laser basics

Optical Sources (laser)

contents'

Laser basic concept.Optical emission from semiconductor.Semiconductor injection laser.Injection laser structure and characteristics.Laser fiber coupling.non semiconductor laser.Laser modulation.

Main function• Convert electrical energy into optical energy (with the condition

that light output to be effectively launched or coupled into optical fiber)

Types • Wideband continuous spectra (incandescent lamps)• Monochromatic incoherent ( LEDs)• Monochromatic coherent (LDs)

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 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.

Fibre Optics Communication

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)

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) Phase Polarisation Direction of propagation

In 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.

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)

Population InversionTherefore we must have a mechanism where N2 > N1

This 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 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.

www.bzupages.com

Basic Concept (LASER)

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

www.bzupages.com

Basic Concept (LASER)Optical feedback and laser oscillations

• Photon striking an excited atom causes emission of a second photon which release two more photons creating an avalanche multiplication.• Amplification &Coherence achieved by (Febry – Perot resonator)

• Placing mirrors at either end of the amplifying medium• Providing positive feedback• Amplification in a single go is quite small but after multiple passes the net gain can be large• One mirror is partially transmitting from where useful radiation may escape from the cavity• Stable output occurs when optical gain is exactly matched with losses incurred (Absorption,

scattering and diffraction)

www.bzupages.com

Semi Conductor Emission

Types of materialConductorInsulatorsSemi conductors (intrinsic & extrinsic)

N type materialDonor impurity addedIncreases thermally excited electrons in the conduction band

P type materialAcceptor impurity addedIncreases positive charges (holes) in the valance band

PN junction

www.bzupages.com

Semiconductor

The properties of Semiconductor lies in between conductor and insulator. The semiconductor has valance band, conduction band and Fermi level ( the gap between two bands ). The Fermi level is relatively small which means that small amount of energy is sufficient to bring about electric current in semiconductors.

Conduction Band

Valance Band

Fermi Level

n-Type Semiconductor

a) Donor level in an n-type semiconductor. b) The ionization of donor impurities creates an increased electron

concentration distribution.

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

p-Type Semiconductor

a) Acceptor level in an p-type semiconductor.

b) The ionization of acceptor impurities creates an increased hole concentration distribution


The pn Junction


Electron diffusion across a pn junction creates a barrier potential (electric field) in the depletion region.

Forward-biased pn Junction


Lowering the barrier potential with a forward bias allows majority carriers to diffuse across the junction.

Reverse-biased pn Junction


A reverse bias widens the depletion region, but allows minority carriers to move freely with the applied field.

The electrical resistance of semiconductor lies in between conductors and insulators. The increase in temperature can lower the resistance in semiconductors. Silicon is the most common semiconductor used.

The Fermi level can be increased or decreased by adding dopants into Silicon.If P-type material such as Aluminum, Gallium or Indium are added, which create holes and

shortage of electrons. Fermi level is increased. If N-type material such as Phosphorus, Arsenic and Boron are added the result is excess of electrons and Fermi level is reduced.P.N Junction If a voltage is applied to P.N Junction ( Forward biased ). The Fermi Level on both sides of Junction will move, so that a current will flow through the P.N Junction. The electrons will flow into the P layer and holes are formed in N layer. The system tries to reach equilibrium by excited electrons flowing to holes . During this process ,energy is released in the form of Photons. It is the mechanism of Light emitting diodes. If p layer and N layer in the P.N Junction are heavily doped and strong current used a population inversion of electrons occurs in an Optical Cavity ,a Laser can be created.

www.bzupages.com

Semiconductor Injection Laser

Stimulated emission by recombination of injected carriersOptical cavity provided in the crystal structureAdvantages are

High radiance due to amplifying effectNarrow line width of the order of 1nmHigh modulation capabilities presently ranging in G HzGood spatial coherence which allows the output to be focused by a lens into a spot to provide high coupling efficiency

Semiconductor Injection Laser

Adv.of the injection laser. over other semiconductor laser(LED)1)High radiance due to the amplifying effect of stimulation emission(supply

miliwatt of o/p current)

2)Narrow line width of the order of (nm (10A)0r less ) which is useful in minimizing the effects of material dispersion

3)Modulation capability which is present extend upto the gigahertz

4)GOOD spatial coherence which allows the o/p to be focused by a lens into spot which has grater intensity then unfocused emission (permit coupling of o/p power into fiber even for fibers with low NA

Stripe geometry

Stripe geometry to the structure to provide optical containment in the horizontal planeOvercome major problem associated with the broad area devices.o/p beam diverges is typically 45 degree perpendicular to the plane of the junction and 9 degree parallel to it

Stripe geometry

injection laser to fiber coupling

One of the major difficulties with using semiconductor laser problem associated with the coupling of light between the laser and the optical fiber(perticularly single mode, low NA).Techniques of coupling of injection laser to optical fibers1)butt coupling2)tapered hemispherical fiber coupling3)confocal lens system

1)butt coupling injection laser are relatively directional they have diverging o/p field Efficiency around10%.(even good alignment and use of a fiber with

a well cleaved end) Positioning the fiber end very close to the laser facts,

2)tapered hemispherical fiber coupling the coupling efficiency can be substaintaily improved when the field

from the laser is matched to the output field of the fiber such achieved by using lens

Coupling efficency 65%.

3)confocal lens systemInjection laser coupling designs based on discrete lenses have also proved fruitfuluse of silicon lens within a confocal system has provide coupling efficiencies of up to 70%

www.bzupages.com

ILD Characteristics

Threshold current Vs TemperatureThreshold current in general tends to increase with temperature

Dynamic responseSwitch-on delay followed by damped oscillations known as relaxation oscillationsSerious deterioration at data rates above 100 Mbps if td is 0.5 ns and RO of twice thistd may be reduced by biasing the laser near threshold current

ROs damping is less straight forward

Temperature variation of the threshold current

0/)( TTzth eITI

Relaxation oscillation peak

www.bzupages.com

ILD Characteristics

Frequency ChirpDirect current modulation of a single mode semiconductor laser causes dynamic shift of the peak resulting in linewidth broadening called Frequency ChirpCombined with chromatic dispersion causes significant performance degradationCan be reduced by biasing the laser sufficiently above the threshold current Low chirp in DFB and quantum well lasers

www.bzupages.com

ILD Characteristics

NoisePhase or frequency noiseInstabilities like kinksReflection of light into the deviceMode partitioning

Mode HoppingMode hopping to a longer wavelength as the current is increased above threshold

www.bzupages.com

ILD Characteristics

ReliabilityMajor problem in ILDsDevice degradation occurTwo types of degradations

Catastrophic degradation– Mechanical damage to mirror facet– Results in partial or complete failure

Gradual degradation– Defect formation in active region– Degradation of the current confining region

laser basics

Science