Traumatic brain injury in the elderly: a significant phenomenon.

INTRODUCTION: Traumatic brain injury (TBI) in the elderly is becoming an increasingly frequent phenomenon. Studies have mainly analyzed the influence of age as a continuous variable and have not specifically looked at geriatric patients as a group. The aim of this study is to map the magnitude and characteristics of geriatric TBI and to identify factors contributing to their poorer outcome. MATERIAL AND METHODS: Based on the ICD-9 register of the University Hospitals Leuven demographic and clinical variables of TBI were analyzed (2002-2008). The influence of older age on physiological variables was assessed using the Brain-IT database. RESULTS: The elderly (aged ≥65 years) accounted for 38.2% of non-concussion TBI and 32.6% of ICU admissions, representing the largest age group. The elderly had a significantly lower ICP (median 10.06 mmHg versus median 14.52 mmHg; p = 0.048), but no difference in their measure of autoregulation (daily mABP/ICP correlation coefficient) compared with 20-35 year-olds. TBI was caused by a fall in 78.9% of elderly patients and 42.3% suffered a mass lesion. 72.1% had cardiovascular comorbidity. Complications did not differ from their younger counterparts. DISCUSSION: Geriatric TBI is a significant phenomenon. Poorer outcomes are not yet sufficiently explained by physiological monitoring data, but reduced vascular versatility is likely to contribute. More research is needed in order to develop specific management protocols.

Enhancement of nonlinear effects at the degenerate band edge of two-dimensional photonic crystals.

The ability of two-dimensional (2D) photonic crystals (PC) for high enhancements of nonlinear processes is analyzed in the case of a degenerate band edge when two symmetrical diffracted beams are generated by Bragg diffraction in the 2D-PC. Calculations are performed using the very simple Bragg-coupled wave theory which only involves three coupled waves (the incident wave and the two diffracted waves) for the linear interaction. The validity of the approximation is proved for wavelengths lying at the neighborhood of the band edges of 2D-PC. Very large local-field intensities are predicted around the band-edge wavelengths, in particular for the upper band edge. Nonlinear propagation is studied through the analysis of degenerate four-wave mixing. For counterpropagating pump beams orthogonally sent onto the 2D-PC huge improvement of the phase conjugate reflectivity are predicted at least for small incidence angles of the signal beam. These results represent an improvement by a factor of 20 when compared to the case of a one-dimensional PC of the same thickness made of the same materials. As three intense phase conjugate beams are generated in the four-wave mixing interaction, the 2D-PC could be very interesting for the purpose of dense parallel optical signal processing. Moreover, the simple theoretical analysis developed in the paper can be used for any kind of 2D-PC.

Detection of the tagged or untagged photons in acousto-optic imaging of thick highly scattering media by photorefractive adaptive holography.

We propose an original adaptive wavefront holographic setup based on the photorefractive effect (PR), to make real-time measurements of acousto-optic signals in thick scattering media, with a high flux collection at high rates for breast tumor detection. We describe here our present state of the art and understanding on the problem of breast imaging with PR detection of the acousto-optic signal.

Efficient Bragg diffraction in thin semiconductor two-dimensional gratings.

Highly improved diffraction properties are demonstrated in a two-dimensional [2D] grating consisting of a transmission grating optically recorded in a semiconductor one-dimensional photonic crystal (1D-PC). Near unity internal diffraction efficiency, high wavelength selectivity, and Bragg diffraction regime operation are demonstrated when the read beam is set at Bragg incidence on the transmission grating while its wavelength corresponds to the band edge of the 3 microm thick 1D-PC. When the 2D grating is grown on a Bragg mirror, a single diffracted beam is obtained, which makes the device promising for optical signal processing.

In situ monitoring of the photorefractive response time in a self-adaptive wavefront holography setup developed for acousto-optic imaging.

The measurement of optical contrasts within thick biological tissues can be performed with the hybrid technique of acousto-optic imaging, but it has been shown that an acquisition rate in the 1-10kHz range is required for a good efficiency. This comes from the interferometric nature of the signal, blurred by speckle decorrelation in a time t(c), due to a decrease of the speckle pattern contrast at the exit of the sample. An holographic setup that associates a fast and large area single photodetector and a photorefractive crystal, can measure in real-time the acousto-optic signal: this is the so-called self-adaptive wavefront holography technique. Nevertheless, it is essential to size the photorefractive response time ( t(PR)) of the crystal with t(c) in order to optimize the signal-to-noise ratio of the measurement. This time mainly depends on the overall light intensity within the crystal. We have developed an original in situ method to determine t(PR) with the combination of acoustic pulses and a frequency de-tuning of the reference beam. We can measure precisely this time but also monitor it according to a theoretical model that we have previously described. We are able to adapt the response time of the setup to the decorrelation time of the medium under study.

Theoretical description of the photorefractive detection of the ultrasound modulated photons in scattering media.

Acousto-optic imaging of thick biological tissues can be obtained in real-time with an adaptive-wavefront holographic setup, where the holographic media is a self-developping photorefractive crystal. As a consequence, the interference signal resulting from the acousto-optic effect can be easily collected with a high etendue and fast single photodetector. We present a statistical model of the field propagating through the scattering media and show why the various acoustic frequency components contained in the speckle output pattern are uncorrelated. We then give a detailed description of the signal measured through the photorefractive effect, in order to explain the quadratic pressure response observed for the two commonly used configurations setup e.g an amplitude or a phase modulation of the ultrasound.

Diffraction-limited polarized emission from a multimode ytterbium fiber amplifier after a nonlinear beam converter.

The multimode and depolarized output beam of a highly multimode diode-pumped Yb-doped fiber amplifier is converted to a diffraction-limited, linearly polarized beam by a self-referencing two-wave-mixing process in an infrared-sensitive photorefractive crystal (Rh:BaTiO3). As much as 11.6 W of single-mode output is achieved with a 78% multimode-to-single-mode photorefractive conversion efficiency.

Photorefractive detection of tagged photons in ultrasound modulated optical tomography of thick biological tissues.

We present a new and simple method to obtain ultrasound modulated optical tomography images in thick biological tissues with the use of a photorefractive crystal. The technique offers the advantage of spatially adapting the output speckle wavefront by analysing the signal diffracted by the interference pattern between this output field and a reference beam, recorded inside the photorefractive crystal. Averaging out due to random phases of the speckle grains vanishes, and we can use a fast single photodetector to measure the ultrasound modulated optical contrast. This technique offers a promising way to make direct measurements within the decorrelation time scale of living tissues.

High-capacity photorefractive neural network implementing a kohonen topological map.

We designed and built a high-capacity neural network based on volume holographic interconnections in a photorefractive crystal. We used this system to implement a Kohonen topological map. We describe and justify our optical setup and present some experimental results of self-organization in the learning database.

Relaxation of the alignment tolerances of a 1.55-mum extended-cavity semiconductor laser by use of an intracavity photorefractive filter.

Commercial grating-tuned single-mode extended-cavity semiconductor lasers (ECLDs) can be tuned over 100 nm near 1.55mum . This continuous tuning with no mode hopping requires delicate factory adjustments and high mechanical stability so that the wavelength precision is kept as high as possible and the mismatch between the lasing wavelength and the wavelength of minimum loss remains as small as possible. The addition of a photorefractive crystal inside the cavity creates an adaptive spectral filter that decreases the loss of the lasing mode and thus enhances its stability. For what is to our knowledge the first time, we demonstrate the extension of the available wavelength-mismatch range without mode hopping by the addition of a CdTe photorefractive crystal inside the cavity of a single-mode grating-tuned ECLD.

Two-wave mixing in photorefractive BaTiO(3):Rh at 1.06 mum in the nanosecond regime.

We present two-beam coupling experiments in the nanosecond regime at 1.06 mum , using photorefractive BaTiO(3):Rh. The maximum observed exponential gain coefficient is 14.2 cm(-1) . No intensity-dependent electron-hole competition and no strong saturation of the photoionized charge carriers are observed for intensities of less than 20MW cm(-2) . The energy required for recording the photorefractive grating is not significantly different in the nanosecond and the cw regimes.

Simultaneous acquisition of pi/2 phase-stepped interferograms with a photorefractive Bi(12)GeO(20) crystal: application to real-time double-pulse holography.

We present a novel method for real-time analysis of vibrations in double-pulse laser holography. Two pi/2 phase-stepped interferograms are obtained simultaneously. An experimental demonstration gives a phase measurement with an accuracy of 4 degrees .

Photorefractive beam-steering system that uses energy transfer in a BaTiO(3) crystal for a fiber-array interconnect.

A beam-control system to write gratings on a holographic plane is studied. The arrangement is designed to interconnect two 1024 monomode fiber arrays. The beam-control system is composed of two subsystems: a beam steerer, which deflects one incident beam toward 1024 positions, and a collimating system, which adapts the shape of the deflected beam to the holographic plane. The collimating system was studied only after the beam steerer had been fully built and tested. It is based on the photorefractive amplification of a beamlet selected by a shutter array. The deflection efficiency is enhanced by a factor 1500 with the photorefractive crystal, and the signal-to-noise ratio is larger than 5500. The influence of the optical aberrations on the coupling losses of the infrared beams in the monomode fibers are evaluated.

High-quality self-pumped phase conjugation of nanosecond pulses at 532 nm using photorefractive BaTiO(3).

We implement a feedback loop oscillator, using a BaTiO(3) crystal that provides stable high-quality phase conjugation for nanosecond pulses, with an efficiency close to the theoretical maximum.

Dislocation density-dependent photorefractive effect in (001)-cut GaAs.

We present studies of the photorefractive effect in nonphotorefractive orientations of liquid-encapsulated Czochralski-grown GaAs crystals. Picosecond diffraction experiments conducted in different samples show that a forbidden photorefractive signal correlates well with dislocation density, which points out that the effect arises from strain fields and growth defects.

Dynamic holographic memory showing readout, refreshing, and updating capabilities.

We propose a new refreshing procedure for reading out a dynamic holographic memory without loss of information. The retrieved images are fed back to the memory after being thresholded and amplified. Experimental demonstration is performed with a LiNbO(3) photorefractive crystal.

Potentialities and limitations of hologram multiplexing by using the phase-encoding technique.

The advantages and limitations of data storage in holographic materials by implementing a pure phase-encoding method of the reference beam are studied. We show that if deterministic orthogonal binary phase addresses are used, such a system is theoretically able to store as many images as the usual angular multiplexing method. However, we demonstrate that imperfections of available optical components generate optical noise and limit the storage capacity. We propose an improved recording technique to overcome some of these limitations.

Influence of alternating field frequency on enhanced photorefractive gain in two-beam coupling.

We derive an analytic expression describing the intensity and field frequency dependence of photorefractive gain in two-beam coupling under an alternating square-wave electric field. We determine the frequency and intensity ranges over which the gain is maximum and time independent. Experimental results for Bi(12)GeO(20) crystals are presented.

Time-resolved buildup of a photorefractive grating induced in Bi(12)SiO(20) by picosecond light pulses.

Photorefractive gratings are induced with picosecond light pulses in a BSO crystal. Both experiment and calculations show a buildup of the effect governed by a diffusion of the excited charge carriers that occurs after illumination.