J. Stiens

An Equivalent Circuit Model of Single Circular Open-Ring Resonators

Circular open-ring resonators (ORR) merit special attention in many applications. They form the backbone of metamaterials. Therefore, a simple and accurate LC model is required to allow the behavior of the structure to be analyzed from the geometric parameters. In this paper, we propose an analytical model to compute the LC resonant frequency response. The model is verified numerically and experimentally in millimeter-wave frequency range with a normal incidence illumination. Moreover, the effect of the geometric parameters has been also studied. The results are very motivating and a good agreement is obtained.

Evanescent wave modulator for medium infrared wavelengths (8-12 μm)

We present a new modulation concept for medium infrared (8 - 12 μm) wavelengths. The operation principle of the presented modulator is based on evanescent wave absorption by means of a bulk, single or multiple quantum well structure. A sub-wavelength grating ensures efficient coupling of the optical field to the absorption medium. Modulation is then achieved by depletion of this absorption medium. We present an analysis of concept parameters and point out their respective advantages and disadvantages with respect to the modulation performance. In this context, we investigated the impact of different absorption media as bulk, single and multiple quantum well structures and found that single quantum well structures are best suited for modulation purposes. Simulations pointed out that an absolute modulation depth of the order of 60% can be achieved. We also investigated the impact of the diffraction order on the modulation performance. Furthermore, some preliminary experimental results on this modulation concept are presented and compared with simulations.

Multispectral Image Fusion for Active Millimeter Wave Imaging Application

The emerging active millimeter wave imaging technology has the potential to yield much more information when one has control over the illumination parameters such as angle and frequency. In this paper, we propose using image fusion techniques to process multispectral millimeter wave images in order to improve the image quality. Three pixel level fusion methods, based on band ratioing, wavelet transform, and principle component analysis, have been applied to multispectral images of two test objects, captured by an experimental active millimeter wave imaging setup. The results show that multispectral image fusion successfully improves the spatial content of the object of interest.

W-band speckle contrast images for inspection of concealed objects

Free-space active W-band millimeter-wave imaging (75-110 GHz) makes possible imaging of phenomena, inaccessible to visible and infrared light. W-band supports the imaging of concealed objects, providing both enough spatial resolution and good penetration. An advantage of mm-wave radiation over X-ray is that it is non-ionizing, and there are no known hazards or risks to human health. When imaging an object with an mmwave coherent beam, this is accompanied with speckle phenomenon. Because mm-wave wavelength is closer to the surface roughness and to the object dimension as by optical imaging, spatial distribution of speckle gives us more information than the image itself. We will use a speckle contrast as a measure of the speckle. Speckle contrast contains useful information when it differs from unity, and has been utilized here to reveal surface roughness of concealed objects. The speckle contrast starts to be reduced from unity when an incoherent part compensates coherent light. A sequence of mm-wave images was acquired with a fixed angle interval. The speckle contrast of each pixel in the image was calculated and a new image was formed: a spatial speckle contrast image. It revealed areas, covered with interference. Comparing the two images together makes all features of the hidden object visible. We also present results, which illustrate mechanical speckle contrast reduction in full W-band by means of phase diversity Hadamard solution. Hadamard principle has been proven by experimental conversion of the coherent sum of the electrical millimeter wave amplitudes into an incoherent sum of intensities. The measured data give results on speckle contrast reduction that match accurately the theoretical statistical estimations. Industrial and medical imaging of concealed objects could benefit both from speckle contrast images and Hadamard speckle reduction.

Theoretical Analysis of Partial-Spatial Q-Switching Dynamics Using a Two-Level

A detailed numerical study of the dynamic behavior of a Q-switched CO2 laser under partial spatial modulation conditions is presented. The Q-switching performance is strongly related to the switching speed and modulation depth of the modulator. The switching speed of most of the modulator principles are typically limited by one or more geometrical dimensions of the modulator. For example, the modulation bandwidth of an acousto-optic modulator is transition time limited. The switching speed is therefore strongly dependent on the optical beam size seen by the modulator along the axis of acoustic wave propagation. This paper is concerned with the impact of transit-time limited modulators with reduced spatial dimensions as compared to the laser beam size, on the Q-switching performance. Numerical simulations on a slit modulated laser beam show that depending on the boundary conditions and the transverse lasing mode, similar or higher Q-switch peak powers can be realized as compared to full-beam modulation.

{\rm CO}_{2}

An analytic model describing the distribution of the electron temperature created by absorption of an optical beam with a cylindrical symmetry in a layered structure was developed. Main attention was paid to the contribution of the lattice heating in the stationary and nonstationary regimes. It was shown that both the spatial distribution of the incident stationary beam and the temporal distribution of the incident pulses can be retrieved from the spatial and temporal electron temperature dependences near the illuminated surface. Electron temperature distributions can be measured using the thermoelectric effect. Experimental results of the spatial and temporal measurements of the thermoelectric voltage were compared with the theoretical calculations and a satisfactory agreement between experimental and theoretical results was found near the incident beam center for the quasistationary regime. The experimentally derived Seebeck detector’s responsivity equals 17.5μV∕Wcm−2.

Laser Model

A waveguide modulator for far-infrared light, based on the plasma resonance properties of a high density, thin electron sheet in semiconductors, is proposed. Parallel-coupled (uncoupled) waveguides can be uncoupled (coupled) by the presence of the electron gas.

CO2 laser induced temperature profiles in n-GaAs: An analytical model probed with the Seebeck effect

This paper presents the design, construction and testing of grounded frequency selective surfaces (FSSs) used to eliminate speckle, produced by coherent mm-wave sources. The reflection amplitude and phase of two dimensional periodic arrays of slot FSSs printed on a grounded Roger 4003C substrate are investigated. It is shown that the reflection phase variation as a function of slot length is controlled by the difference in the central resonance frequencies of different FSS arrays. Simulations and measurements results in W-band demonstrate that this parameter can be used to deduce the gradient and to improve the linearity of the reflection phase. The FSSs were simulated by Finite Integral Technique, fabricated with photolithography-etching technique and measured with MVNA.

Integrated mirror optical switch: resonant semiconductor layer integrated in a far-infrared coupled waveguide modulator

Lumped and 3D-geometrical models are presented for a photo-induced plasma that induces local changes in the dielectric properties of the coplanar waveguide transmission line in order to switch propagating millimeter waves.

Grounded Frequency Selective Surfaces as W-band Quasi-Optical Filters for Active Millimeter Wave Imaging

An infrared modulator of which the working principle is based on evanescent wave generation and intersubband transitions in a single AlGaAs/GaAs quantum well is presented here. CO2 laser light at normal incidence is coupled to an evanescent wave by means of a sub-wavelength diffraction grating. Modulation of the zeroth order reflective mode is achieved by applying an electric field across the quantum well. The model for deriving the complex refractive index of the quantum well region is presented and used for numerical diffraction efficiency simulations as a function of the groove height and period. Two specimens with different groove heights were fabricated. Experiments are conducted at a wavelength of 10.6??m. At this wavelength a relatively strong absolute modulation depth of about 20% could be observed. The experimental results are in good agreement with our model and diffraction efficiency calculations.