We report on novel approach to switch the optical beam profile in a fibre laser using a short piece of special passive fibre. The switching mechanism relies on selective wavelength resonance between the core and a high index surrounding ring in the fibre. Beam switching at 530 kHz from a Gaussian beam shape to a flat-top beam with a ring with corresponding M2 from 1.4 to 3.5 is demonstrated in a modified industrial pulsed fibre laser.
We have designed and built a free space secure key exchange system using weak laser pulses with polarisation modulation by acousto-optic switching. We have used this system to exchange keys over a 1.2km ground range with absolute security. Building from this initial result we analyse the feasibility of exchanging keys to a low earth orbit satellite.
The high performance of fibre lasers is largely due to the outstanding characteristics of fibres as an active medium. However, there is a need to overcome some limits at high optical powers which are imposed by the fibre design. We report on a design and fabrication of a stimulated Raman scattering (SRS) filtering fibre for high average or high peak optical power delivery applications. The fibre geometry is based on the series of circularly arranged high index rod resonators embedded in the silica cladding. The operation principle relies on the resonant coupling of the core and resonators modes. The fabricated fibre demonstrated wide transmission window and filtering of SRS from the output spectra (with the extinction which exceeds 20dB at the Raman Stokes wavelength), robustness for bending and high output beam quality. The fibre has been tested as a beam delivery fibre of a commercial pulsed fibre laser system in order to identify filtering performance and its limitations.
We propose a novel fiber design single-trench fiber (STF) for large mode area fiber laser and amplifier. Fiber offers cylindrical symmetry and also offer higher refractive index of core compared to cladding. This avoids the need of stack and draw process and refractive index compensation of core doped with index raising rare earth and co-dopants ions, which are an indispensable condition in most of other fiber designs. That is why, this fiber design can be fabricated with conventional modified chemical vapour deposition process in conjunction with solution doping process, which can dramatically reduce the fabrication cost, hence suitable for mass production. Fiber offers very high loss (>10dB/m) and low power fraction in core (<50%) to the higher order modes for low loss of fundamental mode (<0.1dB/m) and high power fraction in core (>80%) thanks to the resonant coupling between core and ring modes. We fabricated a preform for 30μm core STF using MCVD process in conjunction with solution doping process in a single step, without using any micro-structuration and pixilation of core. Experiments ensure the robust single mode behaviour irrespective of launching condition of input beam.
Furthermore, this fiber used in three stages MOPA provides 80kW peak pulses with repetition rate of 500 kHz, average power up to 10W, with M2 ~ 1.14. Moreover, all solid structure ensures easy cleaving and splicing. In nutshell, an ultra-low cost, monolithic, compact, and an effective single mode fiber amplifier device can be achieved using single-trench fiber.
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.
Coding data bits in the phase or polarization state of light allows us to exploit the wave particle duality for novel communication protocols. Using this principle the first practical quantum communication systems have been built. These are the fiber and free-space quantum cryptography apparatus used for secure exchange of keys. Beyond this state of the art, various quantum communication schemes are being studied including entangled state key exchange quantum dense coding, state teleportation, and entanglement swapping. The feasibility, advantages and disadvantages of space based realisations of these novel schemes are discussed.
Quantum cryptography bases the security of key exchange on the laws of quantum physics and will become the first application of quantum information methods. Here we present the design of novel hardware components which enabled the demonstration of secure key exchange over a 23.4 km free-space link.