Fow-Sen Choa

Room-temperature continuous-wave quantum cascade lasers grown by MOCVD without lateral regrowth

We report on room-temperature continuous-wave (CW) operation of lambda~8.2 mum quantum cascade lasers grown by metal-organic chemical vapor deposition without lateral regrowth. The lasers have been processed as double-channel ridge waveguides with thick electroplated gold. CW output power of 5.3 mW is measured at 300 K with a threshold current density of 2.63 kA/cm2. The measured gain at room temperature is close to the theoretical design, which enables the lasers to overcome the relatively high waveguide loss

A room-temperature semiconductor spaser operating near 1.5 μm

Room temperature spasing of surface plasmon polaritons at 1.46 μm wavelength has been demonstrated by sandwiching a gold-film plasmonic waveguide between optically pumped InGaAs quantum-well gain media. The spaser exhibits gain narrowing, the expected transverse-magnetic polarization, and mirror feedback provided by cleaved facets in a 1-mm long cavity fabricated with a flip-chip approach. The 1.06-μm pump-threshold of ~60 kW/cm2 is in good agreement with calculations. The architecture is readily adaptable to all-electrical operation on an integrated microchip.

Slow-wave finite-difference beam propagation method

By invoking the slow-wave approximation, the wave equation resumes the form of the Fresnel equation. Codes developed previously for the paraxial beam propagation can be extended to simulate the backward reflection and diffraction at any angle. Results of planar waveguide gratings and a beveled corner bend are presented. >

Midinfrared semiconductor optical metamaterials

We report on a novel class of semiconductor metamaterials that employ a strongly anisotropic dielectric function to achieve negative refraction in the midinfrared region of the spectrum, ∼8.5–13 μm. We present two types of metamaterials, layered highly doped/undoped heterostructures and quantum well superlattices that are highly anisotropic. Contrary to other optical metamaterials these heterostructure systems are optically thick (up to 20 μm thick), planar, and require no additional fabrication steps beyond the initial growth. Using transmission and reflection measurements and modeling of the highly doped heterostructures, we demonstrate that these materials exhibit negative refraction. For the highly doped quantum well superlattices, we demonstrate anomalous reflection due to the strong anisotropy of the material but a determination of the sign of refraction is still difficult. This new class of semiconductor metamaterials has great potential for waveguiding and imaging applications in the long-wave inf...

An optical packet switch based on WDM technologies

Dense wavelength-division multiplexing (DWDM) technology offers tremendous transmission capacity in optical fiber communications. However, switching and routing capacity lags behind the transmission capacity, since most of todays packet switches and routers are implemented using slower electronic components. Optical packet switches are one of the potential candidates to improve switching capacity to be comparable with optical transmission capacity. In this paper, we present an optically transparent asynchronous transfer mode (OPATM) switch that consists of a photonic front-end processor and a WDM switching fabric. A WDM loop memory is deployed as a multiported shared memory in the switching fabric. The photonic front-end processor performs the cell delineation, VPI/VCI overwriting, and cell synchronization functions in the optical domain under the control of electronic signals. The WDM switching fabric stores and forwards cells from each input port to one or more specific output ports determined by the electronic route controller. We have demonstrated with experiments the functions and capabilities of the front-end processor and the switching fabric at the header-processing rate of 2.5 Gb/s. Other than ATM, the switching architecture can be easily modified to apply to other types of fixed-length payload formats with different bit rates. Using this kind of photonic switch to route information, an optical network has the advantages of bit rate, wavelength, and signal-format transparencies. Within the transparency distance, the network is capable of handling a widely heterogeneous mix of traffic, including even analog signals.

Ring-assisted frequency discriminator with improved linearity

A frequency discriminator combining a Mach-Zehnder interferometer with ring resonators is proposed, analyzed and shown to offer a large improvement in linearity without any significant reduction in bandwidth. This can improve the performance of analog optical links by canceling the intermodulation distortion.

TIME-RESOLVED-SPECTRUM STUDIES OF GAN LIGHT EMITTING DIODES

Time‐resolved‐emission spectra of InGaN and GaN light emitting diodes with different device structure are studied. Result shows that the UV generation from bulk materials and blue and green emissions from single quantum well devices are band edge recombination.

A monolithic 5 Gb/s p-i-n/HBT integrated photoreceiver circuit realized from chemical beam epitaxial material

The authors report on a high performance monolithic photoreceiver fabricated from chemical beam epitaxy (CBE) grown InP/InGaAs heterostructures, incorporating a p-i-n photodetector followed by a transimpedance preamplifier circuit configured from heterojunction bipolar transistors (HBTs). The optoelectronic integrated circuit (OEIC) was fabricated on a semi-insulating Fe-doped InP substrate. Microwave on-wafer measurements of the frequency response of the transistors yielded unity current gain cutoff frequencies of 32 GHz and maximum oscillation frequencies of 28 GHz for collector currents between 2 and 5 mA. The photoreceiver was operated up to 5 Gb/s, at which bit rate a sensitivity of -18.8 dBm was measured at a wavelength of 1.5 mu m. The results demonstrate that the CBE growth technique is suitable for high performance HBT-based OEICs.<<ETX>>

Low threshold quantum-cascade lasers of room temperature continuous-wave operation grown by metal-organic chemical-vapor deposition

Very low threshold current densities are reported from buried heterojunction quantum-cascade laser (QCL) grown by metal-organic chemical-vapor deposition at an emission wavelength of 5.07μm. Continuous-wave operation threshold current densities as low as 0.75 and 0.83kA∕cm2, efficiencies of 1079 and 879mW∕A, and total output powers of 116 and 74mW have been achieved for temperatures of 288 and 298K. These cw threshold current densities are lower than all previously reported values for Fabry-Perot QCLs. Internal loss as low as 1.84cm−1 has been extracted; also the lowest value thus far reported in the literature.

Quantum cascade laser based standoff photoacoustic chemical detection

Standoff chemical detection with a distance of more than 41 feet using photoacoustic effect and quantum cascade laser (QCL) operated at relatively low power, less than 40 mW, is demonstrated for the first time. The option of using QCL provides the advantages of easy tuning and modulation besides the benefit of compact size, light weight and low power consumption. The standoff detection signal can be calibrated as a function of different parameters such as laser pulse energy, gas vapor concentration and detection distance. The results yield good agreements with theoretical model. Techniques to obtain even longer detection distance and achieve outdoor operations are in the process of implementation and their projection is discussed.