Robert S. Afzal

Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars

The Mars Orbiter Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor spacecraft, has measured the topography, surface roughness, and 1.064-μm reflectivity of Mars and the heights of volatile and dust clouds. This paper discusses the function of the MOLA instrument and the acquisition, processing, and correction of observations to produce global data sets. The altimeter measurements have been converted to both gridded and spherical harmonic models for the topography and shape of Mars that have vertical and radial accuracies of ~1 m with respect to the planets center of mass. The current global topographic grid has a resolution of 1/64° in latitude × 1/32° in longitude (1 × 2 km^2 at the equator). Reconstruction of the locations of incident laser pulses on the Martian surface appears to be at the 100-m spatial accuracy level and results in 2 orders of magnitude improvement in the global geodetic grid of Mars. Global maps of optical pulse width indicative of 100-m-scale surface roughness and 1.064-μm reflectivity with an accuracy of 5% have also been obtained.

The MESSENGER mission to Mercury: scientific payload

Abstract The MErcury, Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission will send the first spacecraft to orbit the planet Mercury. A miniaturized set of seven instruments, along with the spacecraft telecommunications system, provide the means of achieving the scientific objectives that motivate the mission. The payload includes a combined wide- and narrow-angle imaging system; γ-ray, neutron, and X-ray spectrometers for remote geochemical sensing; a vector magnetometer; a laser altimeter; a combined ultraviolet-visible and visible-infrared spectrometer to detect atmospheric species and map mineralogical absorption features; and an energetic particle and plasma spectrometer to characterize ionized species in the magnetosphere.

MARS OBSERVER LASER ALTIMETER : LASER TRANSMITTER

The Mars Observer Laser Altimeter utilizes a space-qualified diode-laser-pumped Q-switched Nd:YAG laser transmitter. A simple numerical model of the laser energetics is presented, which predicts the pulse energy and pulse width. Comparisons with the measured data available are made. The temperature dependence of the laser transmitter is also predicted. This dependence prediction is particularly important in determining the operational temperature range of the transmitter. Knowing the operational temperature range is especially important for a passive, thermally controlled laser operating in space.

Mars Orbiter Laser Altimeter: receiver model and performance analysis

The design, calibration, and performance of the Mars Orbiter Laser Altimeter (MOLA) receiver are described. The MOLA measurements include the range to the surface, which is determined by the laser-pulse time of flight; the height variability within the footprint determined by the laser echo pulse width; and the apparent surface reflectivity determined by the ratio of the echo to transmitted pulse energies.

Compact, passively Q-switched Nd:YAG laser for the MESSENGER mission to Mercury

A compact, passively Q-switched Nd:YAG laser has been developed for the Mercury Laser Altimeter, an instrument on the Mercury Surface, Space Environment, Geochemistry, and Ranging mission to the planet Mercury. The laser achieves 5.4% efficiency with a near-diffraction-limited beam. It passed all space-flight environmental tests at subsystem, instrument, and satellite integration testing and successfully completes a postlaunch aliveness check en route to Mercury. The laser design draws on a heritage of previous laser altimetry missions, specifically the Ice Cloud and Elevation Satellite and the Mars Global Surveyor, but incorporates thermal management features unique to the requirements of an orbit of the planet Mercury.

Single-mode high-peak-power passively Q-switched diode-pumped Nd:YAG laser

We have demonstrated an efficient, compact, passively Q-switched single-mode diode-pumped Nd:YAG laser that uses Cr(4+):YAG as a saturable absorber. Linear- and ring-cavity configurations were demonstrated. Pulse energies and widths were, respectively, 1.5mJ and 3.9ns for the linear cavity and 2.1mJ and 12ns for the ring cavity.

The Geoscience Laser Altimeter System (GLAS) Laser Transmitter

The Geoscience Laser Altimeter System (GLAS), launched in January 2003, is a laser altimeter and lidar for the Earth Observing Systems (EOS) ICESat mission. GLAS accommodates three, sequentially operated, diode-pumped, solid-state, Nd:YAG laser transmitters. The laser transmitter requirements, design, and qualification test results for this space-based remote-sensing instrument is summarized and presented.

Threshold power and fiber degradation induced modal instabilities in high-power fiber amplifiers based on large mode area fibers

We report on two types of modal instabilities observed in high power Yb amplifiers based on Large Mode Area Fibers. The first is observed to occur at a Threshold Power, which we refer to as Threshold Power Modal Instabilities (TPMI). The modal instability is observed as a decrease in beam quality or reduced core light output as higher order modes leak into the fiber cladding. In PM 25/400 fiber amplifiers, we observe the threshold for the modal instability to vary depending on pump wavelength detuning, with the onset occurring at approximately 15 W/m peak heat load. In PM 20/400 and 25/400 fiber amplifiers without stress rods or other polarization control, we can achieve 1 kW output, limited by available pump power, without modal instabilities. The second type of modal instability is observed for certain cases where the fiber initially operates without any sign of MI but then degrades over an extended operating time, leading to a similar behavior as the TPMI. We refer to the second class as Fiber Degradation Modal Instabilities (FDMI). For these degraded fibers, we observe that fiber performance is unchanged below the critical power for modal instabilities. Experiments on degraded fiber show a wavelength dependent permanent change in the degraded fiber with a memory of the original operating wavelength.

The Geoscience Laser Altimeter System (GLAS) for the ICESat mission

Summary form only given.The Geoscience Laser Altimeter System is a next generation space lidar whose design combines a 10 cm precision surface measurements with a dual wavelength cloud and aerosol lidar in for long-term continuous use in orbit. GLAS is scheduled to be launched into a 598 km circular polar orbit in summer 2001 as part of NASAs ICESat mission. The orbit and mission parameters are summarized. The orbits 94 degree inclination was selected to optimize the crossing ground track patterns over Greenland and Antarctica and to enable data comparison with other NASA Earth Science instruments. GLAS will determine the orbit altitude, position and time from an on-board GPS receiver, and is designed to operate continuously for 3-5 years.

Efficient, diode-laser-pumped, diode-laser-seeded, high-peak-power Nd:YLF regenerative amplifier.

Optical amplification of 11 orders of magnitude in a microlens-collimated, diode-laser-pumped regenerative amplifier has been demonstrated. The amplifier was seeded with 20-ps pulses from an FM mode-locked oscillator and with 0.9-ns pulses from a modulated diode laser. Seed pulses from both sources were amplified to energies exceeding 2.5 mJ. With the thermoelectric coolers and the Pockels cell electronics neglected, the diode-seeded system exhibited an electrical-to-optical efficiency of 2.2%.