Pulsed terahertz systems are currently being deployed for online process control and quality control of multi-layered products for use in the building products and aerospace industries. While many laboratory applications of terahertz can allow waveforms to be acquired at rates of 1 – 40 Hz, online applications require measurement rates of in excess of 100Hz. The existing technologies of thickness measurement (nuclear, x-ray, or laser gauges) have rates between 100 and 1000 Hz. At these rates, the single waveform bandwidth must still remain at 2THz or above to allow thinner layers to be measured. In the applications where terahertz can provide unique capability (e.g. multi-layer thickness, delamination, density) long-term stability must be guaranteed within the tolerance required by the measurement. This can mean multi-day stability of less than a micron. The software that runs on these systems must be flexible enough to allow multiple product configurations, while maintaining the simplicity required by plant operators. The final requirement is to have systems that can withstand the environmental conditions of the measurement. This might mean qualification in explosive environments, or operation in hot, wet or dusty environments. All of these requirements can put restrictions on not only the voltage of electronic circuitry used, but also the wavelength and optical power used for the transmitter and receiver. The application of terahertz systems to online process control presents unique challenges that not only effect the physical design of the system, but can also effect the choices made on the terahertz technology itself.
Terahertz energy, with its ability to penetrate clothing and non-conductive materials, has held much promise in the area of security scanning. Millimeter wave systems (300 GHz and below) have been widely deployed. These systems have used full two-dimensional surface imaging, and have resulted in privacy concerns. Pulsed terahertz imaging, can detect the presence of unwanted objects without the need for two-dimensional photographic imaging. With high-speed waveform acquisition it is possible to create handheld tools that can be used to locate anomalies under clothing or headgear looking exclusively at either single point waveforms or cross-sectional images which do not pose a privacy concern. Identification of the anomaly to classify it as a potential threat or a benign object is also possible.
Terahertz pulse imaging (TPI) is a novel noncontact, nondestructive technique for the examination of cultural heritage
artifacts. It has the advantage of broadband spectral range, time-of-flight depth resolution, and penetration through
optically opaque materials. Fiber-coupled, portable, time-domain terahertz systems have enabled this technique to move
out of the laboratory and into the field. Much like the rings of a tree, stratified architectural materials give the
chronology of their environmental and aesthetic history. This work concentrates on laboratory models of stratified
mosaics and fresco paintings, specimens extracted from a neolithic excavation site in Catalhoyuk, Turkey, and
specimens measured at the medieval Eglise de Saint Jean-Baptiste in Vif, France. Preparatory spectroscopic studies of
various composite materials, including lime, gypsum and clay plasters are presented to enhance the interpretation of
results and with the intent to aid future computer simulations of the TPI of stratified architectural material. The breadth
of the sample range is a demonstration of the cultural demand and public interest in the life history of buildings. The
results are an illustration of the potential role of TPI in providing both a chronological history of buildings and in the
visualization of obscured wall paintings and mosaics.
A portable video rate time-domain terahertz (TD-THz) reflection line-scanner suitable for aerospace destructive
examination (NDE) and security inspection is described. The imager scans a line 6 inches wide and collects a TD-THz
cross-sectional "B-scan" of the sub-surface structure at rates up to 30 Hz. The imager is hand-held. By rolling the
scanner over surface, a radiographic two dimensional "C-Scan" image can be stitched together from the individual lines
at a rate of 1-4 inches per second (depending on desired resolution). The case is 8.7 in. wide (12.9 in. with wheels), 12.5
in. long, and 7.9 in. high. The weight is approximately 11 lbs. Example images taken with the scanner of radome THz
NDE are shown.
Terahertz (THz) imaging is an emerging technique for non-invasive analysis. Since THz waves can penetrate opaque
materials, various imaging systems that use THz waves have been developed to detect, for instance, concealed weapons,
illegal drugs, and defects in polymer products. The absorption of THz waves by water is extremely strong, and hence,
THz waves can be used to monitor the water content in various objects.
THz imaging can be performed either by transmission or by reflection of THz waves. In particular, time domain
reflection imaging uses THz pulses that propagate in specimens, and in this technique, pulses reflected from the surface
and from the internal boundaries of the specimen are detected. In general, the internal structure is observed in crosssectional
images obtained using micro-specimens taken from the work that is being analysed. On the other hand, in THz
time-domain imaging, a map of the layer of interest can be easily obtained without collecting any samples. When realtime
imaging is required, for example, in the investigation of the effect of a solvent or during the monitoring of water
content, a THz camera can be used.
The first application of THz time-domain imaging in the analysis of a historical tempera masterpiece was performed on
the panel painting Polittico di Badia by Giotto, of the permanent collection of the Uffizi Gallery. The results of that
analysis revealed that the work is composed of two layers of gypsum, with a canvas between these layers. In the paint
layer, gold foils covered by paint were clearly observed, and the consumption or ageing of gold could be estimated by
noting the amount of reflection.
These results prove that THz imaging can yield useful information for conservation and restoration purposes.
Terahertz imaging will be presented as a novel method of nondestructively measuring otherwise inaccessible tree-rings
for the purpose of dendrochronologically cross-dating cultural heritage artifacts. Wood specimens were measured using
time-domain terahertz pulse reflectometry. Two-dimensional images of tree-rings were generated through analysis of
both time- and frequency-domain terahertz signals, which changed proportionally to the variations in wood density.
Terahertz pulse separation enabled wood specimens with at least two layers of coatings (primer and/or paint) to be
measured and the terahertz images were quantitatively compared to the optical photographs of related, uncoated
specimen. Tree-ring series and timelines were obtained for each terahertz image with respect to the source (reference)
ring series. Short ring width blocks were aligned to the reference series and combined to create an extended timeline for
each terahertz image. It was determined that while spatial resolution may be improved with analysis at high frequencies,
the lower signal to noise reduces the precision of the ring measurement. Constructing longer timelines from ring blocks,
significantly improves the overall quality of a match.
A fiber Bragg grating sensor array is interrogated using a broad bandwidth passively mode locked fiber laser source. A novel demodulation scheme is demonstrated using highly dispersive fiber to convert the grating wavelength shift to a temporal shift in the arrival time of the reflected pulses. The mode locked fiber laser was then modified and operated in the square pulse regime, where 4 W, 10 ns pulses with bandwidths greater than 60 nm were used successfully to illuminate 2% fiber Bragg gratings.
Microjoule pulse energies are achieved from a single stage upconversion fiber amplifier for the first time in this demonstration of chirped pulse amplification using a multimode Tm:ZBLAN fiber. A Ti:sapphire laser system provides the seed pulse for the upconversion fiber amplifier which produces subpicosecond pulse trains with energies as great as 16 (mu) J at repetition rate of 4.4 kHz. The compressed pulse peak power is more than 1 MW, and the pulse is characterized both temporally and spatially.