Yuehai Wang

Broadband Graphene/Glass Hybrid Waveguide Polarizer

In this letter, a broadband planar-lightwave-circuit-type graphene/glass hybrid waveguide polarizer has been demonstrated. The polarizing effect of the polarizer is based on the difference in the attenuation between the two orthogonally polarized guided modes. The buried depth and the width of the ion-exchange glass waveguide are optimized to 2.6 and 11.5 μm, respectively. The polarization extinction ratio of the polarizer is ~27 dB in the telecommunication band with ~4-mm graphene coating length along the propagation direction. In addition, the polarization extinction ratio can be further increased by decreasing the chemical potential of the graphene film.

Bandwidth and wavelength tunable optical passband filter based on silicon multiple microring resonators

An ultra-compact silicon bandpass filter with wide bandwidth tunability is proposed and experimentally demonstrated. The filter architecture is based on a multiple micro-ring resonator-cascaded structure. A wide bandwidth tunability (from 75 to 300 GHz) can be achieved by controlling the resonant frequency of the microring resonators when a good shape factor (0.24-0.44) is held. The filter has a wide free spectral range (about 1.2 THz). The center wavelength can be tuned over several nanometers linearly. The footprint is only 0.053  mm2.

Low-Coherence Method for Mode Analysis of a Silicon-Based Two-Mode Waveguide

We propose and demonstrate an effective method for mode analysis of a silicon-based two-mode waveguide using time-domain scanning low-coherence interferometry (LCI). An on-chip offset launch technique is implemented to motivate and couple out the two modes. From the low-coherence signal, it is convenient to simultaneously perform the mode identification and analyze the differential modal group delay in a silicon-based multimode waveguide. Our experimental results show good agreement with the simulation results. The low-coherence interferogram shows that only short samples of waveguide (<;220 μm) is sufficient for the LCI to do the mode analysis. This letter provides a feasible method for the mode identification and the transmission channel analysis in silicon-based multimode waveguide used for 2 × 2 on-chip mode-division multiplexing.

Measurement of the intermodal crosstalk of a bent multimode waveguide.

We quantitatively investigate the main source of the intermodal crosstalk of a silicon-based bent multimode waveguide by experiment. The measurement is performed through time-domain scanning low-coherence interferometry. From the measurement results, one can not only calculate the modal crosstalk, but can also locate the position where the crosstalk appears. The results indicate that the modal mismatch at the points where the curvature of the waveguide changes is the main origin of the modal crosstalk. For a two-mode waveguide with a bending radius of 5 μm at 1310 nm, the crosstalk is as high as -20 and -16  dB for the fundamental and first-order mode, respectively. This work gives us a deep insight into how the guided modes actually propagate through the bent waveguide.

Temperature-fluctuation-sensitive accumulative effect of the phase measurement errors in low-coherence interferometry in characterizing arrayed waveguide gratings

We investigate the accumulative effect of the phase measurement errors in characterizing optical multipath components by low-coherence interferometry. The accumulative effect is caused by the fluctuation of the environment temperature, which leads to the variation of the refractive index of the device under test. The resulting phase measurement errors accumulate with the increasing of the phase difference between the two interferometer arms. Our experiments were carried out to demonstrate that the accumulative effect is still obvious even though the thermo-optical coefficient of the device under test is quite small. Shortening the measurement time to reduce the fluctuation of the environment temperature can effectively restrain the accumulative effect. The experiments show that when the scanning speed increases to 4.8 mm/s, the slope of the phase measurement errors decreases to 5.52×10(-8), which means the accumulative effect can be ignored.

Integrated High-Performance Two-Stage Ratiometric Wavelength Monitors On Silicon

We design and fabricate an integrated high-performance two-stage ratiometric wavelength monitors (RMWMs) on silicon. The first stage is a coarse wavelength monitor with a wide working range based on two unbalanced Mach–Zehnder interferometers acting as edge filters. The second stage is a fine wavelength monitor with picometer wavelength resolution utilizing two tunable microrings acting as edgefilters. The highest measured wavelength resolution can reach 1.6 pm in a 0.7-nm-wide wavelength range for the fine wavelength monitor. The working wavelength range can reach about 55 nm with a resolution of 0.1 nm for the coarse wavelength monitor. By thermally tuning the resonant wavelengths of microrings, we can obtain high resolution on the whole 9.6-nm free spectral range of the microrings. The influence of perturbation of temperature is also discussed. This letter can solve the problem that the measurable wavelength range and resolution limit each other in a conventional RMWM.

Integrated high responsivity photodetectors based on graphene/glass hybrid waveguide

We propose and fabricate an integrated graphene/glass hybrid photodetector (PD) with high responsivity and broad spectral bandwidth. The glass straight waveguide enables high absorption of the evanescent light of transverse magnetic (TM) mode propagating parallel to the single layer of graphene. It is based on the mechanism of light-induced change in conductance. As a result, a responsivity as high as 0.72 A/W at a low bias voltage of -0.1  V for a wide wavelength range from 1510 to 1630 nm is experimentally obtained. The proposed graphene/glass hybrid PD could find important applications in graphene-based photonic integrated circuits.

Ultra-sensitive silicon photonic current sensor using a ring resonator

We proposed and experimentally investigated a compact and ultra-sensitive integrated photonic current sensor based on a silicon ring resonator in this paper. The current flowing through the integrated resistive TiN heater produces the Joules heat and changes the temperature, which results in the change of refractive index and physical dimensions of the ring. An optical spectrum analyzer is used to monitor the resonant wavelength shift of the ring. The experiment results show that the sensor achieves an ultra-high sensitivity of 6.8 × 104 nm A−2 and good linearity between real-time current and wavelength shift in the test range of 0–10 mA.