Adam K. Sokół

Double-diamond high-contrast-gratings vertical external cavity surface emitting laser

A new design of vertical external cavity surface emitting laser (VECSEL) with diamond-based high contrast gratings is proposed. The self-consistent model of laser operation has been calibrated based on experimental results and used to optimize the new proposed device and to perform comparative thermal and optical analysis of conventional and double-diamond high-contrast-grating VECSELs. The proposed design considerably reduces the dimensions and complexity of the device and provides up to 80% increase of the maximum emitted power as compared with the conventional design.

Numerical analysis of optically pumped VECSELs

Carrier transport and optical properties of optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) have been analyzed with the aid of the self-consistent numerical model. An influence of active-region design parameters, such as its length as well as number and arrangement of quantum wells, on a carrier distribution and material gain has been investigated. Moreover, a performance of various structures has been compared with the aid of a simple optical model. In particular, increasing number of quantum wells has been found not always to lead to an increasing maximal output power. An arrangement of quantum wells is also of importance. For example, quantum wells located in long active regions far from a chip surface can be not sufficiently pumped. Numerical models described in this paper may give an opportunity to understand more deeply details of VECSEL operation and to design optimal laser structures.

A Possibility to achieve emission in the mid-infrared wavelength range from semiconductor laser active regions

In the present paper the results of the computer analysis of the arsenide-based (GaInNAs/AlGaInAs) and antimonide-based (GaInAsSb/AlGaAsSb) active regions emitting in the mid-infrared wavelength region are presented. Quantum well material contents and strain dependencies on the maximal gain are investigated. It is shown that above 3 μm the maximal gain obtained for arsenide-based active region is very low, irrespective of the nitrogen content and compressive strain in GaInNAs. Much higher optical gain in this wavelength range can be obtained for antimonide-based active region, which offers relatively high gain even at 5 μm, when the indium content in GaInAsSb and compressive strain in this layer are higher than 80% and 1.5%, respectively.

Transverse-mode selectivity in antimonide-based vertical-cavity surface-emitting lasers

In this work results of a threshold operation of antimonide-based tunnel-junction (TJ) VCSEL have been presented with the aid of the comprehensive fully self-consistent optical-electrical-thermal-recombination numerical model. Calculations have been carried out for the structure with GaInAsSb/GaSb active region emitting at 2.6 μm. In order to suppress higher-order transverse modes in the device three different methods have been used. It has been shown that each of these methods allows us to achieve lasing with the LP01 mode for structure with TJ diameter of 8 μm, which has not been possible for the structure without modifications. Although using these methods leads to higher values of the threshold current, the drop of the maximal operating temperature for the structure with TJ diameter of 7 μm has not been higher than 10 K.

DW VECSEL - structure design and MBE fabrication

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

Simulation of 1550-nm diamond VECSEL with high contrast grating

In the following paper a simulation of optically pumped vertical external cavity surface emitting lasers (VECSEL) with a novel approach for the improvement of the heat management is presented. In recent VECSEL structures, it was common to use one top diamond heat spreader in order to decrease the thermal resistance of the device by redistributing the heat flow to the lateral regions and thus transporting heat down to the copper heat sink more efficiently. We present here further improvement of the heat management by eliminating the bottom DBR from the heat flow path and substituting it for a diamond with a High Contrast Grating (HCG). Hence the active region, which consists of 5 pairs of AlGaInAs quaternary alloy quantum wells, is sandwiched between two diamond heat spreading layers. The structure of Si HCG deposited on a diamond provides broad wavelength range in which reflectivity is close to 100% for the emitted beam for perpendicular mode polarization with respect to the direction of the HCG trenches. The HCG assures less than 20% reflection and near zero absorption of pumping light, hence it allows for on-axis bottom pumping scheme and integration of the VECSEL with the pumping laser. According to the simulations 300 μm thick top diamond heat spreader is enough to assure effective heat dissipation mechanism. Replacing the bottom DBR with the diamond heat spreader will provide additional 10% reduction of the thermal impedance. The minimum of thermal impedance is achieved for about 450 μm thick bottom diamond heat spreader.

Concept of the CW GaN-based VECSEL

A concept and numerical study of a continuous-wave (CW) nitride-based vertical-external-cavity surface-emitting laser (VECSEL) with an InGaN/GaN active region is presented. The structure is designed to generate radiation around 450 nm. An array of nitride-based continuous-wave laser diodes is proposed to pump directly the quantum wells in the active region. We expect that it enables CW operation of the presented laser, in contrast to the GaN-based VECSELs demonstrated so far. Moreover, employing in-well pumping instead of barrier pumping reduces pump-laser quantum defect, which contributes to better thermal properties of the device. An external efficiency as high as 26% can be theoretically achieved by using a special multi-pass pump setup.

Metalized monolithic high-contrast grating as a mirror for GaN-based VCSELs

In this paper, we present a novel design of a nitride-based VCSEL emitting at 414 nm and perform numerical analysis of optical, electrical and thermal phenomena. The bottom mirror of the laser is a Al(In)N/GaN DBR (Distributed Bragg Reflector), whereas the top mirror is realized as a semiconductor-metal subwavelength-grating, etched in GaN with silver stripes deposited between the stripes of the semiconductor grating. In this monolithic structure simulations show a uniform active-region current density on the level of 5.5 kA/cm2 for the apertures as large as 10 μm. In the case of a broader apertures, e.g. 40 μm, we showed that, assuming a homogeneous current injection at the level of 5.5 kA/cm2 , the temperature inside the laser should not exceed 360 K, which gives promise to improve thermal management by uniformisation of the current injection.

Optical simulations of blue and green semipolar InGaN/GaN lasers

III-N-based edge-emitting lasers suffer from low refractive index contrast between GaN, AlGaN and InGaN layers, conventionally used in their epitaxial structures. This issue becomes more severe with an increase in wavelength at which those devices operate when tuning from blue-violet to real blue and green light. To overcome this issue and to increase the refractive index contrast other materials must be employed within the epitaxial structures replacing the standard nitride layers with materials with lower refractive index. We demonstrate results of effective-index numerical calculations performed for the state-of-the-art semipolar real blue (471 nm) and green (518 nm) edge-emitting lasers with structural modifications that include ITO, AlInN, plasmonic GaN:Ge and nanoporous GaN layers. Such solutions are extensively investigated for III-N-based EELs operating in blue-violet region but only separately. Using combination of these solutions we managed to increase optical confinement factor over twice in blue- and over 3.5-times in green-EELs.

Numerical study of VECSELs for generation of mid-infrared radiation

Two different approaches to developing new laser sources operating in the mid-infrared range based on vertical-external cavity surface-emitting lasers (VECSELs) are studied with the aid of numerical modelling. The first one consists in enhancing a maximal emission wavelength of currently available GaSb-based structures beyond 3 μm. The second approach consists in using dual-wavelength VECSEL (DW-VECSEL), emitting two coaxial laser beams of different wavelengths, to generate radiation from the 3-5 μm spectral range with the aid of difference frequency generation.