David Maurice Taylor

Characteristics of a Q-switched multicore photonic crystal fiber laser with a very large mode field area

We model and characterize the behavior of a Q-switched fiber laser. The fiber is a doped multicore photonic crystal fiber having six cores in a ring-type geometry. The fiber laser is Q-switched using an intracavity acousto-optic modulator. Using a mode filtering technique in the far field, a mode very close to the fundamental in-phase supermode is obtained with a mode field area of 4200 microm(2) and a divergence of 9 mrad. Pulses with energies of up to 2.2 mJ and durations of 26 ns (limited by end facet damage) at a repetition rate of 10 kHz are obtained.

Multicore Photonic Crystal Fiber Lasers for High Power/Energy Applications

In this paper, the authors discuss the modal and lasing properties of multicore photonic crystal fiber lasers in the context of high power/energy production from fiber cores with very large mode area. Supermode selection methods like Talbot imaging or far-field aperturing are tested using 6-, 7-, and 18-core fibers. It is shown that in-phase mode selection is achieved efficiently by using either method. The fibers have been tested in continuous-wave (CW) and Q-switched laser operation. The mode field area is as large as 4240 mum2 for one of the fibers, providing up to 2 mJ of pulse energy in Q-switched operations with 30 ns pulse duration.

Damage threshold and bending properties of photonic crystal and photonic band-gap optical fibers

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.

Multi-core photonic crystal fibers for high-power laser and amplifiers

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.

Developments towards practical free-space quantum cryptography.

We describe a free space Quantum cryptography system which is designed to allow continuous unattended key exchanges for periods of several days, and over ranges of a few kilometres. The system uses a four laser faint pulse transmission system running at a pulse rate of 10MHz to generate the required four alternative polarization states. The receiver module similarly automatically selects a measurement basis and performs polarization measurements with four avalanche photodiodes. The controlling software can implement the full key exchange including sifting, error correction, and privacy amplification required to generate a secure key.

Damage threshold of microstructured optical fibres

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.

High channel density optical interconnects using photonic crystal fibers

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.

Phase locking of multicore photonic crystal fibers

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.