Idealisierter abstimmbarer Laser 3. Wie erzielt man Wellenlängenabstimmung? 4. Verfügbare Bandbreite der optischen Nachrichtentechnik 1

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1 Verfügbare Bandbreite der optischen Nachrichtentechnik 1 Evolution abstimmbarer Laserdioden (bei 1.55µm) Either: tune the cavity gain characteristic (i. e. shift p ) or: tune the phase shift (i. e. the optical cavity length nl) or both! Idealisierter abstimmbarer Laser 3 Wie erzielt man Wellenlängenabstimmung? 4

2 Discontinuous Tuning! Abstimmung durch Verschiebung der Cavity-Gain-Kurve g c Abstimmverhalten durch Verschiebung der Cavity-Gain-Kurve Discont1 5 6 Stepwise Continuous Tuning! Abstimmung durch Verschiebung des longitudinalen Abstimmung durch Verschiebung des longitudinalen Anwendung Modenkammspektrums Modenkammspektrums 7 8

3 Continuous Tuning! Spektren (schematisch) bei Verschiebung des Synchrone Verschiebung von Cavity-Gain und Anwendung longitudinalen Modenkammspektrums longitudinalem Modenkammspektrum 9 10 Quasi-Continuous Tuning! Intervallweise synchrone Abstimmung 11 Spektren bei Quasi-Kontinuierlicher Abstimmung 1

4 m : Longitudinal Mode Spacing 1.55µm) Verschiebt das Kammspektrum Verschiebt die Cavity-Gain-Kurve Abstimmcharakteristiken (Tuning Modes) 13 Die beiden Grundkomponenten zur elektronischen Wellenlängenabstimmung 14 The Change of Electron and Hole Density by N (= P) yields a Refractive Index Change n (via Carrier Polarisation) of: Plasma-Effekt: nm Temperatur: nm (T = )) QCSE: < 1 nm (Quantum-Confined Stark-Effect e 1 1 n N 8 c n m m = wavelength c = vacuum speed of light n = refractive index 0 = vacuum permittivity m = effective mass of electrons, holes e,h 0 e h 18-3 Typically N of 3x10 cm yields n 004. Physikalische Mechanismen zur zur Wellenlängenabstimmung 15 Der Plasma-Effekt für freie Ladungsträger 16

5 Recombination Rate (undoped tuning region N = P) and typical parameters for InGaAsP at = 1.55 m : nn R = I ev AN + BN CN A : linear recombination coefficient (~ 10 s ) B : bimolecular recombination coefficient (~ 7x10 C : Auger recombination coefficient (~ 6x10 V : tuning region volume (e.g. x 0. x 500m e : elementary charge I : current cm s ) 6 1 cm s = x10-10 ) cm ) I t ev t N 3 / s BN CN Recombination Mechanisms in Tuning Region Rate-Equation 17 Abstimmkurve bei nichtlinearer Rekombination 18 entspricht der Brechzahländerung Änderung der Filterkurve durch Verluste beim Plasma-Effekt 19 Temperaturabhängigkeit der Wellenlänge 0

6 Bei Anlegen eines elektrischen Feldes verringert sich der Überlapp der Wellenfunktionen. Damit reduziert sich die Brechzahl. Der Quantum-Confined Stark-Effekt 1 Vergleich der Abstimm-Mechanismen Elektronisch abstimmbarer Bragg-Reflektor (Prinzip) 3 Integrationstechniken 4

7 1 1 L L 1 4L kl const! No Mode Changes! Front view Top view Abstimmung in transversal integrierten Lasern 5 Thermally Tuned DFB Laser 6 Wavelength and Linewidth of Thermally Tuned DFB Laser 7 Tunable Twin-Guide (TTG) Laser Principle 8

8 TTG-Laser Cross-Section 9 TTG-laser cross-sections 30 Tuning Spectra of a TTG-Laser (Siemens, 1993) 31 TTG-Laser: Tuning Characteristics 3

9 (ATT, 1995) Tunable Multisection DFB Laser 33 6-element DFB-Laser Array (00GHz-spaced channels) 34 Longitudinal Section of 3-Section DBR-Laser 35 Abstimmung in longitudinal integrierten Lasern 36

10 (Murata et. al. Electron. Lett., vol. 3, 1987, pp ) Tuning of a 3-Section DBR-Laser 37 Spectral Linewidth Broadening in a Continuously Tunable Laser (example: TTG Laser) 38 neff L i n n eff eff L L ( i :Mode Number) Linewidth Broadening Mechanisms in (Continuously) Continuous Wavelength Tuning Tunable Laser Diodes 39 40

11 Continuous Wavelength Tuning is Limited by the Maximum Possible Refractive Index Change to n n (~15 nm at 1.5 µm Wavelength) Applying the Vernier Effect: Yields Significantly Larger Tuning Ranges But: Discontinuous Wavelength Tuning Scheme Limitation of the Continuous Tuning Scheme 41 How to Achieve Larger Tuning Ranges than ~ 1%? 4 Amplification factor =x/x=9 Discontinuous Wavelength Tuning 43 The Vernier-Effect 44

12 Tunable Laser Diode Using the Vernier Effect 45 Realization of Comb Reflection Spectra 46 Coupling coefficient (a. u.) I1( T N i) I( r N i) Relative wavelength deviation ( i N ) N 0.1 SSG SG Vertical Coupler Filter Example for Sampled Grating (SG) and Super- Structure Grating (SSG) Coupling Coefficients 47 Codirectional Mode Coupling in VCF 48

13 Wavelength dependence of filter curve for device length L c (schematic) 0 Mode Coupling in VCF 49 Tuning Range of VCF Compared to DFB/DBR 50 (Kim et al., Appl. Phys. Lett., vol. 64, 1994, pp ) Principal Structure of VCF-Laser 51 Basic 1550nm VCF-Laser 5

14 n n c R 0 xl c S fx () f( x) sin x 4 4x x 3 Grating-assisted Codirectional coupled Sampled grating Reflector (GCSR) Laser Die VCF-Übertragungsfunktion bei L c 53 Principle of GCSR-Laser 54 Active Coupler Phase MG-DBR Active CouplerPhase MG-DBR Grating-assisted Codirectional coupled Sampled grating Reflector GCSR-Laser: Structure (ADC) 55 GCSR-Laser: Operation Principlen (ADC) 56

15 Rigole et al GCSR-Laser: SEM-Picture (ADC) 57 Tuning Spectra of GCSR-type Laser 58 Aging at 50 and 70 ºC Coupler current [ma] Frequency Reflector current [ma] [THz] Coupler current [ma] Output power Reflector current [ma] [mw] Frequency drift (GHz) Hours of accelerated aging Lasing parameters of GCSR-Lasers (ADC) 59 Frequency Stability of GCSR Laser (ADC) 60

16 Optical Communications - Wavelength Division Multiplexing (WDM) - Spare Lasers - Local Oscillator in Coherent Detection - FM modulated Source Measurement (Optical Spectrum Analysis) sin : Grating period Sensing and Spectroscopy Optical Radar... External Cavity Tunable Laser Diodes 61 Applications of Tunable Laser Diodes 6 Wavelength Division Multiplexing (WDM) 63 Coherent Multichannel Transmission (example) 64

17 (after ATT, NTT and HHI) Tunable Laser Diode in Coherent Detection 65 Monolithically Integrated Heterodyne Receiver 66 Instantaneous frequency Reference Object signal signal / f f if t 1 t Time =R/c t m Optical Frequency Modulated CW Radar FMCW-Radar: Signals 67 68

18 Instantaneous frequency f Reference signal Approaching Not moving Leaving =R/c Intermediate frequency Irregular IF-signal Time FMCW-Radar: Anemometrie 69 Optical FMCW Radar with TTG-Laser 70 Analyse von Umweltgiften Lecksuche bei Erdgasleitungen Untersuchung von Verbrennungsprozessen Raumluftüberwachung... Laseroptische Gas- und Umweltanalyse Sensorik 71