Phase transitions of the first kind as radiation processes

Crystallization and vapor condensation are considered as the processes of sequential penetration of single atoms/molecules into condensate. In the course of these transitions the transitive radiation must be generated, which would carry away the liberated latent heat by photons of characteristic frequencies. The transient radiation is examined by the general Ginzburg-Frank theory. The emission of defined frequencies determined by the values of liberated latent heat is confirmed by analyses of several experiments of authors and other researchers.     

Improvement in piezoelectric effect of violet InGaN laser diodes

The laser performance of violet InGaN laser diodes is investigated numerically. The polarization-dependent properties, including overlap of electron and hole wavefunctions, threshold current, and slope efficiency, are studied through the use of step-like quantum well structure. Furthermore, the electron and hole wavefunctions, band diagrams, and emission wavelength are compared and analyzed. The simulation results show that the lowest threshold current and the highest slope efficiency are obtained when the step-like quantum well structure is designed as In_0.12Ga_0.88N (2.5 nm)-In_0.18Ga_0.82N (1 nm) or In_0.18Ga_0.82N (2.5 nm)-In_0.12Ga_0.88N (1 nm) for violet laser diodes due to sufficiently enhanced overlap of electron and hole wavefunctions.     

Encryption test of pseudo-aleatory messages embedded on chaotic laser signals: An information theory approach

In this Letter, we make use of two information-theory based indicators to measure the goodness of two encryption schemes commonly used within the context of chaotic communications. In particular, we have shown that the computation of the normalized Shannon entropy and the MPR-Statistical Complexity measure [M.T. Martín, A. Plastino, O.A. Rosso, Phys. Lett. A 311 (2003) 126, P.W. Lamberti, M.T. Martín, A. Plastino, O.A. Rosso, Physica A 334 (2004) 119] for different chaotic laser signals can lead to statistically significant criteria to assess the quality of several encryption techniques. The proposed measures allow, in some cases, to detect the presence of a message embedded within a chaotic carrier. They also reveal that the Chaos Modulation scheme is more reliable from the statistical point of view, when compared with the Chaos Shift Keying.     

CH4/air/SO2 premixed flame spectroscopy with a 7.5-μm diode laser

As sulfur dioxide (SO₂) is often involved in combustion processes, we present here SO₂-concentration measurements in the post-flame region of a CH₄/air/SO₂ premixed flame. SO₂ concentrations were deduced from high-resolution absorption spectra recorded with a mid-infrared tunable diode-laser (TDL) source operating at liquid nitrogen temperature. Single-mode, continuous frequency tuning around 1384.5 cm^-1 (or 7.5 μm) is achieved by a fine TDL temperature ramp. These experiments lead us to develop in situ combustion-pollutant measurements with compact apparatus. We show that this non-intrusive method is efficient for detection and allows the retrieval of SO₂ concentration and temperature. Received: 19 February 2001 / Revised version: 18 April 2001 / Published online: 7 June 2001     

Modeling of InGaAsP–InP 1.55 μm lasers with integrated mode expanders using fiber-matched leaky waveguides

A new concept for InGaAsP–InP 1.55 μm lasers integrated with spot size converters using leaky waveguides is presented. The large fundamental mode size and the high discrimination of the higher order modes make ARROWs (Antiresonant Reflecting Optical Waveguides) and antiguided waveguides useful for fiber coupling functions. Three-dimensional (3-D) beam propagation method (BPM) results show that the devices have transformation losses lower than 0.22 dB. Fiber-coupling efficiencies of 60% are possible with standard cleaved single-mode fibers (SMF). The horizontal and vertical FWHM can be efficiently reduced to 9.70° (horizontal) and 17.80° (vertical). The fabrication of such devices avoids the growth of thick layers of quaternary material with a low Ga and As fraction, and simplifies the fabrication to one planar epitaxial growth step and one non-critical conventional etch. Received: 16 May 2001 / Published online: 30 October 2001     

Abrupt turn-on and hysteresis in a VCSEL with frequency-selective optical feedback

The emission characteristics of a vertical-cavity surface-emitting laser (VCSEL) operated in a single-transverse mode and coupled to an external cavity with a diffraction grating as a frequency-selective element are analyzed experimentally, numerically and analytically. The experiments yield a rather abrupt turn-on of the VCSEL to a high-amplitude emission state and hysteresis phenomena. The experimental results are explained by numerical simulations and analytical calculations demonstrating the possibility of bistability between lasing and non-lasing states close to threshold. Hence, the scheme might be useful in all-optical photonic switching applications. A detailed bifurcation analysis near threshold is given by superimposing the numerical results with analytical steady-state curves. The mode selection and switching behavior obtained in the simulations can be interpreted from the point of view of the preference of states with the minimal total losses.     

Second-harmonic generation of 405-nm light using periodically poled KTiOPO4 pumped by external-cavity laser diode with double grating feedback

A frequency-doubled laser diode system for generation of blue–UV light is described. The system is based on an external-cavity high-power laser diode with double feedback from the zeroth and the first orders of a diffraction grating. Light at 405 nm is generated in a single-pass configuration using periodically poled KTiOPO₄. We show that the double grating feedback improves the second harmonic conversion efficiency by several orders of magnitude as compared to the freely running laser. The conversion efficiency may be improved further such that higher second-harmonic powers may be generated.     

WG-Like Modes Selectivity in a Square Cavity with Posts

We investigate the characteristics of Whispering-Gallery（WG）-like modes in a square cavity with posts by employing the two-dimensional （2D） finite-difference time-domain （FDTD） technique combined with the effective index method. The results indicate that the posts can result in mode selection in the WG-like modes. The WG-like modes with odd mode numbers are not much sensitive to the sizes of the posts. However, the quality factor （i.e. Q-factor） of the WG-like modes with even mode numbers decreases sharply with the increasing size of the posts. The decreasing Q-factor is attributed to mode leakage and scattering loss due to the presence of the post. The mode selection increases the mode spacing of square cavity twice in an optimized structure.     

Deep InP Gratings for Opto-Electronic Devices Etched by C12/CH4/Ar Inductively Coupled Plasma

Deep InP gratings are etched by C12/CH4/Ar inductively coupled plasma （ICP） at room temperature. A comparison is made between SiNz mask patterns formed by wet and dry etching. SF6 reactive ion etching is adopted for smooth and vertical sidewall. The etching conditions of C12/CH4/Ar ICP are optimized for high anisotropy, and a 1.7-μm-deep InP grating with an aspect ratio of 10：1 is demonstrated. The technique is then used for the fabrication of 1.55-μm laterally coupled distributed feedback A1GMnAs-InP laser.     

Optical gain of interdiffused GaInNAs/GaAs quantum wells

A self-consistent model for the band structure and optical gain spectra in interdiffused Ga_xIn_1-xN_0.04As_0.96/GaAs single quantum wells are studied theoretically using Fick’s Law and the Fermi Golden Rule. Due to quantum-well interdiffusion, the peak gain and its peak vary with the annealing time. Our results show that the interdiffusion technique can be used to tune the operation wavelength for multi-wavelength applications without degradation of device performance. Received: 18 April 2001 / Accepted: 19 September 2001 / Published online: 20 December 2001