Gregg W. Switzer

A microbiologically clean strategy for access to the Whillans Ice Stream subglacial environment

Abstract The Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) project will test the overarching hypothesis that an active hydrological system exists beneath a West Antarctic ice stream that exerts a major control on ice dynamics, and the metabolic and phylogenetic diversity of the microbial community in subglacial water and sediment. WISSARD will explore Subglacial Lake Whillans (SLW, unofficial name) and its outflow toward the grounding line where it is thought to enter the Ross Ice Shelf seawater cavity. Introducing microbial contamination to the subglacial environment during drilling operations could compromise environmental stewardship and the science objectives of the project, consequently we developed a set of tools and procedures to directly address these issues. WISSARD hot water drilling efforts will include a custom water treatment system designed to remove micron and sub-micron sized particles (biotic and abiotic), irradiate the drilling water with germicidal ultraviolet (UV) radiation, and pasteurize the water to reduce the viability of persisting microbial contamination. Our clean access protocols also include methods to reduce microbial contamination on the surfaces of cables/hoses and down-borehole equipment using germicidal UV exposure and chemical disinfection. This paper presents experimental data showing that our protocols will meet expectations established by international agreement between participating Antarctic nations.

Extending the continuous tuning range of an external-cavity diode laser

The continuous tuning range of an external-cavity diode laser can be extended by making small corrections to the external-cavity length through an electronic feedback loop so that the cavity resonance condition is maintained as the laser wavelength is tuned. By maintaining the cavity resonance condition as the laser is tuned, the mode hops that typically limit the continuous tuning range of the external-cavity diode laser are eliminated. We present the design of a simple external-cavity diode laser based on the Littman-Metcalf external-cavity configuration that has a measured continuous tuning range of 1 GHz without an electronic feedback loop. To include the electronic feedback loop, a small sinusoidal signal is added to the drive current of the laser diode creating a small oscillation of the laser power. By comparing the phase of the modulated optical power with the phase of the sinusoidal drive signal using a lock-in amplifier, an error signal is created and used in an electronic feedback loop to control the external-cavity length. With electronic feedback, we find that the continuous tuning range can be extended to over 65 GHz. This occurs because the electronic feedback maintains the cavity resonance condition as the laser is tuned. An experimental demonstration of this extended tuning range is presented in which the external-cavity diode laser is tuned through an absorption feature of diatomic oxygen near 760 nm.

Tunable external cavity diode laser based on integrated waveguide structures

A tunable external cavity laser based on integrated optics produced in a KTP substrate is demonstrated. The laser operates single mode with side mode suppression greater than 35dB and an electrooptic tuning response of R=19.5MHz/V.

Blue light generation in an external ring cavity using both cavity and grating feedback

A simple scheme for the efficient frequency doubling of a Fabry–Perot diode laser in a bow tie ring cavity is described. By feeding back the ring cavity transmission into the laser after reflection off a grating, the diode laser operates in single mode and the frequency locks to the cavity resonance. Using a simple analytic form of the beam parameters for the ring cavity, the optimum cavity configuration is found for a high degree of mode matching between the diode output beam and the ring cavity.

Design and performance of a frequency chirped external cavity diode laser

The design of a frequency chirped external cavity diode laser (CECDL) in the Littrow configuration with an intra-cavity electro-optic crystal is presented and its tuning and noise performance is studied. The CECDL has a center wavelength of 793 nm with a 20 nm tuning range by mechanical rotation of the feedback grating about a pivot point. Rapid tuning of the CECDL is achieved electronically by voltage control of the electro-optic crystal. The electro-optic tuning response of this laser is 2.01 MHz/V and linear frequency chirps of 800 MHz ranging in duration from 3 to 337 μs are demonstrated. The maximum electro-optic tuning is set by the external cavity mode spacing of 2.4 G Hz for this laser.

Comparison of xenon lamp-based and led-based solar simulators

Rapid advances in high intensity light emitting diodes (LEDs) have provided sufficient tools to design LED solar simulators to accurately mimic the sun. LEDs offer numerous advantages over lamp-based technology currently used. However, these advantages have not been harnessed because of limitations in creating a solar simulator with the highest rating (AAA) for spectral match, temporal stability, and light uniformity. Oriels VeraSol is one of the first LED, triple A solar simulators. The VeraSol-LED was compared to the equally rated Oriel Sol3A-xenon lamp solar simulator by measuring the current-voltage (I-V) response and spectral response (SR) for a variety of solar cells. Both simulators effectively mimic the sun; however, the results demonstrate the LED-based simulator produced a more stable, flexible, and accurate match to AM1.5G than the xenon lamp-based simulator with similar marks in the quality of PV cell response.

Laser diode facet modal reflectivity measurements

A simple and accurate method for measuring the front facet modal reflectivity of a Fabry-Perot laser diode is presented. In this method, optical feedback from an external mirror of known reflectivity, R(ext), is used to alter the laser diode threshold current. The effect of the external mirror and front facet reflectivities on the threshold current then allows for a measurement of the front facet modal reflectivity of the laser diode and is theoretically and experimentally studied. This method was used to measure a facet reflectivity of R(2) = 0.0151(+0.0018/-0.0032) [R(2) = 0.00592(+0.00085/-0.00123)] for a commercially antireflection-coated facet of a laser diode with a center wavelength of 795 nm (935 nm). The results of the reflectivity measurements based on the threshold current as a function of the external mirror reflectivity are compared with the results of the reflectivity measurements based on modulation depth of the optical spectrum [IEEE J. Quantum Electron. QE-19, 493 (1983)].

Research and development of laser diode based instruments for applications in space

Laser diode technology continues to advance at a very rapid rate due to commercial applications such as telecommunications and data storage. The advantages of laser diodes include, wide diversity of wavelengths, high efficiency, small size and weight and high reliability. Semiconductor and fiber optical-amplifiers permit efficient, high power master oscillator power amplifier (MOPA) transmitter systems. Laser diode systems which incorporate monolithic or discrete (fiber optic) gratings permit single frequency operation. We describe experimental and theoretical results of laser diode based instruments currently under development at NASA Goddard Space Flight Center including miniature lidars for measuring clouds and aerosols, water vapor and wind for Earth and planetary (Mars Lander) use.

Tunable external-cavity diode laser for remote sensing of water vapor

We present a single-longitudinal-mode external-cavity diode laser developed as a tunable laser source for an airborne water vapor differential absorption lidar (DIAL) system. The small efficient laser is ideal for airborne and space-based applications where size, weight, and power consumption is a concern. This system is designed to operate in the 935-nm band, in which the absorption cross sections are /spl sim/20 times stronger than the absorption bands at 720 and 820 nm.