It’s an excellent question.
Most laser users know that a KTP crystal converts 1064 nm into 532 nm by “frequency doubling”. But what about the other wavelengths—585 nm, 650 nm and the various colours advertised by some manufacturers?
Is the laser somehow producing entirely new wavelengths?
Well… yes.
But not quite in the way many people might imagine.
The Nd:YAG laser only geneates one wavelength
The active medium inside a Nd:YAG laser are the neodymium atoms and they naturally emit light at 1064 nm. That’s their job. Everything else happens after the laser has generated its pulse.
Think of the laser as an engine. The various handpieces and optical modules are simply gearboxes that modify the output.
The easy one: 532 nm
This is the wavelength most people know. A nonlinear crystal—usually KTP—is placed in the beam. Inside the crystal, two 1064 nm photons combine to form a single photon with twice the energy. The wavelength is therefore halved: 1064 nm → 532 nm
This process is called frequency doubling (or second harmonic generation).
It’s remarkably efficient and explains why virtually every Q-switched or picosecond Nd:YAG laser offers both 1064 nm and 532 nm.
But what about 585 nm and 650 nm?
This is where things become much more interesting. Older tattoo lasers often used dye handpieces. These contained special laser dyes that absorbed the incoming laser light before re-emitting light at a different wavelength.
They worked. But they were pretty inefficient.
Very inefficient. Typically, only around 5–15% of the original laser energy emerged from the handpiece. The rest of the energy simply became heat, within the dye.
This explains why dye handpieces generally produced much lower fluences than the parent laser.
Modern systems often use Raman wavelength conversion
Many modern picosecond lasers use something far more elegant. Instead of relying on dyes, they employ a Raman crystal. (Raman was a Nobel prize-winning physicist who pioneered the field of spectroscopy – https://en.wikipedia.org/wiki/C._V._Raman). Inside the crystal, the laser light interacts with molecular vibrations. Some photons lose a small amount of energy to the crystal lattice. The result is a new wavelength.
For example: 1064 nm → 1197 nm
This new wavelength can then be frequency doubled to produce approximately: 1197 nm → 598 nm
Similarly: 1319 nm → approximately 660 nm
Manufacturers often round these values and market them as 585 nm or 650 nm handpieces, although the exact wavelength depends upon the optical design. The important point is that these wavelengths are not produced directly by the Nd:YAG crystal. They are created by carefully manipulating the original 1064 nm beam after it leaves the laser cavity.
Is Raman conversion efficient?
Much more so than dye handpieces. Typical external Raman converters achieve around 20–40% conversion efficiency, with overall output after frequency doubling and optical transmission often ending up around 15–20% of the original 1064 nm pulse energy.
That’s still a significant loss. Every optical component extracts its price.
The biggest misconception
This discussion often leads to another question. “If my laser doesn’t produce exactly 650 nm, does that mean it can’t remove green ink?” This is where I think many clinicians have been taught an oversimplified version of the physics. The reality is that every tattoo pigment absorbs every wavelength of light—to some extent.
The difference is not whether absorption occurs. The difference is how much.
Every pigment has an absorption spectrum, not a single magical wavelength. A green pigment may absorb much more strongly at one wavelength than another, but its absorption at 1064 nm is not zero. It is simply lower. Plus a ‘green’ ink may be composed of blue and yellow inks – the laser wavelength will ‘see’ these colours differently!
That single idea explains many clinical observations that otherwise seem mysterious. Why do some green tattoos gradually fade with a 1064 nm laser? Why can some red pigments respond to wavelengths other than 532 nm? Because the governing quantity isn’t the wavelength itself.
It’s the amount of energy that is actually absorbed that is important – which dpeends entirelyon the the absorpton coefficient of that ink colour to the incoming wavelength!
Follow the energy
As physicists, we are taught to follow the energy.
Where did it come from?
Where did it go?
How much was lost along the way?
Tattoo lasers are no different. The electrical energy becomes laser energy. Some is lost in the laser cavity. Some is lost during wavelength conversion. Some is lost in the optics. Some reaches the skin. Some is absorbed by the tattoo pigment. Much of the infra-red light is back-scattered out of the skin – up to 60%!!
Only then does the interesting physics begin.
Understanding that energy pathway tells us far more than simply memorising which wavelength should be used for which ink colour.
And perhaps that’s the most important lesson of all.
Hope this makes sone sense,
Mike.
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