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The Laser-As-Detector Approach Exploiting Mid-Infrared Emitting The laser-as-detector approach exploiting mid-infrared emitting interband cascade lasers: A potential for spectroscopy and communication applications Herdt, Andreas (2020) DOI (TUprints): https://doi.org/10.25534/tuprints-00017369 Lizenz: CC-BY-NC-ND 4.0 International - Creative Commons, Namensnennung, nicht kom- merziell, keine Bearbeitung Publikationstyp: Dissertation Fachbereich: 05 Fachbereich Physik Quelle des Originals: https://tuprints.ulb.tu-darmstadt.de/17369 The laser-as-detector approach exploiting mid-infrared emitting interband cascade lasers: A potential for spectroscopy and communication applications Zur Erlangung des Grades eines Doktors der Naturwissenschaften (Dr.rer.nat.) genehmigte Dissertation von Andreas Herdt geboren in Wetzlar Tag der Einreichung: 13.10.2020, Tag der Prüfung: 09.12.2020 1. Gutachten: Prof. Dr. Wolfgang Elsäßer 2. Gutachten: Prof. Dr. Thomas Walther Darmstadt Fachbereich Physik Institut für Angewandte Physik AG Halbleiteroptik The laser-as-detector approach exploiting mid-infrared emitting interband cascade lasers: A potential for spectroscopy and communication applications Doctoral thesis by Andreas Herdt 1. Review: Prof. Dr. Wolfgang Elsäßer 2. Review: Prof. Dr. Thomas Walther Date of submission: 13.10.2020 Date of thesis defense: 09.12.2020 Darmstadt Bitte zitieren Sie dieses Dokument als: URN: urn:nbn:de:tuda-tuprints-173693 URL: https://tuprints.ulb.tu-darmstadt.de/17369 Dieses Dokument wird bereitgestellt von tuprints, E-Publishing-Service der TU Darmstadt http://tuprints.ulb.tu-darmstadt.de [email protected] Die Veröffentlichung steht unter folgender Creative Commons Lizenz: 2.0 Deutschland Namensnennung – Nicht kommerziell – Keine Bearbeitungen 4.0 International https://creativecommons.org/licenses/by-nc-nd/4.0/ "Alle madri, perchè l’essere in due comincia da loro." Erri de Luca Erklärungen laut Promotionsordnung §8 Abs. 1 lit. c PromO Ich versichere hiermit, dass die elektronische Version meiner Dissertation mit der schriftlichen Version übereinstimmt. §8 Abs. 1 lit. d PromO Ich versichere hiermit, dass zu einem vorherigen Zeitpunkt noch keine Promotion versucht wurde. In diesem Fall sind nähere Angaben über Zeitpunkt, Hochschule, Dissertationsthema und Ergebnis dieses Versuchs mitzuteilen. §9 Abs. 1 PromO Ich versichere hiermit, dass die vorliegende Dissertation selbstständig und nur unter Verwendung der angegebenen Quellen verfasst wurde. §9 Abs. 2 PromO Die Arbeit hat bisher noch nicht zu Prüfungszwecken gedient. Darmstadt, 13.10.2020 A. Herdt v Abstract Interband-Kaskaden-Laser sind außergewöhnlich vielfältig einsetzbare, Hochtechnologiehal- bleiterlaser. Sie stellen die neueste Generation von Halbleiterlasern dar, die Strahlung im mittelinfraroten Wellenlängenbereich erzeugen und werden daher bevorzugt in Spektroskopie und Freistrahlkommunikationsapparaten als kohärente Lichtquellen eingesetzt. In dieser Arbeit wurden Sie eingesetzt um den Laser-als-Detektor Ansatz zu evaluieren. Der Laser-als-Detektor Ansatz ist eine bekannte, aber wenig eingesetzte Technik, die es ermöglicht auf externe optische Detektionseinheiten zu verzichten. Dabei wird ein Halbleiterlaser selber als Detektor genutzt, trotzdem er wie gewöhnlich in Betrieb ist. Das Prinzip beruht dabei darauf, dass externe optische Störungen die Ladungsträger im Interbandkaskadenlaser verändern und sich dementsprechend in Spannungsänderungen manifestieren. Mit Hilfe des Laser-als-Detektor Ansatzes werden anschließend zwei Spektroskopie-Sensoren aufgebaut und deren Detektionslimit anhand von Methanabsorptionslinien evaluiert. Danach wird der Laser-als-Detektor Ansatz ausgenutzt, um einen Freistrahlkommunikationsapparat ohne zusätzliche Detektoren aufzubauen. vii Abstract Interband cascade lasers are exceptionally versatile, high-tech semiconductor lasers. They represent the latest generation of semiconductor lasers that generate radiation in the mid- infrared wavelength domain and are therefore preferred as coherent light sources in spectroscopy and free-space-communication setups. In this thesis they are used to evaluate the so-called laser-as-detector approach. The laser-as-detector approach is a well-known but little used technique that allows dispensing external optical detection units. A semiconductor laser itself is used as a detector, even though it is operating as usual. The principle is based on the fact that external optical perturbation change the charge carrier density in the interband-cascade-laser and manifest themselves accordingly in changes of the terminal voltage. Using the laser-as- detector approach, two spectroscopic methane sensors are then set up and their detection limits are evaluated. The laser-as-detector approach is then used to build a free-space-communication communication device without any detector. ix List of Abbreviations ICL interband-cascade-laser IDL interband-diode-laser QCL quantum-cascade-laser SCL semiconductor-laser QW quantum well LAD laser-as-detector OFN optical frequency noise LEF linewidth enhancement factor SNR signal to noise ratio OFD optical frequency detuning UV ultra-violet VIS visible NIR near-infrared MIR mid-infrared FIR far-infrared arb. u. arbitrary units NLDS nonlinear dynamic systems DDE delay-differential equation LFF low-frequency fluctuations OIL optical injection locking LBW locking bandwidth SVEA slowly varying envelope approximation FTIR Fourier-transform-infrared QWIP quantum-well infrared photodetector MCT mercury cadmium telluride ESA electrical spectrum analyzer OSCI oscilloscope ISO optical isolator x NDF neutral density filter POL polarizer NPBS non-polarizing beam splitter DC direct current AC alternating current HITRAN high-resolution transmission molecular absorption LAS laser absorption spectroscopy TDLAS tunable diode laser absorption spectroscopy SMLAS selfmixing laser absorption spectroscopy ILLAS injection locking laser absorption spectroscopy ppmv parts per million by volume FSO free space optics CLS compound laser state QKD quantum key distribution xi Contents 1 Introduction 1 2 Mid-infrared-emitting Interband Cascade Lasers 7 2.1 Application areas of MIR light . .8 2.2 Coherent MIR sources . 10 2.3 Goals of the chapter . 11 2.4 Laser operation in conventional Interband Diode Lasers . 12 2.5 Quantum Cascade Lasers . 14 2.6 Interband Cascade Lasers . 17 2.7 Rate equation model for Interband Cascade Lasers . 20 2.8 ICL devices - The ILLIAS project . 24 2.9 Opto-electrical characterization . 25 2.9.1 Light-voltage-current characteristics . 28 2.9.2 Optical spectra . 33 2.9.3 Optical linewidth . 37 2.9.4 Linewidth enhancement factor . 42 2.9.5 Carrier lifetime and carrier density . 50 2.10 Conclusion of the chapter . 53 3 The laser-as-detector approach in feedback, unidirectional injection and mutual coupling configurations of interband cascade lasers - theory and experiment 57 3.1 Laser-as-detector approach . 61 3.2 Goals of the chapter . 64 3.3 Rate equations with external optical perturbation . 65 3.3.1 Methods to find the solutions of the rate-equations . 66 3.4 The demonstration of the laser-as-detector approach in a self-mixing experiment 68 3.4.1 Self-mixing theory . 69 3.4.2 Self-mixing experiment . 72 3.4.3 Summary of the section . 75 3.5 Dynamical stability of ICLs subjected to optical feedback . 75 3.5.1 Experimental observation of low-frequency-fluctuations . 77 3.5.2 Dynamical stability as a function of the feedback strength . 78 3.6 Evaluation of the laser-as-detector approach for unidirectional injection of coher- ent light . 81 3.6.1 Goals of the section . 83 xiii 3.6.2 Theory for unidirectionally injected ICLs . 84 3.6.3 Influence of injection strength - experiment and comparison with model 85 3.6.4 Influence of the modulation frequency - The electrical detection bandwidth 89 3.6.5 Summary of the section . 91 3.7 Evaluation of the laser-as-detector approach for bidirectional coupling of two ICLs 93 3.7.1 Rate-equation model for mutually delay-coupled ICLs . 94 3.7.1.1 Compound-laser-states . 96 3.7.1.2 Symmetries of the model . 98 3.7.2 Influence of coupling strength - Experiment and comparison with model 100 3.7.3 Influence of microscopic delay - Experiment and comparison with model 105 3.7.4 Experimental verification of the reflection symmetry . 113 3.7.5 Summary of the section . 115 3.8 Conclusion of the chapter . 115 4 Absorption spectroscopy with interband cascade lasers exploiting the laser-as- detector approach 119 4.1 Molecular absorption spectra . 122 4.1.1 Lineshape of an molecular absorption line . 123 4.1.2 HITRAN Database . 126 4.1.3 The methane molecule . 126 4.1.3.1 Rovibrational states of methane . 127 4.1.4 Absorption lines in Laser spectroscopy . 128 4.1.5 Summary of the section . 128 4.2 Detection limit and the Allan variance . 130 4.3 Laser spectroscopy techniques - A brief overview . 131 4.4 Goals of this chapter . 133 4.5 Tunable diode laser absorption spectroscopy . 134 4.5.1 Summary of the section . 140 4.6 Self-mixing laser absorption spectroscopy . 141 4.6.1 Summary of the section . 148 4.7 Injection Locking Laser Absorption Spectroscopy . 149 4.7.1 Summary of the section . 157 4.8 Conclusion of the chapter . 157 5 Communication using mutually coupled Interband Cascade Lasers 161 5.1 Goals of the chapter . 163 5.2 Bidirectional symmetric communication using mutually coupled twin ICLs . 163 5.2.1 Compound laser states . 163 5.3 Experimental setup to create compound laser states with ICLs . 165 5.4 Data encoding onto compound laser states . 168 5.5 Data decoding from compound laser states
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