Synergistic effect of κ-carrageenan and gelatin blends towards adipose tissue engineering.

The current paper focuses on the functionalization of κ-carrageenan and gelatin as extracellular matrix polysaccharide and protein mimic respectively to produce hydrogel films for adipose tissue engineering. More specifically, κ-carrageenan as well as gelatin have been functionalized with methacrylate and methacrylamide moieties respectively to enable subsequent UV-induced crosslinking in the presence of a photo-initiator. The gel fraction, the mass swelling ratio and the mechanical properties of both the one-component hydrogels and the protein/polysaccharide blends have been evaluated. The mechanical and swelling properties of the blends could be tuned by varying the hydrogel composition as well as the crosslinking method applied. The in vitro biocompatibility assays indicated a significantly higher cell viability of adipose tissue-derived mesenchymal stem cells seeded onto the blends as compared to the one-component hydrogels. The results show that the blends of gelatin and κ-carrageenan clearly outperform the one-component hydrogels in terms of adipose tissue engineering potential.

Soft tissue fillers for adipose tissue regeneration: From hydrogel development toward clinical applications.

There is a clear and urgent clinical need to develop soft tissue fillers that outperform the materials currently used for adipose tissue reconstruction. Recently, extensive research has been performed within this field of adipose tissue engineering as the commercially available products and the currently existing techniques are concomitant with several disadvantages. Commercial products are highly expensive and associated with an imposing need for repeated injections. Lipofilling or free fat transfer has an unpredictable outcome with respect to cell survival and potential resorption of the fat grafts. Therefore, researchers are predominantly investigating two challenging adipose tissue engineering strategies: in situ injectable materials and porous 3D printed scaffolds. The present work provides an overview of current research encompassing synthetic, biopolymer-based and extracellular matrix-derived materials with a clear focus on emerging fabrication technologies and developments realized throughout the last decade. Moreover, clinical relevance of the most promising materials will be discussed, together with potential concerns associated with their application in the clinic.

Compact étendue-preserving light-mixing optics.

We present a compact freeform optic, called "Freeform Shell-Mixer", which, when placed on top of a multicolor light source (particularly, a multi-chip LED), turns it into a virtual source in which colors are uniformly mixed. The optic, compatible with injection molding, makes use of étendue-conserving Köhler integration to provide homogeneous mixing of light. Its minimal size (just 2x larger than the source) makes the concept compatible with many luminaries, as ray tracing models show. Simulations indicate that the Freeform Shell-Mixer can reach efficiencies above 95% and both the size of the virtual source and its emission pattern are very similar to the ones of the original source, so the correct performance of the luminaire is secured.

Physical random bit generation from chaotic solitary laser diode.

We demonstrate the physical generation of random bits at high bit rates (> 100 Gb/s) using optical chaos from a solitary laser diode and therefore without the complex addition of either external optical feedback or injection. This striking result is obtained despite the low dimension and relatively small bandwidth of the laser chaos, i.e. two characteristics that have been so far considered as limiting the performances of optical chaos-based applications. We unambiguously attribute the successful randomness at high speed to the physics of the laser chaotic polarization dynamics and the resulting growth rate of the dynamical entropy.

Light-modulating pressure sensor with integrated flexible organic light-emitting diode.

Organic light-emitting diodes (OLEDs) are used almost exclusively for display purposes. Even when implemented as a sensing component, it is rarely in a manner that exploits the possible compliance of the OLED. Here it is shown that OLEDs can be integrated into compliant mechanical micro-devices making a new range of applications possible. A light-modulating pressure sensor is considered, whereby the OLED is integrated with a silicon membrane. It is shown that such devices have potential and advantages over current measurement techniques. An analytical model has been developed that calculates the response of the device. Ray tracing numerical simulations verify the theory and show that the design can be optimized to maximize the resolution of the sensor.

Sensing characteristics of the rocking filters in microstructured fibers optimized for hydrostatic pressure measurements.

We report on the sensing characteristics of rocking filters fabricated in two microstructured fibers with enhanced polarimetric sensitivity to hydrostatic pressure. The filter fabricated in the first fiber shows a very high sensitivity to pressure ranging from 16.2 to 43.4 nm/MPa, depending on the resonance order and features an extremely low cross-sensitivity between pressure and temperature 28 ÷ 89 × 10(3) K/MPa. The filter fabricated in the second fiber has an extreme sensitivity to pressure ranging from -72.6 to -177 nm/MPa, but a less favorable cross-sensitivity between pressure and temperature of 1.05 ÷ 3.50 × 10(3) K/MPa. These characteristics allow using the rocking filters for pressure measurements with mbar resolution.

Mapping of two-polarization-mode dynamics in vertical-cavity surface-emitting lasers with optical injection.

We report theoretically on the interplay between polarization switching and bifurcations to nonlinear dynamics in a vertical-cavity surface-emitting laser (VCSEL) subject to orthogonal optical injection. Qualitatively different bifurcation scenarios leading to polarization switching are found and mapped out in the plane of the injection parameters, i.e., the frequency detuning vs injection strength plane. A Hopf bifurcation mechanism on the two-polarization-mode solution determines the injection-locking boundaries and influences polarization switching induced by optical injection. We furthermore report on a torus bifurcation emerging from a two-linearly polarized (LP) mode time-periodic dynamics before polarization switching and injection locking appear. It corresponds to an interesting combination of relaxation oscillation dynamics in the x -LP mode together with wave mixing dynamics in the injected y -LP mode. In agreement with recent experiments, we unveil a period-doubling route to chaos that involves both VCSEL orthogonal LP modes. The corresponding region of chaotic dynamics coincides with abrupt changes in the polarization switching boundaries in the plane of the injection parameters.

Fabrication and characterization of microlens arrays using a cantilever-based spotter.

We present a quantitative study on the fabrication of microlenses using a low-cost polymer dispending technique. Our method is based on the use of a silicon micro-cantilever robotized spotter system. We first give a detailed description of the technique. In a second part, the fabricated microlenses are fully characterized by means of SEM (Scanning Electron Microscope), AFM (Atomic Force Microscopy) non contact optical profilometry and Mach-Zehnder interferometry. Diameters in the range [25-130mum] are obtained with an average surface roughness of 2.02nm. Curvature radii, focal lengths as well as aberrations are also measured for the first time: the fabricated microlenses present focal lengths in the range [55-181mum] and exhibit high optical quality only limited by diffraction behaviour with RMS aberration lower than lambda/14.

Experimental investigations of bending loss oscillations in large mode area photonic crystal fibers.

We demonstrate experimentally that bending loss in large mode area photonic crystal fibers oscillates with wavelength. To do so we carried out loss measurements for different fiber bend radii and for different angular orientations. These results confirm the oscillatory behavior of bending loss vs. wavelength as predicted recently by numerical analysis [J. Olszewski et al., Opt. Express 13, 6015 (2005)]. We also found good agreement between our measurement results and our simulations relying on a finite element method with perfectly matched layers and an equivalent index model.

Optical feedback induces polarization mode hopping in vertical-cavity surface-emitting lasers.

Vertical-cavity surface-emitting lasers subjected to weak polarization-insensitive optical feedback are studied experimentally and theoretically. We find that the feedback induces random anticorrelated hopping between the two orthogonal linearly polarized modes. This polarization mode hopping is accompanied by rapid anticorrelated oscillations in the linearly polarized intensities at the external-cavity frequency. The study of a simple stochastic delay differential equation suggests that these oscillations generated by the delay are typical of any hopping phenomenon between states.

Photorefractive beam-fanning effect and self-pulsations in coated LiNbO3 slabs.

We investigate the dynamical behavior of optical Fabry-Perot resonators consisting of LiNbO3 slabs (x and c cut) that are coated with different (absorbing or lossless) dielectric multilayers deposited on both sides of the slabs. Bistable switching is observed experimentally. The buildup of beam fanning with time leads to destructive interference for a portion of the incident beam, inducing a change in absorption and heating, hence to switching off. As a result, self-pulsations appear whose frequency depends strongly on the input light intensity, the spot size, and the focusing. Switching and self-pulsations are not observed in the case of lossless coatings or for bare LiNbO3 slabs, although strong photorefractive beam fanning is still present. We also study the influence of the incident-beam characteristics (width and focusing) on the beam-fanning process and the pulsating behavior.

Optics in computing: introduction to the feature issue.

This issue of Applied Optics features 21 papers that describe the implementation of optics in computer systems and applications. This feature is the eighth in a series on the application of optics in the field of computing.

Architectural approach to the role of optics in monoprocessor and multiprocessor machines.

The relevance of introducing optical interconnects (OI's) in monoprocessors and multiprocessors is studied from an architectural point of view. We show that perhaps the major explanation for why optical technologies have nearly been unable to penetrate into computers is that OI's generally do not shorten the memory-access time, which is the most critical issue for today's stored-program machines. In monoprocessors the memory-access time is dominated by the electronic latency of the memory itself. Thus implementing OI's inside the memory hierarchy without changing the memory architecture cannot dramatically improve the global performance. In strongly coupled multiprocessors the node-bypass latency dominates. Therefore the higher the connectivity (possibly with optics), the shorter the path to another node, but the more expensive the network and the more complex the structure of electronic nodes. This relation leaves the choice of the best network open in terms of simplicity and latency reduction. The bottlenecks resulting from and the benefits of implementing OI's are discussed with respect to symmetric multiprocessors, rings, and distributed shared-memory supercomputers.

Polarization-selective diffractive optical elements with an index-matching gap material.

Highly polarization-selective diffractive optical elements for use in optical interconnection and routing systems have been fabricated by the wet etching of pairs of calcite substrates and characterized experimentally. We show that when an index-matching polymer is used to fill the gap between the two substrates, substrate alignment problems are eliminated and efficiency is greatly increased. This has resulted in first-order diffraction efficiencies of 40.5% and polarization contrast ratios of 450:1 for several off-axis binary-phase elements, allowing these components to be used for practical applications.

Compact optical imaging system for arrays of optical thyristors.

A compact and modular optical system that employs gradient-refractive-index rod lenses to image arrays of Lambertian sources is characterized both experimentally and by ray-tracing simulations. A hybrid optical system that incorporates additional microlens arrays to reduce transmittance losses and aberrations is also modeled, and the two systems are compared.

Optical bistability and switching in nonresonant GaAs:Cr self-electrooptic effect devices.

Using the thermal self-electrooptic effect, optical testability is observed in a hybrid etalon of semi-insulating GaAs:Cr material at the nonresonant wavelength of 1.06 microm. The nonlinearity is traced back to a temperature-dependent index of refraction, largely enhanced by the optoelectromeally induced electrical power arising from phutocurreut flow. The optical absorption is attributed to bandtailing effects due to the trapping centers. Optoelectronic AND gate operation between optical and electrical input signals is achieved.