1. Alan Kost Frontiers in Optics Tucson, AZ October 20, 2005 Monolithically Integrated Semiconductor Components for Coarse Wavelength Division Multiplexing

2. OUTLINE The need for photonic integrated circuits Coarse wavelength division multiplexing - Arrayed waveguide gratings on InP Quantum Well Intermixing - GaAsSb Quantum Wells Low cost is the driver

3. ELECTRONIC ICs

4. A WDM Data Link LASER OPTICAL MODULATOR MULTIPLEXERDE-MULTIPLEXER 1 2 n 1,  … n ADD FIBER POWER AMP DROP i j PRE- AMP IN-LINE AMP 1 2 m VARIABLE ATTENUATOR PHOTODIODE Components are numerous and expensive OUR PROGRAM

5. AWG/SOA Concept Arrayed Waveguide Grating (AWG) 1 to 9 Coupler 9 to 4 Coupler    4 Semiconductor Optical Amplifiers (SOAs)   4  InP Substrate  = 20nm De-multiplexing, amplification, and equalization on one, InP chip

6. Coarse Wavelength Division Multiplexing DWDM Very high throughput (80 channels over C-Band from 1530 to 1565 nm) For use in long haul networks Stabilized lasers and narrow-band filters required 32 nm 50 GHz (~0.4 nm at 1.55  m) CWDM Smaller number of channels and correspondingly smaller throughput For use in short to medium haul networks Compatible with less expensive, un-stabilized lasers and broadband filters 340 nm (1270 – 1610 nm)  = 20 nm

7. InP ARRAYED WAVEGUIDE GRATINGS: “HORSESHOE” TYPE            SYMMETRY LINE Transmission 1 234 1 234 Output Waveguide # Free Spectral Range OPTICAL PATH LENGTH = L L +  L L + 2  L L + 3  L L + 4  L L + 5  L  L = m ~

8. An “S-SHAPED” Arrayed Waveguide Grating This kind of AWG has not been previously fabricated in semiconductor The optical path difference between waveguides in the array can be made arbitrarily small reducing the angle subtended by the arc. Arc Star Coupler Output Waveguides Waveguide Array Input

9. Shallow Ridge Waveguides Bend radius range for AWGs Bend Loss (db/cm) Bend Radius (  m) Fundamental Mode First Higher Order Mode Loss = 4.5 dB/cm

10. Patterned AWGs Star Coupler “S” 1-micron

11. AWG Response Wavelength (nm) Transmitted Power (dB) Channel Width ~ 7 nm (FWHM) Cross talk ~ - 15 dB or less Channel 3 4 56781 1473 1509 1527 1546 15621490 Proper AWG design should include chromatic dispersion Yurt, Rausch, Kost, Peyghambarian, Opt. Express 13, 5535 (2005)  L small  Insensitivity to dimensional error

12. AWG/SOA Concept AWG  A passive device    4 SOAs  Active devices   4  InP Substrate

13. Conventional Approach: Epitaxial Re-Growth Epi-Layers for AWG SUBSTRATE Epi-Layers for AWG SUBSTRATE Epi-Layers for AWG SUBSTRATE Epi-Layers for SOAs 1st growth of epi-layers for AWG Selective area etch to substrate Re-growth of epi-layers for SOAs Technical Problems: Poor morphology for re-grown layers Vertical misalignment of AWG and SOA layers Rough AWG/SOA interface Low Yield

14. Ion-Induced Band Gap Modification HEAT ION MASK BARRIER QUANTUM WELL BARRIER DEFECT HIGH ENERGY IONS DIFFUSION ABSORPTION COEFFICIENT PHOTON ENERGY ABSORPTION EDGE BLUE SHIFT Advantage -No re-growth Disadvantage - Constraints on layers

15. AMPLIFIER BANDWIDTH Useful amplification range for Semiconductor Optical Amplifier (or Erbium-Doped Fiber Amplifier) SOA bandwidth is insufficient to cover all CWDM wavelengths 340 nm (1270 – 1610 nm)  = 20 nm

16. QUANTUM WELL INTERMIXING TO ADJUST AMPLIFICATION RANGE    4    4 ABSORPTION COEFFICIENT ABSORPTION EDGE BLUE SHIFT 4 3 2 1 Candidates Materials InGaAsP - conventional material, limited tuning range GaInNAs GaAsSb

17. GaSb MATERIALS FOR 1.5 MICRON DEVICES GaAs LATTICE CONSTANT IN ANGSTROMS 0.5 1.0 1.5 2.0 5.65.75.85.96.06.16.2 BAND GAP WAVELENGTH (MICRONS) 2.5 3.0 3.5 Substrate AlSb GaSb AlGaSb GaAsSb Candidates AlGaSb (nearly indirect band gap) GaSb Quantum Wells (indirect gap) GaAsSb Quantum Wells

18. GaASSb/AlSb Quantum Wells G.Griffiths, K.Mohanned, S.Subbana, H.Kroemer and J.L.Merz, Appl. Phys. Lett. 43, 1059 (1983) L Г X L Г X Adding Quantum Confinement Indirect band gap GaSb Adding As + Quantum Confinement GaAsSb

19. GaAsSb Quantum Wells Photoluminescence increases dramatically with As content x 10 GaSb Cap AlSb GaAs x Sb 1-x AlSb GaSb Substrate 60X Kost, Sun, Peyghambarian, Eradat, Selvig, Fimland, and Chow, Appl. Phys. Lett. 85, 5631 (2004).

20. GaAsSb Quantum Wells The shift is the largest for any quantum wells (in the telecom band)  = 140 nm,  E = 86 meV  = 195 nm,  E = 123 meV BORON ION IMPLANTATION (~300 keV, 3x10 13 cm -2 ) Sun, Peyghambarian, Kost, Eradat, Appl. Phys. Lett. 86, (2005)

21. Summary AWG for CWDM → Demonstrated first semiconductor AWGs for CWDM using an flexible “S-shape” Band Gap Modification for Heterogeneous Integration → GaAsSb/AlSb quantum wells show promise (enabling technology for PICs) (lower cost devices)

22. Future Directions Application for CWDM AWG - Combined wavelength and time-division multiplexing New materials for intermixing - GaInNAs - Sb quantum wells on InP © 1998 - 2005 Christian L. Deichert