Semiconductor Laser Laboratory Novel Integrable Semiconductor Laser Diodes J.J. Coleman University of Illinois 1998-1999 Distinguished Lecturer Series IEEE Lasers and Electro-Optics Society
Definition of the Problem Semiconductor Laser Laboratory Why aren’t conventional semiconductor diode lasers particularly suitable for integration? 1. Epitaxial structure optimization Lasers and other optical devices generally have very different optimum layer structures 2. Cleaved facet resonators Difficult (impossible) processing Poor optical coupling to other elements
Outline Semiconductor Laser Laboratory • Engineering in the optical path - selective area epitaxy • Integrable laser resonators • Examples of lasers integrated with other optical devices • Summary
Approaches to Wafer Engineering Semiconductor Laser Laboratory • Universal substrate – Compromise epitaxial layer design • Regrowth/overgrowth/multiple growth – Coupling and plane-of-propagation issues • Selective area epitaxy – Multiple regrowths T.L. Koch and U. Koren, OFC’92 Tutorial
Single Stripe Pattern Semiconductor Laser Laboratory SiO 2 • SiO 2 field • single open stripe stripe width 25-150 µ m •
Selective Epitaxy Boundary Conditions Semiconductor Laser Laboratory boundary layer N = constant ∂ N ∂ N = 0 = 0 ∂ x ∂ x ∂ N ∂ N ∂ N = g(x) = 0 = 0 ∂ y ∂ y ∂ y
Selective Epitaxy Boundary Conditions Semiconductor Laser Laboratory boundary layer N = constant ∂ N ∂ N = 0 = 0 ∂ x ∂ x ∂ N ∂ N ∂ N = g(x) = 0 = 0 ∂ y ∂ y ∂ y
Simulation Results Semiconductor Laser Laboratory isoconcentration profiles thickness profile
Modeled and Experimental Data Semiconductor Laser Laboratory 3000 2500 2000 • two stripe widths (50 1500 and 125 µ m) • modeled (dashed) 1000 • experimental (solid) 500 0 -20 0 20 40 60 80 100 120 140 Distance (µm)
Wide Stripe Impracticalities Semiconductor Laser Laboratory • Growth rate enhancement is too large – Poorer quality materials – Less control over thickness, especially for thin layers • Deep bowing – Makes subsequent processing difficult – Yields non-uniform quantum well thicknesses But, the basic parameters determined from these structures can be used to model more complex structures
Dual Stripe Pattern Semiconductor Laser Laboratory • open field • dual stripe • stripe separation 2-5 µm • stripe width 2-25 µm • dimensions small with respect to a diffusion length
Dual Stripe Growth Enhancement Semiconductor Laser Laboratory 3 3 25 µm � 6 µm Enhancement Factor Relative Thickness 2 2 � � w w � � � � � 1 1 0 0 0 5 10 15 20 25 -60 -40 -20 0 20 40 60 Oxide Stripe Width (µm) Distance (µm)
Buried Heterostructure Growth Process Semiconductor Laser Laboratory a) buffer layer and lower confining layer b) selectively grown active layer c) upper confining layer and cap layer
Semiconductor Laser Laboratory
Buried Heterostructure SEM Cross Section Semiconductor Laser Laboratory
Buried Heterostructure L-I Curves Semiconductor Laser Laboratory 180 3 160 140 120 2 100 80 60 1 40 I th = 2.65 mA length = 760 µm 20 0 0 0 5 10 15 0 200 400 600 800 1000 I (mA) Current (mA)
980 nm Wavelength Control Semiconductor Laser Laboratory 10800 9800 � 10600 � � 9700 � 10400 � � � � 10200 � � 9600 � 10000 � � � � � � 9800 9500 S = 5.5-10.5 µ m 9600 � � � 9400 9400 0 5 10 15 20 25 0 2 4 6 8 10 12 Dual oxide stripe width (µm) Element Number
1.55 µ m Wavelength Control Semiconductor Laser Laboratory Masahiro Aoki et al. IEEE J. Quantum Electron. 29 , 2088 (1993) • 110 nm wavelength range • uniform PL intensity • uniform PL half-width
Basic Buried Heterostructure Building Block Semiconductor Laser Laboratory • Engineered transition energy • Automatic lateral optical waveguide • Many relatively lossless coupling schemes are possible
Integrable Resonator Geometries Semiconductor Laser Laboratory • Etched Fabry-Perot facets – Scattering losses, verticality, flatness, coupling • Corner reflectors and ring geometries – Mode selection • Distributed feedback (DFB) resonators – Processing, sensitivity to reflections • Distributed Bragg reflectors (DBR) – Processing, coupling
Tunable DBR Reflectors Semiconductor Laser Laboratory I gain I DBR AlGaAs:p InGaAs-GaAs active layer AlGaAs:n GaAs:n • Asymmetric (thin p-layer) cladding structure • Conventional ridge waveguide or selective-area epitaxy buried heterostructure • Deep surface-etched DBR grating (1st, 2nd, or 3rd order) • Separate grating electrode for tuning purposes
First-Order DBR Grating Lasers Semiconductor Laser Laboratory 25 20 0 Power (mW) Intensity (dB) 15 • First-order gratings formed with -25 RIE 10 -50 • Six grating periods studied -75 5 1.048 1.058 1.068 • Threshold currents below 10 mA Wavelength(µm) 0 and as low as 6 mA 0 10 20 30 40 50 60 70 80 90 100 Current (mA) • Slope efficiencies greater than 0.4 160 W/A 140 � 20° C • Minimum emission linewidth about 120 Linewidth (kHz) the same as the measurement 100 � 80 resolution ~ 40 kHz � 60 � � 40 � � � 20 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Inverse Power (mW -1 )
Spectra versus DBR Tuning Current Semiconductor Laser Laboratory 0 1016 G 20° C -10 I gain = 40 mA 1014 Relative Intensity (dB) -20 G Wavelength (nm) I DBR = 1012 -30 0 mA 50 mA 100 mA G GGG G G G G G G G G G G G G G -40 1010 -50 1008 -60 -70 1006 1000 1005 1010 1015 1020 1025 0 20 40 60 80 100 Wavelength (nm) DBR Current (mA) • T = 20C and drive current fixed at 40 mA • Single mode spectrum preserved over 100 mA tuning current range • SMSR greater than 35 dB over an 8 nm tuning range • Current injection heating dominant tuning mechanism
1.3 µ m InGaAsP Ridge Waveguide DBR Lasers Semiconductor Laser Laboratory 0 First-Order Gain DBR -10 Section Grating -20 Intensity (dB) InP:p 0.5 µm InGaAsP etch stop layer -30 InP:u/p 0.2 µm -40 InGaAsP MQW - - InP:n active region -50 40 -60 I th = 26.5 ± 0.26 mA 1.355 1.36 1.365 1.37 1.375 35 η = 0.290 ± 0.0047 W/A Wavelength (µm) 30 • First-order gratings formed by Power (mW/facet) 25 chemically-assisted ion beam etching 20 (CAIBE) 15 • Compressively-strained MQW active 10 region 5 • SMSR of ~ 40dB 0 0 20 0 20 0 20 0 20 0 20 0 20 0 20 0 20 40 80 120 Current (mA)
Selection of Integrated Photonic Devices Semiconductor Laser Laboratory • Laser - external modulator • Laser - photodiode • Lasers for remote sensing applications • Multiple wavelength sources • An eight-channel transmitter
Example: Integrated Laser-Modulator Semiconductor Laser Laboratory M. Aoki et al. IEEE J. Quantum Electron. 29 , 2088 (1993)
Integrated Laser/External Modulator Semiconductor Laser Laboratory MQW DBR DBR MQW EA Laser Gratings Modulator Al 0.60 Ga 0.40 As:p 0.4 µm • Buried heterostructure laser grown by selective-area Al 0.60 Ga 0.40 As:n 1 µm epitaxy - - GaAs substrate • Tunable DBR grating as part of (a) the resonant cavity λ LD λ M • Blue-shifted electro-absorption SiO 2 modulator SiO 2 150 µm (b)
Integrated Laser/External Modulator Semiconductor Laser Laboratory 7 6 AR/HR Coated 6 3 Uncoated Optical Power (dBm) 0 5 0 Power (mW) -3 4 -20 Power (dB) -6 3 -40 -9 2 -60 -12 1.015 1.025 1.035 1 Wavelength (µm) -15 0 -18 0 10 20 30 40 50 60 70 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Current (mA) Modulator Bias (V) • Thresholds around 10 mA, single longitudinal mode operation • 18 dB extinction ratio at 1 V bias • 40 dB extinction ratio at 1.25 V bias with single mode fiber
Integrated Laser-Photodiode Semiconductor Laser Laboratory AlGaAs-GaAs-InGaAs selective area epitaxy laser structure θ LD p-contact PD p-contact d 18 1 0.9 LD n-contact PD n-contact 16 0.8 14 0.7 0.219 12 mW/mA 0.6 10 GaAs:n + buffer 0.5 8.9 µA/mA semi-insulating GaAs substrate 8 0.4 6 0.3 • RIE etched laser facet 4 0.2 2 0.1 • PD redshifted by 150Å 0 0 0 10 20 30 40 50 60 70 80 90 100 PD input facet angled by θ • LD Current (mA) • four-up contacts for flip chip bonding
DBR Lasers at 915-930 nm Wavelengths Semiconductor Laser Laboratory 0 0.13 0 0.2 0.4 0.6 In Al (a) (b) (c) 0.1 µm -10 Relative Intensity (dB) 0.3 µm -20 700 Å 80 Å -30 -40 20 Å 1.0 µm -50 0.91 0.915 0.92 0.925 0.93 0.935 0.94 20 Wavelength (µm) 18 20C λ = 934 nm 16 • Modified active region for shorter 14 Output Power (mW) λ = 916 nm strained-layer emission wavelength 12 10 λ = 924 nm • Second-order gratings with three 8 different periods 6 4 • Three emission wavelengths (915, 925, 2 935 nm) 0 0 10 20 30 40 50 60 70 80 90 100 Igain (mA)
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