Why does a laser pointer contain a laser diode, and not just an LED?
A laser pointer contains a laser diode, which essentially shines coherent light over a large angle, and a collimating lens, to take that light and focus it into a beam.
If the collimating lens works for coherent photons coming from a laser diode, why wouldn't it work for non-coherent photons coming from an LED? As I understand it, photons don't interact with each other, and so shouldn't care if other photons nearby are in phase with them or not.
Best Answer
Semiconductor light emitters are made of such materials, which have quite large index of refraction. This makes it hard for light to exit the emitter — due to Fresnel equations and low index of refraction of air.
In a laser the light mostly goes back and forth between two mirrors, and reflections only help the lasing. So the light either exits from a tiny area on the side of a laser, or reflects back and continues the lasing cycle.
In a LED, on the other hand, light is incoherent and is emitted in all directions. Only some of the photons generated go at low angle to the normal of the crystal's surface to be efficiently transmitted outside. Most others are reflected back and are likely to eventually be absorbed back, leading to nothing but generation of heat.
To circumvent this, LED crystals are generally packed into a lens, which acts as a buffer between index of refraction of the crystal and air. But this also makes the complete device a much larger light source, so collimation of the light it produces is considerably harder.
The lasers, on the other hand, are used directly as bare crystals behind the collimating lens, so they look much like point light sources, which can easily be collimated.