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

Lecture Contents

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.

For the Laser Course You Need:

A Test on LED and laser will be A Test on LED and laser will be conducted on:conducted on:

2626thth September September

40 objective questions40 objective questions

Group activity 1 (presentation only) = 25% -

done Group activity 2 (open book test) = 25% Test I = 25% Test 2 = 25%

Assignments and Tests

Please study the pass year paper and Please study the pass year paper and all of the ‘typical exam questions’ all of the ‘typical exam questions’ presented to you in the lectures. presented to you in the lectures. There will be 3.5 questions from There will be 3.5 questions from

Optoelectronics Part. Optoelectronics Part. Compulsory for you to answer 2 Compulsory for you to answer 2

questions from both part A and B. questions from both part A and B. Then choose one question from any Then choose one question from any

parts. parts.

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

1.1. Able to state the definition of laserAble to state the definition of laser2.2. Able to state the principle of population Able to state the principle of population

inversioninversion3.3. Able to explain the principle of Able to explain the principle of

semiconducting lasersemiconducting laser4.4. Familiarise with the concept of light Familiarise with the concept of light

simulation and polarisationsimulation and polarisation5.5. Able to list down all materials criteria and Able to list down all materials criteria and

materials selection for a given materials selection for a given semiconducting laser compound.semiconducting laser compound.

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

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

1.1. Able to state the definition of laserAble to state the definition of laser2.2. Able to state the principle of population Able to state the principle of population

inversioninversion3.3. Able to explain the principle of Able to explain the principle of

semiconducting lasersemiconducting laser4.4. Familiarise with the concept of light Familiarise with the concept of light

simulation and polarisationsimulation and polarisation5.5. Able to list down all materials criteria and Able to list down all materials criteria and

materials selection for a given materials selection for a given semiconducting laser compound.semiconducting laser compound.

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

Diode Laser

Typical Application of Laser

The 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!

A laser is a device that generates light by

a process called STIMULATED EMISSION. The acronym LASER stands for Light

Amplification by Stimulated Emission of Radiation

Semiconducting lasers are multilayer semiconductor devices that generates a coherent beam of monochromatic light by laser action. A coherent beam resulted which all of the photons are in phase.

1. Definition of laser

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.

Another Typical Application of Laser – Fibre Optics

Fibre Optics Communication

1. Absorption2. Spontaneous Emission3. Stimulated Emission

1. Absorption2. Spontaneous Emission3. Stimulated Emission

3 Mechanisms of Light EmissionFor atomic systems in thermal equilibrium with their surrounding, the emission of light is the result of:

Absorption

And subsequently, spontaneous emission of energy

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

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

Therefore 3 process of light emission:

Absorption

E1

E2

Spontaneous Emission

Stimulated Emission

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.

Background Physics

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)

Stimulated Emission

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

Background Physics

Einstein;s

Light or photon must be absorbed in order for

us to have a lasing action I(x) = I(o) exp (-x)

Absorption of Light Through a Medium

I(o) I(x)

Light that falls on a piece of material will

decrease exponentially. = (N1-N2)B21(hf) n/c N1 is often more than N2 (N1 < N2)

Example for tungsten

is typically 106m-1 (+ve) If we want implication, must be –ve i.e. N2 > N1

Absorption

Therefore 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 N2 than 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.

Population Inversion

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.

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)

Typical Exam Question…

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

Optical Feedback

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