Nancy A. Morris

Morphology-dependent resonances of a microsphere–optical fiber system

Morphology-dependent resonances of microspheres sitting upon an index-matched single-mode fiber half-coupler are excited by a tunable 753-nm distributed-feedback laser. Resonance peaks in the scattering spectra and associated dips in the transmission spectra for the TE and TM modes are observed. We present a new model that describes this interaction in terms of the fiber-sphere coupling coefficient and the microspheres intrinsic quality factor Q(0). This model enables us to obtain expressions for the finesse and the Q factor of the composite particle-fiber system, the resonance width, and the depth of the dips measured in the transmission spectra. Our model shows that index matching improves the coupling efficiency by more than a factor of 2 compared with that of a non-index-matched system.

High-power separate-confinement heterostructure AlGaAs/GaAs laser diodes with broadened waveguide

AlGaAs/GaAs graded-index separate-confinement heterostructure single quantum well (GRINSCH-SQW) lasers with different waveguide thickness have been analyzed experimentally and compared with results from modeling using transverse optical field distributions. We have found that for GRINSCH lasers the halfwidth of near-field and far-field patterns depends very weakly on the waveguide thickness due to the focusing of the optical field in the transverse direction by the graded-index waveguide. At the same time, the mode intensity in the cladding layers is reduced by two orders of magnitude as the waveguide thickness is increased from 40 nm to 1200 nm. As a result, a 20% improvement in the differential quantum efficiency ((eta) d) is realized, while the threshold current density remains unchanged. Differential quantum efficiency as high as 78% and output power exceeding 4 W cw have been obtained for broadened waveguide lasers.

Single-mode distributed-feedback 761-nm GaAs-AlGaAs quantum-well laser

We have developed single-mode distributed-feedback 761-nm GaAs-AlGaAs quantum well lasers as sources for O/sub 2/ sensing through laser absorption spectroscopy. Devices containing a 4-/spl mu/m-wide ridge waveguide exhibit low threshold currents of 25 mA and quantum efficiencies greater than 35% at output powers in excess of 25 mW. The spectral linewidths of these devices are 12.0 MHz at 15 mW. Temperature- and current-tuning rates are 0.06 nm//spl deg/C and 0.0075 nm/mA (-3.9 GHz/mA), respectively. The devices display smooth, continuous, single-mode wavelength tuning over a 4.2 nm interval.<<ETX>>

Tunable single-frequency III-V semiconductor diode lasers with wavelengths from 0.76 to 2.7 μm

We have fabricated single-frequency diode lasers from a number of III-V semiconducting compounds. These diode lasers were specifically designed for laser absorption spectroscopy. Their emission wavelengths span the internal of 0.76 to 2.7 micrometers . Water vapor, CO, CO2, NH3, CH4 HF, and O2 have been detected using them. After a brief review of their physical structure and principles of operation, we present representative output characteristics of these lasers, along with a discussion of several important applications.

Signal processing strategies in tunable diode laser spectrometers

Abstract Digital signal processing algorithms were applied to diode laser frequency modulation spectrometers in the near- and mid-infrared spectral ranges. These algorithms include digital bandpass filters, a Wiener filter, a matched filter, and a Kalman filter. Digital signal processing is easy to implement and adapt to all experimental configurations without any physical modifications or additions to the spectrometer.

Minority-carrier transport in InGaAsSb thermophotovoltaic diodes

Uncoated InGaAsSb/GaSb thermophotovoltaic (TPV) diodes with 0.56 eV (2.2 {micro}m) bandgaps exhibit external quantum efficiencies of 59% at 2 {micro}m. The devices have electron diffusion lengths as long as 29 {micro}m in 8-{micro}m-wide p-InGaAsSb layers and hole diffusion lengths of 3 {micro}m in 6-{micro}m-wide n-InGaAsSb layers. The electron and hole diffusion lengths appear to increase with increasing p- and n-layer widths. At 632.8 nm the internal quantum efficiencies of diodes with 1- to 8-{micro}m-wide p-layers are above 89% and are independent of the p-layer width, indicating long electron diffusion lengths. InGaAsSb has, therefore, excellent minority carrier transport properties that are well suited to efficient TPV diode operation. The structures were grown by molecular-beam epitaxy.

A novel approach for the improvement of open circuit voltage and fill factor of InGaAsSb/GaSb thermophotovoltaic cells

Heterojunction n-Al0.25Ga0.75As0.02Sb0.98/p-In0.16Ga0.84As0.04Sb0.96 thermophotovoltaic (TPV) cells were grown by molecular-beam epitaxy on n-GaSb-substrates. In the spectral range from 1 μm to 2.1 μm these cells, as well as homojunction n-p-In0.16Ga0.84As0.04Sb0.96 cells, have demonstrated internal quantum efficiencies exceeding 80%, despite about a 200 meV barrier in the conduction band at the heterointerface. Estimation shows that the thermal emission of the electrons photogenerated in p-region over this barrier can provide high efficiency for hetero-cells if the electron recombination time in p-In0.16Ga0.84As0.04Sb0.96 is longer than 10 ns. Keeping the same internal efficiency as homojunction cells, hetero-cells provide a unique opportunity to decrease the dark forward current and thereby increase open circuit voltage (Voc) and fill factor at a given illumination level. It is shown that the decrease of the forward current in hetero-cells is due to the lower recombination rate in n-type wider-bandgap s...

Single-mode distributed-feedback GaAs/AlGaAs quantum well lasers for laser absorption spectroscopy

Laser absorption spectroscopy using III-V semiconductor laser diodes has several advantages for gas sensing applications, as compared with traditional methods employing tunable dye laser and II-VI (e.g., lead salt) laser sources. These advantages include room-temperature operation, reduced cost, and compact size. Limited coverage of spectroscopy wavelengths by high-performance III-V lasers has prevented their widespread application to gas sensing. At those fixed wavelengths, performance of commercially available devices has been limited by multimode emission and/or inadequate wavelength tuning and mode hops. These spectra can, however, be greatly improved by incorporating frequency-selective structures. We have developed single-mode distributed-feedback (DFB) GaAs/AlGaAs quantum well lasers applicable to laser spectroscopy of molecules absorbing in the wavelength interval from 760 to 840 nm. These devices exhibit low threshold current (< 20 mA), high efficiency (> 40%), high output power (> 25 mW), and narrow linewidth (< 3.0 MHz). The lasers display smooth, continuous, single-mode wavelength tuning over 5 nm. Typical temperature and current wavelength-tuning coefficients are 0.065 nm/ degree(s)C and 0.0075 nm/mA (approximately -3.5 GHz/mA), respectively. In preliminary tests, they have been applied to the detection of H2O vapor and O2 gas.