He-Ne Laser

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LASER properties Nearly Monochromatic light
Example He-Ne Laser ?0 632.5 nm ??
0.2 nm Diode Laser ?0 900 nm ?? 10 nm
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Directionality
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Coherence
Incoherent light waves
Coherent light waves
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Atomic Transitions Stimulated absorption
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Stimulated emission
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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 gt N1
then MEDIA IS SAID TO BE ACTIVE
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• 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

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• 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)

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• 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 identical
• Coherent identical photons
• Controllable wavelength/frequency nice colors
• Controllable spatial structure narrow beams
• Controllable temporal structure short pulses

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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
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Dielectric solids neodymium atoms doped in YAG
or glass to make the crystalline NdYAG or
Ndglass lasers Semiconductor materials
gallium arsenide, indium phosphide crystals.
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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)
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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.
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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
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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
confinement Construct 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.
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• Refer to the DH structure
• AlGaAs has Eg of 2 eV
• GaAs 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.

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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.
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Advantages of stripe geometry 1. Reduced
contact reduces threshold current. 2. Reduced
emission area makes light coupling to fiber
easier
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Types based on Oscillations According to the
mode of oscillations lasers are divided into 3
groups Continuous Lasers Emit an continuous
light beam with constant power Require continuous
steady-state pumping of the active medium Pulsed
Lasers Require pulsed operation of the Pumping
system Pumping achieves Population Inversion
periodically for short periods Pulsed Laser with
controlled Losses Concentration of energy
reaches a maximum so that they give rise to 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 switch
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Principle of Q switching The active medium is
excited without feedback by blocking the
reflection from one of the end mirrors of the
cavity The end mirror is then suddenly allowed
to reflect Suddenly applied feedback causes a
rapid population inversion of the lasing
levels Results in a very high peak power output
pulse Duration of the light pulse is in order of
0.1 microseconds
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• Techniques for Q switching
• 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)

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• (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

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• Resonator 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

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• 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.

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• 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.

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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
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Mode 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
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• Mode locking requires various Longitudinal Modes
  to be coupled to each other
• It can be viewed as a condition in which
• A 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

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Multiple oscillating cavity modes
Sum of various modes with same relative phase
MODES LOCKED
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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.
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Active 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 T2L/ c Where L length
of the gain medium c speed of light
in free space
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• Passive 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

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• SATURABLE 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

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• 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

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BEAM PROFILE
MODE LOCKED
CW