Jun 28, 2013

Digging deep down into lasers ... did you know it's an acronym?

4 min read

Let's go back to the beginning of the 20th century. At the same time as the brothers Lumière were inventing cinema, Max Planck was inventing quantum mechanics. For half a century this proved to be the hottest and fastest growing topic in physics, mainly based on Planck's quantum theory (for which he received a Nobel prize in 1918). This theory states that energy is sent out in little packets, called quanta, hence the term ‛quantum mechanics’. The level of radiation energy depends on the wavelength and the factor that links them is now known as Planck's constant. His good friend Albert Einstein (yes, that Albert Einstein, E=mc²) built on Planck's theory and predicted that it should be possible to stimulate that emission. When applying this to ‒ the wavelengths of ‒ visible light, and using Planck's constant, the emitted energy would be low however. For this reason, Einstein's theoretical concept didn't really get off the ground.



Along came World War II and a new acceleration in scientific research and the budgets to support it. US and Russian scientists were working simultaneously on a way to amplify the weak stimulated emission that Einstein came up with. By trapping the light between 2 mirrors, it would bounce back and forth, be amplified and constantly growing in energy as more emitted light is added. This concept could be called ‘Light Amplification by Stimulated Emission of Radiation’... and, coincidentally, that is exactly what the acronym LASER stands for. The first official use and description of this acronym dates from 1958; more than 40 years after Einstein invented the concept.


Laser light vs. white light

Now that we understand the concept and background, let's look at the consequences and the implementation. On the nano-scale: the light (on this scale we talk about one photon) coming from the emission can hit a neighboring electron and create a new photon that is perfectly in phase. This continues across time and space (still on a very small scale of course, inside the material). The fact that the light emission is in phase has two practical consequences. The first is the monochromatic nature of lasers light: it contains only one wavelength. When looking at white light sources such as Xenon lamps (or even the sun), they typically contain all visible wavelengths in their spectrum. Often they also contain unwanted wavelengths such as infrared or UV light. The fact that white light source have this kind of spectrum is no coincidence: it is exactly this wide spectral distribution that gives them their white color. Lasers have the narrowest spectral distribution possible, meaning that a laser ‒ emitting in the visible spectrum ‒ has a very saturated and distinct color. Consequently, when using lasers as light for projectors that expect a white light input, you need more than one laser color to generate white light. A balance between large enough color gamut and low enough cost is reached by using three lasers. This is also perfectly in line with the RGB color processing that is standard in digital imaging.


Coherence length

Another consequence of the in-phase light emission is the coherence of laser light. Coherence is a measure for the amount of randomness present in light waves. It is quantified by a ‛coherence length’ (in m). Lasers have a very high coherence (and a very long coherence length); light coming from more 'chaotic' sources such as Xenon lamps (or the sun) has almost zero coherence. An important application of the coherence of lasers is holography, which would be impossible without a very long coherence length. Unfortunately, in the case of laser projectors, coherence provides one major drawback: speckle. We'll go deeper into the concept of speckle later on in this series. A shorter definition will suffice for the time being. Because of the coherence of the laser light, it easily interferes when bouncing of surfaces, leading to brighter and darker zones. Speckle exactly is this interference pattern.

The fact that we start from a well-controlled and precisely designed environment, makes the laser beams also spatially coherent. Practically, it means that the beam is collimated: when you point it at a wall 10m away, you will see a small dot; unlike the light from a flashlight which is reflected outwards and diverges. It is this spatial coherence that also gives laser light its high energy density. Focused to a tight spot this enables precision applications like laser cutting. Remember James Bond almost being cut in two in Goldfinger?

Making use of laser technology

Let's look at the practical implementation of laser technologies. We now know how it should work: stimulate emission in a material and amplify the light using mirrors. The material can be in a gas, liquid or solid state. The stimulation can happen optically or electrically. The more perceptive reader might have wondered: if this light is constantly bouncing around between these mirrors, how do you make use of it and actually send it into the projector? This is, in practice, solved by making one of the mirroring surfaces semi-transparent.

Lasers are not always the high power light sources that you expect in cinema projectors. They have been all around you for many years in DVD players, laser printers and barcode scanners. What is unique about their application in bright, laser-illuminated projectors is the wavelengths used (visible RGB component) and the total amount of power (cinema projectors need to reach ten thousands of lumens).


To conclude: laser is an acronym that stands for Light Amplification by Stimulated Emission of Radiation. Its unique way of generating (both visible and invisible) light gives it some unique properties. Spatial coherence leads to collimated bundles; we'll explain elsewhere in this series why this is a good thing when designing laser-based projectors. Its monochromatic nature leads to very saturated color gamut: we'll also go deeper into this. An important drawback in our field of application is the temporal coherence that leads to interference, something that can become visible as speckle when not well managed. We hope that this deep dive into the technology has provided you with a better understanding of the fundamentals of laser.

Next week, we'll continue this blog with an inspiring video about contrast. Be sure to tune back in!