Lasers

I have always had a keen interest in lasers - to be honest with light in general, but the purity and brilliance of laser light holds something special.  My uncle (who is no longer with us) spent years working with lasers for all sorts of reasons - everything from a few milliwatt (Helium Neon) up to 2W Argon Ion lasers which on one occasion led to him accidentally cutting his tie in half! - and I spent a lot of time learning from him.

I've owned quite a few lasers - I was gifted a 2mW HeNe laser first off, and I've had quite a few standard laser pointers (all red diode lasers).  But in the last few days I've finally got hold of a couple of diode lasers that are a bit different from the norm - a 20mW green, and a 205mW red one.

I ought to say, I may have got a bit carried away with the technical explanation about lasers having re-read the post.  If you're not interested in that, then skip to the pretty pictures below :-)

Both of the lasers that I have recently acquired are diode lasers, but the green one is a more complex diode pumped solid state (DPSS) laser.  To try and explain: diode lasers create their laser light through the recombination of electrons and holes in a semiconductor material who's recombination causes the emission of photons.  If this makes no sense to you, then I apologise for the various bits of technical gibberish throughout this!  Anyway, the recombination of electron-hole pairs causes a photon to be emitted (spontaneous emission), and this photon can in turn cause other electron-hole pairs to recombine and release a very similar photon (same direction and phase - stimulated emission).  It is this cascading stimulated emissions - contained within an optical cavity - which produces "lasing".

The red laser I have is probably an AlGaInP (Aluminium Gallium Indium Phoshphide) laser which has a single emission frequency of around 660nm:

The diode inside is very small - contained within the aluminium housing at the end of the PCB below (you can just see a hint of the copper casing that the laser is encased in - this is all for heat dissipation):

The PCB consists of a protection diode and a driver IC which generates the appropriate voltage and current to power the laser from the batteries despite their changing (discharging) voltage.  

This laser has an output power of 205mW which puts it into class 3B.  This is the point where I get all serious...  

The lasers I have owned up to now have been less than 5mW total output power.  They are class 1 or class 2 devices, both of which are certainly incapable of burning anything.  With some effort or serious abuse, they would be capable of burning the retina in your (or someone else's) eye, which is the primary cause for concern from most lasers.

These two latest lasers are both powerful enough to do serious damage to the retina without much effort.  At 20-30mW (and potentially more in the infra-red part of the spectrum - explained later), the green laser could cause damage with a very short direct flash into the eye.  The red laser, at 205mW, could do serious damage even through reflected light.  I cannot stress enough that these are not toys.  They may be quite interesting and exciting, but without proper safety precautions they can partially blind you without you even realising until it's too late.

OK, safety talk over (but please take note and don't just go and buy some without some serious research!)

The green laser (below) is considerable more interesting from a technical point of view - it's what's known as a DPSS (diode pumped solid state) laser.  The sort of direct band-gap semiconductors available tend to emit photons towards the red and infra-red end of the spectrum, rather than green or blue light.  It turns out the most efficient way to generate green light is through a 3 stage process involving shifting the frequency of light through the use of two different crystals, starting with an infra-red laser.

So, an infra-red diode laser at 808nm (Gallium Aluminium Arsenide) generated the source light which "pumps" a crystal of Neodymium doped Yttrium Vanadate (Nd:YVO4).  This crystal is excited to emit photos further into the infra-red part of the spectrum at 1064nm.  This sounds completely counter-intuitive (we're trying to get green, after all - why go further away from green), but actually this bit is really clever: you use a crystal of Potassium Titanyl Phosphase (KTP) which doubles the frequency of the light from 1064nm to 532nm - which is green!  So there you go, nice and easy really!

The extended size of the DPSS laser hides it's high power infra-red source (the conversion process is fairly inefficient), and Nd:YVO4 and KTP crystals behind the collimating lens:

Laser light is both monochromatic (a single frequency - think about how it's generated) and coherent (all the wave are in phase), so it doesn't diverge much - the shot below is of the red laser point while on - you can safely look at it from any angle except directly down into the beam:

Now, with all the semiconductor explanation out of the way, here's a few pretty pictures...

This one was outside, a long exposure with the camera on a table, slowly moving the laser back and forth across the sky.  I'm intrigued by the visible "stopping" point of the beam - I don't quite understand why it appears to go a certain distance then stop dead rather than fading into the distance:

Now, the red laser is 205mW, and has a screw-thread collimating lens on the end.  This means you can focus it to a point, and 205mW on a point which has a tiny area means there is a lot of energy on that point, and you  can really heat that point up.  Below is the result of focussing the beam on a piece of wood  (well, the garden furniture, but don't tell the wife!)

The other thing that I had read a fair amount about is using the laser the light a match.  A safety match is dark in colour, and will absorb a lot of the energy from the laser light, so I set up my 7D with the 100mm macro lens focussed on a match head (at 1:1) and started lighting matches.  The shot below shows how visible the beam is in a dark room (note that at 1:1 the 100mm macro lens had a DoF of significantly less than 1mm and the beam is a bit out of this DoF):

This shot shows the match at the point just before it lights:

And the flame is just starting to erupt from the edge of the match here:

Now the match is on it's way to igniting across it's surface:

This shot shows the spent match illuminated with the green laser - it's more for art than any other purpose:

And finally a couple of long exposure shots writing on the Leylandii that make our back fence with both the green and red lasers:

I have a lot more ideas to try, and I can think of all sorts of interesting experiments to perform with these lasers - reflections, combinations, speckled interference patterns, interferometry photos, and apparently popping balloon's is fun!  Hopefully I'll get some interesting shots to post over the next few months.

I think I should just once again re-iterate how these are not toys, and you really should research the safety issues relating to them if you're thinking they look like fun.  At the very least, spend quite a few quid on some safety glasses meant for the frequency of laser that you are using.