Blog Post 33, Free Form: Lasers

Everybody loves lasers. They are just that cool. Let's talk about what they are, what they do, and how they work.

LASER is actually an acronym for Light Amplification by the Stimulated Emission of Radiation.  This is precisely how a laser works.

Here is a laser diagram from Wikipedia.  

1) The Gain Medium

2) Laser Pumping Energy

3) High Reflector

4) Output Coupler

5) Laser Beam

So, how does this all work?  First, the pumping energy is applied to the gain medium.  This causes the medium's electrons to enter excited states, fall back to ground state, and release photons.  These photons will all be of the same wavelength.  The photons will bounce back and forth off of the high reflector and output coupler until they pass through the coupler in the form of a laser beam.  Unlike a flashlight element which emits light in all directions equally, a laser emits light in a single focussed beam.  This allows lasers to achieve very high energy densities at very low powers.  

For example, the flux/area of the sun on a sunny day is about \(1.4kW/m^2\).  Let's assume one uses a 5mW red presentation laser pointer.  This is the cheap kind that you can buy anywhere.  If we assume that the dot from the pointer is about 0.5cm x 0.5cm, it ha an area of \(0.25cm^2=0.000025m^2\).  Thus, the flux/area of our cheap laser is \(0.2kW/m^2\).  That's about 1/7 of the sun's output.  This is why such lasers of 5mW and below are considered pointers, they can accidentally hit a person's eye, and the blink reflex will prevent any sustained damage forth exposure.  It's not good for the ye, but it won't do any permanent damage.  

Of course, 5mW pointers are old school.  Companies sell handheld lasers with powers up to around 3.5W.  For reference, a 1W laser has an output of \(40kW/m^2\), or almost 30 times the energy density of sunlight.  Retinal exposure to a 1W laser almost always results in blindness.  

Laser color has a large impact on its visibility.  Red lasers are very simple in structure, and can be easily made at high powers, but the human eye is not very sensitive to red light.  Green lasers are the exact opposite.  We have not yet developed a direct green laser diode.  This means that we have to use infrared light, and pas it through a crystal to halve its wavelength of 1064nm infrared to 532nm green.  This makes green lasers not very power efficient, and very complex in structure.  However, the human eye is very sensitive to green light, making green lasers beams of a given power far outshine their equal red counterparts.  In recent years, blue 445nm diodes have become available.  These diodes share the best feature of red and green lasers.  They are direct diodes, so it is easy to create high power outputs, and the human eye is sensitive to blue light, making blue lasers show up easily even in daylight.  

Due to user irresponsibility and plane accidents caused by careless pointing of powerful lasers, the government has cracked down on handheld devices, limiting imports, and setting power limitations on what is considered a pointer, and what is not.  Many foreign laser manufacturers will no longer ship to the US due to trouble getting their products through customs.  Amusingly, there has been no such limitations placed on laser components, so the best way to get a high power laser pointer these days is to buy the parts on eBay, and build it yourself.

Note: I do not condone the use or construction of lasers without proper expertise and safety.  Appropriate wavelength safety goggles should be worn at all times when using lasers of output greater than 5mW.  Lasers should never be pointed at people, animals, or highly reflective surfaces.