Piotr Gutowski

Switchable double wavelength generating vertical external cavity surface-emitting laser.

Switchable, double wavelength generation is demonstrated from a single vertical external cavity surface-emitting laser chip. Power of ~0.5 W for two wavelengths λ≈967 nm and 1,018 nm i.e. within the spectral distance of 51 nm were registered. In the semiconductor heterostructure a single set of nominally identical quantum wells was enclosed in a single, two-mode resonant microcavity. The wavelength switching was induced by the change of the pump power. The increase or decrease of the pump power changes the active region temperature and thus tunes spectrally the gain spectrum to the one of two modes.

Impact of Injector Doping on Threshold Current of Mid-Infrared Quantum Cascade Laser–Non-Equilibrium Green’s Function Analysis

Nonequilibrium Greens function modeling is used to study the mechanism through which doping of the core region influences threshold current of quantum cascade laser. For devices emitting in mid-infrared utilizing two-phonon resonance depopulation scheme thermal backfilling of lower laser state is identified as the main interaction channel. Empirical-simulation based-relation between lower thermal population, doping density, bias, and temperature is found. This relation allows to propose new scaling rule that couples threshold bias with temperature and doping density. Scaling rule was tested against experimental threshold-current-voltage-temperature data collected for InGaAs/AlInAs quantum cascade lasers MBE grown on InP. Devices used in this experiment utilize two phonon resonance scheme, emit at either 4.7 or 9.3 μm, and achieve high hot-operating-temperature performance. Test of scaling was passed by the device which has threshold currents limited by thermal backfilling of lower state. The other device contains spurious state in the injector minigap and so its threshold currents are limited by the intersubband absorption. Data collected for this device do not obey the scaling rule.

Room-temperature AlInAs/InGaAs/InP quantum cascade lasers

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

Analysis of InP-based QCLs designed for application in optical transmitter of free-space optics

In this paper, the study of AlInAs/InGaAs/InP Quantum Cascade Lasers application in Free Space Optical data link is performed. Implementation of such FSO link operated in long-wavelength infrared (LWIR: 8-12 μm) will be unique for construction of so-called RF/FSO hybrid communication system. The range of longer wavelengths provides better data transfer performance in the case of severe weather conditions, especially, fog, low haze or air turbulence. In the frame of this work, series of QCLs for application in FSO system were examined. They are characterized by different geometries and constructions towards best performance in optical link systems operated in the wavelength range of 8-12 μm. The preliminary test of QCLs included electrical measurements of pulsed light-current-voltage characteristics and time-resolved spectra. The obtained results made it possible to determine operation point for FSO. Their modulation performances were tested using the laboratory laser drivers. Based on measurements, both power and time parameters of QCLs pulses were investigated. These results defined critical values for FSO system. The second part of the analysis concerned the spatial parameters of QCLs radiation. Knowledge of spatial characteristics of emission is vital for FSO optics construction. To characterize spatial properties of beams, far-field patterns of emission were registered. Finally, the obtained results made it possible to optimize the optical transmitter construction and further performance of FSO laboratory model. This research was supported by The Polish National Centre for Research and Development grant DOB-BIO8/01/01/2016.

Experimental investigation of thermal designs of InP-based quantum cascade lasers (Conference Presentation)

An important issue in the technology of QCLs is the capability to extract heat out of the laser active area in order to reduce the increase of the temperature. High temperature not only reduces most performance metrics but also decreases device lifetime and impairs its reliability, leading to degradation of laser mirrors and destruction of the device. In this paper, we report on the investigation of the temperature of QCLs based on different designs. QCLs are complex, multilayer structures, which require high current and voltage to polarize the structure in order to obtain level alignment. This results in high heat generation. Thermal limitations in case of QCL are the most critical factor decreasing the performance of a device. High electrical power combined with relatively low wall-plug efficiency results in high-temperature increase in the active core. Efficient heat dissipation is difficult due to hundreds of layers impeding thermal conductivity of the structure. Moreover, the materials composing the gain region are ternaries with a composition of roughly 50%, what results in thermal conductivity lower by a factor of 10 than in case of bulk InP. Knowledge of the temperature is gained through unique temperature measurement technique – CCD thermoreflectance (CCD TR). This method allows for rapid thermal characterization of QCLs, as the registration of high-resolution map of the whole facet of the device lasts only several seconds. CCD-TR allows accurate evaluation of the thermal characteristics of quantum cascade lasers. Here, we report on the influence of design on thermal properties of QCLs. The design of waveguides and optical confinement in QCLs is essential. The increase of the optical confinement was frequently achieved by placing the active core between two InGaAs layers. However, low thermal conductivity of InGaAs layers results in inefficient dissipation of heat from the active core. By removing or significant reduction of the layers’ thickness, observed temperatures of the active core are significantly lower. The modifications include Experimental investigation proves that performance improvements can be gained by introducing modifications into the design of the structure. Based on experimental data, methods to further improve the performance of QCLs are discussed.

Heat dissipation schemes in QCLs monitored by CCD thermoreflectance (Conference Presentation)

In this paper we present the development of the instrumentation for accurate evaluation of the thermal characteristics of quantum cascade lasers based on CCD thermoreflectance (CCD TR). This method allows rapid thermal characterization of QCLs, as the registration of high-resolution map of the whole device facet lasts only several seconds. The capabilities of the CCD TR are used to study temperature dissipation schemes in different designs of QCLs. We report on the investigation of thermal performance of QCLs developed at the Institute of Electron Technology, with an emphasis on the influence of different material system, processing technology and device designs. We investigate and compare AlInAs/InGaAs/InP QCLs (lattice matched and strain compensated) of different architectures, i.e., double trench and buried heterostructure (BH) in terms of thermal management. Experimental results are in very good agreement with numerical predictions of heat dissipation in various device constructions. Numerical model is based on FEM model solved by commercial software package. The model assumes anisotropic thermal conductivity in the AR layers as well as the temperature dependence of thermal conductivities of all materials in the project. We have observed experimentally improvement of thermal properties of devices based on InP materials, especially for buried heterostructure type. The use of buried heterostructure enhanced the lateral heat dissipation from the active region of QCLs. The BH structure and epilayer-down bonding help dissipate the heat generated from active core of the QCL.