In this work, we present the development of a multi-sensor system for the detection of objects concealed under clothes
using passive and active millimeter-wave (mmW) technologies. This study concerns both the optimization of a
commercial passive mmW imager at 94 GHz using a phase mask and the development of an active mmW detector at 77
GHz based on synthetic aperture radar (SAR).
A first wide-field inspection is done by the passive imager while the person is walking. If a suspicious area is detected,
the active imager is switched-on and focused on this area in order to obtain more accurate data (shape of the object,
nature of the material ...).
We report on a high-power, passively mode-locked, TEM<sub>00</sub> Nd:YVO<sub>4</sub> oscillator with adjustable pulse duration between
46 and 12ps. The laser is end-pumped by an 888nm laser diode and mode-locking is achieved with a semiconductor
saturable absorber mirror (SESAM). The laser has a repetition rate of 91MHz and the M<sup>2</sup> beam quality factor is better
than 1.2 at 15ps. At the optimum output coupler, it provides a maximum average output power of 45W with 32ps pulse
duration. In literature, the presence of spatial hole burning (SHB) often helps to shorten the pulse length down to few
picoseconds. However, SHB might be an issue for some specific application requiring e.g. low noise picosecond
oscillators. In this contribution, we demonstrate that it is possible to shorten the pulse duration by lowering the
intracavity losses without SHB. Pulse tunability from 46 to 12ps is achieved by changing the output coupler of the cavity
while staying in the continuous-wave mode-locked regime. Pulse duration is almost linear with the output coupler
transmission and increases from 12 to 32ps with average output power ranging from 15 to 45W. In this range of output
power, we demonstrate the shortest pulses directly from a Nd:YVO<sub>4</sub> oscillator.
We demonstrate a high-spectral-purity continuous-wave terahertz source, using a diode pumped Yb<sup>3+</sup>:KGd(WO<sub>4</sub>)<sub>2</sub> dual frequency laser. THz radiation is generated by photomixing the two frequencies in a low temperature grown In<sub>:25</sub>Ga<sub>:75</sub>As photoconductor loading a dipole antenna. The frequency difference between the two optical modes is tuneable by step from d.c. to 3.1 THz. A maximum optical output power of 120 mW CW has been obtained with a beatnote-linewidth narrower than 30 kHz. Preliminary measurements show a tunable THz emission with a maximum output power in the order of a few tens of nW.