David C. Jones

Hybrid organic-inorganic photorefractives

The very high birefringence of liquid crystals makes them attractive for photorefractive applications. However, the drawbacks of using liquid crystals as photorefractives include a small phase shift between the optical and refractive index gratings, coarse grating spacings with narrow beam intersection angles, operation usually restricted to the Raman-Nath regime, a need to apply an external electric field, and, with most geometries, a need to tilt the cell at an angle to the grating k-vector. In this paper, we describe two-beam coupling with hybrid photorefractive cells comprising a nematic liquid crystal layer adjacent to inorganic photorefractive windows. In this arrangement, the underlying photorefractive properties are determined by the inorganic windows while the liquid crystal molecules amplify the overall refractive index modulation. Using this technique we have obtained Bragg matched liquid crystal gain coefficients of more than 1600 cm-1, grating periods of less than 300 nm and a wide range of beam intersection angles without the need to apply an external field.

Phase stabilization of a large-mode-area ytterbium-doped fiber amplifier

Measurements are reported on the open and closed-loop phase stability of a large-mode-area ytterbium-doped fiber amplifier. Phase fluctuations are characterized by a high-frequency low-amplitude jitter superimposed on a slow power-dependent drift. The amplifier may be phase locked to a precision of lambda/20 by using a low-bandwidth feedback loop.

Beam steering of a fiber-bundle laser output using phased array techniques

We describe the experimental study of phase locking of a four element phased array of fibres, in which the output brightness of the bundle is enhanced by phase locking of the individual elements, and steered by controlling the phase of each channel.

A multi-channel phase locked fibre bundle laser

We report on the phase locking of a fibre bundle laser based on a single frequency oscillator coupled into four fibre amplifiers to provide a coherent beam of over 600 W. The oscillator was phase modulated to a width of up to 2 GHz to increase the threshold for stimulated Brillouin scattering and then a fraction split off and frequency shifted to form a reference beam. The oscillator output was amplified by end-pumped fibre amplifiers based on 20 μm core Yb doped fibre to provide a power of up to 260 W per channel. The beams combined to form a coherent output with phase errors of a twentieth of a wave, unaffected by the spectral broadening.

Characterization and stabilising dynamic phase fluctuations in large mode area fibres

Fibre amplifiers exhibit rapid time dependent phase fluctuations due to the environment and to thermal and other effects associated with the pumping and lasing processes. We characterise these effects in a large mode area fibre amplifier having an output power of 260W limited only by pump power. The amplifier retains its coherence even at the highest available output power with negligible linewidth broadening. Phase fluctuations are characterised by a low-amplitude power-independent jitter superimposed on a power-dependent drift due to heating. We also measure the phase fluctuations in a COTS fibre preamplifier and find they are predominantly large amplitude periodic oscillations at 110Hz, probably induced by pump power fluctuations. The two amplifiers were combined in series to give a high gain amplifier chain and actively phase stabilised to high precision (~&lgr;/37 rms) using a piezo-ceramic fibre stretcher incorporated into a PC-based feedback loop.

A model of a fiber amplifier incorporating amplified spontaneous emission

We describe a model of a Yb:glass fibre amplifier incorporating amplified spontaneous emission (ASE). The model is able to predict the output spectrum and power of spectral components in a fibre amplifier, and identifies the optimum configuration for efficiently extracting pump power at a given signal wavelength.

Large area transmissive modulator for a remotely-interrogated MEMS-based optical tag

The development of a micro-opto-electro-mechanical system (MOEMS) technology employing interference effects to modulate incident light in the near-IR band (1550nm) over a wide angular range (120 degrees) is reported. Modulation is achieved by tuning a large array of Fabry-Perot cavities via the application of an electrostatic force to adjust the gap between a moveable mirror and the underlying silicon substrate. The optical design determines the layer thicknesses; however, the speed and power are determined by the geometry of the individual moveable elements. Electro-mechanical trade-offs will be presented as well as a key innovation of utilising overshoot in the device response in reduced pressure environment to reduce the drive voltage. Devices have been manufactured in a modified polysilicon surface micromachining process with anti-reflection coatings on the back of the silicon substrate. Measurements of individual mirror elements and arrays of mirrors at 1550nm show excellent uniformity across the array. This enables good response to an incident signal over a wide field of view when integrated with a silicon retroreflector in a passive optical tag. In conjunction with appropriate anti-stiction coatings, lifetimes of over 100 million cycles have been demonstrated. Key advantages of the modulator are that it is low cost being based on standard polysilicon micromachining; high speed (>100kHz) and robust due to utilising a massively parallel array of identical compact devices; low power for portable applications; and operates in transmission - allowing simple integration with a retroreflector in a passive tag for halfduplex free-space optical communications to a remote interrogator.

Electromagnetic phenomena generated in laser-produced plasmas (Abstract Only)

When a laser plasma is produced on a target, various electromagnetic phenomena can occur. These can produce substantial currents and voltages in nearby structures. The effects depend on the target material and morphology, the pressure and species of the atmosphere, and the nature of the laser pulse. The following mechanisms are known to make a major contribution to electromagnetic signals detected near laser plasmas: (1) UV plume causing transient high conductivity in semiconductor targets, and ionisation in buffer gasses; (2) Laser plasma generating multi-GHz microwaves due to the generation of plasma waves; (3) Space charge and current charge travelling through vacuum due to differences in the electron and ion velocities; (4) Generation of transient magnetic fields that induce anomalous currents in conductors at the target point, and secondary induced current in nearby conductors. Many of which were first reported in the 1970s, and in this report we review their relative contributions and identify regimes where each dominate.

Characterization of photonic bandgap fiber for high-power narrow-linewidth optical transport

An investigation of the use of hollow-core photonic bandgap (PBG) fiber to transport high-power narrow-linewidth light is performed. In conventional fiber the main limitation in this case is stimulated Brillouin scattering (SBS) but in PBG fiber the overlap between the optical intensity and the silica that hosts the acoustic phonons is reduced. In this paper we show this should increase the SBS threshold to the multi-kW level even when including the non-linear interaction with the air in the core. A full model and experimental measurement of the SBS spectra is presented, including back-scatter into other optical modes besides the fundamental, and some of the issues of coupling high power into hollow-core fibers are discussed.