Improved quantum-cascade lasers (QCLs) are being developed as electrically tunable sources of radiation in the far infrared spectral region, especially in the frequency range of 2 to 5 THz. The structures of QCLs and the processes used to fabricate them have much in common with those of multiple- quantum-well infrared photodetectors

Gunapala, Sarath

Mansour, Kamjou

Soidel, Alexander

NASA Technical Reports Server

26 NASA Tech Briefs, January 2006
circularly polarized. The circularly po-
larized laser light passes undisturbed
through a vertical polarizer, then trav-
els along the optical axis to the target.
Reflection from the target reverses the
circular polarization. The reflected
laser light passes again through the ver-
tical polarizer and then through the
quarter-wave retardation plate, which
converts the reverse circular polariza-
tion to horizontal polarization.
The polarizing beam splitter passes
the horizontally polarized reflected laser
light, which then passes through a lens,
a field aperture (which helps to increase
contrast by blocking background light),
a horizontal polarizer, and a filter having
a 20-nm-wide wavelength pass band. The
reflected laser light ultimately comes to
focus in a charge-coupled-device (CCD)
camera. At the same time, the crossed
polarizers and the band-pass filter dis-
criminate the wideband, randomly po-
larized blackbody light from both the
target and the background.
Because the intensity of blackbody
radiation is proportional to the fourth
power of absolute temperature, it
could be necessary to increase the
laser power to maintain adequate con-
trast at higher temperatures. A proto-
type LICES has been found to yield
high-contrast images at temperatures
1,500 °C.
This work was done by William K. With-
erow and Richard R. Holmes of Marshall
Space Flight Center and Robert L. Kurtz
of Pace & Waite, Inc. 
This invention has been patented by NASA
(U.S. Patent No. 6,366,403). Inquiries con-
cerning nonexclusive or exclusive license for its
commercial development should be addressed to
Sammy Nabors, MSFC Commercialization As-
sistance Lead, at sammy.a.nabors@nasa.gov.
Refer to MFS-31561-1.
CCD
Camera
Band-Pass
Filter
Horizontal
Polarizer
Lens
Laser or Other Narrowband
Light Source
Polarizing Beam Splitter
Quarter-Wave Retardation Plate
Vertical Polarizer
Target
Field
Aperture
MFS31561
4-19-02    bs
Only One Optical Path is used for both illuminating and viewing the target with laser light. Blackbody
radiation from the target is suppressed by the crossed (horizontal and vertical) polarizers.
Improved quantum-cascade lasers
(QCLs) are being developed as electri-
cally tunable sources of radiation in the
far infrared spectral region, especially
in the frequency range of 2 to 5 THz.
(Heretofore, the wavelengths of QCLs
have been adjusted by changing tem-
peratures, but not by changing applied
voltages or currents.) In comparison
with gas lasers now used as far-infrared
sources, these QCLs would have larger
wavelength tuning ranges, would be
less expensive, and would be an order
of magnitude less massive and power-
hungry. It is planned to use the im-
proved QCLs initially as the active com-
ponents of local oscillators in
spaceborne heterodyne instruments
for studying infrared spectral lines of
molecules of scientific interest. On
Earth, the QCLs could be used as far-
infrared sources for medical glucose-
monitoring and heart-monitoring in-
struments, chemical-analysis and
spectral-imaging systems, and imaging
instruments that exploit the ability of
terahertz radiation to penetrate cloth
and walls for detection of contraband
weapons.
The structures of QCLs and the
processes used to fabricate them have
much in common with those of multi-
ple-quantum-well infrared photodetec-
tors described in numerous previous
NASA Tech Briefs articles. In one of four
approaches being followed in the pres-
ent development effort, the focus is
upon designing and fabricating the
structures to obtain heterogeneous cas-
cades for different electric fields and
different wavelengths in order to enable
electrical tuning of laser emission wave-
lengths. Both the variation of the emis-
sion wavelength and the targeted range
of the electric-field strength for each
cascade would be kept small so the spec-
tral gains of adjacent cascades at any
given electric-field strength in the target
range would overlap. This approach is
expected to afford the desired variation
of the gain maximum with electric-field
strength, so that a change in applied
bias voltage would result in a wave-
length change.
In the second approach, layers of a QCL
structure are to be graded to modify the
shapes and depths of quantum wells, such
that the electronic wave functions in the
quantum wells and the transition energies
between them would change with an in-
tentional variation of the applied bias elec-
trical field. A change of the bias applied to
such a structure would result in a change
in the energy and, hence, of the wave-
length of the lasing transition.
In the third approach, the focus is on ex-
ploiting distributed-feedback and distrib-
uted-Bragg-reflector QCL architectures to
achieve wavelength tuning through varia-
tion of applied electric current.
In the fourth approach, which is com-
plementary to the other three, the focus
Electrically Tunable Terahertz Quantum-Cascade Lasers
These devices would supplant gas lasers as far-infrared sources.
NASA’s Jet Propulsion Laboratory, Pasadena, California
https://ntrs.nasa.gov/search.jsp?R=20100014079 2019-08-30T09:17:43+00:00Z
NASA Tech Briefs, January 2006 27
Few-Mode Whispering-Gallery-Mode Resonators
Simple structures function similarly to single-mode optical fibers.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Whispering-gallery-mode (WGM) op-
tical resonators of a type now under de-
velopment are designed to support few
well-defined waveguide modes. In the
simplest case, a resonator of this type
would support one equatorial family of
WGMs; in a more complex case, such a
resonator would be made to support
two, three, or some other specified fi-
nite number of modes. Such a resonator
can be made of almost any transparent
material commonly used in optics. The
nature of the supported modes does not
depend on which material is used, and
the geometrical dispersion of this res-
onator is much smaller than that of a
typical prior WGM resonator. Moreover,
in principle, many such resonators
could be fabricated as integral parts of a
single chip.
Basically, a resonator of this type con-
sists of a rod, made of a suitable trans-
parent material, from which protrudes
a thin circumferential belt of the same
material. The belt is integral with the
rest of the rod (see figure) and acts as a
circumferential waveguide. If the depth
(d) and width (w) of the belt are made
appropriately small, then the belt acts
as though it were the core of a single-
mode optical fiber: the belt and its adja-
cent supporting rod material support a
single, circumferentially propagating
mode or family of modes.
It has been shown theoretically that
the fiber-optic-like behavior of the belt-
on-rod resonator structure can be sum-
marized, in part, by the difference, ∆n,
between (1) an effective index of re-
fraction of an imaginary fiber core and
(2) the index of refraction (n) of the
transparent rod/belt material. It has
also been shown theoretically that for a
given required value of ∆n, the re-
quired depth of the belt can be esti-
mated as d ≈ R ∆n, where R is the radius
of the rod. It must be emphasized that
this estimated depth is independent of
n and, hence, is independent of the
choice of rod material.
As in the cases of prior WGM res-
onators, input/output optical cou-
pling involves utilization of evanescent
fields. In the present case, there are
two evanescent fields: one at the
belt/air interface and one in the
boundary region between the belt and
the rest of the rod.
This work was done by Anatoliy
Savchenkov, Dmitry Strekalov, Andrey
Matsko, Vladimir Iltchenko, and Lute
Maleki of Caltech for NASA’s Jet Propul-
sion Laboratory. Further information is
contained in a TSP (see page 1).
In accordance with Public Law 96-517,
the contractor has elected to retain title to this
invention. Inquiries concerning rights for its
commercial use should be addressed to:
Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: iaoffice@jpl.nasa.gov
Refer to NPO-41256, volume and number
of this NASA Tech Briefs issue, and the
page number.
Transparent Rod
2R
d
w
NPO41256
ABPI
11-22-04 bs
An Integral Thin, Narrow Belt around a transpar-
ent rod acts as a circumferential optical wave-
guide. With suitable choice of d and w in con-
junction with R, this structure can be made to act
as a single- or few-mode WGM resonator.
is on increasing maximum operating
temperatures and output power levels
in continuous-wave operation. In a
given QCL, this approach may involve
one or more of the following changes:
reducing the threshold current density,
improving mounting and packaging to
enhance removal of heat and reduce
stresses, and optimizing designs of the
active regions, injectors, and waveguide.
Moreover, optimization of design for in-
creasing maximum operating tempera-
ture and output power must include
finding the most advantageous combi-
nation of the designs of the active re-
gions and waveguide for optimal tuning
performance.
This work was done by Sarath Gunapala,
Alexander Soidel, and Kamjou Mansour of
Caltech for NASA’s Jet Propulsion Labo-
ratory. Further information is contained in
a TSP (see page 1).
In accordance with Public Law 96-517,
the contractor has elected to retain title to this
invention. Inquiries concerning rights for its
commercial use should be addressed to:
Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: iaoffice@jpl.nasa.gov
Refer to NPO-41611, volume and number
of this NASA Tech Briefs issue, and the
page number.


Electrically Tunable Terahertz Quantum-Cascade Lasers

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