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Cascade laser could aid Mars life search

Interband cascade lasers now only require thermoelectric cooling, an advance that has just landed them a place on a 2009 mission to Mars.

A new 3.3 µm interband cascade (IC) laser from the Jet Propulsion Laboratory (JPL) at NASA has achieved CW operation at 264 K - almost 30 K higher than previous efforts.

Although not quite delivering room temperature CW operation just yet, the researchers have crossed a significant threshold: 240 K. Above this temperature, the lasers can be run with a simple one-stage thermoelectric cooler, rather than having to resort to using liquid nitrogen.

IC lasers take advantage of the broken bandgap alignment in Sb-based type-II quantum wells, reusing the injected electrons in the cascade stages for photon generation with a high quantum efficiency. This means that they can offer a wide range of wavelengths without any interruption in the 3 to 5 µm range.

According to Rui Yang of JPL, this range is crucial for the detection of life in space exploration. "Many molecules such as CH4 that contain the chemical building blocks of life - carbon, hydrogen, nitrogen, oxygen and sulfur - possess strong fundamental vibration-rotation lines in the mid-IR.

"By accurately measuring the concentrations of those molecules and their isotopes with the use of mid-IR lasers such as IC lasers, scientists can do analyses and make connections between those data and life evolution."

IC lasers have already been integrated into aircraft and high-altitude balloon instruments (led by Chris Webster of JPL). And if all goes to plan, the lasers will also be selected for use in spectrometers in the NASA Mars Science Laboratory, scheduled to be launched in 2009.

The researchers grew the laser, which has 12 cascade stages, using MBE on an undoped p-type GaSb substrate. For operation at thermoelectrically cooled temperatures, the laser wafer was processed into double-channel mesa stripes using dry etching, and then had its top metal contact electroplated with gold.

It was this 6-7 µm thick gold layer that the researchers expected to provide enhanced thermal dissipation for higher temperature operation.

In tests, the laser operated continuously at 258 K with 120 mA of input current for 1901 hours (79 days). In pulsed mode, it could lase at up to 350 K. It also survived without degradation for 50 days in an air environment at room temperature, although for space exploration and industrial applications the device would normally be hermetically sealed.

Yang s team is now going to concentrate on improving the device s performance further by reducing the threshold current density and power consumption.

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