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Tunable Semiconductor Lasers Continue to Be in Just About Everyone’S List of Important Components for Future Fiber Optic Networks

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Tunable Semiconductor Lasers Continue to Be in Just About Everyone’S List of Important Components for Future Fiber Optic Networks Abstract: Tunable semiconductor lasers continue to be in just about everyone’s list of important components for future fiber optic networks. Various designs will be overviewed with particular emphasis on the widely tunable (>32nm) types. Tunable Semiconductor Lasers a tutorial Larry A. Coldren University of California, Santa Barbara, CA [email protected] Agility Communications, Santa Barbara, CA [email protected] 2 Contents ■ Why Tunable Lasers? ■ Basic Tuning Mechanisms ■ Examples of Tunable Lasers ■ Control of the Wavelength ■ Reliability Issues 3 Optical Network Architecture Core Edge More bandwidth and services/$Æ Low-cost components and agile architectures 4 Introduction ■ Tunable lasers have been of great interest for some time − Dynamic networks with wavelength reconfigurability − Networking flexibility − Reduced cost − One time provisioning (OTP) and sparing seen as side benefits ■ Current market conditions…. − More cautious approach from carriers and system vendors − OTP and sparing are now the leading applications ■ Tunable lasers are compared with DFB or EML − Important to do “apples to apples” comparison − Functionality − Performance − Total Cost of Ownership 5 Why Tunable Lasers? ■ One time provisioning—inventory and sparing ■ Field re-provisioning—new services without hardware change or truck roll ■ Reconfigurable Optical Add/Drop Multiplexers (ROADM)— Drop and add any channel without demux/mux ■ Wavelength conversion—Eliminates wavelength blocking without OEO line cards ■ Photonic Switching—Eliminates many OEO line cards ■ Wavelength Routing—Use passive optical core 6 Applications – One time provisioning—the universal source ■ Laser is provisioned once only ■ Simplifies manufacturing ■ Drastically reduces inventory ■ Minimizes sparing to a manageable level ■ Simplifies forecasting 7 Applications – Re-provisioning ■ Laser is provisioned many times remotely to set up new services − Seconds timeframe − Point and click or ultimately controlled automatically by software ■ Can only be addressed using a widely tunable laser − Without severe constraints ■ Drastically reduces inventory ■ Simplifies forecasting 8 Applications – Re-configurable OADM Tunable filter elements in thru out drop add Tunable Rx Tx ■ Drop and Add without Demux and Mux of all channels ■ Must be “hitless” filter tuning ■ Eliminates mux/demux and OEO ■ Tunable lasers are a key enabler 9 Applications – Photonic Switching 1 123 LR Tx LR SR SR Rx 123 Rx Tx OR Rx Tx 4 4 5 5 6 Grey 6 Optics Tunable 8 7 8 7 ■ Photonic switches require O-E-O on I/O to prevent blocking ■ Tunability reduces O-E-O requirements in half ■ Requires moderately fast switching (ms) 10 Applications – Wavelength Conversion ■ Intersection of metro rings ■ Wavelengths transition between rings − in optical domain ■ Tunable lasers used to resolve wavelength blocking − Alternative is a bank of fixed wavelength lasers Node Node 3B Node 3R Node 4B 4R Node Node 2B Node Node 2R 1B 1R Rx Tx 11 Applications – Wavelength Routing (Optical Packet Switching) Line cards with Line cards Tunable lasers T x N x N Lambda Router (AWG) ■ High capacity, high density router function—need wide tuning ■ Wavelength used to route traffic through passive device ■ For Packets requires very fast switching 12 Contents ■ Why Tunable Lasers? ■ Basic Tuning Mechanisms ■ Examples of Tunable Lasers ■ Control of the Wavelength ■ Reliability Issues 13 Generic Single-Frequency Laser Mode-selection Gain filter Output mλ/2 = nL Mirror-1 Mirror-2 Gain Lasing mode Mode-selection filter Possible modes λ 14 Examples of Single-Frequency Lasers gain ■ DFB Grating (mode-selection − All-elements combined and Light Out & distributed mirror) distributed along length Gain region DFB AR HR Gain Rear Mirror ■ DBR Light Out − Elements separated with individual biases DBR Ext. grating mirror Gain AR ■ External Cavity Light Out − Gain block + external lens & grating Collimating lens ■ VCSEL Active − Short cavity for mode selection DBR mirrors Light Out 15 How Tunable Lasers Tune Mode wavelength: Effective Cavity length mλ/2 = nL Mode number Effective index Wavelength Relative change in wavelength: Tuned by mode-selection filter (via index or grating angle) ∆λ = ∆ n + ∆ L - ∆m λ n L m Tuned by net cavity index change Tuned by physical length change 16 Generic Tunable Single-Frequency Laser Tunable Gain Mode- output Cavity selection phase filter (∆n) (∆m) (∆L) Mirror-1 Mirror-2 mλ/2 = nL Gain Mode-selection Lasing mode filter (∆m) Possible modes(∆n, ∆L) λ 17 Solutions for Tunable Lasers ■ DBR Lasers Gain Phase Rear Mirror Light Out − Conventional DBR (<8 nm) − Extended Tuning DBR’s (≥ 32 nm) DBR ■ External Cavity Lasers (≥ 32 nm) − Littman-Metcalf/MEMs − Thermally tuned etalon Light Out Light Out ■ MEMS Tunable VCSEL (< 32 nm) − Optically or electrically pumped W indow ■ DFB Array (3-4 nm X #DFBs) − On-chip combiner + SOA NEC − Or, off-chip MEMs combiner 8 Microarray MMI DFB-LDs − Thermally tuned S-bent waveguides W indow SOA Chip size: 0.4 x 2.15 mm 2 18 Contents ■ Why Tunable Lasers? ■ Basic Tuning Mechanisms ■ Examples of Tunable Lasers ■ Control of the Wavelength ■ Reliability Issues 19 Examples of Tunable Lasers ■ Narrowly tunable (not discussed further) − Temperature tuned DFBsÆ ~ 3nm − Narrowly tunable 2 or 3 section DBR lasersÆ ~ 8nm ■ DFB selectable arrays − Select DFB array element for coarse tuning + temperature tune for fine cavity mode tuning − Integrated on-chip combiners + SOAs or off-chip MEMs deflectors ■ External-cavity lasers − External grating reflector for mode-selection filter − Angle-tune mirror for mode selection—coarse tuning − Change length and/or phase section for fine tuning ■ MEMS Tunable VCSELs − Move suspended top mirror by electrostatic or thermal tuning − Single knob tuning for both coarse and fine ■ Widely tunable DBR lasers − Coarse tuning by index tuning of compound mirrors/couplers − Fine tuning by index tuning of phase section − Dual SGDBR or vertical-coupler + SGDBR mode selection filters 20 Wavelength-selectable light sources (WSLs) for wide-band DWDM applications Window Feature DFB-LD-array-based structure 8 Microarray Wide-band tunability MMI DFB-LDs S-bent Compact & stable waveguides Window SOA Multi-λ locker module Chip size: 0.4 x 2.15 mm2 Performance Schematic of wide-band WSL WSLs for S-, C-, L- bands (OFC’02) 8 array, ∆λ ~ 16 nm (∆T = 25K) x 6 devices Multi λ-locker integrated Wide-band WSL module (OFC’02) ∆λ ~ 40 nm (∆T = 45K) Multi λ−locker integrated Wide-band WSL module 21 WSLs for S-, C-, L- bands applications - Lasing spectra - 20 10 S-band C-band L-band 0 -10 -20 -30 -40 Intensity (dBm) -50 S １ S2 C1 C2 L1 L2 -60 Wafer 1 Wafer 2 -70 1470 1490 1510 1530 1550 1570 1590 1610 W avelength (nm ) ∆λ ～ 16 nm (∆T 25K) @15 - 40 ℃ 6 devices → 135 channels @100-GHz ITU-T grid SMSR > 42 dB Pf > ～ 10 mW @ IDFB= 100 mA, ISOA= 200 mA 22 Fujitsu DFB Array Integrated Tunable Laser 0.5 × 1.8 mm Fujitsu Laboratories Ltd. 23 Fujitsu Wavelength Tuning Characteristics Temperature tuning Spectra at 32 wavelengths Fujitsu Laboratories Ltd. 24 Santur Switched DFB Array 1 mm 12 element DFB array, each temperature tuned 3nm for 36nm total tuning range - only one laser on at a time MEMS mirror couples the selected laser to fiber Advantages: DFB characteristics (optical quality, reliability, wavelength stability) No SOA, tuning sections, phase-sensitive mechanics High yield, low cost passive alignment (MEMS does the rest) Built-in shutter/VOA 25 Santur 20 mW Module Performance z Full band tunability (36nm C-band, 42nm L-band) z Built-in wavelength locker (25GHz channel spacing) z >50dB SMSR, 2MHz linewidth z Typical tuning time ~ 2sec z Resistant to shock and vibe with no servo (10G causes < 0.2dB fluctuation in power) 26 Intel External-Cavity Approach (acquired from New Focus) • Double sided external cavity laser design, well known in test and measurement applications • Temperature tuned etalon replaces mechanical tuning device • No moving parts, but challenging packaging requirements 27 Littman-Metcalf Cavity (after New Focus) HR Laser Diode Coating Chip Collimating Lens AR Retroreflector Coating Pivot Point Wavelength Laser Tuning Output Diffraction Grating 28 Iolon External-Cavity Laser with MEMs Mirror Movement 29 Tunable VCSELs (optically pumped) ■ Cortek-Nortel-Bookham? ■ Component technologies − MEMS − Thin Film − InP Laser − Packaging ■ Advantages: − High Power − Wide Tuning Range − Continuously Tunable 30 31 Agere “Narrowly” Tunable DBR/SOA/EAM EA-DBR Operation Tuning Current High Low L Gain Tuning EA Modulator Bragg mirror select FP mode Tuning current moves Bragg mirror λ A Five Stage Bell Labs Design Gain DBR Optical Detector Modulator Section Mirror Amplifier “Power Monitor” Many more 7 – 10 nm designs 32 Extended tuning range: SSGDBR--NEL Phase modulated gratings 33 Extended tuning range: GCSR--ADC-Altitun SGDBR + GACC Gain Coupler Phase Reflector S-DBR 30 400µm 600µm 150µm 900µm 25 ] A 20 p-InP 15 10 Reflector current [m current Reflector 5 n-InP 0 QWs structures λg = 1.3 µm λg = 1.38 µm 81515 1525 1535 1545 1555 1565 λg = 1.55 µm 7 ] A 6 5 4 3 Coupler current [m current Coupler 2 1 0 1515 1525 1535 1545 1555 1565 Wavelength [nm] Agility’s Extended Tuning Range Technology: Widely Tunable SGDBR Lasers 35 Sampled Grating Tunable Lasers 10 Front Rear 2 4 1 3 Amplifier Mirror Gain Phase Mirror 0 -10 -20 Light Out -30 Laser Emission, dBm Emission, Laser Relative Power (dB) -40 -501 Back ■ Front 5-10X Tuning Range of DBR 0.8 ■ Reliable, Manufacturable InP Technology 0.6 ■ Can Cover C band, L band
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