Search

A high-performance optical filter is proposed and realized with multimode waveguide grating (MWG) and two-mode multiplexers on the x-cut lithium-niobate-on-insulator (LNOI) platform for the first time, to the best of our knowledge. The present optical filter is designed appropriately to avoid material anisotropy as well as mode hybridness, and has a low excess loss of 0.05 dB and a high sidelobe suppression ratio (SLSR) of 32 dB in theory with Gaussian apodization. The fabricated filters show a box-like response with 1-dB bandwidth of 6–23 nm, excess loss of ∼0.15 dB, sidelobe suppression ratio of >26 dB. The device performance is further improved with a sidelobe suppression ratio as high as 48 dB and a low excess loss of ∼0.25 dB by cascading two identical MWGs.

Electro-optic (EO) modulators with a high modulation bandwidth are indispensable parts of an optical interconnect system. A key requirement for an energy-efficient EO modulator is the low drive voltage, which can be provided using a standard complementary metal oxide semiconductor circuity without an amplifying driver. Thin-film lithium niobate has emerged as a new promising platform, and shown its capable of achieving driverless and high-speed EO modulators. In this paper, we report a compact high-performance modulator based on the thin-film lithium niobate platform on a silicon substrate. The periodic capacitively loaded travelling-wave electrode is employed to achieve a large modulation bandwidth and a low drive voltage, which can support a driverless single-lane 100Gbaud operation. The folded modulation section design also helps to reduce the device length by almost two thirds. The fabricated device represents a large EO bandwidth of 45GHz with a half-wave voltage of 0.7V. The driverless transmission of a 100Gbaud 4-level pulse amplitude modulation signal is demonstrated with a power consumption of 4.49fj/bit and a bit-error rate below the KP4 forward-error correction threshold of 2.4×10−4.

Integrated optical tunable filters are key components for a wide spectrum of applications, including optical communications and interconnects, spectral analysis, and tunable light sources, among others. Compared with their thermo-optic counterparts, integrated acousto-optic (AO) tunable filters provide a unique approach to achieve superior performance, including ultrawide continuous tuning ranges of hundreds of nm, low power consumption of sub-mW and fast tuning speed of sub-µs. Based on suspended one-dimensional (1D) AO waveguides in the collinear configuration, we propose and theoretically investigate an innovative family of integrated AO tunable filters (AOTFs) on thin-film lithium niobate. The AO waveguides perform as tunable wavelength-selective narrow-band polarization rotators, where highly efficient conversion between co-propagating TE0 and TM0 modes is enabled by the torsional acoustic A1 mode, which can be selectively excited by a novel antisymmetric wavefront interdigital transducer. Furthermore, we systematically and quantitatively explore the possibilities of exciting modulated acoustic waves, which contain multiple frequency components, along the AO waveguide to achieve independently reconfigurable multi-band operations, with tunable time-variant spectral shapes. By incorporating a complete set of ultrawide-band polarization-handling components, we have proposed and theoretically investigated several representative monolithic AOTF configurations, featuring different arrangements of single or cascaded identical AO waveguides. One of the present AOTF designs exhibits a theoretical linewidth of ∼8 nm (∼4 nm), a sidelobe suppression ratio of ∼75 dB, and theoretically no excess loss at the center wavelength of 1550 nm (1310 nm), with an ultrawide tuning range of 1.25–1.65 µm (from O-band to L-band), a fast tuning speed of 0.14 µs, and a low power consumption of a few mW.

Photonic crystal (PhC) cavities with high Q factor and low volume have been applied in nonlinear, electro-optic and acoustic-optic devices due to the enhancement of the light-matter interactions. However, there are few devices and research on LiNbO3 (LN) PhC cavities due to the difficulty in making hyperfine structures on LN platform. In this work, we propose a PhC nanobeam cavity on the etchless x-cut LiNbO3-On-Insulator (LNOI). The fabrication-friendly device has been designed based on photonic bound states in the continuum (BICs) exhibiting a high Q factor of over 10,000 with the device length of only about 100 µm. Utilizing the electro-optical effect γ13 of LN, we demonstrate an ultra-compact electro-optic modulator based on the PhC nanobeam cavities, which has the modulation efficiency of 1.5 pm/V and the 3 dB bandwidth of 28 GHz.

We have designed and demonstrated a reconfigurable channel add-drop filter (ADF) based on an antisymmetric multimode photonic crystal nanobeam cavity (AM-PCNC) in a silicon waveguide. The proposed AM-PCNC can realize channel add-drop filtering by selectively filtering and reflecting the fundamental mode (TE0) and 1st-order mode (TE1) in the multimode waveguide. A high-performance add-drop filter has been demonstrated with a high extinction ratio of 28.2 dB and an insertion loss of 0.18 dB. Meanwhile, the reconfigurable add-dropping has been realized by heating the nanobeam cavity to tune the filtering wavelength. A tuning efficiency of 0.464 nm/mW was measured. The rising and falling time are ∼6.5 µs and ∼0.6 µs, respectively, which are at microsecond time scale. The footprint of the involved nanobeam cavity is only 16.5 µm2. The device can potentially provide an integrated component for optical switch array, routers, and wavelength-division multiplexing in the optical networks.

A grating lobe-free silicon optical phased array with large field of view is demonstrated. Antennas with periodically bending modulation are spaced at half wavelength or less. The experimental results show that the crosstalk between adjacent waveguides is negligible at 1550 nm wavelength. Additionally, to reduce the optical reflection caused by the sudden change of refractive index at the output antenna of the phased array, tapered antennas are added to the output end face so that more light will be coupled into the free space. The fabricated optical phased array shows a field of view of 120° without any grating lobes.

A high-performance waveguide Ge/Si avalanche photodiode operating at the O-band (1310 nm) is designed with a Ge/Si ridge waveguide defined by two shallow trenches in the active region and fabricated with simplified processes. The device shows a high primary responsivity of 0.96 A/W at the unit-gain voltage of −7.5 V. It has a large 3-dB bandwidth of >27 GHz and a low dark current of 1.8 µA at a reverse bias voltage of −13 V. When the present Ge/Si APD is used for receiving 25 Gbps data, the eye-diagram is open even for an optical power as low as −18 dBm. Furthermore, 50 Gbps data receiving is also demonstrated with an input optical power of −15 dBm, showing the great potential of the present Ge/Si APD for the application in future high-speed data transmission systems.

Optical phased array (OPA) based on silicon photonics is considered as a promising candidate for realizing solid-state beam steering. However, the high refractive index contrast of the silicon waveguides leads to conventional silicon based OPA suffering from large random phase errors, which require complex post-processing such as time-consuming phase calibration. We propose and demonstrate a calibration-free silicon OPA with optimized optical waveguides width as well as the compact 90° waveguide bends beyond the single mode regime. By using grouped cascaded phase shifters, it is able to reduce the number of control electrodes from N to log2(N). A 16-channel OPA has been demonstrated with continuous beam steering over the field of view controlled by only four control voltages without any calibration.

Low-loss optical waveguides are highly desired for nonlinear photonics such as four-wave mixing (FWM), optical parametric amplification, and pulse shaping. In this work, low-loss silicon photonic spiral waveguides beyond the single-mode regime are proposed and demonstrated for realizing an enhanced FWM process. In particular, the designed 2-µm-wide silicon photonic waveguides are fabricated with standard foundry processes and have a propagation loss as low as ∼0.28 dB/cm due to the reduced light-matter interaction at the waveguide sidewalls. In the experiments, strong FWM effect is achieved with a high conversion efficiency of −8.52 dB in a 2-µm-wide and 20-cm-long silicon photonic waveguide spiral, and eight new wavelengths are generated with the pump power of ∼80 mW (corresponding to a low power density of ∼195 mW/µm2). In contrast, the FWM efficiency for the 0.45-µm-wide waveguide spiral is around −15.4 dB, which is much lower than that for the 2-µm-wide waveguide spiral. It can be seen that silicon photonics beyond the singlemode regime opens a new avenue for on-chip nonlinear photonics and will bring new opportunities for nonlinear photonic applications.

A dual-polarization waveguide superlattice is designed and realized by using 340 nm-thick silicon photonic waveguides. The silicon waveguide superlattices are formed with periodically arranged waveguides. Each period consists of five optical waveguides with core-widths designed optimally for minimizing the crosstalk among the optical waveguides. The optimized core-widths are 390 nm, 320 nm, 260 nm, 360 nm, and 300 nm when the separation between two adjacent waveguides is as small as 0.8 µm. With this design, the silicon waveguide superlattice works with low crosstalk (nearly –18 dB or less) for both polarizations within the range of 1530 nm to 1560 nm, which agrees well with the theoretical analysis.

We propose and theoretically demonstrate an integrated polarization beam splitter on the x-cut lithium-niobate-on-insulator (LNOI) platform. The device is based on a Mach-Zehnder interferometer with an anisotropy-engineered multi-section phase shifter. The phase shift can be simultaneously controlled for the TE and TM polarizations by engineering the length and direction of the anisotropic LNOI waveguide. For TE polarization, the phase shift is −π/2, while for TM polarization, the phase shift is π/2. Thus, the incident TE and TM modes can be coupled into different output ports. The simulation results show an ultra-high polarization extinction ratio of ∼47.7 dB, a low excess loss of ∼0.9 dB and an ultra-broad working bandwidth of ∼200 nm. To the best of our knowledge, the proposed structure is the first integrated polarization beam splitter on the x-cut LNOI platform.

An ultra-sharp multimode waveguide bend (MWB) based on a multimode waveguide corner-bend (MWCB) is proposed and realized. With the present MWCB, total internal reflection (TIR) happens and the light propagation direction of all the mode-channels can be modified with low excess losses (ELs) and low inter-mode crosstalk (CT) in the optical communication bands from 1260 nm to 1680 nm. For the MWCB designed for the TE0 and TE1 modes, the ELs are less than 0.18 dB and the inter-mode CTs are less than −36 dB in the wavelength range of 1260-1680 nm. The measurement results show the fabricated MWCB works very well as predicted by the theory. It is very flexible to extend the present MWCB for more mode-channels by simply adjusting the core width. For example, the MWCB designed with a 35 µm-wide core has an EL less than 0.54 dB and inter-mode CT less than −24 dB for the ten TE-polarization modes (i.e., TE0∼TE9) in the wavelength-band of 1260-1680 nm. For the present MWCB, the fabrication is also very convenient because no tiny nano-structure and no additional fabrication steps are needed. It also shows that the present MWCB is not sensitive to the sidewall angles even when the angle is up to 8°. The proposed MWCB is promising for multimode silicon photonics because of the simple structure, easy design, easy fabrication as well as excellent performances in an ultra-broad wavelength-band.

The O-band coarse wavelength-division (de)multiplexing (CWDM) has been extensively used in data-center optical communications, whereas it’s still challenging to reduce crosstalk and enhance fabrication tolerances for a CWDM filter. In this paper, we propose and experimentally demonstrate a low-crosstalk and fabrication-tolerant four-channel CWDM filter by utilizing dispersion-engineered Mach-Zehnder interferometers. The multi-sectional phase shifters are exploited to eliminate the phase errors induced by width deviations, leading to ultra-precise phase shifts and ultra-large width-error tolerances. The random-phase errors are also inhibited by using multi-mode waveguides as phase-shifting sections. The two-stage-coupler scheme is utilized to flatten the strong coupling-ratio dispersions for directional couplers, so that low crosstalk can be achieved over the whole O-band. The experimental results show both low insertion losses (< 1.2 dB) and low crosstalk (< −22.2 dB) over the whole working wavelength range. The measured width-error tolerance is also as large as ≈ 70 nm.

It is still challenging to realize an on-chip optical sensor that can detect humidity and temperature at the same time. In this paper, we demonstrate a silicon-based dual-polarization micro-ring resonator (MRR) with a polyvinyl-alcohol (PVA) upper-cladding, which enables the simultaneous measurement of humidity and temperature. Due to the significant polarization-dependence of the silicon-on-insulator (SOI) nanophotonic waveguide, the transverse electric (TE) and transverse magnetic (TM) polarization modes have quite different sensitivities towards the changes of ambient relative humidity (RH) and temperature. Sensitivity, resolution, stability and cross-sensitivity are analyzed for the present dual-parameter sensor. The RH and temperature response sensitivity are measured to be 97.9 pm/%RH, 325.1 pm/%RH, 69.0 pm/°C and 30.6 pm/°C for TE and TM polarizations, respectively. To the best of our knowledge, it is the first on-chip optical sensor enabling the simultaneous measurement of RH and temperature.

Optical communication wavelength is being extended from the near-infrared band of 1.31/1.55 µm to the mid-infrared band of 2 µm or beyond for satisfying the increasing demands for high-capacity long-distance data transmissions. An efficient electro-optic (EO) modulator working at 2 µm is highly desired as one of the indispensable elements for optical systems. Lithium niobate (LiNbO3) with a large second-order nonlinear coefficient is widely used in various EO modulators. Here, we experimentally demonstrate the first Mach-Zehnder EO modulator working at 2 µm based on the emerging thin-film LiNbO3 platform. The demonstrated device exhibits a voltage-length product of 3.67 V·cm and a 3-dB-bandwidth of >22 GHz which is limited by the 18 GHz response bandwidth of the photodetector available in the lab. Open eye-diagrams of the 25 Gb/s on-off keying (OOK) signals modulated by the fabricated Mach-Zehnder EO modulator is also measured experimentally with a SNR of about 14 dB.

Graphene has emerged as a promising solution for on-chip ultrafast photodetection for its advantages of easy integration, high mobility, adjustable chemical potential, and wide operation wavelength range. In order to realize high-performance photodetectors, it is very important to achieve efficient light absorption in the active region. In this work, a compact and high-speed hybrid silicon/graphene photodetector is proposed and demonstrated by utilizing an ultra-thin silicon photonic waveguide integrated with a loop mirror. With this design, the graphene absorption rate for the fundamental mode of TE polarization is improved by ∼5 times compared to that in the conventional hybrid silicon/graphene waveguide with hco=220 nm. One can achieve 80% light absorption ratio within the active-region length of only 20 µm for the present silicon/graphene waveguide photodetector at 1550 nm. For the fabricated device, the responsivity is about 25 mA/W under 0.3V bias voltage and the 3-dB bandwidth is about 17 GHz. It is expected to achieve very high bandwidth by introducing high-quality Al2O3 insulator layers and reducing the graphene channel length in the future.

A low-loss and low-crosstalk multimode waveguide bend is proposed and demonstrated for mode-division-multiplexed optical interconnects. The proposed 90°-bend is composed of two identical 45°-bends, which are defined as modified Euler curves. For the designed 90° Euler-bend with a core width of 2.36 μm for supporting four TM-polarization modes, it is allowed to achieve an effective radius as small as 45 μm, which is about 1/4 of the radius (~175 μm) for a regular 90° arc-bend. In theory, this proposed 90° Euler-bend has very low excess losses (

A high-performance silicon-based polarizing beam splitter (PBS) is proposed and demonstrated experimentally by using an improved structure with cascaded bent directional couplers. The measured extinction ratio (ER) is35dB and the excess losses (EL) is0.35dB around the central wavelength for both polarizations. The present PBS has a compact footprint of ~6.9 × 20μmsup2/sup. The measured bandwidths for an ER of20dB,25dB and30dB are ~135nm, ~95nm and ~70nm, respectively, while the measured EL is1dB and0.5dB in a bandwidth of ~140nm and ~85nm, respectively. The fabrication tolerance of the core-width variation is as large as ± 40nm, which makes the fabrication very easy.

A silicon-based polarization-insensitive optical filter is proposed and demonstrated. For the present on-chip polarization-insensitive optical filter, there is a dual-polarization mode (de)multiplexer, a TE-type multimode waveguide grating (MWG) with triangular corrugations and a TM-type MWG with rectangular corrugations. Here the triangular corrugations are introduced to reduce the undesired reflection and suppress the Fabry-Parot resonance. Furthermore, lateral-shift apodization is introduced for both two types of MWGs to suppress the sidelobes. For the fabricated device, the measured 3 dB-bandwidth is as large as ∼11 nm and the excess loss is ∼1.5 dB for both polarizations, while the sidelobe suppression ratios are 23 dB and 17 dB for TE and TM polarizations, respectively.

A low-loss add-drop microring resonator (MRR) with an ultra-large free spectral range (FSR) is demonstrated by introducing an ultra-sharp multimode waveguide bend and bent asymmetrical directional couplers (ADCs). The multimode microring waveguide is introduced to achieve a low bent loss, even with a small radius (e.g., R = 0.8 μm). The bent ADCs are used to suppress the resonance of higher-order modes. For the fabricated device, the transmission at the drop port has a narrow 3 dB-bandwidth of 0.8 nm and a low excess loss of 1.8 dB. A record large FSR of 93 nm is achieved to the best of our knowledge.

The mode property and light propagation in a tapered silicon-on-insulator (SOI) nanowire with angled sidewalls is analyzed. Mode hybridization is observed and mode conversion between the TM fundamental mode and higher-order TE modes happens when light propagates in a waveguide taper which is used very often in the design of photonic integrated devices. This mode conversion ratio is possible to be very high (even close to 100%) when the taper is long enough to be adiabatic, which might be useful for some applications of multimode photonics. When the mode conversion is undesired to avoid any excess loss as well as crosstalk for photonic integrated circuits, one can depress the mode conversion by compensating the vertical asymmetry in the way of reducing the sidewall angle or introducing an optimal refractive index for the upper-cladding. It is also possible to eliminate the undesired mode conversion almost and improve the desired mode conversion greatly by introducing an abrupt junction connecting two sections with different widths to jump over the mode hybridization region.

Third-harmonic generation can be realized via both $\chi^{(3)}$ and cascaded $\chi^{(2)}$ nonlinear processes in a triply-resonant microcavity. It is still unknown how these processes interfere with each other and the optimization of the conversion efficiency still remains as a question. In this work, the interplay between the direct third-harmonic generation and the cascaded process combining of the second-harmonic generation and the sum-frequency generation are investigated. It is found that the interference effect between these two processes can be used to improve the conversion efficiency. By optimizing the cavity resonance and the external coupling conditions, the saturation of the nonlinear conversion is mitigated and the third-harmonic conversion efficiency is increased. A design rule is provided for achieving efficient third-harmonic generation in an optical microcavity, which can be generalized further to the high-order harmonic generations., Comment: 7 pages, 4 figures

A compact silicon hybrid (de)multiplexer is designed and demonstrated by integrating a single bi-directional AWG with a polarization diversity circuit, which consists of an ultra-short polarization-beam splitter (PBS) based on a bent coupler and a polarization rotator (PR) based on a silicon-on-insulator nanowire with a cut corner. The present hybrid (de)multiplexer can operate for both TE- and TM- polarizations and thus is available for PDM-WDM systems. An 18-channel hybrid (de)multiplexer is realized with 9 wavelengths as an example. The wavelength-channel spacing is 400GHz (i.e., Δλ(ch) = 3.2nm) and the footprint of the device is about 530μm × 210μm. The channel crosstalk is about -13dB and the total excess loss is about 7dB. The excess loss increases by about 1~2dB due to the cascaded polarization diversity circuit in comparison with a single bi-directional AWG.

A theoretical analysis is given for the optical forces induced by the Bloch mode propagating along a silicon subwavelength-grating (SWG) waveguide for the first time. As a periodical structure, an SWG waveguide supports periodical light field distribution along the waveguide. This makes it possible to trap many nano-particles stably periodically, which is very different from the case with a conventional optical waveguide. The separation of the trapped nano-particles can be designed easily by modifying the grating period of an SWG waveguide. Furthermore, an SWG waveguide has larger working distance in the lateral direction to trap nano-particles around the waveguide than a conventional optical waveguide.

Polarization-selective microring resonators (MRRs) are designed and demonstrated to work with resonances for only one of TE and TM polarizations for the first time, which can be used for realizing the crossing-free, compact and scalable multi-wavelength coherent receiver with dual polarizations. The present polarization-selective MRRs are realized by manipulating the polarization-dependence of the bending loss of the micro-resonators and the coupling ratio of the bent directional couplers introduced here. The demonstrated TM-type MRR and the TE-type MRR work well as a wavelength-selective optical filter for TM polarization and TE polarization, respectively, as designed. On the other hand, the resonance of the undesired orthogonal polarization mode is depressed significantly with very high extinction ratio. For the realized TM-type MRR, the peak-transmission at the drop port is depressed to be lower than −50dB and the loss of the transmission at the through port is very low (

A vertical multimode waveguide enabling micro-bends is proposed for mode-multiplexed optical interconnect links. The multimode waveguide is designed to be singlemode in the lateral direction and support higher-order modes in the vertical direction. The characteristic analysis for an SOI (silicon-on-insulator)-based vertical multimode waveguide with a ~0.3μm × ~1.5μm cross section is given as an example. The theoretical pure bending loss is negligible for all the lowest eight modes when the bending radius is even less than 5μm. When light goes through the structure consisting of a straight section connected with a bent section, it is found that some inter-mode crosstalk is caused by the significant mode hybridization happening in the sharply bent multimode waveguide. For the designed SOI-based vertical multimode waveguide, the inter-mode crosstalk is lower than −20dB even when the bending radius is chosen as small as R = 10μm, which is one order smaller than that for the traditional lateral multimode waveguide (whose minimal bending radius is about 130μm). The inter-mode crosstalk can be even reduced to −30dB when choosing R = 30μm. Such a multimode optical waveguide microbend with low inter-mode crosstalk is promising for realizing compact mode-multiplexing links.

A theoretical investigation of a nano-scale hybrid plasmonic waveguide with a low-index as well as high-index gain medium is presented. The present hybrid plasmonic waveguide structure consists of a Si substrate, a buffer layer, a high-index dielectric rib, a low-index cladding, a low-index nano-slot, and an inverted metal rib. Due to the field enhancement in the nano-slot region, a gain enhancement is observed, i.e., the ratio ∂G/∂g1, where g and G are the gains of the gain medium and the TM fundamental mode of the hybrid plasmonic waveguide, respectively. For a hybrid plasmonic waveguide with a core width of w(co)=30nm and a slot height of h(slot)=50nm, the intrinsic loss could be compensated when using a low-index medium with a moderate gain of 176dB/cm. When introducing the high-index gain medium for the hybrid plasmonic waveguide, a higher gain is obtained by choosing a wider core width. For the high-index gain case with h(slot)=50nm and w(co)=500nm, a gain of about 200dB/cm also suffices for the compensation of the intrinsic loss.

We characterize an approach to make ultra-low-loss waveguides using stable and reproducible stoichiometric Si3N4 deposited with low-pressure chemical vapor deposition. Using a high-aspect-ratio core geometry, record low losses of 8-9 dB/m for a 0.5 mm bend radius down to 3 dB/m for a 2 mm bend radius are measured with ring resonator and optical frequency domain reflectometry techniques. From a waveguide loss model that agrees well with experimental results, we project that 0.1 dB/m total propagation loss is achievable at a 7 mm bend radius with this approach.

Nano-scale power splitters based on Si hybrid plasmonic waveguides are designed by utilizing the multimode interference (MMI) effect as well as Y-branch structure. A three-dimensional finite-difference time-domain method is used for simulating the light propagation and optimizing the structural parameters. The designed 1×2 50:50 MMI power splitter has a nano-scale size of only 650 nm×530 nm. The designed Y-branch power splitter is also very small, i.e., about 900 nm×600 nm. The fabrication tolerance is also analyzed and it is shown that the tolerance of the waveguide width is much larger than±50 nm. The power splitter has a very broad band of over 500 nm. In order to achieve a variable power splitting ratio, a 2×2 two-mode interference coupler and an asymmetric Y-branch are used and the corresponding power splitting ratio can be tuned in the range of 97.1%:2.9%-1.7%:98.3% and 84%:16%-16%:84%, respectively. Finally a 1×4 power splitter with a device footprint of 1.9 μm×2.6 μm is also presented using cascaded Y-branches.

A new way to make broadband polarizers on silicon-on-insulator (SOI) waveguides is proposed, analyzed and characterized. The characteristics of the eigenmodes in a shallowly-etched SOI ridge optical waveguide are analyzed by using a full-vectorial finite-different method (FV-FDM) mode solver. The theoretical calculation shows that the loss of TE fundamental mode could be made very low while at the same time the TM fundamental mode has very large leakage loss, which is strongly dependent on the trench width. The leakage loss of the TM fundamental mode changes quasi-periodically as the trench width w(tr) varies. The formula of the period ∆w(tr) is given. By utilizing the huge polarization dependent loss of this kind of waveguide, a compact and simple optical polarizer based on a straight waveguide was demonstrated. The polarizer is fabricated on a 700 nm-thick SOI wafer and then characterized by using a free-space optical system. The measured extinction ratio is as high as 25 dB over a 100 nm wavelength range for a 1 mm-long polarizer.

In this work the impedance of separate-absorption-charge-multiplication Ge/Si avalanche photodiodes (APD) is characterized over a large range of bias voltage. An equivalent circuit with an inductive element is presented for modeling the Ge/Si APD. All the parameters for the elements included in the equivalent circuit are extracted by fitting the measured S(22) with the genetic algorithm optimization. Due to a resonance in the avalanche region, the frequency response of the APD has a peak enhancement when the bias voltage is relatively high, which is observed in the measurement and agrees with the theoretical calculation shown in this paper.

A hybrid plasmonic waveguide with a metal cap on a silicon-on-insulator rib (or slab) is presented. There is a low-index material nano-layer between the Si layer and the metal layer. The field enhancement in the nano-layer provides a nano-scale confinement of the optical field (e.g., 50 nm x 5 nm) when operates at the optical wavelength lambda = 1550 nm. The theoretical investigation also shows that the present hybrid plasmonic waveguide has a low loss and consequently a relatively long propagation distance (on the order of several tens of lambda).

An inverse taper on silicon is proposed and designed to realize an efficient mode converter available for the connection between multimode silicon nanophotonic integrated circuits and few-mode fibers. The present mode converter has a silicon-on-insulator inverse taper buried in a 3 × 3μm(2) SiN strip waveguide to deal with not only for the fundamental mode but also for the higher-order modes. The designed inverse taper enables the conversion between the six modes (i.e., TE(11), TE(21), TE(31), TE(41), TM(11), TM(12)) in a 1.4 × 0.22μm(2) multimode SOI waveguide and the six modes (like the LP(01), LP(11a), LP(11b) modes in a few-mode fiber) in a 3 × 3μm(2) SiN strip waveguide. The conversion efficiency for any desired mode is higher than 95.6% while any undesired mode excitation ratio is lower than 0.5%. This is helpful to make multimode silicon nanophotonic integrated circuits (e.g., the on-chip mode (de)multiplexers developed well) available to work together with few-mode fibers in the future.

A Mach-Zehnder Interferometer (MZI) liquid sensor, employing ultra-compact double-slot hybrid plasmonic (DSHP) waveguide as active sensing arm, is developed. Numerical results show that extremely large optical confinement factor of the tested analytes (as high as 88%) can be obtained by DSHP waveguide with optimized geometrical parameters, which is larger than both, conventional SOI waveguides and plasmonic slot waveguides with same widths. As for MZI sensor with 40μm long DSHP active sensing area, the sensitivity can reach as high value as 1061nm/RIU (refractive index unit). The total loss, excluding the coupling loss of the grating coupler, is around 4.5dB.

An improved 8-channel silicon mode demultiplexer is realized with TE-type and TM-type grating polarizers at the output ends, and these gratings serve as fiber-chip couplers simultaneously. The present 8-channel silicon mode demultiplexer includes a three-waveguide PBS (for separating the TE0 and TM0 modes) and six cascaded ADCs (for demultiplexing the high-order modes of both polarizations). The grating polarizers with high extinction ratios are used to filter out the polarization crosstalk in the 8-channel hybrid multiplexer efficiently and the measured crosstalk for all the mode-channels of the improved 8-channel mode multiplexer is reduced greatly to ~-20dB in a ~100nm bandwidth.

Ultra-compact EO polymer modulators based on hybrid plasmonic microring resonators are proposed, simulated and analyzed. Comparing with Si slot microring modulator, hybrid plasmonic microring modulator shows about 6-times enhancement of the figure of merit when the bending radius is around 510 nm, due to its much larger intrinsic quality factor in sub-micron radius range. Influences of the EO polymer height and Si height on the device's performance are analyzed and optimal design is given. When operating with a bias of 3.6 V, the proposed device has optical modulation amplitude of 0.8 and insertion loss of about 1 dB. The estimated power consumption is about 5 fJ/bit at 100 GHz.

Novel suspended SiO(2) ridge optical waveguides on silicon are fabricated and characterized. The present suspended SiO(2) ridge optical waveguide has a SiO(2) ridge core surrounded by air. The propagation loss and the bend loss measured are about 0.385dB/cm and 0.037dB/90° respectively for the fabricated 1 μm-wide waveguides with a bending radius of 100 μm when operating at the wavelength of 1550 nm. With the present suspended SiO(2) optical waveguides, a small racetrack resonator with a radius of 100 μm is also demonstrated and the measured Q-factor is about 3160.

A 16-channel 200 GHz arrayed-waveguide grating (AWG) (de)-multiplexer is demonstrated experimentally by utilizing Si3N4 buried optical waveguides, which have 50 nm-thick Si3N4 cores and a 15 μm-thick SiO2 cladding. The structure with an ultra-thin core layer helps to reduce the scattering due to the sidewall roughness and consequently shows very low loss of about 0.4~0.8 dB/m. When using this type of optical waveguide for an AWG (de)multiplexer, there is no problem associated with gap refill using the upper-cladding material even when choosing a small (e.g., 1.0 μm) gap between adjacent arrayed waveguides, which helps to reduce the transition loss between the FPR (free-propagation region) and the arrayed waveguides. Therefore, the demonstrated AWG (de)multiplexer based on the present Si3N4 buried optical waveguides has a low on-chip loss. The fabricated AWG (de)multiplexer is characterized in two wavelength ranges around 1310 nm and 1550 nm, respectively. It shows that the crosstalk from adjacent and non-adjacent channels are about -30 dB, and -40 dB, respectively, at the wavelength range of 1310 nm. The Si3N4 AWG (de)multiplexer has a temperature dependence of about 0.011 nm/°C, which is close to that of a pure SiO2 AWG device.

A novel concept for an ultracompact polarization splitter-rotator is proposed by utilizing a structure combining an adiabatic taper and an asymmetrical directional coupler. The adiabatic taper structure is singlemode at the input end while it becomes multimode at the other end. When light propagates along the adiabatic taper structure, the TM fundamental mode launched at the narrow end is efficiently (close to 100%) converted to the first higher-order TE mode at the wide end because of the mode coupling between them. By using an asymmetrical directional coupler that has two adjacent waveguides with different core widths, the first higher-order TE mode is then coupled to the TE fundamental mode of the adjacent narrow waveguide. On the other hand, the input TE polarization does not change when it goes through the adiabatic taper structure. In the region of the asymmetrical directional coupler, the TE fundamental mode in the wide waveguide is not coupled to the adjacent narrow waveguide because of phase mismatch. In this way, TE- and TM- polarized light are separated while the TM fundamental mode is also converted into the TE fundamental mode. A design example of the proposed polarization splitter-rotator is given by using silicon-on-insulator nanowires and the total length of the device is less than 100μm. Furthermore, only a one-mask process is needed for the fabrication process, which is compatible with the standard fabrication for the regular photonic integrated circuits based on SOI nanowires.

A hybrid plasmonic waveguide with double low-index nano-slots is introduced. The fabrication is simple and compatible with the standard processes for SOI wafers. The theoretical investigation shows that the present hybrid plasmonic waveguide has a low loss and consequently a relatively long propagation distance (at the order of several tens of lambda). For TE polarization, there is a strong field enhancement in the double nano-slots. More power is confined in the low-index nano-slots for a smaller core width. For a 50 nm-wide hybrid plasmonic waveguide with double 10 nm-wide slots, the power confinement factor in the nano-slots is as high as 85% and the effective area is as small as 0.007 microm(2) at 1550 nm. Consequently, the power density in the nano-slots becomes very high, e.g.,120 microm(-2), which is very desired for many applications. For the present hybrid plasmonic waveguide, the lateral dimension could be less than 50 nm and the calculated decoupled separation for two parallel identical waveguides is only 0.62 microm, which is helpful to realize photonic integration circuits with ultra-high integration density.

A digital optical sensor based on two cascaded rings with different free spectral ranges (FSRs) is proposed. Because of their different FSRs, the major peak of the spectral response from the output port shifts digitally when the effective refractive index of ring #1 changes. And the shift of the major peak is equal to multiple FSRs of ring #2. Since it is easy to design a ring with a FSR of nanometers, the present digital optical sensor shows an ultra-high sensitivity (at the order of 10(5) nm/RIU) which is over two orders higher than that of a regular single-ring sensor. By using the present digital optical sensor, it becomes convenient to use an integrated optical micro-spectrometer (even with a low resolution) to monitor the peak shift of the spectral response. Therefore, it is promising to realize a low-cost and portable highly-sensitive optical sensor system on a single chip.

A novel layout is presented to minimize the size of an arrayed-waveguide grating (AWG) demultiplexer based on Si nanowire waveguides, in particular when a high diffraction order is required. A series of microbends are inserted in the middle of arrayed waveguides to increase the lightpath difference while keeping small separation between arrayed waveguides. A designed ultrasmall AWG with a narrow channel spacing of 0.8 nm has a total size of only about 0.505 mmx0.333 mm (0.165 mm(2)).