WEBVTT

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sleep at time facts, lasers, settle into the bed, and let your breathing become slow, and even

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lasers are quiet examples of light, made orderly, ordinary light spreads out in many directions.

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And mixes many wavelengths together, a laser produces a narrow beam, in which the light waves

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move together. In step, this organization gives the beam a steady direction, and a very pure color,

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you may have encountered lasers. Without thinking about them, they help read the dark bars.

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On grocery packages, they carry soft pulses of light through thin glass fibers,

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that connect phones and computers. In each case, the laser does one gentle job.

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It keeps light aligned as you rest. Imagine that same steadiness. The beam does not rush,

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or scatter. It simply travels along its path. Your thoughts can settle in the same way.

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One soft focus, after another, while the room stays still, and safe, the word laser began,

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as a compact, way to name a very particular kind of light. It is built from the initial letters

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of a longer phrase. And that phrase describes what the device does in calm, physical terms. The phrase

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is light amplification by stimulated emission of radiation over time. The acronym became so familiar

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that people began writing it as an ordinary word. That shift is useful. It reminds us that a laser

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is not a mysterious object. It is a tool for making light behave in an orderly manner. Light is the first

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part of the name. And it is the thing being shaped in everyday life. Light arrives from many sources

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and spreads in many directions. A lamp sends out waves that wander outward. The sun sends a broad

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mix of wavelength, a laser by contrast. Works with light that has been gathered into a much

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narrower pattern. The word light here refers to electromagnetic waves. The same general family

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as the glow from a candle. Or the brightness of a clear afternoon, the laser does not create a different

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kind of substance. It arranges ordinary light into a more focused form. Amplification comes next

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to amplify something is to make it larger or more pronounced and sound. Amplification can make a quiet

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voice carry across a room in light. Amplification means increasing the amount of coordinated

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electromagnetic energy. A weak signal can prompt a larger response. Inside a suitable material,

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the result is a build-up, rather than a chaotic flood, the waves gain strength because they are

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supported by the material. They pass through. The idea is gentle in principle. A small, orderly

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influence can encourage a larger, similar response stimulated is the word that gives the phrase

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it's special character. A stimulus is something that prompts a response in a laser. The stimulus

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is light itself when an atom or molecule in a material is already holding energy. A passing wave

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can encourage it to release that energy as another wave. The new wave matches the first in important

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ways. It travels in step with the original. This matching is what makes the light orderly. The process

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does not need to be forced in a rough way. It is a prompt and a quiet invitation for energy to appear

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in the same pattern that is already present. Amission follows. Amission means sending out. Many things

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emit light, a warm surface, emits infrared, a firefly, emits a soft glow in the laser phrase.

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Amission refers to light leaving matter and entering space. The special part is that the emission is

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stimulated rather than random. It happens in response to a wave that is already there. That response

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adds to the existing light. Each matching addition helps the pattern become clearer. The light

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is not scattered into every corner. It is produced in a way that supports one shared direction

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and rhythm. Radiation is the final word and it can sound more serious than it needs to in physics. Radiation

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simply means energy traveling outward. Sunlight is radiation. The warmth felt nearest stove is

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radiation. The term describes motion. Energy moves from one place to another in the laser acronym.

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Radiation is the light that has been amplified and emitted in a coordinated pattern. The word

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completes the phrase by naming what travels away from the device taken together. The phrase

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describes a sequence of ideas. Light is present. It is amplified. The amplification depends on

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stimulated emission. The emitted energy becomes radiation that moves outward. The acronym places those ideas.

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In a neat row it gives a name to process or light encourages more light of the same kind to appear.

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The device called a laser is the place where this process is arranged with care. The acronym also leaves

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some things open. It does not name the material inside the device. It does not require a single color

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or a single shape for the beam. Those details can vary. What remains constant

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is the method suggested by the phrase light is increased through a process. In which existing light

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prompts new light that is the core of the name. The small word by the phrase carries a quiet load.

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It tells us how the amplification happens. The increase is not produced by simply adding

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unrelated waves. It comes through stimulated emission. One orderly pattern invites another. The phrase

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therefore links cause and effect in a few words. Amplification is the result. Stimulated emission

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is the means because the name is a process. The word laser can point to more than one thing.

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It can mean the instrument that creates the light. It can mean the beam that leaves it. In both cases

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the acronym remains the same guide. The beam is called laser light. Because it has been formed

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through the process named by the phrase, the instrument is called a laser. Because it is built

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to support that process, the phrase also suggests control. Without spelling out, every part of the

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arrangement, amplification can happen in many settings. But a laser is concerned with a disciplined

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kind of increase. The waves are encouraged to build in a shared pattern. The device provides

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conditions where that build-up can continue nothing in the acronym demands a particular size or power.

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It only identifies the orderly route by which the light is made stronger over the years. The word

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has settled into ordinary speech, people use the word for printers and light shows in those

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uses. The acronym has become an adjective or a simple noun. The full phrase can fade from view,

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returning to it, restores the original sense before it settled into everyday use. The phrase was

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formed after the underlying idea was already understood. Scientists had recognized that matter

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could hold energy and release it as light. They also understood that an incoming wave could encourage

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that release once those pieces were clear. The acronym gave them a compact label. It turned

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a chain of physical events into a single term. The label helped people talk about the device.

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Without describing every step each time light can move through matter without leaving a trace.

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Or it can be absorbed and become a slight increase in internal energy between those familiar outcomes,

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sits a quieter possibility, an atom or molecule that already holds extra energy.

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Maybe prompted by passing light to give some of that energy back. The prompt does not strike the particle,

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like a tiny hammer, it acts through the oscillation of the electromagnetic field,

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encouraging the excited system to change to a lower energy state. When that happens,

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the system releases a photon. This process is the central event that makes laser light possible

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an excited atom or molecule. Amits light in response to light already present. The emitted photon

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is not a random answer. It matches the incoming photon in color, which means it has the same

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frequency and energy. It also travels in the same direction and keeps the same phase if the incoming

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light wave has crests and troughs arranged in a certain pattern. The new photon joins that pattern,

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rather than starting a new one to picture this. Think of an atom as having only certain

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allowed amounts of internal energy. It cannot hold just any quantity. It can rest in a low state.

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Or it can hold a definite extra amount after receiving energy from somewhere else. That extra

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energy might have come from an earlier flash of light from an electrical current or from a collision

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with another particle while the atom remains in the higher state. It is ready to release the surplus

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if the right opportunity appears. One opportunity is spontaneous a mission. In that case,

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the excited atom simply drops to a lower state and sends out a photon on its own. The photon

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has the correct color for the energy difference. But its timing, direction, and phase are not tied

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to another light wave. Spontaneous a mission is common. It is why many materials glow softly.

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After being energized, stimulated a mission is different because the presence of a suitable

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photon guides the release the guiding happens because light is an oscillating electric and magnetic field

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and electron in an atom responds to electric fields. Even very weak ones when a photon with the

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right energy passes an excited atom. It's field oscillates at a frequency that corresponds to

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the gap between the atoms higher and lower states that matching rhythm can encourage the electron

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to shift downward. The atom then gives up the energy difference as another photon. The original

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photon continues onward instead of one photon entering the region and being absorbed.

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One photon enters and two photons leave. The two leaving photons are closely matched. They carry

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the same energy. So they have the same color. They move together along the same path. Their wave

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peaks line up with one another and their electric fields oscillate in the same orientation.

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This matching is why stimulated a mission can produce light that is unusually orderly. The process

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adds brightness in a coordinated way. Phase is one of the quieter ideas here. Two waves can have the same

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color and still be out of step if one wave reaches a crest while another reaches a trough.

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They can weaken each other if their crests arrive together. They reinforce each other

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in stimulated a mission. The new photon is born in step with the photon that prompted it. The light

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wave therefore gains a partner that supports its existing rhythm. Direction matters too

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ordinary light from a warm object spreads outward in many directions each atom emits independently.

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And the photons fan away from the source in stimulated a mission. The emitted photon follows the

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direction of the incoming photon. This tendency keeps the light confined to a shared path.

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If many excited atoms lie along that path, each prompted photon can prompt another.

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And the same narrow beam can grow stronger. This growth is often described as amplification, a small

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amount of well chosen light can become larger. Without changing its basic character,

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the added light carries the same frequency and phase as the light that began the process in a laser.

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This principle is placed inside a carefully prepared material where many atoms or molecules are ready to

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the material provides the gain, the stimulated a mission, provides the order, still the heart of

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the matter, remains the single atomic event and incoming photon, prompts an excited system to release

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a matching photon. The idea was worked out by considering how atoms and radiation share energy

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early studies of light and matter had already described absorption. We're a photon is taken up

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and an atom moves to a higher energy state. They also described spontaneous a mission

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where an excited atom emits without outside prompting to make the full accounting of energy exchange

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consistent. Theurists recognized that a third path was needed if light could raise an atom to a higher

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state. Then an excited atom should also be able to fall to a lower state under the influence

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of light already present the symmetry of the situation, pointed toward stimulated a mission,

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and later observations confirmed that it occurs in nature. No dramatic event is required.

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The atom does not need to be shaken. It simply moves from one allowed energy state to another

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and the energy difference appears as light. The process follows the same conservation rules

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that govern all quiet exchanges in physics. Energy is neither lost nor invented. Momentum

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is also accounted for. Which is one reason the emitted photon travels along the same path

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as the prompting photon. The whole interaction is an orderly transfer the match between the incoming

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photon and the atomic transition must be close if the photon has too little energy.

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It cannot prompt the particular downward step if it has too much. It may pass by without causing

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that a mission or it may interact in some other way. Atoms are selective. They're allowed energy gaps

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to determine which colors they can absorb and which colors they can emit stimulated

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a mission. Respects that selectivity a photon of one color can prompt a mission of the same color

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from a suitable excited atom. But it will not make every atom respond this selectivity

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gives stimulated a mission. It's neatness. The emitted light does not smear into many colors.

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It stays tied to a specific transition in some materials. The transition is narrow in others.

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The allowed energy spreads slightly because the atoms are in a solid or a mixture.

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And the emitted color can cover a small range. Even then, the stimulated photons

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follow the incoming light closely. The process preserves the character of the light that stimulates it.

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There is also a timing aspect and excited atom cannot remain excited forever.

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But the length of time it stays ready can vary. Some excited states last. For a tiny fraction

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of a second, others last much longer if a matching photon arrives. While the atom is still excited,

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stimulated a mission can occur if the atom has already released its energy spontaneously.

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The opportunity is gone. The process therefore depends on having excited systems present

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at the same time. As suitable light passes through them, stimulated a mission can happen with molecules

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as well as atoms. Molecules have more ways to store energy because they can vibrate and rotate

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in addition to shifting electronic states. That means they can support stimulated a mission

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across a wider range of wavelengths, including infrared light. The basic rule remains the same

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and excited system releases a photon that agrees with the photon that prompted the release at a deep

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level. The process shows that light can organize light, a passing wave carries energy

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and it can also shape how energy leaves matter. The emitted photon joins the wave with the same

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rhythm and direction. So the passing light becomes a little more unified, atoms hold energy

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in quiet, particular amounts and electron in an atom can rest in a lower energy condition.

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Often called the ground state or it can move to a higher condition if it receives just the right

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amount of energy. These allowed conditions are not continuous like a ramp. They are more like steps

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on a ladder, the spacing between steps gives each material its own set of possible light colors

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when light passes through ordinary matter. Many atoms are in the lower step, a photon with the right

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energy can be absorbed. Lifting an atom upward, this makes the light weaker if the atom later falls back

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down. It may release a photon but in ordinary conditions the release happens in random directions

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and at random times the result is soft. Spread out light rather than a growing beam there is another

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possibility if an excited atom is already sitting on a higher step. A passing photon with the matching

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energy can encourage it to drop down the atom then emits a second photon that matches the first

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in color, direction and phase. This is stimulated emission one photon becomes two and the two

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can encourage more excited atoms to emit in step for this to happen often enough. The material needs

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many atoms already in the excited condition under everyday conditions nature prefers lower energy

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or materials have some excited atoms yet most atoms remain in lower states if a beam of light

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enters such a material. Absorption tends to be stronger than stimulated emission the beam loses

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energy to the atoms instead of gaining energy from them to make light grow. The balance must be

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shifted more atoms need to occupy a chosen excited state than a lower state connected to the transition

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that shifted balance is the condition laser physicists work to create the idea can be compared to a

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quiet hillside if most pebbles lie at the bottom a small push will not send many rolling downward

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if many pebbles are placed higher up a tiny nudge can start a steady cascade in a laser medium

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the pebbles are atoms in energy states and the nudge is a passing photon the cascade is orderly

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it is a coordinated release of light each new photon matching the one that came before creating

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this upward imbalance requires energy from outside the process is called pumping pumping can be done

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with light from a lamp or another laser with an electric current or with chemical reactions that leave

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products in excited forms the purpose is the same energy enters the medium and lifts atoms

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or molecules into higher states faster than they naturally fall back down the medium then

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holds a store of excited particles ready to emit when stimulated not every pair of energy levels

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can support such a store some excited states empty too quickly others decay by releasing heat

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or by emitting light interactions that do not help the beam useful laser transitions

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often involve a metastable state an excited condition where atoms remain for a relatively long time

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a longer stay gives pumping a chance to fill that state it also gives passing photons more

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opportunity to trigger a mission before the energy leaks away a simple two-level arrangement

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is hard to invert if atoms are pumped from a lower level to an upper level the same light that pumps

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them can also stimulate them back down as the upper level fills absorption and stimulated

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emission become balanced the system settles near equal occupancy and the beam does not gain much

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many practical lasers therefore use three or four levels in a three level scheme atoms are

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lifted from the ground state to a high state then they relax into a metastable state the laser

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transition occurs from that metastable state to the ground state because the lower level is the ground

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state a large fraction of atoms must be lifted before a mission can exceed absorption for level

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arrangements make the task gentler the laser transition ends in a level above the ground state

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and that lower level empties quickly since few atoms remain in the lower laser level

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even a modest number in the upper level can create the needed imbalance this helps the laser reach

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amplification with less pumping energy the difference between these level patterns

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explains why some materials are easier to use than others once the inverted medium is prepared

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a photon traveling through it can be amplified if the photon has the right energy

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it meets excited atoms and encourages matching photons the light intensity grows as it travels

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in a laser mirrors often guide light back and forth through the medium

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letting the beam pass many times each pass can add more matched photons one mirror may let a small share

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escape as the useful beam while the rest continues to circulate the balance between pumping

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and lost determines whether amplification continues energy leaves through the output beam

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through absorption in the material and through scattering pumping must replace the excited atoms

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that have emitted if pumping is steady the medium can maintain a stable inverted population

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the beam then settles into a calm continuous output if pumping comes in pulses

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the beam may appear in pulses too in both cases the central requirement remains the same

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the medium must hold enough excited atoms to make stimulated emission stronger than absorption

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the same requirement also shapes which transitions can be used the energy difference between two states

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sets the photon energy to obtain a desired wavelength the medium must have suitable levels separated

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by that amount some materials offer convenient transitions at accessible energies others require

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more complicated media or special gas mixtures the need to maintain an inverted population

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at a specific energy gap is one reason some transitions are harder to sustain still the underlying

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condition is the same a selected upper state must be more populated than a selected lower state

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temperature and material structure influence how easily the condition forms at higher temperatures

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atoms move more vigorously and occupy many energy states this can spread the available energy

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and make a clean imbalance harder solid crystals glasses gasses and semiconductors

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each provide different patterns of levels engineers choose a medium whose levels fit the desired

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wavelength and pumping method they also shape the medium so light can travel through it while gaining

00:31:36.080 --> 00:31:45.040
more than it loses the concept was worked out gradually as physicists studied how atoms exchange

00:31:45.040 --> 00:31:55.360
energy with radiation the key insight was that light amplification requires a departure from ordinary

00:31:55.360 --> 00:32:04.080
thermal balance in normal equilibrium lower states are favored and absorption winds by supplying

00:32:04.080 --> 00:32:12.960
energy selectively a medium can be held in a non-equilibrium state that state allows a weak signal to

00:32:12.960 --> 00:32:23.520
become stronger without changing its color or direction the process is orderly photons do not pile up randomly

00:32:23.520 --> 00:32:31.840
they are matched copies produced by atoms that have been prepared in advance a helpful way to think of it

00:32:32.560 --> 00:32:41.600
is a row of quiet bells if the bells are still a passing sound may be swallowed by soft cushions

00:32:41.600 --> 00:32:50.800
if each bell is already tense with stored energy the same sound can set them ringing in unison the

00:32:50.800 --> 00:33:00.240
incoming sound is carried forward it is repeated and strengthened in a laser medium the stored energy is

00:33:00.240 --> 00:33:09.600
atomic excitation and the ringing is coherent light maintaining the condition does not require force

00:33:10.400 --> 00:33:18.720
in a crude sense it requires steady bookkeeping pumping adds energy a mission removes it losses

00:33:18.720 --> 00:33:27.840
carry some away the medium reaches a working point where enough excited atoms are present at every moment

00:33:27.840 --> 00:33:35.840
small fluctuations may occur but the overall population can remain stable this stability

00:33:36.560 --> 00:33:44.640
allows the beam to have a narrow color and a steady pattern the inverted population is therefore

00:33:44.640 --> 00:33:53.120
the heart of laser action it turns a material from an absorber into an amplifier it lets a single

00:33:53.120 --> 00:34:01.920
photon invite others to match it it makes possible the narrow orderly beams that distinguish laser

00:34:01.920 --> 00:34:10.640
light from ordinary lamp light the condition exists only while energy is supplied when pumping stops

00:34:11.520 --> 00:34:20.960
atoms relax the imbalance fades and the medium returns to its usual preference for lower energy the

00:34:20.960 --> 00:34:29.840
light then quiet leaving the atoms settled once more a laser keeps light in a place long enough for it

00:34:30.240 --> 00:34:39.680
to gather strength the place is usually a space between two mirrors and that space is often called

00:34:39.680 --> 00:34:48.560
an optical cavity the gain medium sits inside the space when atoms are molecules in the medium release light

00:34:48.960 --> 00:34:58.160
the mirrors guide some of that light back through the medium again each return gives the light another chance

00:34:58.720 --> 00:35:05.280
to encourage more a mission the cavity is a path that light follows over and over

00:35:06.560 --> 00:35:16.080
until the path itself shapes what remains the simplest cavity has two facing mirrors one mirror reflects

00:35:16.080 --> 00:35:24.080
almost everything that reaches it the other reflects most of the light but allows a small portion

00:35:24.080 --> 00:35:32.720
to pass that second mirror is sometimes called the output coupler the name fits the action it couples a

00:35:32.720 --> 00:35:41.120
little light out of the cavity while keeping enough inside to continue the process if both mirrors were

00:35:41.120 --> 00:35:50.000
perfectly reflective the light would stay within the cavity and no beam would emerge if the mirrors

00:35:50.000 --> 00:35:58.320
reflected too little the light would fade before it could build the useful arrangement lies between those

00:35:58.320 --> 00:36:07.440
limits inside the cavity light travels back and forth along the line between the mirrors the gain

00:36:07.440 --> 00:36:16.080
medium adds energy to the light on each pass the added light matches the passing light in direction

00:36:16.800 --> 00:36:25.200
and color because the mirrors keep returning the light the matching becomes stronger photons that

00:36:25.200 --> 00:36:33.920
happened to travel along the cavity axis are favored photons that drift sideways soon leave the region

00:36:34.000 --> 00:36:42.320
between the mirrors and do not return this simple preference gives the emerging light a clean

00:36:42.320 --> 00:36:50.480
direction the cavity also favors certain patterns of vibration light is an electromagnetic wave

00:36:51.680 --> 00:36:59.360
and a wave can fit between two mirrors only in particular ways often this means the space

00:37:00.080 --> 00:37:06.960
contains a whole number of half wavelengths the wave must match the distance between the mirrors

00:37:08.320 --> 00:37:14.640
so that repeated reflections line up with themselves when the returning wave arrives in step

00:37:15.360 --> 00:37:24.880
with the wave already present it reinforces it when it arrives out of step it tends to cancel this is

00:37:24.880 --> 00:37:32.880
resonance the cavity simply gives steady support to the waves that fit and less support

00:37:32.880 --> 00:37:41.760
to the waves that do not resonance gives the laser a narrow set of allowed colors the gain medium

00:37:42.480 --> 00:37:50.880
may be able to emit over a range of wavelengths but the cavity can favor only those wavelengths

00:37:51.600 --> 00:38:00.080
that satisfy its spacing in many lasers several resonant wavelengths can exist close together

00:38:00.080 --> 00:38:08.720
the design of the mirrors and the medium can then favor one of them more strongly than the others

00:38:08.720 --> 00:38:16.800
the result is light with a very small spread of wavelengths the beam appears as a single color

00:38:16.960 --> 00:38:25.200
even when the underlying medium could support a broader range the distance between the mirrors

00:38:26.080 --> 00:38:33.120
sets the spacing of these resonances along our cavity allows more closely spaced

00:38:33.120 --> 00:38:40.640
resonant wavelengths a shorter cavity spreads them farther apart builders of lasers choose

00:38:40.640 --> 00:38:48.080
cavity length with the kind of light they want in mind they also choose mirror coatings

00:38:48.800 --> 00:38:57.280
to reflect the desired wavelengths a mirror that works well for one color may not work as well for

00:38:57.280 --> 00:39:06.080
another the coating is made from thin layers that reflect light through controlled interference each

00:39:06.080 --> 00:39:13.920
layer is a quiet adjustment to the way the surface treats different wavelengths partial

00:39:13.920 --> 00:39:21.760
transmission is a careful choice the output mirror must let enough light escape to form the beam

00:39:23.280 --> 00:39:31.200
yet keep enough inside to maintain oscillation the right amount depends on the gain available

00:39:31.200 --> 00:39:40.160
in the medium a medium that adds energy easily can work with a mirror that transmits a little more

00:39:40.160 --> 00:39:48.320
a medium with modest gain needs a mirror that reflects more strongly the aim is a steady rhythm

00:39:49.120 --> 00:39:57.680
between growth and release the cavity also shapes the cross section of the beam light can form patterns

00:39:57.680 --> 00:40:05.600
as it bounces and the mirrors determine which patterns remain stable some cavities

00:40:05.600 --> 00:40:13.360
support a broad spot others support rings or lobes many common lasers are built to favor the

00:40:13.360 --> 00:40:23.120
simplest spot a smooth central region with gentle edges this happens when the mirror curvature

00:40:23.120 --> 00:40:31.280
and spacing guide raise back toward the center rather than letting them wander outward

00:40:31.280 --> 00:40:39.920
the geometry gives the beam its familiar round form mirror curvature helps keep the light centered

00:40:39.920 --> 00:40:49.040
flat mirrors can work but they need very careful alignment curved mirrors can act like gentle guides

00:40:49.120 --> 00:40:56.480
they bend the returning light back toward the axis and make the cavity stable over many

00:40:56.480 --> 00:41:06.320
trips the light still travels in straight lines between reflections yet the overall path remains confined

00:41:06.320 --> 00:41:15.920
this stability is one reason many cavities use one or two curved mirrors the beam that finally passes

00:41:15.920 --> 00:41:22.640
through the output mirror carries this stable shape with it not all light inside the cavity

00:41:23.360 --> 00:41:31.840
becomes the output beam some light is absorbed in the mirrors or the medium some scatters from small

00:41:31.840 --> 00:41:39.920
imperfections some leaves through the sides the cavity quietly sorts these possibilities waves that match

00:41:39.920 --> 00:41:47.520
the resonance continue while waves that do not match fade directions that stay aligned are

00:41:47.520 --> 00:41:56.800
amplified and directions that wander are lost the beam that emerges is the portion of light that

00:41:56.800 --> 00:42:04.640
survived this sorting and passed through the partially transmitting mirror the optical cavity

00:42:05.440 --> 00:42:12.400
also gives the light a shared phase relationship because the same wave pattern repeats

00:42:12.400 --> 00:42:21.200
between the mirrors the emitted light tends to have a regular rhythm this regularity is part of what

00:42:21.200 --> 00:42:29.520
makes laser light different from the mixed light of a lamp the cavity does not create the initial

00:42:29.520 --> 00:42:37.760
emission by itself the gain medium supplies the energy and the first photons the cavity

00:42:37.760 --> 00:42:46.720
organizes those photons into a steady repeating pattern and lets a controlled fraction leave

00:42:46.720 --> 00:42:54.960
when a laser is running the cavity maintains a quiet circulation energy enters the gain medium

00:42:55.920 --> 00:43:02.960
and the medium transfers some of that energy to light the light then returns through the cavity

00:43:04.000 --> 00:43:13.120
and a small part exits as the beam the mirrors make this cycle possible without moving parts in many

00:43:13.120 --> 00:43:22.480
designs they hold the path and set the allowed waves they also decide how much light remains inside

00:43:22.480 --> 00:43:29.840
the beam appears as the visible edge of a steady circulation a lamp gives light the way

00:43:30.640 --> 00:43:38.080
a handful of sand scatters across a table each grain lands where it may the light from a bulb

00:43:38.080 --> 00:43:48.480
a candle or the sun arrives as many small emissions from countless atoms each sending out a wave

00:43:48.480 --> 00:43:57.200
that starts and stops on its own these waves have different colors and point in many directions

00:43:57.200 --> 00:44:05.200
they do not keep a shared rhythm the result is bright and pleasantly familiar it is also mixed

00:44:05.200 --> 00:44:13.840
laser light is different because it is organized the word coherence describes that organization

00:44:13.840 --> 00:44:21.840
in a coherent beam the light waves maintain a steady relationship with one another their peaks

00:44:21.840 --> 00:44:31.840
and troughs line up in a predictable way and they keep that relationship over distance and time this

00:44:31.840 --> 00:44:41.280
is why a laser can form a clean spot on a wall or traces straight line across a room without spreading

00:44:41.280 --> 00:44:49.040
into a soft glow coherence has two sides one side concerns color light of a single color

00:44:49.840 --> 00:44:58.400
corresponds to a narrow range of frequencies a lamp emits a broad band of frequencies even a lamp

00:44:58.400 --> 00:45:06.560
that looks white contains many colors at once a laser by contrast tends to emit light

00:45:07.120 --> 00:45:15.040
within a very small range of frequencies that narrowness means the waves do not drift out of step

00:45:15.040 --> 00:45:23.120
as quickly this steadiness over time is temporal coherence musicians call this kind of steadiness

00:45:23.120 --> 00:45:31.920
staying in tune though with light the idea is about timing rather than pitch the other side

00:45:31.920 --> 00:45:40.320
concerns direction and shape in ordinary light waves move outward in many directions from the

00:45:40.320 --> 00:45:49.920
source a shade or lens can guide some of that light but the underlying emission remains disorderly

00:45:49.920 --> 00:45:59.120
in a laser the light is encouraged to move along one preferred path mirrors inside the device favor waves

00:46:00.000 --> 00:46:08.080
the travel back and forth along the same line waves that wander sideways are not reinforced

00:46:08.080 --> 00:46:16.960
the result is a beam with a steady direction this directional order is called spatial coherence it

00:46:16.960 --> 00:46:25.200
means that different points across the beam are related in a steady way if you could freeze the beam at one

00:46:25.200 --> 00:46:33.520
instant the wave pattern would have a regular shape that regular shape can be focused to a small

00:46:33.520 --> 00:46:41.040
spot it also lets the beam remain narrow over a long distance the beam still spreads a little

00:46:42.000 --> 00:46:50.640
because all waves spread when they travel a wide orderly beam spreads more slowly than a jumbled one

00:46:50.640 --> 00:46:59.120
ordinary lamp light lacks this shared order because of how it is produced in a warm filament

00:46:59.120 --> 00:47:09.520
or a glowing gas atoms release energy independently one atom may emit a wave now another moment later

00:47:10.640 --> 00:47:18.480
and a third in a different direction each tiny wave train has its own phase phase is a way of

00:47:18.480 --> 00:47:27.760
describing where a wave is in its cycle whether it is near a peak a trough or somewhere between

00:47:27.760 --> 00:47:36.640
when many waves have random phases they do not support one another they add up to a broad gentle wash

00:47:36.640 --> 00:47:45.680
a laser arranges emission so that many atoms contribute in step the process depends on stimulated

00:47:45.680 --> 00:47:55.200
emission or an incoming photon encourages an excited atom to release another photon

00:47:56.000 --> 00:48:04.800
with matching properties the new photon matches the original in frequency and direction it also shares

00:48:04.800 --> 00:48:14.000
the same phase one matching photon can prompt another and another over many events a population of

00:48:14.000 --> 00:48:21.280
matching waves grows the light becomes more alike with each pass the structure of a laser

00:48:21.920 --> 00:48:30.560
helps this along again medium sits between mirrors light traveling along the axis bounces back and

00:48:30.560 --> 00:48:39.120
forth on each pass it stimulates more matching emission the mirrors let a small portion escape

00:48:39.680 --> 00:48:46.720
as the output beam because the escaping light comes from waves that have already been selected

00:48:46.720 --> 00:48:56.000
for direction and frequency it carries coherence with it the beam is bright and it is coordinated

00:48:56.000 --> 00:49:04.560
this coordination explains why a laser can stay narrow or lamp light cannot imagine trying to

00:49:04.560 --> 00:49:12.640
send light from a bulb down a long hallway you could place a lens in front of it and the lens

00:49:12.640 --> 00:49:21.040
would form a beam of sorts but the beam would contain many colors and many independent wave fronts

00:49:21.040 --> 00:49:28.560
it would blur and widen quickly a laser beam starts with a wave front that is already orderly

00:49:28.560 --> 00:49:38.240
so it can cross the same hallway as a slim patch of light the difference also appears in shadows

00:49:38.240 --> 00:49:46.320
and edges laser light can produce sharp interference patterns because its waves remain

00:49:46.320 --> 00:49:55.040
related ordinary light can produce interference too but it usually needs careful filtering a

00:49:55.040 --> 00:50:03.200
filter might select one color and one small portion of the source then the light behaves more

00:50:03.200 --> 00:50:11.520
coherently without filtering the many colors and phases wash out fine patterns the lamps light

00:50:12.160 --> 00:50:20.160
is not inferior for general illumination it is simply suited to a different task a cozy reading lamp

00:50:20.800 --> 00:50:27.440
benefits from broad mixed light it fills a room softly it reveals many surfaces

00:50:28.320 --> 00:50:36.160
without demanding alignment a laser beam would not be a comfortable replacement for that it is meant to

00:50:36.160 --> 00:50:44.800
carry order not to spread comfort the two kinds of light answer different needs one is like a crowd

00:50:45.520 --> 00:50:53.280
chatting in a warm hall the other is like a single clear note held by a flute coherence

00:50:53.280 --> 00:51:01.360
also affects how light can be focused a lens gathers light and bends it toward a point if the

00:51:01.360 --> 00:51:10.880
incoming waves are coherent the lens can bring them together into a very small well defined spot

00:51:10.880 --> 00:51:17.840
if the incoming waves are mixed in color and direction the best focus is softer

00:51:17.840 --> 00:51:26.880
different colors bend by slightly different amounts different directions arrive at slightly different places

00:51:26.880 --> 00:51:34.400
the focus becomes a gentle blur rather than a tight point this is one reason laser beams

00:51:35.280 --> 00:51:43.680
are useful for careful measurement a coherent beam can be split and recombined to detect tiny

00:51:43.680 --> 00:51:52.960
changes in distance it can be aimed at a distant reflector and still return enough organized light

00:51:53.600 --> 00:52:02.880
to be read it can be focused onto a very small region without needing enormous lenses none of this

00:52:02.880 --> 00:52:10.400
requires the light to be harsh it only requires the waves to stay in a known relationship

00:52:10.400 --> 00:52:19.520
ordinary light has its own quiet virtues sunlight carries a broad spectrum which helps us see colors

00:52:19.520 --> 00:52:27.840
naturally a warm bulb makes a space feel settled a candle flame flickers with small changes

00:52:28.640 --> 00:52:35.920
that many people find pleasing these sources do not need coherence to be valuable they provide

00:52:35.920 --> 00:52:43.920
illumination by abundance rather than order a chemical laser makes light from the energy

00:52:44.480 --> 00:52:53.760
released when atoms or molecules rearrange themselves into new substances in an ordinary gas laser

00:52:54.720 --> 00:53:02.560
an electric current or another light source lifts atoms into excited states in a chemical

00:53:02.560 --> 00:53:13.040
laser the lifting is done by a reaction the reactants often two compatible gases meet in a chamber

00:53:13.040 --> 00:53:21.280
as they combine the new molecules are often born with extra energy stored in their vibrations

00:53:22.160 --> 00:53:30.160
or electronic states if enough of them occupy those excited states light passing through

00:53:30.160 --> 00:53:38.560
the gas can stimulate them to emit matching photons mirrors at the ends of the cavity guide the light

00:53:38.560 --> 00:53:48.240
back and forth and a small fraction escapes as a beam the central idea is still the same population

00:53:48.240 --> 00:53:56.880
inversion used in other lasers a lower energy level must be less populated than an upper one

00:53:58.240 --> 00:54:06.080
so that photons are more likely to cause emission than absorption chemistry supplies that

00:54:06.080 --> 00:54:13.760
imbalance directly some reactions release energy in a way that places the product molecule

00:54:14.640 --> 00:54:22.880
in a high vibrational level the molecule has just formed and its atoms are still springing

00:54:22.880 --> 00:54:31.520
against one another like a small oscillator that stored motion can become the upper level of a laser

00:54:31.520 --> 00:54:40.800
transition many chemical lasers rely on molecules rather than isolated atoms molecules have

00:54:40.880 --> 00:54:49.600
vibrational and rotational states and the spacing between those states is often small compared

00:54:50.240 --> 00:55:00.240
with electronic gaps in atoms small energy steps correspond to lower photon frequencies that is one

00:55:00.240 --> 00:55:08.880
reason chemical lasers frequently emit in the infrared infrared photons carry less energy

00:55:09.440 --> 00:55:18.160
than visible photons and molecular vibrations are naturally suited to producing them the beam

00:55:18.160 --> 00:55:27.760
may be invisible to the eye yet it can be smooth and steady one familiar family uses hydrogen or

00:55:27.760 --> 00:55:36.640
doterium combined with fluorine when these react the resulting hydrogen fluoride or doterium

00:55:36.640 --> 00:55:45.920
fluoride molecules can be formed in excited vibrational states the molecules then emit infrared

00:55:45.920 --> 00:55:55.120
light as they relax toward lower vibrational levels the exact wavelength depends on the molecule

00:55:56.080 --> 00:56:02.800
and the conditions inside the cavity doterium compounds tend to shift the emission

00:56:03.440 --> 00:56:12.800
to longer wavelengths because the heavier atom changes the vibration frequency this is a quiet sort of

00:56:12.800 --> 00:56:21.680
tuning governed by mass and bond strength rather than by moving parts another chemical laser

00:56:22.640 --> 00:56:32.000
uses excited oxygen to transfer energy to iodine atoms in that system a chemical reaction

00:56:32.000 --> 00:56:41.600
first creates oxygen molecules in an excited electronic state those oxygen molecules collide with

00:56:41.600 --> 00:56:51.600
iodine atoms and pass some of their energy along the iodine atoms become excited and they provide

00:56:51.600 --> 00:57:00.160
the laser transition the emitted light is near infrared this arrangement shows that the reaction

00:57:00.160 --> 00:57:07.520
does not always have to create the lazing particle directly it can create an intermediate carrier

00:57:07.600 --> 00:57:16.640
which then passes energy to the actual laser medium such systems can support steady beams

00:57:17.600 --> 00:57:24.400
because the reactants can be flowed through the device fresh molecules enter the optical region

00:57:25.440 --> 00:57:34.000
emit light and are carried away spent gases leave with the flow and heat can be managed by the

00:57:34.000 --> 00:57:43.120
moving stream this continuous renewal is different from a sealed tube where the gas gradually warms

00:57:44.320 --> 00:57:51.600
and the excited population changes with careful flow design the output can remain stable

00:57:52.400 --> 00:58:00.800
for as long as the supply is maintained the beam becomes a kind of controlled river with chemistry

00:58:00.800 --> 00:58:08.880
feeding it from one side and optics shaping it from the other the distinction from electrically

00:58:08.880 --> 00:58:17.600
pumped lasers is especially clear in the way energy enters the medium a gas discharge

00:58:18.400 --> 00:58:26.880
depends on electrons accelerated by an electric field those electrons collide with atoms or molecules

00:58:27.520 --> 00:58:37.120
and raise them to higher states a chemical laser depends instead on bond formation the energy

00:58:37.120 --> 00:58:45.120
comes from the rearrangement of electrons in chemical bonds and it appears in the product particles

00:58:45.760 --> 00:58:54.240
almost immediately this can be efficient for certain transitions and it can produce large amounts

00:58:54.400 --> 00:59:01.680
of excited material without requiring electrodes to carry high currents

00:59:01.680 --> 00:59:09.920
through the gas optically pumped lasers are also different the absorb light from a lamp or another

00:59:09.920 --> 00:59:19.760
bright source and then give off light at a desired wavelength chemical lasers skip that intermediate

00:59:19.840 --> 00:59:27.840
light step the reaction itself creates the excited state this can simplify some aspects of the design

00:59:29.120 --> 00:59:37.440
because there is no need for a separate pump source of high brightness it also means the laser medium

00:59:38.240 --> 00:59:48.320
is tied closely to reaction chemistry flow patterns and mixing the device is as much a careful

00:59:48.320 --> 00:59:56.080
chemical instrument as an optical one the beam quality depends on keeping the gas uniform

00:59:56.080 --> 01:00:04.960
temperature and pressure affect the population of energy levels and composition does the same if the

01:00:04.960 --> 01:00:13.600
gas becomes too warm collisions can spread energy into unwanted states if mixing is uneven

01:00:13.600 --> 01:00:23.440
some regions may absorb light instead of amplifying it engineers and scientists therefore shape

01:00:23.440 --> 01:00:31.680
nozzles and channels so that the reacting gases move smoothly mirrors are placed where the beam

01:00:31.680 --> 01:00:40.560
can grow in an ordered stream chemical lasers also illustrate why some colors are harder to create

01:00:41.280 --> 01:00:50.320
than others visible light requires larger energy gaps than infrared many reactions release

01:00:50.320 --> 01:01:00.080
energy readily into molecular vibration which matches infrared transitions to reach visible wavelengths

01:01:01.120 --> 01:01:09.520
the reaction must populate higher electronic states and those states often lose energy

01:01:09.600 --> 01:01:15.520
through other paths before they can laser collisions can quench the excitation

01:01:16.880 --> 01:01:24.720
or the products may form in states that do not provide a useful inversion for this reason

01:01:25.760 --> 01:01:32.080
chemical lasers tend to be most successful in the infrared and near infrared

01:01:32.960 --> 01:01:41.440
where the natural motion of molecules aligns with the light being produced the mirrors used with

01:01:41.440 --> 01:01:50.240
these beams are chosen for the wavelength involved infrared light may be reflected by polished metals

01:01:51.520 --> 01:01:56.720
or by coatings designed for long wavelengths since the beam is invisible

01:01:57.680 --> 01:02:05.760
instruments are used to observe its position and power a viewing card or a calibrated detector

01:02:06.640 --> 01:02:14.000
can show where the light is going in a laboratory the setup often sits behind protective

01:02:14.000 --> 01:02:21.600
enclosures keeping the beam path contained and the room calm spectral measurements

01:02:22.400 --> 01:02:30.800
often reveal fine structure within the beam a single vibrational band can contain many rotational lines

01:02:32.240 --> 01:02:40.640
each tied to a slightly different molecular motion reading those lines gives a quiet record

01:02:40.640 --> 01:02:49.200
of the molecules that carried the light the beam in this way carries information about the reaction

01:02:49.200 --> 01:02:56.960
that made it a laser needs a material that can amplify light that material is called the

01:02:56.960 --> 01:03:06.400
gain medium and it sets many of the basic limits on the color a laser can produce inside the gain

01:03:06.400 --> 01:03:16.720
medium atoms ions molecules or semiconductor bands hold energy in discrete amounts when an

01:03:16.720 --> 01:03:24.560
excited particle gives up some of that energy as light the photon carries an amount of energy

01:03:25.360 --> 01:03:33.520
that matches the gap between two allowed states a larger gap means a higher photon energy

01:03:33.520 --> 01:03:39.920
higher photon energy corresponds to a shorter wavelength this simple relation

01:03:40.880 --> 01:03:48.880
is why short wavelength lasers ask so much of their gain media red and infrared lasers

01:03:49.840 --> 01:03:58.000
often rely on modest energy gaps many familiar solid state crystals gas mixtures

01:03:58.960 --> 01:04:08.080
and semiconductor structures support those transitions with stable well understood materials

01:04:08.080 --> 01:04:16.480
the particles can be pumped into excited states and a useful fraction of them return by a

01:04:16.480 --> 01:04:23.920
meeting photons that match the desired transition mirrors guide the light back through the medium

01:04:24.720 --> 01:04:32.640
so the emission grows the process can be efficient enough that heat remains manageable blue and

01:04:32.640 --> 01:04:41.440
ultraviolet light require larger gaps the gain medium must contain states separated by the right amount

01:04:43.040 --> 01:04:52.480
and those states must connect in a way that favors light emission rather than silent relaxation

01:04:52.480 --> 01:05:00.480
silent relaxation happens when excitation turns into vibrations of the surrounding material

01:05:01.440 --> 01:05:10.160
instead of photons those vibrations become heat in a short wavelength material even small losses

01:05:10.160 --> 01:05:20.480
can matter because each photon already represents a large packet of energy if many excitations decay

01:05:20.480 --> 01:05:28.480
without radiating the medium warms and the laser becomes harder to sustain suitable

01:05:28.480 --> 01:05:38.880
host materials also become fewer as the desired wavelength shortens a good laser host must be transparent

01:05:39.520 --> 01:05:47.840
at both the pump wavelength and the laser wavelength it must accept the active ions or molecules

01:05:48.560 --> 01:05:55.600
without degrading it must remain mechanically stable while carrying optical power

01:05:55.600 --> 01:06:04.640
for ultraviolet light many ordinary glasses and crystals absorb strongly absorption turns light

01:06:04.640 --> 01:06:13.280
into heat inside the medium or nearby coatings that makes the choice of host a careful search

01:06:13.280 --> 01:06:22.240
among wide band gap crystals fluorides and special semiconductors each candidate brings its own

01:06:22.240 --> 01:06:31.840
growth habits impurity limits and optical properties semiconductor lasers show the same pattern

01:06:31.840 --> 01:06:40.720
in a different form a semiconductor gain region has a band gap that helps determine the emitted color

01:06:40.800 --> 01:06:49.760
longer wavelength devices can use well established compound semiconductors with narrower gaps

01:06:49.760 --> 01:06:59.280
shorter wavelengths require wider gap materials and the crystal layers must be grown with precise

01:06:59.280 --> 01:07:08.960
composition and low defect density defects can act as places or carriers recombine without a

01:07:08.960 --> 01:07:16.480
midding light they can also scatter light or absorb it producing a smooth clean interface

01:07:16.480 --> 01:07:26.720
between layers becomes more demanding as the required band gap widens doping is another quiet constraint

01:07:26.720 --> 01:07:36.560
many solid state lasers use trace ions dissolved into a crystal or glass those ions provide

01:07:36.560 --> 01:07:44.480
the energy levels needed for gain for short wavelengths the ion must offer a transition at the right

01:07:44.480 --> 01:07:53.680
energy and the host must not disturb that transition too much some ions that seem promising on paper

01:07:54.720 --> 01:08:03.680
lose their excitation through cross relaxation where one excited ion shares energy with a neighbor

01:08:03.840 --> 01:08:12.880
and both end in lower states others absorb the very light they are meant to emit the result

01:08:13.600 --> 01:08:23.040
is a narrow set of practical choices pumping adds more structure to create gain energy must be placed

01:08:23.040 --> 01:08:31.200
into the medium faster than it leaks away pump sources often use flash lamps other lasers

01:08:31.840 --> 01:08:39.840
or electrical current with short wavelength gain media the pump photon may need to be quite

01:08:39.840 --> 01:08:49.920
energetic or several steps may be required to reach the upper laser level each step can introduce laws

01:08:49.920 --> 01:08:58.880
if the pump light is absorbed to near the surface the front of the medium heats more than the interior

01:08:58.880 --> 01:09:06.160
if it passes too far through the gain may be uneven gentle even deposition of energy

01:09:07.040 --> 01:09:16.240
helps the beam stay stable thermal load is therefore a constant companion of short wavelength gain

01:09:16.240 --> 01:09:24.320
the difference between pump energy and laser photon energy often appears as heat

01:09:24.320 --> 01:09:31.840
suppose a pump photon carries more energy than the emitted photon the surplus becomes

01:09:31.840 --> 01:09:41.200
lattice vibrations before or after emission in blue and ultraviolet systems the energy carried by each

01:09:41.200 --> 01:09:51.280
photon is larger so losses that might seem small can still deposit noticeable heat the medium

01:09:51.920 --> 01:09:59.520
may also absorb some of the short wavelength light it produces that absorption adds more heat

01:10:00.400 --> 01:10:08.480
exactly where the optical field is strongest heat changes the optical behavior of the medium

01:10:08.480 --> 01:10:18.480
a warm center and cooler edges can act like a lens bending the beam as it passes temperature gradients

01:10:18.480 --> 01:10:27.440
constrain crystals altering refractive index and changing polarization if heating becomes uneven

01:10:28.480 --> 01:10:35.680
thermal stress can make a material less durable though designers avoid such conditions

01:10:36.480 --> 01:10:45.520
by choosing safe operating levels the practical effect is that a short wavelength gain medium must be

01:10:45.520 --> 01:10:54.720
cooled and shaped with care thin discs slender rods broad slabs and small active regions

01:10:55.600 --> 01:11:02.640
are all ways of letting heat leave while keeping the optical path steady mirrors and coatings

01:11:03.520 --> 01:11:10.880
also feel the influence of short wavelengths a laser cavity depends on reflectors

01:11:11.600 --> 01:11:20.480
that return light with very low loss at blue and ultraviolet wavelengths thin film coatings

01:11:21.200 --> 01:11:29.680
must be designed for materials that do not absorb strongly small absorption can warm the coating

01:11:30.400 --> 01:11:37.040
and shift its properties the surfaces must be smooth on the scale of the shorter wavelength

01:11:37.920 --> 01:11:47.920
because roughness that would be minor for red light can scatter blue or ultraviolet light more noticeably

01:11:48.160 --> 01:11:55.920
this raises the standard for polishing and deposition because direct gain media can be demanding

01:11:57.280 --> 01:12:05.520
many short wavelength lasers are produced indirectly through frequency conversion in this approach

01:12:06.400 --> 01:12:15.920
a laser first generates a longer wavelength or good gain media exist that beam then passes through

01:12:15.920 --> 01:12:24.080
an on-linear crystal the crystal responds to the electric field of the light in a way that can combine

01:12:24.080 --> 01:12:33.520
photons second harmonic generation takes two photons of the original frequency and produces one

01:12:33.520 --> 01:12:41.760
photon with twice the frequency and half the wavelength frequency doubling of infrared light

01:12:42.640 --> 01:12:50.800
can yield green or blue light depending on the starting wavelength additional doubling or mixing

01:12:51.680 --> 01:13:00.720
can reach ultraviolet regions frequency conversion has its own requirements the crystal must be transparent

01:13:01.360 --> 01:13:10.080
at both the input and output wavelengths it must have the correct non-linear properties it also needs

01:13:10.080 --> 01:13:19.040
phase matching which means the interacting waves stay in step as they travel if they drift out of step

01:13:20.160 --> 01:13:27.600
energy flows back and forth instead of building the new color phase matching can be adjusted

01:13:28.240 --> 01:13:36.000
by choosing the crystal orientation or controlling temperature periodic structures inside

01:13:36.000 --> 01:13:45.440
the material can also help these methods allow stable conversion but they add sensitivity a small

01:13:45.440 --> 01:13:54.320
change in temperature or beam angle can reduce efficiency conversion also concentrates attention

01:13:54.960 --> 01:14:01.840
on beam quality a clean narrow spectrum and a well behaved spatial profile

01:14:02.880 --> 01:14:11.360
help the waves overlap in the crystal short wavelengths generated this way often require careful

01:14:11.360 --> 01:14:20.800
filtering to separate the desired color from leftover pump or fundamental light the optics must

01:14:20.800 --> 01:14:28.480
handle both the original beam and the converted beam without absorbing either when the

01:14:28.480 --> 01:14:36.880
target color is deep ultraviolet even tiny absorption along the optical path can matter

01:14:38.240 --> 01:14:48.320
so designers may use special gases purchase or reflective optics the path from pump to output must

01:14:48.320 --> 01:14:56.560
remain transparent all the way to the shortest desired color lasers rest quietly inside the

01:14:56.560 --> 01:15:05.360
instruments that use them waiting as patient sources of ordered light their light wave stay aligned

01:15:06.560 --> 01:15:14.720
keeping a steady rhythm that ordinary light does not usually share in laboratories and small

01:15:14.720 --> 01:15:24.480
devices alike this careful alignment helps instruments measure and guide with soft precision

01:15:24.480 --> 01:15:32.160
now the beam can dim in your thoughts the mirrors and crystals have done their work the colors have shown

01:15:32.880 --> 01:15:41.120
why some wavelengths are easier to find than others there's no hurry left to solve nothing needs to

01:15:41.200 --> 01:15:49.280
be built as the idea of coherent light grows gentle let your breathing slow to the steady path

01:15:49.280 --> 01:15:58.000
of a laser can become a quiet line towards sleep the light keeps its order even as it fades from

01:15:58.000 --> 01:16:02.560
attention may your dreams become and softly focused