Q-switches: Passive vs Active?

 

Throughout the last few years I have heard some people (usually sales people) trying to bamboozle potential customers with various nonsense claims. This is to be expected I suppose, but one which does irritate me greatly is the ‘active’ vs ‘passive’ Q-switch debate!!

So, I thought I’d write a wee piece about this topic, just for clarification purposes….

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What is a Q-switch?

First things first – just what is a Q-switch? A Q-switch is just a device which effectively shortens a laser pulse from (usually) milliseconds (10^-3 s) to nanoseconds (10^-9 s). In most standard laser cavities, the laser medium is optically excited by a high intensity flashlamp. The light from this lamp stimulates the active molecules in the laser medium which causes them to become excited – a higher energy state. When they de-excite they emit photons of a particular wavelength (dependent on the laser medium). These bounce around inside the cavity and some escape which is the beam that we use.

When a Q-switch is introduced into this assembly, the laser beam is not allowed to form properly. The Q-switch prevents ‘normal’ operation of the cavity. At some time the Q-switch’s disruptive action is stopped and the laser energy is ‘released’, which results in an output pulsewidth in the nanosecond range.

Only certain lasers can be Q-switched with a useful output energy. For tattoo removal these include the ruby, alexandrite and the Nd:YAG systems.

It is the Q-switching which generates the very short pulses required to induce the photoacoustic (or photomechanical) effects on the surface of tattoo particles. This cannot be done with longer pulsed lasers or IPL units.

 

What does ‘active’ and ‘passive’ mean exactly?

In both cases the Q-switch is designed to prevent the laser beam from being properly generated and leaving the laser cavity until required.

Active Q-switches are typically acousto-optic modulators found in the laser cavity (see diagram above). These devices prevent a proper build-up of coherent laser energy within the cavity while a voltage is applied to the Q-switch (due to large losses caused by diffraction). When the voltage is turned off the Q-switch suddenly becomes transparent to the laser beam and allows it to generate and leave the cavity in a nanosecond timescale. Active Q-switches require high voltages to ensure a high opacity which necessitates the need for larger voltage supplies.

 

Passive Q-switches can be optical cells filled with a saturable doped crystal (see below) or an organic dye. This material is optically opaque at the laser wavelength until a high laser fluence is incident on it. As the fluence increases the opacity decreases and hence the transmission rate increases. As the fluence peaks the material “saturates” which means that virtually all of the laser energy can pass through unabated.

In many lasers around the 1,000 nm wavelength (including the Nd:YAG laser) the passive Q-switch of choice is made with a chromium:YAG crystal  (Cr^4+:YAG). This device has some advantages over the active Q-switch devices including:

passive Q-switches do not require high voltage supplies;

much simpler designs than active devices;

these units are smaller and lighter;

much lower associated costs;

they can be more reliable and robust with less likelihood of failure.

The main downside with passive Q-switches is that the timing is a function of the material’s absorption properties. It is not triggered as with active Q-switches and, hence, the timing is not as precise. In addition, passive Q-switches are not as efficient as active Q-switches which inevitably results in a lower energy output (due to the residual absorption of the saturated material). However, both active and passive Q-switched lasers can emit more than sufficient energy for laser tattoo removal.

 

What is the difference in terms of treatment application?

In the real world the main difference is that active Q-switched lasers are more expensive than passive lasers. This is simply due to the higher level of technology and bigger high voltage supplies (which cost more…)!

However, in terms of treatment applications there is virtually no difference, certainly none that has been demonstrated clinically to my knowledge. Tattoo removal procedures require very high peak powers to induce the photoacoustic effect on the ink surfaces. This means that the energy must be absorbed by the ink within a very short time – typically nanoseconds. The mainstream, commercially-available QS lasers – ruby, alexandrite, Nd:YAG – vary in pulse duration from around 5 ns to around 75 ns. All of these systems generate photoacoustic reactions in tattoo ink.

All have been clinically demonstrated to remove tattoos, with varying degrees of success which are more dependent on the tattoo ink co0lour and depth.

 

Conclusion

The reality is that it does not matter a hoot whether the laser uses an active or a passive Q-switch. That is merely a bit of technology required to generate sufficient peak powers to achieve the end result. All of the standard QS lasers do this, and have been doing this for many years.

The next time a salesman (or woman) tells you that you should only buy a laser with an active Q-switch, tell them they are talking rubbish!! They’re just trying to sell you a more expensive device!

 

 

 

That’s all for now,

Mike.

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