Download - Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

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Example:

He-Ne Laserλ0 = 632.5 nmΔλ = 0.2 nm

Diode Laserλ0 = 900 nmΔλ = 10 nm

LASER propertiesNearly Monochromatic light

Directionality

Page 4: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Coherence

Incoherent light waves Coherent light waves

Page 5: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Atomic Transitions

Stimulated absorption

Page 6: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Stimulated emission

Page 7: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Condition for Amplification by Stimulated Emission

Population Inversion:More Electrons in higher energy level

Pumping:Process to achieve population

inversion usually through external energy source

In general if N2 > N1 then MEDIA IS SAID TO BE ACTIVE

Page 8: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Page 9: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Basic concepts – Laser resonator

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

Page 10: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

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)

Page 11: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Observations….

Laser medium in a resonator produces oscillations

A spontaneous photon is duplicated over and over

Duplicated photons leak from semitransparent

mirror

Photons from oscillator are identicalCoherent – identical photonsControllable wavelength/frequency – nice colorsControllable spatial structure – narrow beamsControllable temporal structure – short pulses

Page 12: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

ACTIVE MEDIUM

Atoms: helium-neon (HeNe) laser; heliumcadmium

(HeCd) laser, copper vapor lasers

(CVL)

•Molecules: carbon dioxide (CO2) laser, ArF

and KrF excimer lasers, N2 laser

•Liquids: organic dye molecules dilutely dissolved in

various solvent solutions

Page 13: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Page 14: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Dielectric solids: neodymium atoms dopedin YAG or glass to make the crystallineNd:YAG or Nd:glass lasers

•Semiconductor materials: gallium arsenide, indium phosphide crystals.

Page 15: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

LASER action in Semiconductors

Laser diode is similar in principle to an LED.

What added geometry does a Laser diode require? An optical cavity that will facilitate feedback in order to generate stimulated emission

In addition to population inversion laser oscillation must be

sustained.

An optical cavity is implemented to elevate the intensity of

stimulated emission. (optical resonator)

Provides an output of continuous coherent radiation.

A homojunction laser diode is one where the pn junction uses

the same direct bandgap semiconductor material throughout

the component (ex. GaAs)

Page 16: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

LElectrode

Current

GaAs

GaAsn+

Cleaved surface mirror

Electrode

Active region(stimulated emission region)

A schematic illustration of a GaAs homojunction laserdiode. The cleaved surfaces act as reflecting mirrors.

Page 17: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Fundamental Laser diode:

1. Acts like an Edge emitting LED. Edge emission is suitable for

adaptation to feedback waveguide.

2. Polish the sides of the structure that is radiating.

3. Introduce a reflecting mechanism in order to return radiation

to the active region.

Drawback: Excessive absorption of radiation in p and n

layers of diode.

Remedy: Add confinement layers on both sides of active

region with different refractive indexes. Radiation will reflect

back to active region.

Typical output optical power vs. diode current (I) characteristics and the correspondingoutput spectrum of a laser diode.

Laser

LaserOptical Power

Optical Power

LEDOptical Power

Ith

Spontaneousemission

Stimulatedemission

Optical Power

Page 19: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

LASER diode – Heterostructure

The drawback of a homojunction structure is that the threshold

current density ( I th) is too high and therefore restricted to

operating at very low temperatures.

Remedy: Heterostructure semiconductor laser diodes.

What must be accomplished?

- reduce threshold current to a usable level

- improvement of the rate of stimulated emission as well as the

efficiency of the optical cavity

Page 20: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

To improve the performance… …

Carrier confinement Confine the injected electrons and holes to a narrow region

about the junction.

This reduces the amount of current needed to establish the required concentration of electrons for population inversion.

Photon confinementConstruct a dielectric waveguide around the optical gain region

to increase the photon concentration and elevate the probability of stimulated emission.

This reduces the number of electrons lost traveling off the cavity axis.

Page 21: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Refractiveindex

Photondensity

Activeregion

n ~ 5%

2 eV

Holes in VB

Electrons in CB

AlGaAsAlGaAs

1.4 eV

(a)

(b)

pn p

(a) A doubleheterostructure diode hastwo junctions which arebetween two differentbandgap semiconductors(GaAs and AlGaAs).

2 eV

(b) Simplified energyband diagram under alarge forward bias.Lasing recombinationtakes place in the p-GaAs layer, theactive layer

(~0.1 m)

(d) AlGaAs layersprovide lateral opticalconfinement.

(c)

(d)

GaAs

Page 22: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Refer to the DH structure……

AlGaAs has Eg of 2 eVGaAs has Eg of 1.4 eV

P-GaAs is a thin layer (0.1 – 0.2 um) and is the Active Layer where lasing recombination occurs.

Both p regions are heavily doped

With an adequate forward bias Ec of n-AlGaAs moves above Ec of p-GaAs which develops a large injection of electrons from the CB of n-AlGaAs to the CB of p-GaAs.

These electrons are confined to the CB of the p-GaAs due to the difference in barrier potential of the two materials.

Page 23: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Note:

1.Due to the thin p-GaAs layer a minimal amount of current only

is required to increase the concentration of injected carriers at a

fast rate. This is how threshold current is reduced for the purpose

of population inversion and optical gain.

2. A semiconductor with a wider bandgap (AlGaAs) will also have

a lower refractive index than GaAs. This difference in refractive

index is what establishes an optical dielectric waveguide that

ultimately confines photons to the active region.

Page 24: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Schematic illustration of the the structure of a double heterojunction stripecontact laser diode

Oxide insulator

Stripe electrode

SubstrateElectrode

Active region where J > Jth.(Emission region)

p-GaAs (Contacting layer)

n-GaAs (Substrate)

p-GaAs (Active layer)

Currentpaths

Cleaved reflecting surfaceEllipticallaserbeam

p-AlxGa

1-xAs (Confining layer)

n-AlxGa

1-xAs (Confining layer) 12 3

Cleaved reflecting surface

Substrate

Page 25: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Advantages of stripe geometry:

1. Reduced contact reduces threshold current.

2. Reduced emission area makes light coupling to fiber easier

Page 26: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Height, H Width W

Length, L

The laser cavity definitions and the output laser beamcharacteristics.

Fabry-Perot cavity

Dielectric mirror

Diffractionlimited laserbeam

Page 27: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Types based on OscillationsTypes based on Oscillations

According to the mode of oscillations lasers are divided into 3 groups

Continuous Lasers:Emit an continuous light beam with constant powerRequire continuous steady-state pumping of the active medium

Pulsed Lasers:Require pulsed operation of the Pumping systemPumping achieves Population Inversion periodically for short periods

Pulsed Laser with controlled Losses:Concentration of energy reaches a maximum so that they give riseto giant pulses of short duration.Peak power is in the order of 100 Watts or more

Realized by controlling the losses inside the cavity using a device Known as the Q switchQ switch

Page 28: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Principle of Q switchingPrinciple of Q switching

The active medium is excited without feedback by blocking the reflectionfrom one of the end mirrors of the cavity

The end mirror is then suddenly allowed to reflect

Suddenly applied feedback causes a rapid population inversionof the lasing levels

Results in a very high peak power output pulse

Duration of the light pulse is in order of 0.1 microseconds

Page 29: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Techniques for Q switchingTechniques for Q switching

(i) Using a mechanically driven device

E.g., a rotating prism or mirror

Rotate one of the mirrors about an axis perpendicular to the laser

Rotating speed cannot be made very large

Hence Q switching does not take place instantaneously

[semiconductor saturable absorber mirror (SESAM) ]

Page 30: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

(ii) Passive device such as an electro-optical cell: Light from optical cavity passes through a polarizer and an

Electro-optic cell (controlling the phase or polarisation of the

laser beam) When appropriate voltage is applied, the material inside the cell

becomes birefringent By varying the voltage – cell blocks or transmits beam

(iii) Using a cell containing a Dye:

Passive Q switch – cell containing organic dye

Initially light output absorbed by dye, preventing reflection

After a particular intensity is reached, dye is bleached(allows

light)

Now reflection from mirror is possible

Results in rapid increase in cavity gain

Page 31: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Resonator ModesResonator Modes

Since light is a wave, when bouncing between the mirrors of the cavity the light will constructively and destructively interfere with itself.

Leads to the formation of standing waves between the mirrors (a radiation pattern or a field distribution)

These standing waves form a discrete set of frequencies: longitudinal modes of the cavity

These modes are the only frequencies which are self-regenerating and

allowed to oscillate by the resonant cavity

All other frequencies of light are suppressed by destructive

interference

Page 32: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Resonator Modes……Resonator Modes……

Laser resonators have two distinct types of modes, transverse

and longitudinal.

Transverse modes manifest themselves in the cross-sectional

profile of the beam, that is, in its intensity pattern.

Longitudinal modes correspond to different resonances along

the length of the laser cavity which occur at different

frequencies or wavelengths within the gain bandwidth of the

laser.

Modes… …Modes… …

Transverse modes are classified according to the number of noughts that appear across the beam cross section in two directions.

The lowest-order, or fundamental mode, where intensity peaks at the centre, is known as TEM00. [TRANSVERSE ELECTROMAGNETIC WAVE]

A single transverse mode laser that oscillates in a single longitudinal mode is oscillating at only a single frequency – “single mode” operation.

When more than one longitudinal mode is excited, the laser is said to be

in "multi-mode" operation.

Page 34: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

The mode with a single nought along one axis and no nought in the perpendicular direction is TEM01 or TEM10, depending on orientation.

A sampling of these modes, which is produced by stable resonators, is shown below

TEM00

TEM10

TEM01

http://www.fou.uib.no/fd/1996/h/404001/IMG00027.GIF

Page 35: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Mode Locking in LasersMode Locking in Lasers

Mode-locking is a technique in by which a laser can be made to produce

pulses of light of extremely short duration, on the order of picoseconds

(10−12s) or femtoseconds (10−15s).

Need for Mode Locking

When laser is oscillating with various modes and if modes are uncorrelated

The output intensity i.e., The total optic electric field resulting from a multi-

mode oscillation fluctuates with time

To overcome this fluctuation – MODE LOCKING IS DONE i.e., the phase

between the modes is to be fixed

Page 36: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Mode locking requires various Longitudinal Modes to be coupled to each other

It can be viewed as a condition in whichA pulse of light is bouncing back and forth inside the cavity

And every time it hits the mirror, a certain fraction is transmitted as the output pulse

The output of a mode-locked laser will be a series of pulses of extremely short duration

Pulses are separated by a duration tr = 2 L / c

termed the CAVITY ROUND TRIP TIME

Page 37: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

multiple oscillating cavity modes

laser gain

profile

q+1q1

losses

q q+2 q+3

possible cavity modes

Sum of various modes with same relative phase – MODES LOCKED

Multiple oscillating cavity modes

Page 38: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Methods for producing mode-locking in a laser may be classified as either active or passive.

Active methods typically involve using an external signal to induce

a modulation of the intra-cavity light.

Passive methods do not use an external signal, but rely on placing

some element into the laser cavity which causes self-modulation

of the light.

Page 39: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Active Mode LockingActive Mode Locking

The most common active mode-locking technique places a

standing wave acousto-optic modulator into the laser cavity.

This device, when placed in a laser cavity and driven with an

electrical signal, induces a small, sinusoidally varying frequency

shift in the light passing through it.

After some round trips, the oscillating intensity consists of a

periodic train whose modes are locked and the period of the pulses

is T=2L/ c

Where L= length of the gain medium

c = speed of light in free space

Page 40: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Passive Mode LockingPassive Mode Locking

Do not require a signal external to the laser (such as the driving

signal of a modulator) to produce pulses.

An intra-cavity element is introduced in the cavity.

This produces a change in the intra-cavity light.

The most common type of device which will do this is a saturable

absorber

A saturable absorber is used whose absorption coefficient

decreases with Increase in incident light intensity

Page 41: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

SATURABLE ABSORBERSATURABLE ABSORBER

A saturable absorber is an optical device that exhibits

an intensity-dependent transmission.

The device behaves differently depending on the

intensity of the light passing through it.

Will selectively absorb low-intensity light, and transmit

light which is of sufficiently high intensity.

When placed in a laser cavity, a saturable absorber will

attenuate low-intensity constant wave light

Page 42: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

Contd….

As the light in the cavity oscillates, this process

repeats, leading to the selective amplification of the

high-intensity spikes, and the absorption of the low-

intensity light.

After many round trips, this leads to a train of pulses

and mode-locking of the laser.

Mode locked pulse train appear at a frqy of c/2L

Page 43: Example: He-Ne Laser λ0 = 632.5 nm Δλ = 0.2 nm Diode Laser λ0 = 900 nm Δλ = 10 nm LASER properties Nearly Monochromatic light.

BEAM PROFILE

CW MODE LOCKED

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