A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

WDM networks make a very effective utilization of the fiber bandwidth and offer flexible interconnections based on wavelength routing. In high capacity, dynamic WDM networks, blocking due to wavelength contention can he reduced by wavelength conversion. Wavelength conversion addresses a number of key issues in WDM networks including transparency, interoperability, and network capacity. Strictly transparent networks offer seamless interconnections with full reconfigurability and interoperability. Wavelength conversion may be the first obstacle in realizing a transparent WDM network. Among numerous wavelength conversion techniques reported to date, only a few techniques offer strict transparency. Optoelectronic conversion (O/E-E/O) techniques achieve limited transparency, yet their mature technologies allow deployment in the near future. The majority of all-optical wavelength conversion techniques also offer limited transparency but they have a potential advantage over the optoelectronic counterpart in realizing lower packaging costs and crosstalk when multiple wavelength array configurations are considered. Wavelength conversion by difference-frequency generation offers a full range of transparency while adding no excess noise to the signal. Recent experiments showed promising results including a spectral inversion and a 90 nm conversion bandwidth. This paper reviews various wavelength conversion techniques, discusses the advantages and shortcomings of each technique, and addresses their implications for transparent networks.

Wavelength conversion technologies for WDM network applications

This Review Article provides an overview of chaos in laser diodes by surveying experimental achievements in the area and explaining the theory behind the phenomenon. The fundamental physics underpinning laser diode chaos and also the opportunities for harnessing it for potential applications are discussed. The availability and ease of operation of laser diodes, in a wide range of configurations, make them a convenient testbed for exploring basic aspects of nonlinear and chaotic dynamics. It also makes them attractive for practical tasks, such as chaos-based secure communications and random number generation. Avenues for future research and development of chaotic laser diodes are also identified.

Physics and applications of laser diode chaos

The fraction of spontaneous emission going into an oscillating laser mode has been calculated. It is shown that this fraction strongly depends on the strength of astigmatism in the laser output beam. Therefore the spontaneous emission factor in planar stripe lasers with narrow stripe is in the order of 10-4and by one order of magnitude larger than in injection lasers with a comparable active layer volume and with a built-in index waveguide. It is shown that the spontaneous emission factor is approximately proportional to the solid angle of laser radiation and nearly independent of the transverse active layer dimensions. Owing to the large spontaneous emission factor, the spectral width of narrow planar stripe lasers is significantly broader compared to narrow stripe lasers with a built-in index waveguide. In addition the large spontaneous emission coefficient also yields a much stronger damping of relaxation oscillations.

Calculated spontaneous emission factor for double-heterostructure injection lasers with gain-induced waveguiding

Polarization-state selection, polarization-state dynamics, and polarization switching of a quantum-well vertical-cavity surface-emitting laser (VCSEL) for the lowest order transverse spatial mode of the laser is explored using a recently developed model that incorporates material birefringence, the saturable dispersion characteristic of semiconductor physics, and the sensitivity of the transitions in the material to the vector character of the electric field amplitude. Three features contribute to the observed linearly polarized states of emission: linear birefringence, linear gain or loss anisotropies, and an intermediate relaxation rate for imbalances in the populations of the magnetic sublevels. In the absence of either birefringence or saturable dispersion, the gain or loss anisotropies dictate stability for the linearly polarized mode with higher net gain; hence, switching is only possible if the relative strength of the net gain for the two modes is reversed. When birefringence and saturable dispersion are both present, there are possibilities of bistability, monostability, and dynamical instability, including switching by destabilization of the mode with the higher gain to loss ratio in favor of the weaker mode. We compare our analytical and numerical results with recent experimental results on bistability and switchings caused by changes in the injection current and changes in the intensity of an injected optical signal.

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Polarization properties of vertical-cavity surface-emitting lasers

We fabricated a (110)-oriented vertical-cavity surface-emitting laser (VCSEL) with GaAs/AlGaAs quantum wells (QWs) and characterized the lasing properties of the VCSEL under optical spin injection. We demonstrated circularly polarized lasing at a high degree of circular polarization of 0.96 for our VCSEL at room temperature that originated from a long electron spin relaxation time of 0.7 ns in the (110) GaAs QWs despite a really small initial electron spin polarization of 0.04, which was well reproduced by using a rate equation analysis.

Room temperature circularly polarized lasing in an optically spin injected vertical-cavity surface-emitting laser with (110) GaAs quantum wells

Subfemtojoule polarization bistable switching in a vertical-cavity surface-emitting laser (VCSEL) is experimentally demonstrated. All-optical flip-flop operation of the VCSEL was performed using two orthogonally polarized injection light pulses. The optimum wavelengths of the two injection pulses for achieving minimum switching power were different and corresponded to the lasing wavelengths of the two polarization states of the laser. The pulse width/switching frequency dependence of the injection pulses showed that a minimum switching energy was obtained at 1ns∕500MHz. A record low switching energy of 0.3fJ has been achieved as well as a record high switching frequency of 10GHz.

Low-switching-energy and high-repetition-frequency all-optical flip-flop operations of a polarization bistable vertical-cavity surface-emitting laser

Bistable operation in InP/InGaAsP/InP DH lasers with a periodic excitation stripe geometry is reported. Bistability is observed within a current range of about 10% of the threshold current for maximum value. The range strongly depends on the stripe geometry and heat sink temperature, and they are controllable.

Bistable operation in semiconductor lasers with inhomogeneous excitation

Lasing characteristics were investigated on the newly developed very narrow planar stripe lasers. The linearity in the light output vs. current curve is remarkably improved and the critical power at which the kink appears exceeds 70 mW from the facet of the laser when the stripe width w is 2.3 µm. The relaxation oscillation in the pulsed light output becomes less pronounced as w is decreased and disappears when w is less than 2.5 µm. The fundamental transverse-mode is stably excited, but the far-field radiation pattern is rather broad and, at high current, two peaks appear in the pattern owing to the very narrow gain-guidance.

Lasing Characteristics of Very Narrow Planar Stripe Lasers

Bistable operation and self‐sustained pulse oscillation in a semiconductor laser with inhomogeneous excitation are examined. These two characteristics are obtained by the control of the spatial distribution of carrier lifetime in the laser resonator. This laser, which is modulated by injection current, is predicted to have period‐doubling bifurcation, and provides a compact optical chaos emitter.

Hitoshi Kawaguchi

Papers

Room temperature circularly polarized lasing in an optically spin injected vertical-cavity surface-emitting laser with (110) GaAs quantum wells

Low-switching-energy and high-repetition-frequency all-optical flip-flop operations of a polarization bistable vertical-cavity surface-emitting laser

Bistable operation in semiconductor lasers with inhomogeneous excitation

Lasing Characteristics of Very Narrow Planar Stripe Lasers

Optical bistability and chaos in a semiconductor laser with a saturable absorber