EBB424E Dr Zainovia Lockman Principle of Diode LASER Laser 2.
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Transcript of EBB424E Dr Zainovia Lockman Principle of Diode LASER Laser 2.
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 hOut
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 system
To 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 pumping
It 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
h13
E2
Metastablestate
E1
E3
E2
h32
E1
E3
E2
E1
E3
E2
h21
h21
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 pumping
Atoms from E3 decays rapidly to E2 emitting h3
If E2 is a long lived state, atoms from E2 will not decay to E1 rapidly
Condition 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 junction
In 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 doped
By 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
Ec
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 energy
The applied voltage diminished the built-in potential barrier, eVo to almost zero.
Electrons can now flow to the p-side
Holes 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
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 Action
The 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 diode
Give 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 Laser
Direct band gap high probability of electrons-holes recombination radiatively
The 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 AlGaN
InGaN 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 Laser
GaN used