Laser System to Map Ozone Depletion
Brent D. Johnson
Ten years ago, NASA expressed an interest in a laser operating at approximately 120 µm for an orbital mission to map the upper atmosphere. This began an effort that has led to the development of a terahertz gas laser that will be part of the space agency's Earth Observing System Aura satellite. The 2.5-THz Laser Local Oscillator will be used to map the presence of the hydroxyl radical (OH), which is partially responsible for the depletion of ozone in the atmosphere.
The Earth Observing System Aura satellite, slated for launch in 2004, will monitor the composition and chemistry of Earth's atmosphere with a variety of sensors, including a system that incorporates a terahertz gas laser.
In general, NASA has not fielded gas lasers in space missions. Eric Mueller of Coherent DEOS, which developed the new laser system, explained that the first gas lasers were not very reliable for these applications and that this has damaged their reputation in the space community. "Few people seem to realize that this is old history," he said.
In the terahertz laser, a 75-MHz RF source excites a 9.69-µm CO2 laser, which in turn excites a methanol laser from the lowest vibrational state to the first excited vibrational state. The 2.5-THz (118.8 µm) output is obtained from lasing between the rotational states in the excited vibrational state. The final power is 31 mW.
Dennis A. Flower of the Jet Propulsion Laboratory compared the laser system in application to a radio receiver. The hydroxyl molecules of interest emit at a frequency very close to 2.5 THz, so the OH emission from the atmosphere is collected with a photodiode and is mixed with the signal from the Laser Local Oscillator. The resulting output is at a lower frequency (between 8 and 12 GHz) that can be measured using conventional RF electronics.
The biggest challenge in the development of the laser system was making it robust enough to survive the rigors of launch and spaceflight. Because the unit must be able to perform at temperatures from -35 to 60 °C, the differential expansion and contraction of the components in the system must be addressed. The engineers ramped the temperature up and down in a large vacuum chamber and tested to make sure that it did not radiate electromagnetic emissions that might interfere with other systems in the spacecraft.
They also had to consider thermal management. "Because you're operating in a vacuum, your intuition about the way that temperature affects the components is all wrong," Mueller said. In systems on Earth, for example, a component emits heat by conduction and convection in the air. In the vacuum of space, that does not happen, so engineers must consider the conductive heat patterns of every component, down to individual resistors.
Moreover, components must be radiation-hardened to protect them from "event latch-up" when a cosmic ray penetrates the electronics. Most electronic parts are not designed for this task, so the system designers had to employ parts from an approved list, and Coherent DEOS worked in concert with the engineers at the Jet Propulsion Lab, Mueller said.
In 2004, NASA will launch Aura into a near-polar, sun-synchronous orbit that will enable it to view virtually every point on Earth every 16 days. This will give researchers the first complete picture of the critical OH molecule and advance our understanding of the atmospheric environment.
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