Tailorable dual-wavelength-band coupling in a transverse-electric-mode focusing subwavelength grating coupler

A transverse-electric-mode focusing subwavelength grating coupler (FSWGC) is proposed and demonstrated for dual-wavelength-band (DWB) coupling from a single-mode fiber into a suspended-membrane waveguide for the first time, to the best of our knowledge. Location and separation of the two coupling peaks can be flexibly tailored based on a proposed design methodology. As a proof of concept, two DWB FSWGCs working at (1486.0, 1594.5) nm and (1481.5, 1661.5) nm are experimentally demonstrated with coupling efficiencies of (18.3%, 20.1%) and (14.5%, 17.5%), 3-dB bandwidths of (55.0, 30.5) nm and (44.0, >39.5) nm, respectively. A DWB FSWGC working at (1480, 1830) nm with a wavelength separation of 350 nm and coupling efficiencies of (38.0%, 33.2%) is also numerically predicted.

High-performance chemical vapor deposited graphene-on-silicon nitride waveguide photodetectors

Waveguide photodetectors integrated with graphene have demonstrated potential for ultrafast response and broadband operation. Here, we demonstrate high-performance chemical vapor deposited graphene-on-silicon nitride waveguide photodetectors by enhancing the absorption of light propagating in the transverse-magnetic mode through a metal-graphene junction. A doubling in responsivity is experimentally observed. In our zero-biased metal-graphene junction, a 15  mA W^-1 intrinsic responsivity and a 30 GHz bandwidth are achieved at ∼1550  nm. The results are comparable to those obtained from the best pristine graphene-based photodetectors. Our work enables new architectures for high-performance optoelectronic devices based on the graphene-on-silicon nitride platform.

Characterisation of patients with endoscopy-negative, computer tomography-negative midfacial segment pain using the sino-nasal outcome test

BACKGROUND

The purpose of this study was to qualitatively characterise patients with midfacial segment pain (MSP) using the Sino-Nasal Outcome Test (SNOT). The data will provide a detailed overview of the physical and psychological impact on patients'well-being, and how it compares with the normal, healthy population.

METHODS

Suitable patients were prospectively identified from the Multi-disciplinary Facial Pain Clinic at the Royal Liverpool University Hospital, based on the diagnostic criteria for MSP. The pre-treatment SNOT-22 of these patients were also compared to patients with chronic rhinosinusitis and normal healthy volunteers.

RESULTS

Twenty-nine consecutive patients with a diagnosis of MSP were identified, and compared with 30 CRS patients and 34 healthy volunteers. The average SNOT-22 scores of MSP and CRS patients were higher than normal healthy volunteers. Patients with CRS had the highest rhinological subscale SNOT scores compared to normal healthy volunteers and MSP. Conversely, the reported ear and facial symptoms of MSP patients were most unfavourable. A similar trend was observed in reported sleep function where MSP patients recorded higher subscale scores than the other two cohorts. The subscale mean score for psychological function of MSP patients was not significant when compared to the mean score of patients diagnosed with CRS.

CONCLUSION

MSP has an adverse impact on both physical and psychological well-being. The subtle differences in the SNOT subscores between MSP and CRS have provided greater insight into the character and disease impact of MSP. We propose that the SNOT may be suitably utilised in MSP to document disease severity and measure response to treatment.

Mid-infrared germanium photonic crystal cavity

The mid-infrared (MIR) spectral range holds significant potential for spectroscopic and sensing applications because it encompasses the fingerprint region that unveils the vibrational and rotational signatures of molecules. CMOS-compatible on-chip devices that can achieve strong light-matter interaction in the entire fingerprint region are considered a promising way for such applications, but remain unprecedented. Here we present an on-chip MIR germanium photonic crystal cavity that covers the entire fingerprint region. This is made possible by harnessing a homemade air-cladding germanium platform. Our MIR device creates a new avenue toward integrated nonlinear optics and on-chip biochemical sensing in the fingerprint region.

Focusing subwavelength grating coupler for mid-infrared suspended membrane germanium waveguides

We present a focusing subwavelength grating (SWG) for efficient coupling of mid-infrared (mid-IR) light into suspended membrane Ge photonic integrated circuits (PICs) that enable mid-IR applications in the entire fingerprint region. By virtue of their wide spectral transparency window and air-cladding device configuration, the suspended membrane Ge PICs are expected to be effective for mid-IR applications over the spectral region covering from 2 to 15 μm. Specifically, we demonstrate the maximum coupling efficiency of -11  dB with a 1-dB bandwidth of ∼58  nm at the SWG's center wavelength of 2.37 μm. Our focusing SWG is expected to advance the development of on-chip long-wavelength mid-IR applications such as biochemical sensing, thermal imaging, and nonlinear optics in the fingerprint region.

Cavity-enhanced thermo-optic bistability and hysteresis in a graphene-on-Si3N4 ring resonator

Cavity-enhanced thermo-optic bistability is studied in a graphene-on-Si₃N₄ ring resonator. By engineering the coverage of the monolayer graphene on top of the Si₃N₄ ring resonator, we observed a two-fold enhancement in the thermo-optically induced resonance shift rate and an 18-fold increase in the effective thermal nonlinear refractive index compared with the devices without graphene. The thermo-optic hysteresis loop was also characterized in this hybrid structure, where the experimental results agree well with the theoretical calculations. This Letter paves the way for graphene-on-Si₃N₄-based high-speed photodetectors, modulators, and devices for on-chip nonlinear optical applications.

Fully suspended slot waveguides for high refractive index sensitivity

A fully suspended mid-infrared (FSMIR) slot waveguide is proposed and experimentally demonstrated on a silicon-on-insulator (SOI) platform for the first time. The slotted waveguide core is mechanically supported by lateral subwavelength grating claddings. The fabricated waveguides possess low propagation loss, which is measured to be 7.9  dB/cm at the wavelength of 2.25 μm. With the underlying buried oxide (BOX) removed, the FSMIR slot waveguide has a broad spectral range of transparency that is limited only by the absorption of silicon. Numerical simulation shows that its sensitivity, defined as the ratio between the change of the effective index and the ambient refractive index, can reach 1.123, which is 9.7% higher than the maximal sensitivity of conventional SOI slot waveguides on BOX.

Synergistic Effects of Plasmonics and Electron Trapping in Graphene Short-Wave Infrared Photodetectors with Ultrahigh Responsivity

Graphene's unique electronic and optical properties have made it an attractive material for developing ultrafast short-wave infrared (SWIR) photodetectors. However, the performance of graphene SWIR photodetectors has been limited by the low optical absorption of graphene as well as the ultrashort lifetime of photoinduced carriers. Here, we present two mechanisms to overcome these two shortages and demonstrate a graphene-based SWIR photodetector with high responsivity and fast photoresponse. In particular, a vertical built-in field is employed in the graphene channel for trapping the photoinduced electrons and leaving holes in graphene, which results in prolonged photoinduced carrier lifetime. On the other hand, plasmonic effects were employed to realize photon trapping and enhance the light absorption of graphene. Thanks to the above two mechanisms, the responsivity of this proposed SWIR photodetector is up to a record of 83 A/W at a wavelength of 1.55 μm with a fast rising time of less than 600 ns. This device design concept addresses key challenges for high-performance graphene SWIR photodetectors and is promising for the development of mid/far-infrared optoelectronic applications.

Monolithically integrated reconfigurable add-drop multiplexer for mode-division-multiplexing systems

An integrated reconfigurable optical add-drop multiplexer (ROADM) for mode-division-multiplexing systems is proposed and demonstrated for the first time, to the best of our knowledge. The present ROADM with four mode-channels is composed of a four-channel mode demultiplexer, four identical 2×2 thermo-optic Mach-Zehnder switches (MZSs), and a four-channel mode multiplexer, which are integrated monolithically on silicon. All the devices are designed for operation with TM polarization. The ROADM can add/drop any one of the mode channels freely by thermally turning on/off the corresponding MZS. For the added/dropped mode-channels, the excess loss is 1-5 dB, and the extinction ratio is 15-20 dB in the wavelength range of 1535-1565 nm.

High-responsivity graphene-on-silicon slot waveguide photodetectors

We demonstrated strong optical absorption in a graphene integrated silicon slot waveguide. Due to the increase in light intensity and decrease of optical mode confinement in the silicon slot, the graphene experienced an increased interaction to the in-plane light. A waveguide absorption of 0.935 dB μm(-1) was measured at 1.55 μm wavelengths. Based on the graphene-on-silicon slot waveguide, a compact and high-responsivity photodetector was demonstrated. Benefited from the lack of efficient electron cooling in the suspended graphene and the intensity enhancement effect in the nano slot, a maximum responsivity of 0.273 A W(-1) was achieved in the telecommunication band.

Impact of heart disease and calibration interval on accuracy of pulse transit time-based blood pressure estimation

Continuous blood pressure (BP) measurement without a cuff is advantageous for the early detection and prevention of hypertension. The pulse transit time (PTT) method has proven to be promising for continuous cuffless BP measurement. However, the problem of accuracy is one of the most challenging aspects before the large-scale clinical application of this method. Since PTT-based BP estimation relies primarily on the relationship between PTT and BP under certain assumptions, estimation accuracy will be affected by cardiovascular disorders that impair this relationship and by the calibration frequency, which may violate these assumptions. This study sought to examine the impact of heart disease and the calibration interval on the accuracy of PTT-based BP estimation. The accuracy of a PTT-BP algorithm was investigated in 37 healthy subjects and 48 patients with heart disease at different calibration intervals, namely 15 min, 2 weeks, and 1 month after initial calibration. The results showed that the overall accuracy of systolic BP estimation was significantly lower in subjects with heart disease than in healthy subjects, but diastolic BP estimation was more accurate in patients than in healthy subjects. The accuracy of systolic and diastolic BP estimation becomes less reliable with longer calibration intervals. These findings demonstrate that both heart disease and the calibration interval can influence the accuracy of PTT-based BP estimation and should be taken into consideration to improve estimation accuracy.

Experimental demonstration of polarization-insensitive air-cladding grating couplers for silicon-on-insulator waveguides

We present an air-cladding apodized focusing subwavelength grating that can effectively couple two polarizations into a single waveguide. For the transverse magnetic mode, -3.2  dB maximum coupling efficiency with ∼28  nm 1 dB bandwidth is achieved. With the same grating, -4.3  dB maximum coupling efficiency with ∼58  nm 1 dB bandwidth is achieved for the transverse electric mode. The minimum difference between two polarizations' coupling peaks is demonstrated to be ∼32  nm. At the 1525 nm wavelength range, the polarization-insensitive grating is demonstrated with -6.5  dB coupling efficiency. The polarization-insensitive coupling wavelength can be controlled experimentally.

Spectral hole burning in silicon waveguides with a graphene layer on top

We present an experimental study of the nonlinear absorption of graphene-on-silicon waveguides. The wavelength dependence of absorption saturation from a narrow linewidth optical pump is measured. Spectral hole burning (SHB) introduces a decrease in the probe absorption near the pump wavelength, which may be distinguished from the free carrier absorption (FCA) by using the time dependence of the buildup of the free carrier population. Beyond SHB bandwidth, only FCA dominates the waveguide loss. The spectral bandwidth of absorption bleaching from SHB is measured to have a full width at half-maximum of ~12 meV (~20 nm) at 16.8 dBm pump power.

Stable and wavelength-tunable silicon-micro-ring-resonator based erbium-doped fiber laser

In this work, we propose and demonstrate a stable and wavelength-tunable erbium-doped fiber (EDF) ring laser. Here, a silicon-on-insulator (SOI)-based silicon-micro-ring-resonator (SMRR) is used as the wavelength selective element inside the fiber ring cavity. A uniform period grating coupler (GC) is used to couple between the SMRR and single mode fiber (SMF) and serves also as a polarization dependent element in the cavity. The output lasing wavelength of the proposed fiber laser can be tuned at a tuning step of 2 nm (defined by the free spectral range (FSR) of the SMRR) in a bandwidth of 35.2 nm (1532.00 to 1567.20 nm), which is defined by the gain of the EDF. The optical-signal-to-noise-ratio (OSNR) of each lasing wavelength is larger than 42.0 dB. In addition, the output stabilities of power and wavelength are also discussed.

Broadband focusing grating couplers for suspended-membrane waveguides

We propose and demonstrate broadband focusing grating couplers for suspended-membrane waveguides on silicon-on-insulator both in near-infrared (near-IR) and in mid-IR wavelength range. Finite-difference time-domain simulation predicts ∼100  nm 3 dB bandwidth with -1.7  dB coupling efficiency for an apodized grating in near-IR. -3.5  dB maximum coupling efficiency and ∼90  nm 3 dB bandwidth are realized experimentally. In mid-IR, -5.5  dB maximum coupling efficiency from a uniform focusing grating is measured at 2.75 μm, and ∼500  nm 3 dB bandwidth is predicted theoretically.

Tunable integrated variable bit-rate DPSK silicon receiver

We integrate a wavelength-tunable microring resonator with a monolithic Ge-on-Si photodetector for use as a variable bit-rate silicon receiver. The integrated receiver has a responsivity of 0.7 A/W. We achieved error-free operation for a wide range of bit rates from 5 Gb/s to 12.5 Gb/s. The wavelength tuning was realized by a TiN-based thermal heater, which enabled tuning of up to ~1 free spectral range.

Demodulation of 20 Gbaud/s differential quadrature phase-shift keying signals using wavelength-tunable silicon microring resonators

We experimentally demonstrate a demodulation scheme for 20 Gbaud/s nonreturn-to-zero differential quadrature phase-shift keying signals using a silicon microring resonator. Either the I or Q channel can be selected by electrical tuning using the carrier depletion effect. Error-free (bit error rate <10(-9)) operations can be achieved for both the I and Q channels with ~1.5 dB power penalty compared with a commercial 20 GHz Mach-Zehnder delay interferometer. The tuning range of the ring resonator can be up to 60 GHz, which enables operation over a wide range of data rates.

Focusing subwavelength grating coupler for mid-infrared suspended membrane waveguide

A mid-infrared (mid-IR)-focusing subwavelength grating (SWG) coupler and suspended membrane waveguide (SMW) on a silicon-on-insulator wafer are studied. For a transverse-electric mode uniform SWG, finite-difference time-domain simulation predicts 44.2% coupling efficiency with 1 dB bandwidth of about 220 nm and backreflection of 0.78% at 2.75 μm. Then the uniform SWG is curved to a focusing SWG using a phase-matching formula. The SMWs are analyzed by the finite element method and fabricated. An Er3+-Pr3+ co-doped mid-IR fiber laser is used for device characterization. The fabricated mid-IR SWG coupler has 24.7% coupling efficiency.

Long-reach radio-over-fiber signal distribution using single-sideband signal generated by a silicon-modulator

The integration of passive optical network (PON) and radio-over-fiber (ROF) networks could provide broadband services for both fixed and mobile users in a single and low-cost platform. Combining the long-reach (LR)-PON (>100 km) and the LR-ROF can further reduce the cost by simplifying the network architecture, sharing the same optical components and extending the coverage of ROF network. However, the transmission and distribution of ROF signal in LR network is very challenging due to the chromatic dispersion generated periodic power fading and code time-shifting effects in the optical fiber. In this work, we propose and experimentally demonstrate a LR-ROF signal distribution using single-sideband (SSB)-ROF signal generated by a silicon ring-modulator. The silicon modulator is compact and has low power consumption. Besides, one unique feature of the silicon ring-modulator is that it only modulates the signal wavelength at the resonant null. This makes it very suitable for the generation of the SSB-ROF signal. Numerical comparison of the SSB-ROF with the double-sideband (DSB)-ROF and optical carrier suppress (OCS)-ROF signals; as well as the fabrication of the silicon ring-modulator will be discussed.

Dynamic gain-tilt compensation using electronic variable optical attenuators and a thin film filter spectral tilt monitor

An all-optical dynamic gain tilt compensator (DGTC) is proposed and experimentally demonstrated. A single wide-band thin film filter and a pair of photodetector allow the DGTC to distinguish band add/drop position. Power fluctuations from EDFA gain tilt were reduced with fast electronic variable optical attenuators.

Optical label switching of DRZ/DPSK orthogonal signal generated by photonic-crystal fiber

We demonstrate orthogonal label switching by using a dark-return-to-zero (DRZ) payload and differential phase-shift keying (DPSK) label generated by a dispersion-flattened photonic-crystal fiber. The high extinction ratio of both the payload and label improves the receiver margin. The DRZ payload introduces little cross talk to the DPSK label due to the RZ-like output of the demodulated DPSK. Simulations are performed to study the eye-closure penalty of the payload and label at different DRZ pulse widths. We compare the DRZ/DPSK with the RZ and DPSK signals numerically at the same data rate and show that the DRZ/DPSK has a strong tolerance to the polarization-mode dispersion. The DRZ/DPSK has a more compact spectrum suitable for the strong filtering requirements in WDM systems.

Nonlinear polarization rotation in a dispersion-flattened photonic-crystal fiber for ultrawideband (>100 nm) all-optical wavelength conversion of 10 Gbit/s nonreturn-to-zero signals

We study the conversion bandwidth of the cross-polarization-modulation (XPoIM)-based wavelength conversion scheme with a dispersion-flattened highly nonlinear photonic-crystal fiber for signals with a nonreturn-to-zero (NRZ) modulation format. Both theoretical and experimental results show that the conversion bandwidth can be extended to cover a very wide band, including S-, C-, and L-bands for 10 Gbit/s NRZ signals (a total bandwidth of 120 nm is experimentally demonstrated). We also study the theoretical bandwidth limit for 40 Gbit/s NRZ signals. A significant extension of the conversion bandwidth using the XPoIM approach compared with the four-wave mixing approach previously reported is demonstrated.

Nonlinear absorption and Raman gain in helium-ion-implanted silicon waveguides

We study whether helium ion implantation can reduce the carrier lifetime and thus reduce the density of two-photon-absorption-generated free carriers. Our experiments and theoretical model show that helium ion implantation into silicon waveguides can successfully reduce the free-carrier losses and allow net gain to be attained by cw-pumped stimulated Raman scattering without requiring reverse bias to remove the photogenerated free carriers.

Optical label encoding and swapping using half-bit delayed dark RZ payload and DPSK label

Orthogonal labeling is a potential candidate in future opticallabel- switched-networks. Half-bit-delayed-dark-return-to-zero (HBDDRZ) payload with DPSK orthogonal labeling is proposed. The precise payload extinction-ratio adjustment at source nodes and maintenance at each intermediate node is not needed. The high extinction-ratio of both payload and label improves the receiver margin. Owing to the RZ-like nature of the demodulated DPSK label, HBDDRZ payload causes negligible effect on the label. Nonlinear-optical-loop-mirror generates the HBDDRZ payload, which is then encoded by DPSK label. Label swapping is demonstrated by birefringence switching. 10-Gb/s bit-error-rate measurements are performed.