Integrated Photonic Devices and Materials

Transcription

1 Integrated Photonic Devices and Materials Professor Leslie A. Kolodziejski Department of Electrical Engineering and Computer Science MIT in Japan 13 th Annual Symposium for Japanese Industry January 21, 2011

2 Research Highlights Nanoprecision Deposition Laboratory Saturable Bragg Reflectors (SBRs) Long Wavelength Optical Sources Ultra-Broadband Modulators at 800nm

3 Nanoprecision Deposition Laboratory Molecular Beam Epitaxy Platen Sizes: 14x2, 7x3, 4x4, 1x6, 1x8

4 Molecular Beam Epitaxy (MBE) pumping UHV to Torr substrate GaAs/InP shutters molecular beam Ga heating elements

5 Dual-Reactor Multi-Wafer MBE (As,P)-based Reactor sources: 2-Ga, 2-In, Al crackers: As, P dopants: Si, Be, C gas sources: H 2, N 2 Sb-based Reactor sources: 2-Ga, 2-In, Al crackers: As, P, Sb dopants: Si, Be, C, Te gas source: H 2

6 Band Gap Energy (ev) Wavelength (nm) Semiconductor Material Capability AlAs AlGaAs AlSb GaAs InP InGaAs GaSb Lattice Constant (Å)

7 Research Highlights MIT s Nanoprecision Deposition Laboratory Saturable Bragg Reflectors (SBRs) Long Wavelength Optical Sources Ultra-Broadband Modulators at 800nm

8 SBRs to Modelock Short Pulse Lasers a) Example Laser pump pump laser (977nm) output of output laser multiplexer WDM SBR Er-doped EDF fiber SMF output Output coupler coupler (coated ferrule) SBR laser cavity packaged in a box

9 Pulsing with a Saturable Bragg Reflector Saturable Bragg Reflector (SBR) light pulses gain medium mirror + saturable absorber = SBR mirror Purpose: To reflect light back into the laser cavity Purpose: Introduces an intensity-dependent absorption for initiating pulses Low intensities fully absorbed High intensities partially absorbed

10 Short Pulses need Wide Bandwidth 10 fs Pulse-width limited by the component with the smallest bandwidth Dn ~ 40 THz => Dl ~ 200 nm time frequency l c =1230 nm Laser System Gain medium Passive Components Mirror + Saturable Absorber = SBR mirror

11 intensity (dbm) intensity (a.u.) IAC (a.u.) a) a) b) 1 sech 2 fit Example 0.8 measured Laser pump 0.6 laser pump (977nm) 0.4 output of laser output 0.2 SBRs to Modelock Short Pulse Lasers 0 multiplexer WDM 17.5nm wavelength (nm) packaged in a box c) 0 d) frequency (GHz) 8 6 SBR Er-doped EDF fiber SMF 4 output Output coupler coupler 2 (coated ferrule) SBR 0 laser cavity time delay (fs) x187fs NEED -20 for SBRs: -40 self-starting sustain pulses -60 compact -80 robust user-friendly frequency (MHz)

12 intensity (dbm) intensity (a.u.) IAC (a.u.) a) a) b) 1 sech 2 fit Example 0.8 measured Laser pump 0.6 laser pump (977nm) 0.4 output of laser output packaged in a box c) 0 d) SBRs to Modelock Short Pulse Lasers multiplexer WDM 17.5nm 4 SBRs used to modelock : fiber-based lasers solid-state lasers wavelength (nm) frequency (GHz) 8 6 SBR Er-doped EDF Fiber SMF output Output coupler coupler 2 (coated ferrule) SBR 0 laser cavity variety of wavelengths (visible to IR) variety of pulsewidths (fs to ps) time delay (fs) variety of repetition rates (MHz to GHz) x187fs NEED -20 for SBRs: -40 self-starting sustain pulses -60 compact -80 robust user-friendly frequency (MHz)

13 Technology Needing Short Pulse Lasers medical diagnostics (imaging, multi-photon microscopy/spectroscopy) micromachining laser eye surgery photonic A-to-D converters 3D optical data storage optical clocks frequency combs MIT SBR 500 MHz laser SBR pulse interleaver amplifier (14.8-a2/2) cm passive Er-doped Waveguide Laser Chip 5cm gain ßpump loop: a2 cm 10% out 500MHz output 15cm 2GHz output Specs: 10 mw, 200 fsec, 2GHz,1550nm Can ONLY be modelocked with SBR Cost for waveguide laser: $130! ( + $2k for pump laser) 13

14 Design of Saturable Bragg Reflectors pump λ n layers reflecting layers resonant layers to increase absorption saturable absorber for laser cavity: bulk layer or QW* (? thickness) number of QWs absorber placement mirror design: center wavelength λ 0 bandwidth (index contrast) absorber lifetime or response time: strain proton bombardment choice of cladding layer GaAs substrate * QW = quantum well

15 Typical Distributed Bragg Reflector 22 pairs of Al 0.95 Ga 0.05 As & GaAs: Bandwidth: ~ 170nm Peak Reflectivity: 99.8% l 0 = 1550nm bandwidth GaAs

16 Variations to Saturable Absorber InGaAs absorbs at λ = 1580 nm number of absorbing InGaAs layers GaAs GaAs InP InP cladding layer material GaAs GaAs GaAs GaAs

17 Variations to Saturable Absorber InGaAs absorbs at λ = 1580 nm number of absorbing InGaAs layers GaAs GaAs InP InP cladding layer material GaAs GaAs GaAs GaAs thickness of InGaAs layers placement of InGaAs layer(s) AlGaAs AlGaAs x2 resonant layers GaAs GaAs GaAs

18 GaAs AlGaAs GaAs Effect of Adding Resonant Layers InGaAs 60nm InGaAs absorber embedded in a half-wave cladding layer of GaAs Air 22-pair GaAs/Al 0.95 Ga 0.05 As mirror centered at 1550 nm

19 Reflectivity (%) Intensity (arb. units) Characterization of SBRs 1.E+0 1.E-1 Fit Data modulation depth: 2.5% non-saturable loss: 1.5% saturation fluence: 20µJ/cm 2 1.E-2 1.E Fit Data 70 1.E Theta (degrees) high-resolution, triple-axis x-ray diffraction Wavelength (nm) ellipsometry

20 Desired versus Achieved Structure X 22 Structure Desired Reflectivity Fit Composite X-ray Fit Uncoupled Thickness (nm) Al or In content Thickness Al or In content Thickness (nm) Al or In Content Ellipsometer Fit: Coupled Thickness Al or In Content Ellipsometer Fit: Uncoupled Thickness GaAs InGaAs GaAs GaAs AlGaAs GaAs Substrate Al or In Content

21 Proton Bombardment: Modify Carrier Lifetime Generate femtosecond pulses Shortens pulsewidth reduced carrier lifetime reduced two photon absorption (multiple pulsing suppressed)

22 MIT SBR 500 MHz laser SBR Er-doped Waveguide Laser pulse interleaver amplifier SBR Technology Development (14.8-a2/2) cm passive 5cm gain ßpump loop: a2 cm 10% out 500MHz output 15cm 8.8nm FWHM 285fs FWHM pulse duration 0.2mW output for 500MHz 0.4mW output for 2 GHz Pump Output WDM OC 10% Compact Fiber Free Space Fiber Laser 2GHz output 121x94x33mm 3 Output Pump Laser WDM OC 10% Erbium Fiber Collimator Lens MIT SBR 6.4nm FWHM ~400fs FWHM pulse duration 4mW output for 625MHz tunable laser Erbium Fiber + SMF28e (92mm + 11mm) MIT SBR 17.5nm FWHM 187fs FWHM pulse duration 27.4mW output for ~1GHz laser

23 Issues with SBR Robustness 1) Damage by 980 nm pump light Solution: Dichroic dielectric coating 2) Localized Heating Solutions: thin GaAs substrate mount on copper holders fiber spliced onto Er-doped fiber between SBR metal fiber ferrules to conduct heat away thermally-conductive epoxy to conduct heat away 10 mm Image of SBR Degradation

24 Reflection (%) Narrowband SBRs: Various Wavelengths nm SBR 850 nm SBR 910 nm SBR Wavelength (nm) Al 0.95 Ga 0.05 As/GaAs Mirror Stack

25 Intensity (au) Intensity (au) Intensity (au) Intensity (au) Modelocking Results Pulse energies: 1-2 nj, Peak powers: kw, Rep rate: 100 MHz Cr:LiCAF 21nm, 40fs 21 nm Cr:LiSAF 19nm, 40fs 19 nm Wavelength (nm) Wavelength (nm) 1 Cr:LiSAF 1.00 Cr:LiSAF nm, 25fs 34 nm nm, 90fs 10 nm Wavelength (nm) Wavelength (nm)

26 SESAM/SBR Reflectivity (%) Intensity (au) SHG Intensity (au) Cr:LiSAF Laser: 25fs pulses DS1 TM DS2 TE PBS 3 % OC 75 mm 28 cm M1 DCM 75 mm Cr:LiSAF FS prism FS prism M2 DCM 75 mm 75 mm MIT-850-bulk SESAM/SBR MIT SBR DS4 TE PBS DS3 TM M4 25 fs Delay (fs) nm nearly transform limited, 25-fs, 1-nJ pulses at 85 MHz pulsewidth limited by SBR reflectivity and dispersion bandwidth Wavelength (nm)

27 Total cavity GVD (fs^2) _ SBR Reflection (%) Intensity (au) SBR Reflection (%) Intensity (au) SBR Reflectivity (%) Total cavity GVD (fs^2) _ SBR Reflection (%) Wavelength Tuning of Modelocked Laser Cr:LiCAF: nm with <200-fs pulses Cr:LiSAF: nm with <150-fs pulses Wavelength (nm) Cr:LiSAF: nm with <200-fs pulses Wavelength (nm) Cr:LiSAF: nm with <250-fs pulses Wavelength (nm) Wavelength (nm)

28 D4, TE 640 nm D2, TE 640 nm Intensity (au) _ Total cavity GVD (fs^2) _ Intensity (au) Reflection (%) Pulse Energy (nj) _ Pulse Width (fs) 45nm Tuning with sub 200fs Pulses D1, TM 640 nm M5 DCM 100 mm PBS M3 DCM f=65 mm MIT-850-bulk-HR SESAM/SBR MIT SBR M1 DCM 75 mm M4 DCM M2 DCM 75 mm Cr:LiSAF f=65 mm BR tuning plate PBS 3% OC D3, TM 640 nm pulse energy 0.5 pulse width Wavelength (nm) GVD Wavelength (nm) continuous tuning from 828 to 873 nm (45 nm) average pulsewidth = 190 fs average pulse energy = 1.87 nj (91 MHz) Wavelength (nm) tuning range limited by the SBR reflectivity and dispersion bandwidth -950

29 MIT Innovation: Broadband Mirror Stack l 0 = 1550nm 7 pair Al x O y /GaAs stack: Bandwidth*: 600nm Peak Reflectivity: 100% GaAs * determined at 99% reflectivity

30 MIT Innovation: Broadband Mirror Stack l 0 = 1550nm Patent Pending: MIT Case 11667, US Appln. Ser. No. 11/433,736) 7 pair Al x O y /GaAs stack: Bandwidth*: 600nm Peak Reflectivity: 100% GaAs * determined at 99% reflectivity

31 Thermal Oxidation of AlAs AlAs + H 2 0 Al x O y + AsH 3 n = 2.9 n = 1.6 apparatus influence on oxidation rate time, temperature, steam flow sample influence on oxidation rate aluminum content, layer thickness, sample geometry

32 Reflection (%) Benefits to Oxidized AlAs Layers Oxidized Bragg mirror 80 Regular Bragg mirror Wavelength (nm) Replace Al x O y (n 1.6) for AlAs (n 3) Dn increases from 0.5 to 1.9 only 6-7 pairs are required in the Bragg stack >300 nm reflectivity bandwidth less toxic material greater robustness to fabrication

33 Transmission (%) Gain (au) L Broadband Tunability of SBRs E c2 E c1 DE c E g2 E g1 E band-gap-qw Small signal Saturated Cr:LiSAF estimated gain 0 E v E v1 DE v Wavelength (nm) Al 0.19 Ga 0.81 As In 0.14 Ga As Al 0.19 Ga 0.81 As Aim: Tunability in nm region Need flat linear and nonlinear absorption response in nm region 6 nm thick In 0.14 Ga 0.86 As strained absorber sandwiched between Al 0.19 Ga 0.81 As 0.75% modulation depth Absorber transitions are at 920 nm and 803 nm Ref: S. W. Corzine, PhD Thesis, University of California, Santa Barbara (1993).

34 D6, TE 640 nm D3, TE 640 nm Pulse Energy (nj) Pulse Width (fs) Intensity (au) Reflection (%) 105nm Tunability with Broadband SBR 1 Saturated reflection D2, TE Dichroic 660 nm filter M1 f=65 mm 75 mm D1, TM 640 nm PBS Oxidized SAM M2 75 mm Cr:LiSAF f=65 mm M3 DCM DCM 150 mm BR tuning HR plate (cw) Broadband SBR Dichroic filter PBS 3% OC D5, TE 660 nm D4, TM 640 nm Wavelength (nm) Small signal reflection Continuous tuning from 800 to 905 nm Average pulse-width = 140 fs Average pulse-energy = 1.6 nj pulse energy pulse width Wavelength (nm)

35 MIT Broadband SBRs: Wavelength Range Ti:sapphire 15-30fs Er-doped fiber 155fs Cr:Forsterite 20fs

36 MIT Broadband SBRs: Wavelength Range Ti:sapphire 15-30fs Er-doped fiber 155fs Simulated Cr:ZnSe Cr:Forsterite 20fs GaAs 7 Pairs: GaAs 182.7nm AlAs 453.7nm

37 Research Highlights MIT s Nanoprecision Deposition Laboratory Saturable Bragg Reflectors (SBRs) Long Wavelength Optical Sources Ultra-Broadband Modulators at 800nm

38 Near- and Mid-infrared Semiconductor Lasers Infrared laser absorption spectroscopy detection and quantification of molecular trace gases (ppm to ppt by volume) chemical analysis and industrial process control toxic industrial chemical detection petrochemical gas detection combustion sources and processes e.g. early fire detection agriculture and animal facilities environmental gas monitoring spacecraft and planetary surface monitoring Werle et al, Optics and Lasers in Engineering 37 (2002),

39 Near- and Mid-infrared Semiconductor Lasers Biomedical applications eye surgery infrared spectroscopy of biotissues high precision laser ablation of biotissue angioplasty breath analysis Source: NY Times Source: Serebryakov, J. Opt. Technol. 77 (1), (2010) Source: M. Ebrahim-Zadeh and I. T. Sorokina (eds.), Mid-Infrared Coherent Sources and Applications (2008),

40 Long Wavelength Optical Sources In 0.47 Ga 0.53 As:Be 80nm InP:Be 2282nm In 0.52 Ga 0.38 Al 0.1 As:Be 21nm InP:Be 171nm In 0.52 Ga 0.38 Al 0.1 As 190nm In 0.52 Ga 0.38 Al 0.1 As 211nm In x Ga 1-x As 9.5nm In 0.52 Ga 0.38 Al 0.1 As 211nm InP:Si 1141nm InP substrate n Cladding Active Wave Guide Etch Stop Contact C-mounted Laser Device Threshold (ma) VA cw 1502 VA cw 1777 VA cw 1814 VA cw 1879 VA cw 1893 VA cw 1978 VA cw 1990 Wavelength (nm) slightly above threshold

41 Material and Structural Characterization (004)HR-XRD InP substrate 2 InGaAs quantum wells with InGaAlAs cladding layers InGaAs contact layer

42 PL intensity (a.u.) Power (W) Optical Characterization va153 n=2 va154 n=3 va158 n=2 va159 n=3 va162 n=2 va163 n= l (nm) Photoluminescence (PL) 8.0x10-7 VA162 DC1% 1A 7.0x10-7 VA162 DC2% 1A 6.0x x x x x x l (nm) Electroluminescence (EL)

43 Long Wavelength InP-based Lasers L-I-V spectrum Schematic

44 Fabry-Perot Ridge Lasers scanning electron micrograph 3μm ridge width and 2.3μm etch depth In collaboration with P. Heim/A. Cable Thorlabs Quantum Electronics

45 Normalized Intensity 2.6µm Tunable Long Wavelength Laser 1 µm n contact = 3.55 n InP = n BCB = 1.5 n InGaAlAs = 3.62 Schematic of Tunable Laser Design Coupled-mode Analytical Calculation Transmission through 2 rings, Kappa=0.45 Straight WG Loss = 10 db/cm Racetrack: 25µm bends, 75µm coupling regions 0.3µm between WGs l [um]

46 Research Highlights MIT s Nanoprecision Deposition Laboratory Saturable Bragg Reflectors (SBRs) Long Wavelength Optical Sources Ultra-Broadband Modulators at 800nm

47 Modulators: Technology Development Material Design: MBE growth Material characterization Device Design: Modulator- MOS based using embedded Al x O y Multimode Interferometers (MMIs) Y-splitters Fabrication Process Flow: 6 mask levels Self-aligned passive-to-powered sections NR7-3000P Photoresist Process III-V Etching with new ICP-RIE # BCB* Planarization Process Metallization with Significant Topology BCB Etch to Facilitate Cleaving *BCB: benzocyclobutene # ICP-RIE: inductively-coupled plasma reactive-ion etching

48 Intensity (arb. units) 800nm Modulators AlGaAs-based MOS Structure Gold dilute waveguide core Al 0.27 Ga 0.73 As AlAs transformed Al x O y 40nm to Al x O y to improve mode confinement higher operating fields push-pull operation Al 0.17 Ga 0.83 As 60nm 175nm 1E+1 1E+0 1E-1 1E-2 1E-3 (400) x-ray Fit Data 1E-4 500nm 1E Theta (deg) Al 0.27 Ga 0.73 As 1.675µm simulated optical mode GaAs substrate

49 Mach-Zehnder Interferometers: llength: 3mm to 10.5mm aligned to [011] and [01-1] 6 mask level process All waveguides are placed within trenches 800nm Modulators MMI MMI Plan-view Device Schematic Length 3.0 mm Voltage 2.57 V Vπ L = 7.7 volt mm 2 Wafer Outline Vπ L = λ t / n eff3 (r 25 +r 12 +r 32 ) where t = thickness from oxide-to-oxide layers, assuming r s for GaAs

50 Waveguide Etching: Samco Reactive Ion Etcher

51 Waveguide Etching: Samco Reactive Ion Etcher

52 Waveguide Etching: Samco Reactive Ion Etcher

53 Waveguide Etching: Samco Reactive Ion Etcher Al x O y Layers

54 800nm Modulators: Cleaved Facet Al x O y BCB Die size facet-to-facet: 14.5mm Insertion loss: 14 to 23 db 9.7 db/cm loss Y-splitters work successfully Note: mask #6 used to remove BCB facilitating cleaving

55 Research Highlights MIT s Nanoprecision Deposition Laboratory multiwafer MBE, significant material capability Saturable Bragg Reflectors (SBRs) broadband, short pulse generation, wide tunability Long Wavelength Optical Sources widely tunable, InP-based & GaSb-based Ultra-Broadband Modulators at 800nm

56 A Team Effort Students: Dr. Gale Petrich, MIT Professor Erich Ippen, MIT Professor Franz Kaertner, MIT Professor James Fujimoto, MIT Dr. Abdelmajid Salhi, KACST Professor Hamad Brithen, Alfaisal Univ Professor Jaime Viegas, Masdar Institute Sponsors: DARPA, Thorlabs, KACST, AFOSR, QPeak Dr. Umit Demirbas Dr. Ali Motamedi Dr. Hanfei Shen Hyunil Byun Jeff Chen Amy Chi Chun Duo Li Sheila Nabanja Michelle Sander Orit Shamir Ta-Ming Shih