Semiconductor lasers

					Semiconductor lasers



      by: Khanh Kieu




        (12/01/2009)
                 Outlines

• Introduction

• Semiconductor laser: basics

• Types of semiconductor laser

• High power semiconductor lasers

• Applications of semiconductor lasers
Introduction




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



First CW laser diode operating
 at room temperature was
demonstrated in 1970 by
Zhores Alferov
(Double heterostructure )
 Semiconductor laser: doping


As        Ga   As   As        Ga   As


      e                   h
Ga        Se   Ga   Ga        Zn   Ga




As        Ga   As   As        Ga   As



     n-doped             p-doped
p-n junction
             Homojunction diode laser
                 p+         Junction   n+
Ec

       Eg                                                                p+                 n+
                                               eV o           Ec                                           EF n
                                                                        In v ers io n
                                                                            reg io n             Ec
Ev                                                                 Eg
            Ho les in V B                              EF n
EF p                               Electro ns in C B                                                  eV
            Electro ns
                                                       Ec
                                                                                                           EF p


                                                       Ev
        (a)                                                    (b )



                                                                                        V

       The energy band diagram of a degenerately doped p-n with no bias. (b) Band
       diagram with a sufficiently large forward bias to cause population inversion and
       hence stimulated emission.
       © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Homojunction diode laser




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

The region where the population inversion occurs develops a
layer along the junction called an inversion layer or active
region
             Homojunction diode laser
                                                                 Current

                                Cleaved surface mirror


An adequate forward bias is                                           L
required to inject carriers                                                                 Electrode
                                                           p+                    GaAs
across the junction to
                                                                           L
initiate population inversion
The process is called
injection pumping.                                          n+                       GaAs
                                                                                            Electrode

                                                                        Active region
                                                                        (stimulated emission region)

                                A schematic illustration of a GaAs homojunction laser
                                diode. The cleaved surfaces act as reflecting mirrors.
                                © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
              Homojunction diode laser
                                                                  Optical P ower   Laser
                    Optical Power


  Optical P ower   LED
                                                     Stimulated
                                                     emission                         λ
                                                                  Optical P ower   Laser
                              Spontaneous
                      λ       emission

                                                          I
                          0
                                             Ith
                                                                                      λ




Typical output optical power vs. diode current (I) characteristics and the corresponding
output spectrum of a laser diode.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
       Homojunction diode laser

The drawback of a homojunction diode laser is the high threshold
current density therefore it is restricted to operating at very low
temperatures or pulsed mode

   1000 A/cm2 at 77 K temperatures

   100 000 A/cm2 at 300 K temperatures




Solution → Double heterostructure laser
Double heterostructure diode laser

  1.   Carrier confinement: Confine the injected electrons and holes
       to a narrow region about the junction. This requires less current
       to establish the required concentration of electrons for
       population inversion.

  2.   Photon confinement: Construct a dielectric waveguide around
       the optical gain region to increase the photon concentration and
       elevate the probability of stimulated emission. This reduces the
       number of electrons lost traveling off the cavity axis.
Double heterostructure diode laser
                                                                        (a) A double
                           n         p                  p               heterostructure diode has
                                                                        two junctions which are
          (a)            AlGaAs    GaAs            AlGaAs               between two different
                                                                        bandgap semiconductors
                                  (~0.1 µm)                             (GaAs and AlGaAs).
             Electrons in CB                                   Ec
                                                  ∆Ec                   (b) Simplified energy
                 Ec
                                                            2 eV
                                                                        band diagram under a
                  2 eV
                                               1.4 eV                   large forward bias.
                                                                        Lasing recombination
          (b)                                                      Ev   takes place in the p-
                 Ev                                                     GaAs layer, the
                                                                        active layer
                                            Holes in VB

   Refractive                                                           (c) Higher bandgap
   index                                                                materials have a
          (c )                     Active       ∆n ~ 5%                 lower refractive
                                   region                               index
   Photon
   density
                                                                        (d) AlGaAs layers
                                                                        provide lateral optical
          (d)                                                           confinement.


       © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Double heterostructure diode laser


 Monostructure




 Heterostructure
Double heterostructure diode laser

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

 2. The semiconductor with a wider bandgap (AlGaAs) will also have a
    lower refractive index than GaAs. This difference in refractive index is
    what establishes an optical dielectric waveguide that ultimately
    confines photons to the active region
Double heterostructure diode laser
                                               Cleaved reflecting surface
                                                                      W



                                                                 L
                         Stripe electrode

                 Oxide insulator
      p-GaAs (Contacting layer)
 p-Al xGa1-xAs (Confining layer)
          p-GaAs (Active layer)
 n-Al xGa1-xAs (Confining layer)            2     1          3
                                            Current
                                                                                 Substrate
             n-GaAs (Substrate)
                                               Substrate
                                            paths
                                                                            Electrode

                              Elliptical                      Cleaved reflecting surface
                                laser
                                                     Active region where J > Jth.
                               beam
                                                     (Emission region)


     Schematic illustration of the the structure of a double heterojunction stripe
     contact laser diode
     © 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Types of semiconductor lasers

   • Quantum   well lasers

   • Quantum cascade lasers

   • Distributed feedback lasers

   • External-cavity diode lasers

   • VCSELs

   • VECSELs

   • OPSELs
DFB semiconductor lasers
External-cavity diode lasers




   Widely tunable single frequency laser


                                  http://www.rp-photonics.com
                   OPSELs




                        (Credit: D. Payne)


Y. Kaneda at OSC
Emission wavelengths of various
     types of laser diodes




                       http://www.rp-photonics.com
High power diode lasers




                          Credit: Jeff Hecht
High power diode lasers




                 Ralf Ostendorf, Fraunhofer Institute
       Stacked-bar diode laser




Newport illustration




                       Laserline, GmbH, Germany
Stacked-bar diode laser




                          Credit: Jeff Hecht
     Diode laser: Applications

• Telecommunication


• Data storage

• Material processing     ~1   Billion units sale per year!

• Laser pumping

• Medicine

• Laser printers, bar-code readers

•…
High power diode laser




                         Credit: Jeff Hecht
   Diode laser for pumping




Credit: Jeff Hecht
Industrial applications




                          Credit: Jeff Hecht
Industrial applications




                          Dilas photo
Industrial applications




                          Credit: Jeff Hecht
Medical applications




                       Credit: Jeff Hecht
Thank you!

				
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posted:4/23/2012
language:English
pages:34