Photonic Device and System Laboratories Department of Electrical and Computer Engineering Tunable semiconductor lasers Thesis qualifying exam presentation by Chuan Peng B.S. Optoelectronics, Sichuan University(1994) M.S. Physics, University of Houston(2001) Thesis adviser: Dr. Han Le Submitted to the Department of Electrical and Computer Engineering in partial fulfillment of the requirements for the Doctor of Philosophy At the University of Houston Oct. 2003 Photonic Device and System Laboratories Department of Electrical and Computer Engineering Outline 1. Introduction and motivation 2. Semiconductor laser physics 3. Tunable laser fundamentals 4. Technologies for tunable lasers 5. Summary and Conclusion Photonic Device and System Laboratories Department of Electrical and Computer Engineering Introduction: Laser History Milestones: • 1917 Origin of laser can be traced back to Einstein's treatment of stimulated emission and Planck’s description of the quantum. • 1951 Development of the maser by C.H. Townes. • 1958 Laser was proposed by C.H. Townes and A.L. Schawlow • 1960 T.H. Maiman at Hughes Laboratories reports the first laser: the pulsed ruby laser. • 1961 The first continuous wave laser was reported (the helium neon laser). • 1962 First semiconductor laser Photonic Device and System Laboratories Department of Electrical and Computer Engineering Introduction: Laser types and applications Compact disk Basic Scientific Research Laser printer Spectroscopy Free Electron laser (FEL) Scientific Optical disc drives Nuclear Fusion Applications Optical computerLead-salt Sb CoolingAs AtomsN Bar code scanner Short Pulses Common Daily Holograms againstSemiconductor forgery lasers ApplicationsGas Lasers Fiber optic communications Ti-Sphire Free space communications Ruby X-ray lasers Laser shows Surgery:Nd:YAG Liquid Lasers Holograms Alexandrite•Eyes Kinetic sculptures •GeneralOrganic Dye •DentistryAg (Gold) vapor Medical Solid Lasers •DermatologyCu vapor Applications Diagnostic fluorescence Laser range-finder Soft lasers Ar+ Military Target designation Special Lasers Kr+ Applications Laser weapons N2 Laser FIRblinding lasers CO 2 HF He-Ne He-Ne He-Cd Far infaredInfared Measurements Visible Ultraviolet Soft x-rays Energy Transport Straight Lines Industrial Special Laser Gyroscope Material Processing Applications Applications Fiber Lasers30 µµµm 10 µµµm 3µµµmSpectral 1µµµm Analysis300nm 100nm 30nm 10nm λλλ Energy Photonic Device and System Laboratories Department of Electrical and Computer Engineering Introduction: Semiconductor Laser What made the semiconductor lasers the most popular light sources ? • Small physical size • Electrical pumping • High efficiency in converting electric power to light • High speed direct modulation (high-data-rate optical communication systems) • Possibility of monolithic integration with electronic and optical components to form OEICs (optoelectronic integrated circuits) • Optical fiber compatibility • Mass production using the mature semiconductor-based manufacturing technology. Photonic Device and System Laboratories Department of Electrical and Computer Engineering Motivations Application interests in tunable mid-IR semiconductor lasers: • Spectroscopy - Single frequency mode, tunable • Environmental sensing and pollution monitoring - Lidar - Requires Ruggedness, Correct Wavelength • Industrial Process Monitoring - Requirements similar to Environmental Monitoring • Medical Diagnostics - Breath analysis; Non-invasive Glucose monitoring, Cancer Detection, etc. • Military and law enforcement • Optical communication A key requirement: Broad, continuous wavelength tunability Photonic Device and System Laboratories Department of Electrical and Computer Engineering Outline 1. Introduction and motivation 2. Semiconductor laser physics 3. Tunable laser fundamentals 4. Technologies of tunable lasers 5. Conclusion Photonic Device and System Laboratories Department of Electrical and Computer Engineering Laser physics Mirror Active medium R1 R2 Laser output α G, i Oscillation condition is reached when ~ 2/1 2i β L Partially (R R ) e =1 transmitting 1 2 L mirror ~ β = µk − iα 2/ Loop ν = µ Gain 0 gain m mc 2/ L curve µ ∆ν∆ν∆ν =c/2nL=c/2 gL GL 1 1 g = α + ln | | th i 2L R R Real part: Threshold condition Laser 1 2 −α threshold (R R ) 2/1 e( g i )L =1 ννν 1 2 o Laser Longitudinal µ = π == output 2 k0 L 2m , (m 3,2,1 ,..) power modes Imaginary part: wavelength condition Linewidth ννν ννν ννν ννν ννν m-2 m-1 m+1 m+2 Frequency (v) Photonic Device and System Laboratories Department of Electrical and Computer Engineering Semiconductor laser physics • Threshold: carrier density, cavity loss • Wavelength: bandgap physics, or intraband energy level (QC) • Tunability: gain bandwidth: for interband D.H. and quantum well laser: Fermi Distribution for quantum cascade laser: energy level linewidth • Power and efficiency • Operating temperature Photonic Device and System Laboratories Department of Electrical and Computer Engineering Semiconductor laser band diagram (a) Equilibrium band structure Probability of occupancy ∆∆∆ 3 Ec E c ρ (E) 2 c n(E) N-GaAlAs p-GaAs P-AlGaAs 1 E F E Ef v 0 f(E) p(E) ρ Energy(eV) -1 v (E) -2 0.0E+00 1.0E+21 2.0E+21 3.0E+21 4.0E+21 (b) Forward biased -3 -1 Efc ∆∆∆Ec Density (cm eV ) EcEc Carrier concentration in each band: N-GaAlAs P-AlGaAs ρ c (Ec ) Ev n = dE ∫ − + c Efv exp[( Ec EFC /) KT ] 1 ρ (E ) p = v v dE ∫ − + v Energy band diagrams for a double hetero- exp[( Ev EFV /) KT ] 1 structure laser (a) unbiased, (b) forward biased Photonic Device and System Laboratories Department of Electrical and Computer Engineering Threshold current density ∂n J Carrier density rate equation: = D(∇2n) + − R(n) ∂t qd ∂n At steady state, = 0 ∂t Anr nonradiative process 2 J Bn spontaneous radiative rate So: R(n) = 3 qd Cn nonradiative Auger recombination R stimulated recombination that leads = + 2 + 3 + st R(n) Anr n Bn Cn Rst N ph to emission of light and it is proportional to the photon density N ph . n Below or near threshold: R(n) = τ e (n) qdn So, the threshold current density is J = th th τ e (nth ) Photonic Device and System Laboratories Department of Electrical and Computer Engineering Power and efficiency of semiconductor lasers If the current is above threshold, this leads to stimulated emission = + J Jth qdv g gth N ph Optical output power P out vs. injection current is determined by hν α P = v α VN hν = η m (I − I ) out g m ph i α +α th q m i External quantum efficiency is defined as: dP / dI α η = out = m η = η ln( /1 R) e hω α +α i i α + / q m i i L ln( /1 R) Photonic Device and System Laboratories Department of Electrical and Computer Engineering Quantum Well lasers Intersubband transition QC Energy eigenvalues for a particle confined in the quantum well are: h2 = + 2 + 2 E(n,kx ,k y ) En * (kx k y ) 2mn Inerband transition Density of states : >>> ρ = mci ci πh2 Lz Quantum wells are important in semiconductor lasers because they allow some degree of freedom in the design of the emitted wavelength through Compare with Heterostructure: 2m adjustment of the energy levels within the well by ρ (E) = 4π ( c ) 2/3 E 2/1 careful consideration of the well width. c h2 Photonic Device and System Laboratories Department of Electrical and Computer Engineering Quantum cascade lasers Advantages: • It doesn’t depends on material hω = E − E system bandgap making it easy to 3 2 make long wavelength lasers. • In a QCL, each electron can take participate in stimulated emission many times. • QC lasers in mid-IR region have Quantum cascade laser is unipolar intersubband now been demonstrated with CW device operations at room temperature. • It could be used as THz sources. The QC laser relies on only one type of carrier, making electronic transition between conduction band states arising from size quantization. Photonic Device and System Laboratories Department of Electrical and Computer Engineering Outline 1. Introduction and motivation 2. Semiconductor laser physics 3. Tunable laser fundamentals 4. Technologies of tunable lasers 5. Conclusion Photonic Device and System Laboratories Department of Electrical and Computer Engineering Existing Tunable Laser technologies •Distributed Feedback Bragg Grating (DFB) •Sampled grating Distributed Bragg Reflectors (DBR) •MEM-VCSEL •Grating coupled external cavity (ECLD) Photonic Device and System Laboratories Department of Electrical and Computer Engineering Existing Tunable Laser Structures (DFB) Wavelength (um) Cross section Intensity Intensity 4.420 4.440 4.460 4.480 4.500 4.520 wavelength (um) Intensity Intensity 4.420 4.440 4.460 4.480 4.500 4.520 wavelength (um) DFB laser structure. The grating is etched onto one of the cladding layers. Grating period is determined by ΛΛΛ=m λλλ/2. λλλ is the wavelength inside the medium. DFB MEMs tilt Wire Laser Array mirror bond stripes Optical fiber QDI Fujitsu Santur Tunable DFB laser arrays (a) Serial (b) Parallel (c) Mem mirror Photonic Device and System Laboratories Department of Electrical and Computer Engineering Existing Tunable Laser Structures (DBR) SG-DBR laser EAM Amplifier Front mirror Gain Phase Rear mirror Light output Q waveguide MQW active region Agility λ RFP ( ) Longitudinal modes λ λ R1( ) Front mirror Fabry-Perot cavity : λ λ • Modes with FP spacing B R (λ) Rear mirror 2 Sampled Bragg Reflectors : • Spectra with wider spacing and broader profile λ • Filters out 1 Bragg mode R( λ) λ • Increasing I dbr shifts filter profile to shorter Lasing mode λ Phase Section: Fine tuning Photonic Device and System Laboratories