Geometric birefringence in fibres due to asymmetric cladding

A simple and accurate method is devised to calculate the geometric birefringence of optical fibres with asymmetric claddings. The method gives analytical results near the cutoff or transition frequencies of the guided modes. It is found that some asymmetric claddings cannot be used to introduce birefringence effects near the cutoff or transition frequencies.     

Internal-getter formation in nitrogen-doped dislocation-free silicon wafers

An experimental study is reported concerning the formation of defects in nitrogen-doped dislocation-free silicon wafers under a multistage heat treatment to produce an internal getter, the first stage being rapid thermal annealing (RTA) under different conditions. The experiments are conducted on p-Si(100) wafers of diameter 200 mm with an oxygen content of (6−7) × 10¹⁷ cm⁻³ and a doping level of 1.6 × 10¹⁴ cm⁻³, the resistivity being 10–12 ω cm. The processed wafers are examined by optical microscopy and transmission electron microscopy. With normal conditions of RTA (argon, 1250°C, 20 s), the process is found to be incapable of creating a defect-free subsurface layer of adequate thickness, though it is able to provide the desired system of defects in the bulk. The aim is achieved by changing to sequential RTA in oxygen and argon as the first stage. The reasons for the results are presented.     

Tuning and wavelength switching erbium-doped fiber ring lasers by controlled bending in arc-induced long-period fiber gratings

In this paper, we report a simple method for the production of an all-fiber, tunable and wavelength switchable erbium-doped fiber ring laser (EDFRL). We use the shift in resonance wavelength and notch-depth of arc-induced long-period fiber gratings (LPFGs) when it is subject to controlled bend to suppress some wavelength gain and promote lasing in a selective way in a fiber laser system. By changing the bending radius, the operating wavelength of the EDFRL can be tuned from 1526 to 1538 nm and then switched and tuned from 1568 to 1557 nm. The maximum separation measured between laser lines was ～42 nm. This is one of the highest laser separations reported in a wavelength switching operation of an EDFL using a single LPFG as wavelength selective filter.     

Backscattering in single-mode fibres

The scattering process in single-mode optical fibres is considered in terms of wave optics rather than geometrical optics, which is inadequate in this case. Surprisingly, however, the result for the backscattering signal at the input end of the fibre is nearly the same as for multimode fibres.     

强干扰下的波束形成

提出强干扰条件下等距线阵的波束形成方法。该方法将空间角度映射到数字频率,对阵列方向图的设计就等效于对FIR滤波器的零点设计。在强干扰方向形成零陷可采用零点修改法与零点增加法2种设计方法,由于该2种方法均会使波束主瓣方向略有改变,且对旁瓣的抑制能力有所下降,所以要设计方向图指向正确且对旁瓣电平有要求的方向图时需要进行迭代设计。仿真结果表明,该2种设计方法适用于强干扰条件下弱信号的接收。     

Pulse compression in optical fibres

A shift in the emission wavelength of s.h. GaAs lasers has been observed during individual pulses. In conjunction with material dispersion, this leads to pulse compression after transmission through an optical fibre. The use of this device as a source in bandwidth measurements readily leads to erroneous conclusions when results are analysed.     

Anisotropy in spun single-mode fibres

It is shown that the dominant process for the reduction of birefringence in a spun fibre is the averaging of the local fibre anisotropy by the rapid procession of the axes of asymmetry along the fibre. No significant change in cross-sectional geometry occurs.     

Zero-mode dispersion in single-mode fibres

Near cutoff the loss of the LP11-mode in cladded optical fibres becomes very high in the presence of even slight bends. Singlemode operation can thus be achieved with zero-mode dispersion as well as zero material dispersion, thus promising enormously large bandwidths.     

Stress birefringence in single-mode fibres

The core and cladding of a glass fibre exposed to axial tension become optically uniaxial. The modified characteristic equation is presented and, for the fundamental mode, the changes in the most important propagation parameters have been calculated.     

Jointing loss in single-mode fibres

The jointing loss of single-mode fibres is mainly dependent on their numerical apertures, and is largely independent of normalised frequency and fibre diameter. The letter presents simple loss predictions as an aid for practical design work of single-mode fibre joints.     

Microbending loss in monomode fibres

A simple analytic formula for the microbending loss in monomode fibres of arbitrary refractive-index profile is presented. This loss essentially depends on the spot size of the fundamental fibre mode only. Therefore W-fibres, for example, are as sensitive to microbending as conventional cladded-core fibres.     

Dispersion in graded single-mode fibres

The normalised frequency for zero waveguide dispersion of the HE11 mode, called VOD, is highly dependent on profile shape. The more the profile departs from a step, the greater the increase of VOD above the effective single-moded region until, for example, VOD becomes infinite for profiles that fall off at a rate greater than or equal to the parabola.     

Leaky modes in graded fibres

Leaky modes, particularly tunnelling leaky modes, in step-index fibres are well known, but they are also present in graded fibres, in which case they are much more likely to cause errors in calculations and measurements, for example, transmission measurements, since they cannot be distinguished by any simple means from bound modes. The possible and probable extent of their effects are calculated by simple mode theory.     

Polarisation conservation in single-mode fibres

There exist proper orthogonal elliptical polarisation states, which are maintained over long fibre length without converting. When one of the proper polarisation states is excited, orthogonal polarisation less than ?40 dB is confirmed after 5 km single-mode fibre transmission. It is also proposed that twisted single-mode fibres show a high degree of polarisation for any incident polarisation state. The polarisation characteristics are hardly affected by external perturbations.     

Polarisation preservation in multimode fibres

Birefringent multimode fibres have been found to preserve polarisation except for one mode near cutoff. Experimental observations and theoretical considerations show that polarisation ratios better than 20 dB are possible, offering the potential for building polarisation-sensitive multimode fibre components.     

Residual thermomechanical stresses in thinned-chip assemblies

A new technology for the three-dimensional (3-D) stacking of very thin chips on a substrate is currently under development within the ultrathin chip stacking (UTCS) Esprit Project 24910. In this work, we present the first-level UTCS structure and the analysis of the thermomechanical stresses produced by the manufacturing process. Chips are thinned up to 10 or 15 μm. We discuss potentially critical points at the edges of the chips, the suppression of delamination problems of the peripheral dielectric matrix and produce a comparative study of several technological choices for the design of metallic interconnect structures. The purpose of these calculations is to give inputs for the definition of design rules for this technology. We have therefore undertaken a programme that analyzes the influence of sundry design parameters and alternative development options. Numerical analyses are based on the finite element method     

Calculation of thermoelastic stresses in microelectronics

An overview is given of several situations in which thermoelastic stresses can occur in electronics. To gain insight into these situations, simple models are presented allowing analytical or semi-analytical solutions. A first model describes the thermoelastic behaviour of a substrate. A second model describes the attachment between layers. A third model describes thermoelastic effects around small heat sources.     

Recombination activity of nickel in nitrogen-doped Czochralski silicon treated by rapid thermal processing

The recombination activities of nickel (Ni) in p-type nitrogen-doped Czochralski (NCZ) silicon (Si) subjected to the rapid thermal processing (RTP) under different temperatures, atmospheres or cooling rates were investigated by means of microwave photoconductivity decay and scanning infrared microscopy. It was found that the value of the reciprocal of effective minority carrier lifetime (1/τ_eff) of NCZ Si, related to the recombination activity of Ni, increased with the annealing temperature or cooling rate, while, it was almost insensitive of the annealing atmosphere. Moreover, the 1/τ_eff of the Ni-contaminated NCZ Si was lower than that of the Ni-contaminated conventional Czochralski (CZ) Si annealed under the same condition. It is considered that the nitrogen-related defects or large grown-in oxygen precipitates might be the reason of relative lower recombination activity of Ni in NCZ Si.     

Modes of propagation in optical fibres

Light propagates along optical fibres in form of normal modes. As long as fibres are uniform in axial direction, these modes follow as solutions of the eigenvalue problem which Maxwell’s equations together with the boundary conditions of the particular fibre represent. Such modes propagate independently from each other and transmit and distort signals according to their respective delay, dispersion and attenuation. Imperfections of the fibre couple its modes with each other and change its signal attenuation and transmission characteristics. When a randomly imperfect fibre guides many modes, the random coupling leads to a new set of statistical modes which again propagate independently from each other and describe the average power in each of the perfect fibre modes. Interaction between statistical modes occurs only at points along the fibre where the statistics of imperfections change such as at connectors or splices.