David Jui-Yang Feng

Waveguide couplers with new power splitting ratios made possible by cascading of short multimode interference sections

We show that it is possible to obtain 2 x 2 waveguide couplers with new power splitting ratios for cross coupling of 7%, 64%, 80% and 93% by cascading two short MMI sections. These couplers have simple geometry and low loss. They offer valuable new possibilities for designing waveguide power taps, high-Q ring resonators, ladder-structure optical filters, and loop-mirror partial reflectors.

Compact multimode interference couplers with arbitrary power splitting ratio

We show that it is possible to obtain 2 x 2 waveguide couplers with arbitrary power splitting ratios by interconnecting a pair of unequal width waveguides as the phase-tuning section into the middle of two short MMI sections. These couplers have simple geometry and low loss. They offer valuable new possibilities for designing waveguide-based photonic integrated circuits.

Novel Stepped-Width Design Concept for Compact Multimode-Interference Couplers With Low Cross-Coupling Ratio

We show through detailed simulations that the length of conventionally designed multimode-interference couplers for low (<50%) cross-coupling ratios can be shortened significantly by replacing a segment that is 3/2-times the coupling length by a minimum-width design. For the 15% cross-coupling device, this stepped-width design results in 32% length reduction. With additional incorporation of adiabatic tapering, the length reduction can increase to over 44%. Shortening of the device makes it possible to fabricate ring resonators with larger free spectral range, significantly increases the 1-dB bandwidth for the total transmittance, and also helps to reduce the radiative loss

Mode matching and insertion loss in ultrabroadband Cr-doped multimode fibers.

We investigate the fundamental insertion loss due to mode mismatch in an optical link involving a single-mode fiber-optic (SMF) transmission with the insertion of a segment of a multimode Cr(4+)-doped fiber (MMCDF). With an MMCDF core diameter of approximately 15.5 microm that matches the SMF, we obtained coupling efficiencies of 60.3% to 87.5% in the entire transmission spectral range of 1,300 to 1,600 nm. The high coupling efficiency (or low insertion loss) makes it possible for the employment of the MMCDF as an inline ultrabroadband optical amplifier in an optical link for the entire transmission spectral range.

Effects of Doping Ratio and Thermal Annealing on Structural and Electrical Properties of Boron-Doped ZnO Thin Films by Spray Pyrolysis

Boron-doped zinc oxide (BZO) thin films have been fabricated by spray pyrolysis on a glass substrate. The morphology and electrical properties of the thin films were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed. It was found that [B]/[Zn] ratio altered both the microstructure and concentration of the BZO thin films. The film grain size was reduced by increasing the [B]/[Zn] ratio. The highest Hall mobility was 3.65 cm2 V-1 s-1 for the undoped ZnO thin film, and the highest carrier concentration of 1.0×1019 cm-3 was achieved for the as-deposited BZO thin film with [B]/[Zn] = 1.5 at. %. Conductivity was determined at different measurement temperatures and shallow donors provided the dominate conduction mechanism for the as-deposited BZO thin films. After 600 °C annealing, shallow level reduction and donors with a high activation energy of 129±6 meV in the BZO thin films were characterized, and the shallow donors that dominate the carrier concentration for the as-deposited spray-pyrolized BZO thin film were eliminated.

Transmission and Coupling Characteristics of Ultra-Broadband Cr-Doped Multimode Fibers

We report the results of experimental and theoretical investigations of the transmission and coupling characteristics of an in-line multimode Cr-doped fiber (MMCDF) in a single mode fiber (SMF) optical link. This is the situation involved in the employment of a broadband MMCDF optical amplifier in an optical network. The excitation of several modes in the MMCDF can lead to a severe insertion loss. By selecting a proper core diameter of the MMCDF, it is possible to achieve mode-matching at the junctions between the SMF and the MMCDF. This leads to the excitation of predominantly the fundamental LP01 mode in a step-index MMCDF, result in a minimization of the insertion loss at both ends of the MMCDF. Using an MMCDF with a length of 12.5 cm and an average core diameter of 13.5 mum, we obtained a coupling efficiency of 64.6% ~ 74.1% in the spectral region from 1300 to 1600 nm. The measured coupling efficiencies are in good agreement with our theoretical modeling. The result indicates that the low insertion loss of MMCDF may be used as an inline ultra-broadband optical amplifier in an optical link for the entire 1300 to 1600 nm spectral range of low-loss windows of silica fibers. Our results are presented and discussed.

Modulation-Doped InGaAlAs/InP Semiconductor Optical Amplifier Structures Grown by Molecular Beam Epitaxy

Semiconductor optical amplifier (SOA) structures have been designed and grown by molecular beam epitaxy for multi-functional integration of 1.55-µm photonic devices on InP. Triple n-type modulation-doped, strain-balanced compact quantum wells (QWs) are designed to provide desirable gain characteristics under forward bias and clean index changes under reverse bias. The largest refractive index changes with low levels of electroabsorption have been obtained for a sample with modulation doping of 4.27 ×1011 cm-2 per QW and with a hole-stopping barrier.

Photocatalytic study of zinc oxide with different bismuth doping

The zinc oxides (ZnO) films with different bismuth (Bi) doping were prepared by spray pyrolysis deposition at 450°C with aqueous solution contained zinc acetate and bismuth nitrate precursors. Under ultra-violate light illumination, the photodegradation of methyl orange for these ZnO films were studied. In the singly doped ZnO, the 5% Bi doped ZnO shows the relatively high photocatalytic activity. The reaction rate was achieved and possible reason was discussed. It is found that the raction rate can be enhanced by increasing the surface doping. The absorption coefficient was investigated and possible reason was discussed.

InGaAlAs∕InGaAs strain-balanced multi-quantum-well laser/semiconductor optical amplifiers operating at excited transitions

We report the design, growth, and fabrication of strain-balanced n-type modulation-doped InGaAlAs∕InGaAs multiple quantum well laser/semiconductor optical amplifiers on InP. The quantum well contains a lattice-matched InGaAs core, a compressive-strained InGaAs padding, and a tensile-strained InGaAlAs spacer. Electroluminescence spectra show higher optical gain for the quantum well e1-hh2 transition at λ=1460nm than the e1-hh1 transition at λ=1550nm. Ridge-waveguide lasers of Fabry-Perot (FP) type and tilted-end-facet (TEF) type were fabricated. The FP laser shows a lasing peak of λ=1514nm at threshold. Additional lasing wavelengths at λ=1528 and 1545nm were observed sequentially as the injection current increased. For the TEF laser, only the emission at λ=1511nm was observed. These TE-polarized lasing wavelengths are consistent with the δ-like absorption peaks in photocurrent spectra. The lasing performance is attributed to optical transitions within quantum dots/wires which are formed by the strain-field...

GaAsSb spacer effect in quasi-type-II InAs coupled-QDs for intraband absorption enhancement

We designed an InAs coupled quantum-dot (QD) structure with a GaAsSb spacer to form an intermediate band (IB). The electron and hole states are calculated using the k ⋅ p method. The numerical results revealed the band alignment changes to be quasi-type-II with 16% Sb. The 1 nm AlAs layers around the QD and 1 nm GaAs layer help in broadening the intraband absorption spectrum from the far infrared region to infrared range. The coupling QD structure with an 8.5 nm GaAsSb spacer and 16% Sb concentration exhibits better photoelectric efficiency for intermediate band solar cell in the simulation, with a 3.3% enhancement of the same PIN structure with GaAs as the intrinsic region. Introducing a GaAsSb layer in the coupling QD structure will also release the maximum shear stress in QD, exhibiting a 3% release with 16% a GaAsSb spacer.