Dual-wavelength distributed feedback laser array based on four-phase-shifted sampled Bragg grating for terahertz generation.

We propose and experimentally demonstrate a dual-wavelength distributed feedback (DFB) laser array utilizing a four-phase-shifted sampled Bragg grating. By using this grating, the coupling coefficient is enhanced by approximately 2.83 times compared to conventional sampled Bragg gratings. The devices exhibit a stable dual-mode lasing achieved by introducing further π-phase shifts at 1/3 and 2/3 positions along the cavity. These devices require only one stage of lithography to define both the ridge waveguide and the gratings, mitigating issues related to misalignment between them. A dual-wavelength laser array has been fabricated with frequency spacings of 320 GHz, 500 GHz, 640 GHz, 800 GHz, and 1 THz. When integrated with semiconductor optical amplifiers, the output power of the device can reach 23.6 mW. Furthermore, the dual-wavelength lasing is maintained across a wide range of injection currents, with a power difference of <3 dB between the two primary modes. A terahertz (THz) signal has been generated through photomixing in a photoconductive antenna, with the measured power reaching 12.8 µW.

Millimeter-wave generation based on a monolithic dual-wavelength DFB laser with four phase-shifted sampled gratings and equivalent chirp technology.

A dual-wavelength DFB laser array based on four phase-shifted grating and equivalent chirp technology is first proposed, fabricated, and experimentally demonstrated. The dual-wavelength emitting is achieved by symmetrically introducing two π phase shifts into a chirped four phase-shifted sampled grating cavity. Meanwhile, the beating signal of the dual-wavelength output is stabilized by applying an electro-absorption modulator integrated at the rear of the cavity. Under different grating chirp rates, a series of RF signals from 66.8 GHz to 73.6 GHz with a linewidth of less than 210 kHz is obtained.

Sidewall grating slot waveguide microring resonator biochemical sensor.

Integrated microring resonator structures based on silicon-on-insulator (SOI) platforms are promising candidates for high-performance on-chip sensing. In this work, a novel sidewall grating slot microring resonator (SG-SMRR) with a compact size (5 µm center radius) based on the SOI platform is proposed and demonstrated experimentally. The experiment results show that the refractive index (RI) sensitivity and the limit of detection value are 620 nm/RIU and 1.4 × 10-4 RIU, respectively. The concentration sensitivity and minimum concentration detection limit are 1120 pm/% and 0.05%, respectively. Moreover, the sidewall grating structure makes this sensor free of free spectral range (FSR) limitation. The detection range is significantly enlarged to 84.5 nm in lab measurement, four times that of the FSR of conventional SMRRs. The measured Q-factor is 3.1 × 103, and the straight slot waveguide transmission loss is 24.2 dB/cm under sensing conditions. These results combined with the small form factor associated with a silicon photonics sensor open up applications where high sensitivity and large measurement range are essential.

Double slot micro ring resonators with inner wall angular gratings as ultra-sensitive biochemical sensors.

We simulate and demonstrate experimentally an inner-wall grating double slot micro ring resonator (IG-DSMRR) with a center slot ring radius of only 6.72 µm based on the silicon-on-insulator platform. This novel photonic-integrated sensor for optical label-free biochemical analysis boosts the measured refractive index (RI) sensitivity in glucose solutions to 563 nm/RIU with the limit of detection value being 3.7 × 10-6 RIU (refractive index units). The concentration sensitivity for sodium chloride solutions can reach 981 pm/%, with a minimum concentration detection limit of 0.02%. Using the combination of DSMRR and IG, the detection range is enlarged significantly to 72.62 nm, three times the free spectral range of conventional slot micro ring resonators. The measured Q-factor is 1.6 × 104, and the straight strip and double slot waveguide transmission losses are 0.9 dB/cm and 20.2 dB/cm, respectively. This IG-DSMRR combines the advantages of a micro ring resonator, slot waveguide, and angular grating and is highly desirable for biochemical sensing in liquids and gases offering an ultra-high sensitivity and ultra-large measurement range. This is the first report of a fabricated and measured double-slot micro ring resonator with an inner sidewall grating structure.

Asymmetric parameter enhancement in the split-ring cavity array for virus-like particle sensing.

Quantitative detection of virus-like particles under a low concentration is of vital importance for early infection diagnosis and water pollution analysis. In this paper, a novel virus detection method is proposed using indirect polarization parametric imaging method combined with a plasmonic split-ring nanocavity array coated with an Au film and a quantitative algorithm is implemented based on the extended Laplace operator. The attachment of viruses to the split-ring cavity breaks the structural symmetry, and such asymmetry can be enhanced by depositing a thin gold film on the sample, which allows an asymmetrical plasmon mode with a large shift of resonance peak generated under transverse polarization. Correspondingly, the far-field scattering state distribution encoded by the attached virus exhibits a specific asymmetric pattern that is highly correlated to the structural feature of the virus. By utilizing the parametric image sinδ to collect information on the spatial photon state distribution and far-field asymmetry with a sub-wavelength resolution, the appearance of viruses can be detected. To further reduce the background noise and enhance the asymmetric signals, an extended Laplace operator method which divides the detection area into topological units and then calculates the asymmetric parameter is applied, enabling easier determination of virus appearance. Experimental results show that the developed method can provide a detection limit as low as 56 vp/150µL on a large scale, which has great potential in early virus screening and other applications.

Stepped-height ridge waveguide MQW polarization mode converter monolithically integrated with sidewall grating DFB laser.

We report, to the best of our knowledge, the first demonstration of a 1555-nm stepped-height ridge waveguide polarization mode converter monolithically integrated with a sidewall grating distributed-feedback (DFB) laser using the identical epitaxial layer scheme. The device shows stable single longitudinal mode (SLM) operation with the output light converted from TE to TM polarization with an efficiency of >94% over a wide range of DFB injection currents (IDFB) from 140 mA to 190 mA. The highest TM mode purity of 98.2% was obtained at IDFB = 180 mA. A particular advantage of this device is that only a single step of metalorganic vapor-phase epitaxy and two steps of III-V material dry etching are required for the whole integrated device fabrication, significantly reducing complexity and cost.

Co-optimization method to improve lateral resolution in photoacoustic computed tomography.

In biomedical imaging, photoacoustic computed tomography (PACT) has recently gained increased interest as this imaging technique has good optical contrast and depth of acoustic penetration. However, a spinning blur will be introduced during the image reconstruction process due to the limited size of the ultrasonic transducers (UT) and a discontinuous measurement process. In this study, a damping UT and adaptive back-projection co-optimization (CODA) method is developed to improve the lateral spatial resolution of PACT. In our PACT system, a damping aperture UT controls the size of the receiving area, which suppresses image blur at the signal acquisition stage. Then, an innovative adaptive back-projection algorithm is developed, which corrects the undesirable artifacts. The proposed method was evaluated using agar phantom and ex-vivo experiments. The results show that the CODA method can effectively compensate for the spinning blur and eliminate unwanted artifacts in PACT. The proposed method can significantly improve the lateral spatial resolution and image quality of reconstructed images, making it more appealing for wider clinical applications of PACT as a novel, cost-effective modality.

Gold-viral particle identification by deep learning in wide-field photon scattering parametric images.

The ability to identify virus particles is important for research and clinical applications. Because of the optical diffraction limit, conventional optical microscopes are generally not suitable for virus particle detection, and higher resolution instruments such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are required. In this paper, we propose a new method for identifying virus particles based on polarization parametric indirect microscopic imaging (PIMI) and deep learning techniques. By introducing an abrupt change of refractivity at the virus particle using antibody-conjugated gold nanoparticles (AuNPs), the strength of the photon scattering signal can be magnified. After acquiring the PIMI images, a deep learning method was applied to identify discriminating features and classify the virus particles, using electron microscopy (EM) images as the ground truth. Experimental results confirm that gold-virus particles can be identified in PIMI images with a high level of confidence.

DFB laser array based on four phase-shifted sampled Bragg gratings with precise wavelength control.

A four-laser array based on sampled Bragg grating distributed feedback (DFB) lasers in which each sampled period contains four phase-shift sections is proposed, fabricated, and experimentally demonstrated. The wavelength spacing between adjacent lasers is accurately controlled to 0.8 nm ± 0.026 nm and the lasers have single mode suppression ratios larger than 50 dB. Using an integrated semiconductor optical amplifier, the output power can reach 33 mW and the optical linewidth of the DFB lasers can be as narrow as 64 kHz. This laser array uses a ridge waveguide with sidewall gratings and needs only one metalorganic vapor-phase epitaxy (MOVPE) step and one III-V material etching process, simplifying the whole device fabrication process, and meeting the requirements of dense wavelength division multiplexing systems.

Quantitative analysis of errors caused by vibration on polarization parametric indirect microscopic imaging system.

Vibrations cause many problems such as displacement, distortion, and defocusing in microscopic imaging systems. Because vibration errors are random in direction, amplitude, and frequency, it is not known which aspect of the image quality will be affected by these problems and to what extent. Polarization parametric indirect microscopic imaging (PIMI) is a technique that records polarization parameters in a conventional wide-field reflection microscope using polarization modulation of the illumination beam and additional data analysis of the raw images. This indirect imaging technique allows the spatial resolution of the system to be improved. Here, the influence of vibration on the image sharpness and spatial resolution of a PIMI system is analyzed theoretically and experimentally. Degradation in the sharpness of PIMI images is quantified by means of the modulation transfer function and deterioration in the effective spatial resolution by the Fourier ring correlation. These results show that the quality of PIMI images can be improved significantly using vibration isolation.

Characterization of deep sub-wavelength nanowells by imaging the photon state scattering spectra.

Optical-matter interactions and photon scattering in a sub-wavelength space are of great interest in many applications, such as nanopore-based gene sequencing and molecule characterization. Previous studies show that spatial distribution features of the scattering photon states are highly sensitive to the dielectric and structural properties of the nanopore array and matter contained on or within them, as a result of the complex optical-matter interaction in a confined system. In this paper, we report a method for shape characterization of subwavelength nanowells using photon state spatial distribution spectra in the scattering near field. Far-field parametric images of the near-field optical scattering from sub-wavelength nanowell arrays on a SiN substrate were obtained experimentally. Finite-difference time-domain simulations were used to interpret the experimental results. The rich features of the parametric images originating from the interaction of the photons and the nanowells were analyzed to recover the size of the nanowells. Experiments on nanoholes modified with Shp2 proteins were also performed. Results show that the scattering distribution of modified nanoholes exhibits significant differences compared to empty nanoholes. This work highlights the potential of utilizing the photon status scattering of nanowells for molecular characterization or other virus detection applications.

Label-free sensing of virus-like particles below the sub-diffraction limit by wide-field photon state parametric imaging of a gold nanodot array.

A parallel four-quadrant sensing method utilizing a specially designed gold nanodot array is created for sensing virus-like particles with a sub-diffraction limit size (∼100 nm) in a wide-field image. Direct label-free sensing of viruses using multiple four-quadrant sensing channels in parallel in a wide-field view enables the possibility of high-throughput onsite screening of viruses.

Microparticle manipulation using laser-induced thermophoresis and thermal convection flow.

We demonstrate manipulation of microbeads with diameters from 1.5 to 10 µm and Jurkat cells within a thin fluidic device using the combined effect of thermophoresis and thermal convection. The heat flow is induced by localized absorption of laser light by a cluster of single walled carbon nanotubes, with no requirement for a treated substrate. Characterization of the system shows the speed of particle motion increases with optical power absorption and is also affected by particle size and corresponding particle suspension height within the fluid. Further analysis shows that the thermophoretic mobility (DT) is thermophobic in sign and increases linearly with particle diameter, reaching a value of 8 µm2 s-1 K-1 for a 10 µm polystyrene bead.

Visualization of ultrasonic wave field by stroboscopic polarization selective imaging.

A stroboscopic method based on polarization selective imaging is proposed for dynamic visualization of ultrasonic waves propagating in a transparent medium. Multiple independent polarization parametric images were obtained, which enabled quantitative evaluation of the distribution of the ultrasonic pressure in quartz. In addition to the detection of optical phase differences δ in conventional photo-elastic techniques, the azimuthal angle φ and the Stokes parameter S2 of the polarized light are found to be highly sensitive to the wave-induced refraction index distribution, opening a new window on ultrasonic field visualization.

Frequency comb with 100 GHz spacing generated by an asymmetric MQW passively mode-locked laser.

A L-band two-section AlGaInAs/InP asymmetric multiple quantum well (QW) passively mode-locked laser has been used to generate a frequency comb with a 100 GHz spacing at a central wavelength of 1610 nm. The comb contains 10 optical lines within a -3dB bandwidth of 8.05 nm and 34 optical lines within a -20dB bandwidth of 30 nm. The mode-locked pulse duration was 440 fs. To the best of our knowledge, this is the shortest pulse duration from any directly electrically pumped QW semiconductor mode-locked laser.

Multiple-wavelength distributed-feedback laser arrays with high coupling coefficients and precise channel spacing.

Special designs of sampled Bragg gratings have been used to fabricate distributed-feedback semiconductor laser arrays with very precise wavelength spacing and strong coupling coefficients. By dividing one sampling period into two equal sections, each with a grating but with a π-phase shift between the sections, the ±1st-order channels are enhanced while eliminating the zeroth-order reflection. By using multiple phase-shifted sections, only the -1st-order channel is enhanced. Using a single electron beam lithography step and two π-phase-shifted sections, we have fabricated an eight-channel laser array with a channel spacing of 100 GHz.

Improvement of indoor VLC network downlink scheduling and resource allocation.

Indoor visible light communications (VLC) combines illumination and communication by utilizing the high-modulation-speed of LEDs. VLC is anticipated to be complementary to radio frequency communications and an important part of next generation heterogeneous networks. In order to make the maximum use of VLC technology in a networking environment, we need to expand existing research from studies of traditional point-to-point links to encompass scheduling and resource allocation related to multi-user scenarios. This work aims to maximize the downlink throughput of an indoor VLC network, while taking both user fairness and time latency into consideration. Inter-user interference is eliminated by appropriately allocating LEDs to users with the aid of graph theory. A three-term priority factor model is derived and is shown to improve the throughput performance of the network scheduling scheme over those previously reported. Simulations of VLC downlink scheduling have been performed under proportional fairness scheduling principles where our newly formulated priority factor model has been applied. The downlink throughput is improved by 19.6% compared to previous two-term priority models, while achieving similar fairness and latency performance. When the number of users grows larger, the three-term priority model indicates an improvement in Fairness performance compared to two-term priority model scheduling.

Fully integrated multi-optoelectronic synthesizer for THz pumping source in wireless communications with rich backup redundancy and wide tuning range.

We report a monolithic photonic integrated circuit (PIC) for THz communication applications. The PIC generates up to 4 optical frequency lines which can be mixed in a separate device to generate THz radiation, and each of the optical lines can be modulated individually to encode data. Physically, the PIC comprises an array of wavelength tunable distributed feedback lasers each with its own electro-absorption modulator. The lasers are designed with a long cavity to operate with a narrow linewidth, typically <4 MHz. The light from the lasers is coupled via an multimode interference (MMI) coupler into a semiconductor optical amplifier (SOA). By appropriate selection and biasing of pairs of lasers, the optical beat signal can be tuned continuously over the range from 0.254 THz to 2.723 THz. The EAM of each channel enables signal leveling balanced between the lasers and realizing data encoding, currently at data rates up to 6.5 Gb/s. The PIC is fabricated using regrowth-free techniques, making it economic for volume applications, such for use in data centers. The PIC also has a degree of redundancy, making it suitable for applications, such as inter-satellite communications, where high reliability is mandatory.

Home treatment of deep venous thrombosis in the era of new oral anticoagulants.

This is a retrospective cohort study of adults with a primary diagnosis of deep venous thrombosis (DVT) unaccompanied by pulmonary embolism (PE), seen in 4 emergency departments in 2013 and part of 2014. The purpose was to assess the prevalence of home treatment of DVT in the present era of new oral anticoagulants. Among 96 patients with DVT and no PE, 85 (88.5%) were hospitalized and 11 (11.5%) were discharged to home. Most of the patients discharged to home received low-molecular-weight heparin, 9 (81.8%) of 11. None were prescribed new oral anticoagulants. Early discharge in ≤2 days occurred 28 (32.9%) of 85 patients. Most (64.3%) received enoxaparin and/or warfarin at early discharge. Rivaroxaban was prescribed in 7 (25.0%) of those discharged in ≤2 days. We conclude that in some emergency departments, patients with DVT are uncommonly discharged to home even though new oral anticoagulants are available.

Laterally coupled dual-grating distributed feedback lasers for generating mode-beat terahertz signals.

We present a laterally coupled dual wavelength 1.56/1.57-µm AlGaInAs/InP DFB laser, which, by introducing two different grating periods on each sidewall, emits two longitudinal modes simultaneously within the same cavity at a frequency separation of 0.82 THz. The beating signal is stabilized by nonlinear four-wave mixing in an electro-absorption modulator (EAM), located within a monolithically integrated resonant cavity. A stable 0.82-THz beating signal was observed over a wide range of bias parameters in terms of drive currents and bias to the DFB and EAM sections.