All About Lasers

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Podcast Transcript

In 1917, Albert Einstein published a paper whereby he proposed a new method of creating light based on the principles of quantum physics.

Over 40 years later, researchers finally put Einstein’s ideas into practice.

For years it remained a solution in search of a problem. Today, the number of applications for this source of light is almost limitless and includes everything from nuclear fusion, to annoying cats. 

Learn more about Light Amplification by Stimulated Emission of Radiation, or LASERs, on this episode of Everything Everywhere Daily.

The development of lasers dates back to the beginning of the 20th century with the development of quantum physics.

One of the groundbreaking ideas behind quantum physics was that light could be emitted or absorbed in discrete units of energy that were called quanta. 

Before I go any further, I should explain how light is produced at a very base level because it is necessary to understand how lasers work. 

Each atom has electrons orbiting around it at different levels. Technically, they are more probabilistic clouds, but for the purpose of this episode, you can think of them as different orbital levels. 

When all of the electrons in an atom are at the lowest orbits possible, it is known as the ground state. When photons are captured by the electrons, it kicks the electrons up to a higher orbit, putting them at a new energy level and out of the ground state.

This is sort of like when you roll a ball up a hill. The ground state for a ball is at the bottom of the hill. When you put it at the top of a hill, you have added energy. If something disturbs the ball, it will want to roll back downhill.

So too, does the electron want to go back down to its ground state. When it does this, depending on the level it was at, it will emit a photon, aka it will emit light. 

Normally in something like a flashlight, the electricity kicks the electrons up a level, and then they come back down to its ground state somewhat randomly. The result is photons of many different wavelengths, aka colors, with many types of polarization, being emitted in many different directors. 

All of these photons being created without any sort of order is known as incoherence. This is how almost all light is created, both naturally and artificially. 

Einstein posited that it might be possible to create a system where all of the light which was created would be coherent through a quantum physics phenomenon known as stimulated emission. More on that in a bit.

The idea of stimulated emission was just an idea for years until work began on actually trying to implement the ideas in the 1950s. 

The first device which used stimulated emission was constructed by a team led by Charles Hard Townes, which amplified microwaves. They dubbed this device a MASER, an acronym that stood for Microwave Amplification by Stimulated Emission of Radiation. 

The device was seriously limited in what it could do, but it was a huge step forward. 

In the late 50s, many teams around the world were trying to create what they then called an optical MASER. 

The term LASER, replacing the M in microwaves for the L in light, was dubbed in 1957 by a Columbia University graduate student named Gordon Gould. 

Here I should note that while “laser” is technically an acronym, like radar or sonar, its usage has become so common that it is usually not capitalized anymore. 

Gould came up with a host of possible uses for lasers and filed a patent for lasers in 1960. His patent was denied and was given to Bell Labs, who, if you remember from previous episodes, invented everything. He spent the next 27 years fighting the patent before finally winning in 1987.

However, the first group to make a working laser was led by Theodore  Maiman at the Hughes Research Laboratories in Malibu, California. The laser used a synthetic ruby crystal as its medium and produced light at a wavelength of 694 nanometers.

Here I’ll try to explain what exactly this stimulated emission is and what makes a laser a laser and different from a flashlight.

In stimulated emission, light of a very particular type is created inside the laser. The photons in the laser medium tend to cause the atoms inside the medium to release photons just like the photon which are already there.

There is a quantum effect whereby photons want to be with other similar photons, so the new photons have the same wavelength, direction, and polarity as the original photon. In other words, the light is coherent.

These new photons create, even more, amplifying the creation of the light inside the chamber.

The chamber has mirrors on both ends allowing the light to bounce back and forth, but one of the mirrors allows a small amount of light to pass through. 

The creation of a singly type of light that is coherent in direction, polarity, and wavelength is what makes a laser a laser, and why just poking a tiny hole in a box with a lightbulb will not get the same result.

A laser’s wavelength of light is determined by the active laser medium or the gain medium inside the laser. The first laser used ruby, but you can use many different substances. It can be a solid, gas, or liquid. 

In December 1960, Bell Labs created the first gas laser, which is a laser where the medium was a mix of helium and neon.

The discovery of lasers made for great press. They were suspiciously like death rays from science fiction stories. One of the first mentions of lasers in popular culture was in the 1964 movie Goldfinger where James Bond is tied to a table and threatened to be cut in two by a laser. 

Despite the early excitement about lasers, they had few practical applications, which is surprising given how many thousands of uses for lasers there are today. 

It didn’t take long for lasers to find a multitude of uses, however. 

In 1961, the first medical treatment was performed with a laser. It was used to destroy a tumor inside of a retina. 

1962, groups at General Electric, IBM, and MIT all, almost simultaneously, developed a semiconductor laser, which is a type that is used in many electronics.

In 1964, the Carbon dioxide laser was developed that used CO2 as the active medium. CO2 lasers are extremely powerful because the lightwave they create is in the infrared wavelength, which is radiant heat. CO2 lasers were some of the first lasers used for industrial cutting.

In 1966, British physicist George Kao and his team determined how to send information via laser across fiber optic cables. He was awarded the 2009 Nobel Prize for his work on fiber optic communications. 

Today, lasers using fiber optic cables transmit almost all of the internet’s data and are responsible for an ever larger percentage of bandwidth to people’s homes. 

In 1969, Apollo 11 took with them to the moon a special prism called a retroreflector, designed to reflect light directly from any angle from which it came. The purpose of the prism was that a laser would be aimed at it, and a few of the photons would then bounce back directly to the laser. 

They could then measure the time it took for the photons to make the round trip, and from that, they could use the speed of light to measure the distance to the moon. They have been using this technique to measure the distance to the moon for over 50 years, and it is accurate to within centimeters. 

In 1974, the first laser barcode scanner was deployed in a supermarket. The scanner could quickly read a barcode, which I’ve done in a previous episode, and then use the scanned data to interface with a computer. 

In 1982, one of the very first commercial uses of lasers, and the one which gave most people a personal exposure to them, was the release of the audio Compact Disc or CD. 

The CD worked by a red laser with a wavelength of 650 nanometers shining on a spinning disc which would reflect light off pits in the disc. The pits were binary 1s or 0s, which could then decode the digitally encoded music.

Blu-ray discs were literally just a change in lasers to a blue-violet laser with a 405-nanometer wavelength. The shorter wavelength of light allowed for smaller pits on the disc, which allowed for higher data density.

In 1985, David Chu did work using lasers to manipulate atoms, allowing him and his team to set records for the lowest temperature ever recorded. Chu won the Nobel Prize for his work and was later the US Secretary of Energy. 

As lasers became more powerful, their potential applications expanded into the realm of the military. Extremely powerful lasers have been developed that could shoot down rapidly moving missiles out of the sky. 

Another use of high-powered lasers is in the development of nuclear fusion. If several high-powered lasers can be focused on the same spot, it can create temperatures high enough to ignite fusion. 

Less powerful military lasers were developed to measure distances for aircraft and cruise missiles, and other lasers could use their straight-line capabilities to aim a rifle.

One ever-growing use for ranging-finding lasers is called Lidar. It stands for “laser imaging, detection, and ranging” and uses lasers to create a 3D map of the terrain it sees.  There are a host of Lidar uses, including getting accurate maps of buildings.  

Much of the reconstruction of the Notre Dame Cathedral is possible due to high-quality 3D models of it, which had been created beforehand by Lidar. 

Likewise, Lidar is used as the navigation system for projects attempting to create self-driving cars. Lidar allows an onboard computer to get an accurate 3D map of the car’s surroundings. 

I’ll end by discussing the one use of lasers with which many of you have had direct experience: laser pointers.

The availability of commercial laser pointers is a testament to just how cheap and available lasers have become. It is now possible to find cheap laser pointers for about $10 on Amazon. 

Many colors are available, but the most common are red and green. Red has the longest wavelength, and they tend to be cheaper and easier to make.

Green is often used when doing outdoor astronomy simply because green appears to be brighter for the same amount of energy to our eyes. 

Despite being very low power, shining a laser pointer directly into someone’s eyes can be incredibly dangerous. Because of how concentrated the light is, it can be more dangerous than staring directly into the sun.

Lasers have so many uses it would be impossible to list them all. Almost every sector of the economy, from medicine to communications to agriculture to transportation, uses lasers to one degree or another. 

Its widespread use across so many different areas has made the laser one of the most important inventions of the last 100 years.