Laser
Introduction
The acronym LASER, which stands for light amplification by stimulated emission of radiation. Of
central importance are the fundamental processes that allow amplification at optical frequencies to be
obtained. These processes use the energy that is involved when the discrete particles making up
matter, specifically atoms, ions and molecules, move from one energy level to another. These particles
have energies that can have only certain discrete values. This discreteness, or quantization, of energy
is intimately connected with the duality that exists in nature. Light sometimes behaves as if it were a
wave and in other circumstances it behaves as if it were composed of particles. These particles, called
photons, carry the discrete packets of energy associated with the wave. For light of frequency ν, the
energy of each photon is hν, where h is Planck’s constant—6.6×10−34 J s. The energy hν is the
quantum of energy associated with the frequency ν.
On an atomic scale, the amplification of light within a laser involves the emission of such quanta.
Thus, the term quantum electronics is often used to describe the branch of science that has grown from
the development of the maser in 1954 and the laser in 1960. The widespread use of lasers and other
optical devices in practical applications such as communications, signal processing, imaging and data
storage has also led to the use of the term photonics. Whereas electronics uses electrons in various
devices to perform analogue and digital functions, photonics aims to replace the electrons with
photons. Because photons have zero mass, do not interact with each other to any significant extent and
travel at the speed of light, photonic devices promise small size and high speed.
The amplifier–oscillator connection
In ‘conventional’ electronics, where by the word ‘conventional’ for the present purposeswe mean
frequencies where solid state devices such as transistors or diodes will operate, say below 1011 Hz, an
oscillator is conveniently constructed by applying an appropriate amount of positive feedback to an
amplifier. The input and output voltages of the amplifier are Vi and Vo respectively. The voltage gain
of the amplifier is A0 where, in the absence of feedback, A0 = Vo/Vi.
The feedback circuit returns part of the amplifier output to the input. The feedback factor β = |β|ejφ is,
in general, a complex number with amplitude |β| ≤ 1 and phase φ.
and the overall voltage gain is:
As β A0 approaches +1, the overall gain of the circuit goes to infinity and the circuit would generate a
finite output without any input.
This phenomenon is fundamental to all oscillator systems. A laser (or maser) is an optical (microwave)
frequency oscillator constructed from an optical (microwave) frequency amplifier with positive
feedback. Light waves, which are amplified in passing through the amplifier, are returned through the
amplifier by the reflectors and grow in intensity, but this intensity growth does not continue
indefinitely because the amplifier saturates. The arrangement of mirrors (and sometimes other
components) that provides the feedback is generally referred to as the laser cavity or resonator.
If the length of the resonant cavity which provides feedback is L, then for L ˃˃ λ, where λ is the
wavelength at which oscillation occurs, we generally have a laser: for L ~ λ we usually have a maser.
The energy levels of atoms, molecules and condensed matter
All particles in nature have distinct states2 that they can occupy. These states in general have different
energies, although it is possible for particles in different states to have the same energy. The term
‘energy level’ is used to describe a particle with a specific, distinct energy, without implying any
particular information about its (quantum) state. The lowest energy state in which a particle is stable is
called the ground state. All higher energy states are called excited states. Excited states are
intrinsically unstable and a particle occupying one will eventually lose energy and fall to lower energy
states. When a particle falls from a higher energy state to a lower, energy is conserved. The energy ΔE
lost by the particle can be emitted as a photon with energy hν = ΔE: this is radiative energy loss. The
particle can also lose energy non-radiatively, in which case the energy is dissipated into heating.
Atomic systems have only electronic states, which in the simple Bohr model of the atom correspond to
different configurations of electron orbits. The types of energy state that exist in a molecular system
are more varied and include electronic, vibrational and rotational states.
In a molecule, changes in the internuclear separation of the constituent atoms give rise to vibrational
energy states, which have quantized energies. Molecules also have quantized rotational energy levels.
In the gas phase, the energy levels of atoms or molecules are quite sharp and distinct, although we
shall see later that even these precise energies are ‘broadened’. This broadening occurs for several
reasons but perhaps most importantly because of the interactions between neighbouring particles. In
condensed matter, whether this be in the solid or liquid state, there are very many particles close to any
individual particle of interest, and inter-particle interactions are strong. Consequently, the allowed
energies of particles in the medium occupy broad, continuous ranges of energy called energy ‘bands’.
The lowest-lying energy band, which is analogous to the ground state of an isolated particle is called
the valence band. The next highest band of allowed energies is called the conduction band.
Simple energy level diagram for a particle.
Spontaneous and stimulated transitions
To build an amplifier that operates at optical frequencies we use the energy delivered as the particles
that constitute the amplifying medium make jumps between their different energy levels. The medium
may be gaseous, liquid, a crystalline or glassy solid, an insulating material or a semiconductor. The
particles of the amplifying medium, whether these are atoms, molecules or ions, can occupy only
certain discrete energy levels. Consider such a system of energy levels, shown schematically. Particles
can make jumps between these levels in three ways. In the case of an atomic amplifier, these energy
jumps involve electrons moving from one energy level to another.
Spontaneous emission
When a particle spontaneously falls from a higher energy level to a lower one, the emitted photon has
frequency
Representation of the spontaneous emission process for two
levels of energy Ei and E j .
The probability of a spontaneous jump within a small time interval is given quantitatively by the
Einstein A coefficient defined by Ai jΔt = ‘probability’ of a spontaneous jump from level i to level j
during a short time interval Δt.
The total rate at which the population of level i changes by spontaneous emission is:
The population of level i falls exponentially with time as particles leave that level by spontaneous
emission. The time in which the population falls to 1/e of its initial value is called the natural lifetime
of level i, is τi, where τi = 1/Ai .
Levels which can only decay slowly are said to be metastable.
The lineshape function
When a particle loses energy spontaneously the emitted radiation is not, as might perhaps be expected,
all at the same frequency. Real energy levels are not infinitely sharp, they are smeared out or
broadened. A particle in a given energy level can actually have any energy within a finite range. The
frequency spectrum of the spontaneously emitted radiation is described by the lineshape function,
g(ν0, ν), where ν0 is a reference frequency, usually the frequency where g(ν0, ν) has a maximum. The
lineshape function is normalized so that:
A lineshape function g(ν0, ν).
Stimulated emission
As well as being able to make transitions from a higher level to a lower one by spontaneous emission,
particles can also be stimulated to make these jumps by the action of an externally applied radiation
field. The probability of the external radiation field causing stimulated emission depends on its energy
density, which is written as ρ(ν)
Representation of the stimulated emission process for two levels of energy
E2 and E1.
The rate for stimulated emissions to occur within a small band of frequencies dν is:
where B’21(ν) is a function specific to the transition between levels 2 and 1 and N2 is the number of
particles per unit volume in the upper level of the transition. Stimulated emission will occur if the
external radiation field contains energy in a frequency range that overlaps the lineshape function. B21
is called the Einstein B coefficient for stimulated emission.
where ρ(ν) ~ ρ(ν0) is the energy density in the frequency range where transitions take place.
Stimulated absorption
As well as making stimulated transitions in a downward direction, particles may make transitions in an
upward direction between their energy levels by absorbing energy from an electromagnetic field. B12
is a constant specific to the transition between levels 1 and 2 and is called the Einstein coefficient for
stimulated absorption. Here again ρ(ν) is the energy density of the stimulating field. There is no
analogue in the absorption process to spontaneous emission. A particle cannot spontaneously gain
energy without an external energy supply. Thus, it is unnecessary for us to continue to describe the
absorption process as stimulated absorption.
The total rate of change of particle concentration in level 1:
In stimulated emission, the incident photon produces an identical photon by ‘colliding’ with the
particle in an excited level. After the stimulated emission process, both photons are travelling in the
same direction and with the same polarization as the incident photon originally had.
Photon–particle ‘collision’ pictures of the stimulated emission and
absorption processes: (a) stimulated emission, (b) absorption.
Planck’s hypothesis was that an oscillator at frequency ν could only have energies:
The relationship between the Einstein A and B coefficients
Radiative processes connecting two energy levels in thermal equilibrium at temperature T
In thermal equilibrium, the populations N2 and N1 of these two levels are constant, so:
and the rates of transfer between the levels are equal.
In thermal equilibrium, energy levels of high energy are less likely to be occupied then levels of low
energy. Note that for this two-level systems N2 can never be greater than N1. A three or four-level
system is needed for this to occur.
and B12 = B21
Equations are called the Einstein relations. The spontaneous emission rate increases as ν3. This is why
it is much more difficult to build short wavelength lasers (for example, UV and x-ray) than long
wavelength (for example, infrared) lasers. At short wavelengths, electrons jump to a lower energy state
spontaneously at a higher rate; therefore, fewer are available to emit lasing light by stimulation.
The stimulated emission rate is W21,
which is proportional to energy density. The spontaneous emission rate is A21, which is independent of
external radiation.
Ratio of spontaneous and stimulated transitions
In microwave band the spontaneous emission is negligible and creates a source of noise—the
randomly varying component of the optical signal. In the visible and near-infrared region, spontaneous
emission generally dominates.
Optical frequency amplifiers and line broadening
This line broadening affects in a fundamental way not only the frequency bandwidth of the amplifier
but also its gain. There is 2 types of broadening, homogeneous line broadening and natural
broadening. The homogeneously broadened amplifier consists of a number of amplifying particles that
are essentially equivalent while the inhomogeneously broadened amplifier contains amplifying
particles with a distribution of amplification characteristics. This most fundamental source of line
broadening arises, as just mentioned, because of uncertainty in the exact energy of the states involved.
This uncertainty in measured energy,ΔE, arises from the time uncertainty Δt, involved in making such
a measurement. The product of these uncertainties is ΔEΔt ∼ h7. Because an excited particle can only
be observed for a time that is of the order of its lifetime, the measurement time uncertainty, Δt, is
roughly the same as the lifetime. For example, if a photon is observed at time t and is known to have
come from a state with lifetime τ , we know that the probable time t’ at which the atom became
excited was t −τ < t’ ≤ t. The longer the lifetime of the state is, the greater the uncertainty about when
the particle acquired its original excitation becomes. Other homogeneous broadening mechanisms are
collisions with neutral particles and collisions with charged particles.
Doppler broadening is the main factor of inhomogeneous broadening.
Homogeneous broadening of a group of particles in a gas that share the same velocity and
Doppler-broadened distribution of Lorentzian lineshapes.
Resonator
The quality factor Q of a resonator is defined as:
Most lasers have an optical resonator consisting of a pair of mirrors facing each other. One of the
mirrors has an ideal optical reflectivity of 100% and the other less than 100% (the laser beam is
emitted through this second mirror). The mirrors are separated by a distance L and the radii of
curvature of the mirrors are R1 and R2.
The diffraction losses in a laser are characterized by the resonator Fresnel number N. This is a
dimensionless parameter given as:
where a is the radius of the mirror resonator. A large Fresnel number implies small diffraction losses.
A resonator is stable if the stability condition:
0 < g1g2 < 1 is satisfied, where g1 and g2 are the resonator parameters:
The resonator is unstable when:
Some important resonator types are the following:
Plane parallel resonator (a). This resonator has the largest mode volume, but is difficult to keep in
alignment and is used only with high gain medium. Its diffraction losses are larger than those of stable
resonators with spherical mirrors.
Symmetrical confocal resonator, (b). The spot sizes at the mirrors are the smallest of any stable
symmetric resonator. With a Fresnel number larger than unity, the diffraction losses of this resonator
are essentially negligible.
Symmetrical concentric resonator, (c). As with the plane resonator, the exactly concentric resonator is
relatively difficult to keep in alignment; however, when L is slightly less than 2R, the alignment is no
longer critical. The TEM00 mode in this resonator has the smallest beam waist.
Confocal-planar resonator, (d). This resonator is simple to keep in alignment, especially when L is
slightly less than R. Also, small variations in the mirror spacing allows the adjustment of the spot size
w2, so that only the TEM00 mode fills the laser medium or mirror. It is widely used in low-power gas
lasers.
Modes in resonator
The axial or longitudinal modes of the resonator are the resonant frequencies of the cavity fq, where q
is the number of half-waves along the resonator axis.
The frequency spacing fΔq between two axial resonances of the laser cavity is:
Where n is the refractive index. The spacing between modes is independent of wavelength.
The output spot of the laser beam, observed on a screen, is called the transverse electromagnetic mode
(TEM). This intensity curve is a round mode with a Gaussian (bell shaped) profile in crosssection.
However, it is possible to operate a laser having a wide variety, or combination, of other transverse
modes. In these cases, the output beam may have a strange shape. Figure (a) shows that an off-axis
transverse mode is able to self-replicate (resonate) after one round trip along the optical cavity. Figure
(b) shows wavefronts in a self-replicating wave. Figure (c) shows four possible low order transverse
cavity modes. Figure (d) shows the intensity patterns of these modes. Most commercial lasers produce
the transverse electromagnetic TEM00 mode. Figure (e) shows a photograph of the TEM11 mode.
Optical resonators containing an amplifying media
When an optical resonator is filled with an amplifying medium, laser oscillation will occur at specific
frequencies if the gain of the medium is large enough to overcome the loss of energy through the
mirrors and by other loss mechanisms within the laser medium.
The reflection and transmission coefficients (r,t) in the various directions at the mirrors are as shown
in the figure.
The amplitude of the electric field vectors of the successively transmitted, amplified and reflected
waves in a Fabry–P´erot resonator system containing an amplifying (or absorbing) medium. The
absorption factors A, A1 and A2 are not used explicitly in the analysis given in the text, but they
modify the values of t, t1 and, t2.
The presence of a gain medium changes the otherwise passive propagation factor k to:
A distributed loss per pass given by an absorption coefficient α. Such absorption causes a fractional
change in intensity for a single pass through the medium of e−αl. This distributed loss modifies the
complex amplitude by a factor e−iαl/2 per pass. Therefore, the full propagation constant of the wave in
the presence of both gain and loss is:
The ratio of input to output intensities in a passive resonator, which has no gain γ or loss α, Δk = 0,
and if |r1|2 = |r2|2 = R
This function is shown in for three different values of the mirror reflectance R.
Theoretical transmittance (It /I0) characteristic of a laser resonator calculated for different values
of (equal) mirror reflectance R.
In a resonator containing an active medium, as γ − α increases from zero and we have an infinite
amplitude transmitted wave for a finite amplitude incident wave. In other words, a finite amplitude
transmitted wave for zero incident wave—oscillation.
The threshold gain coefficient depends from the population inversion needed for oscillation.
ΔN = N2-N1 ~ 1/A21λ3
Clearly lower inversions are needed to achieve laser oscillation at longer wavelengths. It is much
easier to build lasers that oscillate in the infrared than at visible, ultraviolet or x-ray wavelengths.
Optical gain curve of the lasing medium
Coherence properties
Temporal coherence (ajaline)
If a definite and fixed phase relationship exists between the wave amplitudes at A and B, then the
wave shows temporal coherence for a time c/L. The further apart A and B can be, while still
maintaining a fixed phase relation with each other, the greater is the temporal coherence of the output
beam. The maximum separation at which the fixed phase relationship is retained is called the
coherence length, Lc, which is a measure of the length of the continuous uninterrupted wave trains
emitted by the laser. The coherence length is related to the coherence time τc by Lc = cτc. The
coherence time itself is a direct measure of the monochromaticity of the laser.
To illustrate the concept of temporal coherence. If an unbroken wave train connects the points A and
B then the phase difference (φB − φA) will have a constant value.








Temporal Coherence means that the light has a single wavelength (or a single frequency), and
this is the property of mono-chromaticity;
A wave has complete temporal coherence when the phase of the wave at a certain instant of
time (Δt)
along the travelling wave front, is equal to the phase of the wave after it advance a distance L
at a time L/c for every L;
It means that in a time (Δt), while the wave advances a distance of c(Δt) it keeps its shape as
the original wave;
The narrower the linewidth (Δν) of the light source, the better is its temporal coherence;
The temporal coherence is a measure of the ability of the radiation to perform interference, as
a result of differences in path lengths between the two beams;
This is the reason why it is recommended to use a single (longitudinal) mode laser in
applications related to interference (such as holography);
Narrowing the linewidth, increase its coherence length.
Spatial coherence (ruumiline)
A laser also possesses spatial (lateral) coherence, which implies a definite fixed-phase relationship
between points separated by a distance L transverse to the direction of beam propagation. The
transverse coherence length, which has similar physical meaning to the longitudinal coherence length,
is:





Spatial coherence is measured by a wave front with a constant phase all over space. It means
that between any two points in space the phase difference is constant with time;
A laser light has spatial coherence when the phase difference between two points in space is
constant in time;
When a laser operates in a single basic transverse mode (TEM00), it has the maximum spatial
coherence;
The spatial coherence is the cause of the high directionality of the laser beam;
Spatial coherence is important in applications where all the power is needed in a single spot
(diffraction limited focusing);
Three and four level laser systems
Lasers are classified by the number of energy levels involved in the actual lasing process. Lasers
classified as three or four energy level lasers are shown in Figure. As noted previously, it is not
possible to achieve population inversion with a two-level system.
To produce the required population inversion for laser activity, atoms or molecules must be excited to
specific energy levels. Either light or electricity can provide the energy necessary to excite atoms to
higher energy levels. Excited states commonly have lifetimes of only nanoseconds before they release
their energy by spontaneous emission, a period that is not long enough to likely undergo stimulation
by another photon. A critical requirement for laser action, therefore, is an upper energy state that is
long-lived. Such states do exist for certain materials and are referred to as metastable states. The
average lifetime before spontaneous emission occurs for a metastable state is on the order of a
microsecond to a millisecond, quite a lengthy period of time on the atomic timescale. The longer the
spontaneous emission lifetime, the more suitable the atom is for laser applications.
Three and four level laser systems: (a) three-level laser; (b) four-level laser.
In the three-level system the ground state is the lower lasing level, and a population inversion is
created between the ground level and a higher-energy metastable state. Most of the atoms are initially
excited to a short-lived high-energy state that is higher than the metastable level. From this state they
quickly decay to the intermediate metastable level, which has a much longer lifetime than the higher
state (often on the order of 1000 times longer).
Population inversion
Although the three-level laser system works for all practical purposes, as exemplified by the first laser
(Ruby), a number of problems limit the effectiveness of this approach. The central problem is that the
laser has difficulties operating efficiently. This occurs because the lower lasing level is the ground
level, which is the normal state for most atoms or molecules. In order to produce the population
inversion, a majority of ground state electrons must be promoted to the excited energy level, requiring
a significant input of external energy. In addition, the population inversion is difficult to sustain for an
appreciable time, and, therefore, three-level lasers generally must be operated in pulsed mode rather
than continuously.
In a four-level system the energy level structure is similar to that of the three-level system. The
difference occurs after the atoms drop from the highest level to the metastable upper state; they do not
drop all the way to the ground state in a single step. Because the population inversion is not created
between the ground state and the upper level, the number of atoms that must be elevated is
dramatically reduced in this design. In a typical four-level laser system, if only one or two percent of
the atoms or molecules reside in the lower laser level (which is above the ground state) then exciting
only two to four percent of the total to the higher level will achieve the required population inversion.
Another advantage of separating the lower laser level from the ground level is that the lower level
atoms will naturally fall to the ground state. If the lower laser level has a lifetime that is much shorter
than the upper level, atoms will decay to the ground level at a rate sufficient to avoid accumulation in
the lower laser level. Then, laser emission occurs between the metastable level and the lower level.
Many of the lasers designed under these constraints can be operated in a continuous mode to produce
an uninterrupted laser beam.
Types of lasers
Most lasers are constructed of three elements: an active medium, a pumping source, and a resonant
cavity. The active medium is a collection of atoms or molecules, which can be excited into a
population inversion situation and can release electromagnetic radiation by the stimulated emission
process. The active medium can be in any of the four states of matter: solid, liquid, gas, or plasma. The
wavelengths of the emitted light are determined by the specific transitions between the laser energy
levels in the material. The basic physics of the laser is similar for all types of lasers, and the term
active medium will be used.
The pumping source provides the energy required to pump atoms to higher energy levels so that
stimulated emission can occur. Lasers can be optically pumped, electrically pumped (also by using an
electric discharge), or pumped by a chemical reaction. There are three sources for optical pumping:
1. Flash lamps, which are built from a quartz tube filled with gas (such as mercury vapour) at low
pressure;
2. Noble gas discharge tubes, which usually use xenon gas (but sometimes when higher energy is
required, other noble gases with lower atomic weights, such as krypton or helium, are used);
3. Another laser or any other light source, such as the light from the sun.
The resonant cavity provides a regenerative path for the photons. In essence, the functions of the
resonant cavity are to shorten the laser and to construct the electromagnetic mode. Although the
resonant cavity is a key part of most lasers, there are lasers for which the resonant cavity is not
essential. It is certainly possible to make a laser long enough so that a reasonable intensity emerges
without a resonant cavity. Nitrogen laser and most x-ray lasers are made this way. However, such
lasers tend to have poor output-beam quality.
Helium–Neon (HeNe) Laser
In the HeNe laser, the lasing material is a gas, a mixture of about 15 percent He and 85 percent Ne.
The low pressure gas mixture is placed in a glass tube that has two parallel mirrors one at each end.
Essentially, the helium is used for energizing and the neon for amplification. Atoms in the gas mixture
are excited by applying a high voltage to the tube so that an electric discharge takes place within the
gas mixture. In the process, some of the He atoms are raised to the metastable state E3, as shown in
Figure, which corresponds to a jump of 20.61 eV, almost exactly equal to an excited state in neon,
20.66 eV. The He atoms do not quickly return to the ground state by spontaneous emission but instead
often give their excess energy to the Ne atoms when they collide. In such a collision, the He drops to
the ground state and the Ne atom is excited to the state E3’ (the prime refers to neon states). The slight
difference in energy (20.66-20.61=0.05 eV) is supplied by the kinetic energy of the moving molecules.
In this manner, the state in Ne, which is metastable, becomes more populated than the E2’ level. This
inverted population between E3’ and E2’ is what is needed to produce the laser beam. The most
common HeNe lasing wavelength is red, at 632.8 nm. The HeNe laser will also laze at other
wavelengths (1.15μm and 3.39μm).
Energy levels for the HeNe laser.
HeNe energy level scheme.
Ruby Laser
The first successful optical maser or laser was developed in 1960 by Theodore Maiman using a ruby
crystal. Ruby is an aluminum oxide crystal (Al2O3) in which some of the aluminum atoms have been
replaced with chromium atoms. Chromium gives ruby its characteristic red colour and is responsible
for the lasing behaviour of the crystal. Chromium atoms absorb green and blue light and emit red light.
Figure 31.13 shows the components of the first Ruby laser device. A cylindrical crystal of ruby is
used. A fully reflecting mirror is placed on one end, and a partially reflecting mirror on the other. A
high-intensity lamp is spiraled around the ruby cylinder, to provide a flash of white light that triggers
the laser action. The green and blue wavelengths in the flash light excite electrons in the chromium
atoms to a higher energy level. Upon returning to their normal state, the electrons emit their
characteristic ruby red light. The mirrors reflect some of this light back and forth inside the ruby
crystal, stimulating other excited chromium atoms to produce more red light, until the light pulse
builds up to high power and drains the energy stored in the crystal.
The first ruby laser produces pulses of light.
Energy level diagram for the ruby laser.
As shown in Figure, the atoms are excited from state E1 to state E3. This process is called optical
pumping. The atoms quickly decay either back to E1 or to the intermediate state E2, which is
metastable with a lifetime of about 3x10-3 s (compared to 10-8 s for ordinary levels). With strong
pumping, more atoms can be forced into the E2 state than are in the E1 state. Thus, we have the
inverted population needed for lasing. As soon as a few atoms in the E2 state jump down to E1, they
emit photons that produce stimulated emission of the other atoms, and the lasing action begins. A
Ruby laser thus emits a beam whose photons have 1.8 eV of energy and a wavelength of 694.3 nm
(ruby-red light).
The lasing operation process of the Ruby laser is typical for most laser types, and is illustrated below:
1. The flash tube fires and injects external light into ruby crystal. The chromium ions (Cr3+) in a ruby
crystal are excited by light from the flash tube when high-voltage electricity is applied to the this
external light source, as shown in Figure. This light source must have photons of the right frequency to
raise to the atoms to excited states.
2. The Cr3+ ions are raised in the pumping process to energy level E3, from which they decay to the
metastable level E2 by losing energy to other atoms in the crystal. Some of these atoms emit photons,
as shown in Figure.
Some atoms emit photons spontaneously.
3. A few Cr3+ ions in the E2 level then spontaneously fall to the E1 level. Some of these photons
propagate parallel to the axis of the ruby crystal, so they bounce back and forth between the reflecting
end mirrors of the ruby rod, as shown in Figure. This initiates the continuing process of stimulated
emission and amplification.
Some photons traverse the ruby’s axis and reflect between the mirrors.
4. The presence of light of the right frequency now stimulates the other Cr3+ ions in the E2 level to
radiate; the result is an avalanche of photons that produces a large pulse of red light. Some of the
photons leave through the partially silvered mirror at one end. The red light is the laser light, as shown
in Figure.
Laser light leaves the ruby through the partially silvered mirror.
Semiconductor lasers
All semiconductor lasers, sometimes called laser diodes, are built from semiconductor materials. The
first semiconductor laser was created by Robert Hall in 1961 in the United States of America. In 1975,
the first semiconductor laser capable of operating continuously at room temperature was introduced.
This development has led to the use of semiconductor lasers in the CD-ROM, DVD-ROM, laser
pointer, and many other useful devices. The conductivity of a semiconductor increases with
temperature, up to a certain temperature level. The efficiency of a semiconductor laser decreases when
the temperature increases.
When a diode is forward biased, holes from the p-region are injected into the n-region, and electrons
from the n-region are injected into the p-region, as shown in Figure. If electrons and holes are present
in the same region, they may radiatively recombine; that is, the electron falls into the hole and emits a
photon with the energy of the band gap. This is called spontaneous emission and is the main source of
light in a light-emitting diode.
E-k and energy band diagrams for a diode: (a) the E-k diagram; (b) the energy band diagram.
Under suitable conditions, the electron and the hole may coexist in the same area for quite some time
(on the order of microseconds) before they recombine. If a photon of exactly the correct frequency
happens along within this time period, a recombination (of hole and electron) may be stimulated by
the photon. This causes another photon of the same frequency to be emitted with exactly the same
direction, polarization, and phase as the first photon.
In a laser diode, the semiconductor crystal is fashioned into three layers very thin in thickness and
rectangular in the other two dimensions. An optical waveguide is made in the middle layer such that
the light is confined to a relatively narrow line. This is called the active layer. The top of the crystal is
p-doped, and the bottom is n-doped, resulting in a p–n junction, which forms the diode. The two ends
of the crystal are cleaved so as to form perfectly smooth, parallel edges like mirrors; these two
reflective parallel edges form a resonator called a Fabry–Perot cavity.
Photons emitted in precisely the right direction will travell along the cavity and be reflected several
times from each end before they exit the cavity. Each time photons pass through the cavity, the light is
amplified due to stimulated emission. Hence, if there is more amplification than loss, the diode begins
to laze. The wavelength emitted is a function of the band-gap between the energy levels of the p and n
regions. No photons with energy higher than the band-gap will be emitted.
Laser diode.
A heterostructure laser.
Emitting laser diodes: (a) edge emitting; (b) surface emitting.
Comparison of semiconductor and conventional lasers
1. The gain of the semiconductor laser material is very high and is generated by a population inversion
between the conduction and valence bands of the semiconductors. In some sense, a semiconductor
laser is a two-state laser system.
2. Since the longitudinal mode is on the order of the size of the laser device, the transverse mode of the
semiconductor laser is quite different from that of a conventional laser. In particular, the beam is not
Gaussian, the beam profile tends to be elliptical, and the beam divergence tends to be large.
3. The gain spectrum is quite large (many THz or hundreds of angstroms).
4. The short cavity length (several hundred microns) means that the longitudinal mode spacing is
much larger than that of a conventional gas or solid-state laser (on the order of GHz or angstroms).
5. Due to the small size of semiconductor lasers, they have the potential for mass production and can
be easily integrated on PC boards.
6. The properties of the semiconductor lasers are being rapidly improved. They are becoming
increasingly powerful and efficient laser sources.
Short history of laser development
1916 Albert Einstein proposes stimulated emission;
1954 Charles Townes and James Gordon produce first microwave maser at 24 GHz at Columbia
University;
December 1958 Townes and Arthur Schawlow publish detailed “optical maser” proposal in Physical
Review;
May 16, 1960 Theodore Maiman demonstrates ruby laser at Hughes Research Labs;
November 1960 Peter Sorokin and Mirek Stevenson, IBM, make first four-level solid state laser,
Uranium in CaF2;
December 12, 1960 Ali Javan, William Bennett, and Donald Herriott of Bell Labs make HeNe laser,
the first continuous-wave laser and the first gas laser
November 22, 1961 Ruby laser repairs detached retina in first patient at Harkness Eye Institute in New
York;
1962 First semiconductor diode laser, Robert Hall, GE R&D Labs, followed in weeks by three other
groups;
1964 First 3-D laser holograms displayed by Emmett Leith and Juris Upatnieks;
1964 Kumar Patel makes CO2 laser at Bell Labs;
1966 Ed Gerry and Arthur Kantrowitz invent gasdynamic CO2 laser, which eventually reaches
hundreds of kilowatts;
1969 Ruby laser pulses range the moon by bouncing off retroreflector placed by Apollo 11 astronauts;
1970 Zhores Alferov demonstrates first room-temperature cw diode laser;
1974 First laser scanner demonstrated in a supermarket;
1979 Philips shows prototype compact disk player;
1988 TAT-8, the first transatlantic fiber cable, is completed;
1989 Isamu Akasaki demonstrates blue LED of GaN;
1995 Pulse length of Ti-sapphire reaches 8 fs;
2002 TAT-8 submarine cable retired after failure because its capacity was too small to justify the cost
of repairs.