Quibbles and Bits

Lasers

Ever wondered how lasers work?…Wondered what it is that gives them the interesting properties they exhibit?…You have?….Good! This column’s for you.

Lasers are all about electrons. Atoms are composed of a nucleus surrounded by several electrons, one for each proton in the nucleus. The number of protons in the nucleus determines which element the atom comprises. A nucleus with one proton will be that of a Hydrogen atom; with two protons a Helium atom; with 8 protons, an Oxygen atom, and so forth. The number of electrons which surround the nucleus always matches the number of protons in the nucleus, and although it is the number of protons in the nucleus which determines which element the atom is, it is only the electrons surrounding it which determine an element’s chemical properties.

You can think of an electron as being a bit like a billiard ball. It has mass and energy, and the amount of energy it has depends on how fast it is travelling. If the electron is not orbiting around the nucleus of some nearby atom it is said to be a free electron, and it is free to take on any amount of energy it wishes. But electrons which orbit an atomic nucleus are not so fortunate. Such electrons are constrained to having certain very specific energies [for reasons that we don’t need to go into]. Some of these energies are higher than others, and electrons always want to occupy the lowest available energy level if at all possible. Consequently, the natural state of a substance is that all of its electrons are crammed into the lowest available energy states. Or, to be strictly accurate, most of them are. At any temperature above absolute zero, thermal energy will always manage to jog some of them out of the lowest energy states.

If an electron falls from a higher energy level to a lower one, it obviously has a surplus of energy that it needs to get rid of. There are various ways in which it can do that, but it is easiest to think of it in these terms: it can give up the excess energy as either heat or light. So there is some sort of natural relationship between heat, light, and the energy states of electrons. And it was that man again, Albert Einstein, who first established the basic physics which underpins these relationships. These are called, naturally enough, the “Einstein Equations”, and they are fundamental to understanding how a laser works. But don’t worry…we don’t need to look at them here.

The key to a layman’s understanding of a laser is to understand the interactions between a photon of light and an electron. Like electrons, photons also have energy, and this energy is determined solely by the wavelength of light. The two can interact if the energy of the photon is exactly the same as the energy difference between two states of an electron. Specifically, an electron at a higher energy level (which is termed an “excited state”), can fall to a lower energy level (which is termed the “ground state”) giving up its surplus energy as a photon of light whose energy (and therefore a specific wavelength) will corresponds exactly to the energy difference between the excited state and the ground state. And likewise, if an electron at the ground state encounters a photon of the exact correct energy, it can absorb that photon and use its energy to jump up to the excited state.

There is one additional concept that needs to be grasped if you are to understand how a laser works, and that is the notion of “stimulated emission”. Suppose we have an electron in an excited state. It wants to drop to its ground state, emitting a photon to carry away the excess energy in the process (or, alternatively, giving that energy up as heat). Left to its own devices, it will eventually do just that. But if instead, before it does that, it encounters another photon having that exact energy, it can undergo “stimulated emission”. This is where the incoming photon effectively nudges the electron to fall to its ground state, emitting an identical companion photon. What is really intriguing about this process is that not only do the two photons have the same energy, they also have the same phase and travel together in the same direction. In fact they are two absolutely identical copies of each other. Essentially, the photon has been amplified by a factor of two.

So what do we need to do if we want to use this process to amplify a beam of light into something seriously powerful? Well, it is clear that we require a combination of a huge assemblage of electrons in their excited states, and a huge flux of photons of the exact correct energy (i.e. wavelength). The problem boils down to this – we need enough electrons to hang around in their excited states long enough for them to encounter photons to amplify. And this turns out to be a major problem. When you measure these “excited state lifetimes” they inevitably turn out to be far too short – by several orders of magnitude – to be of any use in making a laser. In other words, they don’t stay excited long enough to encounter a photon. But actually, if you look carefully enough, there are certain very specific atoms – and some molecular structures too – which turn out to have excited state lifetimes long enough to make a practical laser out of them.

The other important aspect of laser design is that you want all the photons to have the exact same phase and direction. You do this by placing the laser material within a so-called “optical cavity”, which is effectively the space between two parallel mirrors. The light bounces back and forth between the two mirrors, passing through the laser medium each time, and thereby being amplified along the way. Finally, if you make one of the mirrors only partially-silvered, some of the light can escape though it and form the output of the laser.

The thing that makes a laser so special, and so practically useful, is this fact of all the photons having the same phase and direction of propagation. We refer to this as “coherence”. It is solely this property of coherence that allows a laser beam to propagate over huge distances in a pencil-thin beam, something that an incoherent light source – such as a flashlamp, or an LED – cannot do.

The laser was invented in 1960, but it is intriguing to speculate whether Einstein could have built a laser as early as 1926. For sure, his theory was complete, and if he had the materials to hand he would have had everything he needed – in principle at least – to invent the laser. It appears that Einstein never grasped the full practical significance of the fact that stimulated emission produced pairs of photons of identical phase and direction of propagation, and never contemplated what it would mean if one could amplify enough of these photons to generate a beam of light. Had he stopped to consider that, it would have been a relatively trivial exercise to work out the extraordinary properties that such a beam of light might possess. A thornier question is whether he (or his contemporaries worldwide) would have been able to identify a suitable material from which to make a practical laser, but it has been argued that it need not have proved insurmountable at that time, given sufficient impetus to work on it. Finally, it is intriguing to speculate what he might have called the device. Whatever the answer, it probably wouldn’t have been anything as romantic as LASER! [To be clear: LASER is an acronym, each letter representing the first letter of the words Light Amplification by Stimulated Emission of Radiation. The laser has a cousin which amplifies microwaves, and as you might expect, it’s called MASER.Ed.]

Getting back to the real world, a related device is the “optical amplifier”. This is essentially a laser but without the “optical cavity” formed by the back-and-forth reflection of the mirrors. In an optical amplifier the light passes just once through the laser medium, and is amplified. Optical amplifiers are almost exclusively used in high-speed long-distance fiber-optic data links, and you wouldn’t be able to watch Netflix without them.

One thing that inspired me to write about lasers and optical amplifiers was a point Paul McGowan regularly makes about the fundamental structure of an audio amplifier. Regardless of whether we are talking about a preamplifier or a power amplifier, a point he likes to emphasize is that these circuits are not, strictly speaking, actually amplifying anything. They are circuits whose function is to take a power supply, and get a small signal to make use of it to fashion a bigger copy of itself.

That may seem like an arcane observation, but its true power becomes evident when you compare an electrical amplifier with an optical amplifier. The latter is a true amplifier. Photons go in at one end, and each time one of these photons interacts with an excited electron in the amplification medium, it causes the excited electron to fall to its ground state and give up a direct copy of the incident electron. In fact, calling it a copy is itself an understatement. It is a complete replication, in all respects, of the incoming photon. If it were possible to inspect the two photons – one incoming and the other replicated – as they propagate away from the electron they just interacted with, you would not be able to determine which was which.

By comparison, an electrical amplifier is like Paul McGowan (I hope he doesn’t mind me taking his name in vain!) heading off to MOMA and sitting in front of Van Gogh’s small masterpiece Starry Night, where he proceeds to paint a giant-sized forgery as accurately as he possibly can. He may be a damn good forger, and as a result you may not be able to tell them apart (other than the fact that Paul’s copy is really big), but it will be a forgery nonetheless. With an optical amplifier, you have genuinely made the original incoming signal itself bigger, and there just isn’t an equivalent process in the electronic domain. Even a triode tube comes close, but gets no cigar.

Good luck getting close enough to forge that!—Ed.

I spent the majority of my career (well, the productive parts at any rate) in the field of lasers and fiber optics, and as a result it is quite cool to be able to compare and contrast these aspects of the laser and audio worlds. I mention this merely because it strikes me that none of my old non-audiophile laser industry colleagues would find my comparison of optical and electrical amplification at all worthy of mention. Whereas I think (or maybe I just hope) the audiophile community might find it more interesting.