The osseointegrated trans-femoral implant system provides a direct anchoring technique to attach prosthetic limb. This technique was first introduced PI Brenmark in Sweden. The UK had the first clinical trial in 1997 and currently has 6 active limb wearers. The success of this procedure has the potential for improved gait function and mobility, increased employability and significant long-term improvements in the quality of life for above knee amputees. However, the significant load involved in the trans-femoral implant system has caused permanent deformation and/or fractures of the implant abutment in several occasions. To protect the implant system, the implant abutment in particularly, an overloading protection device was introduced. The device uses mechanical mechanism to release torsion overload on the abutment. However, the bending overload protection remains unsolved. To solve the problem, a new overload protection device was developed. This device uses SMA component for bending overload protection. In this paper, the results of non-linear finite element modelling of the SMA and steel (AISI 1040) components were presented. Experiments were also carried out using steel components to assess the design which is based on the non-linear property of the materials.
Silica glass can be poled either thermally or with UV exposure during application of a strong electric field. Such treatment allows electret formation. So normally isotropic glass can become anisotropic via formation of a frozen-in field. This produces non-zero second-order nonlinearity in glass. After such poling treatment a change in the third- order nonlinearity has been observed. In this paper we examine if modification of the third-order nonlinearity is real or some artifact. To do this the DC third-order nonlinearity was measured before poling, after poling and then after erasure of the second-order nonlinearity. It was found that modification of the third-order nonlinearity remains after erasure of the frozen-in field. The reason for modification of the third-order nonlinearity is still not understood. It may be due to some kind of structural modification of the glass. It is known that impurity ionic species are moved through the glass during poling. This movement of ions under the high field may be sufficient to modify the glass structure. From our results, it is clear that the second-order nonlinearity is predominantly caused by formation of a frozen-in field. The increase of the third-order nonlinearity is independent of existence of a frozen-in field after poling.
Silica glass plays a key role in photonic systems because of its excellent optical properties, such as low loss, low fabrication cost and high photo-refractive damage threshold. Unfortunately, silica, being centrosymmetric, has no intrinsic linear electro-optic (LEO) coefficient or second-order nonlinearity (SON). However, thermal poling has been demonstrated to produce a LEO coefficient and SON of approximately 1 pm/V in silica glass and fiber. It is necessary to understand the mechanism of thermal poling in order to achieve a larger, stable and reliable LEO effect. A series of thermal poling experiments on silicate fiber was carried out. The in situ measurements of the total LEO coefficients (the sum of the poling field induced LEO coefficient and the thermal poling induced residual LEO coefficient) suggest movement of charges. Thermal poling induced residual LEO coefficients are measured in situ during prolonged negative thermal poling. Both the shielding field and the ionization field are frozen-in at room temperature and lead to LEO effect. The time evolution of the residual LEO coefficients shows that the competition between the shielding and ionization fields is a linear process. Using this new understanding, a specialty optical fiber was developed for the production of thermally poled optical fiber devices.
Recent work on the thermal poling of silicate optical fiber is presented. This paper includes the background on thermal poling, optimization of poling conditions, lifetime of the electro-optic effect as well as discussions on the change in (chi) (3) after poling and the validity of the frozen-in field model for thermal poling.
Electro-optic modulators and switches are important components for photonic systems. Existing technologies (such as lithium niobate) have significant drawbacks in terms of coupling losses to fiber, photorefractive damage and cost. Silica, especially fiber, based devices would be ideal; unfortunately silica, being centrosymmetric, has no intrinsic bulk electro-optic characteristics. However an electro-optic coefficient can be induced by silica by poling. By thermal poling, an electro-optic coefficient of around 1 pm/V has been produced in bulk silica; whilst scientifically interesting, this is insufficient for practical devices, especially as the values for fiber are much smaller. Recent work on UV excited poling has produced values around 6 pm/V in fiber; sufficient to realize fiber devices a few centimeters long, requiring only a few volts for switching. To perform UV-poling, optical fiber with internal electrodes is produced by milling holes into the fiber preform, close and parallel to the core, and then pulling at sufficiently low temperature to avoid closure of the holes into which the fine wire electrodes are subsequently introduced. A voltage is applied to the electrodes to produce a strong electric field (approximately 100/V/micrometers ) in the fiber core which is also exposed to UV laser radiation. After removal of irradiation and field an electro-optic coefficient of around 6 pm/V is measured. By applying a modulating voltage to the internal electrodes this electro-optic coefficient may be used to effect phase or polarization modulation (and amplitude modulation in a suitable structure). Furthermore, by periodic UV-poling, electrically tunable in-fiber Bragg gratings may be produced. Details of the fiber and device geometry, the processing conditions, the measurement techniques, the device performance and potential applications of this exciting new technology will be presented.
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