# How do lasers work?

## The Electromagnetic Spectrum and Quantum Energy

The electromagnetic spectrum consists of the complete range of frequencies from radio waves to gamma rays.  All electromagnetic radiation consists of photons which are individual quantum packets of energy.

For example, a household light bulb emits about 1,000,000,000,000,000,000,000 photons of light per second!  In this course we will only concern ourselves with the portion of the electromagnetic spectrum where lasers operate – infared, visible, and ultraviolet radiation.

 Name Wavelength Ultaviolet 100 nm – 400 nm Visible 400 nm – 750 nm Near Infrared 750 nm – 3000 nm Far Infrared 3000 nm – 1 mm

Einstein was awarded the Nobel Prize for his discovery and interpretation of the formula – E=mc2 – right?  Wrong.

He won the Nobel Prize for his explanation of the phenomena referred to as the photoelectric effect.  When light (electomagnetic energy) is shined on a metal surface in a vacuum, it may free electrons from that surface.  These electrons can be detected as a current flowing in the vacuum to an electrode.

The light was not always strong enough to cause this effect, however.  When the scientists made the light brighter, no increase in electrons was seen.  Only when they changed the color of the light (the wavelength) did they see a change in photoemission of electrons.

This was explained by Einstein using a theory that light consists of photons, each with discrete quantum of energy proportional to their wavelength.  For an electron to be freed from the metal surface it would need a photon with enough energy to overcome the energy that bound it to the atom.

So, making the light brighter would supply more photons, but none would have the energy to free the electron.  Light with a shorter wavelength consisted of higher energy photons that could supply the needed energy to free the electron.

Now, you ask, “What the heck does this idea of quantum energy have to do with a laser?”.  Well, with this background under our belts we will continue.

Laser Components

### The Lasing Medium

A substance that when excited by energy emits light in all directions.   The substance can be a gas, liquid, or semi-conducting material.

### The Excitation Mechanism or Energy Pump

The excitation mechanism of a laser is the source of energy used to excite the lasing medium.  Excitation mechanisms typically used are: electricity from a power supply, flash tubes, lamps, or the energy from another laser.

### The Optical Cavity

The optical cavity is used to reflect light from the lasing medium back into itself.  It typically consists of two mirrors, one at each end of the lasing medium.

As the light is bounced between the two mirrors, it increases in strength, resulting in amplification of the energy from the excitation mechanism in the form of light.

The output coupler of a laser is usually a partially transparent mirror on one end of the lasing medium that allows some of the light to leave the optical cavity to be used for the production of the laser beam.

### How it Works

The lasing medium will normally emit photons in specific spectral lines when excited by an energy source.  The wavelength is determined by the different quantum levels, or energy states, of the material.  Normally, most atoms in a medium are in the ground state.  Some small percentage will exist at higher energies as well.

Normally, these higher energy states are unstable and the electrons will release this excess energy as photons almost immediately and return to the ground state.   In some materials, specifically those chosen as lasing medium, a metastable state is possible where the atom or molecule will remain at an excited state for some time.

Energy is supplied to the laser medium by the energy pumping system.   This energy is stored in the form of electrons trapped in the metastable energy levels.  Pumping must produce a population inversion (i.e., more atoms in the metastable state than the ground state) before laser action can take place.

When population inversion is achieved, the spontaneous decay of a few electrons from the metastable energy level to a lower energy level starts a chain reaction.

The photons emitted spontaneously will hit (without being absorbed)other atoms and stimulate their electrons to make the transition from the metastable energy level to lower energy levels – emitting photons of precisely the same wavelength, phase, and direction.

This action occurs in the optical cavity.

When the photons that decay in the direction of the mirrors (most are lost – lasers are not as efficient as one would believe) reach the end of the laser material, they are reflected back into the material where the chain reaction continues and the number of photons increase.  When the photons arrive at the partially-reflecting mirror, only a portion will be reflected back into the cavity and the rest will emerge as a laser beam.

Now that we know the basics, lets discuss the Types and Classifications of lasers.

## Case study: Lasers

Lasers provide the archetypal example of how a discovery in basic physics led to an invention, several decades later, that was unpredictably world-changing.

What are lasers?

Credit: Shutterstock

Lasers are devices that emit narrow beams of intense electromagnetic radiation (light). The term laser originated as an acronym for “light amplification by stimulated emission of radiation”.

A laser beam has the special property that the light waves emitted are all in step with one another – coherent – and usually of one wavelength, or colour.

There are many different kinds of lasers, from giant installations emitting powerful pulses of high-energy radiation, such as X-rays, to tiny devices etched onto semiconductor chips producing infrared light.

Many different kinds of material can be made to “lase” – such as gases, crystalline solids, glasses and polymers – and which one is used depends on the application.

Some lasers are designed to emit a continuous beam while others can spit out rapid pulses of light that are ultra-short.

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The wavelengths of light generated by certain types of laser can even be “tuned” for specific applications, making them extremely versatile.

Lasers offer a way of generating, controlling and directing intense light in remarkable ways, yet when they were first invented, physicists were not sure what they could be used for – they were famously described as a “solution looking for a problem”.

In fact, although the first laser was constructed in the 1950s, practical applications did not appear until a couple of decades later – as is often the case in science.

Since then, thanks to research activity in both university physics departments and companies, including those in the UK, lasers have become ubiquitous and are central to many technologies that are used in manufacturing, communications, medicine and entertainment.

Today, lasers are key tools in manipulating and communicating information (in CD and DVD players, supermarket barcode readers and broadband telecommunications), in measurement (surveying and environmental studies), chemical analysis (of foods, medical specimens and materials) and, increasingly, in transforming materials (welding, cutting and etching, printing, and surgery).

Research into lasers continues apace – new types of laser are being developed with a variety of characteristics and potential applications.

In some cases, the result is a cheaper, more compact portable device designed for a specific use, or a more powerful laser used to generate power, for instance. UK university physics departments are at the forefront of many of these areas.

In particular, physicists in the Central Laser Facility (CLF) at the Rutherford Appleton Laboratory develop novel high-powered laser systems and make them available for both pure and applied research.

The science

Credit: Shutterstock

The laser would never have been developed without a profound understanding of an area of fundamental physics – quantum theory.

The principle behind the laser goes back to the world’s most famous physicist, Albert Einstein, who in 1917 proposed a theory of stimulated light emission.

Einstein had previously shown that light was composed of tiny packets of wave energy called photons (the wavelength depending on the energy).

He theorised that if the atoms that make up a material are given excess energy and so emit photons, these photons could stimulate nearby atoms to emit further photons, creating a cascade effect. All the photons would have the same energy and wavelength and move off in the same direction.

However, it was not until 40 years later that physicists were able to convert this idea into a practical laser. The principle is that the “active” material has first to be pumped with energy from another light source or an electrical current.

The resulting stimulated light emission is then amplified by bouncing the light back and forth through the lasing material in a mirrored cavity, so stimulating more emission, before it escapes through a transparent mirror section as a laser beam.

A device that amplified microwaves was constructed in 1953 by Charles Townes and colleagues at Columbia University.

Townes shared a Nobel Prize in Physics in 1964 with Nikolay Basov and Aleksandr Prochorov of the Lebedev Institute in Moscow (who independently also demonstrated what came to be called a maser).

The next few years saw a race to build the first visible light laser. Theodore Maiman at Hughes Research Laboratories in California pipped Townes and his team at the post when he built the first working laser in 1960 using ruby as a lasing medium – although who should be credited for the laser’s invention was then hotly contested.

Initially the laser concept was not taken very seriously, nevertheless the 1960s saw a huge expansion in laser research including the development of high-power gas lasers, chemical lasers and semiconductor lasers. However, they were still rather specialised research tools. By the 1970s, semiconductor lasers that worked at room temperature had been developed and this led to the advent of the compact disc (CD).

Without the discovery of lasers, the entire fundamental field of cold atoms would never have opened up. Research in this field has led to the award of several Nobel Prizes in Physics, including the discovery of Bose–Einstein condensates (BEC). BEC has opened the door to a host of applications such as atom lasers, improved atomic clocks and quantum computers.

Today, semiconductor diode lasers are the most common type, found in industry, commerce and the home.

Applications

• Information TechnologyThe largest application of lasers is in optical storage devices (e.g. CD and DVD players), in which a focused beam from a semiconductor laser, less than 1 mm wide, scans and reads the disc surface. Other everyday uses include barcode readers, laser printers and laser pointers. Over the past 25 years the publishing and newsprint industries have been revolutionised by the use of lasers, which have replaced traditional “hot metal” printing.
• TelecommunicationsThe second largest application is in fibre-optic communications. Broadband depends on the transmission of light pulses alongoptical fibres, which are generated and relayed via lasers. This is made possible by fibre amplifiers, invented in the UK, which are an important component in long-distance fibre links.
• MedicineLasers can deliver concentrated energy in the form of fine controllable light beams, so physicians soon took advantage of them to perform micro-surgery, which involves less pain and scarring, lower blood loss and shorter recuperation time in hospital. Laser beams delivered via flexible optical fibres allow surgeons to reach inside the gut, for example, and seal a bleeding ulcer. One of the most publicised uses of lasers is in eye surgery to treat disease and, increasingly, improve bad eyesight.
• ManufacturingLasers can deliver enough power to heat and melt metal joints, and so are used for welding, as well as for cutting. When controlled by a computer, a laser can cut complex designs into a material such as wood or paper, as is increasingly being seen in furniture and other home goods.
• Measurement and analysisLasers have long been used by the military for range-finding, but now even estate agents employ laser tape measures. Because lasers can be tailored to produce specific wavelengths, they are used to analyse chemical and physical structure, and so are used in factory quality control and to monitor environmental pollutants remotely. Lasers can be used for a type of measurement called interferometry which can measure tiny changes in distance.
• Scientific researchVirtually every university science department in the UK relies on lasers for some aspect of its research programmes – they have become indispensible research tools. Without lasers, many recent discoveries would never have been made, which illustrates the synergic relationship between developments in physics and other fields. Lasers interact with matter at the quantum level in very specific ways and so are important probes in research. They can be used to follow chemical reactions and elucidate structure at the atomic and molecular scale. Increasingly, life scientists are employing lasers in new types of microscopy designed to highlight cellular structures.
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Current developments

Credit: Shutterstock

Physicists are continually developing new lasers and many UK teams are involved in these projects. These include nanoscale devices that emit light and that are expected to find use in chemical and biological sensors on “lab-on-a-chip” devices.

The University of St Andrews, for example, has developed laser optical tweezers to manipulate biological cells to contribute to the burgeoning area of biophotonics.

Several UK research groups are developing a new semiconductor laser called the quantum cascade laser, which promises to be an excellent source of terahertz radiation (between infrared and microwaves) now being introduced for national security screening. New laser technology will also play a role in developing the all-optical computer.

## What Is a Laser?

The letters in the word laser stand for Light Amplification by Stimulated Emission of Radiation.

A laser is an unusual light source. It is quite different from a light bulb or a flash light. Lasers produce a very narrow beam of light.

This type of light is useful for lots of technologies and instruments—even some that you might use at home!

### How does a laser work?

Light travels in waves, and the distance between the peaks of a wave is called the wavelength.

Each color of light has a different wavelength. For example, blue light has a shorter wavelength than red light. Sunlight—and the typical light from a lightbulb—is made up of light with many different wavelengths. Our eyes see this mixture of wavelengths as white light.

This animation shows a representation of the different wavelengths present in sunlight. When all of the different wavelengths (colors) come together, you get white light. Image credit: NASA

A laser is different. Lasers do not occur in nature. However, we have figured ways to artificially create this special type of light.

Lasers produce a narrow beam of light in which all of the light waves have very similar wavelengths. The laser’s light waves travel together with their peaks all lined up, or in phase.

This is why laser beams are very narrow, very bright, and can be focused into a very tiny spot.

This animation is a representation of in phase laser light waves. Image credit: NASA

## How do lasers work? | Who invented the laser?

by Chris Woodford. Last updated: July 10, 2019.

Lasers are amazing light beams powerful
enough to zoom miles into the sky or cut through lumps of metal.
Although they seem pretty recent inventions, they've actually
been with us over half a century: the theory was figured out in 1958; the first
practical laser was built in 1960.

At that time, lasers were
thrilling examples of cutting-edge science: secret agent 007,
James Bond, was almost chopped in half by a laser beam in the 1964
film Goldfinger. But apart from Bond villains, no-one
else had any idea what to do with lasers; famously, they were described as “a solution looking for a problem.

Today, we all have lasers in our homes (in CD and DVD players), in our offices (in
laser printers), and in the stores where we shop (in barcode scanners). Our clothes are cut with lasers, we fix our eyesight with
them, and we send and receive emails over the Internet with signals
that lasers fire down fiber-optic cables.

Whether we realize it or
not, all of us use lasers all day long, but how many of us really
understand what they are or how they work?

The basic idea of a laser is simple. It's a tube that concentrates light over and over again until it
emerges in a really powerful beam. But how does this happen, exactly? What's going on inside a laser? Let's take a closer look!

Photo: A scientific experiment to check the alignment of optical equipment
using laser beams, carried out at the US Navy's Naval Surface Warfare Center (NSWC).
Photo by Greg Vojtko courtesy of US Navy.

## How Things Work: Lasers

You know,
I have one simple request.
And that is to have sharks
with frikkin laser beams

attached to their heads!

~Dr. Evil

Always look on the bright side
…unless you’re holding a laser pointing device.
~Unknown

The death star superlaser embodies our fascination with lasers. But how do they work? (Source: Death Star PR)

The laser is, without a doubt, one of the most ubiquitous, archetypal technologies of modern times. And it is one of the most direct applications of quantum mechanics.  But how do lasers work?

It All Starts In The Atom

The story starts deep within the atom. I’ve previously discuss the fact that particles are waves and that this forces electrons to have only certain specific energies inside an atom. The energy and momentum of a particle control how many times the corresponding wave wiggles. And these must fit in a circle around the nucleus of the atom, as shown below.

## How Does a Laser Work?

Lasers are small yet powerful beams of concentrated light. These beams of light are so powerful that they are able to perform a number of tasks that one would think were impossible for a simple beam of light. Since the creation of lasers, laser technology has continued to grow and flourish. Today, these powerful, concentrated beams of light are used for a variety of purposes.

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### How Do Lasers Work?

Lasers work to amplify a light source and turn it into one powerful, concentrated beam. Electricity must be supplied to the laser through a power supply. Lasers can be powered through the use of batteries, electricity, or even another laser.

Lasers also must have a medium that produces amplification of light. Once a laser has power and something to pass through, it becomes a concentrated beam. This beam can then be emitted outward in a single line of bright light.

The word “laser” is an acronym that stands for “light amplification by the stimulated emission of radiation.”

### Who Invented the Laser?

The invention of lasers is a controversial area. The first laser is said to have been invented by a man named Charles H. Townes. But while some people say that there was one sole inventor or team of inventors of the laser, others argue that this is not the case.

Several patents were secured in the creation of the laser, and some argue that there is no single inventor of lasers; rather it was a group effort. Despite the controversy, laser technology has grown and improved rapidly throughout the past several years.

Those who come up with new forms or uses for laser lights are highly regarded.

### How Is Laser Light Different From Ordinary Light?

There are big differences between ordinary lights, or natural lights, and laser lights. Natural light generally illuminates a large area.

This is because the natural light and other light sources are designed to distribute light to larger areas.

With natural light being distributed and not focused into one beam, it is much less powerful than a laser light. Laser lights are much more focused than natural light sources.

• What Is Laser Light, and How Is it Different?
• Lasers and Light

### What Do We Use Lasers For?

In the beginning, the use of a laser was limited to areas of science and exploration. Today, as laser technology and technology in general has advanced, the uses are numerous. Many average people use lasers as a source of entertainment. This is possible through devices such as laser pointers and can be very fun and educational when they are used safely and responsibly.

Other uses for lasers are found in the medical and chemistry fields. Lasers are often used in communications and information processing devices. They are frequently used during experiments in science fields, particularly in chemistry. Lasers can also be useful to surgeons and in diagnosing and treating various types of cancers. Laser-cutters are also a common use for lasers.

### What Other Kinds of Lasers Are There?

While there are highly powerful types of lasers that are used in medical work, chemistry labs, and the development of technology, there are other types as well. A popular type of laser is a laser pointer. Laser pointers are designed for public use and are nowhere near as powerful as the lasers used in professional settings.

When it comes to professional lasers, there are several varieties that can be used dependent upon the specific need. Lasers can be divided into gas, liquid, or solid lasers. In gas lasers, the medium consists of a single gas or vapor or a combination of more than one. In liquid lasers, a dye is generally used as the medium.

Solid lasers use a medium such as crystal or glass.

## How Lasers Work

Laser light is very different from normal light. Laser light has the following properties:

• The light released is monochromatic. It contains one specific wavelength of light (one specific color). The wavelength of light is determined by the amount of energy released when the electron drops to a lower orbit.
• The light released is coherent. It is “organized” — each photon moves in step with the others. This means that all of the photons have wave fronts that launch in unison.
• The light is very directional. A laser light has a very tight beam and is very strong and concentrated. A flashlight, on the other hand, releases light in many directions, and the light is very weak and diffuse.

To make these three properties occur takes something called stimulated emission. This does not occur in your ordinary flashlight — in a flashlight, all of the atoms release their photons randomly.

In stimulated emission, photon emission is organized.

The photon that any atom releases has a certain wavelength that is dependent on the energy difference between the excited state and the ground state.

If this photon (possessing a certain energy and phase) should encounter another atom that has an electron in the same excited state, stimulated emission can occur.

The first photon can stimulate or induce atomic emission such that the subsequent emitted photon (from the second atom) vibrates with the same frequency and direction as the incoming photon.

The other key to a laser is a pair of mirrors, one at each end of the lasing medium. Photons, with a very specific wavelength and phase, reflect off the mirrors to travel back and forth through the lasing medium.

In the process, they stimulate other electrons to make the downward energy jump and can cause the emission of more photons of the same wavelength and phase. A cascade effect occurs, and soon we have propagated many, many photons of the same wavelength and phase.

The mirror at one end of the laser is “half-silvered,” meaning it reflects some light and lets some light through. The light that makes it through is the laser light.

You can see all of these components in the figures on the following page, which illustrate how a simple ruby laser works.