Waveguide lasers in Er:Yb-doped phosphate glass fabricated by femtosecond laser writing

Advanced waveguide lasers, operating both in continuous wave and pulsed regimes, have been realized in an active phosphate glass by direct writing with femtosecond laser pulses. Stable single mode operation was obtained; the laser provided more than 50 m W in single longitudinal and transverse mode operation with 21% slope efficiency. Furthermore, by combining a high gain waveguide and an innovated fiber-pigtailed saturable absorber based on carbon nanotubes, a mode-locked ring laser providing transform limited 1.6 ps pulses was demonstrated. © 2007 IEEE.

Implementation of semi-active damping on a tri-axle heavy-vehicle suspension

A semi-active truck damper was developed in conjunction with a commercial shock absorber manufacturer. A linearized damper model was developed for control system design purposes. Open- and closed-loop damper force tracking control was implemented, with tests showing that an open-loop approach gave the best compromise between response speed and accuracy. A hardware-in-the-loop test facility was used to investigate performance of the damper when combined with a simulated quarter-car model. The input to the vehicle model was a set of randomly generated road profiles, each profile traversed at an appropriate speed. Modified skyhook damping tests showed a simultaneous improvement over the optimum passive case of 13 per cent in vertical body acceleration and 8 per cent in dynamic tyre forces. Full-scale vehicle tests of the damper on a heavy tri-axle trailer were carried out. Implementation of modified skyhook damping yielded a simultaneous improvement over the optimum passive case of 8 per cent in vertical body acceleration and 8 per cent in dynamic tyre forces. © IMechE 2008.

Broadband ultrafast pulse generation with double wall carbon nanotubes

Materials with nonlinear optical properties are much sought after for ultrafast photonic applications. Mode-locked lasers can generate ultrafast pulses using saturable absorbers[1]. Currently, the dominant technology is based on semiconductor saturable absorber mirrors (SESAMs). However, narrow tuning range (tens of nm), complex fabrication and packaging limit their applications[2]. Single wall nanotubes (SWNTs) and graphene offer simpler and cost-effective solutions[1]. Broadband operation can be achieved in SWNTs using a distribution of tube diameters[1,3], or by using graphene[4-8], due to the gapless linear dispersion of Dirac electrons[8,9]. © 2011 IEEE.

Nanotube-based passively mode-locked Ytterbium-pumped Raman laser

Over the past decades mode-locked fibre lasers have been extensively refined and developed, with most research efforts focussing on employing rare-earth doped fibres as the active elements [1]. This presents the problem that operation is limited to regions of the spectrum where such elements exhibit gain [1]. Raman amplification in silica fibre is an attractive way to overcome this spectral limitation, with gain available across the entire transparency window (300 nm - 2300 nm) [2-4]. There have been a number of reports utilising Raman gain in ultrashort pulse sources [2-4], however none using a broadband saturable absorber, such as carbon nanotubes [5-7] and graphene [7-9]. A broadband saturable absorber is an essential pre-requisite in order to fully exploit the wavelength flexibility provided by the Raman gain in short pulse mode-locked fiber lasers. © 2011 IEEE.

Wideband tunable, high-power, graphene mode-locked ultrafast lasers

Ultrafast passively mode-locked lasers with spectral tuning capability and high output power have widespread applications in biomedical research, spectroscopy and telecommunications [1,2]. Currently, the dominant technology is based on semiconductor saturable absorber mirrors (SESAMs) [2,3]. However, these typically have a narrow tuning range, and require complex fabrication and packaging [2,3]. A simple, cost-effective alternative is to use Single Wall Carbon Nanotubes (SWNTs) [4,10] and Graphene [10,14]. Wide-band operation is possible using SWNTs with a wide diameter distribution [5,10]. However, SWNTs not in resonance are not used and may contribute to unwanted insertion losses [10]. The linear dispersion of the Dirac electrons in graphene offers an ideal solution for wideband ultrafast pulse generation [10,15]. © 2011 IEEE.

Nanotube-based passively mode-locked Raman laser

We demonstrate passive mode-locking of a Raman fiber laser using a nanotube-based saturable absorber. The normal dispersion cavity generates highly-chirped 500 ps pulses that are compressed down to 2 ps, with 1.4 kW peak power. © 2011 OSA.

Theoretical model for dicke superradiance in a semiconductor laser device

A theoretical model for Dicke superradiance (SR) in diode lasers is proposed using the travelling wave method with a spatially resolved absorber and spectrally resolved gain. The role of electrode configuration and optical bandwidth are compared and contrasted as a route to enhance femtosecond pulse power. While pulse duration can be significantly reduced through careful absorber length specification, stability is degraded. However an increased spectral gain bandwidth of up to 150 nm is predicted to allow pulsewidth reductions of down to 10 fs and over 500-W peak power without further degradation in pulse stability. © 2011 IEEE.

Microwave characterization of multi-walled carbon nanotube arrays

A vertically aligned multi-walled carbon nanotube (VACNT) film has been characterized by rectangular waveguide measurements. The complex scattering parameters (S-parameters) are measured by a vector network analyzer at X-band frequencies. The effective complex permittivity and permeability of the VACNT film have been extracted using the Nicolson-Ross-Weir (NWR) approach. The extracted parameters are verified by full wave simulations (CST Microwave Studio) and very good agreement has been obtained. A systematic error analysis is presented and the errors are within the acceptable range. The performance of VACNT films as an absorber is examined, and comparison with the conventional carbon loaded materials shows that a 90% size reduction is possible whilst maintaining the same absorption level. © 2011 EUROPEAN MICROWAVE ASSOC.

500 fs wideband tunable fiber laser mode-locked by nanotubes

Sub-picosecond tunable ultrafast lasers are important tools for many applications. Here we present an ultrafast tunable fiber laser mode-locked by a nanotube based saturable absorber. The laser outputs ∼500fs pulses over a 33 nm range at 1.5μm. This outperforms the current achievable pulse duration from tunable nanotube mode-locked lasers. © 2012 Elsevier B.V. All rights reserved.

Graphene composites for ultrafast photonics

Ultrafast lasers play a key role in a variety of devices, from basic research to materials processing and medicine. Graphene has great potential as saturable absorber for ultrafast lasers. Here we present an overview of graphene-based ultrafast lasers, from solution processing of the raw materials, to their incorporation into polymers, device fabrication and testing. © 2011 The Japan Society of Applied Physics.

Tm-doped fiber laser mode-locked by graphene-polymer composite

We demonstrate mode-locking of a thulium-doped fiber laser operating at 1.94 μm, using a graphene-polymer based saturable absorber. The laser outputs 3.6 ps pulses, with ∼0.4 nJ energy and an amplitude fluctuation ∼0.5%, at 6.46 MHz. This is a simple, low-cost, stable and convenient laser oscillator for applications where eye-safe and low-photon-energy light sources are required, such as sensing and biomedical diagnostics. © 2012 Optical Society of America.

Sintered metal foams for acoustic absorption

Metal foams fabricated via sintering offer novel mechanical and acoustic properties. Previously, polymer foams have been used as a means of absorbing acoustic energy. However, the structural applications of these foams are limited. The metal sintering approach offers a cost-effective means for the mass-production of open-cell metal foams. The static flow resistance of sintered metal foams was characterized for a range of practical pore sizes and porosities. The measured values for the flow resistance were subsequently used in a phenomenological acoustic model to predict the impedances and propagation constants of the foams. The predictions were then compared to acoustic measurements. At low frequencies (0-1000Hz), the phenomenological model captures the magnitude and frequency dependence of the absorption. At higher frequencies, as expected, the phenomenological model underpredicted the acoustic properties of the foams. However, an alternative microstructural model demonstrated good correlation to the measured results in this frequency range. The effects of foam type and arrangement on the absorption pattern were examined. General trends were identified for enhancing the low frequency performance of an acoustic absorber incorporating sintered foams.

Change propagation analysis in complex technical systems

Understanding how and why changes propagate during engineering design is critical because most products and systems emerge from predecessors and not through clean sheet design. This paper applies change propagation analysis methods and extends prior reasoning through examination of a large data set from industry including 41,500 change requests, spanning 8 years during the design of a complex sensor system. Different methods are used to analyze the data and the results are compared to each other and evaluated in the context of previous findings. In particular the networks of connected parent, child and sibling changes are resolved over time and mapped to 46 subsystem areas. A normalized change propagation index (CPI) is then developed, showing the relative strength of each area on the absorber-multiplier spectrum between -1 and +1. Multipliers send out more changes than they receive and are good candidates for more focused change management. Another interesting finding is the quantitative confirmation of the "ripple" change pattern. Unlike the earlier prediction, however, it was found that the peak of cyclical change activity occurred late in the program driven by systems integration and functional testing. Patterns emerged from the data and offer clear implications for technical change management approaches in system design. Copyright © 2007 by ASME.

Graphene passively Q-switched two-micron fiber lasers

We demonstrate a passively Q-switched thulium fiber laser, using a graphene-based saturable absorber. The laser is based on an all-fiber ring cavity and produces ∼2.3 μs pulses at 1884nm, with a maximum pulse energy of 70 nJ. © 2012 OSA.

Hybrid passive/active inertial actuators to attenuate the structural response of a submerged hull

This paper theoretically investigates the application of tuned vibration absorbers and hybrid passive/active inertial actuators to reduce the vibrational responses of plates and shells. The passive/active actuators are initially applied to a simple plate. A model of a submerged hull consisting of a ring stiffened finite cylinder with bulkheads and external fluid loading is then considered. The fluctuating forces from the propeller result in excitation of the low frequency global hull modes. Inertial actuators and tuned vibration absorbers are located at each end of the hull and in circumferential arrays to reduce the hull structural response at its axial resonances. The control performance of the hybrid passive/active inertial actuator, where the passive component is tuned to a structural resonance, is compared to the attenuation achieved by a fully passive tuned vibration absorber. This work shows the potential of using hybrid passive/active inertial actuators to attenuate the global structural responses of a submerged vessel.