Superluminescent diode (SLD/SLED) lighting principles unveiled
Nowadays, lasers and LEDs are part of the everyday life thanks to the Star Wars lightsabers or the low consumption bulbs, although a non specialized public will find difficult to explain accurately the difference between both. Nevertheless, if you google SLED without any reference to optoelectronics, the result will be related to a sledge, as sled is another way to call it. What’s more, even some researchers in the field of optoelectronics aren’t familiar with the term SLED, also known as SLD or superluminescent LED. From now on, we will employ the term SLD, because it is the most commonly used one.
The purpose of this article is to shed some light on the concept of SLD and to explain in which applications it might be useful. We will try to keep it simple and we will take advantage of this explanation to revise some basic concepts of the optoelectronic components.
Although at first it might be seen a little far from our final goal, let’s start by remembering that there are 3 phenomena that describe the interaction between electrons and photons:
- Absorption of a photon by an electron in the valence state, which produces the generation of an electron-hole pair.
- Spontaneous emission of a photon due to the spontaneous recombination of an electron-hole pair, the opposite of the absorption.
- Stimulated emission of a photon originated by a triggering photon caused by an electron-hole recombination. The new photon is an exact copy of the first one: same energy, wavelength λ, direction and phase of the electric field, that is, an amplification.
After introducing these basic concepts, we can already explain the fundamentals of LEDs and lasers.
A LED (light emitting diode) is basically a PN junction (junction with a positively doped area and a negatively doped area, see Figure 2) through which an electric current is sent. This current generates electron-hole pairs that recombine producing the spontaneous emission of photons, which is the main emission phenomenon in the LED. This results in a propagation of light in every direction and in a broad spectral width.
In the case of the laser (or LASER, if you prefer), the meaning behind the abbreviation, that is, light amplification by stimulated emission of radiation, gives us an idea of the phenomenon responsible for its operation. We have already explained what stimulated emission is, but we haven’t mentioned how to induce it.
The laser diode is also a PN junction but in this case there are 2 mirrors and a waveguide. The waveguide is a region of increased refractive index between the P and N sections and it restricts the direction in which the light can propagate. The 2 mirrors (often referred as facets) make a cavity that is selective in wavelength, which explains the narrow spectral width of the laser (see Figure 2).
One of the mirrors is totally reflective and the other one is partially reflective. Part of the light goes through this last mirror and provides the effective optical power of the laser. The rest of the light is recirculated inside the cavity as a feedback to ensure that stimulated emission carries on. The amplification in one round trip between the 2 mirrors must overcome all the losses for the stimulated emission to become the predominant emission phenomenon.
Now that we have fully grasped the behaviour of LED and lasers, we can turn to SLDs. SLD stands for superluminescent LED, but it is also called superluminescence diode and sometimes the abbreviation SLED is employed instead of SLD, as we have mentioned before. The name is a bit tricky, as it seems to suggest that a SLD is a normal LED with a high output power, and that is far from reality, as SLDs and LEDs are completely different devices.
So, what is a SLD and how does it work? Well, in a SLD we have a PN junction, a waveguide and the facets. Until now, it seems there is no much difference with the laser. However, SLDs are designed to have a single pass amplification of the spontaneous emission generated along the waveguide but (key point that establishes the difference with lasers) not enough feedback to make the stimulated emission the predominant phenomenon. How do we achieve this? Simply by tilting the facets with respect to the waveguide (see Figure 2). If it’s necessary, we will further reduce the reflectivity of the facets (and therefore the feedback) with an anti-reflection coating (AR coating).
The phenomenon we have explained in the last paragraph (single pass amplification with not enough feedback) is called superluminescence (now the name SLD makes sense) or amplified spontaneous emission (ASE). Nevertheless, instead of remembering this long pedantic name, we believe it’s simpler and clearer to think of a SLD as a laser that doesn’t achieve stimulated emission due to the lack of optical feedback.
Therefore, on the one hand a SLD has a high optical power density as the light is predominantly emitted in one direction thanks to the waveguide. It’s the same to say that the SLD has a high spatial coherence, an aspect in which is similar to a laser. Remember that the coherence (see our article about coherence) of a light source basically determines its ability to generate interferences.
On the other hand, as the SLD doesn’t have a cavity that is selective in wavelength, it emits a broad spectrum. In this case, it’s the same to talk about low temporal coherence. Regarding this aspect, the SLD is nearer the LED than the laser. The low temporal coherence also allows to avoid the problem of speckle noise, a random pattern that can be observed when a highly coherent beam is diffusely reflected by a rough surface, affecting the quality of the interference.
That’s why the SLD is usually described as a high power broadband light source with a behaviour just in the middle of the LED and the laser. It can be considered an acceptable definition for a first approximation to the topic, although by now, we should know better.
In a future article we will approach the applications of SLDs.
Check also our FJORD series of SLD light sources, with a wide variety of spectrum, ideal for your needs.