wavelength λ.

[0040] It will be appreciated that when the above arrangement is realized, it becomes possible to propagate light in core 134 that exhibits different behaviors depending on the polarization of light in use. For example, in the embodiment of FIG. 1 , device layer 130 is shown to consist mainly of Film C, except for a narrow rectangular core 134 (corresponding to a rectangular waveguide extending along axis Z), which consists of the Film B. For light that is polarized in the vertical direction, corresponding to the TM optical mode, the effective refractive index of the surrounding Film C is lower than the refractive index of the core region 134, causing the optical mode to be confined to core 134 through total internal reflection. However, for light that is polarized in the horizontal direction, corresponding to the TE mode, the effective refractive index of the surrounding Film C is greater than that of the core 134. Thus, TE- polarized light is not confined to core 134, and is radiated out of core 134; in other words, core 134 does not support a mode for the TE polarization, although core 134 does support a mode for TM-polarized light. Device 100 can then be considered a "TM-only" waveguide. Such polarization-dependent operation holds for all wavelengths λ for which the effective medium approximation is satisfied (meaning that there is a short- wavelength cutoff but there is no cutoff in the long-wavelength regime) and for which the relationship n™ < ΠΜ < ητε is maintained.

[0041] It can also be recognized that the same approach allows for construction of "TE- only" waveguides. An example of a TE-only device 300 is illustrated in FIG. 3, which is the "complement" of the arrangement in FIG. 1 (i.e., the result of switching the material of core 334 and cladding 332a, 332b materials relative to those of the TM-only arrangement of device 100). Accordingly, in device 300, because the refractive index of the cladding is lower than that of the core, TM-polarized light is no longer guided, but the TE-polarized light is. Thus the techniques of the present invention readily enable broadband "TM-only" and/or "TE-only" single- polarization waveguides to be realized, on a common layer, without any additional processing beyond normal fabrication flow required to define each type. Also, the index of refraction of the upper cladding and the lower cladding is less than nnvi and ητε (i.e., the indices of the core) at wavelength λ, and the index of refraction of the upper cladding and the lower cladding is typically less than ΠΜ at wavelength λ.

[0042] It will be appreciated that in each of devices 100 and 300, the core 134, 334 comprises one of (1) a material having an index of refraction n_M, and (2) alternating layers of a first material having a first index of refraction nn and second material having a second index of refraction ¾ different than the first material. The alternating layers have an effective index of refraction for TE-polarized light ηχε and an effective index of refraction for TM-polarized light njM, where ηχε and nTM are defined as follows. (n-m)² = f x (n_H)² + (1 - f)(n_L)² Equation 1 (a) l/(niM)² = f / (n_H)² + (1 - f)/(n_L)² Equation 1 (b) where ηχΜ< ΠΜ < ηχε at the wavelength λ, and

f is the fill factor of the nn material in an nn - ¾ pair of layers.

[0043] Each of first cladding film 132a, 332a and second cladding film 132a, 332a comprise the other of (1 ) the material having an index of refraction n_M and (2) the alternating layers.

[0044] It will be appreciated that embodiments of systems employing polarization- dependent devices as described above, also support the integration of more conventional waveguides (i.e., that support propagation of both polarizations TE and TM) onto device layer. It will be appreciated that such ability is desirable since for many applications, such as highspeed communication or remote sensing, it is desirable to transmit and subsequently process both polarization states in order to preserve flexibility of design. For example, conventional waveguides can be achieved by arranging the materials (of devices 100 and 300) as in FIG. 4A or FIG. 4B.

[0045] FIGs. 4A and 4B show a waveguide core 434, 434' consisting of Film B and Film C, respectively, with the cores surrounded by a cladding of Film D on sides 432a and 432b and top 440, and Film A on the bottom 420. In these arrangements, the refractive indices nA and ri are sufficiently low in comparison to the birefringent indices ηχε and njM of Film C such that modes of both polarizations are supported. The horizontal extent of Film D need only be large enough to prevent evanescent coupling from the waveguide core into the rest of the device layer. For example, typical minimum widths may be from 0.5 to 3 microns. Waveguides capable of supporting propagation of both polarizations will be referred herein as E/M waveguides.

[0046] It is to be appreciated that combining the arrangements discussed with reference to FIGs. 1 , 3 , 4A and 4B enables layout and usage of unique waveguide devices that can achieve any desired polarization state, from pure TE operation, to bi-polarized operation, to pure TM operation. To achieve polarization-selective photonic devices using this technology, transitions between waveguides of these types may be exploited, which divide light based on polarization. During transitions to and from any of the arrangements of FIGs. 1 , 3 , 4A and 4B, a hybrid arrangement of materials may be employed, such as those in FIGs. 5A - 5C. In all three hybrid arrangements, the relative widths of the Film B and Film C in core region 534 are varied along the z-direction and may take on any value in-between initial and final widths corresponding to other arrangements as described herein. These particular transition regions will be referred to in the context of examples of polarization-selective devices achievable using techniques according to aspects of the present invention. Typically, transition regions include a lower cladding 520, side claddings 532a and 532b, an upper cladding 540, all on a substrate 510.

[0047] One such device is a polarizer. Functionally, a polarizer is a device that is inserted into an optical waveguide path, which effectively attenuates light of one polarization but leaves the other polarization unaffected. It will be appreciated that, in a polarizer device, an upper cladding and lower cladding are present, but omitted from the FIG. 6 for ease of description. FIG. 6 is a top-view of a device layer of an example of a polarizer according to aspect of the present invention. For example, a TE-blocking polarizer 600 could be implemented as follows. Light is received from an input 602, which is an E/M waveguide (e.g., having a width 300 to 3000 nm) which undergoes an adiabatic transformation to a TM-only waveguide 606 by means of tapering away the Film D cladding and replacing it with Film C in an area surrounding the core, over a characteristic length (e.g., between 5 to 100 microns) in a transition region 604. The transition region is sufficiently long so as to prevent coupling into higher-order modes. It will be appreciated that the arrangement corresponds to that seen in FIG. 5B. During propagation along the transition region, the TE light begins to radiate away as it does not belong to a supported mode in the waveguide. After the transition is complete (at which point the arrangement corresponds to FIG. 1), a TM-only waveguide is formed which propagates for some distance (e.g., between 10 to 300 microns) to ensure that no stray TE-polarized light remains, and then it undergoes a reciprocal transformation back (in transition region 608) to the E/M waveguide arrangement before reaching an output region 603. Thus TM light is unaffected, while TE light is completely removed from the path, representing ideal polarizing behavior. It will be appreciated that propagation in regions 604, 606 and 608 is polarization-dependent, and regions 604, 606 and 608, individually and together, form a polarization-dependent region of polarizer 600. It will also be appreciated that although the above description assumed all light of a selected polarization was removed, in devices according to aspects of the present invention, the light of selected polarization may be removed to any suitable degree. [0048] A snapshot of a two-dimensional simulation of a polarizer as described with reference to FIG. 6 is provided in FIG. 7 A and 7B. The simulated device showed < 0.1 dB loss for the TM polarization and 23 dB attenuation for the TE polarization at a wavelength of 633 nm. When the wavelength is doubled (1266 nm), the loss for the TM polarization was 0.05 dB and the attenuation for the TE polarization was 31 dB. To date, no simulated or fabricated integrated polarizer has achieved such a wide bandwidth. It will be appreciated that higher extinction ratios can be achieved trivially by increasing the length of the single-polarization section. For example, simulations can be performed using the Multiphysics^® simulation software available from COMSOL^®, Inc of Burlington, MA.

[0049] Polarizers that block TM light are also readily achieved using techniques according to aspects of the present invention. FIG. 8 is a top-view of a device layer of an appropriate arrangement of material of an example of a polarizer according to aspects of the present invention. An input E/M waveguide 802 and output E/M waveguide 803 utilize Film B as the core. The core is replaced by Film C at the onset of the initial taper of transition region 804 and likewise at the end of the taper of transition region 808 at output 803. It will be appreciated that propagation in regions 804, 806 and 808 is polarization-dependent, and regions 804, 806 and 808, individually and together, form a polarization-dependent region of polarizer 800.

[0050] Another integrated photonics device according to aspects of the present invention is a polarization beam-splitter (PBS). A PBS is capable of taking a common input 904 (i.e., an input of TE-polarized and TM-polarized light) and splitting light of each polarization into a separate output 906, 908. It is typically desirable that the splitting occur with low intrinsic losses and low crosstalk (undesired polarization in a given output). FIG. 9 is a top-view of an example of an arrangement of materials of a PBS 900 according to aspects of the present invention. The input is an E/M waveguide 902 with core 934 consisting of Film B and side claddings 932a and 932b of film D, with the waveguide core having a width of 300 - 3000 nm and a thickness of 100 - 2000 nm). Structurally, PBS 900 is similar to a conventional "Y-junction splitter," a component that acts as a 50:50 splitter. Core 934 is gradually widened by a factor between 1.3 to 3 times over a transition region 907 (e.g., having a length between 5 to 100 microns). In the illustrated embodiment, core 934 comprises an input core 936 and a transition core 938. In the illustrated embodiment, in transition region 907, the input core 936 has a uniform width and the transition core 938 has a width that increases along the direction of propagation of light. In the illustrated embodiment, core 934 is widened on its lower side as transition core 938 gets wider. In the illustrated embodiment the transition core consists of Film C. In the illustrated

embodiment, once the transition core reaches its maximum width, the waveguide core consists of equal parts of Film B and Film C. In the transition region 907, the core has a hybrid arrangement comprising film B and film C. Next, in the separation region 909, the two core materials (i.e., Film B and Film C) are split apart by a triangular wedge as the Film D cladding is introduced between core materials. For example, the internal angle at which the two cores are split may be from 0.1 to 10 degrees. The above arrangement results in the TM and TE polarizations splitting into separate arms with a high degree of efficiency. Once the arms are diverged by a sufficient spacing so as to prevent coupling between the arms, the lower arm corresponding to where the TE output is diverted may be replaced with Film B again (not shown). The upper arm with its Film B core is where the TM output light is diverted.

[0051] At 633 nm wavelength, an insertion loss of 0.16 dB was calculated for the TE output, and 0.05 dB for the TM output. For 1266 nm wavelength, the loss for both ports becomes negligible (< 0.01 dB). FIGs. 10A and 10B are snapshots of splitting simulations for the TM- polarized and TE-polarized 1266 nm light, respectively, for a device as shown in FIG. 9. It is apparent that the TM light is diverted into the upper waveguide and the TE light is directed into the lower waveguide.

[0052] FIG. 1 1 is a top view of another embodiment of a PBS 1 100 according to aspects of the present invention. An input E/M waveguide 1102 having a core 1134 (Film B) providing an input into PBS 1100. Core 1 134 is widened in a first transition region 1 104 using Film C. In a second transition region 1106, core 1134 is widened using Film B, while keeping the width of the Film C fixed. Multiple spatial modes may be supported in the waveguides in this particular embodiment, although the length of the structure can be made sufficiently long to avoid coupling into those modes; a total PBS length of 30 - 500 microns may be suitable depending on the wavelength of interest. The maximum width of each core should be large enough such that the optical modes for TE and TM polarizations are well-separated once the two core materials are split apart in separation region 1 108 by a wedge of Film D. A suitable maximum width could be from 800 to 3000 nm for each core area, and each widening section could be between 10-100 microns in length. Although in the embodiments described above the input cores were made of Film B and the transition cores were made of Film C, it will be appreciated that an input core can be made of Film C and that a transition core can be made of Film B.

[0053] Techniques according to aspects of the preset invention also enable optical devices that are not possible with conventional integrated photonics. The ability to design anisotropy into specific structures can be used to change coupling conditions between different waveguides. One example apparatus/application of this is a "polarization-cloaked resonator". A polarization- cloaked resonator consists of a circular ring waveguide (ring resonator) coupled to a "bus" waveguide. The nominal widths of both waveguides are ideally chosen to confine only a single transverse optical mode in the horizontal (in the plane of the surface) direction.

[0054] FIG. 12 is a top view schematic of a TE-cloaked resonator 1200. In one embodiment of a "TE-cloaked resonator," the bus waveguide core 1220 consists of film C. The ring waveguide core 1210 is of film B having a width W transverse to core 1210. Both waveguides are of the E M type, having a surrounding cladding 1230 consisting of Film D. The bus waveguide passes within a small gap G (between 100 - 3000 nm) away from the ring waveguide to control the amount of light that is evanescently coupled in and out of the resonator 1210. In the vicinity of their smallest separation, a tapering or "clipping" of the ring waveguide is applied to gradually reduce its width (in a direction transverse to the core longitudinal axis) in region Ci, and then in a region C₂ the width (transverse to the core longitudinal axis) is increased it as it passes away from bus 1220. Light travels in direction Z. It will be appreciated that region C₂ is further along direction Z than region Ci. The extent of clipping should consist of a reduction in the ring waveguide width from W, by a fraction between 0.1 to 0.6. The clipping, combined with the intrinsic anisotropy due to film C in the bus waveguide results in only TM-polarized light being coupled into ring resonator 1210. For TE-polarized light injected through the input, almost none is coupled into the resonator and no losses result from it. Simulation results are provided in FIG. 13 for light having a wavelength 1000 nm show that the TM power coupling can be 38 dB stronger than the TE power coupling.

[0055] FIG. 14 is a top view schematic of a TM-cloaked resonator 1400 achieved by reversing the core materials of the bus waveguide 1420 and ring waveguide 1410 relative to the bus waveguide 1220 and ring waveguide 1210 of resonator 1200. In resonator 1400, the opposite relationship between polarizations is achieved relative to resonator 1200. It is to be appreciated that the polarization-selective nature of this coupling strength is maintained over a broad wavelength range since it does not depend on resonant effects. It is also to be appreciated that the same "clipped" coupling approach is useful for directional couplers, which are designed to transmit a fraction of power from one waveguide to another.

[0056] FIG. 15 is a top view of a direction coupler 1500 employing "clipped" coupling for coupling-out a fraction of energy of TE-polarized light from bus waveguide 1520 to second waveguide 1510, but not TM-polarization light. It will be appreciated that film B and Film C materials of the bus waveguide and second waveguide may be swapped to couple the orthogonal polarization of light to couple-out a fraction of energy of TM-polarized light, but not TE- polarized light.

[0057] Examples of methods of fabricating the previously described devices and material arrangements according to aspects of the present invention are discussed below. It will be appreciated that the devices and material arrangements described above are not limited to those constructed using methods described. Selected steps of methods of fabricating are described below with reference to FIGs. 16-26.

1. In FIG. 16, a handle wafer is coated with Film A, followed by Film C;

2. In FIG. 17, a suitable etch mask resist is coated and patterned with lithography to facilitate formation of features to be etched into Film C;

3. In FIG. 18, dry etching of Film C is performed all the way to the top interface of Film

A;

4. In FIG. 19, the etch mask resist is removed using solvent cleaning or plasma cleaning;

5. In FIG. 20, Film B is deposited onto the wafer surface, to a sufficient thickness that any etched surfaces are completely filled and to a minimum height of 300 nm above the top surface of Film A;

6. In a next step, excess material of Film B is removed to flatten the overall surface at the level of the top surface of Film C (commonly referred to as planarization). Planarization may be achieved, for example, using one of the following techniques:

(i) In FIG. 21 A, the surface of Film B is coated with a polymer film with suitable planarization characteristics, followed by plasma etching with equal selectivity between the resist and Film B (as illustrated in FIG. 2 IB) until the top surface of Film C is reached, or

(ii) In FIG. 22, Film B is chemical-mechanical polished (CMP) to bring it down to the top surface of Film C and thereby flatten it;

7. In FIG. 23, an additional lithography is performed to pattern an etch mask resist into some areas of the surface, wherever prescribed by the necessary design;

9. In FIG. 24, exposed areas are plasma etched to form trenches down to the top interface

10. In FIG. 25, the resist is removed using a solvent or plasma cleaning;

11. In FIG. 26, a 0.5-5 um thick of Film D is deposited to cover the etched sidewalls of any features, and to fully cover the top surface, making the structure essentially optically symmetric in the vertical direction.

[0058] The method of fabrication described above can be applied using any of a plurality of materials as films A-D, as well as the handle/substrate. Films A-D may comprise either dielectric or semiconductor materials, or some combination thereof. Dielectric materials, could include (but are not limited to) silicon-based compounds such as amorphous silicon, silicon dioxide, silicon nitride, silicon oxynitride, silicon carbide or silicon monoxide. Other materials of interest could include tantalum pentoxide, titanium dioxide, zinc sulfide, zinc selenide, hafnium oxide, aluminum oxide, aluminum nitride, silicate compounds (including glasses such as Hydex), or fluoride compounds such as magnesium fluoride or calcium fluoride. In principle, any dielectric materials may be used for films A-D, provided their combination satisfies the refractive index relationships as set forth above. Chalcogenide materials may be employed as well given their large tunability in refractive index; such materials may include variable glass compositions employing germanium, arsenic, sulfur, antimony and/or selenium. Semiconductor material systems may also be employed, including materials and alloys such as silicon, silicon- germanium, and germanium, or of Group ΠΙ-V compounds (where Group ΙΠ includes elements such as Germanium, Aluminum, Indium, etc. and Group V includes elements such as nitrogen, phosphorus, arsenic, etc.). Such semiconductor systems are suitable for embodiments of this invention which employ epitaxial growth methods.

[0059] Concerning Film C, which itself consists of an alternating stack of two different materials with refractive indices nn and ¾, it may be deposited on the substrate either by means of epitaxial growth, sputtering, metalorganic chemical vapor deposition (MOCVD), vacuum evaporation, plasma-enhanced chemical vapor deposition (PECVD), or low-pressure chemical vapor deposition (LPCVD), inductively-coupled plasma-enhanced chemical vapor deposition (ICP-PECVD), or any other technique of depositing materials with suitable refractive indices in an alternating combination as prescribed, typically with low interface roughness < 50 angstroms and in thicknesses ranging from 5 - 300 nm for each layer.

[0060] Concerning Film B, it typically consists of a material that can be deposited into the etched trenches of Film C, such that a conformal and smooth coating of the sidewalls is achieved without voids or inhomogeneities. Additionally, it is desirable that it is optically isotropic in order to maintain the proper relationship between refractive indices of the core and cladding. To suitably deposit Film B, vacuum evaporation methods may be applied assuming the substrate is rotated or translated during the process in order to expose the etched sidewalls of trenches to incoming material evenly. Chemical vapor deposition methods as described above are all generally suitable as they may provide conformal coating on sidewalls. Epitaxial growth may also be applied, provided that low-stress growth on the etched surfaces of Film C and potentially the exposed Film D is possible.

[0061] Concerning Film A, the lower cladding film, it can either be the same material as the substrate if the refractive index is suitable, or it may be achieved by partial oxidation of the substrate (as in the oxidation of silicon to achieve silicon dioxide), or by any deposition means mentioned prior. If single-crystal growth quality is required (in the case of epitaxial growth or MOCVD), it typically consists of a suitable single-crystal material upon which to grow the constituents of Film C.

[0062] Concerning Film D, a similar characteristic of being able to conformally coat steep sidewalls (similar to Film B) is typically desirable, and the same suite of deposition methods and characteristics applies to Film D as to Film B. For both Films B and D, the depositable thickness should be able to exceed the etched trench depth in order to achieve the desired optical properties.

[0063] Although many such combinations may be possible as described above, it is useful to detail particular embodiments that are readily envisioned. For example, Films A and D may consist of silicon dioxide, Film B of silicon oxynitride (with a suitable composition to achieve the refractive index requirements stated earlier), and Film C being comprised of alternating films of silicon nitride (to achieve nn) and silicon dioxide (to achieve ¾). Films A, B, C, and D may all be deposited by any means of chemical vapor deposition (excluding MOCVD). [0064] Concerning the handle wafer, it may consist of any mechanically stable semiconductor or dielectric material, but advantageously either silicon dioxide or silicon as they are more affordable to manufacture. However, this invention represents a self-contained system of three layers (lower cladding, device layer, and upper cladding) that can in principle augment any substrate underneath. The substrate itself may already possess a variety of materials and devices on its top surface prior to the addition of the embodied technology. For example, it could consist of an integrated silicon photonic chip possessing silicon waveguides and metal interconnect features. Typically, the only requirements set forth prior to the addition of this technology is that the top surface is planarized or flat, that it provides sufficient adherence to subsequent films that are deposited, and that none of its materials break down in the process of depositing films A-D. An example of a suitable arrangement for the case of augmenting a pre- processed substrate is given in FIG. 27.

[0065] Those skilled in the art will recognize that the arrangement and composition of the pre-processed substrate is not limited to that pictured. Other relevant arrangements may include a photonic integrated circuit comprising Indium Phosphide- or Gallium Arsenide-based photonic devices. Interfacing of the pre-processed substrate with the augmented layer of this invention may be achieved, for example, by various means such as grating couplers, tapered waveguide directional couplers, or angled reflectors, which are well-known in this field of research.

[0066] Example 1

In this example, dielectric materials were chosen to implement the platform. Silicon Dioxide was selected for upper cladding (Film A), lower cladding (Film D) and layer nL of Film C). Silicon Nitride was selected for layer njj of film C; and silicon oxynitride was selected for side claddings (film B). Low slab propagation losses were measured in the multilayer stacks as well as the silicon oxynitride layers.

[0067] Waveguides were fabricated using these material selections for Films A-D, and tested at 633 nm wavelength. FIG. 28A shows low-loss propagation as observed from the top surface of a chip having a TM-only waveguide; and FIG. 28B shows low-loss propagation as observed from the top surface of the chip having a TE-only waveguide. These results validate that the multilayer structure exhibits the anisotropy that is required to achieve polarization- selective waveguiding. [0068] A test embodiment of a PBS was fabricated using these material selections. The PBS showed good efficiency at routing each polarization into the desired output channel.

Additionally, negligible crosstalk was present. The device was tested at 633 and 1110 nm wavelengths for both polarization inputs. An estimated extinction ratio > 10 dB and insertion losses of < 1.2 dB were achieved for both polarizations at both wavelengths, confirming the expected broadband performance.

[0069] FIGs. 29A and 29B are optical images of light exiting the ports of a PBS according to aspects of the present invention. On the right side of each of FIGs. 29A and 29B are digital images of the light exiting the TE and TM output ports, respectively. On the left side of each of FIG. 29A and 29B there is included a "reference" port with no PBS, showing the fraction of power routed to each port. The similar brightness of light from each of the output ports as the corresponding reference port is indicative of very low losses.

[0070] Although various embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the claims which follow.

Claims

WHAT IS CLAIMED

1. A photonic device to guide light in a first direction, the light having a wavelength λ, the device having a polarization-dependent region comprising:

a lower cladding layer;

a device layer disposed on the lower cladding layer, the device layer comprising a first cladding film and a second cladding film, and a core film extending in the first direction between the first cladding film and the second cladding film; and

an upper cladding layer disposed on the device layer,

the core film comprising one of (1) a material having an index of refraction n_M and (2) alternating layers of a first material having a first index of refraction and second material having a second index of refraction different than the first material, the alternating layers having an effective index of refraction for TE polarized light nj_E and an effective index of refraction for TM polarized light ηχΜ,

each of the first cladding film and the second cladding film comprising the other of (1) the material having the index of refraction n_M and (2) the alternating layers,

where ηχ_Μ< n_M < ηχε at the wavelength λ, and the index of refraction of the upper cladding and the lower cladding is less than n™, n_Mand ητε at wavelength λ.

2. The device of claim 1 , wherein the difference between ηχ_Μ and n_M is substantially equal to the difference between n_M and nj_E, at the wavelength λ.

3. The device of claim 2, wherein the difference between ηχ_Μ and nj_E is in the range 0.01 to 0.8.

4. The device of claim 1, wherein the core film comprises the material having an index of refraction n_M.

5. The device of claim 1, wherein the core film comprises the alternating layers.

6. The device of claim 1 , further comprising an input E/M waveguide having an input core optically coupled to the core film.

7. The device of claim 6, wherein the input E/M waveguide comprises:

an input E/M waveguide lower cladding layer;

an input E/M waveguide device layer disposed on the input E/M waveguide lower cladding layer, the input E/M waveguide device layer comprising an input E/M waveguide first cladding film and an input E/M waveguide second cladding film, and the input core extending in the first direction between the input E/M waveguide first cladding film and the input E/M waveguide second cladding film; and

an input E/M waveguide upper cladding layer disposed on the input E/M waveguide device layer,

the input core comprising a material having index of refraction greater than the index of refraction of the input E/M waveguide lower cladding layer, the input E/M waveguide first cladding film, the input E/M waveguide second cladding film and the upper cladding layer at wavelength λ.

8. The device of claim 7, wherein the device is a polarizer, the device further comprising: a first transition region disposed between the input waveguide and the polarization- dependent region where a width of the first cladding film is disposed between the core film and a width of the input E/M waveguide first cladding film, and a width of the second cladding film is disposed between the core film and a width of the input E/M waveguide second cladding film, wherein, in the first transition region, (1) the width of the first cladding film increases and the width of the input E/M waveguide first cladding film decreases along the first direction and (2) the width of the second cladding film increases and the width of the input E/M waveguide second cladding film decreases along the first direction.

9. The device of claim 8, further comprising an output E/M waveguide having an output core optically coupled to the core film, at an opposite end from the input E/M waveguide.

10. The device of claim 9, wherein the output E/M waveguide comprises: an output E/M waveguide lower cladding layer;

an output E/M waveguide device layer disposed on the output E/M waveguide lower cladding layer, the output E/M waveguide device layer comprising an output E/M waveguide first cladding film and an output E/M waveguide second cladding film, and the input core extending in the first direction between the output E/M waveguide first cladding film and the output E/M waveguide second cladding film; and

an output E/M waveguide upper cladding layer disposed on the output E/M waveguide device layer,

the ouput core comprising a material having index of refraction greater than the index of refraction of the output E/M waveguide lower cladding layer, the output E/M waveguide first cladding film, the output E/M waveguide second cladding film and the upper cladding layer at wavelength λ.

11. The device of claim 10, the device further comprising:

a second transition region disposed between the output waveguide and the polarization- dependent region where a width of the first cladding film is disposed between the core film and a width of the output E/M waveguide first cladding film, and a width of the second cladding film is disposed between the core film and a width of the output E/M waveguide second cladding film, wherein, in the second transition region, (1) the width of the first cladding film decreases and the width of the output E/M waveguide first cladding film increases along the first direction and (2) the width of the second cladding film decreases and the width of the E/M waveguide second cladding film increases along the first direction.

12. The device of claim 11 , wherein the polarizer is a TE-pass polarizer.

13. The device of claim 11 , wherein the polarizer is a TM-pass polarizer.

14. A photonic device to guide light in a first direction, the light having a wavelength λ, comprising:

a bus waveguide;

a second waveguide having a core characterized by a width equal to W transverse to the core and the bus waveguide, the second waveguide having a first tapered region proximate the bus waveguide in which the width is reduced along the first direction, and a second tapered region proximate the bus waveguide in which the width is increased along the first direction back to W, the second waveguide being evanescently coupled to the bus waveguide between the first tapered region and the second tapered region; and

a cladding material disposed between the bus waveguide and the second waveguide ; and the second waveguide comprising one of (1) a material having an index of refraction n_M and (2) alternating layers of a first material having a first index of refraction and second material having a second index of refraction different than the first material, the alternating layers having an effective index of refraction for TE polarized light ηχε and an effective index of refraction for TM polarized light ^ηχΜ_,

the bus waveguide comprising the other of (1) the material having an index of refraction n_M and (2) the alternating layers,

15. The device of claim 14, wherein second waveguide is a ring waveguide.

16. The device of claim 14, wherein the bus waveguide comprises the material having an index of refraction n_M.

17. The device of claim 14, wherein the bus waveguide comprises the alternating layers.

18. A photonic device to guide light in a first direction and to divide the light into a first output having only TE-polarized light and a second output having only TM-polarized light, the light having a wavelength λ, the device comprising:

a lower cladding layer;

a device layer disposed on the lower cladding layer,

an upper cladding layer disposed on the device layer,

the device layer comprising a first cladding film and a second cladding film, and a core film extending in the first direction between the first cladding film and the second cladding film, the core film comprising a transition region and a separation region

in the transition region, the core film comprises an input core having a first width transverse to the first direction and a transition core contacting the input core and the transition core having a second width that increases along the first direction until core film has a width equal to the 1.3 to 3.0 times the input core width; and

in the separation region, the input core is separated from the transition core by a separation distance that increases along the first direction to a size that prevents coupling of the light of wavelength λ between the input core and the transition core;

the input core comprising one of (1) a material having an index of refraction n_M and (2) alternating layers of a first material having a first index of refraction and second material having a second index of refraction different than the first material, the alternating layers having an effective index of refraction for TE polarized light ητε and an effective index of refraction for TM polarized light ηχ_Μ,

the transition core comprising the other of (1) the material having an index of refraction n_Mand (2) the alternating layers,

where ηχ_Μ< n_M < ηχε at the wavelength λ, and each of the upper cladding layer, the lower cladding layer, the first cladding film, the second cladding film and the separation cladding film have an index of refraction less than each of nj_M, n_M , nj_E at wavelength λ whereby the input core forms the output of only a first of TE -polarized light and the TM-polarized light, and the transition core forms an output of only a second of the TE-polarized light and the TM-polarized light.

19. The device of claim 19, wherein the input core and the transition core are separated from one another in the separation region at an angle from 0.1 to 10 degrees.

20. The device of claim 19, wherein the first width equals the second width where the transition region and the separation region meet.

21. The device of claim 19, further comprising a second transition region where the transition core has a constant width and the input core has a width that increases along the first direction.