M. Wasiak

Numerical Self-Consistent Analysis of VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) yield single-longitudinal-mode operation, low-divergence circular output beam, and low threshold current. This paper gives an overview on theoretical, self-consistent modelling of physical phenomena occurring in a VCSEL. The model has been experimentally confirmed. We present versatile numerical methods for nitride, arsenide, and phosphide VCSELs emitting light at wavelengths varying from violet to near infrared. We also discuss different designs with respect to optical confinement: gain guidance using tunnel junctions and index guidance using oxide confinement or photonic crystal and we focus on the problem of single-transverse-mode operation.

Wafer-Fused Optically Pumped VECSELs Emitting in the 1310-nm and 1550-nm Wavebands

1300-nm, 1550-nm, and 1480-nm wavelength, optically pumped VECSELs based on wafer-fused nInAlGaAs/InP-AlGaAs/GaAs gain mirrors with intracavity diamond heat spreaders are described. These ndevices demonstrate very low thermal impedance of 4u2009K/W. Maximum CW output of devices with 5 groups nof quantum wells shows CW output power of 2.7u2009W from 180u2009 μm apertures in both the 1300-nm and the n1550-nm bands. Devices with 3 groups of quantum wells emitting at 1480 nm and with the same aperture nsize show CW output of 4.8u2009W. These VECSELs emit a high-quality beam with 𝑀 2 beam parameter nbelow 1.6 allowing reaching a coupling efficiency as high as 70% into a single-mode fiber. Maximum value nof output power of 6.6u2009W was reached for 1300u2009nm wavelength devices with 290u2009 μm aperture size. Based non these VECSELs, second harmonic emission at 650u2009nm wavelength with a record output of 3u2009W and nRaman fiber lasers with 0.5u2009W emission at 1600u2009nm have been demonstrated.

High-power optically-pumped VECSELs emitting in the 1310-nm and 1550-nm wavebands

1300-nm, 1550-nm and 1480-nm wavelength, optically-pumped VECSELs based on wafer-fused InAlGaAs/InPAlGaAs/ GaAs gain mirrors with intra-cavity diamond heat-spreaders demonstrate very low thermal impedance of 4 K/W. Maximum CW output of devices with5 groups of quantum wells show CW output power of 2.7 W from 180μm apertures in both 1300-nm and 1550-nm bands. Devices with 3 groups of quantum wells emitting at 1480 nm and with the same aperture size show CW output of 4.8 W. These devices emit a high quality beam with M² beam parameter below 1.6 allowing reaching a coupling efficiency into a single mode fiber as high as 70 %. Maximum value of output power of 6.6 W was reached for 1300nm wavelength devices with 290μm aperture size.

Modification of the Two-Point Scaling Theory for the Description of the Phase Transition in Solution. Analysis of Sodium Octanoate Aqueous Solutions

On the basis of conventional scaling theory, the two-point scaling theory was modified in order to describe the influence of composition on the partial molar heat capacity and volume during the micellization process. To verify the theory, isobaric heat capacities and densities of aqueous sodium octanoate solutions were measured over wide composition and temperature ranges and the modified approach was used to analyze the calculated partial molar heat capacities and volumes of the surfactant in water. The results obtained indicate that the micellization process is subject to the scaling laws. The results were compared with those for other systems. Peculiar behavior of the critical indices was observed and correlated with the structure of the micelles.

Influence of van der Waals interactions on volumetric properties of cholesterol in solvents of linear structure

Abstract The densities of solutions of cholesterol in several primary amines and 1-chloroalkanes have been measured at T =298.15xa0K. The values of partial molar volumes of cholesterol have been calculated and the effect of solute–solvent and solvent–solvent interactions on the partial molar volume of cholesterol have been discussed. For the solvents studied the increase of alkyl chain length enhances the dispersion solvent–solvent interactions and weakens (per one cholesterol molecule) the contribution of dispersion forces in the solute–solvent interactions. Both effects bring about an increase in the partial molar volume of cholesterol with increasing the alkyl chain length. The relation between interaction volume of cholesterol (s.p.t. calculation) and the alkyl chain length have been also discussed.

Thermal induced structural and magnetic transformations in Fe73.5−xCex=0,3,5,7Si13.5B9Nb3Cu1 amorphous alloy

Structural and magnetic properties of amorphous and partly crystallized Fe73.5−xCex=0,3,5,7Si13.5B9Nb3Cu1 alloys, were analysed in the temperature ranging from RT to 800xa0°C with scanning calorimetry and magnetometry. The Fe(Si) and Fe(B) structures were identified and characterised with set of crystallization temperatures and activation energies. Also, Curie temperatures for amorphous and for crystalline structures were determined and analysed as functions of Ce content.

Temperature increase within quantum-cascade lasers originating from their incomplete soldering

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Structure optimisation of modern GaAs-based InGaAs/GaAs quantum-dot VCSELs for optical fibre communication

Room-temperature (RT) continuous-wave (CW) performance of modern 1300-nm oxide-confined In(Ga)As/GaAs quantum-dot (QD) vertical-cavity surface-emitting diode lasers (VCSELs) taking advantage of many QD sheets is investigated using our comprehensive self-consistent simulation model to suggest their optimal design. Obviously, quantum dots should be as uniform as possible and as dense as possible to ensure high enough optical gain. Besides, our simulation reveals that efficient and uniform current injection into VCSEL active regions necessary to enhance excitation of the desired fundamental LP01 mode is accomplished in the VCSEL configuration with the broad-area bottom contact and the ring upper one as well as with the oxide aperture localized within the first period of the upper p-type DBR. The doping of the DBR mirrors is chosen as a compromise between their high enough electrical conductivity and low enough free-carrier absorption. The oxide aperture is additionally introducing the radial optical waveguiding. Moreover, our analysis has been concluded that VCSEL active regions should be composed of at least 9 QD sheets to acquire efficient RT CW operation. Furthermore, rather longer optical cavities are recommended in this case because localization of QD sheets should be adjusted to the anti-node positions of the optical cavity standing wave.

Design guidelines for fundamental-mode-operated cascade nitride VCSELs

Design guidelines for manufacturing fundamental-mode-operated nitride vertical-cavity surface-emitting lasers (VCSELs) are presented. Results of comparative analysis of various possible VCSEL configurations carried out with the aid of an advanced self-consistent optical-electrical-thermal device modeling are given. Our computer simulations suggest that the properly arranged design with a semitransparent contact, a double (cascade) active region, and two tunnel junctions ensures the best laser performance characteristics, in particular, single-fundamental-mode room-temperature continuous-wave operation.

Single-photon devices in quantum cryptography

Modern communication in absolute secrecy requires creation of new intrinsically secure quantum communication channels. It is particularly necessary during the first connection between two parties establishing then in assumed unconditional security the secret cryptographic key which is supposed to be used afterwards during normal information exchanging. This new emerging field of quantum information technology is based on a new type of light sources, in which numbers of emitted photons can be carefully controlled. Especially advantageous are sources of single photons emitted at strictly predetermined moments, so called single-photon devices. Then any possible eavesdropper activity will be followed by some unavoidable disturbance which alerts both communication parties to an event. In the present paper, the Purcell effect associated with enhancement of spontaneous emission coupled to a resonator is explained, methods used to produce streams of antibunched photons are given, mechanisms applied to control carrier injection into quantum dots are shown and some possible designs of single-photon devices are presented and described. These devices are based on taking advantage of both the Purcell effect and the atom-like energy spectrum of quantum dots.