A broadband external-cavity diode laser (ECDL) has been invented for use in spectroscopic surveys preparatory to optical detection of gases. Heretofore, commercially available ECDLs have been designed, in conjunction with sophisticated tuning assemblies, for narrow- band (and, typically, single-frequency) operation, as needed for high sensitivity and high spectral resolution in some gas-detection applications. However, for preparatory spectroscopic surveys, high sensitivity and narrow-band operation are not needed; in such cases, the present broadband ECDL offers a simpler, less-expensive, more-compact alternative to a commercial narrowband ECDL

Pilgrim, Jeffrey S.

NASA Technical Reports Server

28 NASA Tech Briefs, September 2005
The present compact windscreen is
based on an entirely different principle:
that infrasound at sufficiently large wave-
length can penetrate any barrier of prac-
tical thickness. Thus, a windscreen having
solid, non-porous walls can block con-
vected pressure fluctuations from the
wind while transmitting infrasonic
acoustic waves. The transmission coeffi-
cient depends strongly upon the ratio be-
tween the acoustic impedance of the
windscreen and that of air. Several mate-
rials have been found to have impedance
ratios that render them suitable for use in
constructing walls that have practical
thicknesses and are capable of high trans-
mission of infrasound. These materials
(with their impedance ratios in parenthe-
ses) are polyurethane foam (222), space-
shuttle tile material (332), balsa (323),
cedar (3,151), and pine (4,713).
A small wind tunnel was built to test the
acoustical properties of a variety of wind-
screen materials. A fan generated wind at
speeds up to 21 mph (9.4 m/s) across an
infrasonic microphone. Tests were con-
ducted with and without the windscreen;
the difference in the noises detected in
the presence and absence of the wind-
screen was used as a measure of the atten-
uation of wind noise by the windscreen.
The windscreen that performed best
in the wind-tunnel tests was a cylinder
made of polyurethane foam of a type
known in the industry as “eight-
pounder,” having an inside diameter of
3 in. (7.62 cm), a wall thickness of 0.5 in.
(1.27 cm), and a length of 12 in. (30.48
cm). The attenuation of wind-generated
noise was quantified as the ratio between
the wind noises measured without and
with this windscreen. The results, plot-
ted in the upper part of Figure 1, show
that this windscreen attenuated wind
noise by amounts ranging from 12 to 20
dB at frequencies ranging from 0.7 to 20
Hz. The large spikes in the spectrum
represent aeolian tones generated by
the wind passing over the windscreen,
but these lie above the infrasonic range.
For measurements of the infrasound-
transmission coefficient of this wind-
screen, a subwoofer was placed at an end
of the wind tunnel and used to generate
a tone that was swept over the frequency
band from 10 to 200 Hz. In this case, the
ratio between detected sounds, with and
without the windscreen, was taken as a
measure of the transmission through the
windscreen. The results for the portion
of the spectrum from 10 to 100 Hz, plot-
ted in the lower part of Figure 1, show
that this windscreen had a large trans-
mission coefficient at frequencies below
25 Hz, even exhibiting a gain as high as
8 dB at 10 Hz, but then attenuated
sound at higher frequencies. Finally, a
soak test revealed that the water ab-
sorbed by the polyurethane windscreen
material amounted to only 2.1 percent
by weight.
Figure 2 shows a windscreen installed
over a microphone mounted on a pole
in the field. The windscreen has proved
robust in weather conditions of all sea-
sons and it survived Hurricane Isabel
with wind gusts up to 67 mph (30 m/s).
This work was done by Allan J. Zuckerwar,
Qamar A. Shams, Bradley S. Sealey, and
Toby Comeaux of Langley Research Cen-
ter. For further information, contact the Lan-
gley Innovative Partnerships Office at (757)
864-3521. 
LAR-16833-1
Broadband External-Cavity Diode Laser
This relatively simple, inexpensive device is suitable for use in survey spectroscopy. 
John H. Glenn Research Center, Cleveland, Ohio
A broadband external-cavity diode
laser (ECDL) has been invented for use
in spectroscopic surveys preparatory to
optical detection of gases. Heretofore,
commercially available ECDLs have
been designed, in conjunction with so-
phisticated tuning assemblies, for nar-
row-band (and, typically, single-fre-
quency) operation, as needed for high
sensitivity and high spectral resolution
in some gas-detection applications. How-
ever, for preparatory spectroscopic sur-
veys, high sensitivity and narrow-band
operation are not needed; in such cases,
the present broadband ECDL offers a
simpler, less-expensive, more-compact
alternative to a commercial narrowband
ECDL.
To be precise, the output of the tun-
able, broadband ECDL consists of many
narrow spectral peaks spaced at narrow
wavelength intervals that, taken to-
gether, span a broad wavelength band.
The broadband ECDL can, therefore, be
likened to a light-emitting diode except
that the spectrum incorporates the ex-
ternal-cavity mode structure.  Unlike
light-emitting diodes, the ECDL offers
the greater brightness, simpler fiber
coupling, and superior spatial propaga-
tion properties of a laser.  For example,
the broadband ECDL is easily coupled
into multiple-pass optical-path-length-
enhancement cells.  A tunable filter —
preferably, a monochromator or a spec-
trometer — is used to select a portion of
the output spectrum.
The optical configuration of the
broadband ECDL (see figure) is based
The Feedback Mirror Is Made Curved (in contradistinction to flat) to make it select a range of wave-
lengths (in contradistinction to a single wavelength).
Diode Laser
Collection Optic
Output
Diffraction
Grating
Feedback
Mirror
https://ntrs.nasa.gov/search.jsp?R=20110015140 2019-08-30T17:12:57+00:00Z
Solar photovoltaic cells would be de-
signed to exploit photonic-bandgap
(PBG) materials to enhance their en-
ergy-conversion efficiencies, according
to a proposal. Whereas the energy-con-
version efficiencies of currently available
solar cells are typically less than 30 per-
cent, it has been estimated that the en-
ergy-conversion efficiencies of the pro-
posed cells could be about 50 percent or
possibly even greater.
The primary source of inefficiency of
a currently available solar cell is the mis-
match between the narrow wavelength
band associated with the semiconductor
energy gap (the bandgap) and the
broad wavelength band of solar radia-
tion. This mismatch results in loss of
power from both (1) long-wavelength
photons, defined here as photons that
do not have enough energy to excite
electron-hole pairs across the bandgap,
and (2) short-wavelength photons, de-
fined here as photons that excite elec-
tron-hole pairs with energies much
above the bandgap. It follows that a
large increase in efficiency could be ob-
tained if a large portion of the incident
solar energy could be funneled into a
narrow wavelength band corresponding
to the bandgap. In the proposed ap-
proach, such funneling would be ef-
fected by use of PBG materials as inter-
mediaries between the Sun and
photovoltaic cells.
The approach involves a thermopho-
tovoltaic principle in addition to the use
of PBG materials. The basic idea is to tai-
lor the wavelength- and direction-de-
pendent emissivity of one or more PBG
material(s) such that as much as possible
of the wavelength-mismatched portion
of the incident broad-band solar power
would be absorbed — the absorbed
power would cause heating, and the re-
sulting thermal radiation would be fun-
neled into a narrow band corresponding
to the bandgap of the semiconductor
material of a solar cell. Recent experi-
ments unrelated to the development of
solar cells have shown that as much as
half of the thermal power could be thus
re-routed into the bandgap.
The figure depicts two of many con-
ceivable configurations for implement-
ing the proposal. In one configuration,
NASA Tech Briefs, September 2005 29
on, but differs from, a standard configu-
ration known in the art as that of the
Littman-Metcalf design. Whereas
heretofore, a flat feedback mirror would
be used to select a single laser output
wavelength, in the present case, a curved
feedback mirror is used to select multi-
ple wavelengths. Preferably, the feed-
back mirror is cylindrical or spherical
and is positioned with its center of curva-
ture at the point of diffraction (the in-
tersection of the laser beam with the dif-
fraction grating).
In this configuration, each wave-
length component diffracted from the
grating is reflected from the mirror
back to the point of diffraction. Thus,
many wavelength components are si-
multaneously oscillating in the external
cavity. The wavelength range is deter-
mined by the range of angles inter-
cepted by the mirror; hence, the wave-
length range can be adjusted by moving
the mirror to a different position on the
diffraction circle. The zeroth-order out-
put of the diffraction grating is used as
the laser output.
The length of the external cavity (in-
cluding the mirror radius) determines
the longitudinal mode spacing. It is
preferable to make this spacing smaller
than the wavelength resolution of tun-
able filter, so that for the purpose of fil-
tering, the ECDL spectrum can be re-
garded as continuous. 
This work was done by Jeffrey S. Pilgrim of
Southwest Sciences, Inc. for Glenn Research
Center. Further information is contained in
a TSP (see page 1).
Inquiries concerning rights for the commer-
cial use of this invention should be addressed
to NASA Glenn Research Center, Innovative
Partnerships Office, Attn: Steve Fedor, Mail
Stop 4–8, 21000 Brookpark Road, Cleve-
land, Ohio 44135. Refer to LEW-17486-1.
PBG Materials Could Be Utilized in these and other configurations to increase the energy-conversion
efficiencies of solar cells.
Sunlight
PBG Filter
Absorber Emitter Solar Cell Mirror
Sunlight
PBG Absorber/Emitter/Filter Solar Cell PBG Mirror
High-Efficiency Solar Cells Using Photonic-Bandgap Materials
Energy-conversion efficiencies exceeding 50 percent may be possible.
NASA’s Jet Propulsion Laboratory, Pasadena, California


Broadband External-Cavity Diode Laser

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