Francis Généreux

Bragg gratings made in reverse proton exchange lithium niobate waveguides with a femtosecond IR laser and a phase mask

Three-decibel reflecting Bragg gratings were made in buried reverse proton exchange lithium niobate waveguides with high-power femtosecond infrared radiation from a regenerative Ti : sapphire amplifier and a phase mask. Two superposed gratings were observed: a relief grating on the surface of the crystal with a 300-nm corrugation depth and a subsurface grating in phase with the surface grating. Atomic force and optical microscopy were used to characterize the gratings physical structure.

Influence of pixel geometry on the 1/f noise coefficient

This paper presents a systematic study of the 1/f noise coefficient as a function of pixel geometry for microbolometer structures. Structures with various VOx widths, electrode gaps, electrode widths and via hole sizes were fabricated and characterized. The experimental results show that the 1/f noise coefficient is adversely affected by current non uniformity, in agreement with model predictions. Design parameters that significantly impact current non uniformity are identified and approaches to minimize their importance are proposed.

Introducing a 384x288 pixel terahertz camera core

Terahertz is a field in expansion with the emergence of various security needs such as parcel inspection and through-camouflage vision. Terahertz wavebands are characterized by long wavelengths compared to the traditional infrared and visible spectra. However, it has recently been demonstrated that a 52 μm pixel pitch microscanned down to an efficient sampling pitch of 26 μm could provide useful information even using a 118.83 μm wavelength. With this in mind, INO has developed a terahertz camera core based on a 384x288 pixel 35 μm pixel pitch uncooled bolometric terahertz detector. The camera core provides full 16-bit output video rate.

Case study of concealed weapons detection at stand-off distances using a compact, large field-of-view THz camera

The detection of concealed weapons in crowd situations is a critical need and solutions are being sought after by security agencies at the federal, state and municipal levels. Millimeter waves have been evaluated for these kinds of applications, but the currently available technologies are typically too large and bulky to allow for widespread deployment. Alternatively soft X-rays have been considered but safety issues hinder their acceptance. Terahertz technology is ideally suited for such an application as it has the ability to see through clothing, and offers higher resolution than in the millimeter band, also being more compact. THz photons have lower energy than infrared and do not show the ionizing properties of X-ray radiation. The longer Terahertz waves penetrate deeper into various materials then their visible and infrared counterparts. Though the wavelength is longer it has been shown that high resolution in a small form factor can be obtained in the THz wavebands thanks to the use of small pixel pitch detectors. In this paper, a case study for the use of a compact THz camera for active see-through imaging at stand-off distances is presented. More specifically, the cases of seeing through packages and clothing are analyzed in the perspective of concealed weapons detection. The paper starts with a review of the characteristics of a high resolution THz camera exhibiting small pixel size and large field-of-view. Some laboratory results of concealed object imaging along with details of a concept for live surveillance using a compact see-through imaging system are reviewed.

Deep x-cut PPLN for efficient frequency conversion

A new technique is presented for the fabrication of deep x-cut PPLN. The method takes advantage of a ridge structure to improve the field penetration into the crystal. PPLN deeper than 6.5 microns is obtained.

Hyperspectral fluorescence lifetime lidar for geological exploration

We have developed a small, relatively lightweight and efficient lidar instrument for remotely detecting and classifying minerals. The system is based on a pulsed, eye-safe, diode pumped Nd:YAG laser, tripled (355nm) or quadrupled (266nm), for UV excitation of minerals, which then fluoresce with a typical spectrum and lifetime. Fluorescence is detected through a telescope / filter / fiber bundle / spectrograph / multi-channel detector system capable of photon counting. Transmission and detection efficiency have been optimized to reduce the need for high optical excitation energy. Detection electronics are based on gated charge integration using a multi-anode photomultiplier tube. Spectra shown are measured in the 420 to 720 nm visible range with 355 nm laser excitation. Results show that it is relatively easy to distinguish between vegetation and non-vegetation spectra using lifetime data. Lifetime of vegetation is relatively short when compared to the mineral samples investigated. Although results shown are measured in a controlled environment on the ground, the system is being developed for eventual use in a low altitude airborne application. System parameters are presented and upgrade paths are discussed.

Widely tunable SHG in a PPLN using a low voltage

We report a new technique based on domain inversions in x-cut LiNbO3 to tune the quasi-phasematching condition of a SHG process. The tuning range covers 58 nm with an applied voltage of plusmn150 V.

Fabrication of periodically poled lithium niobate on x-cut substrates

Periodically Poled Lithium Niobate (PPLN) components are very promising for the development of active photonic devices. Most of the PPLN devices currently fabricated are based on the use of z-cut wafers. We report on the fabrication of Periodically Poled Lithium Niobate on x-cut substrates by the application of a high electric pulse. The technique is taking advantage of the use of high voltage amplifier to generate the needed high voltage waveform. The shape of the pulse is controlled by feedback to maintain the poling charge constant. This approach makes the total charge independent from the electric field amplitude and the pulse duration. Using this system we show evidence that a long poling pulse improves the domains wall propagation in the forward direction e.g. from Z+ to the Z- side. We also observed an improvement of the quality of the inverted domains when controlling the poling current to a low constant value. High nucleation spike was also critical to obtain uniform inversion and repeatable poling curves for different samples. Using adequate pulse duration, nucleation spike and charges amount, uniform 1 micrometer deep PPLN were successfully fabricated on x-cut substrates.

Small uncooled bolometers with a broad spectral response

This paper reports the infrared spectral responses of 17 and 35 μm uncooled bolometers fabricated at INO. They are measured by making use of an external readout circuit along with a monochromator. As expected, the spectral absorption strongly depends on the bolometer stack as well as the pixel layout. By proper selection of design parameters, the spectral response can be made flat from 3 to 14 μm without significant deterioration of the detector figure of merit.

Customized packaged bolometers in niche applications at INO

This paper reviews recent developments in customized packaged bolometers at INO with an emphasis on their applications. The evolution of INOs bolometer packages is also presented. Fully packaged focal plane arrays of broadband microbolometers with expanded absorbing range are shown, for applications in spectroscopic and THz imaging. This paper also reports on the development of customized packaged bolometer focal plane arrays (FPAs) for space applications such as a multispectral imaging radiometer for fire diagnosis, a far infrared radiometer for in-situ measurements of ice clouds and a net flux radiometer for Mars exploration.