Lasers

Lasers are one of two technologies that led to a revolutionary advance in optics (the other being semiconductors, which I'll discuss in a later post). The word 'laser' was originally meant as an acronym for a curious piece of engineering, that gained such wide traction it has become a word. Technically, it stands for "light amplification by stimulated emission of radiation". Radiation here is the generic term for electromagnetic waves, which encompasses visible light, UV, infrared, microwaves, gamma rays, x-rays and radio waves, among many other types. Since their invention, laser's have become staples of the modern world. They are what make cds, dvds and blurays possible. They are central to anything fiber optic, and have become so cheap and ubiquitous we use them instead of the tradition wooden pointer for presentations, and use them as levels. Their uses in science and engineering are bordering on innumerable, not to mention their use in medicine for precision surgery and low collateral damage treatments. They range from size and potency from smaller than a AAA battery and some temporary blindness if you look straight into it (e.g. laser mice) to the size of a small house and the ability to fuse atoms (lasers at the National Ignition Facility).

Via WikiCommons

Lasers come in several different basic types:dye, gas, solid state, LED/semiconductor, chemical, fiber,  free electron, and more recently 'exotic material' lasers. They each have their advantages and disadvantages.

•  Dye lasers can give very tunable wavelengths, which is an issue with most lasers, but the materials used are also (more often than not) highly toxic and annoying to work with. They are dropping out of favor as other types of laser become more tunable without the toxicity issues. 
• Gas lasers are bulky, but low cost, very narrow bandwidth  and very very common. Helium Neon (HeNe) lasers are used for a variety of research and education purposes. Carbon Dioxide (CO2) lasers are used for welding, cutting and telecom purposes (the latter at obviously much lower powers). Depending on the gas used they have a range of available wavelengths. Also, far less toxic than dye lasers. 
• Solid state lasers use crystals or doped glass as the gain medium. The first laser used ruby. These are typically bulky, and are limited by thermal considerations, but they can output very high powered pulses. They typically have very narrow bandwidths, and are not tunable except by integer multiples of the frequency. 
• Fiber lasers are a subtype of solid state lasers where the gain medium is many loops of  a fiber. They have  a distinct advantage over bulk crystal lasers in that, since the fiber is very thin, they can be efficiently cooled. However, they cannot operate as at high powers, since the intensity of the light passing through the fiber can cause distorting and non-linear effects. 
• Semiconductor or LED lasers are lasers that use light emitting diodes to create laser light. Below a certain threshold, the LED acts as a normal LED. Above a certain amount of current, the material begins to lase. This type of laser can be very compact (they are used in laser pointers for example), and offers a broad range of available wavelengths, presently from near UV to  near IR. They are relatively inexpensive compared to other types of laser media. 
• Free electron lasers are kind of an oddball laser, because it uses a relativistic beam of electrons as its lasing medium. They are huge (garage sized), incredible expensive, but also high powered and highly tunable. They are not as popular now that LEDs can offer much the same tunability. 
• Chemical lasers are used for applications were very high powers are needed, such as military applications. Instead of pumping a lasing medium with light or electricity, a violent chemical reaction is used. 
• Exotic material lasers use different types of radioactivity to pump a medium. These are more lab experiments at the moment (Although I'm sure someone would think of a use, I can't think of why I would want to use radioactivity and not light or electricity).

 With the exceptions of the free electron laser and chemical laser, lasers work in a weird but fairly simple manner. The gain medium is confined and two partially silvered, concave mirrors are put at either end (in the case of electrically stimulated lasing, only one mirror need be silvered). Because the mirrors are partially reflective, partially transmissive, pumping light can come in, and laser light can leave, but roughly half the light can be contained to continue the population inversion necessary for lasing. Curving the mirrors inward, even slightly, helps to avoid light lost at the edges and diffraction effects. Outside of the cavity, there will be a series of lens to produce a clean light beam, and make sure it is going in the right direction. In the case of a solid state laser, there is usually an optical diode to prevent light from coming back in and destroying the crystal. 

1) gain medium
2) pumping energy (electricity here)
3) Back mirror
4) Front mirror (and lens, it looks like)
5) Laser beam
(via WikiCommons)

Inside the crystal, the atoms are undergoing population inversion. This nifty little video gives a good visualization of the process. Every atom has discrete energy levels, and in jumping between a higher level and a lower level, can release a photon. In a laser, the idea is to put enough energy into the system that most of the atoms are in the higher energy state, and then releasing that energy in the form of light. Not every material is capable of lasing, because it must be capable of staying in that higher energy state for  some (atomically long) time period. Otherwise, it is impossible to achieve population inversion. 

That being said, you could make a laser, given the right resources, out of a surprising number of things, including but certainly not limited to, a glass of beer or a gin and tonic. The first thing to do would be to cap the container somehow. In either case, you could with a partially silvered mirror, a fully reflective mirror and an electrical supply use the carbon dioxide dissolved in the liquid to create a laser (albeit not a very good one). In the case of the gin and tonic, tonic water contains a small amount of quinine (the  drink is said to have been invented as a way to get British troops to take their malaria medicine, since quinine itself is very bitter), which will fluoresce under ultraviolet illumination. Fluorescence alone will not cause lasing unless you filter the reflected light for a specific wavelength and force all the atoms to the same energy level. This can be achieved using traditional filters, Bragg gratings or nano-fabricated mirrors, which can be tuned to reflect only narrow band light. 

That is an extremely brief look at lasers and how they work. But I need to move onto other topics, so this will have to do for now!

~PhysicsGal