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 microstructured fiber. In this type of fiber, a photonic crystal fiber (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/cm2 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/cm2 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.
Materials that have periodic microstructure on a given length scale display a strong modification of electromagnetic density of states for radiation wavelength at the corresponding scale. These structures are known as photonic crystals. Under certain circumstances the density of states vanishes completely for a range of wavelengths, and the material is said to have a photonic band gap, whereby optical propagation is completely suppressed. This affords the possibility of optical control useful for a range of applications including novel filters, waveguides, and efficient laser structures. In this paper, a range of fabrication methods of these crystals is described, together with basic theory and some properties and applications. Particular attention is given to two-dimensionally periodic materials in the form of optical fibre (the photonic crystal fibre and photonic band gap fibre), which have the potential for high-power optical guidance. Three-dimensionally periodic materials designed to control microwave radiation are also described.
The sequence of Givens rotations used to update the upper triangular matrix R in a recursive QR decomposition may also be used to update the inverse transpose matrix R-H. Alternative forms of square-root-free Givens rotation for updating the inverse transpose matrix are derived by representing and storing it in a different factorized form from that used before. The modified Givens rotations do not involve explicit division by the exponential forget factor and lead to an update algorithm equivalent to the one derived by Sakai using Recursive Modified Gram Schmidt orthogonalization.
A procedure for multidimensional nonlinear modeling and interpolation is described which employs the method of radial basis function analysis. A systolic array for efficiently performing the associated computation for both the modeling and interpolation modes recursively in time is also described. Conditions are given for the further improvement of efficiency in the algorithm when the input data constitute a time series, and an associated processing structure is outlined.
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