Wednesday, April 13, 2011

Office Physics: The Laser Pointer

The original laser pointer

Back in the days of star trek, the laser was first discovered. They were first used as weapons for obliterating alien space ships. Much like guns, they had amazing accuracy, which was valuable for combat and lecturing. In the ancient days, guns were used for pointers in the classroom. The teacher would pull out her revolver, and shoot wherever on the board she wanted to point. The accuracy was impeccable, but it caused a disruption in the neighboring classroom the first day it was used and was never used again. From then on, wooden pointing sticks were used even though they were helplessly inaccurate. When laser guns came around one anonymous entrepreneur realized they could be used in the classroom for pointers. Luckily he had spent several years scouring the microfiches of thousands of libraries and came across the story of the teacher using a gun for a pointer. So he decided to make a laser pointer with lower power so as not to damage the young minds he taught. The laser pointer has evolved some over time, but here's a diagram of what the modern green laser pointer looks like:
There are three important parts to a laser.

1. The pump: This just a light with a single color. It could be a laser, an LED, a laser diode, whatever. In the case of this laser pointer, the pump is a laser diode with a wavelength of 808nm. It's effectively just a red light bulb. The light from the pump is sent through a focusing lens, into the resonant cavity.
A laser pump

2. The resonant cavity: The resonant cavity is made of two mirrors and inside the resonant cavity sits a material called the gain medium. If the separation between the two mirrors is right, then one particular wavelength of light will resonate in the cavity and will cause the output light to be very intense. The resonant cavity's job is to allow only one wavelength of light to pass through the gain medium several times, so that you only get one color of light coming out the other side.

A resonant cavity for kids

3. The gain medium: The gain medium is some material that's placed inside the resonant cavity so that the right wavelength of light comes out the other side and it causes the intensity to increase. In the case of this green laser pointer, there's two materials inside the cavity. One is Nd:YVO4, which is used to create infrared 1064nm light. What happens in this step is that high energy (red) light comes in and is absorbed by electrons in the crystal. This left the electrons with a high energy for a bit. The electrons soon decide to release their energy in the form of a photon of wavelength 1064nm. After converting the light into 1064nm, the light goes into the KTP crystal. This is a nonlinear crystal, which means that it acts differently for different intensities of light. Inside the KTP crystal, a process called second harmonic generation happens. This picture shows how it works:

One low energy photon comes in and excites an electron to a higher energy state--an energy of E. Immediately, a second identical photon comes in and excites the electron to a higher energy to an energy of 2E. Like before, this electron decides to relax and gives off a photon with twice the energy of the original photons. This gives a photon coming out that is half the wavelength of the infrared photon, so you get a green light with wavelength 532nm (half of 1064nm).

The green light is expanded and collimated so you get a nice green beam coming out the laser pointer. Tada!

If you made it this far, you probably know more than you ever wanted to know about a laser pointer.

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