Adrian Dobroiu

Terahertz imaging system based on a backward-wave oscillator

We present an imaging system designed for use in the terahertz range. As the radiation source a backward-wave oscillator was chosen for its special features such as high output power, good wave-front quality, good stability, and wavelength tunability from 520 to 710 GHz. Detection is achieved with a pyroelectric sensor operated at room temperature. The alignment procedure for the optical elements is described, and several methods to reduce the etalon effect that are inherent in monochromatic sources are discussed. The terahertz spot size in the sample plane is 550 microm (nearly the diffraction limit), and the signal-to-noise ratio is 10,000:1; other characteristics were also measured and are presented in detail. A number of preliminary applications are also shown that cover various areas: nondestructive real-time testing for plastic tubes and packaging seals; biological terahertz imaging of fresh, frozen, or freeze-dried samples; paraffin-embedded specimens of cancer tissue; and measurement of the absorption coefficient of water by use of a wedge-shaped cell.

Terahertz-wave sources and imaging applications

An overview is given on the field of the terahertz-frequency electromagnetic waves, their properties and emerging applications. Some widespread sources with their advantages and drawbacks are presented; an emphasis is placed on the parametric generation sources that we build and use in our research. Several applications are then described: imaging techniques based on transmission, reflection and scattering, results in chemical imaging and electric field imaging, as well as linear scanning and the measurement of optical properties of highly-absorbing liquids.

Terahertz imaging with a direct detector based on superconducting tunnel junctions

We demonstrated terahertz imaging using a direct detector based on niobium superconducting tunnel junctions (STJs). The detector is composed of linearly distributed junctions placed on a superconducting microstrip line and is integrated on two wings of a log-periodic antenna. We succeeded nondestructive imaging for an integrated-circuit card and dry material using the detector around its sensitivity peak (∼0.66THz). The dynamic range was measured to be higher than 4×107 (76dB). Thus, the STJ detector is applicable to high-sensitivity and high-speed terahertz imaging for various nondestructive inspection applications.

THz-Wave Spectroscopy Applied to the Detection of Illicit Drugs in Mail

A series of experiments in the field of THz-wave spectroscopy is described. The measurement of absorption spectra of solid samples, powders, and liquids is demonstrated using various optical setups. One of the sources we used is a widely tunable coherent THz-wave generator relying on an optical parametric process: nanosecond Q-switched Nd : YAG laser light scattering from the polariton mode of a MgO-doped LiNbO3 crystal. For the measurement of the absorption of THz waves in liquids, a train of waves is allowed to oscillate inside a silicon prism. A liquid placed on a total internal reflection surface lowers the quality factor of the resonator, which allows determining the complex refraction index of the sample by frequency scanning. We have also developed a technique for THz chemical imaging, by introducing the component spatial pattern analysis. The spatial distribution of the chemicals, such as illicit drugs concealed in an envelope, is obtained from terahertz multispectral transillumination images and absorption spectra. To compensate for the low imaging speed, a prescreening step is proposed, by first detecting the presence of powders in the envelopes, which is achieved by measuring in real time the amount of scattering produced by the envelopes. Details and examples are provided.

Terahertz technique for detection of microleaks in the seal of flexible plastic packages

A method to detect production faults in plastic packages using terahertz (THz) radiation is presented. Relying on the large difference between the absorption coefficients of plastic and water (for water-filled defects), and on the refraction index difference between plastic and air (for air-filled defects), our technique consists of focusing and scanning a terahertz beam on the sealed area of the package, followed by the detection of the transmitted signal. Compared to previous methods such as visual and ultrasound inspection, our technique can be applied for optically opaque packages and does not require immersion in a matching liquid. We tested our terahertz system on defects simulated by water-filled and air-filled round channels imbedded in polyethylene films, with diameters ranging from 10 to 100 µm. The results show that detection is possible down to 30 µm for water-filled and 40 µm for air-filled channels. The results are the same for both transparent and opaque packages.

Terahertz-wave absorption in liquids measured using the evanescent field of a silicon waveguide

A technique is presented for the measurement of the absorption properties of liquids in the terahertz wave range, based on the interaction between the liquid and the evanescent wave covering the surface of a cylindrical waveguide made of high-resistivity silicon. The terahertz wave propagates inside the waveguide as the fundamental EH11 mode. The presence of a medium around the silicon rod has a measurable effect on the overall end-to-end transmission of the waveguide. As demonstration, we report the measurements of a D-glucose aqueous solution with a concentration ranging from 0% to 45%, using a backward-wave oscillator as the terahertz wave source.

Monolithic Fabry-Perot resonator for the measurement of optical constants in the terahertz range

A prism-shaped monolithic Fabry-Perot resonator is shown to allow absorption measurements of liquid samples in the terahertz wave range. As the radiation source a backward-wave oscillator was chosen for its tunable, highly monochromatic, continuous wave. The resonator is made of high-resistivity silicon, with two surfaces acting as partial mirrors and forming the cavity, while a third one contributes with a total internal reflection. When a liquid sample is placed on this latter surface the total reflection is attenuated and the finesse of the resonator decreases, providing absorption information about the sample. The measurement method relying on Fourier processing of the signal, as well as experimental data are presented.

Tilt-compensating algorithm for phase-shift interferometry

A self-calibrating algorithm for phase-shift interferometry is described that is able to cancel the effect of accidental relative tilts that may occur during phase stepping. The algorithm is able to retrieve both the phase steps and the tilts that accompany them. Only three phase-shifted interferograms are needed, and no other information about the intentional phase shifts or possible tilts has to be supplied. This purpose is achieved by division of the interferogram space into blocks on which a previously reported self-calibrating algorithm is applied and the actual values of the local phase shifts are calculated. The information thus obtained is used for extracting the global shift and tilt values. Further improvement in the results is achieved by means of a fitting routine.

Self-calibrating algorithm for three-sample phase-shift interferometry by contrast levelling

A new statistical self-calibrating algorithm for phase-shift interferometry is presented. The algorithm can be applied to the case of three phase-shifted interferograms for which the assumption of a constant fringe contrast over the viewing field can be made. Self-calibration is achieved by minimizing the non-uniformity that appears in the map of the calculated contrast when incorrect phase shifts are supposed. No information on the actual phase shifts has to be supplied, the only input needed is the set of three interferograms. The residual errors of the algorithm are as low as in usual conditions.

Statistical self-calibrating algorithm for three-sample phase-shift interferometry

A new self-calibrating algorithm is described that succeeds in reconstructing an almost error-free wavefront from only three interferograms. The algorithm is based on the assumption that the optical phase, taken modulo , is quasi-uniformly distributed in the range [0, ) over the field of the interferograms. When the actual reference phases differ from those considered in the phase computation program a non-uniform histogram of the computed phase results. An analysis of this histogram allows a fitting procedure to find the actual phase shifts. Eventually an accurate shape of the wavefront can be calculated or a corrected signal can be sent to the phase-shifting device.