Manyalibo Joseph Matthews

10-Gb/s upgrade of bidirectional CWDM systems using electronic equalization and FEC

We discuss options for upgrading coarse wavelength-division multiplexed (CWDM) optical access links over standard single-mode fiber (SSMF) by increasing per-channel data rates from 2.5 to 10 Gb/s. We identify electronic equalization and forward error correction (FEC) as the enabling technologies to overcome the dispersion limit of SSMF. In addition, we show how FEC enhances the tolerance to in-band crosstalk, and paves the way toward fully bidirectional CWDM transmission. Due to the lack of CWDM sources rated for 10-Gb/s operation, we demonstrate full-spectrum (1310 to 1610 nm) 10-Gb/s CWDM transmission over standard-dispersion fiber using uncooled, directly modulated lasers specified for 2.5 Gb/s. All 16 CWDM channels could be transmitted over more than 40 km, yielding a capacity-times-distance product of 6.4 Tb/s/km. The longest transmission distance (80 km) was achieved at 1610 nm, equivalent to 1600 ps/nm of chromatic dispersion.

Spatial variation of electrical properties in lateral epitaxially overgrown GaN

Using confocal Raman and scanning probe microscopies, we show that the electrical properties of lateral epitaxial overgrown GaN films vary at the submicron scale. Wing regions, which are located directly above the SiOx stripes, contain carrier densities ∼1020 cm−3, but possess a Fermi level deep in the band gap. This cannot be explained by having a high density of free electrons in the conduction band, but is consistent with high levels of compensation and impurity band transport. In the coalescence region, stripes of different electrical properties are evident, indicating the incorporation of impurities and defects being dictated by the growth dynamics.

Gain in a quantum wire laser of high uniformity

A multi-quantum-wire laser operating in the one-dimensional (1D) ground state has been achieved in a very high uniformity structure that shows free-exciton emission with unprecedented narrow width and low lasing threshold. Under optical pumping, the spontaneous emission evolves from a sharp free exciton peak to a redshifted broad band. The lasing photon energy occurs about 5 meV below the free exciton. The observed shift excludes free excitons in lasing and our results show that Coulomb interactions in the 1D electron-hole system shift the spontaneous emission and play significant roles in laser gain.

Characterization of phosphosilicate thin films using confocal Raman microscopy

We have demonstrated a characterization tool based on confocal Raman microscopy capable of studying the vibrational spectrum of silica-on-silicon-based thin films within a confined, ∼1 μm3-size volume beneath the sample surface. The Raman spectra of a set of phosphosilicate thin film samples have been quantitatively analyzed and correlated with both phosphorus concentration CP and refractive index n, as determined by conventional methods. The normalized intensity of the P=O vibration scaled linearly with CP and n, and allowed for the calibration of the Raman measurements to a precision of ∼0.2 wt. %P and ∼10−4 in index. The capability of this technique for studying index and dopant profiles in arbitrary systems is also discussed.

Carrier density imaging of lateral epitaxially overgrown GaN using scanning confocal Raman microscopy

GaN thin films, grown by the lateral epitaxial overgrowth (LEO) method, are studied by scanning confocal Raman microscopy. By measuring changes in coupled longitudinal-optical phonon–plasmon frequencies and using a standard harmonic oscillator dielectric function, detailed images of carrier density could be formed. Carrier concentrations are extremely high (∼1020 cm−3) immediately above SiOx mask layers and decrease abruptly when the SiOx mask are not directly exposed to the growth surface, implying that SiOx masks are the source of dopants. Images of intergrated E1 longitudinal-optical phonon intensities could be compared with free-carrier images and showed a clear anticorrelation throughout the LEO structure.

A microscope for imaging, spectroscopy, and lithography at the nanometer scale: Combination of a two-photon laser scanning microscope and an atomic force microscope

We designed and built a unique instrument that combines a two-photon laser scanning microscope (LSM) with an inverted atomic force microscope (AFM). Local photoluminescence (PL) spectroscopy and three-dimensional lithography are demonstrated using the two-photon LSM. High spatial resolution topographic images from the AFM can be recorded simultaneously with the PL images of the same region, allowing us to correlate PL variation and surface features of the sample. The wavelength of the short-pulse laser excitation can be varied continuously from 700 to 800 nm while the detection setup is optimized for signals between 350 and 650 nm. We demonstrate the performance of this instrument by examining the spatial variation of PL signals in GaN samples and by fabricating photonic crystal structures in polymer films.