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Semiconductor lasers: What's next? Grigorii Sokolovskii Ioffe Institute, St Petersburg, Russia

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Outline Applications of semiconductor lasers. ‘Revolution’ of light. Basic principles of laser operation (only to remind). Absorption and gain in semiconductors, inversion of population and conditions for it's achievement. Rate equations. Lasing threshold. Laser efficiency. Modulation of the laser signal. Gain clamping. Fiber-optical applications. DFB and DBR lasers. VCSELs and VECSELS Waveguide in LD structure. Modes of the waveguide. Beam-propagation (‘beam-quality’) parameter M 2 and how to measure it. Achieving maximum power density with LDs. Interference focusing of LD beams. What’s next? New applications and problems to solve.

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Applications of LDs Telecoms Data reading/recording Laser printing Pumping of the solid-state and fiber lasers ‘Direct’ applications: cutting/drilling/welding Biology and medicine $7B → $1T

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Applications of LDs Telecoms Data reading/recording Laser printing Pumping of the solid state and fiber lasers ‘Direct’ applications: drilling/cutting etc Biomedicine

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1. Surface and edge-emitting (e.g. VCSELs, VECSELs, GCSELs, etc and ‘striped’ LDs) 2. Broad and narrow area Broad: high power, poor beam quality; Narrow: low power, better beam quality Types of LDs

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LDs for Mid-IR ( nm) In Mid Infrared spectral range nm lies strong absorption bands of such important gases and liquids as: CH 4, H 2 O, CO 2, CO, C 2 H 2, C 2 H 4, C 2 H 6, CH 3 Cl, OCS, HCl, HOCl, HBr, H 2 S, HCN, NH 3, NO 2, SO 2, glucose and many others.

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Optical tweezers Waveguide: light in matter ‘Inverse’ waveguide: matter in light – optical tweezers

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Laser projectors Microvision SHOWWX Laser Pico Projector MacWorld 2010 Best of Show award Samsung H03 Pico Pocket Sized LED Projector Aiptek PocketCinema T30

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Basic principles of laser operation Gain + Feedback

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Basic principles of laser operation Lasing threshold

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Spontaneous and stimulated emission E1E1 E2E2 E1E1 E2E2 Spontaneous Stimulated Absorption Basic principles of laser operation

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‘Negative’ temperature Gain: inversion of population Basic principles of laser operation E1E1 E2E2 T n

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3- & 4-level systems E1E1 E2E2 E3E3 E1E1 E2E2 E3E3 E4E4

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Laser diodes: Some basics Vibronic laser E1E1 E2E2 E3E3 E4E4 EvEv EcEc EgEg Diode laser E c – conduction band E v – valence band E g – energy gap

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QWs and QDs are the ‘artificial atoms’ :) Laser diodes: Some basics

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Light generation and absorption in semiconductors ‘Golden’ rule of Quantum mechanics: Absorption probability: Radiation probability: The total radiation probability:

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Light generation and absorption in semiconductors What is the condition for stimulated emission? Inversion of population:

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Inversion of population Forward-biased p-n-junction is both the source for the inversion of population and for the name of the “laser diodes”

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Stimulated and spontaneous emission rate: 3D ћω=E r r st r sp fcefce fvhfvh ρ(E) -ρ(E) T↑ EgEg

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Density of states: Low-Dimensional vs 3D Bulk semiconductor Quantum Well Quantum Wire Quantum Dot

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Evolution of the threshold current density: from 3D to Low-Dimensional

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Density of states: Low-Dimensional vs 3D Bulk semiconductor ~1 kA/cm 2 Quantum Well ~100 A/cm 2 Quantum Wire ??? Quantum Dot ~10 A/cm 2

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3D E r st QDs vs 3D Lower threshold Lower temperature dependence of the laser parameters But: at low QD density threshold may NOT be reached Narrow spectrum for IDENTICAL QDs But: broad spectrum for inhomogeneously broadened QDs QD

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k0nfk0nf kzkz kxkx ncnc nsns nfnf f kxfkxf kxfkxf φcφc φsφs Laser diodes: The waveguide E g ↓ ↔ n↑

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x k0nfk0nf z k0nsk0ns k0nck0nc x k0nfk0nf z k0nsk0ns k0nck0nc Modes of the waveguide

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Composing Rate Equations The simplest model J is the average pump current density n is the average carrier concentration in the active region S is the average photon concentration (average intensity) Carriers, steady-state: Pump rate = Recombination rate Carriers, time-dependent:

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Composing Rate Equations Photons, steady-state: Total radiation probability = Recombination rate Photons, time-dependent: Г - confinement factor β - spontaneous emission factor n t - transparency concentration A - linear gain coefficient τ s - spontaneous recomb. time τ p - photon lifetime

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Lifetimes Spontaneous:Photon lifetime: τ s ~ 1 nsif n ~ n t, then: τ p ~ 1 ps L ↑ → τ p ↑ α in ↑ → τ p ↓ R ↑ → τ p ↑

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Rate Equations for QD LD GS ES τ WG τ WE τEτE τGτG …where f is the filling factor

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Steady-state solution of the Rate Eqs 0

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J J S J th n n th β↑β↑ β↑β↑ ntnt

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Lasing Threshold R↑R↑R↑R↑ R↑R↑

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Threshold vs Temperature

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Laser efficiency Pumping efficiency Internal quantum efficiency EvEv EcEc EvEv EcEc EvEv EcEc E vv free-carriersAuger

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Laser efficiency differential efficiency: R↑R↑ I P I th ηdηd

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Laser efficiency Measuring IQE: L 1/η d Lasing efficiency: I P I th U r serial ↑ I P I th η (1/2) r serial ↑

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Turn-on delay t J J th t S t S ΔtΔt ?

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Turn-on delay If J = 2J th, τ s = 1 ns: Δt = 0.7 ns t J J th Non-return-to-zero (NRZ) modulation t S ΔtΔt t J J th t S RZ modulation

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Turn-on delay of QD LDs J/J th ΔtΔt CτpCτp Non-QD laser diode: QD LD:

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Small-signal moduation

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Amplitude-freq. response of photons

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Phase-frequency response of photons

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Amplitude-freq. response of carriers

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Phase-frequency response of carriers

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π/2 π/2 shift in the phase-frequency responses means the energy flow between the photons and carriers (similar to the kinetic and potential energy in pendulum) which is called ‘relaxation oscillations’. Phase-frequency response

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n(t) S(t) Relaxation oscillations NB: no relaxation oscillations in QD lasers! Typical explanations: 1) QD LDs are too fast; 2) QD LDs are too slow

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Eye-diagram

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Gain clamping (gain saturation) Laser diodes at extremely high pumping: pulsed vs CW I P I th I CW pulsed I P I th CW pulsed

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Gain clamping Quasi-analytical: Asymptotical: (I-I th )/I th Concentration. N ε =0 S

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Gain clamping J S J th

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=0, =0 =2 10 23, =1 =0 Gain clamping Frequency, GHz Response, dB

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Application to the fiber optical communications

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Double power: Double distance? Typical attenuation: -20dB if:

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Wavelength / Time Division Multiplexing

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Wavelength Selection Gain Chip: 4mm length, 5µm wide waveguide 10 layers InAs QDs, grown on GaAs substrate waveguide angled at 5 o facets AR coated: R angled < R front ~ 2 ·10 -3

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Distributed feedback (DFB) laser diodes p-InP n-InP p-InP n-InP p-InP n-InP p-InP n-InP

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Distributed Bragg reflector (DBR) LDs

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Diffraction grating in a waveguide x k0nfk0nf z k0nsk0ns k0nck0nc x z k0Nmk0Nm

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x z k0Nmk0Nm x z k0Nmk0Nm 1 st order2 nd order

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Distributed feedback Р.Ф.Казаринов, Р.А.Сурис, ФТП, 1972, т.6(7), с H.Kogelnik, C.V.Shank, Journal of Appl. Phys, 1972, v.43(5), pp

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Distributed feedback

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Vertical-cavity surface-emitting lasers VCSELs (AlGa)O xy ,0 0,2 0,4 0,6 0,8 1,0 Radial distance [nm] N o r m a l i z e d o p t i c a l f i e l d i n t e n s i t y C u r r e n t d e n s i t y p r o f i l e s E 2 LP 01 J

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Vertical-extended-cavity surface-emitting lasers (VECSELs) A.Mooradian, “High brightness cavity-controlled surface emitting GaInAs lasers operating at 980 nm”, Proceedings of the Optical Fiber Communications Conference, March 2001

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Power Beam quality Spectral quality LD ‘bar’ Problems of LDs Broad-stripe LD Power density &intensity gradient

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Optical confinement. Double confined Separately confined

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Modes of the waveguide x k0nfk0nf z k0nsk0ns k0nck0nc x k0nfk0nf z k0nsk0ns k0nck0nc

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Optical confinement.

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Managing mode structure with optical confinement

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Higher power does not mean higher power density … :( Power density and LD power ‘Not impossible’ near-field distribution of the broad-stripe LD Higher power: Higher modes Spots (filaments) Astigmatism

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Multi-moded and ‘spotted’ (filamented) radiation is difficult (and sometimes impossible) to focus! Multimode LDs

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Propagation of the Gaussian beam exp(-r 2 /w 2 ) Gaussian beams Gaussian exp(-r 2 /w 2 ) is the most dense distribution. Therefore, Gaussian beams are the beams of the highest ‘quality’. Even the ‘ideal’ power density is limited due to the quantum mechanical uncertainty principle ΔpΔx=h

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exp(-r 2 /w 2 ) Beam waist size on the level of 1/e 2 (86%) Propagating beam size (beam diameter) Wavefront curvature Raleigh range (distance of √2-fold increase of diameter of the beam) Gaussian beams

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LD beams: From multi-mode to quasi-Gaussian exp(-2r 2 /w 2 ) M 2 – beam propagation parameter

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Second moment properties Definition of the second moment (or variance): Leads to a universal, rigorous parabolic propagation rule: which holds for any arbitrary real beam (Gaussian or non-Gaussian, spatially coherent or incoherent) in free space Can develop such a universial propagation rule only for the second-moment beam width definition Actually holds true for propagation through arbitrary paraxial optical systems as well A.E.Siegman, How to (Maybe) Measure Laser Beam Quality, OSA Annual Meeting, 1998

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Second-moment-based beam width definition For a zero-mode Gaussian beam the standard deviation is related to the Gaussian spot size w by Therefore for arbitrary real beams similar beam width definitions can be adopted: These second-moment-based beam widths will propagate exactly quadratically with distance in free space. For any arbitrary beam (coherent or incoherent), one can then write using the second-moment width definition A.E.Siegman, How to (Maybe) Measure Laser Beam Quality, OSA Annual Meeting, 1998 using this width definition,

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Basic properties of the M 2 parameter M 2 is a “times-diffraction-limited” parameter based on measured near and far field second-moment beam widths M 2 =1 for a zero-mode Gaussian beams Arbitrary real beam widths can then be fully described by six parameters: A.E.Siegman, How to (Maybe) Measure Laser Beam Quality, OSA Annual Meeting, 1998 Requires time-averaged intensity measurements only. NO phase or wavefront measurements!

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Quasi-Gaussian beams Beam waist size on the level of 1/e 2 (15%) Propagating beam size Wavefront curvature Raleigh range exp(-2r 2 /w 2 )

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Focusing of the quasi-Gaussian beams exp(-2r 2 /w 2 )

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How to measure M 2 parameter? follows: with Measurable: beam diameter as a function of propagation length:

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How to measure M 2 parameter? M 2 is different at fast and slow axis!

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How to measure M 2 in your lab? 1. Use house-built setup with any CAL or CAD software (e.g. MicroCal Origin) 2. Use specialist hard- and soft-ware (e.g. DataRay) Approx. time for one laser: 3 hrs (when you’ve got some experience) Approx. time for one laser: 15 mins Standard: ISO 11146

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Measuring M 2 parameter Fast axis Slow axis M 2 varies with current! For broad-stripe laser diodes, M 2 at slow axis is an order of magnitude higher comparing to that at slow axis… M 2 can be NOT measurable! 100 um high-power LD, 1.06 um

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Don’t call it ‘Gaussian’ before you are sure… Light emitting diode Lumiled, 0.63 um 350 mA, M 2 =500

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Don’t overestimate the M 2 parameter of the ‘nasty’ LD beams… Narrow stripe LD, 1.06 um, 100 mA, M 2 =4

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Interference focusing (Generation of Bessel beams) Prism: interference of plane waves Conical lens (axicon): interference of conical waves J. Durnin // J. Opt. Soc. Am. 1987, A 4, P B.Ya.Zel’dovich, N.F.Pilipetskii // Izvestia VUZov, Radiophysics, 1966, 9(1), P cos 2 (x) J 0 2 (x)

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Bessel beams vs Gaussian beams Axicon Bessel beams are ideal for optical tweezers, micromanipulation and lab-on-a-chip applications Lense distance power density distance power density Bessel beams are typically generated with vibronic lasers

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VCSEL DO-701d, Innolume GmbH, axicon 170 0, I=1 mА G.S.Sokolovskii et al. // Tech Phys Lett, 2008, 34(24) Main problem: Low coherence??? Interference focusing with Laser Diodes

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Interference focuing with High-power Laser Diodes Main problem: Poor beam quality Radiation wavelength λ = 1.06 µm. Axicon apex angle Central lobe size d 0 = 10 µm. Multimode FilamentedOblique

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Axicon apex angle Bessel beams from the broad-stripe LD G.S.Sokolovskii et al. // Tech Phys Lett, 2010, 36(1) 22-30

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Propagation length of Bessel beams generated from Laser Diodes M 2 LD =10-50 M 2 LED =

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Achieving ‘non-achievable’ intensity gradients with broad-stripe LDs Broad-stripe LD 1.06 um, М 2 =22, interference focal spot dia = 4 um LED 0.63 um, М 2 =200, interference focal spot dia = 6 um

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(Fiber axicon (Fiber axicons manufactured by Maria Farsari and colleagues, FORTH) Fiber axicons 1

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Applications of LDs What’s next? Data reading/recording Telecoms Laser printing ‘Direct’ applications: cutting/drilling/welding Pumping of other lasers and frequency conversion Biology and medicine $7B → $1T

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Laser projectors: Green Light? Microvision SHOWWX Laser Pico Projector MacWorld 2010 Best of Show award Samsung H03 Pico Pocket Sized LED Projector Aiptek PocketCinema T30 No need for focusing!

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From laser printers to 3D printing

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Telecoms/Datacoms: Silicon Photonics?

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On-chip Datacoms: Size matters? Energy matters! What counts? fJ/bit! Polariton Laser?

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Automotive applications: laser ignition?

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Optical tweezers: lab-on-a-chip? Waveguide: light in matter ‘Inverse’ waveguide: matter in light – optical tweezers

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‘Lab’ ‘Chip-in-a-lab’‘Lab-on-a-chip’ ‘Laboratory’ Lab-on-a-chip

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Fluorescent microscopy: need for color?

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Extracellular heat shock proteins (Hsp70) and ceramides are known to possess high adjuvant activity for cancer vaccines and low- immunogenic vaccines against dangerous infections. Hsp70 and ceramide can activate receptors of the innate immunity (TLR4) and boost protective immune response reactions against abiotic stress factors and biopathogens. Generation of Heat-Shock Proteins and Ceramides with Laser Diodes Laser diode Power supply Hsp70 Pulse duration 100 ns Frequency 10 kHz Wavelength 1060 nm

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LDs for Mid-IR (1600 nm nm) In Mid Infrared spectral range nm lies strong absorption bands of such important gases and liquids as: CH 4, H 2 O, CO 2, CO, C 2 H 2, C 2 H 4, C 2 H 6, CH 3 Cl, OCS, HCl, HOCl, HBr, H 2 S, HCN, NH 3, NO 2, SO 2, glucose and many others. 16 μm

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Thank you for your attention! Applications of semiconductor lasers. ‘Revolution’ of light. Basic principles of laser operation (only to remind). Absorption and gain in semiconductors, inversion of population and conditions for it's achievement. Rate equations. Lasing threshold. Laser efficiency. Modulation of the laser signal. Gain clamping. Fiber-optical applications. DFB and DBR lasers. VCSELs and VECSELS Waveguide in LD structure. Modes of the waveguide. Beam-propagation (‘beam-quality’) parameter M 2 and how to measure it. Achieving maximum power density with LDs. Interference focusing of LD beams. What’s next? New applications and problems to solve.

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