The advent of non-linear photonic crystal fibres with engineered optical properties has enabled the production of
compact high-brightness super-continuum sources with a spectral power density in excess of 1 mW/nm throughout
the visible and near-infrared spectral regions. Such sources have intrinsically good beam quality and, when properly
collimated, the various spectral components propagate in a co-linear fashion, thus retaining spectral fidelity along the
beam path. These properties are ideal for an active hyper-spectral remote sensing system.
We report the construction and testing of a white light transceiver for measurement of the spectral reflectivity of
remote targets. The transmission section of the transceiver comprises a commercial white light source with
apochromatic optics to ensure simultaneous collimation at all wavelengths. The receiver section comprises a
telescope coupled to a fibre-optic visible band spectrometer. A portion of the received light is directed onto a camera
to facilitate accurate pointing of the system.
The transceiver has been used to measure the spectral reflection from both diffuse and retro-reflecting targets at an
outdoor range. The spectral return from retro-reflective targets was successfully measured at ranges up to 1.2 km.
For diffuse targets, the useful range was limited to a few hundred metres, beyond which the signal was dominated by
ambient daylight. The propagation of the white light beam along the 1.2 km has been studied. The fidelity of
measured spectra was affected by atmospheric turbulence which caused the beam to break up into a time-varying
pattern of coloured regions. This effect imposed a lower limit on the integration time required to measure individual
spectra, independent of the signal to noise ratio.
A review of the development of nonlinear materials suitable for use in the mid-IR (3 to 5 μm) is presented. This examines the properties, performance, limitations and availability of a range of materials, including birefringently phasematched crystals and engineered quasi-phasematched materials. Higher-order nonlinear processes in alternative materials are also considered and a discussion on material suitability for down-conversion of near-IR lasers into the mid-IR to meet various application requirements is also presented.
Demanding real-time data processing applications are driving the need for high-throughput programmable logic. Improvements to computing speed from reduction of processor feature sizes are predicted, but these are expected to be hampered within the next 2-5 years by the limitations of metallic interconnects between processors. Optical interconnect alternatives have been attempted, but independent optical channel densities are, at present, restricted by conventional fiber dimensions. In this paper a novel solution to this problem is presented employing a multi-core <i>microstructured</i> fiber. In this type of fiber, a <i>photonic crystal fiber</i> (PCF), the core is a solid silica region surrounded by air holes shot through the length of the fiber. This is created by stacking capillaries and solid canes of silica to create a preform, with the structure preserved after drawing down; a core may be created by replacing an air hole by a solid cane. The criteria for the fiber design are discussed: a bit error rate restriction leads to an upper limit for cross-coupling between cores and hence the distance (or number of air holes) between each channel. Modeling indicates a final fiber design containing 37 cores 31.25 microns apart, equivalent to a density of 1150 independent channels per millimeter squared. Details of an optical transmitting/receiving system utilizing four of the channels and arrays of VCSELs as transmitters and receivers are described. Future improvements to the system are discussed.
In this paper, we show that it is possible to arrange for an 18-core photonic crystal fibre (PCF) laser to operate in the fundamental in-phase supermode. The mode divergence is as small as 12.5 mrad. The equivalent mode field diameter is about 52 μm. Mode filtering is provided by a pinhole in the far field. The laser is Q-switched using an Acousto-Optic Modulator (AOM). An output power up to 65 W at a repetition rate of 50 kHz (corresponding to 1.3 mJ per pulse), with 22 ns short pulses, has been obtained with a slope efficiency of 46%. Ongoing amplification experiments are briefly described. Limiting factors (end facet damage threshold and thermal dissipation) are discussed for further scaling of this laser concept.
Laser damage thresholds of 10 and 20 micron-core diameter solid-core photonic crystal fibres (PCF) and hollow-core photonic band gap (PBG) fibres have been measured. The studies were carried out using a Nd:Yag laser (30nsec pulses at 10Hz), which is optimally coupled into the fibres by careful mode matching, providing a coupling efficiency greater than 90%. It has been shown that the damage threshold of the 10-micron PBG fibre occurs for pulse energies close to 1 mJ, equivalent to a fluence well in excess of 1kJ/cm<sup>2</sup> propagating down the fibre. This is a factor of 4 larger than the damage threshold of the 10-micron diameter solid-core PCF. However, the damage threshold of the large-core PBG is smaller than that of the PCF.
Theoretical modelling based only on the optical modal properties of the single-mode PBG fibre shows that an enhancement by a factor 25 should be obtainable. Thus there are different mechanisms potentially responsible for the fragility of larger core PBG fibres. In an experimental study of bend losses it ahs been found that it is possible to bend the 10-micron PBG fibre up to the breaking point bend radius (less than 1mm). The critical bend radius for the 20-micron PCF. A summary will be presented of the results of the experimental and theoretical studies, highlighting possible reasons for the observed trends for the two different forms of fibre.
Laser damage thresholds of 8μm- and 22μm-core diameter solid-core photonic crystal fibres (PCF) and hollow-core photonic band gap (PBG) fibres have been measured. The studies were carried out using a 1.06μm Nd:Yag laser (30nsec pulses at 10Hz), which is optimally coupled into these fibres by careful mode matching, providing a coupling efficiency greater than 90%. It has been shown that the damage threshold of the 8µm core PBG fiber occurs at pulse energies close to 1 mJ, equivalent to a fluence well in excess of 1kJ/cm<sup>2</sup> propagating down the fibre. This is a factor of 4 larger than the damage threshold of a solid-core PCF of similar core diameter. In comparison, the damage threshold of the large-core PBG is smaller than that of the equivalent PCF. Theoretical modelling based only on the optical modal properties of the single mode PBG fibre shows that an enhancement of a factor of 25 should be obtainable. Thus there are different damage mechanisms potentially responsible for the fragility of larger core PBG fibres. In an experimental study of bend losses it has been found that it is possible to bend the 8μm PBG fibre up to the breaking point bend radius (<1mm). The critical bend radius for the 22μm core PBG is close to 2 mm, which is 50 times smaller than the critical bend radius of a 20μm core PCF.
We have analysed different 1D and 2D arrays of evanescently coupled cores within a fibre laser structure. The supermodes (phase-locked modes) have been calculated using coupled mode theory. We show that without a Talbot mirror, the out-of-phase supermode has the lowest threshold. Supermode selection is obtained using a Talbot cavity. A threshold analysis is carried out and it is shown than the in-phase supermode can be selected for a densely packed array of cores. 2D core structures are much more effective than 1D core structures for in-phase supermode selection. The influence of parameters like the strength of the evanescent coupling constant or the core-to-core detunings of propagation constant on the dynamical stability of the supermodes is investigated. We give figures of the minimum bend radius for phase locking. We show that large multicore structures can potentially be bent tighter than the equivalent single large core fibre laser.
In the near future several astronomical observatories in Chile are planning to use sodium laser guide stars to increase the sky coverage provided by their adaptive optics facilities. Knowledge of the mesospheric sodium layer behavior is crucial to predict the performance of future laser guide star adaptive optics systems. Whereas the sodium layer has been observed quite extensively at several locations, many of them in the Northern Hemisphere, very little measurements have been made in Chile. The Gemini Observatory therefore initiated a year-long sodium monitoring campaign at the Cerro Tololo Inter-American Observatory located only a few kilometers away from the Gemini South telescope where a conventional laser guide star facility will be offered to the community in 2005, soon to be upgraded to a multi-conjugate adaptive optics system with five laser guide stars. This paper reports on the laser-based sodium monitoring experimental set up and data reduction techniques, and presents some preliminary results on the sodium column density and layer altitude variations observed from February 2001 to February 2002. Implications for the Gemini South Adaptive Optics system expected performance are presented as well.
We report the results of the first laser beacon experiment at the astronomical site of La Palma (Canary Islands). A continuous wave low power laser (a few hundreds of mW) system has been set up. The laser, tuned on the sodium D<SUB>2</SUB> line at 589 nm, is launched close to zenith angle. The emission of the mesospheric sodium layer is observed from a telescope located 160 meters away from the laser. The layer is therefore resolved in altitude and the different features of its dynamics are investigated.