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.

Variable repetition frequency femtosecond-pulse surface emitting semiconductor laser

We report a femtosecond-pulse vertical-external-cavity surface-emitting laser with a continuous repetition frequency tuning range of 8 near 1 GHz. A constant average output power of 56 ± 1 mW and near-transform-limited pulse duration of 450 ± 20 fs were observed across the entire tuning range. © 2011 American Institute of Physics.

External-cavity mode-locked quantum-dot laser diodes for low repetition rate, sub-picosecond pulse generation

This paper presents an investigation of the mode-locking performance of a two-section external-cavity mode-locked InGaAs quantum-dot laser diode, focusing on repetition rate, pulse duration and pulse energy. The lowest repetition rate to-date of any passively mode-locked semiconductor laser diode is demonstrated (310 MHz) and a restriction on the pulse energy (at 0.4 pJ) for the shortest pulse durations is identified. Fundamental mode-locking from 310 MHz to 1.1 GHz was investigated, and harmonic mode-locking was achieved up to a repetition rate of 4.4 GHz. Fourier transform limited subpicosecond pulse generation was realized through implementation of an intra-cavity glass etalon, and pulse durations from 930fs to 8.3ps were demonstrated for a repetition rate of 1 GHz. For all investigations, mode-locking with the shortest pulse durations yielded constant pulse energies of ∼0.4 pJ, revealing an independence of the pulse energy on all the mode-locking parameters investigated (cavity configuration, driving conditions, pulse duration, repetition rate, and output power). © 2011 IEEE.

Algorithm-based continuous pulse duration tuning and performance control of a mode-locked laser diode.

A control algorithm is presented that addresses the stability issues inherent to the operation of monolithic mode-locked laser diodes. It enables a continuous pulse duration tuning without any onset of Q-switching instabilities. A demonstration of the algorithm performance is presented for two radically different laser diode geometries and continuous pulse duration tuning between 0.5 ps to 2.2 ps and 1.2 ps to 10.2 ps is achieved. With practical applications in mind, this algorithm also facilitates control over performance parameters such as output power and wavelength during pulse duration tuning. The developed algorithm enables the user to harness the operational flexibility from such a laser with 'push-button' simplicity.

Ultrashort superradiant pulse generation from a GaN/InGaN heterostructure.

Dicke superradiance from a two-section violet GaN/InGaN semiconductor laser diode is demonstrated for the first time. In the superradiance regime, optical pulses with peak powers in excess of 2.8 W and durations as short as 1.4 ps are generated at repetition rates of up to 10 MHz at the emission wavelength of 408 nm. The properties of superradiant pulse generation from these GaN/InGaN laser diodes are very similar to those reported for infrared AlGaAs/GaAs laser diodes.

Fibre laser microvia drilling and ablation of Si with tuneable pulse shapes

The dramatic increase in hole quality on single crystalline silicon with an 1 μm fiber laser has been reported recently, it redefines the processing options for Si at that wavelength. This study investigated the effects of the MOPA based pulse tuning on the changes of the machined depth and the mass removal mechanism for the generation of microvia holes. Hole depths were measured and surface morphology studied using SEM and optical interferometric profilometry. The pulse peak power was found to strongly influence the material removal mechanism with fixed pulse duration. High peak powers (>1 kW) gave vaporization dominated ablation, left a limited re solidified molten layer and clean hole formation. The pulse duration was found to strongly influence the machined depth. Longer pulse durations generated deeper holes with constant peak power (>1 kW). In comparison with the DPSS UV laser, the IR fiber laser of longer pulse durations machined deeper holes and generated less resolidifed melt beyond the hole rim at high fluencies. The comparison suggests that some applications (microvia drilling) of the DPSS UV laser can be replaced with the more flexible, low cost IR fiber laser. © KSPE and Springer 2012.

New application of temperature-dependent modelling of high temperature superconductors: Quench propagation and pulse magnetization

We present temperature-dependent modeling of high-temperature superconductors (HTS) to understand HTS electromagnetic phenomena where temperature fluctuation plays a nontrivial role. Thermal physics is introduced into the well-developed H-formulation model, and the effect of temperature-dependent parameters is considered. Based on the model, we perform extensive studies on two important HTS applications: quench propagation and pulse magnetization. A micrometer-scale quench model of HTS coil is developed, which can be used to estimate minimum quench energy and normal zone propagation velocity inside the coil. In addition, we study the influence of inhomogeneity of HTS bulk during pulse magnetization. We demonstrate how the inhomogeneous distribution of critical current inside the bulk results in varying degrees of heat dissipation and uniformity of final trapped field. The temperature- dependent model is proven to be a powerful tool to study the thermally coupled electromagnetic phenomena of HTS. © 2012 American Institute of Physics.

Ultrashort pulse generation in diode laser devices

The generation of ultrashort optical pulses by semiconductor lasers has been extensively studied for many years. A number of methods, including gain-/Q-switching and different types of mode locking, have been exploited for the generation of picosecond and sub-picosecond pulses [1]. However, the shortest pulses produced by diode lasers are still much longer and weaker than those that are generated by advanced mode-locked solid-state laser systems [2]. On the other hand, an interesting class of devices based on superradiant emission from multiple contact diode laser structures has also been recently reported [3]. Superradiance (SR) is a transient quantum optics phenomenon based on the cooperative radiative recombination of a large number of oscillators, including atoms, molecules, e-h pairs, etc. SR in semiconductors can be used for the study of fundamental properties of e-h ensembles such as photon-mediated pairing, non-equilibrium e-h condensation, BSC-like coherent states and related phenomena. Due to the intrinsic parameters of semiconductor media, SR emission typically results in the generation of a high-power optical pulse or pulse train, where the pulse duration can be much less than 1 ps, under optimised bias conditions. Advantages of this technique over mode locking in semiconductor laser structures include potentially shorter pulsewidths and much larger peak powers. Moreover, the pulse repetition rate of mode-locked pulses is fixed by the cavity round trip time, whereas the repetition rate of SR pulses is controlled by the current bias and can be varied over a wide range. © 2012 IEEE.

Investigations of current limiting properties of the MgB2 wires subjected to pulse overcurrents in the benchtop tester

A laboratory scale desktop test system including a cryogenic system, an AC pulse generation system and a real time data acquisition program in LabView/DAQmx, has been developed to evaluate the quench properties of MgB 2 wires as an element in a superconducting fault current limiter under pulse overcurrents at 25K in self-field conditions. The MgB2 samples started from a superconducting state and demonstrated good current limiting properties characterized by a fast transition to the normal state during the first half of the cycle and a continuously limiting effect in the subsequent cycles without burnouts. The experimental and numerical simulation results on the quench behaviour indicate the feasibility of using MgB 2 for future superconducting fault current limiter (SFCL) applications. © IOP Publishing Ltd.