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.
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.
We report on a laser communications experiment over a kilometre optical range where we have used a retro-reflective
transponder incorporating an optical modulator based on silicon micro-electro-mechanical systems (MEMS) device. This
employs interference to provide modulation and relies on performing as a coherent array to modulate incident light in the
near-IR band (1550nm) over a wide angular range (120 degrees). 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 micro-mirrors have a strong mechanical resonance, and modulate light by adjusting the spacing between the micromirrors
and the substrate. We use a 'release and catch' technique to exploit the mechanical resonance, and we time the
motion of the micro-mirrors to be synchronised with the arrival of an interrogator pulse to ensure that the etalon spacing
provides the required modulation, whatever the angle of incidence.
We describe experiments over a one kilometre path where simple strings were sent at 200kbit per second. We also
discuss approaches to adapting the link to a given angle of incidence.
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.
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.
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.
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.
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.
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.
Fe:LiNbO3 in a simple focal plane geometry has already demonstrated its potential as an efficient optical limiter of low power continuous wave visible lasers. However, widespread use of this material is hampered by a severe reduction in performance in fast optical systems. Coherent bundles of photorefractive optical fibers, used in place of bulk crystalline material, will allow efficient optical limiting in fast optical systems. Owing to the crystalline nature of the photorefractive media, the fabrication of large numbers of high quality photorefractive fibers is extremely challenging. This paper describes some of the options available, including novel composite photorefractive materials.
Fe:LiNbO3 in a simple focal plane geometry has demonstrated efficient optical limiting through two-beam coupling. The magnitude of the observed optical limiting implies an optical gain coefficient which greatly exceeds that predicted by standard photorefractive diffusion theory. Experimental measurements have confirmed that the optical gain coefficient is approximately five times greater than can be accounted for through normal change diffusion. The photovoltaic effect has been identified as the most likely mechanism for generating the observed high optical gain. We have made a direct observation of the role of the photovoltaic effect in counter-propagating two-beam coupling in photorefractive iron doped lithium niobate. We have found experimentally that the photovoltaic effect is indeed the dominant mechanism for two beam coupling in an optical limiting geometry. The contribution to optical limiting from the photovoltaic effect is approximately five times greater than that arising form diffusion mechanisms alone, in agreement with earlier optical gain measurements.
Fe:LiNbO3 in a simple focal plane geometry has demonstrated efficient optical limiting through two-beam coupling. The performance is largely independent of the total Fe concentration and the oxidation state of the Fe ions, providing the linear optical transmission of uncoated crystals is between 30% and 60%. Both the maximum change in optical density ((Delta) OD) and the speed improve with increasing pumping intensity, and neither the (Delta) OD or the speed have shown any signs of saturation for local cw pumping intensities up to 10 kW/cm2. Fe has been found to be the best dopant for LiNbO3, giving the widest spectral coverage and the greatest optical limiting. Optical limiting in Fe:LiNbO3 has been shown to be very much grater than predicted by simple diffusion theory. The reason for this is a higher optical gain than expected. It is suggested that this may be due to an enhancement of the space-charge field from a combination of hot diffusion with the photovoltaic effect. The standard two-beam coupling equations have been modified to include the effects of the dark conductivity. This has produced a theoretical intensity dependence on the (Delta) OD which closely follows the behavior observed in the laboratory. A further modification to the theory has also shown that the focusing lens f-number greatly affects the optical limiting characteristics of Fe:LiNbO3. A lens f-number of approximately 20 gives the best results.