Download pdf, 400 KB

To be published in: IEEE Computer Society, Proceedings of the 34th Applied Imagery Recognition Workshop
(AIPR ’05), Washington, D.C.
Terahertz Laser Based Standoff Imaging System
Kurt J. Linden, William R. Neal
Spire Corporation
[email protected], [email protected]
Jerry Waldman, Andrew J. Gatesman, Andriy Danylov
University of Massachusetts Lowell, Submillimeter Wave Technology Laboratory
[email protected], [email protected], [email protected]
Abstract
Definition and design of a terahertz standoff imaging system has been theoretically investigated. Utilizing terahertz quantum cascade lasers for transmitter
and local oscillator, a detailed analysis of the expected
performance of an active standoff imaging system
based on coherent heterodyne detection has been carried out. Five atmospheric windows between 0.3 THz
and 4.0 THz have been identified and quantified by
carrying out laboratory measurements of atmospheric
transmission as a function of relative humidity. Using
the approximate center frequency of each of these windows, detailed calculations of expected system performance vs target distance, pixel resolution, and relative humidity were carried out. It is shown that with
1.5 THz laser radiation, a 10m standoff distance, 1 m x
1m target area, and a 1cm x 1cm pixel resolution, a
viable imaging system should be achievable. Performance calculations for various target distances, target
pixel resolution, and laser frequency are presented.
1. Introduction
There is strong interest in developing standoff
methods for detecting hidden weapons and explosives.
At the present time it is virtually impossible to detect
concealed weapons and explosives on people or in
containers at a distance. Close-proximity detection
schemes utilizing x-ray imaging techniques and hand
searches are widely used at airports and in some government buildings, but such close proximity detection
is often too late to prevent terrorists from firing
weapons or detonating explosives in crowded areas.
What is needed is a method whereby concealed
weapons and explosives can be detected at distances
such that use of the weapons or detonation of the
explosives are far enough away to prevent or greatly
reduce human injury.
Currently existing hidden weapon detection
schemes such as x-ray imaging or trace gas spectroscopy do not work at large distances. Visible and
infrared detection schemes have limited capability
because this radiation cannot easily distinguish
between different materials such as clothing, paper,
metals, or a human body. Beyond the far-infrared
spectral region however, in the sub-millimeter
wavelength region (known as the terahertz region
because the frequencies are between 0.3 and 4 THz),
radiation is strongly absorbed by polar molecules such
as water and certain explosives. Terahertz radiation is
reflected by metals and heavy organic molecules (such
as contained in certain explosive materials), but
materials such as clothing, paper, and most packaging
materials are relatively transparent to it. The energy of
this radiation is very low (in the meV range), being
orders of magnitude below that of x-rays, and is
therefore non-ionizing. Thus, with no anticipated
safety issues at power levels well below 1 W, it is an
ideal type of electromagnetic radiation for detection of
concealed weapons and explosives.
Terahertz technology involves electromagnetic radiation with wavelengths ranging approximately from
1000 µm to 75 µm (corresponding to frequencies ranging from 0.3 THz to 4 THz). Also known as the submillimeter wavelength spectral region, the technology
of terahertz radiation has been extensively investigated, and a number of sources and detectors of such
radiation have evolved over the past 50 or so years [1].
Current and prior terahertz technology applications
include astronomy, plasma fusion diagnostics and gas
diagnostics based on terahertz spectroscopy, and
scaled radar simulation. Scaled radar simulation using
terahertz laser radiation has been extensively carried
out over the past 25 years [2].
One of the primary problems that kept the terahertz
technology from being more widely used has been the
lack of convenient and affordable terahertz radiation
sources. There are currently a number of sources of
terahertz radiation, but all of the current sources have
disadvantages such as large size, high cost, complex
equipment, or the need for cryogenic cooling [3].
Terahertz frequencies occur in the most underdeveloped region of the electromagnetic spectrum, even
though their potential applications are promising for
biological and medical imaging, as well as for
detection of chemical and biological agents. This
underdevelopment is primarily due to the lack of coherent solid-state terahertz sources that can provide
convenient radiation intensities (greater than a mW)
and continuous-wave (CW) operation. The terahertz
frequencies fall between the microwave and the infrared regions of the electromagnetic radiation spectrum,
two regions in which conventional semiconductor
devices are well developed. In the microwave region,
semiconductor electronic devices (such as transistors,
Gunn oscillators, IMPATT devices, Schottky-diode
frequency multipliers, and photomixers) have operating frequencies that are usually limited by carrier transit times or parasitic RC time constants. Consequently,
the power level of these classical devices decreases as
1/f4, or even faster, as the frequency f, increases above
1 THz. In the infrared region, semiconductor optoelectronic devices based on quantum-mechanical interband
transitions are limited to frequencies higher than those
corresponding to the semiconductor energy gap, which
is higher than 4 THz even for the narrowest-bandgap
materials. Thus, the frequency range of 0.3-4 THz is
generally inaccessible for conventional microwave and
infrared semiconductor devices. A list summarizing
currently available terahertz radiation sources is shown
in Table 1. In this table, coherent implies that the listed
source is appropriate for use in a coherent heterodyne
transmitter/receiver system.
The quantum cascade laser has the favorable attributes of small size (similar to diode lasers), potentially
low manufacturing cost (using standard semiconductor
foundry operations with thousands of devices on a
single wafer), low power requirements, and frequency
tunability. The remaining challenge is to obtain thermoelectric temperature (250 K) or room temperature
(300 K) operation. Several laboratories and companies
(including Spire) are currently working in this area.
2. Identification of atmospheric terahertz
transmission windows
Atmospheric transmission measurements were
carried out at the University of Massachusetts Lowell,
Submillimeter-wave Technology Laboratory (UMLSTL) using a 1.7 m FTIR interferometer. Atmospheric
relative humidity (RH) values ranged from 5 % to
58 % with air temperatures ranging from 25.5˚C to
28.7˚C. The results obtained from these measurements
(the details of which are to be published elsewhere) are
summarized in Figure 1.
Lower terahertz windows, even though they clearly
have higher transparency, were not considered in the
calculations for two reasons: (a) the longer wavelengths require unrealistically large antenna dish sizes,
and (b) from basic physical reasons it is expected to be
very difficult to obtain efficient quantum cascade
lasers operating at frequencies below 1.5 THz.
Optical radiation loss as a function of propagation
distance through a medium can be calculated using the
Beer-Lambert law, which states that radiation intensity
decreases exponentially with distance according to
(1)
I = I0 exp (- αx)
where I is the intensity of the radiation at a position, x,
I0 is the intensity of the radiation for x = 0, and α is the
optical absorption coefficient (with dimensions of
1/distance). Since the measured atmospheric transmission values shown in Figure 1 were measured for
atmospheric path lengths of 1.7 m, it is possible to
calculate α for x = 1.7 m for any terahertz frequency
and RH value. From these values, it is then possible to
extrapolate the atmospheric attenuation for any distance.
For example, the calculated optical loss in the five
identified atmospheric windows for a 50 m path is
summarized in Table 2.
The entries for α correspond to the atmospheric attenuation values (m-1), while the dB values are the calculated radiation loss due primarily to the absorption
of terahertz radiation by the atmospheric water content
over a distance of 50 m. Table 2 illustrates the expected tradeoff between lower terahertz frequency (with
expected larger antenna size and greater difficulty in
quantum cascade laser fabrication) and improved
atmospheric transmission. It is because of this trade off
that the method of heterodyne detection is selected as
the most favorable choice of detection mechanisms.
Table 1 List of some currently available terahertz radiation sources. None of these sources has
all the attributes of low cost, convenience, coherence, and adequate (mW or greater) power. The
quantum cascade laser appears as a strong candidate for a low-cost, mass-produced terahertz
radiation source.
THz source
Details
Characteristics
Mercury lamp
Mercury arc lamp in water cooled housing
Low pressure (1E-3 Torr), 75 - 150 W
Sciencetech SPS-200, 300, low spectral power
density. Low cost - used in THz spectroscopy.
Optically pumped gas laser
Grating-tuned CO2 laser and far-IR gas
cell. Most mature device.
> 100 mW, 0.3 - 10 THz, discrete lines, CW or pulse
Commercially avail., coherent rad'n, large footprint
Optically pumped PC switch
Mode locked Nd:YAG pumped, low-temp
photoconducting InGaAs-antenna shaped
Imaging apparatus produced, 0.1 to 3 THz
Commercially avail, low ave. power/spectral density
Photomixing of near-IR lasers ErAs:GaAs photomixer
Four-wave mixing, two-color 835 nm diode
Room temp, frequency tunability,< 1 uW THz output
Room temp, sub nW power out, 2-3.5 THz output
Optical parametric oscillator
Phase matching in GaP with YAG laser
Not commercial. GaP gave 480 mW @ 1.3 THz
and OPO. Difference freq generation
GaSe crystal, Nd:YAG/OPO difference freq Tunable 58-3540 um (5.17 - 0.1 THz), 209 W pulsed
Electrically pumped Ge
Electric field injects electrons, magnetic
field splits hole levels for low-E transitions
Requires electric and magnetic fields
Up to hundreds of mW, cryo cooled, multi-mode
Direct multiplied mm waves
multiplied to low-THz region
Microwatt level, commercially available (VA Diodes)
Coherent, heterodyne local oscillators in astronomy
Quantum cascade (QC) laser
First announced in 2002, semiconductor,
AlGaAs/GaAs-based, MBE (presently)
3-4 THz region, mW power level, to 165K (MIT)
Coherent, not yet commercially available
Transistor
InGaAs channel HEMT with 60 nm gate
Plasma waves provide oscillation
Under development at Inst. Elec. Micro, Lille, France
and U. Illinois - InGaAs/InP (600-650 GHz)
Backward-wave oscillators
Vacuum tube, requires homogeneous mag Tunable output possible. Homogeneous magnetic
field of ~ 10 kG
field awkward. Commercially available
Traveling Wave Tube (TWT)
Difficult to get small dimensions, MEMS
in Si being explored
Theoretical, 1-100 mW to 0.5 THz, U. WI
Free electron laser
Benchtop design at Univ. Essex, UK
Brookhaven/Lawrence Berkeley/Jefferson:
Tunable over entire THz region, limited avail., large
Achieved 20 W
Synchrotron
Coherent synchrotron produces very high
photon flux, including THz region
Very broadband source, limited instrument availab.,
40 MeV elec bent in H-field = 20 W pulsed, large
3. Terahertz imaging system candidates
and system selection
The standoff imaging system being considered is
intended to be capable of detecting hidden objects
approx. the size of a hand weapon (gun, knife, or
hidden explosives) on a person ideally at distances of
approx. 50 m. Candidate system designs include
single detector systems operating in either scanning
mode with homodyne detection, or scanning mode
with heterodyne detection, as well as 2-dimensional
detector array detection methods with full-scene
illumination. These options are summarized in
Table 3.
Heterodyne detection, though somewhat more
complex, increases system sensitivity by approximately 8 to 9 orders of magnitude. Such terahertz
detection systems have been previously demonstrated
at UML-STL using terahertz, CO2-optically-pumped
gas lasers [4]. Substitution of a large, bulky gas laser
with a miniature quantum cascade semiconductor
laser will significantly reduce system size and cost.
Quantum cascade lasers can operate at voltages
below approx. 20 V, and are very small. They have
been operated at temperatures of up to approx.
165 K, and could reach thermoelectric temperatures
of approx. 250K in the near future. Thermoelectric
coolers are small and inexpensive, and readily
available.
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9
Frequency (THz)
Fig. 1 Atmospheric terahertz spectral transmission (0.3 – 4.0 THz), normalized to 1 at 0.3 THz,
under RH values ranging from 5 % (upper plot) to 58 % (lowest plot). The five identified
atmospheric windows are 1.47 – 1.56 THz, 1.96 – 2.0 THz, 2.09 – 2.12 THz, 2.51 – 2.55 THz, and
3.39 – 3.47 THz. The instrument spectral resolution is 0.15 cm-1
Table 2 Calculated values of terahertz attenuation over a distance of 50 m, for laser frequencies
corresponding to the five measured atmospheric window regions, as a function of RH.
THz Window
Region
3.39 – 3.47 THz
2.51 – 2.55 THz
2.09 – 2.12 THz
1.96 – 2.0 THz
1.47 – 1.56 THz
RH
= 6.4%
α = 0.062
- 13.5 dB
α = 0.062
- 13.5 dB
α = 0.049
- 10.6 dB
α = 0.038
- 8.2 dB
α = 0.012
- 2.6 dB
RH
= 11.8%
α = 0.069
- 15 dB
α = 0.077
- 16.8 dB
α = 0.075
- 16.3 dB
α = 0.049
- 10.64 dB
α = 0.021
- 4.56 dB
RH
= 25.9%
α = 0.110
- 23.9 dB
α = 0.131
-28.5 dB
α = 0.117
- 25.4 dB
α = 0.072
- 15.6 dB
α = 0.0333
- 7.23 dB
RH
= 39.4%
α = 0.131
- 28.51 dB
α = 0.161
- 35.05 dB
α = 0.1462
- 31.75 dB
α = 0.103
- 22.28 dB
α = 0.0555
- 12.05 dB
RH
= 52%
α = 0.193
- 41.95 dB
α = 0.218
- 47.40 dB
α = 0.218
- 47.40 dB
α = 0.139
- 30.12 dB
α = 0.0819
- 17.78 dB
RH
= 58%
α = 0.359
- 78 dB
α = 0.359
- 78 dB
α = 0.359
- 78 dB
α = 0.218
- 47.4 dB
α = 0.15
- 32.6 dB
Table 3 Three candidate system designs for terahertz imaging of objects at standoff distances
of 50 m.
THz imaging instrument option
Single detector, scanning mirror, with
heterodyne detection
Single detector, scanning mirror, without
heterodyne detection
2-D array detector with laser source as
illuminator
Attributes
High sensitivity but also high level of complexity
Simplicity in design, but reduced system sensitivity
Reduced system complexity and sensitivity
Terahertz detector types and associated sensitivity
values have been reviewed, and a number of detector
options will be investigated [1]. A wide variety of
terahertz detectors are available. Room temperature
detectors such as Schottky diodes and bismuth or
tellurium composite bolometers have been extensively used, as have microbolometers and Golay cells.
More recently, cooled germanium-on-silicon bolometers and metal tunnel junction composite bolometers
have demonstrated noise equivalent power (NEP)
values of 10-17 to 10-18 W Hz-1/2. Superconducting
bolometers have demonstrated NEP values as low as
10-20 W Hz-1/2. A major advantage of the heterodyne
detection system described here is that room
temperature Schottky diode detectors can be used.
Beam shaping and signal gathering optics will be
required to direct the master oscillator laser beam to
the subject and to collected the terahertz radiation
reflected from and scattered by the target object. The
most promising candidate heterodyne detection
system type is one consisting of a laser transmitter
antenna whose diameter is designed to flood illuminate the 1 m x 1 m target area. This antenna will
accordingly be relatively small. The receiver antenna
is designed to scan the target area with a 1 cm x 1 cm
imaging resolution (pixel size). The receiver antenna
will therefore be much larger than the transmitter
antenna, as discussed in the following section.
Data shown in Figure 1 indicate the existence of
five distinct terahertz window regions between 1.4
and 4 THz. These window regions are centered
around 1.488 THz, 1.979 THz, 2.11 THz, 2.52 THz,
and 3.42 THz. Good terahertz transmission windows
at currently-achievable cascade laser emission wavelengths (2 – 4 THz), exist at 2.11 THz, 2.52 THz, and
3.42 THz. It is anticipated that terahertz quantum
cascade lasers emitting in the 1.488 THz and
1.979 THz regions will soon become available as
well. For RH values of 52% or less, the 2.11, 2.2, and
3.42 THz window regions appear reasonable,
although at much higher humidity values, there may
be detection problems. The 1.488 THz window is
expected to provide the best system performance.
4. Terahertz imaging system design and
system sensitivity calculations
This section contains estimates of achievable
system performance and sensitivity for the selected
1.488 THz atmospheric window, using ideal
Gaussian beam approximations for optical and
infrared laser resonators as outlined by Kogelnik and
Li [5].
The candidate terahertz standoff detection system
design is illustrated in Figure 2.
Fig. 2 Schematic illustration of a candidate
terahertz standoff detection system for a
50 m target distance, with an assumed suicide bomber target. The heterodyne detection system includes two frequency-locked
terahertz quantum cascade lasers, one master oscillator (MO), and one local oscillator
(LO). The detector is a Schottky barrier diode
mixer operating at room temperature.
This figure describes the candidate terahertz
standoff imager. Representative target distances of 50
m and 10 m are analyzed and described, and the
design tradeoffs are presented. The heterodyne detection system incorporates a master oscillator (MO)
cascade laser and a local oscillator (LO) cascade
laser, and these lasers are frequency locked at a given
frequency separation. The Schottky detector mixer
operates at room temperature, and feeds a 15 GHz
(down-converted from 1.5 THz) signal to a conventional microwave IF strip, video detector and video
amplifier. The video signal is then digitized and
computer processed to form a real-time display.
The terahertz reflectance properties of explosives
of the type commonly used by suicide bombers, cyclotrimethylenetrinitramine (RDX) for example, have
been measured and their imaging properties have
been discussed by Federici, et. al [6]. RDX has a
fairly high reflectivity in the 1.5 THz spectral
region [7]. This material, when hidden, should therefore be distinguishable under clothing, similar to a
metal.
The initial 1.5 THz system performance calculations (for 1 cm x 1 cm resolution, and 1 m x 1 m target size) are shown in Figure 3. This figure shows the
power received by the receiver antenna as it scans the
target area as a function of distance to target and
atmospheric RH level. The specific atmospheric ab-
sorption coefficient values used in the calculations of
Figure 3 are taken from the third-order least-squares
curve fit to measured data points shown in Figure 4.
Fig. 3 Terahertz standoff imager performance (received power) vs distance to target
for various atmospheric RH levels. Scene
pixel size is 1 cm x 1 cm. A video bandwidth
of 0.5 MHz is assumed.
Fig. 4 Atmospheric absorption coefficient
vs. RH for the 1.5 THz atmospheric window.
The shown data points are used for Figure 3
and later for Figure 5.
The calculations summarized in Figure 3 assume a
terahertz cascade laser power of 10 mW. Future
quantum cascade laser devices may well have
100 mW or higher output power, shifting the curves
of received power up by 10 dB or more.
The lower, horizontal line of Figure 3 indicates
the purely thermal noise floor for a video bandwidth
of 0.5 MHz. To this thermal noise floor one must
add a receiver noise figure, typically about 10 dB. In
addition, one must add 10 to 20 dB of loss to account
for clothing attenuation. Accordingly, the effective
noise floor can range from 20 to 30 dB above the
indicated thermal noise line of Figure 3. Therefore,
for example in Figure 3, it can be seen that at a target
distance of 50 m, the 20 dB above thermal noise floor
would accommodate a system operating at 30%
relative humidity, and the 30 dB above thermal noise
floor would accommodate a system operating at only
10% relative humidity.
In order to achieve viable standoff detection
system operation, it is necessary to either reduce
target distance or reduce system resolution. As the
distance to target is reduced, higher RH values can be
accommodated. The second parameter that could be
relaxed to provide increased system sensitivity is the
imaging resolution. Reduced resolution results from
increased pixel size. Increasing the pixel size while
operating at the same scan rate leads to reduced video
bandwidth. A reduction in video bandwidth leads to
a reduction in the thermal noise floor (horizontal line
in Figure 3).
In order to achieve system sensitivities more in
line with the original goal (i.e., 50 m standoff
distances), the target linear pixel size was increased
by a factor of 10 (from 1 cm to 10 cm). This however
would make it difficult to detect a hand weapon, for
example, but is expected to be adequate to detect an
explosive belt. Increasing the pixel size and decreasing the scene scan rate to yield a 1 kHz video bandwidth results in a factor of 500 in bandwidth
reduction and thereby a 27 dB increase in system
sensitivity. It also has the additional important benefit
of significantly reducing the required size of the
receiver antenna. Since the receiver antenna must be
horizontally and vertically rocked to produce a raster
scan of the target, its size and weight reduction are
expected to be important benefits in terms of
minimizing system size, weight, and cost.
System sensitivity calculations were carried out
for the reduced resolution pixel size of 10 cm x
10 cm. The imager performance (received power)
calculations under these conditions are presented in
Figure 5.
From this figure it can be seen that significant
improvement in system performance is obtained. For
example, at a 50 m target distance, with 10 dB
receiver noise figure and 10 to 20 dB clothing attenuation, it is possible to obtain an image with 20% to
35% relative atmospheric humidity.
The standoff imager performance plots for both
high imaging resolution (1 cm x 1 cm pixel size) and
lower resolution (10 cm x 10 cm pixel size) shown in
Figures 3 and 5, respectively, were generated with
the assumption that the antenna dish diameters were
optimized for each target distance for the following
reason. If a given divergent transmitter beam just
flood-illuminates a target at a given distance, then
bringing the target closer to the source results in only
the inner portion of the target being illuminated.
Fig. 5 Terahertz standoff imager received
power vs distance to target for various atmospheric RH levels, for an assumed scene
pixel size of 10 cm x 10 cm. A video bandwidth of 1 kHz is assumed. This results in a
system thermal noise floor reduction of 27 dB (500 x) below that shown in Figure 3.
Accordingly, for optimal performance, the transmitter antenna size (and thus the flooding angle)
must be modified for each target distance considered.
Similar considerations hold for the receiver antenna.
The impact of these considerations for both the 1 cm
x 1 cm and 10 cm x 10 cm pixel imaging resolution
and for both transmitter and receiver antennas is
shown in Figure 6.
Fig. 6 Calculated optimal transmitter and
receiver antenna dish diameter vs. distance
to target for a high-resolution (1 cm x 1 cm
pixel size with 1 m x 1 m target area) and
lower resolution (10 cm x 10 cm pixel size
with 1 m x 2 m target area) system.
Comparable system sensitivity calculations were
also carried out for the case of reduced target distances, but with the original pixel imaging resolution
of 1 cm x 1 cm.
The imager performance
calculations under these conditions are presented in
Figure 7.
Fig. 7 Terahertz standoff imager performance vs. distance to target for various relative humidity levels. Pixel resolution is 1 cm
x 1 cm. For a 10 m target distance predicted
system performance is 37 dB above the
thermal noise floor for a 50 kHz bandwidth.
At a target distance of 10 m, if one includes a
receiver noise figure of 10 dB and clothing losses of
20 dB, there is still a noise margin of approx. 10 dB
for a relative atmospheric humidity level of 60%.
This suggests acceptable system performance.
The optimal size of the transmitter and receiver
antennas as a function of distance to target for this
scenario, is shown in Figure 8.
Fig. 8 Transmitter and receiver antenna
dish diameter vs distance to target for 1 m x
1 m target size and 1 cm x 1 cm pixel size, in
a standoff imaging system designed for use
around 10 to 20 meter target distances.
For a system designed for 10 m target distances, a
2.5 mm transmitter antenna diameter and a 25 cm
receiver antenna diameter is optimal. The slopes of
the two curves reflect the previously-noted fact that
different target distances require different optimal
antenna sizes. Use of antennas of sizes that deviate
from these values can be expected to result in system
performance degradation. For example, for a system
designed for operation at a 10 m target distance,
operation at shorter distances results in a reduction in
the area of the illuminated target, while operation at
longer distances results in a reduction of flood
illumination intensity. This does not mean that the
system will not operate at distances other than those
for which it is designed. It means that the system
performance will be somewhat degraded according to
the degree of deviation from the design distance.
Since moderate to high atmospheric humidity levels
must realistically be dealt with, two possible
tradeoffs were investigated. Reducing either imaging
resolution or operational target distance results in
improved system sensitivity. A reduction in imaging
resolution from 1 cm x 1 cm to 10 cm x 10 cm,
results in acceptable sensitivity at 50 m target
distances, whereas maintaining the 1 cm x 1 cm
image resolution results in the achievement of
adequate sensitivity at distances of about 10 m.
Intermediate target resolution/target distance
combinations could also provide adequate system
sensitivity. For hidden hand weapon detection, the
higher (1 cm x 1 cm) imaging resolution will be
required.
5. Summary and conclusions
The authors are indebted to Thomas Goyette and
Jason Dickinson of UML-STL for technical input on
antenna design issues. This work was supported by
the Homeland Security Advanced Research Projects
Agency (HSARPA). The authors are grateful to Peter
Costianes of AFRL for his support and encouragement of this work.
Atmospheric transmission measurements of terahertz radiation over the 0.3 – 4.0 THz spectral range
were carried out at sea-level for RH values ranging
from 5 % to 58 %. These previously unpublished
terahertz atmospheric absorption coefficient data are
needed to enable accurate estimation of terahertz
standoff imager performance, and are of value to the
general scientific community. Five transmission windows were found to occur in the spectral regions
between 3.39 – 3.47 THz, 2.51 – 2.55 THz, 2.09 –
2.12 THz, 1.96 – 2.0 THz, and 1.47 – 1.56 THz. A
performance tradeoff study involving atmospheric
attenuation, antenna size, and terahertz laser
technology suggests that the 1.5 THz atmospheric
window region is the most promising, if lower
frequency windows are excluded. Portable standoff
imaging system definition was carried out, and a
comparison of general imaging system performance
for several basic imaging system designs was
completed and tabulated. A detailed analysis of
standoff imaging system performance was carried out
for a system based on the ultra-sensitive heterodyne
detection technique, wherein both the transmitter and
the local oscillator in the receiver utilize terahertz
quantum cascade lasers. Such lasers have demonstrated power levels adequate for the proposed
system. This standoff detection system appears to be
the most promising for detection of hidden weapons
and explosives on individuals at standoff distances.
The 1.5 THz design, which involved a 1 cm x 1 cm
target imaging resolution (pixel size) at a target
distance of 50 m, does not provide adequate system
sensitivity at normal atmospheric humidity levels.
Acknowledgment
References
[1] Peter H. Siegel, “Terahertz Technology,” IEEE Trans.
Microwave Theory and Techniques 50, 910 (2002).
[2] J. Waldman, H.R. Fetterman, P.E. Duffy, T.G. Bryant,
and P.E. Tannenwald, “Submillimeter model measurements
and their applications to millimeter radar systems,” Proc.
4th Int. Infrared Near-Millimeter Waves Conf., 49
(Dec. 1979.)
[3] J.M. Chamberlain, “Where optics meets electronics:
recent progress in decreasing the terahertz gap,” Phil.
Trans. R. Soc. Lond. A 362, 199-213 (2004).
[4] D.R. Culkin, G.B. DeMartinis, T.M. Goyette, J.C.
Dickinson, J. Waldman and W.E. Nixon, “W-Band
Polarimetric Scattering Features of a Tactical Ground
Target Using a 1.56 THz 3D Imaging Compact Range,”
Proc. SPIE 4382, 241 (2001).
[5] H. Kogelnick and T. Li, “Laser Beams and
Resonators,” Appl. Opt. 5, 1550 (1966).
[6] J.F. Federici, D. Gary, B. Schulkin, F. Huang, H. Altan,
R. Barat, and D. Zimdars, “Terahertz imaging using an
interferometric array,” Appl. Phys. Lett. 83, 2477 (2003).
[7] M.C. Kemp, P.F. Taday, B.E. Cole, J.A. Cluff, A.J.
Fitzgerald, and W.R. Tribe, Proc. SPIE 5070, 44 (2003).