Robert P. Sarzala

Modeling of the threshold operation of 1.3-/spl mu/m GaAs-based oxide-confined (InGa)As-GaAs quantum-dot vertical-cavity surface-emitting lasers

In the paper, the self-consistent optical-electrical-thermal-gain threshold model of the oxide-confined (OC) quantum-dot (QD) (InGa)As-GaAs vertical-cavity surface-emitting diode laser (VCSEL) is demonstrated. The model has been developed to enable better understanding of physics of an operation of GaAs-based OC QD VCSELs in a full complexity of many interactions in its volume between individual physical phenomena. In addition, the model has been applied to design and optimize the low-threshold long-wavelength 1.3-/spl mu/m GaAs-based OC QD VCSELs for the second-generation optical-fiber communication systems and to examine their anticipated room-temperature (RT) performance. An influence of many construction parameters on device RT lasing thresholds and mode selectivity has been investigated. Some essential design guidelines have been proposed to support efforts of technological centers in producing low-threshold single-mode RT devices.

Optimal photonic-crystal parameters assuring single-mode operation of 1300 nm AlInGaAs vertical-cavity surface-emitting laser

We present a self-consistent analysis of InP-based 1300 nm AlInGaAs photonic-crystal vertical-cavity surface-emitting lasers (PhC VCSELs) and tunnel-junction PhC VCSELs, and analyze the influence of the electrical confinement, the PhC hole diameter and etching depth, and the size of the single defect optical aperture on the threshold current and the transverse mode discrimination. We also investigate the thermal performance of the two VCSEL configurations. As a result we determine the optimal PhC parameters assuring stable, single-mode operation in a broad range of driving currents.

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.

Precise Lateral Mode Control in Photonic Crystal Vertical-Cavity Surface-Emitting Lasers

We demonstrate an oscillatory behavior of resonant wavelengths and threshold gains of the fundamental and first-order transverse mode of photonic crystal vertical-cavity surface-emitting laser (PhC VCSEL) with the PhC etching depth. To this aim we use a full vectorial, 3-D optical model (plane wave admittance method) combined with a thermal model and an exemplary 1300 nm quantum-dot VCSEL structure. Such oscillatory behavior poses a strong restriction on the etching depth precision: about 0.1 μm in order to achieve a threshold gain not higher than 10% the threshold gain minimum and twice higher precision for simultaneous strong discrimination of the first-order transverse mode.

Highly doped GaN: a material for plasmonic claddings for blue/green InGaN laser diodes

Highly n-doped GaN is a material of a reduced refractive index which may substitute AlGaN as a cladding layer in InGaN laser diodes. In this study we focus on the determination of the optical absorption and the refractive index of GaN:O having the electron concentration between 1·1018 - 8·1019 cm-3. Though the measured absorption coefficient for the highest doped GaN are rather high (200 cm-1) we show, using an optical mode simulation, that you can design a InGaN laser diode operating in blue/green region with decent properties and low optical losses. We propose to use relatively thin AlGaN interlayer to separate plasmonic GaN from the waveguide and thus to dramatically reduce the optical losses.

Finite-element thermal model for buried-heterostructure diode lasers

First results of our new finite-element modelling of thermal properties of GaAs/(AlGa)As buried-heterostructure (BH) lasers are reported. The calculus procedure is very efficient, so we have used a standard IBM PC/XT microcomputer. For the stripe active-region width of 1 μm, the thermal resistance of the laser was determined to be about 70 KW-1, whereas its electrical resistance was about 6 ohms. To the best of our knowledge, isothermal lines within BH lasers have been obtained for the first time. The isothermal configuration enables us to analyse heat-spreading phenomena in BH lasers, which makes possible thermal optimization of the laser construction.As the first application of the model, the relative influence of the oxide layer thickness on the laser thermal resistance was examined. Because of relatively large lateral dimensions of the laser crystal as compared to the active region, this influence is often neglected, whereas our detailed calculations reveal its importance. An increase in this thickness from 0.1 μm to 0.5 μm is followed by over 15% increase in the laser thermal resistance.

Threshold simulation of 1.3-μm oxide-confined in-plane quantum-dot (InGa)As/GaAs lasers

In the paper, the self-consistent optical–electrical–thermal-gain model of the oxide-confined long-wavelength 1.3-μm quantum-dot (InGa)As/GaAs diode laser is demonstrated. The model has been applied to analyse room-temperature (RT) threshold-operation characteristics of the advanced laser of this kind. It may be used to describe physics of the above arsenide-based diode lasers to better understand their threshold performance and finally to optimize their structures.

Analysis of anticipated performance of 650-nm GaInP/AlGaInP quantum-well GaAs-based VCSELs at elevated temperatures

The possibility of application of the 650-nm oxide-confined GaInP/AlGaInP quantum-well vertical-cavity surface-emitting diode lasers (VCSELs) at elevated temperatures as sources of the carrier 650-nm wave in the fibre optical communication using POFs has been investigated with the aid of the comprehensive self-consistent model. An increase in the VCSEL threshold current at higher temperatures has been found to be mostly associated with both the carrier leakage from the valley of the Ga0.43In0.57P quantum-well material to the X-valley of the (Al0.67Ga0.33)0.52In0.48P barriers and the band-to-band absorption within the Ga0.52In0.48P layer of the band-gap comparable with the energy of emitted radiation. Nevertheless, the AlGaInP VCSELs exhibit encouraging thermal behaviour with the characteristic temperature T0 equal to as much as 134 K for the active-region temperatures up to 357 K. For the 5-μm devices, the maximal achievable output has been determined to decrease from a quite high value of 1.0 mW for 293 K to 0.6 mW for 320 K and to still high 0.33 mW for 340 K. However, an efficient operation of the above VCSEL at elevated temperatures requires still some structure modifications leading to a reduction of both the above effects, the electron leakage from the valley and the band-to-band absorption within GaInP layers.

Physics of mode selectivity of vertical-cavity surface-emitting diode lasers

The single-fundamental-mode operation of vertical-cavity surface-emitting diode lasers (VCSELs) is essential for most of their applications. It is well known, that it is enhanced in VCSELs exhibiting uniform current injection into their active regions. But usually this injection is far from being uniform, sometimes even with a distinct current-crowding effect at active-region edges. Therefore, in the present paper, excitation of successive cavity modes is investigated with the aid of the comprehensive VCSEL physical model taking the modern GaAs-based oxide-confined intracavity-contacted quantum-well GaInNAs/GaAs VCSEL emitting the 1.3u2002μm radiation as a typical VCSEL example. VCSEL operation depends on many optical, electrical, thermal, and recombination phenomena. But numerous and usually nonlinear interactions between these phenomena have been found to be equally important. Therefore, the main intention of this work is to demonstrate that, for any VCSEL design, an analysis of its mode selectivity require...

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