Novel Integrable Semiconductor Laser Diodes

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1 Novel Integrable Semiconductor Laser Diodes J.J. Coleman University of Illinois Distinguished Lecturer Series IEEE Lasers and Electro-Optics Society

2 Definition of the Problem 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

3 Outline Engineering in the optical path - selective area epitaxy Integrable laser resonators Examples of lasers integrated with other optical devices Summary

4 Approaches to Wafer Engineering 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

5 Single Stripe Pattern SiO 2 SiO 2 field single open stripe stripe width mm

6 Selective Epitaxy Boundary Conditions N = constant boundary layer N x = 0 N x = 0 N y = 0 N y = g(x) N y = 0

7 Selective Epitaxy Boundary Conditions N = constant boundary layer N x = 0 N x = 0 N y = 0 N y = g(x) N y = 0

8 Simulation Results isoconcentration profiles thickness profile

9 Modeled and Experimental Data two stripe widths (50 and 125 mm) modeled (dashed) experimental (solid) Distance (µm)

10 Wide Stripe Impracticalities 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

11 Dual Stripe Pattern open field dual stripe stripe separation 2-5 µm stripe width 2-25 µm dimensions small with respect to a diffusion length

12 Dual Stripe Growth Enhancement µm 6 µm Relative Thickness 2 1 Enhancement Factor 2 1 w w Distance (µm) Oxide Stripe Width (µm)

13 Buried Heterostructure Growth Process a) buffer layer and lower confining layer b) selectively grown active layer c) upper confining layer and cap layer

15 Buried Heterostructure SEM Cross Section

16 Buried Heterostructure L-I Curves I th = 2.65 ma length = 760 µm I (ma) Current (ma)

17 980 nm Wavelength Control Dual oxide stripe width (µm) 9400 S = µm Element Number

18 1.55 mm Wavelength Control Masahiro Aoki et al. IEEE J. Quantum Electron. 29, 2088 (1993) 110 nm wavelength range uniform PL intensity uniform PL half-width

19 Basic Buried Heterostructure Building Block Engineered transition energy Automatic lateral optical waveguide Many relatively lossless coupling schemes are possible

Integrable Resonator Geometries Etched Fabry-Perot facets Scattering losses, verticality, flatness, coupling Corner reflectors and ring geometries

20 Integrable Resonator Geometries 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

waveguide or selective-area epitaxy buried heterostructure Deep

21 Tunable DBR Reflectors Igain IDBR 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

22 First-Order DBR Grating Lasers Power (mw) Linewidth (khz) Intensity (db) Current (ma) Wavelength(µm) 20 C First-order gratings formed with RIE Six grating periods studied Threshold currents below 10 ma and as low as 6 ma Slope efficiencies greater than 0.4 W/A Minimum emission linewidth about the same as the measurement resolution ~ 40 khz Inverse Power (mw -1 )

23 Spectra versus DBR Tuning Current Relative Intensity (db) I DBR = 20 C I gain = 40 ma 0 ma 50 ma 100 ma Wavelength (nm) 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

24 1.3 mm InGaAsP Ridge Waveguide DBR Lasers InP:p InP:u/p Gain Section 0.5 µm 0.2 µm First-Order DBR Grating InP:n InGaAsP etch stop layer InGaAsP MQW active region Intensity (db) Power (mw/facet) I th = 26.5 ± 0.26 ma η = ± W/A Current (ma) First-order gratings formed by chemically-assisted ion beam etching (CAIBE) Compressively-strained MQW active region SMSR of ~ 40dB Wavelength (µm)

25 Selection of Integrated Photonic Devices Laser - external modulator Laser - photodiode Lasers for remote sensing applications Multiple wavelength sources An eight-channel transmitter

26 Example: Integrated Laser-Modulator M. Aoki et al. IEEE J. Quantum Electron. 29, 2088 (1993)

27 Integrated Laser/External Modulator MQW DBR Laser DBR Gratings MQW EA Modulator Al 0.60 Ga 0.40 As:p 0.4 µm Al 0.60 Ga 0.40 As:n 1 µm GaAs substrate (a) λ LD λm SiO 2 SiO 2 Buried heterostructure laser grown by selective-area epitaxy Tunable DBR grating as part of the resonant cavity Blue-shifted electro-absorption modulator 150 µm (b)

28 Integrated Laser/External Modulator AR/HR Coated Uncoated Power (mw) Power (db) Wavelength (µm) Optical Power (dbm) 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

29 Integrated Laser-Photodiode AlGaAs-GaAs-InGaAs selective area epitaxy laser structure LD p-contact θ PD p-contact d LD n-contact GaAs:n + buffer PD n-contact semi-insulating GaAs substrate RIE etched laser facet PD redshifted by 150Å PD input facet angled by q four-up contacts for flip chip bonding mw/ma 8.9 µa/ma LD Current (ma)

30 DBR Lasers at nm Wavelengths In 80 Å Å Al 0.1 µm 0.3 µm 700 Å 1.0 µm Relative Intensity (db) (a) (b) (c) Output Power (mw) C λ = 934 nm λ = 916 nm 10 λ = 924 nm I gain (ma) Wavelength (µm) Modified active region for shorter strained-layer emission wavelength Second-order gratings with three different periods Three emission wavelengths (915, 925, 935 nm)

31 Application to Remote Sensing DBR Current Source Variable Attenuator Gain Current Source Balanced Receiver ADC White Cell White cell filled with humid air Path length 85 m Absorption (%) TE Cooler Mounted Laser Coarse tuning with temperature, fine tuning with DBR current Lorentzian fit to the data within 6% of HITRAN values for halfwidth -0.7 center wavelength = nm Wavenumber (1/cm)

32 Dual (Redundant) Source Channel 1 V 2 =-2 V 2 =0 V 2 =0 Channel 2 V =0 1 V =-2 1 V = Wavelength (µm) Current (ma)

33 Multiple Wavelength Laser Conventional growth for straincompensated MQW active region Selectively-grown passive regions Grating pitches adjusted for 4 nm spacing A. Talneau et. al. Photon. Technol. Lett. 11, 12 (1999)

34 Dual Wavelength DBR Laser L 2 L 1 Λ2 Λ 1 Gain Section 1 µm Single ridge waveguide output aperture Separate grating elements Three grating period combinations Sample L 1 L

35 Dual Wavelength DBR Laser CW, 15 C CW, 15 C I = 45 ma (a) 42 ma Intensity (10 db/div) (b) Intensity (Log scale) 37 ma. 35 ma 32 ma (c) 30 ma Wavelength (µm) Wavelength (µm) Single mode operation (~30 db SMSR) Dual operation over ma range Sample Dl

36 Example: 8-Channel Transmitter C.H. Joyneret al. IEEE Photonics Technol. Lett. 7, 1013 (1995)

37 Summary Precise control of emission wavelengths can be obtained by selective area epitaxy The DBR reflector is a good candidate for an integrable resonator High performance novel integrated photonic devices are possible

38 I would like to thank. M. Aoki (Hitachi) J.A. Dantzig (Illinois) P.D. Dapkus (USC) J.G. Eden (Illinois) C. Jagadish (ANU) A.M. Jones (Illinois) C.H. Joyner (Lucent) S.M. Kang (Illinois) R.M. Lammert (Ortel) P.V. Mena (Illinois) M.L. Osowski (Illinois) S.D. Roh (Illinois) G.M. Smith (ATMI) T. Tanbun-Ek (Lucent) W.T. Tsang (Lucent) G.S. Walters (LEOS)

Optoelectronics ELEC-E3210

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