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Abstract In this paper, we present the potential of Terahertz Time-Domain Imaging (THz-TDI) as a tool to perform non-invasive 3D analysis of an ancient enamel plate manufactured by Longwy Company in France. The THz data collected in the reflection mode were processed using noise filtering procedures and an advanced imaging approach. The results validate the capability to identify glaze layers and the thickness of ceramic materials. To characterize the nature of the pigments, we also use with X-ray images, visible near-infrared hyperspectral imaging spectroscopy, and p-XRF (portable X-ray fluorescence) to qualitatively and quantitively identify the materials used. The obtained information enables a better understanding of the decoration chromogens nature and, thus, to determine the color palette of the artists who produced such decorative object. We also establish the efficiency of a focus, Z-tracker, which enables to perform THz imaging on non-flat samples and to attenuate artifacts obtained with a short focus lens. Then, 3D images are extracted and generated, providing a real vision. We also report the evaluation of the internal damage state through the detection of fractures.

Abstract Typical eruptions of Taal Volcano in the Philippines are frequent and violently explosive. The last devastating phreatomagmatic eruption from 1965 to 1977 resulted in the death of about 200 persons. For this and other earlier events, no warnings of impending eruptions were issued. Since this time, volcanic crises involving episodes of seismicity, deformation, gas release, temperature change, etc., have occurred frequently. Each of these did not, but possibly could have, evolved into an explosive eruption. One such event occurred between April 2010 and March 2011. This paper reports various types of geophysical data (electric/magnetic field, ground temperature, seismicity and ground deformation) taken during this period of activity together with the current state of geophysical knowledge about the volcano in order to throw light on activity evolution. While the 2010 seismovolcanic crisis was preceded by unusual changes in electric field, ground tilting and uplift, the major signals from all data were concomitant with the evolution of the crisis. On April 20, 2010, felt earthquakes first started to occur. The numbers increased sharply after April 29 with repeated pulses of seismicity that ended finally in March 2011. Overall, we can identify three periods of seismic activity, from April 20 to August 4–8, from August 4–8 to November 18, and from November 18 to March 28, 2011. Although only a single tiltmeter was in operation, it recorded three phases of deformation that correspond generally with the pulses in seismic activity. Phase 1 appears to correspond to inflation at a depth of about 5 km just to the NW of Main Crater Lake from April 20 to June 11, 2010. This resulted in an eastward tilting of the northern flank of the volcano. This apparently triggered rapid but shallower inflation (Phase 2) in the hydrothermal region of the volcano under the northern edge of Main Crater Lake from June 11 to July 13 just to the SE of the initial deeper source as indicated by northward tilting of the volcano’s northern flank. Progressive deflation began after July 13. By March 28, 2011, the deformation state recovered to the pre-crisis level and felt seismic activity ceased. Apparent related changes in seismicity, tilt, magnetic and electric fields occur during smaller NS inflations in the Phase 3 deflation—Phase 3a (July 13 to August 4–8), Phase 3b (August 4–8 to November 18) and Phase 3c (November 18 to March 28, 2011). These likely result from interrelated stress change, fluid/gas migration and thermal activity at depth and in the hydrothermal system. The rapidity of changes in the state of the volcano over a matter of hours during this entire process indicates rapid communication occurs between processes at depth and in the hydrothermal system and this could easily result in the dramatic evolution to violent phreatic explosions with little warning.

Terahertz (THz) generation via photomixing on photoconductive antenna using twin delayed chirped pulses provides a long THz pulse with a narrow bandwidth. To generate a long pulse with a broad bandwidth, we propose a new, to the best of our knowledge, method that combines two long optical pulses with opposite chirps. The pulses exhibit temporal distributions of their instantaneous frequencies with opposite slopes. As a result, the interaction between the time-evolving beat frequency and a photoconductor produces a broad THz spectrum with temporal variations. In our experimental setup, we generate a 12-ps-long pulse with a 1 THz bandwidth spectrum, featuring a frequency ramp of 90 GHz/ps, resembling a chirped THz pulse. By lowering optical peak power, this approach signifies a major advancement toward integrating photomixer technology, particularly in THz ranging applications.

Four-wave mixing is investigated when chirped pump and signal pulses are injected in a photonic crystal fiber. The shot-to-shot stability of the amplified coherent signal was measured by using the dispersive Fourier transform method and compared with numerical simulations. We highlight that the signal-to-noise ratio (SNR) of the pulsed signal increases with the injected power and show that it is not deteriorated through the amplification when the fiber optical parametric amplifier is strongly saturated. The SNR of the signal remains nearly constant after the amplifier.

This paper presents the development, performance, integration, and implementation of a 150 GHz FMCW radar based on a homodyne harmonic mixing scheme for noncontact, nondestructive testing. This system offers high-dynamic-range measurement capabilities up to 100 dB and measurement rates up to 7.62 kHz. Such interesting characteristics make this system attractive for imaging applications or contactless sensing. Numerous samples of different materials and geometries were imaged by taking advantage of the radar's performance. By taking into account the nonionizing capability of the system, new applicative fields such as food industry and pharmaceutical packaging were explored.

Developments toward the implementation of a terahertz pulse imaging system within a guided reflectometry configuration are reported. Two photoconductive antennas patterned on the same LT-GaAs active layer in association with a silica pipe hollow-core waveguide allowed us to obtain a guided optics-free imager. Besides working in a pulsed regime, the setup does not require additional optics to focus and couple the terahertz pulses into the waveguide core, simplifying the global implementation in comparison with other reported guided terahertz reflectometry systems. The system is qualified for imaging purposes by means of a 1951 USAF resolution test chart. An image resolution, after a 53 mm propagation length, by about 0.707 LP/mm over the 400-550 GHz integrated frequency band, was obtained, thus providing a promising basis to pursue efforts toward compact guided pulse imagers for sample inspection within the terahertz range.

Reflection terahertz frequency modulated continuous waves scanner (300 GHz) has been proficiently optimized for imaging two easel paintings of different age. The information content of the obtained THz images has been fully inspected by selecting the appropriate THz image parameter. At the same time, a new data processing has been developed for improving the level of detail held by the axial parametric THz images by means of Gaussian fit of the reflected signals. By carefully weighting the reflected signals as a function of the optical path, the reflected amplitude has been corrected for the positioning of the object surface with respect to the beam focal point. The artifact affecting the THz images recorded from an uneven painting surface have been resolved and the obtained images fairly represent to the original painting.