LASER(semiconducting Lasers)

LASER 1 EBB 424E

Dr Zainovia Lockman

Lecture Contents

Definition of lasers

Emission and absorption of radiation

Population Inversion

Semiconducting lasers

Materials used for semiconducting laser

Laser for fibre optics communication

Quantum Well devices

For the Laser Course You

Need:

A general reading on lasers:A photocopy from a book by Watson p23-64 (easy read)

Population Inversion and Diode Laser:A photocopy from Wilson and Hawkes

p 169- 182 (more advance reading)

P 204-223 (more advance reading)

A general reading + the optical fibre application + on laser diode

A photocopy from Kasap p.159-166 (optical fibre)

P.181-196

EBB 424 Lecture Presentation

EBB 424 Short Lecture Notes summarising all of the above.

Important

Announcement 1:

Test schedule

A Test on LED and laser will be

conducted on:

26th September

40 objective questions

Assignments and Tests

Group activity 1 (presentation only) =

25% - done

Group activity 2 (open book test) =

25%

Test I = 25%

Test 2 = 25%

Information

about the examPlease study the pass year paper and all of the‘typical exam questions’ presented to you in thelectures.

There will be 3.5 questions fromOptoelectronics Part.

Compulsory for you to answer 2 questionsfrom both part A and B.

Then choose one question from any parts.

Lecture: Laser

Objectives (by the end of the lectures on laser student will be<)

1. Able to state the definition of laser

2. Able to state the principle of population inversion3. Able to explain the principle of semiconducting

laser

4. Familiarise with the concept of light simulation and polarisation

5. Able to list down all materials criteria and materials selection for a given semiconducting laser compound.

6. Able to highlight several examples of the application of laser.

Diode Laser

Typical Application of LaserThe detection of the binary data stored in the form of pits on the compact disc is

done with the use of a semiconductor laser. The laser is focused to a

diameter of about 0.8 mm at the bottom of the disc, but is further focused to

about 1.7 micrometers as it passes through the clear plastic substrate to strike

the reflective layer. The reflected laser will be detected by a photodiode. Moral

of the story: without optoelectronics there will no CD player!

1. Definition of laser

A laser is a device that generates light by aprocess called STIMULATED EMISSION.

The acronym LASER stands for LightAmplification by Stimulated Emission ofRadiation

Semiconducting lasers are multilayersemiconductor devices that generates acoherent beam of monochromatic light bylaser action. A coherent beam resultedwhich all of the photons are in phase.

Another Typical Application

of Laser – Fibre Optics

An example of application is for the light source for fibre optics communication. Light travels down a fibre optics glass at a speed, = c/n, where n = refractive index.

Light carries with it information

Different wavelength travels at different speed.

This induce dispersion and at the receiving end the light is observed to be spread. This is associated with data or information lost.

The greater the spread of information, the more loss

However, if we start with a more coherent beam then loss can be greatly reduced.

Fibre Optics Communication

3 Mechanisms of Light Emission

For atomic systems in thermal equilibrium with their

surrounding, the emission of light is the result of:

�Absorption

�And subsequently, spontaneous emission of energy

There is another process whereby the atom in an upper energy

level can be triggered or stimulated in phase with the an

incoming photon. This process is:

�Stimulated emission

�It is an important process for laser action

1. Absorption

2. Spontaneous Emission

3. Stimulated Emission

Therefore 3 process of light emission:

Absorption

E1

E2

Spontaneous Emission

Stimulated Emission

Background Physics

In 1917 Einstein predicted that:

� under certain circumstances a photon

incident upon a material can generate a

second photon of

�Exactly the same energy (frequency)

�Phase

�Polarisation

�Direction of propagation

� In other word, a coherent beam

resulted.

Background Physics

Consider the ‘stimulated emission’ as

shown previously.

Stimulated emission is the basis of the

laser action.

The two photons that have been produced

can then generate more photons, and the 4

generated can generate 16 etc< etc<

which could result in a cascade of intense

monochromatic radiation.

E1

E2

hυ

(a) Absorption

hυ

(b) Spontaneous emission

hυ

(c) Stimulated emission

Inhυ

Out

hυ

E2

E2

E1 E

1

Absorption, spontaneous (random photon) emission and stimulatedemission.

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

Stimulated Emission

Background Physics

In a system, all three mechanisms occur.

However the stimulated emission is very

very sluggish compared to the

spontaneous emission

We need to have a much stimulated

emission as possible for lasing action

How?

Refer to the board for the derivation of the

Einstein’s

Einstein;s

Absorption of Light Through

a Medium

Light or photon must be absorbed in

order for us to have a lasing action

I(x) = I(o) exp (-αx)

I(o) I(x)

Absorption

Light that falls on a piece of material will decrease exponentially.

α = (N1-N2)B21(hf) n/c

N1 is often more than N2 (N1 < N2)

Example for tungstenα is typically 106m-1 (+ve)

If we want implication, α must be –ve

i.e. N2 > N1

Population InversionTherefore we must have a mechanism where N2 > N1

This is called POPULATION INVERSION

Population inversion can be created by introducing a so call metastable centre where electrons can piled up to achieve a situation where more N2than N1

The process of attaining a population inversion is called pumping and the objective is to obtain a non-thermal equilibrium.

It is not possible to achieve population inversion with a 2-state system.

If the radiation flux is made very large the probability of stimulated emission and absorption can be made far exceed the rate of spontaneous emission.

But in 2-state system, the best we can get is N1 = N2. To create population inversion, a 3-state system is required.

The system is pumped with radiation of energy E31 then atoms in state 3 relax to state 2 non radiatively.

The electrons from E2 will now jump to E1 to give out radiation.

3 states system

Population Inversion

When a sizable population of electrons resides in upper levels,

this condition is called a "population inversion", and it sets the

stage for stimulated emission of multiple photons. This is the

precondition for the light amplification which occurs in a LASER

and since the emitted photons have a definite time and phase

relation to each other, the light has a high degree of coherence.

Typical Exam Question…

Define the term population inversion

for a semiconducting laser (diode)

explain what is the condition of

population inversion.

Why is population inversion required

for a lasing action?

(40 marks)

Optical Feedback

The probability of photon producing a stimulated emission event can be increased by reflecting back through the medium several times.

A device is normally fashioned in such a way that the 2 ends are made higly reflective

This is term an oscillator cavity or Fabry Perot cavity

Therefore in a laser….

Three key elements in a laser

•Pumping process prepares amplifying medium in suitable state

•Optical power increases on each pass through amplifying medium

•If gain exceeds loss, device will oscillate, generating a coherentoutput