Daigao Chen

Highly efficient silicon optical polarization rotators based on mode order conversions.

We design and demonstrate the novel silicon optical polarization rotators (PRs) based on the TM(0)-TE(n)-TE(0) mode conversions inside the waveguide. The TM(0)-TE(n) mode converters are realized by the mode hybridization of the tapered rib waveguides. The TE(n)-TE(0) mode converters based on the beam shaping method are followed to complete the PRs function. By using the TE(1), TE(2), and TE(3) mode as the transitional mode, the fabricated PRs show the insert losses of less than 0.4, 0.5, and 1 dB, respectively. The corresponding polarization extinction ratios of larger than 21, 18, and 23 dB, over a wavelength range of 100 nm.

Low-loss and fabrication tolerant silicon mode-order converters based on novel compact tapers

We propose large bandwidth and high fabrication-tolerance mode-order converters on the silicon-on-insulator platform based on novel compact tapers structures. Each of the converters is in a single waveguide. Designs of different symmetries with and without breaking the parities between odd and even modes are illustrated. The fabrication tolerances of the devices are also investigated. The simulation results show that high conversion efficiencies can be readily achieved over a wavelength range from 1520 nm to 1580 nm for all of the proposed devices. The average conversion efficiencies of TE1-to-TE0, TE2-to-TE0, TE3-to-TE0, TE2-to-TE1, TE3-to-TE1, and TE3-to-TE2 converters are -0.061 dB, -0.052 dB, -0.11 dB, -0.119 dB, -0.168 dB, and -0.154 dB, respectively. The conversion efficiencies have negligible degradations under normal width and thickness deviations.

Mode-selective wavelength conversion of OFDM-QPSK signals in a multimode silicon waveguide

We experimentally demonstrate on-chip mode-selective wavelength conversions based on the degenerate four-wave mixing (FWM) nonlinear effect in a few-mode silicon waveguide. A multimode waveguide with tapered directional coupler based mode (de)multiplexers is designed and fabricated. Using signals with advanced modulation formats all-optical wavelength conversions of 102.6-Gb/s OFDM-QPSK signals are verified. Experimental results show that only small optical signal-to-noise ratio (OSNR) penalties are observed after wavelength conversion of both modes, which are less than 2 dB for OFDM-QPSK at 7% forward error correction (FEC) threshold.

Femtogram scale nanomechanical resonators embedded in a double-slot photonic crystal nanobeam cavity

An optomechanical device that contains a nanomechanical resonator with an ultralow effective mass of 6.42 fg is designed and demonstrated. The femtogram scale nanomechanical resonator is embedded in a double-slot photonic crystal nanobeam cavity. Optical resonance provides efficient readout of the nanomechanical resonator movements. The fabricated device is optically and mechanically characterized in atmosphere. In the measured radio-frequency power spectral density, a peak at 3.928 GHz is identified to be the mechanical mode with an effective mass of 6.42 fg. The measured room-temperature mechanical Q-factor is 1255, and a displacement sensitivity of 0.13 fm/ Hz, which is 22 times beyond the standard quantum limit, is obtained. These demonstrated on-chip integrated optomechanical devices combining high Q-factor optical cavities and nanomechanical resonators with ultralow effective masses are promising in ultrasensitive measurements.

Broadband, Fabrication-Tolerant Polarization Beam Splitters Based on a Tapered Directional Coupler

We propose and demonstrate a polarization beam splitter (PBS) based on a tapered directional coupler (DC). The tapered DC structure has the advantage of being insensitive to variations of the coupling length and the local coupling coefficient, and thus can significantly increase the bandwidth and polarization extinction ratio (ER). The fabricated PBS with one waveguide width tapered from 0.55 to 0.41 μm and another tapered reversely in a length of 29 μm shows a polarization ER above 16 dB and an insertion loss less than 0.4 dB over a wavelength range of 100 nm. The fabrication tolerance of the proposed PBS is also analyzed to be ±50 nm, ensuring the compatibility with the standard 180-nm complementary metal- oxide-semiconductor process.

Ultra-broadband 3dB power splitter based on silicon slot waveguide

We present the design of the ultra-broad bandwidth 3dB splitter based on silicon slot waveguide. The bandwidth of the proposed splitter is simulated as large as 600 nm, with the insertion loss less than 1dB.

A 32Gb/s-NRZ, 15GBaud/s-PAM4 DFB laser driver with active back-termination in 65nm CMOS

A 32Gb/s-NRZ, 15GBaud/s-PAM4 configurable DFB Laser Diode Driver (LDD) is presented in standard 65nm CMOS. The driver employs a balanced-input, single-ended-output topology to deliver large current output, and integrates a tunable pre-emphasis to extend the bandwidth. An on-chip active back-termination (ABT) is proposed which absorbs loading reflections without sacrificing the effective modulation current. Directly wire-bonded to a DFB laser chip, the measurement results show 25.78Gb/s NRZ optical eye-diagram with 4dB Extinction Ratio (ER) and a 19.3% eye-mask margin referred to the 100G specification. The LDD delivers 60mA bias and 60mApp modulation current, consuming only 550mW power consumption.

Silicon nitride O-band (de)multiplexers with low thermal sensitivity

In this paper, four-channel cascaded Mach-Zehnder interferometer-based wavelength (de)multiplexers in the O-band are demonstrated experimentally by utilizing silicon nitride (SiN) optical waveguides. By reference to the commonly used 100 Gigabit Ethernet standards, two types of (de)multiplexer devices with different channel spacings are designed and fabricated. Both the devices exhibit low insertion loss and flat passbands. The lower thermo-optical coefficient provided by SiN brings benefits of reduction in thermal sensitivity. The fabricated (de)multiplexers show a temperature-dependent wavelength shift of about 18.5 pm/°C, which is reduced by 75% compared to the standard silicon-based devices.

High speed silicon optical modulators: Applications, technologies and integrations

Silicon optical modulators, one of the essential building blocks of silicon photonics technology, have made rapid progress in the last decade and start to show the competitive performances when comparing to traditional LiNbO3 and III-V optical modulators.

Broadband polarization beam splitter based on a tapered mismatched directional coupler

We design and fabricate a broadband micro polarization beam splitter on SOI (Silicon-On-Insulator) substrate which is compatible with the 180 nm COMS process. The polarization splitter is based on mismatch coupling in which a tapered directional coupler structure with slowly varying waveguide width is used. The device is fabricated in Astar-IME silicon photonics platform. When the waveguide width of the wider port of the tapered waveguide is fixed at 550 nm, by sweeping the taper waveguide length in several waveguide width of the narrower port, the polarization splitter can realize high extinction ratios of the both polarizations in a broad wavelength range. When the waveguide width of the narrow port is 410 nm, the device can split the TE and TM polarizations in a 30 μm length with the extinction ratios of the both polarizations more than 15 dB over a wavelength range from 1500 nm to 1600 nm. The proposed polarization beam splitter is not sensitive to the fabrication error and has large tolerance. The polarization extinction ratios are still more than 10 dB in a wavelength range of 100 nm even if the waveguide width of the taper has a 50 nm fabrication error. Due to its simple structure and high performance, we believe this micro polarization beam splitter will be widely used in photonic integrated circuit, optical signal processing, and optical communication devices.