Nowadays, it is possible to purchase Q-switched Nd:YAG lasers with a single YAG rod, or with two rods. So, what is the difference and is it important?
To understand why a laser might have two rods instead of one we should look at the basic design of a laser first.
Basic laser cavity
A basic laser design is seen in the diagram below. A laser ‘material’, which can be a gas, solid, liquid or a microchip, is ‘stimulated’ by an external ‘energy source’ so that it produces lots of little photons of the same wavelength (colour).
These photons bounce around inside the cavity in all directions. However, some are reflected off the two mirrors which causes them to go through the laser material again and again. These photons can bump into atoms inside the material causing them to generate yet more photons. This is the ‘stimulated emission’ and ‘amplification’ bit of a laser! (Remember – Light Amplification by the Stimulated Emission of Radiation).
One of the mirrors is not a fully 100% reflector, so some of the photons escape. This is what we call the laser beam – the “useful” stuff!
The laser material (or, medium) is responsible for the wavelength and energy output. Increasing the ‘energy source’ will increase the total laser beam energy output.
The duration of the laser energy output determines its power (since power is just the energy over time). Q-switched lasers use methods to reduce the pulse duration to billionths of a second (nanoseconds). By doing so the power is huge, typically millions of watts. Picosecond lasers have even higher powers because their pulse duration are less than 1 nanosecond.
Why double rods?
The main benefit of using two laser rods rather than one is the increase in total output energy. There won’t be a doubling of energy – that’s very tricky due to geometrical considerations. However, two rods will generate more laser energy than one, from one flashlamp. Utilising one flashlamp means less in the way of electronics, capacitors, weight, cost etc.
Clinically, this means that larger spot diameters can be achieved, than with a one rod laser. Not only does this mean quicker treatment times (due to the larger spot area on the skin surface) but, it has been shown, using Monte Carlo modelling, that larger spot diameters result in useful fluences deep within the skin.
Ash et.al. found that larger spot diameters significantly increase the ‘depth of penetration’ of light into the skin (see their excellent report here) for the same incident energy density at the skin surface.
This, clearly, has beneficial effects when treating deeper targets, such as tattoo ink or deep blood vessels.
Using two laser rods in a laser has some benefits, particularly on the spot diameter applied to the skin. Using larger spots diameters can help in some treatments. It’s worth considering such a system if you are looking to ‘upgrade’.
Thanks for reading,