Semiconductor Lasers Leture 12-01-09

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    Semiconductor lasers

    by: Khanh Kieu

    (12/01/2009)

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    Outlines

    Introduction

    Semiconductor laser: basics

    Types of semiconductor laser

    High power semiconductor lasers

    Applications of semiconductor lasers

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    Introduction

    Laserfocusworld.com

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    Introduction

    First demonstration in 1962 by Robert N. Hall(homojunction diodes)

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    Introduction

    First CW laser diode operatingat room temperature wasdemonstrated in 1970 byZhores Alferov(Double heterostructure )

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    Semiconductor laser: doping

    As Ga As

    SeGa

    As As

    Ga

    Ga

    e

    As Ga As

    ZnGa

    As As

    Ga

    Ga

    h

    n-doped p-doped

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    p-n junction

    http://upload.wikimedia.org/wikipedia/commons/4/48/PnJunction-PV-E.PNG
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    Homojunction diode laser

    p+ n+

    EF n

    (a)

    Eg

    Ev

    Ec

    Ev

    Holes in VB

    Electrons in CB

    Junction

    ElectronsE

    c

    p+

    Eg

    V

    n+

    (b)

    EF n

    eV

    EF p

    The energy band diagram of a degenerately dopedp-n with no bias. (b) Banddiagram with a sufficiently large forward bias to cause population inversion andhence stimulated emission.

    Inversionregion

    EF p

    Ec

    Ec

    eVo

    1999 S.O. Kasap, Optoelectronics (Prentice Hall)

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    Homojunction diode laser

    Population Inversion: More electrons in the CB at energiesnear Ec than electrons in VB near Ev

    The region where the population inversion occurs develops alayer along the junction called an inversion layeror activeregion

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    Homojunction diode laser

    LElectrode

    Current

    GaAs

    GaAsn+

    p+

    Cleaved surface mirror

    Electrode

    Active region(stimulated emission region)

    A schematic illustration of a GaAs homojunction laserdiode. The cleaved surfaces act as reflecting mirrors.

    L

    1999 S.O. Kasap, Optoelectronics (Prentice Hall)

    An adequate forward bias is

    required to inject carriersacross the junction toinitiate population inversionThe process is calledinjection pumping.

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    Homojunction diode laser

    Typical output optical power vs. diode current (I) characteristics and the correspondingoutput spectrum of a laser diode.

    Laser

    LaserOptical Power

    Optical Power

    I0

    LEDOptical Power

    Ith

    Spontaneous

    emission

    Stimulated

    emission

    Optical Power

    1999 S.O. Kasap, Optoelectronics (Prentice Hall)

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    Homojunction diode laser

    The drawback of a homojunction diode laser is the high thresholdcurrent density therefore it is restricted to operating at very lowtemperatures or pulsed mode

    1000 A/cm2 at 77 K temperatures

    100 000 A/cm2 at 300 K temperatures

    Solution Double heterostructure laser

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    Double heterostructure diode laser

    1. Carrier confinement:Confine the injected electrons and holesto a narrow region about the junction. This requires less currentto establish the required concentration of electrons for

    population inversion.

    2. Photon confinement:Construct a dielectric waveguide aroundthe optical gain region to increase the photon concentration andelevate the probability of stimulated emission. This reduces thenumber of electrons lost traveling off the cavity axis.

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    Double heterostructure diode laser

    Refractiveindex

    Photondensity

    Active

    region

    n ~ 5%

    2 eV

    Holes in VB

    Electrons in CB

    AlGaAsAlGaAs

    1.4 eV

    Ec

    Ev

    Ec

    Ev

    (a)

    (b)

    pn p

    Ec

    (a) A doubleheterostructure diode hastwo junctions which arebetween two differentbandgap semiconductors(GaAs and AlGaAs).

    2 eV

    (b) Simplified energy

    band diagram under alarge forward bias.Lasing recombinationtakes place in the p-GaAs layer, theactive layer

    (~0.1 m)

    (c) Higher bandgapmaterials have a

    lower refractiveindex

    (d) AlGaAs layersprovide lateral opticalconfinement.

    (c )

    (d)

    1999 S.O. Kasap, Optoelectronics (Prentice Hall)

    GaAs

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    Double heterostructure diode laser

    Heterostructure

    Monostructure

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    Double heterostructure diode laser

    1. Due to the thin p-GaAs layer a minimal amount of current is requiredto increase the concentration of injected carriers. This is how thethreshold current for population inversion and optical gain is reduced

    2. The semiconductor with a wider bandgap (AlGaAs) will also have alower refractive index than GaAs. This difference in refractive index iswhat establishes an optical dielectric waveguide that ultimatelyconfines photons to the active region

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    Double heterostructure diode laser

    Schematic illustration of the the structure of a double heterojunction stripecontact laser diode

    Oxide insulator

    Stripe electrode

    SubstrateElectrode

    Active region where J>Jth.

    (Emission region)

    p-GaAs (Contacting layer)

    n-GaAs (Substrate)

    p-GaAs (Active layer)

    Current

    paths

    L

    W

    Cleaved reflecting surfaceEllipticallaser

    beam

    p-AlxGa

    1-xAs (Confining layer)

    n-AlxGa

    1-xAs (Confining layer)

    12 3

    Cleaved reflecting surface

    Substrate

    1999 S.O. Kasap, Optoelectronics (Prentice Hall)

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    Types of semiconductor lasers

    Quantum well lasers

    Quantum cascade lasers

    Distributed feedback lasers

    External-cavity diode lasers

    VCSELs

    VECSELs

    OPSELs

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    DFB semiconductor lasers

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    External-cavity diode lasers

    http://www.rp-photonics.com

    Widely tunable single frequency laser

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    OPSELs

    (Credit: D. Payne)

    Y. Kaneda at OSC

    E i i l h f i

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    Emission wavelengths of various

    types of laser diodes

    http://www.rp-photonics.com

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    High power diode lasers

    Credit: Jeff Hecht

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    High power diode lasers

    Ralf Ostendorf, Fraunhofer Institute

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    Stacked-bar diode laser

    Newport illustration

    Laserline, GmbH, Germany

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    Stacked-bar diode laser

    Credit: Jeff Hecht

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    Diode laser: Applications

    Telecommunication

    Data storage

    Material processing

    Laser pumping

    Medicine

    Laser printers, bar-code readers

    ~ 1 Billion units sale per year!

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    High power diode laser

    Credit: Jeff Hecht

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    Diode laser for pumping

    Credit: Jeff Hecht

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    Industrial applications

    Credit: Jeff Hecht

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    Industrial applications

    Dilas photo

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    Industrial applications

    Credit: Jeff Hecht

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    Medical applications

    Credit: Jeff Hecht

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    Thank you!