Spectral measurement of diode lasers with a single-mode birefringent fibre

A new, and very simple spectrometer based on birefringent fiber is described. A resolution of 0.02 angstrom has been achieved, and the system has been used to measure diode laser chirp. A length of 10km of fiber would be sufficient to resolve single mode line widths.

BISTABLE PERFORMANCE OF CLOSELY COUPLED TWIN STRIPE LASERS.

Results are given for bistable effects in closely coupled twin stripe lasers. These devices use controlled adjustment of asymmetric transverse optical gain to obtain bistability. Various bistable effects have been observed. Initially the authors reported a large light/current hysteresis loop obtained as the drive current to the laser was raised and lowered. Information concerning the bistable mechanisms was then obtained by applying small current pulses into each stripe. It was thus found that bistability was involved with the switching from one stable laser waveguiding mechanism to another. More recently the experimental measurement system has been much improved. Through the use of computer control of motorised micromovements and computer controlled data management, time resolved near and far field, and charge carrier concentration distribution measurements have been more accurately carried out. The paper will outline briefly this system, and report on how it has helped to reveal new mechanisms of bistability in twin stripe lasers.

Characterization of the temperature sensitivity of gain and recombination mechanisms in 1.3-μm AlGaInAs MQW lasers

The potential of 1.3-μm AlGaInAs multiple quantum-well (MQW) laser diodes for uncooled operation in high-speed optical communication systems is experimentally evaluated by characterizing the temperature dependence of key parameters such as the threshold current, transparency current density, optical gain and carrier lifetime. Detailed measurements performed in the 20°C-100°C temperature range indicate a localized T0 value of 68 K at 98°C for a device with a 2.8μm ridge width and 700-μm cavity length. The transparency current density is measured for temperatures from 20°C to 60°C and found to increase at a rate of 7.7 A·cm -2 · °C-1. Optical gain characterizations show that the peak modal gain at threshold is independent of temperature, whereas the differential gain decreases linearly with temperature at a rate of 3 × 10-4 A-1·°C-1. The differential carrier lifetime is determined from electrical impedance measurements and found to decrease with temperature. From the measured carrier lifetime we derive the monomolecular (A), radiative (B), and nonradiative Auger (C) recombination coefficients and determine their temperature dependence in the 20 °C-80 °C range. Our study shows that A is temperature independent, B decreases with temperature, and C exhibits a less pronounced increase with temperature. The experimental observations are discussed and compared with theoretical predictions and measurements performed on other material systems. © 2005 IEEE.

35 GHz passive mode-locking of InGaAs/GaAs quantum dot lasers at 1.3 μm with Fourier-limited pulses

We report 35 GHz passive mode-locking and 20 GHz hybrid mode-locking of quantum dot (QD) lasers at 1.3 μm. Our investigations show ultrafast absorber recovery times and for the first time transform-limited mode-locked pulses. © 2003 Optical Society of America.

Long-wavelength monolithic mode-locked diode lasers

A detailed study of the design issues relevant to long-wavelength monolithic mode-locked lasers is presented. Following a detailed review of the field, we have devised a validated travelling wave model to explore the limits of mode-locking in monolithic laser diodes, not only in terms of pulse duration and repetition rate, but also in terms of stability. It is shown that fast absorber recovery is crucial for short pulse width, that the ratio of gain to absorption saturation is key in accessing ultrashort pulses and that low alpha factors give only modest benefit. Finally, optimized contact layouts are shown to greatly enhance pulse stability and the overall operational success. The design rules show high levels of consistency with published experimental data.

Paintable band-edge liquid crystal lasers.

In this paper we demonstrate photonic band-edge laser emission from emulsion-based polymer dispersed liquid crystals. The lasing medium consists of dye-doped chiral nematic droplets dispersed within a polymer matrix that spontaneously align as the film dries. Such lasers can be easily formed on single substrates with no alignment layers. The system combines the self-organizing periodic structure of chiral nematic liquid crystals with the simplicity of the emulsion procedure so as to produce a material that retains the emission characteristics of band-edge lasers yet can be readily coated. Sequential and stacked layers demonstrate the possibility of achieving simultaneous multi-wavelength laser output from glass, metallic, and flexible substrates.

Tunable hydrodynamic chromatography of microparticles localized in short microchannels.

This paper describes a new way to perform hydrodynamic chromatography (HDC) for the size separation of particles based on a unique recirculating flow pattern. Pressure-driven (PF) and electro-osmotic flows (EOF) are opposed in narrow glass microchannels that expand at both ends. The resulting bidirectional flow turns into recirculating flow because of nonuniform microchannel dimensions. This hydrodynamic effect, combined with the electrokinetic migration of the particles themselves, results in a trapping phenomenon, which we have termed flow-induced electrokinetic trapping (FIET). In this paper, we exploit recirculating flow and FIET to perform a size-based separation of samples of microparticles trapped in a short separation channel using a HDC approach. Because these particles have the same charge (same zeta potential), they exhibit the same electrophoretic mobility, but they can be separated according to size in the recirculating flow. While trapped, particles have a net drift velocity toward the low-pressure end of the channel. When, because of a change in the externally applied PF or electric field, the sign of the net drift velocity reverses, particles can escape the separation channel in the direction of EOF. Larger particles exhibit a larger net drift velocity opposing EOF, so that the smaller particles escape the separation channel first. In the example presented here, a sample plug containing 2.33 and 2.82 microm polymer particles was introduced from the inlet into a 3-mm-long separation channel and trapped. Through tuning of the electric field with respect to the applied PF, the particles could be separated, with the advantage that larger particles remained trapped. The separation of particles with less than 500 nm differences in diameter was performed with an analytical resolution comparable to that of baseline separation in chromatography. When the sample was not trapped in the separation channel but located further downstream, separations could be carried out continuously rather than in batch. Smaller particles could successfully pass through the separation channel, and particles were separated by size. One of the main advantages of exploiting FIET for HDC is that this method can be applied in quite short (a few millimeters) channel geometries. This is in great contrast to examples published to date for the separation of nanoparticles in much longer micro- and nanochannels.

Emission characteristics of a homologous series of bimesogenic liquid-crystal lasers.

In this study we have fabricated eight different liquid-crystal lasers using the same gain medium but different homologues from the bimesogenic series alpha-(2',4-difluorobiphenyl-4'-yloxy)-omega-(4-cyanobiphenyl-4'-yloxy)alkanes, whereby the number of methylene units in the spacer chain varied from n=5 to n=12. To quantify the performance of these lasers, the threshold energy and the slope efficiency were extracted from the input-output characteristics of each laser. A clear odd-even effect was observed when both the excitation threshold and the slope efficiency were plotted as a function of the number of methylene units in the spacer chain. In all cases, the bimesogen lasers for which n is even exhibit lower threshold energies and higher slope efficiencies than those for which n is odd. These results are then interpreted in terms of the macroscopic physical properties of the liquid-crystalline compounds. In accordance with a previous study [S. M. Morris, A. D. Ford, M. N. Pivnenko, O. Hadeler, and H. J. Coles, Phys. Rev. E. 74, 061709 (2006)], a combination of a large birefringence and high order parameters are found, in the most part, to correlate with low-threshold energy and high slope efficiency. This indicates that the threshold and slope efficiency are dominated by the host macroscopic properties as opposed to intermolecular interactions between the dye and the liquid crystal. However, certain differences in the slope efficiency could not be explained by the birefringence and order parameter values alone. Instead, we find that the slope efficiency is further increased by increasing the elastic constants of the liquid-crystal host so as to decrease the scattering losses incurred by local distortions in the director field under high-energy optical excitation.

Tunable electronic transport characteristics of surface-architecture-controlled ZnO nanowire field effect transistors.

Surface-architecture-controlled ZnO nanowires were grown using a vapor transport method on various ZnO buffer film coated c-plane sapphire substrates with or without Au catalysts. The ZnO nanowires that were grown showed two different types of geometric properties: corrugated ZnO nanowires having a relatively smaller diameter and a strong deep-level emission photoluminescence (PL) peak and smooth ZnO nanowires having a relatively larger diameter and a weak deep-level emission PL peak. The surface morphology and size-dependent tunable electronic transport properties of the ZnO nanowires were characterized using a nanowire field effect transistor (FET) device structure. The FETs made from smooth ZnO nanowires with a larger diameter exhibited negative threshold voltages, indicating n-channel depletion-mode behavior, whereas those made from corrugated ZnO nanowires with a smaller diameter had positive threshold voltages, indicating n-channel enhancement-mode behavior.