Automatic Categorization and Scoring of Solid, Part-Solid and Non-Solid Pulmonary Nodules in CT Images with Convolutional Neural Network.

We present a computer-aided diagnosis system (CADx) for the automatic categorization of solid, part-solid and non-solid nodules in pulmonary computerized tomography images using a Convolutional Neural Network (CNN). Provided with only a two-dimensional region of interest (ROI) surrounding each nodule, our CNN automatically reasons from image context to discover informative computational features. As a result, no image segmentation processing is needed for further analysis of nodule attenuation, allowing our system to avoid potential errors caused by inaccurate image processing. We implemented two computerized texture analysis schemes, classification and regression, to automatically categorize solid, part-solid and non-solid nodules in CT scans, with hierarchical features in each case learned directly by the CNN model. To show the effectiveness of our CNN-based CADx, an established method based on histogram analysis (HIST) was implemented for comparison. The experimental results show significant performance improvement by the CNN model over HIST in both classification and regression tasks, yielding nodule classification and rating performance concordant with those of practicing radiologists. Adoption of CNN-based CADx systems may reduce the inter-observer variation among screening radiologists and provide a quantitative reference for further nodule analysis.

MR image segmentation and bias field estimation based on coherent local intensity clustering with total variation regularization.

Though numerous segmentation algorithms have been proposed to segment brain tissue from magnetic resonance (MR) images, few of them consider combining the tissue segmentation and bias field correction into a unified framework while simultaneously removing the noise. In this paper, we present a new unified MR image segmentation algorithm whereby tissue segmentation, bias correction and noise reduction are integrated within the same energy model. Our method is presented by a total variation term introduced to the coherent local intensity clustering criterion function. To solve the nonconvex problem with respect to membership functions, we add auxiliary variables in the energy function such as Chambolle's fast dual projection method can be used and the optimal segmentation and bias field estimation can be achieved simultaneously throughout the reciprocal iteration. Experimental results show that the proposed method has a salient advantage over the other three baseline methods on either tissue segmentation or bias correction, and the noise is significantly reduced via its applications on highly noise-corrupted images. Moreover, benefiting from the fast convergence of the proposed solution, our method is less time-consuming and robust to parameter setting.

Integrated photonics with programmable non-volatile memory.

Silicon photonics integrated circuits (Si-PIC) with well-established active and passive building elements are progressing towards large-scale commercialization in optical communications and high speed optical interconnects applications. However, current Si-PICs do not have memory capabilities, in particular, the non-volatile memory functionality for energy efficient data storage. Here, we propose an electrically programmable, multi-level non-volatile photonics memory cell (PMC) fabricated by standard complementary-metal-oxide-semiconductor (CMOS) compatible processes. A micro-ring resonator (MRR) was built using the PMC to optically read the memory states. Switching energy smaller than 20 pJ was achieved. Additionally, a MRR memory array was employed to demonstrate a four-bit memory read capacity. Theoretically, this can be increased up to ~400 times using a 100 nm free spectral range broadband light source. The fundamental concept of this design provides a route to eliminate the von Neumann bottleneck. The energy-efficient optical storage can complement on-chip optical interconnects for neutral networking, memory input/output interfaces and other computational intensive applications.

Analysis of the polarization rotation effect in the inversely tapered spot size converter.

Inversely tapered spot size converter (SSC) is widely used to connect silicon waveguide with fiber in silicon photonics. However, the tapered structure may cause polarization rotation and further generate interference fluctuation in the transmission spectrum even of a straight waveguide. We analyzed the light propagation in a straight waveguide with SSC at the both ends with coupling matrix and transmission matrix methods. The analysis results matched with the phenomena we observed in the transmission spectrum. Combining the analysis with the measurement results, we calculated the polarization rotation efficiency of the SSC in different samples and analyzed the origin of the polarization rotation effect. Finally, we discussed the influence of the effect to the DP-QPSK signal and proposed several methods to release the impact.

Mach-Zehnder interferometer (MZI) point-of-care system for rapid multiplexed detection of microRNAs in human urine specimens.

MicroRNAs have been identified as promising biomarkers for human diseases. The development of a point-of-care (POC) test for the disease-associated miRNAs would be especially beneficial, since miRNAs are unexpectedly well preserved in various human specimens, including urine. Here, we present the Mach-Zehnder interferometer-miRNA detection system capable of detecting multiple miRNAs in clinical urine samples rapidly and simultaneously in a label-free and real-time manner. Through measurement of the light phase change, the MZI sensor provides an optical platform for fast profiling of small molecules with improved accuracy. We demonstrate that this system could specifically detect target miRNAs (miR-21, and let-7a), and even identify the single nucleotide polymorphism of the let-7 family of miRNAs from synthetic and cell line samples. The clinical applicability of this system is confirmed by simultaneously detecting two types of miRNAs in urine samples of bladder cancer patients in a single reaction, with a detection time of 15 min. The POC system can be expanded to detect a number of miRNAs of different species and should be useful for a variety of clinical applications requiring at or near the site of patient care.

Low-loss partial rib polarization rotator consisting only of silicon core and silica cladding.

We present a low-loss and small-footprint polarization rotator based on mode evolution. The polarization rotator is composed of an asymmetric-rib waveguide and a tapered waveguide, both of which consist only of a silicon core and a silica cladding. The rotator is fabricated under the same design rules as other device blocks, such as rib-waveguide phase shifters for photonic integration. The polarization rotator is fabricated using CMOS-based processes and provides polarization rotations with an on-chip insertion loss lower than 0.5 dB from transverse-electric (TE) to transverse-magnetic (TM) polarization and a loss lower than 1.0 dB from the TM to TE polarization in a 200 nm wavelength range extending over C and L bands.

Microring resonator photodetector for enhancement in L-band performance.

We proposed a microring resonator (MRR) enhanced photodetector (PD) structure. Resonance wavelength enhanced by the MRR amplifies the PD response. At L-band wavelengths, responsivity was doubled for an ultra-short germanium PD of 4 µm employing the MRR structure. Data rates of up to 40 Gb/s were also demonstrated at 1600 nm.

Efficient silicon nitride grating coupler with distributed Bragg reflectors.

In this paper we have designed, fabricated and characterized a high efficiency Silicon nitride grating coupler at 1490 nm. Distributed Bragg reflectors as bottom mirrors are employed to improve the coupling efficiency by reflecting the downward traveling light. The peak coupling efficiency obtained is about -2.5 dB and the 1-dB bandwidth is 53 nm. The fabrication process is CMOS-compatible and is ready to be integrated with photonic circuits.

Mode size converter between high-index-contrast waveguide and cleaved single mode fiber using SiON as intermediate material.

High-index-contrast (HIC) waveguide such as Si and Si3N4 has small mode size enabling compact integration. However, the coupling loss with single mode fiber is also remarkable owning to the mode mismatching. Therefore, mode size converter, as the interface between HIC waveguide and optical fiber, takes an important role in the field of integrated optics. The material with refractive index (RI) between HIC waveguide and optical fiber can be used as a bridge to reduce the mode mismatching loss. In this letter, we employ silicon oxynitride (SiON) with RI about 1.50 as the intermediate material and optimize the structure of the SiON waveguide to match with cleaved single mode fiber and HIC waveguide separately. Combined with inverse taper and suspended structure, the mismatching loss is reduced and the dependence to the dimension of the structure is also released. The coupling loss is 1.2 and 1.4 dB/facet for TE and TM mode, respectively, with 3 dB alignment tolerance of ± 3.5 µm for Si(3)N(4) waveguide with just 200 nm-wide tip. While for Si waveguide, a critical dimension of 150 nm is applied due to the higher index contrast than Si(3)N(4) waveguide. Similar alignment tolerance is realized with coupling loss about 1.8 and 2.1 dB/facet for TE and TM mode. The polarization dependence loss (PDL) for both platforms is within 0.5 dB.

Silicon optical modulator with shield coplanar waveguide electrodes.

A silicon Mach-Zehnder Interferometer (MZI) optical modulator with a shield coplanar waveguide (CPW) transmission line electrode design was demonstrated. This shield-CPW electrode suppresses the signal distortion caused by the parasitic slot-line (SL) mode and improves the electrical bandwidth and the electro-optical (EO) bandwidth. With the shield-CPW electrodes and 5.5 mm-long phase shifters, the silicon MZI optical modulator delivered an EO bandwidth of above 24 GHz and a V (π) = 3.0 V was achieved at λ = 1310 nm. When modulated at 28-Gb/s data rate, it achieved an extinction ratio of 5.66 dB under a driving voltage of V (pp) = 1.3 V, corresponding to a power consumption of 0.8 pJ/bit.

Loop coupled resonator optical waveguides.

We propose a novel coupled resonator optical waveguide (CROW) structure that is made up of a waveguide loop. We theoretically investigate the forbidden band and conduction band conditions in an infinite periodic lattice. We also discuss the reflection- and transmission- spectra, group delay in finite periodic structures. Light has a larger group delay at the band edge in a periodic structure. The flat band pass filter and flat-top group delay can be realized in a non-periodic structure. Scattering matrix method is used to calculate the effects of waveguide loss on the optical characteristics of these structures. We also introduce a tunable coupling loop waveguide to compensate for the fabrication variations since the coupling coefficient of the directional coupler in the loop waveguide is a critical factor in determining the characteristics of a loop CROW. The loop CROW structure is suitable for a wide range of applications such as band pass filters, high Q microcavity, and optical buffers and so on.

Three-dimensional (3D) monolithically integrated photodetector and WDM receiver based on bulk silicon wafer.

We propose a novel three-dimensional (3D) monolithic optoelectronic integration platform. Such platform integrates both electrical and photonic devices in a bulk silicon wafer, which eliminates the high-cost silicon-on-insulator (SOI) wafer and is more suitable for process requirements of electronic and photonic integrated circuits (ICs). For proof-of-concept, we demonstrate a three-dimensional photodetector and WDM receiver system. The Ge is grown on a 8-inch bulk silicon wafer while the optical waveguide is defined in a SiN layer which is deposited on top of it, with ~4 µm oxide sandwiched in between. The light is directed to the Ge photodetector from the SiN waveguide vertically by using grating coupler with a Aluminum mirror on top of it. The measured photodetector responsivity is ~0.2 A/W and the 3-dB bandwidth is ~2 GHz. Using such vertical-coupled photodetector, we demonstrated an 8-channel receiver by integrating a 1 × 8 arrayed waveguide grating (AWG). High-quality optical signal detection with up to 10 Gbit/s data rate is demonstrated, suggesting a 80 Gbit/s throughput. Such receiver can be applied to on-chip optical interconnect, DRAM interface, and telecommunication systems.

Silicon-based traveling-wave photodetector array (Si-TWPDA) with parallel optical feeding.

We demonstrate silicon-based traveling-wave photodetector arrays (Si-TWPDAs) with parallel optical feeding by integrating multiple Germanium photodetectors. Such Si-TWPDAs feature the merit of high optical saturation power with remaining the large operation bandwidth. The impedance-matched traveling-wave electrode design takes into account the individual Ge photodetector loading effect. Optical waveguide delay lines are designed in order to balance the electrical phase delay of the traveling-wave electrode. The maximum linear photocurrent at -4V biased voltage are respectively 16 mA, 38 mA, and 65 mA with integrating 1, 2, and 4 photodetectors, upon the saturation power of 40 mW, 100 mW, and 160 mW. This corresponds to a normalized photocurrent generation of >0.32 mA/µm3 and a normalized saturation power of 0.8 mW/µm3. The extracted fiber access responsivity is ~0.42 A/W and the intrinsic responsivity of ~0.82 A/W. The measured 3-dB bandwidth for 4-channel TWPDA is ~20 GHz.

Low-loss high-speed silicon IQ modulator for QPSK/DQPSK in C and L bands.

A low-loss high-speed silicon in-phase (I) quadrature (Q) modulator is designed, fabricated and characterized. The fabricated IQ modulator has a low passive optical loss of 9 dB in C and L bands. Using the modulator, differential quadrature phase-shift keying (DQPSK) transmission at 44.6 Gb/s with differential detection is confirmed with an optical signal-to-noise ratio (OSNR) of 16.3 dB for a bit error rate (BER) of 10(-3) and a dispersion tolerance of -96 to 107 ps/nm. Moreover, in digital coherent detection, quadrature phase-shift keying (QPSK) up to 64 Gb/s are achieved with an OSNR of 11.6-11.8 dB for a BER of 10(-2) at 1530, 1550, and 1610 nm.

PN-type carrier-induced filter with modulatable extinction ratio.

We demonstrate the first PN-type carrier-induced silicon waveguide Bragg grating filter on a SOI wafer. The optical extinction ratio of this kind of filter can be efficiently modulated under both reverse and forward biases. The carrier-induced Bragg grating based on a PN junction is fabricated on the silicon waveguide using litho compensation technology. The measured optical bandwidth and the extinction ratio of the filter are 0.45 nm and 19 dB, respectively. The optical extinction ratio modulation under the reverse bias is more than 11.5 dB and it is more than 10 dB under the forward bias. Only 1-dB optical transmission loss is realized in this Bragg grating under a reverse bias. The shifting rates of the central wavelength under forward and reverse biases are ~-1.25 nm/V and 0.01 nm/V, respectively. The 3-dB modulation bandwidth of this filter is 5.1 GHz at a bias of -10 V.

High-efficiency Si optical modulator using Cu travelling-wave electrode.

We demonstrate a high-efficiency and CMOS-compatible silicon Mach-Zehnder Interferometer (MZI) optical modulator with Cu traveling-wave electrode and doping compensation. The measured electro-optic bandwidth at Vbias = -5 V is above 30 GHz when it is operated at 1550 nm. At a data rate of 50 Gbps, the dynamic extinction ratio is more than 7 dB. The phase shifter is composed of a 3 mm-long reverse-biased PN junction with modulation efficiency (Vπ·Lπ) of ~18.5 V·mm. Such a Cu-photonics technology provides an attractive potentiality for integration development of silicon photonics and CMOS circuits on SOI wafer in the future.

Demonstration of a vertical pin Ge-on-Si photo-detector on a wet-etched Si recess.

In this paper, we demonstrate a vertical pin Ge-on-Si photo-detector on a wet-etched Si recess on a SOI wafer. A 120 nm-deep Si recess is etched on the SOI wafer with a 340 nm-thick top Si layer by the TMAH solution. The measured results show that the responsivity is more than 0.60 A/W for TE polarization and is more than 0.65 A/W for TM polarization at 1550 nm wavelength. Compared to the photo-detector without the Si recess, the responsivities for both TE and TM polarizations are improved by ~10%. A low dark current of 170 nA is achieved at a bias voltage of -1 V. And, the 3 dB-bandwidth at a bias voltage of -3 V is 21.5 GHz. This approach can be used to improve the coupling and absorption for high responsivity of photo-detector while maintain its high speed on a thick SOI platform based on the simulation results.

Slope efficiency and spurious-free dynamic range of silicon Mach-Zehnder modulator upon carrier depletion and injection effects.

We investigate the performances of a silicon PN-junction Mach-Zehnder modulator for analog application. The slope efficiency and spurious-free dynamic range (SFDR) of such a modulator upon carrier depletion and carrier injection effects are characterized and compared. Input RF frequency-dependence measurements show that the depletion-type modulator is usually with ~20 dB ∙ Hz(2/3) higher SFDR comparing to the injection-type modulator, yet with an order-of-magnitude lower slope efficiency. For the depletion-type and injection-type modulators, the measured maximum SFDRs are respectively ~95 dB ∙ Hz(2/3) and 75 dB∙Hz(2/3), with maximum slope efficiency of 0.3 V(-1) and 8 V(-1<). We numerically model the SFDR by using the experimentally extracted effective refractive index change, which shows good agreement with the measurements.

Silicon-based optoelectronic integrated circuit for label-free bio/chemical sensor.

We demonstrate a silicon-based optoelectronic integrated circuit (OEIC) for label-free bio/chemical sensing application. Such on-chip OEIC sensor system consists of optical grating couplers for vertical light coupling into silicon waveguides, a thermal-tunable microring as a tunable filter, an exposed microring as an optical label-free sensor, and a Ge photodetector for a direct electrical readout. Different from the conventional wavelength-scanning method, we adopt low-cost broadband ASE light source, together with the on-chip tunable filter to generate sliced light source. The effective refractive index change of the sensing microring induced by the sensing target is traced by scanning the supplied electrical power applied onto the tracing microring, and the detected electrical signal is read out by the Ge photodetector. For bulk refractive index sensing, we demonstrate using such OEIC sensing system with a sensitivity of ~15 mW/RIU and a detection limit of 3.9 µ-RIU, while for surface sensing of biotin-streptavidin, we obtain a surface mass sensitivity of S(m) = ~192 µW/ng·mm(-2) and a surface detection limit of 0.3 pg/mm(2). The presented OEIC sensing system is suitable for point-of-care applications.