Lasers

Laser is an acronym of Light Amplification by Stimulated Emission of Radiation. These days you will find lasers for cutting, welding, barcode readers, in CD and DVD players, in guidance systems, lithography, holography, surgery,  scanners and even optical tweezers, plus about a thousand other uses and places.

There are lots of different types including gas lasers, chemical lasers, solid state lasers, fiber lasers, excimer lasers, photonic crystal lasers, semiconductor lasers, dye lasers, free electron laser and bio lasers.

In the modern world the laser can be found just about everywhere it seems. Due to mass production of diode lasers you can now get them for very little. I bought a laser guided spirit level the other day for under $10! Diode lasers out sell all other laser put together by about 5000 to 1!

It is due to their wide scale use of lasers that I thought it may be worth putting together a blog to this amazing piece of physics.

The original  idea for a laser was published in 1917 by Albert Einstein after he'd done some pondering on work by Max Planck. You have to give it to Albert Einstein, he had the knack of investigating and contributing to many of the great aspects of physics, you have to wonder if he was just really lucky, or a really bright guy, a genius perhaps? Anyway, back to lasers.

Light is made up of photons of all different frequencies. The different frequencies appear to our light detectors (more commonly known as eyes) as the different colours.

Electrons orbit round a positive nucleus in something we call atoms. An electron can move from a lower energy state into a higher energy state and visa versa. When it moves from the high energy state to the low energy state it gives out a photon. If you get lots of these photons you have something the eye can detect. In a light bulb, electricity is used to energize the electrons and when they drop into lower energy levels we get light. It comes out in all different frequencies and so it appears to our eyes as white light. This is known in the trade as the spontaneous emission of photons.

A laser is a special version of this process. In a laser the electrons become energized in much the same way, through electricity or some other source. But that is where the similarity ends. In the case of a laser, light is emitted by a process we call stimulated emission. In this case an electron in the higher orbit drops into the lower orbit because of the presence of a photon of exactly the same energy as the difference between the two energy levels.

A photon causes a stimulated emission resulting in 2 photons

When the electron drops a photon is produced that is identical to the first and they add like waves to give a more intense wave. This process continues in something similar to an avalanche and before you know it you have a really intense beam of light with a single frequency (this is known as monochromatic) and that is a laser.


While this process may seem relatively simple to understand now, it was a remarkable piece of deduction by Einstein and some equally clever practical physics from the people who actually turned the theory into working lasers.

3 level laser

The trick is how to get the electrons into the higher energy state. See the problem is that as fast as you push them up they will start to fall back down. Thermodynamics shows that this is the case and that you can never get more electrons in the E₂ level than in the E₁ level. Which means that you don't get a a laser.

The solution is really clever. Find a material that has 3 energy levels. Pump from level 1 into the level 3 using light or electricity. The electrons fall back into level 2 (not level 1) giving out heat. By doing this you can get more electrons in level 2 than level 1. When this happens you have something called population inversion, which is the required condition for a laser. You then introduce photons with the same energy as the gap between level 2 and level 1 and bingo, you cause stimulated emissions to occur.

That's all well and good, now all we have to do is find a material with this magical level structure and they do actually exist, a ruby is a good example. The first lasers were based on ruby. They were very inefficient, but they did work. Further investigation lead to the realization that there were a number of materials that could be used to create lasers, including semiconductors.

Semiconductors define 20th century electronics for me, which is odd in a way because they are actually very poor conductors, they have been used in just about every computerized electrical device in the world. Given that there are tens of billions of devices based on semiconductor electronics you soon see that they are literally everywhere.

The semiconductor lasers work on something called a p-n junction diode, which will be covered in a post of its own. Experiments on this material led to the discovery that it could be used as a light source, converting electrical energy into light energy. It was then only a smallish step to create a p-n diode laser. These are by far the most common lasers in the world. While these may be made of silicon and not ruby and while they cost less than $10 they are based on the same principle, pumping electrons into an energized state that the drop down into the lower energy state giving off light of a fixed frequency.

It is odd that these days people take lasers pretty much for granted, you can buy them on the internet.  We have lasers in just about every computer and every CD or DVD player. Yet they are based on some brilliant theoretical and practical physics. We really should not forget this because without these brilliant men we would not have all these fantastic toys.