Inside a small Laser

Lasers seem mysterious to most people and the fact that they even work is even more so. I had the opportunity to dissect a simple crystal driven cavity style laser. My photos and explanation are here for your learning experience. If I’m wrong about anything please let me know!

Let me start by saying that I have no idea what kind of laser this is. I believe this is a YAG laser, but I don’t know the crystal type. I also don’t know what it was used for, but I do know that it follows all the normal rules for a flash bulb pumped YAG rod based laser.

Did I loose you? YAG (Nd:YAG is probably more formal) refers to the crystal type and stands for neodymium-doped yttrium aluminum garnet. This is basically a crystal that the light is “pumped” into. There are other types of YAGs, such as Er:YAG, Yb:YAG, Tm:YAG and on and on (more info here). The crystal types determine the color or wavelength of the beam. The Nd:YAG that I’m referring to produces a wavelength of light at 1064 nanometers and is in the infrared area of the spectrum. What does all that mean? Just that it’s invisible to the naked eye.

Ok, the pumped part. Pumped simply means that light is forced into the crystal. The cavity (see below) is mirrored on all sides except the ends. This causes all the light created by the flash lamp (or diode) to enter the crystal rod at some point. Remember the wavelength part before? The lamp produces full spectrum light, meaning all wavelengths are present, or at least the visible ones. The crystal takes all this and puts out only one wavelength.

Now that the crystal is pumped up (insert dumb joke here), the light exits the crystal at both ends. One end returns the light to the cavity and the other lets it out. What comes out is your laser beam of approximately one wavelength.

Simple? Not done yet. This beam now has to be focused, bent and put where you want it. The unfocused beam is pretty useless. A series of lenses, colluminators, up scopes, down scopes and other optical goodies focus that beam down to a point so it can do something. All that light focused to a small point can do a lot. Most people think the beam can only burn. Not true. The reason for all kinds of lasers is to do different things. Lasers can burn, etch, cut, weld, solder, engrave, anneal and drill. The coolest in my opinion is when a UV (ultraviolet) laser marks on plastic. It doesn’t really cut it, it more sunburns it and literally changes the color of the plastic.

The laser below has all the optics with it but I have no idea what it’s original function was so I don’t know where it is focusing the beam.

This is the laser. It’s very compact.

The most important part here is the cavity where the crystal and flash lamp are. Below is a shot of the cavity open.

The cavity is basically two half cylinder mirrors. The concept here is that the flash bulb flashes at a high rate and almost all of the light is “pumped” into the crystal. The light/photons exit at each end of the crystal and through a series of half silvered mirrors and light bending prisms/optics the light is focused, multiplied and sent out.

The picutres below should allow you to see the path of the beam before it totally exits the unit.

Below are the crystal and flash lamp. The crystal is optically clear end to end and appears frosted on the outside. The lamp is about what you’d expect from a flash lamp.

Other neat laser stuff

High power lasers used in industry are usually water cooled. In the YAG style as described above, water is circulated INSIDE the cavity where the crystal is. The amount of heat generated is HUGE and sadly is one of the inefficient byproducts of lasers.

Lasers are usually between .1% and 30% efficient. That means, for example, that a laser that is 10% efficient takes in say 100 watts of electricity, but only puts out 10 watts at the focused point of the beam. Where does the other 90% of the electricity go? Heat. Not real high on the “Energy Star” approval list… Now realize that all that heat has to be removed from the laser. That takes fans (more power there) or a water chiller if the laser is water cooled (even more power). The heat removal just makes things worse.

Consider a laser that is rated at 400 watts. That normally means that it can output the heat equivalent of 400 watts at the focused point of the beam. To get that at 10% efficiency, you need to provide it with 4,000 watts!! Ouch!

Generally the more efficient lasers are “diode pumped”. This means they aren’t using flash bulbs but something more akin to an LED.

Want more? Visit the Wikipedia article on lasers.

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