A Look At Lasers | Ruby Laser, Fiber Optics, Laser Cleaning

For years lasers have been a hallmark of science fiction. Though, much of our
modern technology depends on them even throughout our daily lives. Some noteworthy examples include range finding devices, optical communications, and of course, even barcode scanners. The unique properties of laser light that make this possible are:

Single wavelength
Narrow beam and
Great Intensity

Additionally, laser stands for "light amplification by stimulated emission of radiation". The unique characteristics of lasers even allow surgeons to reattach retinas, further underscoring their value to modern medicine. An injury can cause the eye’s retina to peel away from the supporting tissue. And without rapid treatment, the entire retina can become detached resulting in blindness. Surgeons can use green laser light from an excimer laser of nearly a single wavelength since that color passes through the eyes lens and vitreous humor and is not strongly absorbed, avoiding further damage.

The laser beam then strikes the retina where the tissue greatly absorbs the light. At this point, the high-intensity light welds the detached retina back into place. The narrowness of the
laser beam enables the surgeon to affect only the area of the retina that needs to
be repaired; in areas as small as 30 microns. How a laser creates light with these three
characteristics is a marvel of engineering.

In 1960 the first demonstrated use of a functional laser was credited to Theodore Maiman at Hughes Research Laboratories, in which he took a ruby cylinder and surrounded it with a xenon arc flash lamp typically used in aerial photography. Maiman’s ruby laser, as it was known, was based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow. The intense bursts of light from the lamp initiated the laser process. To better understand how it works let’s look at what happens when applying light from a weaker lamp.

A flash would promote a few electrons from the ground state to an excited state. Subsequently, they would lose a bit of energy, fall to a lower energy state without emitting light and then drop from there to the ground state giving off a burst of light.

Thereafter, the light produced would be called incoherent light which is a spectrum of colors and intensities. However, in order to create a laser, a much more powerful lamp is necessary. In the ruby laser repeated flashes called pumping creates an amazing phenomenon. So much energy is applied that a population inversion occurs.

Population inversion is essentially when more atoms exist in the higher, excited state than in lower, unexcited, or ground state.

Electrons from a population inversion that are returning to the ground state release light that starts an avalanche called stimulated emission. We’ll go into more detail on stimulated emission later in this video. The photon produced when an electron decays, induces other excited electrons to simultaneously decay and release nearly identical photons. Thereafter, coherent light is created which means that the crests and troughs of every light wave in the beam match up. At this point, we have coherent light but not yet the other two properties needed for laser light.

To get a narrow beam with all the light rays parallel and a nearly single wavelength requires an addition to our ruby laser example. To achieve this, Maiman incorporated silvered ends to reflect the light within the ruby cylinder, creating a resonance cavity. He made the two ends of the rod parallel to each other. From top to bottom the distance between these two mirrors differs by no more than two hundred nanometres.

Two important things take place inside the resonance cavity. First, any light rays that fail to line up with the axis eventually exit out from the side of the cylinder, and more importantly the light parallel to the axis becomes intensified and narrowed in wavelength.

At this point, we now meet the three characteristics of a laser.

The way it works is that the mirrored ends create a standing wave. This means that only light at a particular wavelength can exist inside the cavity. By selecting the correct rod length we’re able to get the nearly single wavelength of light, characteristic of a laser. The addition of a small opening in one of the mirrors allows the light to escape, creating the quintessential laser beam. In technical terms, what allows the light to escape is that the mirrors are very highly reflective and only the light that meets the resonance condition will be transmitted out of a pinhole aperture.

Now that we have a better understanding of the fundamentals of how a laser works. Let’s take look at the two main flavors of a laser.

Generally, the operation of a laser can either be continuous wave or pulsed. The primary determinant rests on whether the power output is essentially continuous over time or whether its output takes the form of pulses of light, on some variation of a time scale.