一种高速率、高精度的全光纤偏振控制方法

提出了一种结构简单、高速率、高精度的全光纤偏振控制方法, 并对方案进行了详细的理论分析. 理论和实验结果表明:本方案可以自动补偿光纤系统由于环境变化使光纤发生形变产生的额外双折射效应, 提高了系统的抗干扰能力. 实验获得了31 dB消光比, 此方案有望在实际中得到广泛应用. 关键词： 光纤偏振控制器 双折射效应 法拉第旋转镜     

V型高双折射光子晶体光纤超连续谱产生的实验研究

高非线性高双折射光子晶体光纤是超连续谱产生的最有效介质之一, 因此本文选取V型光子晶体光纤作为研究对象. 通过多极理论数值模拟的结果, 确定V型光纤具有高双折射、高非线性等特性. 通过实验手段, 发现入射光中心波长和光纤的双折射效应对产生的超连续谱有很大的影响: 当入射光波长处于光子晶体光纤大的反常色散区时, 脉冲相对展得比较宽, 长轴要比短轴方向的超连续谱更宽, 频谱成分更加丰富. 在同一波长下光偏振方向越接近45°时, 超连续谱谱宽范围越大. 随着入射脉冲功率的增加, 超连续谱展得越宽, 但是当功率比较大时会达到功率饱和. 关键词： 光子晶体光纤 高双折射 多极理论 超连续谱     

Photoinduced color centers creation in superionic crystals RbAg 4 I 5

Abstract

A new phenomenon of a reversible photoinduced coloration caused by light irradiation is discovered and investigated in superionic RbAg ₄ I ₅ crystals. The reversible photoinduced absorption is found to be a result of irradiation by light with wavelengths in the region from 420 nm to 450 nm. The proposed mechanism of the discovered effect is associated with ambipolar diffusion of screened by mobile ions optically excited electronic carriers. The processes of color centers creation in superionic crystals RbAg ₄ I ₅ due to additive coloring in iodine vapours, ionic implantation and γ-ray irradiation are considered.     

Optical studies of the ferroelastic phase transitions in Rb3H(SeO4)2, (NH4)3H(SO4)2 and (NH4)3H(SeO4)2

Optical observations of growth twins and ferroelastic domains and measurements of the rotation of the optical indicatrix were carried out for Rb₃H(SeO₄)₂ and (NH₄)₃H(SO₄)₂ using an optical microscope. Taking into account the symmetry reduction from the rhombohedral (Rm) to the monoclinic phase (B2/a) the occurrence of domains and growth twins can be well described. The orientations of oblique ferroelastic walls are well determined by the spontaneous strains ^s e ₁₁ and ^s e ₂₃ at room temperature.     

Precursor dynamics to the structural instability in SrTiO3

The structural instability of SrTiO₃ at T _S?=?105?K is reinvestigated experimentally and theoretically by birefringence measurements and within the self-consistent phonon approximation. Analogous to BaTiO₃ the phase transition shows precursor dynamics starting approximately 60?K above the phase transition temperature. These rotational dynamic clusters compete with precursor effects stemming from the polar soft mode and appear on different length scales as compared to BaTiO₃.     

Phase diagram and domain structure of the Rb2 x Tl2(1− x )Cd2(SO4)3 solid solutions

From measurements of temperature dependences of the birefringence and from observations of the domain structure the x, T–phase diagram for the Rb_{2 x} Tl_{2(1? x )}Cd₂(SO₄)₃ solid solutions have been obtained. Ferroelastic domain structure and movement of phase boundaries have been studied in the course of phase transitions in Rb_{2 x} Tl_{2(1? x )}Cd₂(SO₄)₃ crystals. Possible reasons for the appearance of mechanically non-compatible (‘forbidden’) ferroelastic domain walls are given.     

Wavelength Shift of Cladding Mode Resonances in a Mechanically Induced LPFG by Twisting the Fiber

Abstract

The long-period fiber grating is mechanically induced over a twisted fiber and its characteristics are investigated. The amplitude as well as the wavelength shift of the resonance is studied in response to the applied twist and pressures. These resonances decrease in amplitude and shift to shorter wavelength side as the applied twist increases. The spectral responses of a grating assembly formed by two grating sections in series, one section with a twisted fiber and other with an untwisted fiber, are also investigated. Shearing stress and photo-elasticity causes the fiber to be circularly birefringent and the mechanically induced grating formed over the twisted fiber region causes the appearance of two resonances shifted away from the resonances of the untwisted grating section. At higher twist rates, resonant wavelength shift becomes insensitive to applied pressures, showing a reduction in the induced linear birefringence. The wavelength shift is almost symmetric with respect to the applied twist rate in clockwise and counterclockwise directions.     

Molecular Properties and Order Parameters of Schiff's Base Compounds

The refractive indices n_o, n_c and the order parameters are measured as a function of temperature for the Schiffs base compound PBBA. From the experimental data the molecular polarisabilities and its anisotropies at different wavelengths are calculated using different internal field models. The merits of each model are discussed. Order parameters (P ₂), (P ₄) and other molecular parameters of PBBA and EBBA are also calculated using X-ray diffraction techniques at different temperatures. Comparison with theoretical values shows the relative merits of different experimental techniques. The (P ₂) and (P ₄) values of other members of the Schiff's base compounds, already reported from our laboratory, have been given herewith to show the disagreement with the simple mean field theory. Possible causes for such disagreement have been discussed.     

Structural Studies of 1:1 Quinone-Hydroquinone Complexes

The structures of the 1:1 quinone-hydroquinone complexes of 2-phenyl and of 2-(4′-chloro) phenylbenzoquinone have been studied by X-ray methods. A superficial study would indicate that the quinhydrones are centrosymmetric and belong to the space group P2₁/c. However, other evidence indicates that the true crystal structure may belong to either the space group P2₁ or to Pc, or that the crystal may contain regions that would coresspond to each of these two non-centrosymmetric space groups. Some possible consequences of such a structural arrangement are briefly discussed.     

Phase shift of polarized light through sculptured thin films: Experimental measurements and theoretical study

A slanted columnar TiO2 sculptured anisotropic thin film （ATF） is prepared via the glancing angle deposition technique and used as the phase retardation plate. The tilted nanocolumn microstructures of thin film induce the optical anisotropy. With the biaxial birefringent model, the optical constants dispersion equations of TiO2 ATF are derived by fitting the transmittance spectra for s-and p-polarized waves measured at normal and oblique incidence within 400 nm–1200 nm. The phase shift of polarized light after reflection and/or transmission through the TiO2 ATF is analyzed with the characteristic matrix employing the extracted structure parameters. The theoretical studies reasonably well accord with the experimental results measured with spectroscopic ellipsometry. In addition, the dependences of the phase shift on the coating physical thickness and oblique incidence angle are also discussed. Birefringence of the biaxial ATF provides a sophisticated phase modulation by varying incidence angles over a broad range to have a wide-angle phase shift.