Fangfei Liu

Compact optical temporal differentiator based on silicon microring resonator

We propose and experimentally demonstrate a temporal differentiator in optical field based on a silicon microring resonator with a radius of 40 microm. The microring resonator operates near the critical coupling region, and can take the first order derivative of the optical field. It features compact size thus is suitable for integration with silicon-on-insulator (SOI) based optical and electronic devices. The performance of this optical differentiator is tested using signals with typical shapes such as Gaussian, sinusoidal and square-like pulses at data rates of 10 Gb/s and 5 Gb/s.

System performance of slow-light buffering and storage in silicon nano-waveguide

We experimentally demonstrate optically tunable buffer in a nano-scale silicon microring resonator with a 20-μm radius. The delay-tuning mechanism is based on the red shift of the resonance induced by the thermal nonlinear effect. We use a non-return-to-zero (NRZ) pseudo random bit sequence (PRBS) signal with different data rates as the probe signal, and investigate its delay performance under different pump powers.

Optically Tunable Delay Line in Silicon Microring Resonator Based on Thermal Nonlinear Effect

We experimentally demonstrate optically tunable delay line in a silicon microring resonator with a 20- m radius. The delay-tuning mechanism is based on the red shift of the resonance induced by thermal nonlinear effect. We investigate the delay performance of three modulation formats-non-return-to-zero (NRZ), return-to-zero (RZ), and differential phase-shift keying (DPSK) signals at different data rates. Tunable delay is achieved by controlling the power of the continuous-wave (CW) pump with very low tuning threshold, which could be used in microring-resonator-based slow-light structure.

Dense wavelength conversion and multicasting in a resonance-split silicon microring

We experimentally demonstrate all-optical wavelength conversions in a 10 μm radius resonance-split silicon microring resonator based on free carrier dispersion effect. The split resonance is caused by the mutual coupling between the two countertraveling modes inside the ring resonator. Dense wavelength conversions are performed at data rates from 500 Mbytes/s to 5 Gbytes/s and a dual-channel wavelength multicasting is realized at a data rate of 1.25 Gbytes/s. The resonance splitting phenomenon opens up opportunities to convert more closely spaced wavelengths, thus effectively increasing the system capacity.

Stringent cessation criterion results in better durability of lamivudine treatment: a prospective clinical study in hepatitis B e antigen‐positive chronic hepatitis B patients

Summary.  The cessation criteria for lamivudine treatment vary in published articles and their results are contradictory, especially factors predicting relapse. To clarify these contradictions, this long‐term follow‐up study of 125 Chinese hepatitis B e antigen (HBeAg)‐positive chronic hepatitis B patients was designed with stringent cessation criterion. All patients received lamivudine and achieved HBeAg seroconversion (group A, n = 82) or loss (group B, n = 43) with undetectable hepatitis B virus (HBV) DNA by PCR assay during the treatment. Lamivudine was withdrawn ≥6 months after HBeAg seroconversion/loss occurred. The median treatment durations were 24 (12–54) months and 36 (18–89) months in group A and group B, respectively. Patients were followed up for median 24 (2–84) months. The cumulative relapse (defined as serum HBV DNA ≥104 copies/mL) rates in the two groups at months 12, 24, 36 and 48 were 23.4%vs 35.0%, 25.0%vs 37.7%, 25.0%vs 41.1% and 29.4%vs 41.1%, respectively (log‐rank test, P = 0.119). For patients whose total treatment duration ≥18 months in group A, the cumulative relapse rates at months 12, 24, 36 and 48 were 18.3%, 20.1%, 20.1% and 25.1%, which was significantly lower than those with a shorter duration (log‐rank test, P = 0.002). The mean age and median total duration were statistically different between relapsers and nonrelapsers in group A (33.9 ± 13.6 vs 23.1 ± 11.0 years, P < 0.001 and 24 vs 26 months, P = 0.003). Cox regression revealed that age was the only predictive factor for relapse (RR, 1.069; 95% CI, 1.032–1.106, P < 0.001). Patients aged <30 years relapsed less frequently in 5 years (12.3%vs 53.5%, P = 0.001). In conclusion, for patients who maintained HBeAg seroconversion for ≥6 months and total duration for ≥18 months, lamivudine withdrawal is a reasonable option. Prolonged treatment may be required for patients aged greater than 30 years to reduce relapse.

All-optical format conversions from NRZ to BPSK and QPSK based on nonlinear responses in silicon microring resonators

We propose and numerically verify a novel scheme of all-optical format conversion from non-return-to-zero (NRZ) to binary phase-shift- keying (BPSK) at 160 Gb/s using cascaded microring resonators (CMRR) on a single silicon chip. The conversion is based on large phase shift and flattened intensity-response characteristics in the CMRR. A continuous-wave light experiences different phase shifts controlled by the power of an input NRZ signal with an ~8.8-dB extinction ratio, while maintaining approximately the constant intensity. All-optical format conversion from NRZ to quadrature phase-shift-keying (QPSK) is also demonstrated based on parallel NRZ/BPSK converters in a Mach-Zehnder interferometer structure.

Pulse Delay and Advancement in SOI Microring Resonators With Mutual Mode Coupling

In this paper, we experimentally demonstrate continuously tunable pulse propagation in SOI microring resonators with mutual mode coupling, which is induced by nanosized gratings along the ring sidewalls. In the presence of the mutual mode coupling, dispersion-induced group delay is investigated in single-waveguide and double-waveguide coupled single-ring resonators, respectively. In particular, pulse propagation in the drop channel of a double-waveguide coupled resonator is observed and exhibits continuously tunable delay and advancement features in the experiment. Delayed pulses are also obtained at the reflection port of a single-waveguide coupled resonator due to the existence of backward whispering gallery mode (WGM).

Enhanced fast light in microfiber ring resonator with a Sagnac loop reflector

We fabricate a microfiber knot-type ring resonator with a Sagnac loop reflector, and control the light velocity using the device. In this structure, light is reflected by the Sagnac loop and passes through the ring resonator twice. Thus, it possesses doubled transmission and group delay comparing with the microfiber ring resonator without the Sagnac loop. We experimentally demonstrate pulse advancement in an under-coupled microfiber knot-type ring resonator with a Sagnac loop reflector. In the experiment, a maximum of approximately 25 ps pulse advancement was achieved for a 5-Gb/s RZ signal.

All-optical regenerative NRZ-to-RZ format conversion using coupled ring-resonator optical waveguide

This paper proposes a scheme for format conversion from a distorted non-return-to-zero (NRZ) signal to a high-quality return-to-zero (RZ) signal, using the nonlinearity in a coupled ring-resonator optical waveguide (CROW) on a silicon chip. In this method, a distorted NRZ signal is amplified and fed into the CROW together with an RZ pulse train. The CROW performs as a nonlinear step gate for the RZ pulse train, which outputs amplitude-equalized RZ pulses inheriting the information from the NRZ signal. Clearly, the integration of the format conversion and regeneration simplifies the system. Our simulations performed at 10 Gb/s and 40 Gb/s verify the feasibility of our proposal.

Sensitive label-free and compact biosensor based on concentric silicon-on-insulator microring resonators.

We propose an ultracompact parallel label-free biosensor based on concentric silicon-on-insulator microring resonators. Our numerical studies show that the proposed biosensor offers higher sensitivity and a larger detection area than conventional single-ring-based sensors. We analyze the detection sensitivity of the DNA hybridization by immobilizing the probe on top of the double concentric ring resonators and in the ring-ring air gap. For a concentric double-ring system with an area of 27.646 microm2, the bulk detection sensitivity is 683 nm RIU(-1). The sensitive uniformity of the system related to the optical field distribution is also studied.