Quantum cascade lasers (QCLs) are unipolar semiconductor lasers, where the wavelength of emitted radiation is determined by the engineering of quantum states within the conduction band in coupled multiple-quantum-well heterostructures to have the desired energy separation. The recent development of terahertz QCLs has provided a new generation of solid-state sources for radiation in the terahertz frequency range. Terahertz QCLs have been demonstrated from 0.84 to 5.0 THz both in pulsed mode and continuous wave mode (CW mode). The approach employs a resonant-phonon depopulation concept. The metal-metal (MM) waveguide fabrication is performed using Cu-Cu thermo-compression bonding to bond the GaAs/AlGaAs epitaxial layer to a GaAs receptor wafer

Chattopadhyay, Goutam

Kawamura, Jonathan H.

Lin, Robert H.

Williams, Benjamin

NASA Technical Reports Server

NASA Tech Briefs, July 2012 23
commonly known as electrostatic precip-
itation, is a mature technology in air at
one atmosphere. In this case, the high
voltages required for the method to
work can easily be achieved. However, in
carbon dioxide at low pressures, such as
those found on Mars, large voltages are
not possible.
The innovation reported here consists
of two concentric cylindrical electrodes set
at specific potential difference that gener-
ate an electric field that produces a corona
capable of imparting an electrostatic
charge to the incoming dust particles. The
strength of the field is carefully balanced
so as to produce a stable charging corona
at 5 to 10 mbars, and is also capable of im-
parting a force to the particles that drives
them to the collecting electrode.
There are only two possible ways that
dust can be removed from Martian at-
mospheric gas intakes: with this elec-
trostatic precipitator design, and with
the use of filters. However, filters re-
quire upstream compression of the gas
to be treated because the atmospheric
pressure on Mars is too close to vac-
uum to use a vacuum pump down-
stream to the filter to draw the gas
through the filter. The electrostatic
precipitator is the best and more effi-
cient solution for this environment. No
other precipitator designs have been
developed for the environment of Mars
due to the challenges of the low atmos-
pheric pressure.
Dust particles are charged using co-
rona generation around the high-volt-
age discharge electrode, which ionizes
gas molecules. Since the atmospheric
gas intakes for the ISRU processing
chambers will likely be cylindrical, cylin-
drical precipitator geometry was cho-
sen. The electrostatic precipitator de-
sign presented here removes simulated
Martian dust particles in the required
range in a simulated Martian atmos-
pheric environment. The current-volt-
age (I-V) characteristic curves taken for
the nine precipitator configurations at 9
mbars of pressure showed that a cylin-
drical collecting electrode 7.0 cm in di-
ameter with a concentric positive high-
voltage electrode 100 µm thick provides
the best range of voltage and charging
corona current. This precipitator de-
sign is effective for the size of the dust
particles expected in the Martian atmos-
phere. Mass determination, as well as
microscopic images and particle size
distributions of dust collected on a sili-
con wafer placed directly below the pre-
cipitator with the field on and off,
showed excellent initial results.
This work was done by Carlos Calle of
Kennedy Space Center, and Sid Clements of the
Appalachian State University Department of
Physics and Astronomy. Further information is
contained in a TSP (see page 1). KSC-13657
Quantum cascade lasers (QCLs) are
unipolar semiconductor lasers, where
the wavelength of emitted radiation is
determined by the engineering of
quantum states within the conduction
band in coupled multiple-quantum-well
heterostructures to have the desired en-
ergy separation. The recent develop-
ment of terahertz QCLs has provided a
new generation of solid-state sources
for radiation in the terahertz frequency
range. Terahertz QCLs have been
demonstrated from 0.84 to 5.0 THz
both in pulsed mode and continuous
wave mode (CW mode). 
A 2.7-THz QCL structure uses a metal-
metal waveguide QCL with multiple-
quantum-well cascade medium to pro-
vide terahertz gain for subbands
engineered to have the desired energy
separation. The approach employs a res-
onant-phonon depopulation concept.
The metal-metal (MM) waveguide fabri-
cation is performed using Cu-Cu
thermo-compression bonding to bond
the GaAs/AlGaAs epitaxial layer to a
GaAs receptor wafer. A laterally corru-
gated distributed feedback (DFB) grat-
ing is etched into a MM waveguide, as
Terahertz Quantum Cascade Laser With Efficient Coupling and
Beam Profile 
High-power QCLs can be used in medical instruments, security screening equipment, and illicit
material detection. 
NASA’s Jet Propulsion Laboratory, Pasadena, California
Ti/Au Top Contact
DFB
GaAs Receptor Substrate
Probe Bottom Contact
3-µm GaAs Membrane
45 µm
45 µm
Integrated 
Probe
Bias
Channel
Active Region
Waveguide
Block
QCL Device
Waveguide to
Integrated
Diagonal Horn
MM-Waveguide QCL Laser shown in (top) a processing schematic for fabrication of the laser with in-
tegrated waveguide probe; and (bottom) in a waveguide mount with the integrated radial probe. The
top half of the block is removed to show the QCL device inside the waveguide. 
https://ntrs.nasa.gov/search.jsp?R=20120011902 2019-08-30T21:00:25+00:00Z
24 NASA Tech Briefs, July 2012
this is easily performed in a single pho-
tolithographic and etch step. Extended
modeling is done for both the DFB cav-
ity and the coupling with the waveguide
via the integrated probe. The DFB struc-
ture QCL has an integrated waveguide
probe suitable for mounting in a ma-
chined waveguide block. Following fab-
rication of the MM-waveguide, the wafer
can be mounted top-down on a tempo-
rary support wafer, and the GaAs recep-
tor substrate is thinned to a membrane
with the assistance of an etch-stop layer. 
Development of a demonstrator horn-
antenna coupled QCL at 2.7 THz with
Gaussian output beam profile and high
coupling efficiency capable of effectively
pumping mixers at these frequencies is a
major breakthrough in the spectro-
scopic studies for the Earth-observation
and astrophysics community. The ap-
proach, which includes an integrated
probe on the QCL device in a waveguide
enclosure transitioning to a diagonal
horn, may lead to compact, coherent,
continuous-wave solid-state sources. 
A phase-locked terahertz QCL source
with high-quality beam profile and ex-
cellent output coupling efficiency oper-
ating at or above liquid nitrogen temper-
atures will be of great strategic
importance for NASA’s astrophysics,
Earth, and planetary mission capabili-
ties. This will make these QCLs the local
oscillator source of choice for the future
NASA and European suborbital and or-
bital terahertz instruments for astro-
physics missions such as the interfero-
metric (ESPRIT) and other single- and
multi-pixel heterodyne spectroscopic
missions, as well as for Earth observing
and planetary missions. A high-power
QCL with good beam profile can also be
used in biological and medical science
instruments, security screening and il-
licit material detection, and nondestruc-
tive evaluation applications. 
This work was done by Goutam Chattopad-
hyay, Jonathan H. Kawamura, and Robert
H. Lin of Caltech, and Benjamin Williams of
UCLA for NASA’s Jet Propulsion Laboratory.
For more information, contact iaoffice@
jpl.nasa.gov. NPO-46980
A subaperture stitching interferome-
ter system provides near-nulling of a sub-
aperture wavefront reflected from an ob-
ject of interest over a portion of a
surface of the object. A variable optical
element located in the radiation path
adjustably provides near-nulling to facili-
tate stitching of subaperture interfero-
grams, creating an interferogram repre-
sentative of the entire surface of
interest. This enables testing of aspheric
surfaces without null optics customized
for each surface prescription.  
The surface shapes of objects such as
lenses and other precision components
are often measured with interferometry.
However, interferometers have a limited
capture range, and thus the test wave-
front cannot be too different from the
reference or the interference cannot be
analyzed. Furthermore, the performance
of the interferometer is usually best
when the test and reference wavefronts
are nearly identical (referred to as a
“null” condition). Thus, it is necessary
when performing such measurements to
correct for known variations in shape to
ensure that unintended variations are
within the capture range of the interfer-
ometer and accurately measured.
This invention is a system for near-
nulling within a subaperture stitching in-
terferometer, although in principle, the
concept can be employed by wavefront-
measuring gauges other than interferom-
eters. The system employs a light source
for providing coherent radiation of a sub-
aperture extent. An object of interest is
placed to modify the radiation (e.g., to re-
flect or pass the radiation), and a variable
optical element is located to interact
with, and nearly null, the affected radia-
tion. A detector or imaging device is situ-
ated to obtain interference patterns in
the modified radiation. Multiple subaper-
ture inter fero grams are taken and are
“stitched,” or joined, to provide an inter-
ferogram representative of the entire sur-
face of the object of interest.  
The primary aspect of the invention is
the use of adjustable corrective optics in
the context of subaperture stitching
near-nulling interferometry, wherein a
complex surface is analyzed via multiple,
separate, overlapping interferograms.
For complex surfaces, the problem of
managing the identification and place-
ment of corrective optics becomes even
more pronounced, to the extent that in
most cases the null corrector optics are
specific to the particular asphere pre-
scription and no others (i.e. another as-
phere requires completely different null
correction optics). In principle, the
near-nulling technique does not require
subaperture stitching at all.
Building a near-null system that is
practically useful relies on two key fea-
tures: simplicity and universality. If the
system is too complex, it will be diffi-
cult to calibrate and model its manu-
facturing errors, rendering it useless as
a precision metrology tool and/or pro-
hibitively expensive. If the system is
not applicable to a wide range of test
parts, then it does not provide signifi-
cant value over conventional null-cor-
rection technology. Subaperture stitch-
ing en ables simpler and more
universal near-null systems to be effec-
tive, because a fraction of a surface is
necessarily less complex than the
whole surface (excepting the extreme
case of a fractal surface description).
The technique of near-nulling can sig-
nificantly enhance aspheric subaper-
ture stitching capability by allowing
the interferometer to capture a wider
range of aspheres. More over, subaper-
ture stitching is essential to a truly ef-
fective near-nulling system, since look-
ing at a fraction of the surface keeps
the wavefront complexity within the
capability of a relatively simple near-
null apparatus. Furthermore, by reduc-
ing the subaperture size, the complex-
ity of the measured wavefront can be
reduced until it is within the capability
of the near-null design.
This work was done by Greg Forbes, Gary
De Vries, and Paul Murphy of QED Technolo-
gies, Inc.; and Chris Brophy of Optical Engi-
neering Services for Goddard Space Flight
Center. Further information is contained in a
TSP (see page 1). GSC-16152-1
Measurement Via Optical Near-Nulling and 
Subaperture Stitching
This simple and universal technique uses adjustable corrective optics.
Goddard Space Flight Center, Greenbelt, Maryland


Terahertz Quantum Cascade Laser With Efficient Coupling and Beam Profile

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