sleep at time facts, lasers, settle into the bed, and let your breathing become slow, and even
lasers are quiet examples of light, made orderly, ordinary light spreads out in many directions.
And mixes many wavelengths together, a laser produces a narrow beam, in which the light waves
move together. In step, this organization gives the beam a steady direction, and a very pure color,
you may have encountered lasers. Without thinking about them, they help read the dark bars.
On grocery packages, they carry soft pulses of light through thin glass fibers,
that connect phones and computers. In each case, the laser does one gentle job.
It keeps light aligned as you rest. Imagine that same steadiness. The beam does not rush,
or scatter. It simply travels along its path. Your thoughts can settle in the same way.
One soft focus, after another, while the room stays still, and safe, the word laser began,
as a compact, way to name a very particular kind of light. It is built from the initial letters
of a longer phrase. And that phrase describes what the device does in calm, physical terms. The phrase
is light amplification by stimulated emission of radiation over time. The acronym became so familiar
that people began writing it as an ordinary word. That shift is useful. It reminds us that a laser
is not a mysterious object. It is a tool for making light behave in an orderly manner. Light is the first
part of the name. And it is the thing being shaped in everyday life. Light arrives from many sources
and spreads in many directions. A lamp sends out waves that wander outward. The sun sends a broad
mix of wavelength, a laser by contrast. Works with light that has been gathered into a much
narrower pattern. The word light here refers to electromagnetic waves. The same general family
as the glow from a candle. Or the brightness of a clear afternoon, the laser does not create a different
kind of substance. It arranges ordinary light into a more focused form. Amplification comes next
to amplify something is to make it larger or more pronounced and sound. Amplification can make a quiet
voice carry across a room in light. Amplification means increasing the amount of coordinated
electromagnetic energy. A weak signal can prompt a larger response. Inside a suitable material,
the result is a build-up, rather than a chaotic flood, the waves gain strength because they are
supported by the material. They pass through. The idea is gentle in principle. A small, orderly
influence can encourage a larger, similar response stimulated is the word that gives the phrase
it's special character. A stimulus is something that prompts a response in a laser. The stimulus
is light itself when an atom or molecule in a material is already holding energy. A passing wave
can encourage it to release that energy as another wave. The new wave matches the first in important
ways. It travels in step with the original. This matching is what makes the light orderly. The process
does not need to be forced in a rough way. It is a prompt and a quiet invitation for energy to appear
in the same pattern that is already present. Amission follows. Amission means sending out. Many things
emit light, a warm surface, emits infrared, a firefly, emits a soft glow in the laser phrase.
Amission refers to light leaving matter and entering space. The special part is that the emission is
stimulated rather than random. It happens in response to a wave that is already there. That response
adds to the existing light. Each matching addition helps the pattern become clearer. The light
is not scattered into every corner. It is produced in a way that supports one shared direction
and rhythm. Radiation is the final word and it can sound more serious than it needs to in physics. Radiation
simply means energy traveling outward. Sunlight is radiation. The warmth felt nearest stove is
radiation. The term describes motion. Energy moves from one place to another in the laser acronym.
Radiation is the light that has been amplified and emitted in a coordinated pattern. The word
completes the phrase by naming what travels away from the device taken together. The phrase
describes a sequence of ideas. Light is present. It is amplified. The amplification depends on
stimulated emission. The emitted energy becomes radiation that moves outward. The acronym places those ideas.
In a neat row it gives a name to process or light encourages more light of the same kind to appear.
The device called a laser is the place where this process is arranged with care. The acronym also leaves
some things open. It does not name the material inside the device. It does not require a single color
or a single shape for the beam. Those details can vary. What remains constant
is the method suggested by the phrase light is increased through a process. In which existing light
prompts new light that is the core of the name. The small word by the phrase carries a quiet load.
It tells us how the amplification happens. The increase is not produced by simply adding
unrelated waves. It comes through stimulated emission. One orderly pattern invites another. The phrase
therefore links cause and effect in a few words. Amplification is the result. Stimulated emission
is the means because the name is a process. The word laser can point to more than one thing.
It can mean the instrument that creates the light. It can mean the beam that leaves it. In both cases
the acronym remains the same guide. The beam is called laser light. Because it has been formed
through the process named by the phrase, the instrument is called a laser. Because it is built
to support that process, the phrase also suggests control. Without spelling out, every part of the
arrangement, amplification can happen in many settings. But a laser is concerned with a disciplined
kind of increase. The waves are encouraged to build in a shared pattern. The device provides
conditions where that build-up can continue nothing in the acronym demands a particular size or power.
It only identifies the orderly route by which the light is made stronger over the years. The word
has settled into ordinary speech, people use the word for printers and light shows in those
uses. The acronym has become an adjective or a simple noun. The full phrase can fade from view,
returning to it, restores the original sense before it settled into everyday use. The phrase was
formed after the underlying idea was already understood. Scientists had recognized that matter
could hold energy and release it as light. They also understood that an incoming wave could encourage
that release once those pieces were clear. The acronym gave them a compact label. It turned
a chain of physical events into a single term. The label helped people talk about the device.
Without describing every step each time light can move through matter without leaving a trace.
Or it can be absorbed and become a slight increase in internal energy between those familiar outcomes,
sits a quieter possibility, an atom or molecule that already holds extra energy.
Maybe prompted by passing light to give some of that energy back. The prompt does not strike the particle,
like a tiny hammer, it acts through the oscillation of the electromagnetic field,
encouraging the excited system to change to a lower energy state. When that happens,
the system releases a photon. This process is the central event that makes laser light possible
an excited atom or molecule. Amits light in response to light already present. The emitted photon
is not a random answer. It matches the incoming photon in color, which means it has the same
frequency and energy. It also travels in the same direction and keeps the same phase if the incoming
light wave has crests and troughs arranged in a certain pattern. The new photon joins that pattern,
rather than starting a new one to picture this. Think of an atom as having only certain
allowed amounts of internal energy. It cannot hold just any quantity. It can rest in a low state.
Or it can hold a definite extra amount after receiving energy from somewhere else. That extra
energy might have come from an earlier flash of light from an electrical current or from a collision
with another particle while the atom remains in the higher state. It is ready to release the surplus
if the right opportunity appears. One opportunity is spontaneous a mission. In that case,
the excited atom simply drops to a lower state and sends out a photon on its own. The photon
has the correct color for the energy difference. But its timing, direction, and phase are not tied
to another light wave. Spontaneous a mission is common. It is why many materials glow softly.
After being energized, stimulated a mission is different because the presence of a suitable
photon guides the release the guiding happens because light is an oscillating electric and magnetic field
and electron in an atom responds to electric fields. Even very weak ones when a photon with the
right energy passes an excited atom. It's field oscillates at a frequency that corresponds to
the gap between the atoms higher and lower states that matching rhythm can encourage the electron
to shift downward. The atom then gives up the energy difference as another photon. The original
photon continues onward instead of one photon entering the region and being absorbed.
One photon enters and two photons leave. The two leaving photons are closely matched. They carry
the same energy. So they have the same color. They move together along the same path. Their wave
peaks line up with one another and their electric fields oscillate in the same orientation.
This matching is why stimulated a mission can produce light that is unusually orderly. The process
adds brightness in a coordinated way. Phase is one of the quieter ideas here. Two waves can have the same
color and still be out of step if one wave reaches a crest while another reaches a trough.
They can weaken each other if their crests arrive together. They reinforce each other
in stimulated a mission. The new photon is born in step with the photon that prompted it. The light
wave therefore gains a partner that supports its existing rhythm. Direction matters too
ordinary light from a warm object spreads outward in many directions each atom emits independently.
And the photons fan away from the source in stimulated a mission. The emitted photon follows the
direction of the incoming photon. This tendency keeps the light confined to a shared path.
If many excited atoms lie along that path, each prompted photon can prompt another.
And the same narrow beam can grow stronger. This growth is often described as amplification, a small
amount of well chosen light can become larger. Without changing its basic character,
the added light carries the same frequency and phase as the light that began the process in a laser.
This principle is placed inside a carefully prepared material where many atoms or molecules are ready to
the material provides the gain, the stimulated a mission, provides the order, still the heart of
the matter, remains the single atomic event and incoming photon, prompts an excited system to release
a matching photon. The idea was worked out by considering how atoms and radiation share energy
early studies of light and matter had already described absorption. We're a photon is taken up
and an atom moves to a higher energy state. They also described spontaneous a mission
where an excited atom emits without outside prompting to make the full accounting of energy exchange
consistent. Theurists recognized that a third path was needed if light could raise an atom to a higher
state. Then an excited atom should also be able to fall to a lower state under the influence
of light already present the symmetry of the situation, pointed toward stimulated a mission,
and later observations confirmed that it occurs in nature. No dramatic event is required.
The atom does not need to be shaken. It simply moves from one allowed energy state to another
and the energy difference appears as light. The process follows the same conservation rules
that govern all quiet exchanges in physics. Energy is neither lost nor invented. Momentum
is also accounted for. Which is one reason the emitted photon travels along the same path
as the prompting photon. The whole interaction is an orderly transfer the match between the incoming
photon and the atomic transition must be close if the photon has too little energy.
It cannot prompt the particular downward step if it has too much. It may pass by without causing
that a mission or it may interact in some other way. Atoms are selective. They're allowed energy gaps
to determine which colors they can absorb and which colors they can emit stimulated
a mission. Respects that selectivity a photon of one color can prompt a mission of the same color
from a suitable excited atom. But it will not make every atom respond this selectivity
gives stimulated a mission. It's neatness. The emitted light does not smear into many colors.
It stays tied to a specific transition in some materials. The transition is narrow in others.
The allowed energy spreads slightly because the atoms are in a solid or a mixture.
And the emitted color can cover a small range. Even then, the stimulated photons
follow the incoming light closely. The process preserves the character of the light that stimulates it.
There is also a timing aspect and excited atom cannot remain excited forever.
But the length of time it stays ready can vary. Some excited states last. For a tiny fraction
of a second, others last much longer if a matching photon arrives. While the atom is still excited,
stimulated a mission can occur if the atom has already released its energy spontaneously.
The opportunity is gone. The process therefore depends on having excited systems present
at the same time. As suitable light passes through them, stimulated a mission can happen with molecules
as well as atoms. Molecules have more ways to store energy because they can vibrate and rotate
in addition to shifting electronic states. That means they can support stimulated a mission
across a wider range of wavelengths, including infrared light. The basic rule remains the same
and excited system releases a photon that agrees with the photon that prompted the release at a deep
level. The process shows that light can organize light, a passing wave carries energy
and it can also shape how energy leaves matter. The emitted photon joins the wave with the same
rhythm and direction. So the passing light becomes a little more unified, atoms hold energy
in quiet, particular amounts and electron in an atom can rest in a lower energy condition.
Often called the ground state or it can move to a higher condition if it receives just the right
amount of energy. These allowed conditions are not continuous like a ramp. They are more like steps
on a ladder, the spacing between steps gives each material its own set of possible light colors
when light passes through ordinary matter. Many atoms are in the lower step, a photon with the right
energy can be absorbed. Lifting an atom upward, this makes the light weaker if the atom later falls back
down. It may release a photon but in ordinary conditions the release happens in random directions
and at random times the result is soft. Spread out light rather than a growing beam there is another
possibility if an excited atom is already sitting on a higher step. A passing photon with the matching
energy can encourage it to drop down the atom then emits a second photon that matches the first
in color, direction and phase. This is stimulated emission one photon becomes two and the two
can encourage more excited atoms to emit in step for this to happen often enough. The material needs
many atoms already in the excited condition under everyday conditions nature prefers lower energy
or materials have some excited atoms yet most atoms remain in lower states if a beam of light
enters such a material. Absorption tends to be stronger than stimulated emission the beam loses
energy to the atoms instead of gaining energy from them to make light grow. The balance must be
shifted more atoms need to occupy a chosen excited state than a lower state connected to the transition
that shifted balance is the condition laser physicists work to create the idea can be compared to a
quiet hillside if most pebbles lie at the bottom a small push will not send many rolling downward
if many pebbles are placed higher up a tiny nudge can start a steady cascade in a laser medium
the pebbles are atoms in energy states and the nudge is a passing photon the cascade is orderly
it is a coordinated release of light each new photon matching the one that came before creating
this upward imbalance requires energy from outside the process is called pumping pumping can be done
with light from a lamp or another laser with an electric current or with chemical reactions that leave
products in excited forms the purpose is the same energy enters the medium and lifts atoms
or molecules into higher states faster than they naturally fall back down the medium then
holds a store of excited particles ready to emit when stimulated not every pair of energy levels
can support such a store some excited states empty too quickly others decay by releasing heat
or by emitting light interactions that do not help the beam useful laser transitions
often involve a metastable state an excited condition where atoms remain for a relatively long time
a longer stay gives pumping a chance to fill that state it also gives passing photons more
opportunity to trigger a mission before the energy leaks away a simple two-level arrangement
is hard to invert if atoms are pumped from a lower level to an upper level the same light that pumps
them can also stimulate them back down as the upper level fills absorption and stimulated
emission become balanced the system settles near equal occupancy and the beam does not gain much
many practical lasers therefore use three or four levels in a three level scheme atoms are
lifted from the ground state to a high state then they relax into a metastable state the laser
transition occurs from that metastable state to the ground state because the lower level is the ground
state a large fraction of atoms must be lifted before a mission can exceed absorption for level
arrangements make the task gentler the laser transition ends in a level above the ground state
and that lower level empties quickly since few atoms remain in the lower laser level
even a modest number in the upper level can create the needed imbalance this helps the laser reach
amplification with less pumping energy the difference between these level patterns
explains why some materials are easier to use than others once the inverted medium is prepared
a photon traveling through it can be amplified if the photon has the right energy
it meets excited atoms and encourages matching photons the light intensity grows as it travels
in a laser mirrors often guide light back and forth through the medium
letting the beam pass many times each pass can add more matched photons one mirror may let a small share
escape as the useful beam while the rest continues to circulate the balance between pumping
and lost determines whether amplification continues energy leaves through the output beam
through absorption in the material and through scattering pumping must replace the excited atoms
that have emitted if pumping is steady the medium can maintain a stable inverted population
the beam then settles into a calm continuous output if pumping comes in pulses
the beam may appear in pulses too in both cases the central requirement remains the same
the medium must hold enough excited atoms to make stimulated emission stronger than absorption
the same requirement also shapes which transitions can be used the energy difference between two states
sets the photon energy to obtain a desired wavelength the medium must have suitable levels separated
by that amount some materials offer convenient transitions at accessible energies others require
more complicated media or special gas mixtures the need to maintain an inverted population
at a specific energy gap is one reason some transitions are harder to sustain still the underlying
condition is the same a selected upper state must be more populated than a selected lower state
temperature and material structure influence how easily the condition forms at higher temperatures
atoms move more vigorously and occupy many energy states this can spread the available energy
and make a clean imbalance harder solid crystals glasses gasses and semiconductors
each provide different patterns of levels engineers choose a medium whose levels fit the desired
wavelength and pumping method they also shape the medium so light can travel through it while gaining
more than it loses the concept was worked out gradually as physicists studied how atoms exchange
energy with radiation the key insight was that light amplification requires a departure from ordinary
thermal balance in normal equilibrium lower states are favored and absorption winds by supplying
energy selectively a medium can be held in a non-equilibrium state that state allows a weak signal to
become stronger without changing its color or direction the process is orderly photons do not pile up randomly
they are matched copies produced by atoms that have been prepared in advance a helpful way to think of it
is a row of quiet bells if the bells are still a passing sound may be swallowed by soft cushions
if each bell is already tense with stored energy the same sound can set them ringing in unison the
incoming sound is carried forward it is repeated and strengthened in a laser medium the stored energy is
atomic excitation and the ringing is coherent light maintaining the condition does not require force
in a crude sense it requires steady bookkeeping pumping adds energy a mission removes it losses
carry some away the medium reaches a working point where enough excited atoms are present at every moment
small fluctuations may occur but the overall population can remain stable this stability
allows the beam to have a narrow color and a steady pattern the inverted population is therefore
the heart of laser action it turns a material from an absorber into an amplifier it lets a single
photon invite others to match it it makes possible the narrow orderly beams that distinguish laser
light from ordinary lamp light the condition exists only while energy is supplied when pumping stops
atoms relax the imbalance fades and the medium returns to its usual preference for lower energy the
light then quiet leaving the atoms settled once more a laser keeps light in a place long enough for it
to gather strength the place is usually a space between two mirrors and that space is often called
an optical cavity the gain medium sits inside the space when atoms are molecules in the medium release light
the mirrors guide some of that light back through the medium again each return gives the light another chance
to encourage more a mission the cavity is a path that light follows over and over
until the path itself shapes what remains the simplest cavity has two facing mirrors one mirror reflects
almost everything that reaches it the other reflects most of the light but allows a small portion
to pass that second mirror is sometimes called the output coupler the name fits the action it couples a
little light out of the cavity while keeping enough inside to continue the process if both mirrors were
perfectly reflective the light would stay within the cavity and no beam would emerge if the mirrors
reflected too little the light would fade before it could build the useful arrangement lies between those
limits inside the cavity light travels back and forth along the line between the mirrors the gain
medium adds energy to the light on each pass the added light matches the passing light in direction
and color because the mirrors keep returning the light the matching becomes stronger photons that
happened to travel along the cavity axis are favored photons that drift sideways soon leave the region
between the mirrors and do not return this simple preference gives the emerging light a clean
direction the cavity also favors certain patterns of vibration light is an electromagnetic wave
and a wave can fit between two mirrors only in particular ways often this means the space
contains a whole number of half wavelengths the wave must match the distance between the mirrors
so that repeated reflections line up with themselves when the returning wave arrives in step
with the wave already present it reinforces it when it arrives out of step it tends to cancel this is
resonance the cavity simply gives steady support to the waves that fit and less support
to the waves that do not resonance gives the laser a narrow set of allowed colors the gain medium
may be able to emit over a range of wavelengths but the cavity can favor only those wavelengths
that satisfy its spacing in many lasers several resonant wavelengths can exist close together
the design of the mirrors and the medium can then favor one of them more strongly than the others
the result is light with a very small spread of wavelengths the beam appears as a single color
even when the underlying medium could support a broader range the distance between the mirrors
sets the spacing of these resonances along our cavity allows more closely spaced
resonant wavelengths a shorter cavity spreads them farther apart builders of lasers choose
cavity length with the kind of light they want in mind they also choose mirror coatings
to reflect the desired wavelengths a mirror that works well for one color may not work as well for
another the coating is made from thin layers that reflect light through controlled interference each
layer is a quiet adjustment to the way the surface treats different wavelengths partial
transmission is a careful choice the output mirror must let enough light escape to form the beam
yet keep enough inside to maintain oscillation the right amount depends on the gain available
in the medium a medium that adds energy easily can work with a mirror that transmits a little more
a medium with modest gain needs a mirror that reflects more strongly the aim is a steady rhythm
between growth and release the cavity also shapes the cross section of the beam light can form patterns
as it bounces and the mirrors determine which patterns remain stable some cavities
support a broad spot others support rings or lobes many common lasers are built to favor the
simplest spot a smooth central region with gentle edges this happens when the mirror curvature
and spacing guide raise back toward the center rather than letting them wander outward
the geometry gives the beam its familiar round form mirror curvature helps keep the light centered
flat mirrors can work but they need very careful alignment curved mirrors can act like gentle guides
they bend the returning light back toward the axis and make the cavity stable over many
trips the light still travels in straight lines between reflections yet the overall path remains confined
this stability is one reason many cavities use one or two curved mirrors the beam that finally passes
through the output mirror carries this stable shape with it not all light inside the cavity
becomes the output beam some light is absorbed in the mirrors or the medium some scatters from small
imperfections some leaves through the sides the cavity quietly sorts these possibilities waves that match
the resonance continue while waves that do not match fade directions that stay aligned are
amplified and directions that wander are lost the beam that emerges is the portion of light that
survived this sorting and passed through the partially transmitting mirror the optical cavity
also gives the light a shared phase relationship because the same wave pattern repeats
between the mirrors the emitted light tends to have a regular rhythm this regularity is part of what
makes laser light different from the mixed light of a lamp the cavity does not create the initial
emission by itself the gain medium supplies the energy and the first photons the cavity
organizes those photons into a steady repeating pattern and lets a controlled fraction leave
when a laser is running the cavity maintains a quiet circulation energy enters the gain medium
and the medium transfers some of that energy to light the light then returns through the cavity
and a small part exits as the beam the mirrors make this cycle possible without moving parts in many
designs they hold the path and set the allowed waves they also decide how much light remains inside
the beam appears as the visible edge of a steady circulation a lamp gives light the way
a handful of sand scatters across a table each grain lands where it may the light from a bulb
a candle or the sun arrives as many small emissions from countless atoms each sending out a wave
that starts and stops on its own these waves have different colors and point in many directions
they do not keep a shared rhythm the result is bright and pleasantly familiar it is also mixed
laser light is different because it is organized the word coherence describes that organization
in a coherent beam the light waves maintain a steady relationship with one another their peaks
and troughs line up in a predictable way and they keep that relationship over distance and time this
is why a laser can form a clean spot on a wall or traces straight line across a room without spreading
into a soft glow coherence has two sides one side concerns color light of a single color
corresponds to a narrow range of frequencies a lamp emits a broad band of frequencies even a lamp
that looks white contains many colors at once a laser by contrast tends to emit light
within a very small range of frequencies that narrowness means the waves do not drift out of step
as quickly this steadiness over time is temporal coherence musicians call this kind of steadiness
staying in tune though with light the idea is about timing rather than pitch the other side
concerns direction and shape in ordinary light waves move outward in many directions from the
source a shade or lens can guide some of that light but the underlying emission remains disorderly
in a laser the light is encouraged to move along one preferred path mirrors inside the device favor waves
the travel back and forth along the same line waves that wander sideways are not reinforced
the result is a beam with a steady direction this directional order is called spatial coherence it
means that different points across the beam are related in a steady way if you could freeze the beam at one
instant the wave pattern would have a regular shape that regular shape can be focused to a small
spot it also lets the beam remain narrow over a long distance the beam still spreads a little
because all waves spread when they travel a wide orderly beam spreads more slowly than a jumbled one
ordinary lamp light lacks this shared order because of how it is produced in a warm filament
or a glowing gas atoms release energy independently one atom may emit a wave now another moment later
and a third in a different direction each tiny wave train has its own phase phase is a way of
describing where a wave is in its cycle whether it is near a peak a trough or somewhere between
when many waves have random phases they do not support one another they add up to a broad gentle wash
a laser arranges emission so that many atoms contribute in step the process depends on stimulated
emission or an incoming photon encourages an excited atom to release another photon
with matching properties the new photon matches the original in frequency and direction it also shares
the same phase one matching photon can prompt another and another over many events a population of
matching waves grows the light becomes more alike with each pass the structure of a laser
helps this along again medium sits between mirrors light traveling along the axis bounces back and
forth on each pass it stimulates more matching emission the mirrors let a small portion escape
as the output beam because the escaping light comes from waves that have already been selected
for direction and frequency it carries coherence with it the beam is bright and it is coordinated
this coordination explains why a laser can stay narrow or lamp light cannot imagine trying to
send light from a bulb down a long hallway you could place a lens in front of it and the lens
would form a beam of sorts but the beam would contain many colors and many independent wave fronts
it would blur and widen quickly a laser beam starts with a wave front that is already orderly
so it can cross the same hallway as a slim patch of light the difference also appears in shadows
and edges laser light can produce sharp interference patterns because its waves remain
related ordinary light can produce interference too but it usually needs careful filtering a
filter might select one color and one small portion of the source then the light behaves more
coherently without filtering the many colors and phases wash out fine patterns the lamps light
is not inferior for general illumination it is simply suited to a different task a cozy reading lamp
benefits from broad mixed light it fills a room softly it reveals many surfaces
without demanding alignment a laser beam would not be a comfortable replacement for that it is meant to
carry order not to spread comfort the two kinds of light answer different needs one is like a crowd
chatting in a warm hall the other is like a single clear note held by a flute coherence
also affects how light can be focused a lens gathers light and bends it toward a point if the
incoming waves are coherent the lens can bring them together into a very small well defined spot
if the incoming waves are mixed in color and direction the best focus is softer
different colors bend by slightly different amounts different directions arrive at slightly different places
the focus becomes a gentle blur rather than a tight point this is one reason laser beams
are useful for careful measurement a coherent beam can be split and recombined to detect tiny
changes in distance it can be aimed at a distant reflector and still return enough organized light
to be read it can be focused onto a very small region without needing enormous lenses none of this
requires the light to be harsh it only requires the waves to stay in a known relationship
ordinary light has its own quiet virtues sunlight carries a broad spectrum which helps us see colors
naturally a warm bulb makes a space feel settled a candle flame flickers with small changes
that many people find pleasing these sources do not need coherence to be valuable they provide
illumination by abundance rather than order a chemical laser makes light from the energy
released when atoms or molecules rearrange themselves into new substances in an ordinary gas laser
an electric current or another light source lifts atoms into excited states in a chemical
laser the lifting is done by a reaction the reactants often two compatible gases meet in a chamber
as they combine the new molecules are often born with extra energy stored in their vibrations
or electronic states if enough of them occupy those excited states light passing through
the gas can stimulate them to emit matching photons mirrors at the ends of the cavity guide the light
back and forth and a small fraction escapes as a beam the central idea is still the same population
inversion used in other lasers a lower energy level must be less populated than an upper one
so that photons are more likely to cause emission than absorption chemistry supplies that
imbalance directly some reactions release energy in a way that places the product molecule
in a high vibrational level the molecule has just formed and its atoms are still springing
against one another like a small oscillator that stored motion can become the upper level of a laser
transition many chemical lasers rely on molecules rather than isolated atoms molecules have
vibrational and rotational states and the spacing between those states is often small compared
with electronic gaps in atoms small energy steps correspond to lower photon frequencies that is one
reason chemical lasers frequently emit in the infrared infrared photons carry less energy
than visible photons and molecular vibrations are naturally suited to producing them the beam
may be invisible to the eye yet it can be smooth and steady one familiar family uses hydrogen or
doterium combined with fluorine when these react the resulting hydrogen fluoride or doterium
fluoride molecules can be formed in excited vibrational states the molecules then emit infrared
light as they relax toward lower vibrational levels the exact wavelength depends on the molecule
and the conditions inside the cavity doterium compounds tend to shift the emission
to longer wavelengths because the heavier atom changes the vibration frequency this is a quiet sort of
tuning governed by mass and bond strength rather than by moving parts another chemical laser
uses excited oxygen to transfer energy to iodine atoms in that system a chemical reaction
first creates oxygen molecules in an excited electronic state those oxygen molecules collide with
iodine atoms and pass some of their energy along the iodine atoms become excited and they provide
the laser transition the emitted light is near infrared this arrangement shows that the reaction
does not always have to create the lazing particle directly it can create an intermediate carrier
which then passes energy to the actual laser medium such systems can support steady beams
because the reactants can be flowed through the device fresh molecules enter the optical region
emit light and are carried away spent gases leave with the flow and heat can be managed by the
moving stream this continuous renewal is different from a sealed tube where the gas gradually warms
and the excited population changes with careful flow design the output can remain stable
for as long as the supply is maintained the beam becomes a kind of controlled river with chemistry
feeding it from one side and optics shaping it from the other the distinction from electrically
pumped lasers is especially clear in the way energy enters the medium a gas discharge
depends on electrons accelerated by an electric field those electrons collide with atoms or molecules
and raise them to higher states a chemical laser depends instead on bond formation the energy
comes from the rearrangement of electrons in chemical bonds and it appears in the product particles
almost immediately this can be efficient for certain transitions and it can produce large amounts
of excited material without requiring electrodes to carry high currents
through the gas optically pumped lasers are also different the absorb light from a lamp or another
bright source and then give off light at a desired wavelength chemical lasers skip that intermediate
light step the reaction itself creates the excited state this can simplify some aspects of the design
because there is no need for a separate pump source of high brightness it also means the laser medium
is tied closely to reaction chemistry flow patterns and mixing the device is as much a careful
chemical instrument as an optical one the beam quality depends on keeping the gas uniform
temperature and pressure affect the population of energy levels and composition does the same if the
gas becomes too warm collisions can spread energy into unwanted states if mixing is uneven
some regions may absorb light instead of amplifying it engineers and scientists therefore shape
nozzles and channels so that the reacting gases move smoothly mirrors are placed where the beam
can grow in an ordered stream chemical lasers also illustrate why some colors are harder to create
than others visible light requires larger energy gaps than infrared many reactions release
energy readily into molecular vibration which matches infrared transitions to reach visible wavelengths
the reaction must populate higher electronic states and those states often lose energy
through other paths before they can laser collisions can quench the excitation
or the products may form in states that do not provide a useful inversion for this reason
chemical lasers tend to be most successful in the infrared and near infrared
where the natural motion of molecules aligns with the light being produced the mirrors used with
these beams are chosen for the wavelength involved infrared light may be reflected by polished metals
or by coatings designed for long wavelengths since the beam is invisible
instruments are used to observe its position and power a viewing card or a calibrated detector
can show where the light is going in a laboratory the setup often sits behind protective
enclosures keeping the beam path contained and the room calm spectral measurements
often reveal fine structure within the beam a single vibrational band can contain many rotational lines
each tied to a slightly different molecular motion reading those lines gives a quiet record
of the molecules that carried the light the beam in this way carries information about the reaction
that made it a laser needs a material that can amplify light that material is called the
gain medium and it sets many of the basic limits on the color a laser can produce inside the gain
medium atoms ions molecules or semiconductor bands hold energy in discrete amounts when an
excited particle gives up some of that energy as light the photon carries an amount of energy
that matches the gap between two allowed states a larger gap means a higher photon energy
higher photon energy corresponds to a shorter wavelength this simple relation
is why short wavelength lasers ask so much of their gain media red and infrared lasers
often rely on modest energy gaps many familiar solid state crystals gas mixtures
and semiconductor structures support those transitions with stable well understood materials
the particles can be pumped into excited states and a useful fraction of them return by a
meeting photons that match the desired transition mirrors guide the light back through the medium
so the emission grows the process can be efficient enough that heat remains manageable blue and
ultraviolet light require larger gaps the gain medium must contain states separated by the right amount
and those states must connect in a way that favors light emission rather than silent relaxation
silent relaxation happens when excitation turns into vibrations of the surrounding material
instead of photons those vibrations become heat in a short wavelength material even small losses
can matter because each photon already represents a large packet of energy if many excitations decay
without radiating the medium warms and the laser becomes harder to sustain suitable
host materials also become fewer as the desired wavelength shortens a good laser host must be transparent
at both the pump wavelength and the laser wavelength it must accept the active ions or molecules
without degrading it must remain mechanically stable while carrying optical power
for ultraviolet light many ordinary glasses and crystals absorb strongly absorption turns light
into heat inside the medium or nearby coatings that makes the choice of host a careful search
among wide band gap crystals fluorides and special semiconductors each candidate brings its own
growth habits impurity limits and optical properties semiconductor lasers show the same pattern
in a different form a semiconductor gain region has a band gap that helps determine the emitted color
longer wavelength devices can use well established compound semiconductors with narrower gaps
shorter wavelengths require wider gap materials and the crystal layers must be grown with precise
composition and low defect density defects can act as places or carriers recombine without a
midding light they can also scatter light or absorb it producing a smooth clean interface
between layers becomes more demanding as the required band gap widens doping is another quiet constraint
many solid state lasers use trace ions dissolved into a crystal or glass those ions provide
the energy levels needed for gain for short wavelengths the ion must offer a transition at the right
energy and the host must not disturb that transition too much some ions that seem promising on paper
lose their excitation through cross relaxation where one excited ion shares energy with a neighbor
and both end in lower states others absorb the very light they are meant to emit the result
is a narrow set of practical choices pumping adds more structure to create gain energy must be placed
into the medium faster than it leaks away pump sources often use flash lamps other lasers
or electrical current with short wavelength gain media the pump photon may need to be quite
energetic or several steps may be required to reach the upper laser level each step can introduce laws
if the pump light is absorbed to near the surface the front of the medium heats more than the interior
if it passes too far through the gain may be uneven gentle even deposition of energy
helps the beam stay stable thermal load is therefore a constant companion of short wavelength gain
the difference between pump energy and laser photon energy often appears as heat
suppose a pump photon carries more energy than the emitted photon the surplus becomes
lattice vibrations before or after emission in blue and ultraviolet systems the energy carried by each
photon is larger so losses that might seem small can still deposit noticeable heat the medium
may also absorb some of the short wavelength light it produces that absorption adds more heat
exactly where the optical field is strongest heat changes the optical behavior of the medium
a warm center and cooler edges can act like a lens bending the beam as it passes temperature gradients
constrain crystals altering refractive index and changing polarization if heating becomes uneven
thermal stress can make a material less durable though designers avoid such conditions
by choosing safe operating levels the practical effect is that a short wavelength gain medium must be
cooled and shaped with care thin discs slender rods broad slabs and small active regions
are all ways of letting heat leave while keeping the optical path steady mirrors and coatings
also feel the influence of short wavelengths a laser cavity depends on reflectors
that return light with very low loss at blue and ultraviolet wavelengths thin film coatings
must be designed for materials that do not absorb strongly small absorption can warm the coating
and shift its properties the surfaces must be smooth on the scale of the shorter wavelength
because roughness that would be minor for red light can scatter blue or ultraviolet light more noticeably
this raises the standard for polishing and deposition because direct gain media can be demanding
many short wavelength lasers are produced indirectly through frequency conversion in this approach
a laser first generates a longer wavelength or good gain media exist that beam then passes through
an on-linear crystal the crystal responds to the electric field of the light in a way that can combine
photons second harmonic generation takes two photons of the original frequency and produces one
photon with twice the frequency and half the wavelength frequency doubling of infrared light
can yield green or blue light depending on the starting wavelength additional doubling or mixing
can reach ultraviolet regions frequency conversion has its own requirements the crystal must be transparent
at both the input and output wavelengths it must have the correct non-linear properties it also needs
phase matching which means the interacting waves stay in step as they travel if they drift out of step
energy flows back and forth instead of building the new color phase matching can be adjusted
by choosing the crystal orientation or controlling temperature periodic structures inside
the material can also help these methods allow stable conversion but they add sensitivity a small
change in temperature or beam angle can reduce efficiency conversion also concentrates attention
on beam quality a clean narrow spectrum and a well behaved spatial profile
help the waves overlap in the crystal short wavelengths generated this way often require careful
filtering to separate the desired color from leftover pump or fundamental light the optics must
handle both the original beam and the converted beam without absorbing either when the
target color is deep ultraviolet even tiny absorption along the optical path can matter
so designers may use special gases purchase or reflective optics the path from pump to output must
remain transparent all the way to the shortest desired color lasers rest quietly inside the
instruments that use them waiting as patient sources of ordered light their light wave stay aligned
keeping a steady rhythm that ordinary light does not usually share in laboratories and small
devices alike this careful alignment helps instruments measure and guide with soft precision
now the beam can dim in your thoughts the mirrors and crystals have done their work the colors have shown
why some wavelengths are easier to find than others there's no hurry left to solve nothing needs to
be built as the idea of coherent light grows gentle let your breathing slow to the steady path
of a laser can become a quiet line towards sleep the light keeps its order even as it fades from
attention may your dreams become and softly focused