Zhenzhou Cheng

Wideband subwavelength gratings for coupling between silicon-on-insulator waveguides and optical fibers.

We propose and experimentally demonstrate a novel subwavelength grating coupler on silicon-on-insulator, for coupling to optical fibers with a wide optical bandwidth. Theoretical analysis and design optimization of the coupler are described. About 73 nm 1 dB bandwidth was experimentally demonstrated with -5.6  dB coupling efficiency. Better than -3.4  dB efficiency with 86 nm 1 dB bandwidth is predicted for these structures with optimized buried oxide thickness.

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.

Mid-Infrared Grating Couplers for Silicon-on-Sapphire Waveguides

Theoretical and experimental results of mid-infrared (mid-IR) gratings for coupling between transverse-electric (TE)/transverse-magnetic (TM) mode silicon-on-sapphire (SOS) waveguides and zirconium, barium, lanthanum, aluminum, and sodium fluoride (ZBLAN) fibers are presented. The shallow-etched uniform grating, full-etched subwavelength grating, and apodized grating are analyzed theoretically. TE mode shallow-etched apodized gratings with coupling efficiency of 80.6% and TM mode full-etched apodized subwavelength gratings with coupling efficiency of 39.6% are predicted at the wavelength of 2.75 by finite-difference time-domain (FDTD) simulation. An co-doped ZBLAN fiber laser is set up to be used as a mid-IR light source at 2.75 . Coupling efficiencies of 32.6% and 11.6% to TE mode and TM mode SOS waveguides, respectively, were obtained experimentally with uniform grating couplers. The dependence of coupling efficiency on etch depth, grating period, fill factor, and incident angle are also studied.

Apodized focusing subwavelength grating couplers for suspended membrane waveguides

We demonstrate an apodized focusing subwavelength grating (SWG) for suspended membrane waveguides on silicon-on-insulator. Finite-difference time-domain simulation predicts −1.7 dB coupling efficiency and a 3 dB bandwidth of ∼50 nm for the transverse-magnetic mode apodized SWG, which has 98% field overlap with propagation mode in the single mode fiber. A modified phase matching formula is proposed to design the focusing apodized SWG. Better than −3.0 dB coupling efficiency and a 3 dB optical bandwidth of ∼50 nm is demonstrated experimentally.

In-Plane Optical Absorption and Free Carrier Absorption in Graphene-on-Silicon Waveguides

We experimentally study the in-plane optical absorption and free carrier absorption (FCA) in graphene-on-silicon waveguides using a pump-probe measurement over microsecond timescales. The silicon waveguide is fabricated using complementary metal-oxide-semiconductor compatible processes, and directly covered by a graphene layer. Saturable absorption in the graphene is observed at the beginning of the pump pulse followed by an increase in absorption. The increase in absorption builds up over several microseconds, and is experimental evidence that free carriers generated by the pump absorption in graphene can transfer into silicon waveguides. The FCA in silicon waveguides eventually dominates the optical loss, which reaches ~9 dB, after several microseconds. All-optical modulations of the probe light are thus demonstrated. There is also a large thermally induced change in waveguide effective refractive index because of the optical absorption in the graphene.

Optical time-stretch imaging: Principles and applications

Breathtaking innovations in optical imaging have opened new exciting avenues for science, industry, and medicine over the last few decades. One of such innovations is optical time-stretch imaging—an emerging method for ultrafast optical imaging that builds on temporally stretching broadband pulses by using dispersive properties of light in both spatial and temporal domains. It achieves continuous image acquisition at an ultrahigh frame rate of 10–1000 million frames per second by overcoming technical and fundamental limitations that exist in traditional imaging methods. By virtue of its inherent affinity with optical signal processing, optical time-stretch imaging can be combined with various optical techniques such as amplification, nonlinear processing, compressive sensing, and pattern correlation to realize unique capabilities that are not possible with the traditional imaging methods. Applications enabled by such capabilities are versatile and include surface inspection, surface vibrometry, particle analysis, and cell screening. In this paper, we review the principles and limitations of conventional optical imaging, the principles and applications of optical time-stretch imaging, and discuss our future perspective.

Design of electro-optic modulators based on graphene-on-silicon slot waveguides

We present a graphene-on-silicon (GoS) suspended vertical slot waveguide. By changing the Fermi level of graphene, the variation in the effective refractive index (RI) of the waveguide is a factor of two larger than that in the traditional GoS rib waveguide. The improvement is due to the light-intensity enhancement and the poor confinement of the optical mode in the slot nanostructure. We design Mach-Zehnder interferometer (MZI) and microring modulators based on the GoS suspended vertical slot waveguide. Our calculations show that the modulators can be energy-efficient and footprint-compact due to the large phase shift of the propagating mode in the waveguide after applying a gate voltage on the graphene. Fabrication of our design is easy and CMOS-compatible. It paves the way for chip-integrated electronic-RI modulators.

Graphene photodetector integrated on silicon nitride waveguide

We demonstrated a graphene photodetector integrated on silicon nitride waveguide. The photodetector worked in the photoconductor mode. The detection mechanisms of the device were based on photo-thermoelectric effect and bolometric effect. The waveguide absorption (0.025 dB/μm) with a chemical vapor deposition grown monolayer graphene on top was studied experimentally. The measurement agreed well with the simulation result. The Fermi level of the top layer graphene in the photodetector was analyzed by using the field-effect transport measurement. A maximum internal responsivity of 126 mA/W with dynamic response of 1 K Hz was achieved in the telecommunication band. The unique combination of graphene and silicon nitride integrated circuit can potentially lead to unprecedented nonlinear and optoelectronic applications.

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.

Bit-Rate-Variable DPSK Demodulation Using Silicon Microring Resonators With Electro-Optic Wavelength Tuning

We demonstrate a bit-rate-variable differential phase-shift keying (DPSK) demodulator based on silicon microring resonators (MRRs) with an integrated diode for wavelength tuning. The resonance wavelength of the MRR may be tuned by carrier injection and depletion. Error-free operations (bit error rate <;10-9) have been achieved for all tuning wavelengths. In addition, the proposed scheme can demodulate DPSK signals over a wide range of bit rates from below 25 to 40 Gb/s.