In earlier experiments [1] we found there was significant transmission of THz radiation through carbon-fiber enforced composites, despite that fact that the dc conductivity of the carbon fibers is expected to be good and hence should prevent penetration of electro-magnetic radiation. To study the relationship between absorption of THz radiation and electrical conductivity we performed measurements on samples with different concentrations of graphene in an epoxy matrix. We observed an increased absorption of THz radiation with increased graphene concentration. Our conductivity measurements (simple transverse DC measurements using tin foil as electrodes that cover the two sample surfaces) showed the typical increase of several orders of magnitude with graphene concentration. Although both the conductivity and the THz absorption increase with graphene concentration, there is no direct cause-and-effect relation between the two quantities. Careful analysis shows that even the highest dc conductivity values cannot explain even the lowest observed values for the THz absorption coefficient.
We report on the development and application of a random forest regressor that not only identifies but also estimates the relative concentrations of substances (one explosive and two simulants), both in one-substance and two-substance samples. Performance of the regressor is quantified using Receiver Operating Characteristics and the performance is contrasted with that of a simple Spectral Angle Mapping technique that worked well on single-substance samples [1-3].
We present a pulsed all-fiber Er/Yb-doped master oscillator power amplifier at 1.55 μm wavelength. In a simple two-stage amplifier design, the source delivers 140 μJ pulses at 25 kHz and 50 kHz pulse repetition frequency, and 100 μJ at 100 kHz, with pulse durations of ∼10 ns and beam quality of M2 = 1.3. The MOPA is suitable as a robust field source for ladar applications, and has been tested in an in-house developed scanning ladar system.
Several types of infrared sensors are based on sensitive focal plane arrays. In such sensors, the intensity will typically increase by a factor ~107 at the focal plane, compared to the intensity of the incoming radiation. Such arrays are thus vulnerable when illuminated with high-intensity laser pulses. One solution for protecting the array against such pulses is to use an optical limiter. We here present results where carbon disulfide (CS2) has been tested as an optical limiting material against high-energy laser pulses at 2.05 μm wavelength.
A fiber-coupled Terahertz time domain spectroscopy (THz-TDS) system based on photoconductive antennas, pumped by a 100-fs fiber laser, has been used to characterize materials in transmission and reflection mode. THz images are acquired by mounting the samples under investigation on an x-y stage, which is stepped through the beam while the transmitted or reflected THz waveform is captured. The samples include a carbon fiber epoxy composite and a sandwich-structured composite panel with an aramid fiber honeycomb core in between two skin layers of fiberglass reinforced plastic. The former has an artificially induced void, and from a comparison of recorded reflected time-domain signals, with and without the void, a simple model for the structure of the composite is proposed that describes the time-domain signals reasonably well.
False alarm rates must be kept sufficiently low if a method to detect and identify objects or substances is to be implemented in real life applications. This is also true when trying to detect and identify dangerous substances such as explosives and drugs that are concealed in packaging materials. THz technology may be suited to detect these substances, especially when imaging and spectroscopy are combined. To achieve reasonable throughput, the detection and identification process must be automated and this implies reliance on algorithms to perform this task, rather than human beings. The identification part of the algorithm must compare spectral features of the unknown substance with those in a library of features and determining the distance, in some sense, between these features. If the distance is less than some defined threshold a match is declared. In this paper we consider two types of spectral characteristic that are derived from measured time-domain signals measured in the THz regime: the absorbance and its derivative. Also, we consider two schemes to measure the distance between the unknown and library characteristics: Spectral Angle Mapping (SAM) and Principal Component Analysis (PCA). Finally, the effect of windowing of the measured time-domain signal on the performance of the algorithms is studied, by varying the Blackman-Harris (B-H) window width. Algorithm performance is quantified by studying the receiver-operating characteristics (ROC). For the data considered in this study we conclude that the best performance is obtained when the derivative of the absorbance is used in combination with a narrow B-H window and SAM. SAM is a more straight-forward method and requires no large training data sets and tweaking.
We present a high energy infrared laser source where a Tm:fiber laser is used to pump a high-energy 2-μm cryogenically
cooled Ho:YLF laser. We have achieved 550 mJ of output energy at 2.05 μm, and through non-linear conversion in
ZnGeP2 generated 200 mJ in the 3-5-μm range. Using a numerical simulation tool we have also investigated a setup
which should generate more than 70 mJ in the 8-12-μm range. The conversion stage uses a master-oscillator-power-amplifier
architecture to enable high conversion efficiency and good beam quality.
We present results on the comparison of different THz technologies for the detection and identification of a variety of
explosives from our laboratory tests that were carried out in the framework of NATO SET-193 “THz technology for
stand-off detection of explosives: from laboratory spectroscopy to detection in the field” under the same controlled
conditions. Several laser-pumped pulsed broadband THz time-domain spectroscopy (TDS) systems as well as one
electronic frequency-modulated continuous wave (FMCW) device recorded THz spectra in transmission and/or
reflection.
We have performed intracavity molecular spectroscopy of water vapor, isotopic carbon dioxide, methane, acetylene,
carbon monoxide, formaldehyde and other gases using broadband mid-IR sync-pumped OPO sources (PPLN-based OPO pumped by a femtosecond Er-fiber laser, or OP-GaAs-based OPO pumped by a femtosecond Tm-fiber laser) operating near degeneracy, with up to 2.5–6.1 μm instantaneous bandwidth. We found that the measured spectral line shapes may show dispersive features. The measured spectra were compared to a simple model, based on the intracavity round-trip dispersion, and excellent agreement between theory and measurements was found. Detection limits in the ppb-range were demonstrated.
We describe experiments where different explosives were hidden under common barrier materials, and THz radiation
was used to detect and identify these explosives. Our THz system, a time-domain spectroscopy (TDS) system, is based
on a femtosecond laser whose radiation is converted into THz radiation by a low-temperature grown GaAs
photoconductive switch. A similar switch detects the reflected signal. The advantage of using a TDS system is that
pulses reflected from the barrier and the actual explosive, arrive at different instances at the detector. This simplifies the
separation of the barrier signature from the explosive signature, compared to a frequency domain system. However,
partial temporal overlap between the two pulses makes it challenging to completely separate the spectral characteristics
of the explosive from the characteristics of the barrier. Also, in addition to attenuating the THz-pulses, transmission
through barrier materials may add spectral features to the reflected signal, hampering recognition of the explosive. On
top of that, the explosive may have a rough surface, which reduces the strength of the reflected signal. In this
contribution we shall address these issues and discuss strategies that may be used to face these challenges.
We demonstrate numerically that acoustooptic interaction between two co-propagating modes in an optical
fiber can be utilized to obtain positive and negative delays. A single acousto-optic interaction region, and a
configuration based on two acousto-optic interaction regions, separated by a section of unperturbed fiber, are
simulated. It is found that the delays and advancements can be several pulse lengths in the configuration with
two acoustooptic coupling regions separated by a section of unperturbed fiber. These results are not in conflict
with relativistic causality, but are a consequence of the difference in group velocity between the two coupled
modes.
Nonlinear optical conversion of high-energy 1.064 μm pulses from a Q-switched Nd:YAG laser to the mid-infrared
is demonstrated. The experimental setup is based on a two-stage master-oscillator/power-amplifier (MOPA)
design with a KTiOPO4 based MOPA in the first stage and a KTiOAsO4/ZnGeP2 based MOPA in the second
stage. The setup can be tuned to provide output at wavelengths within the transparency range of ZnGeP2. We
obtain more than 8 mJ at 8 μm, and up to 33 mJ in the 3-5 μm wavelength region. The measured beam quality
factors are in the range M2 =2-4 for both wavelength regions.
Nonlinear optical conversion of 1.064 μm pulses from a Q-switched Nd:YAG laser to the mid-infrared is demonstrated
experimentally. The setup is based on a two-stage master-oscillator/power-amplifier (MOPA) design
with a KTiOPO4 based MOPA in the first stage and a KTiOAsO4/ZnGeP2 based MOPA in the second stage.
We obtain more than 8 mJ at 8 μm with a beam quality factor M2 ≈ 3.6.
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