Shuh-Ying Lee

High-index-contrast waveguides and devices

We present a theoretical and experimental study of high-index-contrast waveguides and basic (passive) devices built from them. Several new results are reported, but to be more comprehensive we also review some of our previous results. We focus on a ridge waveguide, whose strong lateral confinement gives it unique properties fundamentally different from the conventional weakly guiding rib waveguides. The ridge waveguides have distinct characteristics in the single-mode and the multimode regimes. The salient features of the single-mode waveguides are their subwavelength width, strong birefringence, relatively high propagation loss, and high sensitivity to wavelength as well as waveguide width, all of which may limit device performance yet provide new opportunities for novel device applications. On the other hand, wider multimode waveguides are low loss and robust. In addition, they have a critical width where the birefringence is minimal or zero, giving rise to the possibility of realizing intrinsically polarization-independent devices. They can be made effectively single mode by employing differential leakage loss (with an appropriate etch depth) or lateral mode filtering (with a taper waveguide). Together these waveguides provide the photonic wire for interconnections and the backbone to build a broad range of compact devices. We discuss basic single-mode devices (based on directional couplers) and multimode devices (multimode interferometers) and indicate their underlying relationship.

Dry Etched Waveguide Laser Diode on GeOI

We demonstrate a top-top contact, dry etched mirror facet III-V waveguide laser diode grown on germanium-on-insulator (GeOI). A 3 × InGaAs/GaAs quantum well designed to lase at 985 nm was grown on a GaAs buffer, which was lattice matched to the GeOI platform in order to realize the monolithic integration of III-V electronic and photonic devices with silicon and SiO2. Lasing occurred at ~ 985 nm with a threshold current density of ~ 2 kA/cm2. Pulsed measurements, and SEM and TEM characterization methods were used. The issue of heat transfer limited the performance of the laser.

Transformation between directional couplers and multi-mode interferometers based on ridge waveguides

We present a unique comparison of ridge-type directional couplers (DC) and multi-mode interferometers (MMI) in terms of their transformational relationship. The two devices are intimately related as the MMI is derived from the DC. We show for the first time the continuous evolution from the two-mode coupling characteristic of DC to the multimode mixing and interference characteristic of MMI, as the DC is structurally transformed into the MMI. We also show that DC can be designed to have the MMI features of compactness and polarization-insensitivity, two traits that reflect their shared lineage. However, the design of DC requires careful control of a large set of design parameters, while the MMI design is more robust and involves fewer design variables.

Polymeric Optical Splitter Based on Multimode Interference Mechanism

A 1 times 2 and 1 times 3 planar optical splitter based on BenzoCyclobutene (BCB 4024-40) polymeric material is proposed. The device is designed based on symmetric interference mechanism, utilizing a BK7 glass as a substrate and thin layer of SiO2 as a cover. Simulation results show that the splitting uniformity is better than 0.5 dB at 1550 nm optical wavelength.

A rigorous comparative analysis of directional couplers and multimode Interferometers based on ridge waveguides

We present a rigorous comparison of the unique characteristics of directional couplers and multimode interferometers based on the unique properties of high-index contrast ridge waveguides. The two devices are intimately related as the multimode interference (MMI) is derived from the directional couplers (DCs). We show for the first time the continuous evolution from the two-mode coupling characteristic of DC to the multimode mixing and interference characteristic of MMI, as the DC is structurally transformed into the MMI. We also show that DC can be designed to have the MMI features of compactness and polarization-insensitivity, two traits that reflect their shared lineage. The performance of MMI and DC are compared in terms of coupling length, polarization dependence, crosstalk, excess loss, and fabrication tolerances. We show that the DC, as long as it is designed to have nearly the same coupling length for transverse electric and transverse magnetic, can potentially have better performance than the MMI in terms of crosstalk and polarization sensitivity. Such a DC, however, requires careful control of many design parameters, while the MMI design is more robust and involves fewer design variables. Finally, the effect of higher-order modes and mode filtering are also considered.

A traveling-wave electroabsorption modulator with a large optical cavity and intrastep quantum wells

This paper reports a novel traveling-wave electroabsorption modulator (TWEAM) with a large optical cavity waveguide and an intrastep quantum well structure designed to achieve a high bandwidth, high saturation power and better fiber-matched optical profile, which is good for high coupling efficiency. The optical mode characteristic shows a great improvement in matching the circular mode of the fiber and the saturation power of 21 dBm, and a 3 dB bandwidth of 23 GHz was achieved for the fabricated TWEAM.

Two-micron-wavelength germanium-tin photodiodes with low dark current and gigahertz bandwidth

We report the demonstration of a germanium-tin (Ge0.9Sn0.1) multiple-quantum-well p-i-n photodiode on silicon (Si) substrate for 2 μm-wavelength light detection. Characterization of the photodetector in both direct current (DC) and radio frequency (RF) regimes was performed. At the bias voltage of -1 V, a dark current density of 0.031 A/cm2 is realized at room-temperature, which is among the lowest reported values for Ge1-xSnx-on-Si p-i-n photodiodes. In addition, for the first time, a 3 dB bandwidth (f3dB) of around 1.2 GHz is achieved in Ge1-xSnx photodetectors operating at 2 μm. It is anticipated that further device optimization would extend the f3dB to above 10 GHz.

Integration of multimode interference device with electroabsorption modulators as simple switches

A 1×2 multimode interference device is monolithically integrated to two electroabsorption modulator based switches at the output ports by means of quantum-well intermixing. At the wavelength range of 1550–1570nm, both arms act as separate DC switches which can be modulated with the extinction ratios of up to 10dB. Monolithic integration of active and passive photonic components is attractive due to its miniaturized size which can simultaneously satisfy multifunctional purposes. The potential of compacted optical circuits along with low-cost batch fabrications and high stability ensures the viability of such endeavours. The integration of an MMI device (passive device) with EAMs (active devices) enables it to be used as a compact multiplexer or optical transmitter. The prototype made is of a relatively larger size (∼2000µm) but its size can be potentially reduced to around 500µm or less. The material structure employed is the InGaAlAs/InGaAlAs MQW with dual depletion structure (DDR) and large optical cavity (LOC) structure design. The LOC material structure was designed for the realization of lower insertion loss. The QWI technology utilized here is the impurity-free vacancy diffusion (IFVD) method and it resulted in a 56nm blue-shift in the bandgap structure. The etching process executed here was the dry etching process using inductively coupled plasma (ICP) etching based on a CH4/Cl2/H2 gas combination. The MMI length is 1150µm whereas the modulators are both 500µm long.

Integration of InGaAs MOSFETs and GaAs/ AlGaAs lasers on Si Substrate for advanced opto-electronic integrated circuits (OEICs)

Lasers monolithically integrated with high speed MOSFETs on the silicon (Si) substrate could be a key to realize low cost, low power, and high speed opto-electronic integrated circuits (OEICs). In this paper, we report the monolithic integration of InGaAs channel transistors with electrically pumped GaAs/AlGaAs lasers on the Si substrate for future advanced OEICs. The laser and transistor layers were grown on the Si substrate by molecular beam epitaxy (MBE) using direct epitaxial growth. InGaAs n-FETs with an ION/IOFF ratio of more than 106 with very low off-state leakage and a low subthreshold swing with a minimum of 82 mV/decade were realized. Electrically pumped GaAs/AlGaAs quantum well (QW) lasers with a lasing wavelength of 795 nm at room temperature were demonstrated. The overall fabrication process has a low thermal budget of no more than 400 °C.

Demonstration of a single-mesa vertical coupler and its integration with a multimode interferometer

We demonstrate for the first time, to the best of our knowledge, a vertical coupler based only on a single-mesa structure. The vertical coupler requires simpler fabrication steps as compared to the conventional double-mesa structure. We are proposing a concept of realizing a vertical coupler based on the single-mesa structure, regardless of the material system, and the device performance shown is by no means the best the design could give. By conservative estimation from experimental data, the transfer efficiency is at least 16%. The device integration with a 1 × 2 multimode interferometer is also demonstrated, giving a splitting ratio of almost 50:50. The versatile design concept could be implemented in other material systems.