Andrew M. Scott

High-power Yb-doped multicore ribbon fiber laser

A highly elongated double-clad ribbon fiber that comprises a pure-silica inner cladding with transverse dimensions of approximately 1.4 mm by 0.23 mm with a linear array of ten ytterbium-doped cores has been fabricated and operated in a simple laser configuration pumped by two diode stacks. The fiber laser yielded 320 W of output power at a center wavelength of 1045 nm in a combined beam with beam propagation factors of approximately 2 (perpendicular to the array) and approximately 150 (parallel to the array) for 576 W of launched pump power. The slope efficiency with respect to absorbed pump power was 62%. The prospects for further power scaling and improved beam quality and efficiency are discussed.

Manufacturing by laser direct-write of three-dimensional devices containing optical and microfluidic networks

The index of refraction of most glasses can be permanently changed by exposure to femtosecond laser pulses. This effect allows for the fabrication of various two-dimensional or three-dimensional light guiding structures. Passive and active optical devices have been manufactured using this femtosecond direct-write technique. A closely related technique has recently been demonstrated to manufacture three-dimensional microfluidic networks. We describe recent work at Translume and RPI in femtosecond direct write to produce devices which incorporate on a single glass chip optical network with microfluidic network.

Phase stabilization of a large-mode-area ytterbium-doped fiber amplifier

Measurements are reported on the open and closed-loop phase stability of a large-mode-area ytterbium-doped fiber amplifier. Phase fluctuations are characterized by a high-frequency low-amplitude jitter superimposed on a slow power-dependent drift. The amplifier may be phase locked to a precision of lambda/20 by using a low-bandwidth feedback loop.

Beam steering of a fiber-bundle laser output using phased array techniques

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.

A multi-channel phase locked fibre bundle laser

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.

Micromachined modulator arrays for use in free-space optical communication systems

A summary is presented of some of the design criteria relevant to the realisation of silicon micromachined modulator arrays for use in free-space optical communication systems. Theoretical performance levels achievable are compared with values measured on experimental devices produced using a modified Multi-User MEMS Process (MUMPS). Devices capable of realising modulation rates in excess of 300 kHz are described and their optical characteristics compared with published data on devices based on multiple quantum well technology.

Effect of signal frequency on four-wave mixing through stimulated Brillouin scattering

We present measurements of the dependence of the phase-conjugate reflectivity on signal frequency for Brillouinenhanced four-wave mixing at pump intensities above the threshold instability. The measurements were made in TiC1(4) at lambda = 1 microm and are consistent with a computer model of the reflectivity. We have observed that the frequency of the conjugate beam is independent of the frequency of the input signal beam in the unstable regime.

Determination of the threshold for instability in four-wave mixing mediated by Brillouin scattering

The threshold for instability in Brillouin-enhanced four-wave mixing has been experimentally determined as a function of both the phase mismatch and the ratio of the pump beam intensities and is shown to agree with theoretical modeling. The effective input noise intensity for four-wave mixing in the unstable regime is compared to the noise in a stimulated Brillouin scattering amplifier and is found to be higher by a factor of three in the forward direction. Competition between two input signals has been investigated, and it is shown that the signal which arrives first dominates the interaction in the unstable regime. >

Characterization and stabilising dynamic phase fluctuations in large mode area fibres

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.

300 W quasi-continuous-wave diffraction-limited output from a diode-pumped Nd:YAG master oscillator power amplifier with fiber phase- conjugate stimulated Brillouin scattering mirror

A master oscillator power amplifier system comprising a cw Nd:YAG master oscillator, a quasi-cw diode-pumped power amplifier, and a fiber-based stimulated Brillouin scattering phase-conjugate mirror is reported. A 12-pass amplifier configuration is employed to achieve high gain in a small-length slab amplifier. Residual and pump-induced optical inhomogeneities in the slab are corrected by the fiber phase conjugator to achieve diffraction-limited output beam quality. 300 W quasi-cw output and a maximal attainable gain of approximately 150 for the system are obtained.