Lasers

Traditional gas lasers that many people are familiar with have almost been buried by the number of solid state (semiconductor) lasers now flooding the market. The prices of these are plummeting, and it isn't at all hard to extract free lasers from broken cd and dvd players.

A typical laser pointer

When people think of lasers, they think one of two ways; laser pointers, or big lasers that burn clean holes through steel. These are all correct, as they're all lasers. What's different is their power output. Laser optical power output is usually specified in watts (W) and milliwatts (mW).

To put into perspective... the typical laser pointer you buy at some random supermarket has an output of 1mW, while the lasers which burn holes through steel typically have power outputs of a few thousand WATTS.

Optical Power Output	Class	Precautions
<0.1mW	I	No real danger
0.1mW - 1mW	II	Avoid direct eye exposure
1mW - 5mW	IIIa	Avoid direct eye exposure
5mW - 500mW	IIIb	Avoid direct skin and eye exposure
>500mW	IV	Avoid eye and skin exposure to direct and scattered laser radiation

Image courtesy of www.handprint.com

In Australia, the maximum legal power output for a laser pointer is restricted to less than 1mW. In the USA, its 5mW. Don't ask why, that's just the way it is. However, non laser pointer units are allowed to have as much power as you like, such as those used in disco lighting and on top of buildings.

The wavelength of a laser is also important. The usual wavelength for a red laser pointer is 670nm, whereas other lasers have other wavelengths.

The eye has different sensitivities to different wavelengths, so a 635nm laser seems about four times as bright as a 670nm laser of similar power output.

The human eye is most sensitive to green/yellow light. (Image courtesy of www.tedmontgomery.com)

Earlier last year, I managed to get a 5mW red laser diode, at 635nm. Since this wavelength appeared 4 times as bright as the normal laser pointer, and also 5 times the power, all in all it appeared about 20 times as bright as your normal laser pointer.

And just to further demonstrate the stuff everyone learns in physics (all that reflection, refraction blah blah blah)... this one is the laser beam being reflected several times off two front surface mirrors (mirrors with the silver coating at the front). This bouncing of light is kind of like in those supermarkets where there's two mirrors on either side of a product line.

This time the laser beam is reflected off a mirror into a CD, where the the beam is refracted before it comes out, where it is again refracted.

Those pictures might look pretty awesome, but I have to admit I cheated. The laser beam is visible because its grazing the surface of the table. No, you can't see the beam of a 5mW red laser in midair. But with my newer 50mW DPSS (diode pumped solid state) laser, you can. With this one, you can also burn through black stuff- pretty fast.

The 50mW DPSS laser head and driver circuitry mounted in a custom case.

And the resultant laser beam at night- the very bright green beam visible in midair. The wavelength is 532nm, which appears 20 times as bright as a standard red laser pointer, and 50 times the power output. So, in perspective? It appears about 1000 times brighter than a red laser.

This is me playing with homebrew optics. Although this isn't a focused beam. I've put a diverging lens (scrounged from a scanner) in front of the output so I don't completely screw up the camera. Here the beam is being split by a prism (window pane) and reflected off a back surface mirror (personal mirror) so that the two beams are parallel.

The camera is mounted behind the laser unit, which is why the beam seems to start out of nowhere.

Ever seen those Hong Kong laser light shows at night? Well stuff that, here's the Darwin version!

The beam seems to disappear in the photo, but you can actually see it hit the clouds!

Night sky beam observation from leanyer, 2.1km away, was carried out on 10 April '06. The beam was described as being a thin green line in the air (mm yeah, what else?) which was just visible. Funnily enough, the synchronized observation at another location, ~100m away, was unable to see the laser beam. Probably due to trees.

This image was taken in Jan 2008 during Cyclone Helen's raid on Darwin. Some pretty fresh gales are evident from the blurred palm leaves in the photo. This is a long exposure - the laser was shone for a few seconds in three separate points, with the beam quickly moving between to draw a nice pretty triangle on the trees. Where the beam has shone in one position for more than a second or two, a beam line is visible in the photo.

Similar to writing with a torch, here is a (very neatly) hand written laser trail on palm trees. You can get pretty proficient at writing like this. This was a long exposure of 6 seconds.

The beam is frightingly intense, and without proper laser goggles, it is fairly uncomfortable to see the beam hitting a white surface (and only just bearable looking at it hitting a completely black surface).

So far it has burned through black electrical tape, lit matches, and blasted pits into hard black plastics. When it burns through tape, the cut is precise and clean. Not bad for a fairly modest 50mW I have to say.

Lasers are closely tied in with the holography industry. Check out some of my homemade holograms here: