Polarization-independent fiber-optic filters are presented that replace the polarizers in traditional birefringence filters with a Sagnac interferometer. The performance of these filters is discussed when Solc and Lyot type birefringence combinations are used. A high-order, compound filter combining these two filters is presented, along with experimental verification.

A broadband, electrically controlled polarization- independent intensity modulator (PIM) based on a Sagnac inter- ferometer is presented. Influences from asymmetric modulation and traveling wave effects are analyzed and methods to elim- inate these effects are proposed. Compared to Mach-Zehnder interferometer-based modulators, this modulator is independent of the input polarization state and less sensitive to ambient per- turbations due to its reciprocity property. A prototype modulator operating at 1 and 2.488 Gb/s modulation rates is demonstrated.

A broadband, electrically controlled polarization-independent intensity modulator (PLM) based on a Sagnac interferometer is presented. Influences from asymmetric modulation and traveling wave effects are analyzed and methods to eliminate these effects are proposed. Compared to Mach-Zehnder interferometer-based modulators, this modulator is independent of the input polarization state and less sensitive to ambient perturbations due to its reciprocity property. A prototype modulator operating at 1 and 2.488 Gb/s modulation rates is demonstrated.

考虑到有效钉扎势U（J）与临界电流密度的非线性关系,将U（J）展开为三级泰勒级数,最后推导出一个普遍适用于描述高温超导体磁弛豫的等温衰减方程,并用这一方程研究了具有高J c ,强钉扎的区域熔炼法制备YBa 2 Cu 3 O 6+x 的磁弛豫,而且对目前被普遍采用的几个钉扎模型进行了讨论.

Considering the nonlinear dependence of the effective pinning potential U(J) on the critical current Jc, a more general isothermal decay equation to describe the magnetization relaxation in high Tc superconductors was derived, which takes into account the first two nonlinear terms in the Taylor expansion. With this equation, the magnetization relaxation on zone-melted YBa2Cu3O6+x samples which show high Jc and strong flux pinning was studied; and three flux pinning models, i.e. Anderson-Kim model, collective pinning model and the logarithmic form of U(J) were discussed.

> 1 Introduction It is very important for practical application of HTSC to increase the critical cur-rent density （J c ）by artificially introducing pinning centers to enhance flux pinning.Ex-periment results show that neutron radiation is 1000 times more effective than theelectron and proton radiation.So in recent years,the neutron radiation has becomepopular,theoretically and practically.After irradiation of fast neutrons in high flux,the magnetization ratio of polycrystal samples of Y and Bi Systems are M irr /M unirr ≈2.8—3.1.But T c is descending with the neutron flux increasing and the decreasingratio is about（2.6—3.0 K）/(10 18 n/cm 2 ).

The time and field sweep rate dependencies of magnetization in high temperature superconductors are derived from a combined view based on the thermally activated flux creep. The tau(0) in the magnetization relaxation expression M=M(0)+M(1) ln(1+t/tau(0)) is a macroscopic quantity related primarily to the sample geometry and other experimental conditions via the effective activation energy U(J(0)). This activation energy provides an alternative interpretation to the nearly inverse proportionality of tau(0) to the field sweep rate. The values of tau(0), in the magnetization relaxation of single crystal YBa2Cu3O7-delta are consistent with the calculated ones. As the nonlinear U(J) is taken into account, the curvature in the \M\-ln t plot may change from negative to positive over a sufficiently long time window, and the magnetization hysteresis M(H) depends on the field sweep rate H by const+A ln H+B ln(2) H, where A is slightly larger than M(1) and B>0. A discussion of the temperature dependency of the normalized flux creep rate is at the conclusion of the article.

This paper presents systematically the doping effects of halogens in YBa2Cu3O7-δ polycrystalline samples, with a series of concentrations of fluorine, chlorine, bromine or idodine. All samples were prepared by chemical method of citrate pyrolysis, which makes the halogen concentration in the samples more uniform. Samples doped with F have Tc as high as 94.5 K, Cl, Br and I dopings also raise Tc to a certain extent. In addition, the influence of halogen doping on critical current density, morphology, constitution and so forth is discussed.

Irradiation effect of low-fluence (∼108 n/cm2) slow neutrons on halogen-doped superconductors is presented in this paper. And the mechanism of the effect is also described from the viewpoint of nuclear physics for both fast and slow neutrons on high-temperature superconductors (HTSC). It is shown experimentally and theoretically that slow neutrons of low fluence has a similar irradiation effect to that of fast neutron beams with an energy En > 0.1 MeV and fluence 1016 - 1018 n/cm2. However, quite different mechanisms are involved in them: Fast neutrons transfer their energies through elastic scattering in HTSC, whereas slow neutrons give off their energies during the slow neutron capture (n, γ) reaction.

This work studies the effect and mechanism of slow neutron irradiation of low fluence (～108n/cm2) in normal resistivity of Y and Bi system and the doped high Tc superconductors. The experimental results indicate that the normal resistivity R not only follows the law that R is increasing exponentially with the enhancement of fluence φn, but also decreases greatly after irradiation of slow neutron with low fluence.

The effect of slow neutron irradiation at a low fluence of about 108n/cm2 on the microstructures of Bi-2223 polycrystalline samples was observed by transmission electron microscopy. By comparative study of the defect structures in a given area in one sample, we get evidence for the flux pinning enhancement by low fluence slow neutron irradiation.

This paper studies a new electron irradiation effect of the formation, movement and growth of stacking fault in YBa2Cu3O6+x. The energy of electron irradiation is about 160 keV. It would be a promising way to enhance the density of pinning centers.

Ⅰ. INTRODUCTION Critical current density J_c varies with structural imperfections in non-ideal type-Ⅱ superconductors, and it is sensitive to the defect concentration. Neutron irradiation has been established as a useful tool to increase the defect concentration in superconducting materials. Many experiments have shown that neutron irradiation at a suitable fluence would enhance

Effect of slow neutron irradiation of low fluence on bismuth-based superconductors is studied. It's shown through experiments that (1) both critical current density Jc and zero-resistance temperature Tc0 are increased, though to different extents, after the Bi-based superconductors were irradiated with slow neutrons to suitable fluence and (2) slow neutrons of low fluence and fast neutrons of high fluence have similar irradiation effects on Bi-based superconductors.

We measured the time decay of the magnetic moment of high Tc superconductors YBa2Cu3O7-δ ceramic samples and found the memory effect which has hitherto been reported only in single crystals. The temperature and field ranges, especially the memory time are different from those of the latter. We believed that the memory phenomenon in ceramic samples at liquid nitrogen temperature can only be understood in a combined picture of both flux creep and glass behaviour.