Berhanu Bulcha shows off his terahertz laser technology in his lab at
NASA’s GoddardSpace Flight Center in Greenbelt, Md. Credits: NASA/Michael
Giunto
Finding water on the Moon could be easier with a Goddard technology that uses an effect called quantum tunneling to generate a high-powered terahertz laser, filling a gap in existing laser technology.
Locating water and other resources is a
NASA priority crucial to exploring Earth’s natural satellite and other objects
in the solar system and beyond. Previous experiments inferred, then confirmed
the existence of small amounts of water across the Moon. However, most
technologies do not distinguish among water, free hydrogen ions, and hydroxyl,
as the broadband detectors used cannot distinguish between the different
volatiles.
Goddard engineer
Dr. Berhanu Bulcha said a type of instrument called a heterodyne spectrometer
could zoom in on particular frequencies to definitively identify and locate
water sources on the Moon. It would need a stable, high-powered, terahertz
laser, which was prototyped in collaboration with Longwave Photonics through
NASA’s Small Business Innovation Research (SBIR) program.
“This laser allows us to open a new window
to study this frequency spectrum,” he said. “Other missions found hydration on
the Moon, but that could indicate hydroxyl or water. If it’s water, where did
it come from? Is it indigenous to the formation of the Moon, or did it arrive
later by comet impacts? How much water is there? We need to answer these
questions because water is critical for survival and can be used to make fuel
for further exploration.”
As the name implies, spectrometers detect
spectra or wavelengths of light in order to reveal the chemical properties of
matter that light has touched. Most spectrometers tend to operate across broad
sections of the spectrum. Heterodyne instruments dial in to very specific light
frequencies such as infrared or terahertz. Hydrogen-containing compounds like
water emit photons in the terahertz frequency range — 2 trillion to 10 trillion
cycles per second — between microwave and infrared.
Like a microscope for subtle differences
within a bandwidth like terahertz, heterodyne spectrometers combine a local laser source with
incoming light. Measuring the difference between the laser source and the
combined wavelength provides accurate readings between sub-bandwidths of the
spectrum.
Traditional lasers generate light by
exciting an electron within an atom’s outer shell, which then emits a single
photon as it transitions, or returns to its resting energy level. Different
atoms produce different frequencies of light based on the fixed amount of
energy it takes to excite one electron. However, lasers fall short in a
particular portion of the spectrum between infrared and microwave known as the
terahertz gap.
“The problem with existing laser
technology,” Dr. Bulcha said, “is that no materials have the right properties
to produce a terahertz wave.”
Electromagnetic oscillators like those
that generate radio or microwave frequencies produce low-powered terahertz
pulses by using a series of amplifiers and frequency multipliers to extend the
signal into the terahertz range. However, this process consumes a lot of
voltage, and the materials used to amplify and multiply the pulse have limited
efficiency. This means they lose power as they approach the terahertz
frequencies.
From the other side of the terahertz gap,
optical lasers pump energy into a gas to generate photons. However,
high-powered, terahertz-band lasers are large, power hungry, and not suitable
for space exploration purposes where mass and power are limited, particularly
hand-held or Small Satellite applications. The power of the pulse also drops as
optical lasers push towards the terahertz bandwidths.
This tiny laser capitalizes on quantum-scale effects of materials just tens
of atoms across to generate a high-powered beam in a portion of the spectrum
where traditional lasers fade in strength. Credits: NASA/Michael Giunto
To fill that gap, Dr. Bulcha’s team is developing quantum cascade lasers
that produce photons from each electron transition event by taking advantage of
some unique, quantum-scale physics of materials layered just a few atoms thick.
In these materials, a laser emits photons in a specific frequency
determined by the thickness of alternating layers of semiconductors rather than
the elements in the material. In quantum physics, the thin layers increase the
chance that a photon can then tunnel through to the next layer instead of bouncing
off the barrier. Once there, it excites additional photons. Using a generator
material with 80 to 100 layers, totaling less than 10 to 15 microns thick, the
team’s source creates a cascade of terahertz-energy photons.
This cascade consumes less voltage to generate a stable, high-powered
light. One drawback of this technology is its beam spreads out in a large
angle, dissipating quickly over short distances. Using innovative technology
supported by Goddard’s Internal Research and Development (IRAD) funding, Dr.
Bulcha and his team integrated the laser on a waveguide with a thin optical
antenna to tighten the beam. The integrated laser and waveguide unit reduces
this dissipation by 50% in a package smaller than a quarter.
He hopes to continue the work to make a flight-ready laser for NASA’s
Artemis program.
The laser’s low size and power consumption allow it to fit in a 1U CubeSat,
about the size of a teapot, along with the spectrometer hardware, processor,
and power supply. It could also power a handheld device for use by future
explorers on the Moon, Mars, and beyond.
By Karl B. Hille NASA’s Goddard Space Flight Center in Greenbelt, Md.
Source: Tiny,
High-Powered Laser to Find Water on the Moon | NASA
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