Can ordinary light be focused to become a laser?

A laser beam is more than just focused light. A laser beam is coherent light. Furthermore, no matter how hard you try, you can't create a laser beam by cleverly focusing ordinary light. You create a laser beam using stimulated emission. Stimulated emission is what causes the light in a laser beam to coherent, and coherence is what makes a laser beam more useful than ordinary light. In fact, the word "laser" is actually an acronym that stands for "Light Amplification by Stimulated Emission of Radiation."

What is coherence? In the simplest picture, you can imagine a beam of light as a bunch of small sine waves traveling through space. In this picture, coherence means that all of the individual peaks of the various sine waves line up in space and continue to stay aligned as the waves propagate. The phrase "aligned" means that if you were able to take a snapshot of the different wave components in the beam at a specific time, you would find that all the first peaks are in the same place in space, all the second peaks are the same distance away, and so on. In order for the peaks to align perfectly everywhere, a few things have to happen:

1. The waves have to have approximately the same wavelength (temporal coherence)

If consecutive peaks of one wave are 600 nm apart, and the peaks of the other wave are 830&;nm apart, then obviously if you align one pair of peaks, you can't align any of the other pairs of peaks. Ideally, if all wave components had exactly the same wavelength (and met the other criteria listed below), then all peaks could line up perfectly together forever. This situation is actually physically impossible. An infinitely long beam would be required for all wave components to have exactly the same wavelength (the proof of this statement is not obvious and requires Fourier analysis). Although a beam of perfectly single wavelength is physically impossible, we can get very, very close. In fact, having a beam that is very close to a single wavelength (called "monochromatic") is one of the main reasons why lasers are so useful. Using monochromatic light allows us to measure or trigger very specific responses in materials (e.g. spectroscopy, laser cooling).

2. Waves Must Be In Phase (Spectrally Coherent)

The phase of a wave describes what fraction of a sine wave's cycle is present at some reference point. Two waves that are 180掳 out of phase will have one wave peaking at the same point in space while the other wave dips at the bottom. So even if two waves have the same wavelength, if one wave is shifted a little bit forward relative to the other, their peaks will not line up. The phases of the various waves in a beam must be the same in order for their peaks to line up and for the beam to be spectrally coherent. A stable phase for a coherent beam is very useful. The phase of a wave tends to change when it interacts with a material, so using a beam with a stable phase allows us to measure the phase shift due to the material and thus learn something about the material (e.g. ellipsometry).

3. The waves must be locally propagating in the same direction (spatial coherence)

If one wave is travelling north and the other is travelling northeast, then their peaks cannot align. The peaks can only align if the waves at every point in the beam are travelling in the same direction. Note that some people restate this principle as "all light rays are parallel". This is an oversimplification to the point of being wrong. If a coherent beam (like a laser beam) is made up of perfectly parallel beams, then those beams don't spread out as they travel. In fact, the beams always move away from straight lines as they travel through space (we call this "diffraction"). You probably won't notice the divergence of a laser beam with your naked eye, but it's there. Rather than saying that all the rays in a coherent beam are parallel, it's more accurate to say that the wave components at one point in space are parallel in a single coherent beam, but not parallel from point to point. Furthermore, if two waves travel in different directions but meet all other criteria for coherence, we treat those two waves as completely separate beams, and their combination results in an interference pattern. For coherent beams where the beam width is very large compared to its wavelength, the diffraction is very small, so all the waves at different locations are very close to being parallel. Such beams can be used for pointing or scanning (e.g. laser printing, 3D; laser scanning, barcode scanning, laser guidance for missiles, lidar).

4. Waves must be the same polarization (polarization coherence)

The polarization of a light wave describes the direction in which its electric field oscillates in space. If one wave's electric field oscillates up and down, and another wave's electric field oscillates left and right, their peaks cannot align because the peaks exist in different directions. The different wave components in a beam of light must have their electric fields pointing in the same direction in order for their peaks to align and for the beam to be coherent. Polarized light is useful because we can learn something about the object the light is shining on by the way the object changes the polarization of the light (e.g., polarimetry).

If all of the above criteria are met, then the peaks are aligned everywhere, and they remain aligned over time. Therefore the beam is perfectly coherent. (Note that perfect coherence is physically impossible, but many light beams, such as laser beams, can come very close to being perfectly coherent). Ordinary light, such as the light from an incandescent light bulb or a fire, is incoherent. The light from a fire contains different light waves that have different frequencies, different phases, travel in different directions, and have different states of polarization. Focusing incoherent light, such as with a glass lens, does not make the light waves have the same frequency, the same phase, the same local direction, or the same polarization. Therefore, focusing incoherent light does not make it coherent like a laser beam.

The phenomenon of stimulated emission in lasers is very useful because the light it produces is usually coherent in time, spectrum, space, and polarization. Stimulated emission means that when a bit of light (a photon) comes along and knocks an electron down to an unexcited state, the electron is in an excited state, causing it to emit another bit of light (another photon) in the process. In the process of knocking the electron down, the original photon causes a new photon to become coherent with it. This process repeats in a domino fashion; each time a new photon is added to the beam that is coherent with the original photon. Stimulated emission is not as exotic as it sounds, it actually happens all the time. The difficulty in designing a working laser is to make stimulated emission the dominant way that electrons become de-excited. In a regular piece of matter, excited electrons are most often de-excited by colliding with other electrons or atoms, thereby losing their energy to heat, or by spontaneously emitting some incoherent light. Designing a laser, therefore, involves making the stimulated emission transition of the excited electrons very likely, and making other transitions less likely.

Note that stimulated emission is not the only way to create a coherent beam of light. For high-frequency waves, such as visible light, stimulated emission is the most efficient way to create a coherent beam of light. But for low-frequency waves, such as radio waves, it is much easier to create a coherent beam of light simply by driving a sine-wave current into an antenna. Waves produced by an antenna driven at a single frequency, such as those that carry radio station broadcasts, are coherent. Such broadcast radio waves are not technically laser beams, since they are not produced by stimulated emission, but they have all the useful coherence properties of a laser beam. A policeman's radar speed gun is more similar to a demonstration laser pointer than most people realize. Both are handheld devices that emit a beam of coherent electromagnetic waves. Neither uses focusing to create the coherence of the beam.