Tunable Diode Laser Absorption Spectroscopy (TDLAS) is finding ever increasing utility for industrial process measurement and control. The technique’s sensitivity and selectivity benefit continuous concentration measurements of specific gas components in complex gas mixtures which are often laden with liquids or solid particulates. Tradeoff options among optical path length, absorption linestrength, linewidth, cross-interferences, and sampling methodology enable sensor designers to optimize detection for specific applications. Emerging applications are demanding increasing numbers of distributed miniaturized sensors at diminishing costs. In these applications, the TDLAS specificity is a key attribute, and its high sensitivity enables novel sampling package designs with short optical path lengths. This paper describes a miniature hermetically-sealed backscatter TDLAS transceiver package designed for high-volume production at acceptable cost. Occupying a volume less than 1in3 and weighing less than 0.06 lb, the transceiver is a key component of TDLAS sensors intended for in-situ measurements of potentially explosive gas mixtures.
NASA's Goddard Space Flight Center is developing the Geoscience Laser Altimeter System (GLAS) employing a diode pumped, Q-switched, Nd:YAG laser operating at 40 Hz repetition rate. To meet the five-year mission lifetime goal, a single transmitter would accumulate over 6.3 billion shots. Cr4+:YAG is a promising candidate material for passively Q- switching the laser. Historically, the performance of saturable absorbers has degraded over long-duration usage. To measure the multi-billion shot performance of Cr4+:YAG, passively Q-switched GLAS-like oscillator was tested at an accelerated repetition rate of 500 Hz. The intracavity fluence was calculated to be approximately 2.5 J/cm2. The laser was monitored autonomously for 165 days. There was no evidence of change in the material optical properties during the 7.2 billion shot test. All observed changes in laser operation could be attributed to pump laser diode aging. This is the first demonstration of multi-billion shot exposure testing of Cr4+:YAG in this pulse energy regime.
NASA's Goddard Space Flight Center is developing technology for satellite-based, high peak power, LIDAR transmitting requiring 3-5 years of reliable operation. Semi-conductor laser diodes provide high efficiency pumping of solid state lasers with the promise of long-lived, reliable operation. 100-watt quasi-CW laser diode bars have been baselined for the next generation laser altimeters. Multi-billion shot lifetimes are required. The authors have monitored the performance of several diodes for billions of shots and investigated operational modes for improving diode lifetime.
As part of NASA's suite of Earth Observing Satellites, Goddard Space Flight Center is developing a laser altimeter for measuring the polar ice sheet mass balance. The Geoscience Laser Altimeter System (GLAS) requires a diode pumped, Q-switched, Nd:YAG laser transmitter producing 150 mJ, 4 ns pulses at a 40 Hz repetition rate in a single transverse mode. The mission lifetime goal is five years (6.3 billion shots). The projected performance of the GLAS laser can be limited by a number of failure sources including optical damage to the components and degradation of the pump laser diodes. To the authors' knowledge, no data exists describing the multi-billion shot accumulative exposure effect Q-switched, 4 ns pulses have on intracavity optical components. To obtain multi-billion shot results in a reasonable time, an accelerated repetition rate (500 Hz) version of the GLAS oscillator was built with modifications for thermal management. An Accelerated GLAS Exposure System (AGES) was developed to autonomously monitor the laser's vital signs. Upon analysis of the stored data, the system dynamics were decoupled to identify the sources of degradation. Over 7 billion shots were accumulated during AGES' non-stop 5.5 month operation.
The Geoscience Laser Altimeter System (GLAS) is being developed by NASA/GSFC to measure the dynamics of the ice sheet mass balance, land, cloud, and atmospheric properties. An instrument altimetric resolution of 10 cm per shot is required. The laser transmitter will be a diode-pumped, Q-switched, Nd:YAG laser producing 1064 nm, 100 mJ, 4 ns pulses at 40 Hz repetition rate in a TEM(infinity) mode. A minimum lifetime goal of 2 billion shots is required per laser transmitter. The performance of the GLAS laser can be limited by physical damage to the optical components caused by the interaction of intense laser energy with the optical coatings and substrates. Very little data exists describing the effects of long duration laser exposure, of 4 ns pulses, on an optical component. An Accelerated GLAS Exposure Station (AGES) is being developed which will autonomously operate and monitor the GLAS laser at an accelerated rate of 500 Hz. The effects of a large number of laser shots will be recorded. Parameters to be monitored include: laser power, pulsewidth, beam size, laser diode drive current and power, Q-switch drive voltage, temperature, and humidity. For comparison, one set of AGES sister optical components will be used in the nonaccelerated GLAS laser and another will be evaluated by a commercial optical damage test facility.
Some of the tradeoffs involved in selecting a laser source for space-based laser ranging are outlined, and some of the recent developments in the laser field most relevant to space-based lasers for ranging and altimetry are surveyed. Laser pulse width and laser design are discussed. It is argued that, while doubled/tripled ND-host lasers are currently the best choice for laser ranging in two colors, they have the shortcoming that the atmospheric transmission at 355 nm is significantly poorer than it is at longer wavelengths which still have sufficient dispersion for two-color laser ranging. The life requirement appears to demand that laser diode pumping be used for space applications.
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