Commonly used for removal of Tattoos
Lasers have been used for tattoo removal for more than 20 years. For the last 10 years, the Q-switched laser has been the acknowledged laser of choice for all tattoos, excepting some reds, which may be best treated with an Nd:Yag laser.
Why is a tattoo so difficult to remove? The tattoo pigment injected into the lower layers of skin (the dermis) becomes sealed away by a tough network of collagen fibres. It is very difficult to remove the tattoo pigment without affecting the surrounding tissue.
The alternatives to laser treatment involve either physically removing the pigment and the surrounding skin by surgical excision and skin grafting, or chemically destroying the pigment using salts, acids or 'organic' solutions. In either case the surrounding skin is damaged and will require significant aftercare. Significant scarring would be an expected outcome of non-laser removal methods.
Tattoo removal is most commonly performed using lasers that react with the ink in the tattoo, and break it down. The broken-down ink is then absorbed by the body, mimicking the natural fading that time or sun exposure would create. All tattoo pigments have specific light absorption spectra. A tattoo laser must be capable of emitting adequate energy within the given absorption spectrum of the pigment in order to provide an effective treatment. Certain tattoo pigments, such as yellows, greens and fluorescent inks are more challenging to treat than the darker blacks and blues. These pigments are more challenging to treat because they have absorption spectra that fall outside or on the edge of the emission spectra available in the respective tattoo removal laser.
Laser tattoo removal requires repeat visits to remove a tattoo. A brand of ink, InfinitInk, was developed to enable easier tattoo removal with a single laser treatment. The newer Q-switched lasers are said by the National Institutes of Health to result in scarring only rarely, however, and are usually used only after a topical anaesthetic has been applied. Areas with thin skin will be more likely to scar than thicker-skinned areas.
There are several types of Q-switched lasers, and each is effective at removing a different range of the colour spectrum.
Types of Q-switched Lasers
The most common type is the actively Q-switched solid-state bulk laser.
Solid-state gain media have a good energy storage capability, and bulk lasers allow for large mode areas (thus for higher pulse energies and peak powers) and shorter laser resonators (e.g. compared with fibre lasers).
The laser resonator contains an active Q switch - an optical modulator, which in most cases is an acousto-optic modulator.
For wavelengths in the 1-µm spectral region, the most common pulsed lasers are based on a neodymium-doped laser crystal such as Nd:YAG, Nd:YVO4, or Nd:YLF, although ytterbium-doped gain media can also be used. A small actively Q-switched solid-state laser may emit 100 mW of average power in 10-ns pulses with a 1kHz repetition rate and 100 µJ pulse energy. The peak power is then ≅9 kW. The highest pulse energies and shortest pulse durations are achieved for low pulse repetition rates (below the inverse upper-state lifetime), at the expense of somewhat reduced average output power. A somewhat larger Nd:YAG laser with a 10-W pump source (e.g. a diode bar) can reach pulse energies of several millijoules. Nd:YVO4 is attractive particularly for short pulse durations and high pulse repetition rates, or for operation with low pump power.
Q-switched lasers with longer emission wavelengths are often based on erbium-doped gain media such as Er:YAG for 1.65 or 2.94 µm, or thulium-doped crystals for ≅2 µm. Significantly larger pulse energies can be obtained from amplifier systems (MOPAs). For high average powers combined with moderate pulse energies, fibre MOPAs, also called MOFAs, can be used.
Schematic setup of a passively Q-switched laser.
The saturable absorber is a crystal (e.g. of Cr:YAG) within the laser resonator. Particularly for low pulse repetition rates, lamp pumping can be an economically favourable option, since discharge lamps are much cheaper than laser diodes for a given peak power. For higher powers, however, diode pumping becomes more attractive, also because thermal effects in the laser crystal are strongly reduced. A passively Q-switched laser contains a saturable absorber (passive Q switch) instead of the modulator. For continuous pumping, a regular pulse train is obtained, where the timing of the pulses usually cannot be precisely controlled with external means, and the pulse repetition rate increases with increasing pump power. The most frequently used saturable absorbers for 1-µm lasers are Cr:YAG crystals.
Microchip laser, passively Q-switched with a SESAM.
The left-hand side of the laser crystal has a dielectric coating, serving as the output coupling mirror. Passively Q-switched microchip lasers have particularly compact setups. Such lasers typically emit pulses with energies between nanojoules and a few microjoules, average output powers of some tens of milliwatts, and repetition rates between a few kilohertz and a few MHz. Generally, passively Q-switched lasers are more limited in average output power than actively Q-switched versions, since saturable absorbers dissipate some of the energy, so that limiting thermal effects can occur. Note that saturable absorbers usually have some non-saturable losses, which often increase the dissipated energy well beyond the level which is unavoidable in principle.
An actively Q-switched fibre laser.
Particularly some of the smaller Q-switched lasers, but also some lasers with longer resonators containing an optical filter such as a volume Bragg grating, operate on a single axial resonator mode. This leads to a clean temporal shape and to a small optical bandwidth, often limited by the pulse duration. Other lasers oscillate on multiple resonator modes, which leads to mode beating effects: the optical power is modulated with frequencies which are integer multiples of the resonator round-trip frequency.
Fibre lasers can also be actively or passively Q-switched. However, all-fibre devices are fairly limited in terms of performance, whereas Q-switched fibre lasers containing bulk-optical elements (e.g. an acousto-optic Q switch, see Figure 4, left) are less robust and still less powerful than bulk lasers. The relatively small mode areas (even when using large mode area fibres) introduce problems with fibre nonlinearities and damage, which set limits on the pulse energies and particularly the peak powers achievable.
Note also that the typically very high laser gain in a fibre laser has important effects on the laser dynamics; in particular, it can lead to the formation of a complicated temporal sub-structure. On the other hand, high-power fibre amplifiers are suitable for amplifying pulse trains with high average power but moderate pulse energy. Some degree of nonlinear pulse distortion in such an amplifier is often acceptable for applications.
edited by John Sandham, June 11.