Gencheng Wang

Silicon three-mode (de)multiplexer based on cascaded asymmetric Y junctions

A three-mode (de)multiplexer based on two cascaded asymmetric Y junctions is proposed and experimentally demonstrated on a silicon-on-insulator platform for mode-division multiplexing applications. Within a bandwidth from 1537 to 1566 nm, the best demultiplexing crosstalk of the fabricated device, composed of a three-mode multiplexer, a multimode straight waveguide, and a three-mode demultiplexer, is up to -31.5  dB, while in the worst case it is -9.7  dB. The measured maximum insertion loss is about 5.7 dB at a wavelength of 1550 nm. The mode crosstalk and insertion loss can be further improved by high-quality fabrication processes.

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.

Graphene-Based Floating-Gate Nonvolatile Optical Switch

We proposed a nonvolatile optical waveguide switch by utilizing a floating-gate (FG) configuration whose FG layer is a single-layer graphene. The switching signal can be removed after the optical switching is accomplished. The propagation state of light then can be retained by charges trapped in the graphene layer until the next erasing signal. Depending on waveforms of driving signals, the device can work as either a phase shifter or an intensity switch. In the phase shifter mode, a 646-μm-long device can achieve a phase shift of π. Corresponding energy consumptions to program/erase, the π phase shift is 82.8 and 118.2 pJ, respectively. In the intensity switch mode, a 328-μm-long device is able to attenuate the light by 20 dB. Energies consumed by the programming and the erasing operations are 35.7 and 45.4 pJ, respectively.

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.

An integrated high-performance ratio-metric wavelength measurement device on glass

The measurable wavelength range and the resolution of the ratio-metric wavelength monitor are limited by each other in a conventional structure. To solve this problem we designed and fabricated a high-performance integrated double ratio-metric wavelength measurement device on glass by the method of ion-exchange. It consists of four unbalanced Mach–Zehnder interferometers (MZIs) to form a rough wavelength measurement with a wide range and a fine wavelength measurement with high resolution. The highest measured resolution can reach 10 pm in a 1.6 nm-wide wavelength range for the fine wavelength measurement together with a 45 nm-wide wavelength range for the rough measurement. By heating the unbalanced MZI, the performance of the fine wavelength monitor can be improved.

Broadband tunable filter based on the loop of multimode Bragg grating

A broadband tunable silicon filter has been demonstrated on silicon-on-insulator platform. The device is based on the loop of multimode anti-symmetric waveguide Bragg grating. A wide bandwidth tunability about 1.455 THz (0.117-1.572 THz) is achieved. The device, functions like a ring, can realize the bandwidth tunable of the drop port and the through port. And, its feature has simultaneous wavelength tuning and no free space ranges limitation. A high out-of-band contrast of 30 dB is achieved with a bandwidth of 1.572 THz (Δλ = 13 nm). The out-of-band contrast is 18 dB at the minimum bandwidth 0.117 THz (Δλ = 1.0 nm).

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.

Wavelength Tunable Cavity Mirror for Silicon Micro-Ring-Based Hybrid Integrated Lasers

Two types of wavelength tunable cavity mirrors have been presented and silicon-on-insulator (SOI) platform. Both cavity mirrors show high reflectivity and large Q magnitude. They are used for realizing the silicon hybrid integrated lasers. The Q magnitude of the mirrors is about 6 × 104, and the reflectivity is more than 95%.We use a suspended edge coupler to connect the III-V semiconductor optical amplifier chip and the cavity mirrors. The measured highest laser output power is 6.1 mW in uncooled condition. A wide bandwidth tenability (the whole FSR) can be achieved, the tuning efficiency is about 6.7 mW/nm, and over 35-dB side mode suppression ratio is obtained.

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.

Graphene-based nonvolatile terahertz switch with asymmetric electrodes

We propose a nonvolatile terahertz (THz) switch which is able to perform the switching with transient stimulus. The device utilizes graphene as its floating-gate layer, which changes the transmissivity of THz signal by trapping the tunneling charges. The conventional top-down electrode configuration is replaced by a left-right electrode configuration, so THz signals could transmit through this device with the transmissivity being controlled by voltage pulses. The two electrodes are made of metals with different work functions. The resultant asymmetrical energy band structure ensures that both electrical programming and erasing are viable. With the aid of localized surface plasmon resonances in graphene ribbon arrays, the modulation depth is 89% provided that the Femi level of graphene is tuned between 0 and 0.2 eV by proper voltage pulses.