The History & Basics of Laser Cutters
Lasers are a powerful component of our everyday lives, whether we realize it or not — they’re not only in a lot of our movies, but in our barcode scanners, DVD players, and other physical items. Many of us might be familiar with the use of lasers in the popular machine known as the laser cutter. Laser cutters play an integral role in the world of engineering, so let’s dive into the laser cutter’s history and how it works!
The history actually goes back to the 1900s…
NOTE: We recognize that there are a lot of figures who have contributed to the development of lasers and laser cutters, including those who may not have gotten credit for their work. Unfortunately, not all contributors’ names will be mentioned below for the sake of brevity.
Albert Einstein theorized the principle of a laser. He described a theory of "stimulated emission of radiation."
What is stimulated emission of radiation?
To understand this, we need to understand electrons, energy levels, and photons. According to the Bohr model, we know that an atom looks something like this:
The different rings around the nucleus represent different energy levels. Think of energy levels as stairs. You can’t step “in between” stairs, only on the stairs themselves. Likewise, electrons can’t exist in between energy levels. The amount of energy an electron has dictates which energy level it is on. The farther away from the nucleus, the higher the energy level. The typical electron rests in what is called its ground state, which is the lowest energy level possible.
Moving between energy levels involves photons (we'll learn about that soon). A photon is essentially a particle of light, or a packet of light energy. Electromagnetic radiation travels in the form of photons.
For background, let’s first describe a process called spontaneous emission of radiation. When an electron of an atom absorbs energy such as light energy (in the form of a photon), heat energy, or electrical energy, it can be excited, which means it jumps to a higher energy level. This can only occur if the amount of energy the electron absorbs equals the exact same amount of energy as the difference between the level the electron is currently in and a higher level (remember the stair analogy?). Being in a higher energy level is usually unstable, so the electron wants to very quickly return to its “home” (its ground state) in about a few nanoseconds, although the time varies from case to case. When it drops down to its ground state, the electron releases its excess energy in the form of a photon. The whole process of an electron returning to its ground state and emitting one photon occurs naturally and requires no interference from anything else — that’s why this is known as spontaneous emission.
But stimulated emission of radiation is a bit different. It occurs when photon emission, instead of happening naturally, takes place due to the interference of other photons. We know that an electron can be excited after absorbing one photon. It turns out that if an electron absorbs another photon while it is in the excited state, the electron will actually be stimulated to drop to a lower energy level and give off TWO photons. The photon which originally stimulated the electron will pass through, and the electron will emit a “daughter photon” with the same wavelength, energy, and direction as the other. These photons are said to be “in phase” and coherent.
So, to recap: when an electron absorbs energy, it can jump to a higher level. Spontaneous emission occurs the electron emits one photon naturally, while stimulated emission occurs if another photon causes the electron to emit two photons.
NOTE: The names of these processes include “emission of radiation” because radiation is emitted when photons (which are particles of light) are given off by atoms! By the way, the color of the light is determined by the wavelength of the photons. The wavelength of the photons, then, is determined by the amount of energy released when the excited electron drops to a lower orbit.
Gordon Gould, a graduate student of Columbia University, proposed that stimulated emission could be used to amplify light. He described a theory of Light Amplification by Stimulated Emission of Radiation. (The term “LASER” is an abbreviation for this!)
What is light amplification by stimulated emission of radiation?
This is how a laser works!
A laser beam is a very concentrated light beam. From what we learned of spontaneous and stimulated emission of radiation, light (in the form of photons) can be emitted by excited electrons. So to produce an extremely strong beam of light, we need (1) a lot of atoms with excitable electrons in some medium AND (2) something to stimulate, or excite, the atoms with.
This is what a laser looks like up close:
The laser medium (AKA laser material, lasing medium, or gain medium) can be a gas, liquid, or semi-conducting solid (such as glass or crystal) which consists of many atoms. When excited by energy, this substance will emit light in many random directions.
The energy pump (AKA excitation mechanism) is the source of energy used to excite the atoms in the laser medium. Some common energy pumps are electrical currents, flash tubes, and energy from another laser.
The optical cavity is created by the two mirrors on both ends of the laser medium. One end is fully reflective while the other is only partially reflective.
In short, a laser is made up of a bunch of excitable atoms within a cavity that has mirrors on both sides. The magic happens when the energy pump excites those atoms!
The process of how a laser beam is generated is described below:
- Electrons in the atoms in the laser medium absorb energy from the energy pump. They become excited and jump to a higher energy level.
- The laser medium allows excited electrons to remain at their higher energy states for an extended period of time in what is called a metastable state.
- The energy pump keeps supplying energy to the laser medium until there is a population inversion. A population inversion occurs when there are more atoms with electrons in the metastable state than atoms with electrons in the ground state.
- Some electrons in the metastable state quickly return to their ground states and each emit one photon through spontaneous emission. The photons that are given off move inside the medium in random directions at the speed of light. Meanwhile, some electrons remain in the metastable state for longer periods of time.
- Most of the photons which were emitted in the previous step are absorbed by the walls containing the medium. (Sadly, lasers are actually not very efficient).
- Meanwhile, photons that perpendicularly hit the mirrors of the optical cavity will be reflected by the mirrors back into the medium and will promote stimulated emission, as seen in the next step.
- The photons which are reflected end up hitting electrons still in the metastable state, stimulating them to drop to their ground states and each emit two photons through stimulated emission. In a sort of chain reaction, stray photons stimulate electrons to emit more photons, which stimulate more electrons, etc.
- The fully reflective mirror reflects all the photons that hit it perpendicularly back into the medium. On the other side of the optical cavity, the partially reflective mirror reflects some photons back while allowing other photons to leave.
- The photons that leave the laser medium through the partially reflective mirror form a concentrated beam of laser light that is very bright and straight.
NOTE: The term light amplification comes from the fact that the amount of photons increases from stimulated emission. With an increase in the amount of photons comes an increase in the amount of light. As the light bounces between the two mirrors, it becomes stronger.
The first ever working laser was built by Theodore Maiman at Hughes Research Laboratories in California. The laser medium used was synthetic ruby, and this laser emitted a deep red beam. However, the news of the development of a laser was met with skepticism, and the laser was described as “a solution looking for a problem.”
The CO2 laser and gas laser cutting process using a CO2 mixture was invented by Kumar Patel at Bell Labs in New Jersey. The CO2 laser was found to be cheaper and more efficient than the ruby laser. The crystal laser and crystal laser cutting process was also invented at Bell Labs in this year by J. E. Geusic. (Also, James Bond was nearly sliced in half by a laser in Goldfinger.)
How do laser cutters work?
Laser cutters can cut, engrave, and mark a variety of materials — including (but not limited to) wood, metal, fabric, and plastic. Laser cutting, compared to mechanical cutting using cutting tools, is a contact-free process of cutting materials into precise shapes. It works on many different thicknesses and can cut slits with widths as small as 0.1mm.
The process of how laser cutting works is described below:
- A laser beam is generated. (See above for a description of how this happens.) The beam’s intensity, length, and heat length may be adjusted depending on the material that is being cut. Also, the beam can be a pulsed beam, which delivers light in short bursts, or a continuous wave beam, which delivers light continuously.
- The beam is focused. Several mirrors and a lens guide and focus an intense beam of light so that the beam’s energy becomes more concentrated. The beam’s intensity increases because it has the same amount of power aimed at a smaller area. The focused laser beam is “assisted” by some sort of compressed gas (e.g., oxygen, nitrogen) as it goes through the bore of a nozzle.
- The concentrated beam heats and melts the material underneath.
- The material directly targeted by the laser beam is removed. Wood, for example, is burnt and reduced to carbon and smoke.
- The beam moves to another portion of the material. This movement can be achieved by adjusting the mirrors, controlling the laser cutting head, moving the workpiece, or a combination of these.
Here's our diagram of a laser cutter!
1967 - 1979
Lasers gained popularity throughout the world and among manufacturers, including Western Electric and Boeing. Laser cutting, one of the first uses of laser beams that was discovered, was adopted on a much wider scale by the British. In this period of the late 1960s and early 1970s, gas laser cutting continued to advance and was used to cut through new materials such as metal.
By now, about 20,000 laser cutting machines, which were collectively worth about $7.5 billion, had been installed throughout the world for various industries.
Professor William Steen commented in his book, Laser Material Processing, about how humans had entered a new industrial revolution since the invention of the laser.
Fiber lasers grew in popularity as the newest method of laser cutting.
What are the different types of laser cutting?
Remember how the laser medium can be a gas, liquid, or solid? Well, lasers are classified based on the state of their laser medium. A solid-state laser uses a solid (glass or crystal) as its laser medium and a light source (e.g., flashtube, flash lamps, etc.) as its energy pump. A gas laser uses a gas mixture as its laser medium and an electric current as its energy pump. A liquid laser uses a liquid as its laser medium and a light source as its energy pump.
Note: In solid-state lasers, the solid medium has to be "doped" to be effective. Basically, some of the solid’s atoms are replaced with ions of impurities to adjust the energy levels of the solid.
There are three main types of laser cutting:
- Gas laser cutting (AKA CO2 laser cutting): This type of cutting involves a gas laser. The laser medium is a CO2 mixture which is electrically stimulated by an electric current discharged through it.
- Gas laser cutting is most often used on nonmetals and is seen in industrial settings, especially the medical sector.
- Crystal laser cutting: This type of cutting involves a solid-state laser. The laser medium is a crystal (e.g., ruby).
- Crystal laser cutting is useful for cutting both metals and nonmetals. It is seen in dentistry, manufacturing, medical, and military industries. However, crystal laser cutters have short life expectancies (8,000-15,000 hours) and are expensive.
- Fiber laser cutting: This type of cutting involves a solid-state laser. The laser medium is made of optical fibers.
- Fiber laser cutting is the newest method of laser cutting! It is used on both metals and nonmetals. Fiber laser cutters have stronger laser beams, longer life cycles, and are cheaper. Their beams are about 100 times more intense than those of CO2 lasers.
Three scientists received the Nobel Prize in physics for advancing the science of lasers and creating useful tools out of laser beams (such as optical tweezers, AKA laser tweezers). They were Arthur Ashkin, Gerard Mourou, and Donna Strickland. Donna Strickland was the 3rd woman to ever win a Nobel Prize in physics and the first woman to do so in 55 years!