How do Lasers Work Page 2
To fully answer the question How do Lasers Work we have to zoom in even closer to the Laser Diode (for magnification like this you would need an electron microscope) –
How do semiconductors work?
The material on the top is a P Type semiconducting material, this has been engineered in a Laboratory to be missing one electron it would like to gain an electron, for this reason it is sometimes said to have a “hole”.
The material on the bottom is an N Type semiconductor material, it has been engineered with an extra electron, therefore it would like to lose one.
When P Type and N Type semiconductor materials are placed side by side to each other as shown above you create what is called P-N Junction. Things start to get interesting in a P-N Junction when a voltage is put across the two materials –
When the P Type material is connected to the positive side of the battery and the N Type to the negative terminal, the density of holes and free electrons increases near the junction of both materials. Soon, the free electrons begin to fill the empty holes, however, if you imagine the electrons were small marbles and the holes were slightly too small for them, the electron must lose some mass (energy) in order to fit into the holes, when it does this it must release the energy it loses in some way, it does this by giving off a photon of light. When the photon of light is given off the electron can fit into its hole on the opposite side of the P-N junction.
If we were trying to understand how an LED works this would be the end of the story, however a laser is slightly more complicated. When the photon of light is given off it begins to bounce around inside the P-N Junction, this is because the inside walls have been given a mirrored surface, this causes several things to happen –
- The photon bounces around and “encourages” other electrons and holes to combine, when they do so they are the give off a photon that is same phase, polarization and direction as the original photon
- This means that the light being produces is all the exact same, this is how laser light is so powerful, whereas in an LED the photons produced are scattered in all different directions and are not in phase with each other.
- The amount of light inside the laser diode begins to increase until the whole diode is filled with laser light.
As you also may have noticed, at either end of the laser diode there are two small slits, one at the back and one at the front. This is where the laser light can exit the diode. The light at the rear exits the diode and hits the photo diode. The photo diode “reads” the amount of light hitting it and regulates the current going through the laser diode if its predetermined strength is passed This time i have zoomed back out to regular scale so you can see the various components –
From the slit at the front the laser light can also escape, however, there is a slight problem still remaining. As you can see from the laser light exiting the rear, it comes out diffracted (splayed out to the sides), this is due to the laser having to exit such a small slit.
As we all know laser light comes out in a beam, so this needs to be rectified. This is where the collimating lens that i mentioned earlier comes in. It takes the diffracted light, pass it through a series of lenses and straighten out the light into a strong narrow beam –
Once the Laser light has been straightened by the collimating lens it exits at the end of the key chain laser –
How lasers work animation
Below is an animation showing how lasers work, it may show you in more detail how they operate rather than the text above. You can watch the animation on the original video page here – How lasers Work, or watch below.