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.

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.

We demonstrated terahertz imaging using a direct detector based on niobium superconducting tunnel junctions ͑STJs͒. 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 ͑ϳ0.66 THz͒. The dynamic range was measured to be higher than 4 ϫ 10 7 ͑76 dB͒. Thus, the STJ detector is applicable to high-sensitivity and high-speed terahertz imaging for various nondestructive inspection applications.

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We present the principle of a terahertz-wave radar and its proof-of-concept experimental verification. The radar is based on a 522 GHz resonant-tunneling-diode oscillator, whose terahertz output power can be easily modulated by superimposing the modulation signal on its bias voltage. By using one modulation frequency and measuring the time delay of the returning signal, a relative measurement of the propagation distance is possible; adding a second modulation frequency removes the ambiguity stemming from the periodicity of the modulation sine wave and allows an absolute distance measurement. We verified this measurement method experimentally and obtained a submillimeter precision, as predicted by theory.

We present the principle and the experimental verification of a distance measurement method based on the propagation of terahertz waves. The method relies on modulating the amplitude of a resonant-tunneling-diode (RTD) oscillator used as terahertz-wave source and on measuring the phase of the detected wave by applying a quadrature mixing technique. The distance measurement is found to have a residual error as small as 0.063 mm (standard deviation), which is a record for an RTD-based terahertz-wave radar system. This is almost five times better than our previous record of 0.29 mm, when an oscilloscope was used for phase measurements; additionally, the quadrature mixing brings about numerous practical benefits, such as greatly reduced cost, size, weight, complexity, and power consumption.

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 2π , is quasi-uniformly distributed in the range [0, 2π) 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.

We designed, fabricated, and characterized a superconducting detector array for terahertz imaging applications. To evaluate the optical performance as an imaging array, we measured the spectral response of a 5-pixel linear array detector, and confirmed its sensitivity peaks to be at the same frequency within accuracy of one percent in the range of 0.65 THz. The frequency peaks are also in good agreement with a numerical calculation. This linear array was tested in the nondestructive imaging of a metallic pattern and an integrated-circuit card, and allowed the shortening of the total acquisition time. High fabrication yield of about 99% was achieved for single superconducting tunnel junctions, which led to 90% yield for the detectors.