Download lecture 3 Introduction to Laser

LASER properties
Nearly Monochromatic light
Example:
He-Ne Laser
λ0 = 632.5 nm
Δλ = 0.2 nm
Diode Laser
λ0 = 900 nm
Δλ = 10 nm
Directionality
Coherence
Incoherent light waves
Coherent light waves
Atomic Transitions
Stimulated absorption
Stimulated emission
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
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
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)
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
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
Dielectric solids: neodymium atoms doped
in YAG or glass to make the crystalline
Nd:YAG or Nd:glass lasers
•Semiconductor materials: gallium arsenide, indium
phosphide crystals.
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)
Current
Cleaved surface mirror
L
p+
GaAs
Electrode
L
n+
GaAs
Electrode
Active region
(stimulated emission region)
A schematic illustration of a GaAs homojunction laser
diode. The cleaved surfaces act as reflecting mirrors.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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.
Optical P ower
Optical Power
Optical P ower
Laser
LED
Stimulated
emission

Optical P ower
Spontaneous
emission

0
Laser
I
Ith

Typical output optical power vs. diode current (I) characteristics and the corres ponding
output spectrum of a laser diode.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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
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.
(a)
n
p
p
AlGaAs
GaAs
AlGaAs
(a) A double
heterostructure diode has
two junctions which are
between two different
bandgap semiconductors
(GaAs and AlGaAs).
(~0.1 m)
Electrons in CB
Ec
Ec
1.4 eV
2 eV
Ev
Holes in VB
(c)
Photon
density
2 eV
Ev
(b)
Refractive
index
Ec
Active
region
n ~ 5%
(d)
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
(b) Simplified energy
band diagram under a
large forward bias.
Lasing recombination
takes place in the pGaAs layer, the
active layer
(c) Higher bandgap
materials have a
lower refractive
index
(d) AlGaAs layers
provide lateral optical
confinement.
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
where lasing recombination occurs.
Layer
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.
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.
Cleaved reflec ting surface
W
L
Stripe elec trode
Oxide insulator
p-Ga As (Contac ting layer)
p-AlxGa1-xAs (Confining laye r)
p-Ga As (Ac tive layer)
n-AlxGa1-xAs (Confining laye r)
n-Ga As (Substrate)
Elliptical
laser
beam
2
1
Current
paths
Substrate
3
Substrate
Elec trode
Cleaved reflec ting surface
Ac tive region where J > Jth.
(Emission region)
Schematic illustration of the the structure of a double heterojunction stripe
contact laser diode
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Advantages of stripe geometry:
1. Reduced contact reduces threshold current.
2. Reduced emission area makes light coupling to
fiber easier
Dielectric mirror
Fabry-Perot cavity
Length, L
Height, H
Width W
Diffraction
limited laser
beam
The laser cavity definitions and the output laser beam
characteristics.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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
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
Techniques 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) ]
(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
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
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… …
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.
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
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 multimode oscillation fluctuates with time
To overcome this fluctuation – MODE LOCKING IS DONE i.e., the phase
between the modes is to be fixed
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
Multiple oscillating cavity modes
multiple oscillating
cavity modes
laser
gain
profile
losses
wq-2 wq-1 wq wq+1 wq+2 wq+3
possible cavity modes
Sum of various modes with same
relative phase – MODES LOCKED
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.
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 T=2L/ c
Where L= length of the gain medium
c = speed of light in free space
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
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
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 lowintensity 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
BEAM PROFILE
CW
MODE LOCKED