Examples of Semiconductor Lasers

Quantum well lasers Advanced lasers: Quantum dot lasers

Next time: Advanced lasers QCLs Single mode lasers DFB lasers VCSELs

Reminder from last lecture: Design considerations for laser diode performance

• Low threshold current– low threshold can be generated by electronic devices which can be modulated at

high speed to provide a high speed modulation in the output(1) reducing the active layer thickness↣ Quantum-Well (~ 50 - 100 Å), Strained Quantum-Well

(2) cavity design

• Lateral confinement reduce the lateral dimension of the Fabry-Perot cavity (1) Stripe geometry (Gain-guided cavity)

(2) Buried heterostructures

• Selective Optical Cavity– to reduce the laser linewidth (1) Distributed Feedback (DFB) structures

(2) Vertical cavity surface emitting lasers (VCSELs)

Evolution of the threshold current of the semiconductor lasers

Quantum Well Laser

• Constant 2D density of states means a large concentration of electrons can easily occur at E1 (and holes at the minimum valence band energy)

• Population inversion occurs quickly without the need for a large current to bring a large number of electrons

• Benefits: Threshold current reduced, linewidth is narrower

Multiple Quantum Well (MQW) Laser

• Several single quantum wells are coupled into a “multiple quantum well (MQW)” structure.

• The significantly reduced temperature sensitivity of MQW lasers has been related to the staircase density of states distribution and the distributed electron and photon distributions of the active region.

• The optical confinement helps to contain the otherwise large losses from a narrow active region, leading to low threshold currents.

Bandgap engineering: Visible-UV-IR range

Red QW Laser DiodeDiagram of red GaInP DQW laser

Diagram showing the alloy composition through the layer structure of a two-well, separate confinement (AlyGa1−y)In1−xP quantum well laser. The vertical distance axis is not to scale: the wells are each about 6.5 nm wide, the y=0.5 waveguide core is about 200 nm thick, and the cladding layers are each about 1 μm thick.

Violet QW Laser Diode

Diagram of deep violet InGaN DQW laser structures

From: “Performance enhancement of deep violet indium gallium nitride double quantum well lasers using delta barrier close to electron blocking layer”, J. Nanophoton. 2012;6(1):063514-1-063514-12. doi:10.1117/1.JNP.6.063514.

Modes: longitudinal and transverse

Transverse modes

Longitudinal modes

Laser waveguides design for transverse confinement

• Vertical confinement• Lateral confinement

– Gain-guided– Index guided: ridges, ribs– Buried heterostructure lasers

Graded Index Separate Confinement Heterostructure (GRINSCH) Laser

• GRaded INdex Separate Confinement Heterostructure (GRINSCH) Laser

• A narrower carrier confinement region (d) of high recombination is separated from a wider optical waveguide region

• Optical confinement can be optimized without affecting the carrier confinement

• GRINSCH-SQW and GRINSCH-MQW

• The threshold current for a GRINSCH is much lower than that of a DH laser

Vertical confinement

GRINSCH Laser

Lateral confinement

Efficient operation of a laser diode requires reducing the # of lateral modes, stabilizing the gain for lateral modes as well as lowering the threshold current.

These are met by structures that confine the optical wave, carrier concentration and current flow in the lateral direction. Important types of laser diodes are: gain-guided, positive index guided, and negative index guided.

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

Oxide insulator

Stripe electrode

SubstrateElectrode

Active region where J > Jt h.(Emission region)

p -GaAs (Contacting layer)

n -GaAs (Substrate)

p -GaAs (Active layer)

Currentpaths

L

W

Cleaved reflecting surfaceEllipticallaserbeam

p -Al xGa 1-xAs (Confining layer)

n -Al xGa 1-xAs (Confining layer) 12 3

Cleaved reflecting surface

Substrate

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

• Stripe contact increases current density in the active region.

• The widths of the active region or the optical gain region is defined by current density from the stripe

Gain guided: optical gain is highest where current density is greatest

Ridge laser

Oxide insulation

n- AlGaAs

p + -AlGaAs (Contacting layer)

n -GaAs (Substrate)

p -GaAs (Active layer)n -AlGaAs (Confining layer)

p -AlGaAs (Confining layer)

Schematic illustration of the cross sectional structure of a buriedheterostructure laser diode.

Electrode

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

• Active layer is surrounded by lower index AlGaAs and behaves like a dielectric waveguide

• Ensures that photons are confined to the active or optical gain region• Increases rate of stimulated emission

Index guided: optical power confined to waveguide

Buried heterostructure laser

Laser Diodes (temperature characteristics)The output characteristics of an LD are sensitive to

temperature.=>As temperature increases threshold current increases

exponentially.Output spectrum also changes.A single mode LD will mode hop (jump to a different

mode) at certain temperatures.This results in a change of laser oscillation wavelength. increases slowly due to small change in refractive index

and cavity length.

0 20 40 60 800

2

4

6

8

10

Po (mW)

I (mA)

0 C25 C

50 C

Output optical power vs. diode current as three different temperatures. Thethreshold current shifts to higher temperatures.

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

o(nm)

Mode hopping

20 30 40 50Case temperature ( C)

Single mode

776

778

780

782

784

786

788

20 30 40 50Case temperature ( C)

Single mode

20 30 40 50

Multimode

Case temperature ( C)

Peak wavelength vs. case temperature characteristics. (a) Mode hops in the outputspectrum of a single mode LD. (b) Restricted mode hops and none over the temperaturerange of interest (20 - 40 C). (c) Output spectrum from a multimode LD.

(a) (b) (c)

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

Laser Diodes (temperature characteristics)

Remedies if mode hopping is undesirable:1. Adjust device structure.2. Implement thermoelectric (TE) cooler.Gain guided LDs inherently have many modes therefore the

wavelength vs. temperature behaviour tends to follow the bandgap (optical gain curve as opposed to the cavity properties).

Advanced semiconductor lasers

Quantum dot (QD) lasers

Evolution of the threshold current of the semiconductor lasers

0-D (Quantum dot): An artificial atom

)()( iEEE

E

Ei

Areal density:

Theoretical quantum dots

(a) Structure of a 4nm-high, 10 nm-wide hexagonal GaN quantum dot embedded in AlN.

(b) Profile of the conduction band edge.

(c) Maps of the dot electron ground state,

(d) Map of the first excited state.

In Stranki-Krastanov growth of QDs: strain-mediated intra- and inter-layer interactions between the QDs

Aligned array of GaN QDs in AlN

QDL – Predicted Advantages• Wavelength of light determined by the energy

levels not by bandgap energy:– improved performance & increased flexibility

to adjust the wavelength

• Maximum material gain and differential gain• Small volume:

– low power high frequency operation – large modulation bandwidth

• Superior temperature stability of I threshold

I threshold (T) = I threshold (T ref).exp ((T-(T ref))/ (T 0))– High T 0 decoupling electron-phonon interaction

by increasing the intersubband separation. – Undiminished room-temperature performance

without external thermal stabilization

• Suppressed diffusion of non-equilibrium carriers Reduced leakage

QDL – Basic characteristics

• An ideal QDL consists of a 3D-array of dots with equal size and shape• Surrounded by a higher band-gap material

– confines the injected carriers. • Embedded in an optical waveguide

– Consists lower and upper cladding layers (n-doped and p-doped shields)

Edge emitting QDL

http://qdlaser.com/

QDL – Application Requirements• Same energy level

– Size, shape and alloy composition of QDs close to identical– Inhomogeneous broadening eliminated real

concentration of energy states obtained• High density of interacting QDs

– Macroscopic physical parameter light output• Reduction of non-radiative centers

– Problem for nanostructures made by high-energy beam patterning since damage occurs during fabrication

• Electrical control – Electric field applied can change physical properties of QDs – Carriers can be injected to create light emission

Comparison of QD Laser with QW laser

http://qdlaser.com/

QD Laser vs. QW Laser

• Comparison of efficiency: QWL vs. QDL

Bottlenecks • First, the lack of uniformity.• Quantum Dots density is

insufficient• the lack of correlation

between QDs

Single dot

Ensemble of QDsFWHM = 20-30 meV

Breakthroughs

Fujitsu Temperature Independent QD laser2004

Temperature dependence of light-current characteristics

Fujitsu's quantum dot laser fires data at 25Gbps (2010)

BreakthroughsInP instead of GaAs

Can operate on ground state for much shorter cavity length High T0 is achieved First buried DFB DWELL operating at 10Gb/s in 1.55um range Surprising narrow linewidth-brings a good phase noise and time-

jitter when the laser is actively mode locked

Alcatel Thales III–V Laboratory, France2006

High-Performance Quantum Dot Lasers and Integrated Optoelectronics on Si

Market demand of QD lasers

QD Lasers

Microwave/Millimeter wave transmission with optical fibersD

atacom netw

ork

Telecom netw

ork

Optics

Promising properties

High speed quantum dot lasers

Advantages

Directly Modulated Quantum Dot Lasers

•Datacom application•Rate of 10Gb/s

Mode-Locked Quantum Dot Lasers

•Short optical pulses•Narrow spectral width•Broad gain spectrum•Very low α factor-low chirp

InP Based Quantum Dot Lasers

•Low emission wavelength•Wide temperature range•Used for data transmission

Promising properties

High power Quantum Dot lasers

Advantages

QD lasers for Coolerless Pump Sources

•Size reduced quantum dot

Single Mode Tapered Lasers

•Small wave length shift•Temperature insensitivity

Future Directions• Widening parameters

range

• Further controlling the position and dot size

• Decouple the carrier capture from the escape procedure

• Combination of QD lasers and QW lasers

• Reduce inhomogeneous linewidth broadening

• Surface Preparation Technology

• Allowing the injection of cooled carriers

• Raised gain at the fundamental transition energy

using

by

In term of

to

Conclusions: QD Laser vs. QW LaserIn order for QD lasers compete with QW lasers: • A large array of QDs since their active volume is small• An array with a narrow size distribution has to be produced to

reduce inhomogeneous broadening• Array has to be without defects

– may degrade the optical emission by providing alternate nonradiative defect channels

• The phonon bottleneck created by confinement limits the number of states that are efficiently coupled by phonons due to energy conservation – Limits the relaxation of excited carriers into lasing states– Causes degradation of stimulated emission – Other mechanisms can be used to suppress that bottleneck effect

(e.g. Auger interactions)