“Frickin’ Sharks with Frickin’ Lasers”
As a laser engineer, I have to endure what that movie has done to that field at least once a month, and without fail on every plane ride. But many people simply do not understand, from either a theoretical or a practical standpoint, what a laser does, or why they are important. It is because not only does it represent impressive technology and innovation, but also because they allow us to interact with the world around us in new and fascinating ways in the fields of pure research, medicine, industry, and many more.
In order to understand how light interacts with materials, a brief overview of exactly what constitutes laser light is warranted. There are several characteristics that make it unique:
(1) Monochromatic. Light from a laser occupies a very narrow line on the electromagnetic spectrum, and the light emitted from a laser can be generally thought of as nearly completely of a single wavelength. Technically, the light is more likely bound within finite limits, usually about one nanometer wide, depending on whether Gaussian or Lorentzian broadening is taking place in the gain medium. This can be contrasted with typical light sources such as incandescent light bulbs or sunlight (broadband, or “blackbody” sources), whose light can spread across a very large range along that spectrum.
(2) Unidirectional. When light is emitted from an incandescent light bulb, for instance, that light radiates more or less equally in all directions. The intensity of light that we can observe from that source is equal to only a fraction of the actual total power emitted from the bulb, since we can only view a single section at any given moment. A laser source, however, will concentrate all of the light into a single, small area – usually only millimeters across or less. This property essentially increases the energy density of the light by many orders of magnitude.
(3) Coherence. When a light source emits light, that light can be thought of as an oscillating wave. The direction of that oscillation in both time in space may or may not be the same as the other waves near it. In the case of the sun, it averages out such that the oscillation directions are essentially random after only a few millimeters. But in the case of a laser, by the very nature of the process, all the waves share both spatial and temporal coherence, which gives us properties such as uniform polarization of the light and control over interference. This is a very general overview of coherence, and sufficient to the purposes of this discussion, as a more detailed treatment of coherence would require hundreds of pages. I have first-hand knowledge of that fact.
The physics of lasers are complex and lengthy. A quick, but useful overview of lasers and their basic construction can be found here: <http://en.wikipedia.org/wiki/Laser>. For the remainder of the discussion, we will assume that the basics of the function of a laser are already known.
So with this understanding of how laser light differs from normal, everyday light sources, what happens when that light interacts with a material? It’s a complex question. Based on the intensity of the light, how tightly it’s focused, whether it’s continuous-wave (CW) or pulsed, and how short or long those pulses are (from microseconds to femtoseconds), the color – or wavelength – of the laser, and, of course, the composition of the material, the outcome is incredibly varied. We’ll start with common applications and proceed toward more esoteric applications.
Bar code scanners are ubiquitous, and their function is relatively simple. A series of rotating, polygonal mirrors are arranged such that a low-power red laser will shine onto them at an oblique angle. This angle changes as the mirror rotates, and does so at such a speed that the single point reflection appears to be a line. As the light hits the bar code, it is either reflected by the light parts of the bar code, or absorbed by the darker section. The reflected light moves into a detector within the system and the “value” of the bar code is deciphered, using the known speed of the rotating mirrors, and the order in which the dark/light (zero and ones, respectively) signals are received. This same principle, applied slightly differently, applies also to CD’s, DVD’s, and Blu-Ray discs.
Medicinal uses of lasers rely on several factors. Typical LASIK procedures use either excimer (excited dimer) lasers – usually ArF, which lases at around 193 nm (deep UV) or pulsed lasers, firing light in pulses only several hundred femtoseconds (several hundred quadrillionths of a second). The procedure uses the laser to cut the cornea, and this wavelength reduces burning by literally destroying the molecular bonds between the cells, causing them to be ablated (essentially, vaporized) instead of heating, reducing both complications and recovery time.
Tattoo removal is, somewhat counter-intuitively, a more complex type of interaction. The color of laser (wavelength) required will vary depending on the color of the target ink, as well as the person’s complexion. These lasers have substantially higher power than barcode scanners, and come in a variety of flavors. For people with darker skin, a neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) crystal at its fundamental frequency (1064 nm) is one of the only choices available, since this infrared wavelength is very poorly absorbed by melanin, and will be more preferentially absorbed by the ink. The doubled version of this laser is obtained by adding a second-harmonic generating (SHG) crystal, usually lithium triborate (LBO), potassium titanyl phosphate (KTP) or some similar material. The crystal absorbs the input light at 1064 nm, which then causes the electrons within the medium to oscillate at twice the frequency, causing a reduction in wavelength by one-half. The doubled version is at 532 nm, which is a bright green light, easily seen. This is usually good for red and orange targets on people with fairer complexions. Red and orange lasers, as well as tunable-wavelength (dye) lasers may also used at the doctor’s discretion, depending on the pigment of both the ink and of the patient’s skin.
Interestingly, the physics behind tattoo removal are analogous to the principles used in industrial laser drills. Based on the material being processed, the quality of the processing (such as smoothness or roundness of the final part), the speed required, and the required polarization, different lasers must be chosen.
For example, in many industrial applications, much attention is given to the “heat-affected zone,” which is the local region around where a laser pulse hits a material. The zone will then suffer localized heating, and may contract, deform, or chip, depending on how much heat has been absorbed, what kind of material, etc. Glass in particular is highly affected by this, as the beam must be so heavily focused so that the energy density in the material causes a non-linear reaction (otherwise, the material would just let it pass through). But, the pulse width must be short enough that when the glass is no longer transmissive, the laser light isn’t being scattered, causing excessive heating of the surrounding material. Since glass suffers from crack propagation, localized stresses from ultrafast heating and cooling will cause chipping and sometimes even large-scale breaks.
All metals are difficult to machine. In many automotive industries, high-power lasers with hundreds or thousands of watts are required. Here, they usually utilize extremely long pulse widths (hundreds of microseconds) to cut and weld aluminum and steel. Since the requirements are so much more lax than those in other, smaller-scale industries, beam quality and consistency are less heavily weighted compared to the total power being delivered These are just some examples of how lasers are used. There are many more, but unfortunately, they are usually obscured by trade secrets or intellectual property law.
Additional uses for lasers are being discovered constantly. In experiments at the Lawrence Livermore National Laboratory, experiments using laser power of 370-terawatts (370,000 billion watts) in order to perform fusion experimentation. Universities studying the phenomena of super-cooled particles actually have learned to manipulate light in order to extract energy from particles, allowing us to observe their behavior at temperatures near absolute zero (-273.15 degrees Celsius). And lasers can be used to create what are called “optical tweezers,” essentially trapping charged particles in an electromagnetic field, and allowing people to maneuver the particle as they see fit within a medium, completely independently of its surroundings, opening new avenues in biomedical imaging and diagnostics.
It’s unfortunate that such a ubiquitous and significant tool as a laser is typically only recognized as an object of idle enjoyment for one with active pets, or someone who believes they are only used in security systems in scientifically inaccurate spy films. But while we are still quite a way from creating a light saber (which, honestly, is not a practical weapon by any standard), lasers are helping us unravel the secrets of the universe, both massive and tiny, one joule at a time.
