laser 267

EBB424E Dr Zainovia Lockman

Principle of Diode LASER

Laser 2

Recap

Light amplification by Stimulated Emission of Radiation

What is the word LASER stands for?

What is Population Inversion?

Laser 2. Lecture Content:Principle of the laser diodes (semiconducting laser) importantHeterostructure laser diodesMaterials requirements and materials selectionsLight emitters for fibre optics communications

Principle of Laser Diode

Stimulated Emission

E1

E2

h

(a) Absorption

h

(b) Spontaneous emission

h

(c) Stimulated emission

In h

Out

h

E2 E2

E1 E1

Absorption, spontaneous (random photon) emission and stimulatedemission.© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

In stimulated emission, an incoming photon with energy h stimulates the emission process by inducing electrons in E2 to transit down to E1.

While moving down to E1, photon of the same energy h will be emitted

Resulting in 2 photons coming out of the system

Photons are amplified – one incoming photon resulting in two photons coming out.

Population InversionNon equilibrium distribution of atoms among the various energy level atomic systemTo induce more atoms in E2, i.e. to create population inversion, a large amount of energy is required to excite atoms to E2

The excitation process of atoms so N2 > N2 is called pumpingIt is difficult to attain pumping when using two-level-system.Require 3-level system instead

E2

E1

More atoms here

N2

N1

N2> N1

E2

E1

E3

There level system

Principles of Laser

E1

h13E

2Metastablestate

E1

E3

E2

h32

E1

E3

E2

E1

E3

E2

h21h21

Coherent photons

OUT

(a) (b) (c) (d)

E3

.

IN

In actual case, excite atoms from E1 to E3.Exciting atoms from E1 to E3 optical pumpingAtoms from E3 decays rapidly to E2 emitting h3

If E2 is a long lived state, atoms from E2 will not decay to E1 rapidlyCondition where there are a lot of atoms in E2 population inversion achieved! i.e. between E2 and E1.

Coherent Photons Production (explanation of (d))

When one atom in E2 decays spontaneously, a random photon resulted which will induce stimulated photon from the neighbouring atomsThe photons from the neighbouring atoms will stimulate their neighbours and form avalanche of photons. Large collection of coherent photons resulted.

Laser Diode Principle

Consider a p-n junctionIn order to design a laser diode, the p-n junction must be heavily doped.In other word, the p and n materials must be degenerately dopedBy degenerated doping, the Fermi level of the n-side will lies in the conduction band whereas the Fermi level in the p-region will lie in the valance band.

Diode Laser Operationp+ n+

EFn

(a)

Eg

Ev

Ec

Ev

Holes in VBElectrons in CB

Junction

Electrons Ec

p+

Eg

V

n +

(b)

EFn

eV

EFp

Inversionregion

EFp

Ec E

c

eVo

•P-n junction must be degenerately doped.

•Fermi level in valance band (p) and conduction band (n).

•No bias, built n potential; eVo barrier to stop electron and holes movement

•Forward bias, eV> Eg

•Built in potential diminished to zero

•Electrons and holes can diffuse to the space charge layer

Application of Forward Bias

Suppose that the degenerately doped p-n junction is forward biased by a voltage greater than the band gap; eV > Eg

The separation between EFn and EFp is now the applied potential energyThe applied voltage diminished the built-in potential barrier, eVo to almost zero.

Electrons can now flow to the p-sideHoles can now flow to the n-side

Population Inversion in Diode Laser

hEg

Optical gain EF n EF p

Optical absorption

0

Energy

Ec

Ev

CB

VB

(a) The density of states and energy distribution of electrons and holes inthe conduction and valence bands respectively at T 0 in the SCLunder forward bias such that EFn EFp > Eg. Holes in the VB are emptystates. (b) Gain vs. photon energy.

Density of states

Electronsin CB

Holes in VB= Empty states

EF n

EF p

eV

At T > 0

At T = 0

(a) (b)

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

Population Inversion in Diode Laser

Electrons in CBEFn

EFp

CB

VB

Eg

Holes in VB

eV

EFn-EfP = eV

eV > Eg

eV = forward bias voltage

Fwd Diode current pumping injection pumping

More electrons in the conduction band near EC

Than electrons in the valance band near EV

There is therefore a population inversion between energies near EC and near EV around the junction.

This only achieved when degenerately doped p-n junction is forward bias with energy > Egap

The Lasing ActionThe population inversion region is a layer along the junction also call inversion layer or active regionNow consider a photon with E = Eg

Obviously this photon can not excite electrons from EV since there is NO electrons thereHowever the photon CAN STIMULATE electron to fall down from CB to VB. Therefore, the incoming photon stimulates emission than absorptionThe active region is then said to have ‘optical gain’ since the incoming photon has the ability to cause emission rather than being absorbed.

Pumping Mechanism in Laser Diode

It is obvious that the population inversion between energies near EC and those near EV occurs by injection of large charge carrier across the junction by forward biasing the junction. Therefore the pumping mechanism is FORWARD DIODE CURRENT Injection pumping

For Successful Lasing Action:1. Optical Gain (not absorb)

Achieved by population inversion2. Optical Feedback

Achieved by device configurationNeeded to increase the total optical amplification by making photons pass through the gain region multiple timesInsert 2 mirrors at each end of laserThis is term an oscillator cavity or Fabry Perot cavityMirrors are partly transmitted and party reflected

Reflection of Photons Back and Forth, Higher Gain

Fabry-Parrot Cavity

The photons vibrates to and forth with resonant wavelength

Laser

Gain + Feedback = laser

Optical FeedbackIn diode laser it is not necessary to use external mirrors to provide positive feedback. The high refractive index normally ensure that the reflectance at the air/material interface is sufficiently high The diode is often cleaved at one end and roughened at the other end. This results in the radiation generated within the active region spread out into the surrounding lossy GaAs, and there is a confinement of the radiation within a small region called the mode volume, In the a mode volume, there are additional carriers present which increases the refractive index of the material as compared to the surrounding material. This produces a dielectric waveguide similar to the heterojunctuction LED. However the difference is too small to be an efficient waveguide.

Typical Exam Questions!!What is laser diode?Describe the principle of a laser diodeGive some examples of applications of laser diode.What is the pumping mechanism in a laser diode and explain in term of the p-n junction.

Laser Diode Materials

Basically all of the materials are similar to that of LED.

UV, Vis and IR Laser can be produced by materials as discussed in the LED lectures

Materials for LED and Laser Diodes - summary

Optical Power in Laser is Very High due to Optical Feedback and Higher Forward Bias Current.

Threshold current density

Direct Gap Diode LaserDirect band gap high probability of electrons-holes recombination radiativelyThe recombination radiation may interact with the holes in the valance band and being absorbed or interact with the electrons in the conduction band thereby stimulating the production of further photons of the same frequency stimulated emission

Materials Available

Technologically Important Material for Blue Laser

InGaN and AlGaNInGaN and AlGaN have been produced over the entire composition range between their component binaries; InN, GaN, AlN InAlN is less explored. GaN and AlN are fairly well lattice-matched to SiC substrates, SiC has substrate is better as it can be doped (dopability) and high thermal conductivity relative to more commonly used Al2O3 substrates.AlN and GaN can be used for high temperature application due to wide bandgaps and low intrinsic carrier concentrations.

Blue/Violet Laser

Blue LaserGaN used