T. Y. Chang

400-nm-Bandwidth Emission From a Cr-Doped Glass Fiber

Cr4+-doped glass fiber by a laser-heated pedestal growth method is reported. By analyzing the absorption spectra, the transitions of Cr3+ and Cr4+ in the fibers are identified. With appropriate pumping wavelength and divalent doping concentration, both Cr3+ and Cr4+ emission bands can be excited simultaneously, and become comparable in their fluorescent intensities. As a result, more than 400-nm-width emission peaked at 1144 nm was generated at room temperature. As much as 12muW of amplified spontaneous emission was obtained by pumping with a 900-nm Ti : sapphire laser

Efficient Nd:Y3Al5O12 Crystal Fiber Laser

Efficient Nd:Y3Al5O12 (Nd:YAG) lasers were demonstrated using gradient-index crystal fibers. A laser output power of 145 mW was achieved along with a slope efficiency of 28.9%. Up to 40% higher Nd ion concentration compared to that in the source rod at the center of the crystal fiber was obtained. Measurement by low-coherence reflectometry indicated an index difference of 0.0284 between the center and the edge of the crystal fiber.

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.

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

Accurate model including Coulomb-enhanced and Urbach-broadened absorption spectrum of direct-gap semiconductors

An accurate absorption model including Coulomb interaction and Urbach-broadened band edge has been demonstrated for direct bandgap semiconductors. We propose a steep-edged compound Lorentzian line-shape function for modeling the Urbach tail and the line broadening of exciton absorptions. The results of applying this fitting procedure to room-temperature and low-temperature absorption spectra of GaAs, InP, and InAs are presented, and a consistent set of band parameters are extracted. The analytical absorption model is suitable for a complete closed-form Kramers-Kronig transform of the absorption spectrum to obtain the refractive-index spectrum.

Fabrication of Cr-doped fibers by drawing tower

We report on the first fabrication of a Cr-doped fiber using a drawing-tower method with Cr:YAG as the core of the preform. The Cr-doped fibers have potential for use as fiber amplifier to cover the bandwidths in the whole 1.3-1.6 mum range

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.

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...

Ring resonator adopting multimode waveguide turning mirrors

The characteristic response of In_0.532Ga_0.468As/In_0.528Ga_0.257Al_0.215As on InP single ring resonators in the form of race tracks with a radius of 260(15⁰), 110(60⁰) and 260(15⁰) mum are presented. The used 2times2 multimode interference (MMI) couplers with coupling factor 0.85 are as short as 128 mum. Deep etching vertical mirror was used in the middle of MMI crossing. The free spectral ranges of 83 GHz and the through output (max)-drop (min) ratio of 21 dB have been achieved for the single ring resonator.

Compact 2×2 Couplers for unequal Splitting of Power obtained by Cascading of Short MMI Sections

More than 32% length reduction for 0%, 15%, and 28% cross-coupling multimode-interference couplers is obtained by cascading two short MMI sections. Previously unavailable cross coupling ratios of 7% and 64% are also obtained.