The History of LASERS
The word LASER stands for Light Amplification by Stimulated Emission of Radiation, and the first-ever laser was built in 1960 by Theodore H. Maiman, an American engineer and physicist who received numerous awards for his invention. However, it was Elias Snitzer who, in 1961, demonstrated the first optical fiber laser and fiber amplifier. Since then, fiber laser technology has only evolved to encompass a diversity of uses in many fields, including industry, medicine, and telecommunications.
How a laser works
First, laser diodes from a power source transform electricity into photons. So, they basically create light which is then pumped into a fiber-optic cable.
But if not controlled by a specific mechanism, light will go in every direction and we have seen how narrow and focused a laser beam is. We'll get there, but first we need to understand how light travels through the fiber optic cable.
- The core is the conduit through which light travels. It's made of silica glass and coated with rare earth elements (in this case, Ytterbium) and has a high refractive index.
- The cladding is the layer surrounding the core with a low refractive index.
- The coating is a thicker plastic layer that acts as a buffer to absorb shocks and prevent the core from bending.
The importance of refraction
When you think about refraction, you usually visualize light entering from the air into glass or water and changing its angle. This happens because both glass and water are denser than air and have higher refractive indexes, so light travels slower when passing through them. This change in speed is what causes the change in angle. When light exits the denser medium and goes out into the air again, it gains speed, and the angle changes back to its original degree, as seen in the image:
So, what does refraction have to do with our fiber optic cable? We don't want the light to go through the cladding and out at a different angle. We want to keep it inside the core.
Here is where we get into a bit of physics. According to the law of refraction (Snell's Law), there is something called the "critical angle". The critical angle is the largest angle of incidence for which refraction can still occur in a particular medium. So, the light will only refract as long as the angle of incidence (the angle at which the light hits the cladding) is smaller than the critical angle.
If the angle of incidence surpasses the critical angle, the light ray will bend so much that a phenomenon called "total internal reflection" occurs. Total internal reflection means the light bounces back into the first medium (the core), which is exactly what we need. Image 4 in the diagram below shows total internal reflection:
Without the cladding, light would go in all directions and exit the core. Thanks to the low refractive index of the cladding, the high refractive index of the core, and how narrow the cable is, light hits the cladding at an angle greater than the critical angle and bounces back over and over, causing it to travel through the optical fiber.
How is light amplified?
Remember that LASER stands for "Light Amplification," which is achieved by "Stimulated Emission"? Let's delve into how amplification is attained.
As we mentioned before, the fiber's core is coated with the rare element Ytterbium. In physics, this is called a "doped" fiber. As particles from this doped fiber interact with light (photons), their electrons are excited, which means they rise to a higher energy level. Eventually, the electrons drop back to their original level, but because energy is never lost, when dropping, they release energy in the form of photons thus, producing more (or amplifying) light.
But light amplification doesn't stop there. The fiber core also has "Fiber Bragg Gratings" (FBG), which, in simple terms, are a set of regularly spaced mirrors that bounce photons back and forth.
Most light wavelengths are allowed through the gratings, but a specific light wavelength is bounced back. So, now we have these reflected photons joining the constant flow of photons pumped by the diodes in the source. The increased number of photons all hit excited particles creating even MORE photons for exponential light amplification. As a result of this stimulated emission, the laser beam is created.
How is light turned into a focused beam?
To generate this ultra-focused beam which lasers are famous for, you need a collimating lens. Collimating is the process of aligning light so that its rays are parallel and have minimal spread. Depending on the lens, the collimated beam can be calibrated to a specific diameter and focused on a specific point. This super focused beam then exits the fiber into the open air, and comes in contact with the metal sheet, cutting it at exceptional accuracy.
Each kind of fiber laser generates a beam of one particular wavelength only, depending on the doping element of the core. For this reason, visible laser light is monochromatic and can be blue, green, or red. However, Ytterbium-doped fiber lasers like those used for metal cutting generate a wavelength between 1000 and 1100 nanometers, making them near-infra-red and, therefore, invisible to the human eye. So, when cutting metal with a fiber laser, the only light the eye can see is that from the sparks created by the laser when in contact with the metal.
Now that you know how fiber lasers work, it will be easier to understand why fiber lasers are are replacing CO2 lasers in metal cutting shops worldwide.