Qingzhong Deng

Strip-slot waveguide mode converter based on symmetric multimode interference.

Optical mode mismatch makes coupling between strip and slot waveguides a tough issue in integrated photonics. This Letter presents both numerical and experimental results of a strip-slot mode converter based on symmetric multimode interference (MMI). Distinct from previous reported converters which gradually convert the mode through sharp tips, the proposed solution makes full use of the symmetry of the two-fold image of MMI, and its field distribution similarity with a slot waveguide to convert the mode. A converter based on this mechanism is able to convert light from a TE-polarized fundamental mode of a strip waveguide to that of a slot waveguide, and vice versa. Strip-slot waveguide coupling though this mode converter has a measured efficiency of 97% (-0.13  dB), and the dimensions are as small as 1.24×6  μm. Further analysis shows that the proposed converter is highly tolerant to fabrication imperfections, and is wavelength-insensitive.

Subwavelength-grating-assisted broadband polarization-independent directional coupler.

This Letter presents both numerical and experimental results of a polarization-independent directional coupler based on slot waveguides with a subwavelength grating. The measured coupling efficiency is 97.4% for TE and 96.7% for TM polarization at a wavelength of 1550 nm. Further analysis shows that the proposed subwavelength grating directional coupler has a fabrication tolerance of ±20  nm for the grating structure and that the coupling efficiencies for the two polarizations are both higher than -0.5  dB (∼89%), exceeding the entire C-band (1525-1570 nm) experimentally.

Arbitrary-ratio 1 × 2 power splitter based on asymmetric multimode interference

Free choice of splitting ratio is one of the main properties of a power splitter required in integrated photonics, but conventional multimode interference (MMI) power splitters can only obtain a few discrete ratios. This Letter presents both numerical and experimental results of an arbitrary-ratio 1×2 MMI power splitter, which is constructed by simply breaking the symmetry of the multimode region. In the new device, the power splitting ratio can be adjusted continuously from 100:0 to 50:50, while the dimension of the multimode section stays in the range of 1.5×(1.8-2.8)  μm. The experimental data also indicate that the proposed arbitrary-ratio splitter keeps the original advantages of MMI devices, such as low excess loss, weak wavelength dependence, and large fabrication tolerance.

Active unidirectional propagation of surface plasmons at subwavelength slits

Highly efficient, active and compact, unidirectional surface plasmon (SP) propagator composed of double subwavelength slits; filled with organic electro-optic (EO) material is proposed and investigated. By selecting appropriate structure parameters, obtained by solving phase relations between slits, the relative phase of SP generated at the slit exit aperture can be tailored. Simulation results show under normal illumination and external voltage of 8.7 V, SP launching efficiency of 55% and unidirectional SP extinction ratio about 47dB at wavelength of 632.8 nm is achieved. The power consumption of the structure is on the order of 9 fJ/bit which meet the power consumption limitation for optical devices. Moreover, the structure is very compact with effective total length of 1.2 µm and thickness of 0.6 µm.

Robust polarization-insensitive strip-slot waveguide mode converter based on symmetric multimode interference

Strip-slot waveguide mode converters for TE0 have been widely investigated. Here we demonstrate a polarization-insensitive converter numerically and experimentally. The polarization-insensitive performance is achieved by matching the optical field distribution of the 2-fold image of the Multimode Interference (MMI) and the TE0 (TM0) mode of a slot waveguide. The working principle for this MMI-based mode converter is thoroughly analyzed with the quantitatively evaluated optical field overlap ratio that is theoretically derived from the orthonormal relation of eigenmodes. Based on the analysis, the MMI-based polarization-insensitive converters are then simulated and fabricated. The simulation and measurement results indicate that the proposed scheme is a robust design since it is not only polarization-insensitive but also wavelength-insensitive and fabrication-tolerant. Moreover, the mode converter is as small as 1.22 μm × 4 μm while the measured conversion efficiencies are 95.9% for TE0 and 96.6% for TM0. All these excellent properties make the proposed mode converter an ideal solution for coupling light between strip and slot waveguides when both TE and TM polarizations are considered.

Low loss, high-speed single-mode half-disk resonator.

This work proposes a new type of resonator: a single-mode half-disk resonator. Half of the resonator is solid, allowing large electrical or mechanical contacts, and a single-mode operation can be retained. A systematic design method is demonstrated and analyzed. For a 3 μm radius, the simulation predicts an internal Q-factor as high as 2.4×10⁵, and a loaded Q-factor of ~9000 is measured in experiments, comparable even with uncontacted microrings of bigger radii. The large contacts will improve the performance of a wide range of active devices. We present the contact resistance of a vertical PN junction modulator based on this structure, which can be reduced to nearly an order of magnitude, enabling a much faster modulation speed.

Athermal and flat-topped silicon Mach-Zehnder filters

Athermal and flat-topped transmissions are the two main requirements for a silicon WDM filter. A Mach-Zehnder (MZ) filter which simultaneously satisfies these two requirements has been experimentally demonstrated in this paper. A combination of strip waveguide and hybrid strip-slot waveguide is introduced for athermalization, and two-stage interference is utilized for flat-topped transmission. The temperature dependent wavelength shift is measured to be ~-5 pm/K while the best 1 dB bandwidth is 5.5 nm with 14.7 nm free spectral range (FSR). The measured minimum insertion loss is only 0.4 dB with a device dimension of 170 μm × 580 μm. Moreover, Such a MZ filter is compatible with the state-of-art CMOS-fabrication process and its minimum feature size is as large as 200 nm.

An ultra-compact MMI-based wavelength diplexer employing subwavelength grating

Through the refractive index engineering of the subwavelength grating, an ultra-compact multimode interference based diplexer is proposed and demonstrated. It is 43.4 μm in length, only ~30% of its conventional counterpart and displays a wide 1dB bandwidth >120nm.

Linear-regression-based approach for loss extraction from ring resonators

This Letter proposes and demonstrates a linear-regression-based loss-extraction model that is applicable for all-pass rings and symmetrically and asymmetrically coupled add-drop rings. The model is derived by transforming the transmission spectra of a ring into linear relationships with no approximation. An all-pass ring resonator is fabricated to verify the model, and the experimental results indicate that the proposed model is more reliable than previously reported models [Opt. Express14, 3872 (2006)OPEXFF1094-408710.1364/OE.14.003872; Opt. Express15, 10553 (2007)OPEXFF1094-408710.1364/OE.15.010553; and J. Lightwave Technol.16, 1308 (1998)JLTEDG0733-872410.1109/50.701410].

An Ultra-Compact Wavelength Diplexer Engineered by Subwavelength Grating

With the assistance of subwavelength grating (SWG), an ultra-compact multimode interference (MMI)-based diplexer is proposed. A certain number of SWG pitches are implemented in the middle of the MMI section, which can engineer the effective refractive index for the modes and, therefore, tune the beat length. With proper tailoring of the grating parameters, the beat lengths of the two wavelengths can be reduced as well as the device length, which has to match several odd or even times of beat lengths of both wavelengths. As a result, the proposed wavelength diplexer is