Laser 1.ppt

LASER(semiconducting Lasers)Pertemuan 4

Topik Definisi laserEmisi dan absorpsi radiasi Inversi PopulasiLaser semiconduktorMaterial untuk laser semicondutorLaser untuk komunikasi serat optikPerangkat Quantum Well

Lecture: LaserObjectives (by the end of the lectures on laser student will be)Able to state the definition of laserAble to state the principle of population inversionAble to explain the principle of semiconducting laserFamiliarise with the concept of light simulation and polarisationAble to list down all materials criteria and materials selection for a given semiconducting laser compound.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 laserA laser is a device that generates light by a process called STIMULATED EMISSION.The acronym LASER stands for Light Amplification by Stimulated Emission of RadiationSemiconducting 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.

Another Typical Application of Laser Fibre OpticsAn 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 informationDifferent 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 lossHowever, if we start with a more coherent beam then loss can be greatly reduced.

Fibre Optics Communication

3 Mechanisms of Light EmissionFor atomic systems in thermal equilibrium with their surrounding, the emission of light is the result of:AbsorptionAnd subsequently, spontaneous emission of energyThere 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 emissionIt is an important process for laser actionAbsorptionSpontaneous EmissionStimulated EmissionTherefore 3 process of light emission:

AbsorptionE1E2

Spontaneous Emission

Stimulated Emission

Background PhysicsIn 1917 Einstein predicted that: under certain circumstances a photon incident upon a material can generate a second photon of Exactly the same energy (frequency)PhasePolarisationDirection of propagationIn other word, a coherent beam resulted.

Background PhysicsConsider 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.

Stimulated Emission

Background PhysicsIn a system, all three mechanisms occur. However the stimulated emission is very very sluggish compared to the spontaneous emissionWe need to have a much stimulated emission as possible for lasing actionHow?Refer to the board for the derivation of the Einsteins

Einstein;s

Absorption of Light Through a MediumLight or photon must be absorbed in order for us to have a lasing actionI(x) = I(o) exp (-x)

I(o)I(x)

AbsorptionLight that falls on a piece of material will decrease exponentially. = (N1-N2)B21(hf) n/cN1 is often more than N2 (N1 < N2)Example for tungsten is typically 106m-1 (+ve)If we want implication, must be vei.e. N2 > N1

Population InversionTherefore we must have a mechanism where N2 > N1This is called POPULATION INVERSIONPopulation inversion can be created by introducing a so call metastable centre where electrons can piled up to achieve a situation where more N2 than N1The 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 InversionWhen 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 QuestionDefine 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 FeedbackThe 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 reflectiveThis 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