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Surface grating couplers are diffractive periodic structures that enable efficient coupling of light between optical fibers and planar waveguides. Conventional grating couplers have polarization specific and limited wavelength operation, because of the intrinsic radiation angle dependency on both wavelength and polarization. In this Letter, we propose, to the best of our knowledge, the first polarization-independent surface fiber-chip grating coupler, behaving as a wavelength splitter for the O and C communication bands. Polarization insensitivity is achieved by subwavelength segmentation of the silicon gratings, together with a novel design to allow the dual wavelengths to propagate along two opposite directions in the chip. For the TE and TM polarizations, coupling efficiencies around −4.5 dB are achieved at both 1310 nm and 1550 nm, with an average 1-dB bandwidth of ~45 nm and ~60 nm, respectively. This grating coupler concept can be used as a part of transceivers to increase the data rate of wavelength-division-multiplexing (WDM) systems for fiber-to-the-home (FTTH) network services.

We present an open-source deep artificial neural network (ANN) model for the accelerated design of polarization-insensitive subwavelength grating (SWG) couplers on the silicon-on-insulator platform. Our model can optimize SWG-based grating couplers for a single fundamental-order polarization, or both, by splitting them counter-directionally at the grating level. Alternating, SWG sections are adopted to reduce the reflections (loss) of standard, single-etch devices—further accelerating the design time by eliminating the need to process a second etch. The model of this device is trained by a dense uniform dataset of finite-difference time-domain (FDTD) optical simulations. Our approach requires the FDTD simulations to be made up front, where the resulting ANN model is made openly available for the rapid, software-free design of future standard photonic devices, which may require slightly different design parameters (e.g., fiber angle, center wavelength, and polarization) for their specific application. By transforming the nonlinear input–output relationship of the device into a matrix of learned weights, a set of simple linear algebraic and nonlinear activation calculations can be made to predict the device outputs 1830 times faster than numerical simulations, within 93.2% accuracy of the simulations.

We present a compact silicon-based surface grating antenna design with a high diffraction efficiency of 89% (-0.5 dB) and directionality of 0.94. The antenna is designed with subwavelength-based L-shaped radiating elements in a 300-nm silicon core, maintaining high efficiency with a compact footprint of 7.6 µm × 4.5 µm. The reflectivity remains below -10 dB over the S, C and L optical communication bands. A broad 1-dB bandwidth of 230 nm in diffraction efficiency is achieved with a central wavelength of 1550 nm.

We experimentally demonstrate, to the best of our knowledge, the first microdisk-based silicon-photonic mode-division (de)multiplexer circuit, which is compatible with wavelength-division multiplexing for high aggregate bandwidth on-chip optical communications. This circuit uses waveguide-wrapped microdisk resonators, featuring low levels of intermodal crosstalk and insertion loss within an ultracompact footprint. In addition, the proposed device presents an increased free spectral range, allowing for 530 combined data channels. Furthermore, the microdisk structure naturally supports vertically oriented depletion-type pn junctions, which have been shown to reach subfemtojoule-per-bit modulation efficiencies. The high modulation efficiency, compactness, and wide free spectral range of waveguide-wrapped microdisk resonators present the potential for higher bandwidth and lower energy consumption in next-generation data processing and communication applications.

Spectral filters are important building blocks for many applications in integrated photonics, including datacom and telecom, optical signal processing and astrophotonics. Sidewall-corrugated waveguide grating is typically the preferred option to implement spectral filters in integrated photonic devices. However, in the high-index contrast silicon-on-insulator (SOI) platform, designs with corrugation sizes of only a few tens of nanometers are often required, which hinders their fabrication. In this work, we propose a novel geometry to design complex Bragg filters with an arbitrary spectral response in silicon waveguides with laterally coupled Bragg loading segments. The waveguide core is designed to operate with a delocalized mode field, which helps reduce sensitivity to fabrication errors and increase accuracy on synthesized coupling coefficients and the corresponding spectral shape control. We present an efficient design strategy, based on the layer-peeling and layer-adding algorithms, that allows to readily synthesize an arbitrary target spectrum for our cladding-modulated Bragg gratings. The proposed filter concept and design methodology are validated by designing and experimentally demonstrating a complex spectral filter in an SOI platform, with 20 non-uniformly spaced spectral notches with a 3-dB linewidth as small as 210 pm.

Integrated optical antennas are key components for on-chip light detection and ranging technology (LIDAR). In order to achieve a highly collimated far field with reduced beam divergence, antenna lengths on the order of several millimeters are required. In the high-index contrast silicon photonics platform, achieving such long antennas typically demands weakly modulated gratings with lithographic minimum feature sizes below 10 nm. Here, we experimentally demonstrate a new, to the best of our knowledge, strategy to make long antennas in silicon waveguides using a metamaterial subwavelength grating (SWG) waveguide core loaded with a lateral periodic array of radiative elements. The mode field confinement is controlled by the SWG duty cycle, and the delocalized propagating mode overlaps with the periodic perturbations. With this arrangement, weak antenna radiation strength can be achieved while maintaining a minimum feature size as large as 80 nm. Using this strategy, we experimentally demonstrate a 2-millimeter-long, single-etched subwavelength-engineered optical antenna on a conventional 220 nm SOI platform, presenting a measured far-field beam divergence of 0.1° and a wavelength scanning sensitivity of 0.13°/nm.

We experimentally demonstrate a ring-assisted Mach–Zehnder interferometer (RAMZI) design with slot waveguides for bulk index sensing using sucrose solution. Devices were implemented with both directional couplers (DCs) and multimode interferometer couplers (MMIs). The RAMZI designs with DCs were found to have a sensitivity of 203.3 ± 15.3 nm/RIU, while 177.2 p 12.0 nm/ RIU was demonstrated with MMIs-coupled RAMZI. The free spectral ranges (FSRs) of DCs and MMIs-coupled RAMZI were measured to be 24.3 and 34.3 nm, respectively. The FSRs demonstrated can be as wide as 7.9 times larger than the equivalent microring resonator.

International audience; In this work we investigate the implementation of ultra-wideband polarization rotator in the mid-infrared spectral region. A new design method of the rotation section is proposed, yielding a polarization rotator with an extinction ratio of at least 15 dB in a wavelength range of 2 µm. For a spectral range wider than 3.8 µm, an extinction ratio of at least 10 dB is achieved for this design. The device is 1660 µm long and the associated insertion loss is below 1.2 dB on the full operational wavelength range. The influence of geometrical parameters with respect to the design method to obtain such a broadband behavior is discussed. Finally, to increase the tolerance to fabrication errors, a tapered rotator design is proposed. Such a device can support up to ± 100 nm fabrication errors and still guarantees remarkable broadband behavior. To the best of our knowledge, this is the first time an integrated polarization rotator is designed to operate for the wavelength range of 4 to 9 µm with a bandwidth exceeding 2 µm.

Existing OPAs are typically based on 2D rectangular arrays or 1D linear arrays. Both approaches present a limited field-of-view (FOV) due to the presence of the grating lobes when the element spacing is larger than λ/2. To address the need for an increased steering range, we propose a new design strategy of an OPA system utilizing a 2D circular phased array, with a substantially increased FOV. We present a circular OPA using a demonstrated antenna element design, with an 820-element array. A steering range ΩSR calculated as a solid angle of 0.51π sr, and an angular beamwidth of 0.22°, was achieved. The array exhibits a sidelobe suppression larger than 10 dB, and a FOV of 2π sr. Although the performance is limited by the far field pattern of the individual antenna we chose, our circular OPA achieved, to the best of our knowledge, the largest steering range reported to date compared to the state-of-the-art integrated optical phased arrays reported in literature.

A novel broadband multimode waveguide bend is proposed that supports the propagation of multiple TE modes on a silicon-on-insulator platform. The gradient curvature bend utilizes trapezoidal subwavelength grating (SWG) segments, connected by adiabatically tapered radial strips to achieve efficient mode (de)multiplexing. The inclusion of the radial strips offers an extra degree of design freedom, allowing the realization of a multimode bend with only one single full etch step. The access waveguide has a width of 2.075 μm with an effective radius of 10 μm. Propagation loss for all modes remains below 2.96 dB, and intermodal crosstalk has a maximum of -19 dB across a broad bandwidth of 100 nm, centred at 1550 nm. This work presents an excellent design choice for broadband mode-division multiplexing operations.

We demonstrate the design and experimental results of a silicon-on insulator (SOI) polarization rotator design based on asymmetric periodically loaded waveguides. The rotator design features a compact device footprint of 15.78 $\mu\mbox{m}$ , with a measured polarization rotation extinction ratio of 11.8 dB at 1525 nm while maintaining at least 6 dB ER over the entire C-band. The fabrication of this rotator is fully compatible with the standard complementary metal-oxide semiconductor (CMOS) process with a SiO2 top cladding. So far, this is the most compact polarization rotator demonstration utilizing the asymmetric periodically loaded SOI waveguides. However, the device is sensitive to fabrication errors; thus, the performance is limited by the current fabrication technology.

Optical antennas are key components in optical phased arrays for light detection and ranging technology requiring long sensing range and high scanning resolution. To achieve a narrow beam width in the far-field region, antenna lengths of several millimeters or more are required. To date, such long antennas have been impossible to achieve in silicon waveguides because currently demonstrated technologies do not allow accurate control of grating strength. Here, we report on a new type of surface-emitting silicon waveguide with a dramatically increased antenna length of L = 3.65 m m . This is achieved by using a subwavelength metamaterial waveguide core evanescently coupled with radiative segments laterally separated from the core. This results in a far-field diffracted beam width of 0.025°, which is a record small beam divergence for a silicon photonics surface-emitting device. We also demonstrate that by using a design with L -shaped surface-emitting segments, the radiation efficiency of the antenna can be substantially increased compared to a conventional design, with an efficiency of 72% at the wavelength of 1550 nm.

We propose and experimentally demonstrate a novel approach to implement a low-loss, broadband, and compact transverse electric (TE)-pass polarizer on a silicon-on-insulator platform. The TE-polarizer utilizes a subwavelength grating (SWG) structure to engineer the waveguide equivalent material index. In this paper, the SWG-based polarizer only supports its fundamental TE mode, whereas the transverse magnetic (TM) mode is suppressed under the cutoff condition, i.e., the TM mode leaks from the waveguide with low reflection. The simulations predict that the bandwidth to achieve a polarization extinction ratio (ER) of 35 dB exceeds 200 nm. Experimentally, the measured polarization ER is ∼30 dB, and the average insertion loss is 0.4 dB in the wavelength range of 1470–1580 nm. The fabricated TE-polarizer has a compact length of 60 $\mu\textrm{m}$ .

Small footprint photonic integrated interferometer based on silicon-on-insulator platform is reported. A passive interferometer comprising small footprint $$4\times 4$$ and $$2\times 2$$ multi-mode interference (MMI) couplers is designed and tested. The $$4\times 4 \,(2\times 2)$$ MMIs feature 0.16 dB (0.06 dB) excess loss, 0.22 dB (0.09 dB) power imbalance, and 1.7 $$^{\circ }$$ (0.12 $$^{\circ }$$ ) phase error. An efficient use of the small design area is accomplished by positioning the input and output ports of the passive interferometer along the same side of the chip with a separation of $$127\,\upmu \hbox {m}$$ between each other (pitch size). A passive-active interferometer terminated with Ge photodiodes has also been designed and tested. Sinusoidal fitting of the phase differences between output port number 1 and all other output ports are estimated to be under 2 $$^{\circ }$$ margin, i.e., $$90^{\circ }\pm 1^{\circ },180^{\circ }\pm 1^{\circ }$$ and $$270^{\circ }\pm 1^{\circ }$$ .

We propose a robust polarization rotator based on the mode-evolution mechanism. The polarization rotation in a silicon wire waveguide is achieved by forming an amorphous silicon (a-Si) overlayer and an SiO2 spacer on top of the waveguide. A strip pattern of a constant width is designed to be etched through the overlayer at a specific angle with respect to the Si waveguide. The asymmetry in the a-Si overlayer affects the waveguide mode by rotating the modal axis. This polarization rotator design is amenable to comparatively simple fabrication compatible with standard silicon photonic processing for integration. The length of the rotation section is 17 μm, and the broadband operation is achieved with a rotation efficiency higher than 90% for a wavelength range exceeding 135 nm. A maximum polarization rotation efficiency of 99.5% is predicted by calculation.

The design of silicon-on-insulator (SOI) rib waveguide structures requires careful selection of dimensions for single-mode and birefringence-free operation, which is often necessary in optical devices. At large SOI heights, the single-mode condition (SMC) is well understood. Here, we present a SMC study extending to ultrasmall waveguides, from slab heights of the order of a wavelength down to 220 nm at the 1310 nm wavelength. In addition, the condition for achieving zero-birefringence is presented alongside the SMC for a complete design guideline when both conditions must be met simultaneously. Simulation results show that at small SOI heights (800 nm), previous SMCs no longer apply, and birefringence-free operation becomes difficult to achieve. New expressions are proposed that define the single-mode cut-off condition at these dimensions.

We experimentally demonstrate a ring-assisted MZI (RAMZI) design with slot waveguides for bulk index sensing using sucrose solution and compare it to the results of a single MRR design. The devices were implemented with multimode interferometer couplers (MMIs). The RAMZI design were found to have a sensitivity of 169.9±24.1nm/RIU and the free spectral ranges (FSRs) of RAMZI was found to be 34.3nm. The FSRs demonstrated was found to be 7.9 times larger than that of the single MRR design.

We experimentally demonstrate a compact, low-cross talk and fabrication-tolerant two-mode (de)multiplexer on the silicon-on-insulator platform. The device consists of a silicon wire waveguide coupled to a taper-etched waveguide. The partially etched taper structure is used to relax fabrication tolerance and thus to ensure high mode-conversion efficiency. The device is 68 μm in length, with a TE0-to-TE1 mode conversion loss of better than −0.8 dB demonstrated over the C-band wavelengths. In addition, the device demonstrates a low TE0-to-TE0 through waveguide insertion loss of better than −1.3 dB with modal cross talk lower than −26 dB, over a 65 nm wavelength range. Finally, we have experimentally demonstrated that the device is tolerant of fabrication errors of up to 20 nm. Better than −6 dB TE0-TE1 conversion loss with a cross talk lower than −23 dB over a 55 nm bandwidth has been obtained with a fabrication tolerance as large as 40 nm.

Subwavelength engineering in silicon photonic integrated circuits is a powerful design tool that allows one to synthesize an effective photonic medium with adjustable refractive index. This creates a new degree of freedom in photonic circuit design. We present an overview of the fundamental concept and its application to address several important practical challenges in the implementation of silicon photonics as a next generation photonics platform for telecom, datacom and sensing. In particular, we report our results in developing highly efficient and broadband fiber-chip couplers for silicon photonic wire waveguides using subwavelength engineered edge coupling structures. We experimentally demonstrate a coupling efficiency of -0.4 dB and polarization independent operation for a broad spectral range exceeding 100 nm for optical fiber with a core diameter of 3.2 μm. For coupling to standard SMF-28 fiber with 10.4 μm mode field diameter we numerically demonstrate a subwavelength engineered overlayer structure composed of SiO2 and Si3N4 which exhibits an overall coupling efficiency exceeding 90%. We have also used our subwavelength structure for coupling experiments with a conventional InGaAsP/InP buried heterostructure laser at λ = 1.3 μm with a measured near field mode size of 2.1 μm×2.8 μm. Peak coupling efficiency is 1.5 dB with 1-dB alignment tolerance of approximately ±1.2 μm horizontally and ±0.8 μm vertically. We further present subwavelength engineered grating couplers fabricated in a single-etch step for the telecom (1.55 μm) and datacom (1.3 μm) wavelengths with efficiencies exceeding -0.5 dB. Further applications that will be discussed include waveguide crossings, microspectrometers, ultra-fast optical switches, athermal waveguides, evanescent field sensors, polarization rotators and colorless interference couplers., 2016 Progress in Electromagnetic Research Symposium (PIERS), August 8-11, 2016, Shanghai, China

We demonstrate the design and experimental results of two polarization rotator designs based on asymmetrical periodic loaded SOI waveguides. The devices feature CMOS compatible fabrication process, with compact footprints (

We report our recent advances in development of subwavelength engineered dielectric metamaterial structures for integrated photonics., 18th International Conference on Transparent Optical Networks (ICTON), July 10 -14, 2016, Trento, Italy

Silicon-on-insulator (SOI) technology has been a promising platform for photonic applications. However, the high index-contrast between silicon and the top cladding (SiO2 or air) of the SOI waveguides makes the modal birefringence hard to control. Consequently, SOI based photonics integrated circuits (PICs) are in general highly polarization-sensitive, making polarization management important. In this paper, a polarization rotator (PR) design on the 220 nm SOI platform is demonstrated through numerical simulations and experiments. The demonstrated PR design is based on asymmetrical periodic loaded waveguide structures. The demonstrated design features compact device footprint and can be fabricated by CMOS compatible process. In addition, no special cladding is required to break the vertical symmetry of the waveguide. The design has shown promising performance over the C-band wavelengths (1530 nm-1565 nm) by simulations. However, the fabrication requirements are stringent for the design, thus the performance of the fabricated devices are limited by the current fabrication technology.

The penetration depth of light plays a crucial role in therapeutic medical applications. In order to design effective medical photonic devices, an in-depth understanding of light’s ability to penetrate tissues (including bone, skin, and fat) is necessary. The amount of light energy absorbed or scattered by tissues affects the intensity of light reaching an intended target in vivo. In this study, we examine the transmittance of light through a variety of cranial tissues for the purpose of determining the efficacy of neuro stimulation using a transcranial laser. Tissue samples collected from a pig were irradiated with a pulsed laser. We first determine the optimal irradiation wavelength of the laser to be 808nm. With varying peak and average power of the laser, we found an inverse and logarithmic relationship between the penetration depth and the intensity of the light. After penetrating the skin and skull of the pig, the light decreases in intensity at a rate of approximately 90.8 (±0.4) percent for every 5 mm of brain tissue penetrated. We also found the correlation between the irradiation time and dosage, using three different lasers (with peak power of 500, 1000, and 1500mW respectively). These data will help deduce what laser power is required to achieve a clinically-realistic model for a given irradiation time. This work is fundamental and the experimental data can be used to supplement existing and future research on the effects of laser light on brain tissue for the design of medical devices.

We review the use of the oxide cladding stress-induced photoelastic effect to modify the polarization dependent properties in silicon-on-insulator (SOI) waveguide components, and highlight characteristics particular to this high index contrast (HIC) systems. The birefringence in SOI waveguides has its origin in the electromagnetic boundary conditions at the waveguide boundaries, and can be further modified by the presence of stress in the waveguiding materials. With typical stress levels in SiO2 films, which are often used as the upper cladding, the waveguide effective index can be altered anisotropically up to the order of 10−3 for ridges with heights ranging from 1 μm to 5 μm. This effect can be used effectively to counter the waveguide geometrical birefringence, allowing the waveguide cross-section profiles to be optimized for design criteria other than null geometrical birefringence. Design strategies are developed for using stress engineering to achieve a variety of functions. Polarization insensitive arrayed waveguide gratings (AWGs), polarization insensitive ring resonators, and polarization splitters and filters are demonstrated using these design principles.

We report our advances in development of subwavelength engineered waveguide structures. This unique NRC patented technology [1,2] allows synthesis of a metamaterial with an unprecedented control of material properties, constituting a powerful tool for a designer of photonic integrated circuits. We have demonstrated a number of subwavelength engineered devices operating at telecom wavelengths [3-7], for example fibre-chip couplers, waveguide crossings, WDM multiplexers, ultra-fast optical switches, athermal waveguides, evanescent field sensors, polarization rotators, transceiver hybrids and ultra-broadband interference couplers. The subwavelength metamaterial concept has been adopted by industry (IBM) for fibre-chip coupling and subwavelength structures are likely to become key building blocks for the next generation of integrated photonic circuits. Here we present an overview of recent examples of our subwavelength engineered structures, with an emphasis on couplers for optical interconnects and evanescent field sensors. We demonstrate an unprecedented control over the light coupling between optical fibers and silicon chips by constructing metamaterial couplers operating at telecom (1.55 μm) and datacom (1.3 μm) wavelengths. We also show that by subwavelength patterning of silicon-wire waveguides the field delocalization can be engineered to increase the sensitivity of evanescent field waveguide sensors [8]. Finally, we discuss some emerging applications of subwavelength engineered structures in mid-infrared photonics., 2015 17th International Conference on Transparent Optical Networks (ICTON), July 5-9, 2015, Budapest

We propose a polarization rotator design based on an asymmetrical periodic loaded waveguides on SOI platforms. The device features high conversion efficiency. The fabrication is CMOS compatible with a two-step lithography etching.

We demonstrate the working principle of a silicon-based all-optical half-adder using AND-XOR Boolean logic. Ultrafast switching speeds are achieved by applying the Kerr effect to hydrogenated amorphous silicon microring resonators.

Fiber-chip edge couplers are extensively used in integrated optics as one of the key structures for coupling of light between planar waveguide circuits and optical fibers. In this work, a new fiber-chip edge coupler concept with large mode size for coupling to submicrometer silicon photonic wire waveguides is presented. The coupler allows direct coupling to conventional SMF-28 optical fiber and circumvents the need for lensed fibers. We demonstrate by simulations a 95% mode overlap between the mode at the chip facet and a high numerical aperture single mode optical fiber with 6 μm mode field diameter (MFD). We also demonstrate a modified design with 89% overlap between the mode at the chip facet and a standard SMF-28 fiber with 10.4 μm MFD. The coupler is designed for 220 nm silicon-oninsulator (SOI) platform. An important advantage of our coupler is that large mode size is obtained without the need to increase buried oxide (BOX) thickness, which in our design is set to 3 μm. This remarkable feature is achieved by implementing in the SiO2 upper cladding two thin high-index Si3N4 layers. The high-index layers increase the effective refractive index of the upper cladding layer near the facet and are gradually tapered out along the coupler to provide adiabatic mode transformation to the silicon wire waveguide. Simultaneously, the Si-wire waveguide is inversely tapered along the coupler. The mode overlap at the chip facet is studied using a vectorial 2D mode solver and the mode transformation along the coupler is studied by 3D Finite-Difference Time-Domain simulations. The couplers are optimized for operating with transverse electric (TE) polarization and the operating wavelength is centered at 1.55 μm., SPIE Optics + Optoelectronics, April 13-15, 2015, Prague, Czech Republic, Series: Proceedings of SPIE; no. 9516

This paper presents an overview of our recent work on several fundamental optical elements and their integration on the silicon-on-insulator (SOI) waveguide platform. Theory, design and experimental results are presented for monolithically integrated asymmetric graded-index waveguide couplers, as well as output couplers based on total internal reflection mirrors. Design strategies for dispersive elements on SOI, for example, ring resonators and arrayed waveguide gratings, are discussed with special emphasis on methods to eliminate the polarization sensitivity. Finally, the properties and applications of evanescent fields in SOI waveguides are reviewed

We propose a novel method to implement a compact and fabrication-tolerant polarization splitter and rotator (PSR) on the silicon-on-insulator platform. The PSR consists of a silicon wire waveguide coupled to a subwavelength grating (SWG) waveguide in an asymmetrical directional coupler. The SWG effect allows an additional degree of design freedom to engineer the equivalent material refractive index. This is advantageously used to effectively compensate for fabrication inaccuracies in PSRs. Our simulation results show that the PSR has a low TM-to-TE polarization conversion loss of -0.13 dB (a conversion efficiency of 97%) at the wavelength of 1550 nm, and better than -0.4 dB conversion loss over the entire C-band wavelength range. Compared to the PSRs made of conventional wire waveguides, the use of SWG index engineering improves the waveguide width fabrication tolerance substantially, from ±3 nm to ±40 nm. A compact device size with a coupling length of 25 μm is achieved.

We propose an original two-mode (TE0/TE1) division multiplexer using a taper-etched directional coupler. The multiplexer has a low insertion loss of -0.1 dB within C-band with a minimum loss of -0.05 dB, a modal crosstalk below -40 dB and a robust fabrication tolerance (>100 nm)., 11th International Conference on Group IV Photonics, GFP 2014, 27 August 2014 through 29 August 2014

SOI-based microring resonators (MRRs) have attracted extensive attentions as ultra-compact sensors. Recently, a new structure design combining a ring and a Mach-Zehnder interferometer (MZI) was proposed as sensors for biomedical applications, and as modulators for communications applications. In this design, the MZI uses two identical couplers, where one arm is formed by connecting the access waveguide of the couplers, while the other arm is part of the microring. Such a device may have only one major resonance with a high extinction ratio in a very broad wavelength span (quasi-free spectral range, quasi- FSR ), which offers a very large measurement range for sensing applications. 2×2 multimode interference (MMI) couplers are used to couple the microring and the bus waveguides as MMI couplers have broader wavelength responses. We present the first experimental demonstration of the MMI-coupled MZI racetrack microrings for sensing applications. Two types of MMI-coupled MZI racetrack microrings are discussed: one with wire waveguides, and the other using slotted waveguides. For the MZI racetrack microring using wire waveguides, we achieve a quasi-FSR of 34.3 nm near the wavelength of 1520 nm. The corresponding major resonance of the MZI racetrack microring demonstrates a high extinction ratio of ~22.4 dB with a full-width-half-maximum (FWHM) of 1.94 nm, and a quality factor Q of ~800. On the other hand, the quasi-FSR of the MZI racetrack microring with slot waveguides is 23.2 nm near the wavelength of 1540 nm; and the extinction ratio of the major resonance is ~24.5 dB with λ FWHM =0.82 nm and Q=~1,900. To demonstrate the uses for sensing applications, we measure the resonance shifts corresponding to the concentration change of the ambient aqueous solutions of sucrose. DI water is used as the reference for calibration to avoid any other variations, e.g. temperature change. Experiments show that the sensitivities of the MZI racetrack microring sensors with wire and slot waveguides are 101.7 nm/RIU and 166.7 nm/RIU, respectively.

We present experimentally a two-mode ring resonator add/drop filter which exhibits distinct resonances for each mode. This device demonstrates strong potential for channel selective filtering in mode-division multiplexing systems.

We present generalized design rules for athermal performance and materials compatibility in high-index-contrast (HIC) waveguides. Thermal stability of integrated photonic devices is one critical limitation in the development of commercially viable integrated optoelectronic circuits. Thermooptically neutral designs are achieved by choosing a cladding material whose thermooptic coefficient is opposite to that of the waveguide core. We derive analytical expressions of athermal conditions for both symmetric and asymmetric channel waveguide structures. The equations apply to general HIC systems such as silicon-on-insulator and SiN-based structures.

The 2x2 optical switch is a crucial component to the future of optical communications and integrated optics. Optical switches on the silicon-on-insulator (SOI) platform have shown advantages in terms of device footprint and switching speed. However, due to the intrinsic properties of SOI rib waveguides, these devices suffer from a strong wavelength and polarization dependent response. Our work presents an SOI based Mach-Zehnder interferometer (MZI) switch which is both polarization and wavelength insensitive over a large bandwidth of 1260-1360 nm. We have completed detailed analyses on the polarization and wavelength performance of the MZI, and obtained optimized parameters in a novel design to reduce the crosstalk f or transverse electric (TE) and transverse magnetic (TM) modes over the wavelength range 1260-1360 nm. Our simulations suggest that we successfully obtained a polarization and wavelength insensitive MZI. A crosstalk level below -18 dB is achieved for both the TE and TM modes in the on-state and the off-state, across the 100 nm bandwidth. Such a polarization and wavelength insensitive switch has a variety of applications in wavelength division multiplexing and other communication systems.

A photonic integrated interferometer based on silicon-on-insulator 4×4 and 2×2 nano-scale MMI couplers is designed and characterized. Small footprints MMIs featuring 0.28 dB excess loss and 2.85-degrees phase error in a 20 nm span are reported.

We demonstrate a SOI slotted microring resonator using a multimode interferometer (MMI) coupler. We achieved high bandwidth of 0.25 nm, and a quality factor Q of ~6000 for rings with a radius of 20 μm.

A method for the optimization of strain-induced Pockels effect in silicon waveguides is proposed. We introduce a new figure of merit and show a 35% enhancement in FOM compared to most efficient reported devices. © 2013 IEEE., 2013 IEEE 10th International Conference on Group IV Photonics, GFP 2013, 28 August 2013 through 30 August 2013, Seoul

Recently developed subwavelength structures in silicon waveguides are presented. Subwavelength grating photonic structures are discussed along with emerging practical components that exploit the principle of subwavelength refractive index engineering, including polarization-independent athermal silicon waveguides, ultra-broadband and compact multimode interference couplers, fabrication tolerant polarization rotators and interferometer arrays with subwavelength optical delay lines.

We review our recent results on subwavelength grating structures for engineering the refractive index of silicon waveguides. Various applications are presented, such as fiber-chip coupling, waveguide crossings and athermal waveguides with a polymer-silicon hybrid core.

Fiber-chip edge couplers are extensively used in integrated optics for coupling of light between planar waveguide circuits and optical fibers. In this work, we report on a new fiber-chip edge coupler concept with large mode size for silicon photonic wire waveguides. The coupler allows direct coupling with conventional cleaved optical fibers with large mode size while circumventing the need for lensed fibers. The coupler is designed for 220 nm silicon-on-insulator (SOI) platform. It exhibits an overall coupling efficiency exceeding 90%, as independently confirmed by 3D FiniteDifference Time-Domain (FDTD) and fully vectorial 3D Eigenmode Expansion (EME) calculations. We present two specific coupler designs, namely for a high numerical aperture single mode optical fiber with 6 µm mode field diameter (MFD) and a standard SMF-28 fiber with 10.4 µm MFD. An important advantage of our coupler concept is the ability to expand the mode at the chip edge without leading to high substrate leakage losses through buried oxide (BOX), which in our design is set to 3 µm. This remarkable feature is achieved by implementing in the SiO2 upper cladding thin high-index Si3N4 layers. The Si3N4 layers increase the effective refractive index of the upper cladding near the facet. The index is controlled along the taper by subwavelength refractive index engineering to facilitate adiabatic mode transformation to the silicon wire waveguide while the Siwire waveguide is inversely tapered along the coupler. The mode overlap optimization at the chip facet is carried out with a full vectorial mode solver. The mode transformation along the coupler is studied using 3DFDTD simulations and with fully-vectorial 3D-EME calculations. The couplers are optimized for operating with transverse electric (TE) polarization and the operating wavelength is centered at 1.55 µm. Web of Science 24 5 5038 5026

Athermal operation of silicon waveguides for the TM and TE mode is achieved using the bridged subwavelength grating (BSWG) waveguide geometry. For the TM mode the experimental results show that the temperature-induced wavelength shift (dλ/dT) is an order of magnitude smaller for the BSWG waveguides with grating duty cycle, waveguide and bridge widths of 42%, 490 nm and 220 nm, respectively, as compared to standard photonics wires (PW). For the TE mode similar results are achieved by using the bridge width of 200 nm and similar duty cycle and waveguide width. A temperature-induced shift of only -2.5 pm/°C is reported for the TM polarized light. Propagation losses of BSWG waveguides for both polarizations were measured to be about 8 dB/cm, comparable to that of PWs. © 2012 SPIE., Photonics North 2012, June 6-8, 2012, Montreal, QC, Canada, Series: Proceedings of SPIE; no. 8412

We demonstrate low-loss photonic wire waveguides, in both the straight and bent waveguide configurations, fabricated by the LOCal Oxidation of Silicon (LOCOS) process, using the standard optical lithography. The oxidation in the LOCOS process produces waveguides in submicron dimensions with ultra-smooth sidewalls. The Full-Width Half- Maximum (FWHM) of the fabricated LOCOS wire waveguide is approximately 650 nm and the height is 280 nm. We used the cut-back method to measure the propagation loss of the TE (x-polarized) mode. The average propagation loss measured by the cut-back method was 8.78 dB/cm, while the minimum measured propagation loss achieved was 7.18 dB/cm for simple straight waveguides. The propagation loss is expected to be lower, as we include the scattering loss in the measurements. The measured bending loss of the LOCOS wire waveguide with a bending radius of 5 um is as low as 0.0089 dB/90° bend for the TE mode. To the best of knowledge, this is the first direct measurement in propagation loss and bending loss for LOCOS wire waveguides.

Our progress in subwavelength structures in silicon waveguides is reviewed. Several practical implementations of subwavelength grating waveguides are discussed, including fibre-chip couplers, waveguide crossings and athermal waveguides. © 2011 IEEE., 8th IEEE International Conference on Group IV Photonics, GFP 2011, 14 September 2011 through 16 September 2011, London

In this paper, athermal subwavelength grating (SWG) waveguides are investigated. Both numerical simulations and experimental results show that a temperature independent behaviour can be achieved by combining two materials with opposite thermo-optic coefficients within the waveguide. SU-8 polymer with a negative thermo-optic coefficient (dn/dT = -1.1×10-4 K -1) is used in our silicon SWG waveguides to compensate for silicon's positive thermo-optic coefficient of 1.9×10-4 K-1. The grating duty ratio required to achieve an athermal behavior is reported to vary as a function of the operating wavelength and the waveguide dimensions. For example, for athermal waveguides of 260 nm in height, duty ratios of 61.3% and 83.3% were calculated for TE and TM polarized light respectively for a 450 nm wide waveguide, compared to ratios of 79% and 90% for a 350 nm wide waveguide. It is also reported that with increasing width, and increasing height, a smaller grating duty ratio is necessary to achieve an athermal behaviour. A smaller fraction of silicon would hence be needed to compensate for the polymer's negative thermo-optic effect in the waveguide core. Subwavelength sidewall grating (SWSG) waveguides are also proposed here as alternatives to high duty ratio SWG waveguides that are required for guiding TM polarized light. Assuming a duty ratio of 50%, the width of the narrow segments for temperature- independent behavior is found by numerical simulations to be 125 nm and 143 nm for TE and TM polarized light, respectively. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)., Photonics North 2011, May 16-18, 2011, Ottawa, Ontario, Canada, Series: Proceedings of SPIE; no. 8007

We present a method for designing temperature independent silicon waveguide devices using the subwavelength grating effect. Photonic wire waveguides are patterned with periodic gaps and filled with SU-8 polymer to cancel the silicon thermo-optic effect., Integrated Photonics Research, Silicon and Nanophotonics, 12–15 June 2011, Toronto, ON, Canada

Various embodiments for etching of silicon nitride (SixNy) lightpipes, waveguides and pillars, fabricating photodiode elements, and integration of the silicon nitride elements with photodiode elements are described. The results show that the quantum efficiency of the photodetectors (PDs) can be increased using vertical silicon nitride vertical waveguides.