Table of contents

The authors give the fundamental concepts of all-fibre devices and the basic technology that has been developed to realise those concepts. In addition, they present a review of the state of the art in the design and performance of linear and nonlinear optical all-fibre devices with particular emphasis on recent progress in such devices in he field of lightwave technology including optical fibre communications, optical fibre sensing and optical signal processing.

The first results from the Mk-6 Strathclyde University electron cyclotron maser are presented. The maser is based upon a lightweight two-electrode system making use of a field-immersed, field-emission cold cathode with conventional pulsed coils producing the required spatially homogeneous magnetic field. The maser was operated in the W band (75-110 GHz) and single-mode oscillation in the TE₀₃ mode was observed. This had a centre frequency of 96 GHz and developed up to 100 kW in a 400 ns pulse. Multimode oscillation was subsequently observed, on tuning the intracavity B field with the simultaneous excitation of the TE₁₃ mode (centred at 81 GHz) and the TE₄₂ mode (centred at 88 GHz).

A general approach to continuum wavefront motion in anisotropic, inhomogeneous and time-dependent systems based on the Huygens' principle of wave propagation is presented. The fundamental nature of the elementary Huygens' wavelet is discussed and its derivation also presented. Specific examples of surface evolution during anisotropic chemical etching, ion bombardment-induced erosion and curvature-dependent wavefront propagation are considered as illustrations of the general approach.

Kendall has used contact mechanics to show that surface energy affects the friction coefficient of a spherical particle on a smooth substrate. In particular, the friction coefficient is shown to depend on the particle diameter, the applied force and the interfacial energy. For small particles the friction coefficient can be twice that measured for large block samples. In the present analysis, contact mechanics is applied to determine the friction coefficient of an incompressible flat particle on a smooth compressible substrate. It is shown that the static friction coefficient of an incompressible truncated sphere (i.e. flat on the bottom) depends not only on the particle diameter, the applied force and the surface energy, but also on the elastic properties of the substrate. The author shows that a large enhancement in friction coefficient over that observed for bulk samples, in some cases a factor of two, can result for small flat particles on soft substrates. The implications for the effects of plasticisers and adsorbed solids and gases, and the choice of substrate material parameters, on friction are discussed.

Sharp current pulses, with amplitudes of the order of hundreds of mA and frequencies of the order of 100 kHz, occur in SF₆ glow discharges in the pressure range 1 to 10 Torr for average discharge currents up to 20 mA. The pulses are shown to be a phenomenon associated with a constricted positive column and a negative voltage-current discharge characteristic. A series of experiments is reported in which the amplitudes and frequencies of the current pulses are measured as functions of average discharge current, SF₆ pressure, electrode separation, tube diameter and interelectrode capacitance. The results are described in terms of the charge stored on the interelectrode capacitance, and the dependence of the discharge processes of ionisation and attachment on E/N (axial electric field/neutral density) in the positive column. Further support for this description is provided by an examination of current pulses in high-pressure glow discharges through argon and neon. In these discharges electron-ion recombination plays a similar role to that of attachment in SF₆ discharges.

Experimental and theoretical investigations have been carried out on the effect of N₂ gas in a transverse-flow CO₂ laser excited by silent discharge (SD). It has been found that the addition of N₂ is a cause of the increase of excitation efficiency compensating the effect of the increase in reduced electric field strength E/N. The excitation efficiency expressed as a function of N₂ gas concentration agrees qualitatively with the results of a Boltzmann equation analysis in which the electric field is assumed to be temporally stationary and spatially uniform. The decrease in gain coefficient with N₂ concentration is explained well by a model in which the stream of excited molecules due to transverse flow of gas is taken into account. The optimum gas composition for SD excitation is found to be a mixture of CO₂-CO-N₂-He=8-4-60-28. The optimum molar fraction of N₂ is extremely high in comparison to those values reported for other types of discharge excitation.

A numerical method is presented for studying an RF discharge caused by secondary electron emission from the electrode, i.e. the multipactoring discharge. Using this method, the precise thresholds of the electron multiplication are obtained systematically. Also, this method can be extended to a nonuniform configuration of the electrode and the applied field. For example, the simulation results are compared with typical experimental results in the previous work. Good agreement is obtained for parallel electrodes. Furthermore, an intermediate (subharmonics) mode is found between the usual resonant modes. In the case of the coaxial electrodes, the agreement was obtained when the reflection of low-energy electrons on the surface of the electrode was assumed.

The momentum transfer cross section of krypton is deduced from experimental electron drift velocity data over the energy range from 0.01 to 100 eV by using an algorithm based on a Boltzmann equation method. This energy range covers the Ramsauer-Townsend minimum and the peak of the momentum transfer cross section. The results show that the Ramsauer-Townsend minimum is located at an electron energy of 0.6 eV and the magnitude of the cross section there is 2.3*10^-17 cm², while the peak is located at an electron energy of 8.0 eV with a magnitude of 2.0*10^-15 cm². For electron energies above 9.91 eV, the threshold of inelastic collisions, a set of cross sections for these collisions is necessary. Since the total excitation cross section used seemed less accurate than the ionisation cross section, the momentum transfer cross section was determined by consulting the experimental ionisation coefficient data as well as electron drift velocity data and by modifying the excitation cross section. It is found that the momentum transfer cross section is not affected by choice of excitation cross section for electron energies below 30 eV. The diffusion coefficient and the characteristic energy are also calculated by using the momentum transfer cross section determined here to find good agreement with experiment. The energy distributions are calculated and compared with the energy distributions proposed by Morse et al. (1935). The agreement of these two energy distributions is very good at E/N=0.1 Td.

Discharge phenomena for smaller pd values than the Paschen minimum include many interesting problems from engineering and physical points of view. However, it was difficult to maintain steady-state discharges under these conditions, since sparks occurred along longer paths more easily than along the shortest path which corresponded to the gap distance. The authors designed a concave electrode to solve this problem and measured the current-voltage characteristics in this region. The authors deal with the design of the concave electrode and the results of I-V characteristics of the discharge for small pd values. The I-V characteristics of discharge in this region were similar to those of abnormal glow. However, a dark space was not observed. The results of spectrum observations showed that highly excited ions were brought about by discharge under the low-pd conditions.

Two samples consisting of silicon oxide layers, of different thicknesses, on a crystalline silicon substrate have been investigated using the technique of neutron reflectivity. By simulating the neutron reflectivity from a theoretical scattering length density profile, and altering the profile so that the closest possible fit to the experimental data is achieved, the structures of the samples normal to the reflecting surfaces have been determined. This analysis has been performed using two different approaches, the results of which are compared. Finally, the advantages and disadvantages of the technique are discussed.

A group of tensile microstructures are modelled that exhibit negative Poisson's ratios. These microstructures are a combination of anisotropic or isotropic particles, tensile springs and topologically constraining rods or strings. A range of structures are modelled, one of which contains an internal rotational degree of freedom. In all cases topological interactions occur between first- and second-nearest neighbour particles to produce cooperative transverse expansions under longitudinal loading.

A microporous, anisotropic form of expanded polytetrafluoroethylene has been found to have a large negative major Poisson's ratio. The value of Poisson's ratio varies with tensile strain and can attain values as large as -12. The microporous structure of the material is described and the mechanisms that lead to this large negative Poisson's ratio are identified. Micro-rotational degrees of freedom are observed, suggesting that a micropolar elasticity theory may be required to describe the mechanical properties.

For pt.I see ibid., vol.22, p.1877-82 (1989). In a previous paper the morphology of a microporous material made from expanded poly(tetrafluoroethylene) was described and results presented for its mechanical behaviour. The material was shown to be highly anisotropic and exhibited a large negative Poisson's ratio. In this paper a simple model for the microstructure is described to account for this effect. The model is based on an interconnected array of anisotropic particles that deforms so as to produce a large transverse displacement under longitudinal tensile loading. Very good agreement is found between the model and experimental results, providing an explanation for the variation of Poisson's ratio with tensile strain, in terms of changes in material morphology.

Previous theories of the inertial contribution to measured stress in dynamic compression of cylindrical specimens have been extended to allow for anvil motion as well as specimen strain. A numerical model has been developed to estimate the inertial stress from this theory, and it was found that the new term can have the largest contribution. Because of the effects of wave propagation and dispersion found in experimental records, an estimate of the inertial errors from this numerical simulation is probably more reliable than an estimate derived directly from the experimental measurements.

The authors have studied the variation of the electron acoustic signal in various materials as a function of the beam accelerating voltage and current. The results show that in most materials examined, e.g. copper, CuZnAl alloy and silicon, it is only the total incident beam power which determines the magnitude of the electron acoustic signal produced. In compensated GaAs the electron acoustic signal depends only on the beam current and is independent of the accelerating voltage which is unlike the case for highly doped GaAs which resembles the metals and silicon. Both semi-insulating and highly doped InP behave as the highly doped GaAs sample. The results are discussed in relation to some of the signal generation mechanisms thought to operate in the electron acoustic (or thermal wave) microscope.

Hysteresis in the Seebeck coefficient has been studied for type K and type N thermocouples. The temperature range was 0 to 1200 degrees C, heating periods were up to 530 h and the alloys were supplied from a number of different sources (five for type K and two for type N). In both types, changes of about 1% in Seebeck coefficient were found but the hysteresis extended over a larger temperature range for the type N thermocouple. Thermoelectric hysteresis is a major cause of instability in Ni-based thermocouples, especially in the more stable, Nicrosil-sheathed mineral-insulated configuration. It is shown that it is possible to produce type K thermocouples with a performance comparable with that previously reported for type N.

The irreversible changes in the Seebeck coefficient that occur in mineral-insulated Nicrosil-sheathed thermocouples were measured during 530 h of heating at temperatures up to 1200 degrees C. Type K and type N alloys from seven sources were examined. Most change occurred in the negative thermoelements whose behaviour depended on whether Mn and Al was present and not on whether they were type K or type N alloys. The presence of Mn and Al gave the more complex behaviour but reduced instability at high temperatures. It is shown that Nicrosil-sheathed temperature probes of 6 mm diameter and having Mn- and Al-bearing type K alloys would drift less than about 3 degrees C during a approximately 1000 h use at temperatures up to 1200 degrees C. Similar probes with type N thermoelements would drift up to 5 degrees C.

The field and concentration dependence of chain formation in magnetic fluids is investigated. The results show that the chain length increases rapidly with the field and then continues to increase gradually at higher fields approaching saturation. Moreover, the results show that the chain length at a given field is linear with the concentration of the fluid. The correlation between chain formation and some magneto-optic effects is discussed.

Positive temperature coefficient of resistance (PTCR) BaTiO₃ ceramics were prepared from commercially available BaTiO₃ powder, to which a suitable quantity of donor dopant was added. The samples were sintered at 1320 degrees C for 20 min and, while cooling to room temperature, annealed at 1220 degrees C for a further 20 min. The apparent relative permittivity, epsilon '_app, was measured at a frequency of 30 kHz between ambient and approximately=360 degrees C. Plots of 1/ epsilon '_app versus temperature were found to follow the Curie-Weiss law only within a small temperature range ( approximately 60 degrees C) above the ferroelectric transition temperature. At higher temperatures 1/ epsilon '_app rose more slowly with temperature, giving a second region with smaller slope. This behaviour can be explained on the basis of the well known Heywang model, following narrowing of the grain boundary barrier layers as a result of the depopulation of the surface states at high temperatures.

NaOH solution has been used to study the dissolution rates on boron-doped Si(100) wafers as a function of molarity (0.02 to 10 M) and temperature in the range from 30 degrees C to boiling point. For dissolution at boiling point, two distinct ranges of solution normalities have been observed. For M<or=4.5, the dissolution rate increases logarithmically and is defect-dependent. For higher values of M>or=4.5 the dissolution rate becomes a linear function of molarity and reaction is defect independent. The reaction activation energies for these regions have been measured to be 0.77+or-0.05 and 0.69+or-0.01 eV respectively.

In the Schottky limit, electron temperature T_e in a gas discharge positive column can be written as a simple function of pressure-radius product pR: eV_i/kT_e=a+b In pR where V_i is the ionisation potential of the gas, a is the constant that depends on the gas and b is a universal numerical constant.

Optimisation of volume-produced hydrogen negative ions for a tandem two-chamber system is studied numerically using a set of particle balance equations in a steady-state hydrogen plasma. Dependences of the production of hydrogen negative ions and vibrationally excited hydrogen molecules on some key parameters are explored. In addition, in order to clarify the effectiveness of the tandem two-chamber system, the relative hydrogen negative ion yields for the single-chamber and the two-chamber systems are compared.

High-T_c screen-printed Y-Ba-Cu-O films were prepared on YSZ substrates by a liquid-phase processing method. The films which were heated rapidly at 1030-1060 degrees C under helium flow and then annealed in oxygen showed relatively high-T_c values of 1200-1600 A cm^-2 at 77 K in zero magnetic field. SEM micrographs showed that these films have an aligned grain structure consisting of long, needle-shape grains over a wide area.

Scanning tunnelling microscopy images of stoichiometric and thus electrically insulating Ga₂O₃ thin films are presented for the first time. These ceramic films were deposited by RF sputtering. Characterisation was by means of RBS (stoichiometry) and X-ray fluorescence (contaminants). The scanning tunnelling microscopy results are consistent with those of high-resolution scanning electron microscopy. For scanning electron microscopy the specimens had to be covered by a thin metallisation. This was not necessary for the scanning tunnelling microscopy images. The presented results show that scanning tunnelling microscopy is suitable for the detailed study of insulating thin-film ceramics. This is a prerequisite for future studies of ultra-thin metallisation on thin film ceramics (supported catalysts) with superior vertical resolution.