Haike Zhu

Design and Analysis of a Miniature Intensity Modulator Based on a Silicon-Polymer-Metal Hybrid Plasmonic Waveguide

We propose a miniature optical intensity modulator based on a silicon-polymer-metal hybrid plasmonic waveguide. Benefiting from the high mode confinement of hybrid plasmonic waveguide and the high linear electro-optic effect of polymer material, the intensity modulator is ultra-compact with a length of only ~ 13 μm. The device is optimized using numerical simulations based on the finite element method (FEM). The modulator exhibits a large modulation bandwidth of 90 GHz, a modulation depth of 12 dB at 6 V, and low power consumption of 24.3 fJ/bit.

Tunable two-stage self-coupled optical waveguide resonators.

We report tunable two-stage self-coupled optical waveguide (SCOW) resonators composed of a pair of mirror-imaged single SCOW resonators connected by a phase shifter in between. Experimental results show that the coupled-resonator-induced-transparency and high-order bandstop filtering characteristics can be obtained in the transmission spectra of the devices with two different configurations. The resonance spectrum can be tuned by using either a p-i-p microheater or a p-i-n diode in the phase shifter. Our theoretical modeling based on the transfer matrix method has a good agreement with the experimental results.

Tunable silicon Fabry-Perot comb filters formed by Sagnac loop mirrors.

We experimentally demonstrate tunable silicon comb filters based on Fabry-Perot (FP) resonators composed of two Sagnac loop mirrors. The comb filter resolves up to 54 comb lines with a 115 GHz channel spacing over a spectral range from 1510 to 1560 nm. The comb line extinction ratio is ~26.3 dB and the quality factor is ~57,000 around 1550 nm wavelength. Electrical tuning is enabled via periodically interleaved PN junctions embedded inside the FP resonator. The comb lines are blue shifted by ~0.92 nm (one channel spacing) with a 5 mA forward-bias current and red-shifted by ~0.05 nm with a -10 V reverse-bias voltage.

CMOS-compatible temperature-independent tunable silicon optical lattice filters.

We present a CMOS-compatible athermal tunable silicon optical lattice filter composed of 10 cascaded 2 × 2 asymmetric Mach-Zehnder interferometers. Active tuning experiments show that the filter central wavelength can be red-/blue-shifted by 13.1/21.3 nm with power consumption of 77/96 mW on top/bottom arms. Temperature shift measurements show that the thermal-sensitivity of the filter central wavelength before active tuning is as low as -1.465 pm/°C. The thermal-sensitivity is varied within 26.5 pm/°C to -27.1 pm/°C when the filter central wavelength is tuned in the wavelength range of 1534 nm to 1551 nm. We use the transfer matrix method to theoretically model the lattice filter and its thermal-sensitivity before and after tuning is analyzed and discussed.

On-Chip Optical Power Monitor Using Periodically Interleaved P-N Junctions Integrated on a Silicon Waveguide

We investigate the photocurrent generation with surface-state absorption effect in a silicon waveguide integrated with periodically interleaved p-n junctions. Due to the high electric field (~5 × 105 V/cm) and large depletion area coverage in the waveguide, our device can collect more photocurrent than regular p-i-n and p-n structures. The responsivity of our device is optical power dependent with a higher value at a lower power level. The measured 3-dB bandwidth of the frequency response is 11.5 GHz. Although its responsivity is low compared to that of III-V and Ge photodiodes, its simple fabrication and compatibility with all-silicon photonic devices makes it suitable as on-chip optical power monitors.

Photoconductive effect on p-i-p micro-heaters integrated in silicon microring resonators

We study the photoconductive effect of a p-i-p micro-heater integrated in a microring resonator. Due to the surface state absorption (SSA) and two photon absorption (TPA) of optical wave around 1550 nm, free carriers are generated in the silicon waveguide, leading to the modulation of silicon conductivity and thus the current flowing through it. The current-voltage (I-V) response of the p-i-p diode is dependent on the bias voltage and can be divided into ohmic-law regime and space-charge-limited regime. The resonance peak current is more sensitive to optical power in the ohmic-law regime. Such a phenomenon can also be utilized to monitor the optical power in the waveguide.

Investigation of Coupling Tuning in Self-Coupled Optical Waveguide Resonators

We investigate the influence of coupling strength on self-coupled optical waveguide (SCOW) resonators. Experimental results reveal that the SCOW resonator transmission spectrum can exhibit single-channel or dual-channel stopbands with the band splitting level determined by the two coupling coefficients. Electrically tunable SCOW resonators comprising 2×2 Mach-Zehnder interferometer couplers are also demonstrated, which shows a similar change trend upon coupling tuning.

Optical signal processing using silicon resonance and slow-light structures

We present our recent work on silicon resonance and slow-light based devices for optical signal processing. Waveguide self-coupling and mutual coupling are used to tailor the waveguide spectral and dispersion characteristics. With selfcoupling, optical resonances are generated with unique transmission performances. Electromagnetically induced transparency (EIT)-like effect appears in cascaded self-coupled waveguides. With mutual coupling between ridge and slot waveguides, group velocity experiences a big jump and optical signal can be delayed for a large range with low distortion by using thermo-optic tuning.

Experimental demonstration of self-coupled optical waveguide (SCOW)-based resonators

We experimentally demonstrate a self-coupled optical waveguide (SCOW)-based optical resonator. Transmission spectra reveal the resonators can exhibit split, broadened, or enhanced resonance dips. The SCOW resonators can be used for second-order optical filters.

Optimized Silicon QPSK Modulator With 64-Gb/s Modulation Speed

We demonstrate a compact silicon quadrature phase-shift keying (QPSK) modulator consisting of two nested Mach-Zehnder interferometers with a size of 4.5-mm². The V₁₁ · L of the modulator is 1.4 V · cm with an insertion loss of 8 dB. In addition, 64-Gb/s QPSK modulation is achieved with an error vector magnitude (EVM) of 24.4% and bit error rate (BER) of 2.1 × 10^-5 at the received optical power of -10 dBm after the device and an EVM of 31.7% and BER of 8.4 × 10^-4 at the received optical power of 0 dBm after 10-km single-mode fiber transmission. The estimated power consumption is 7.1 pJ/bit for the 64-Gb/s QPSK modulation.