Most of us use them or are at least fascinated by them, but do we all know the basics of today's lasers? This month's Light on Lasers column is devoted to the basic laser terms that are critical to understanding, naming and operating any laser or light-based system.
Power, energy and fluence are features that many people talk about, but these things are really just a small part of a laser's total parameters. Proper adjustment of pulse duration, cooling and wavelength will increase treatment efficacy and safety, so understanding them is crucial.
A light source can qualify as a "laser" (light amplification by stimulated emission of radiation) by meeting three criteria:
Many of the devices we commonly call lasers are not in fact true "lasers." For example, an intense pulsed light (IPL) device typically emits light in a range of wavelengths and therefore does not meet the criteria of a true laser.
The fact that some of these devices are not true lasers does not make them any safer or easier to use than true lasers. Despite not being lasers, IPLs are some of the most complicated systems to date, allowing for multiple or single pulses of varying duration and allowing for thousands of possible combinations of treatments.
Laser devices are similar in that all have the following three main components:
1. Pump - The device that generates the energy to power the laser, the pump excites the electrons that are contained in the lasing medium.
2. Lasing medium - This medium can be gas (CO2), liquid (pulsed dye laser) or solid (Nd:YAG). The type of medium determines the wavelength of the light that passes through; hence the name of the laser.
3. Optical cavity - The chamber containing the lasing medium and two mirrors, one on each end of the cavity. Light amplification is created by reflection of the photons between these mirrors. Because the back mirror is 100 percent reflective and the front mirror is partially reflective, the amplified beam is released through the front mirror.
In addition to being named by the type of lasing medium they use, many lasers are named by the type of pulse they emit. The pulse of a laser can be continuous, quasicontinuous or pulsed. With the introduction of the theory of selective photothermolysis (Anderson and Parrish, 1983), pulsed laser systems gained popularity over the other methods of delivery. This theory explains how we can use lasers to selectively destroy a target without damaging the surrounding tissues.
A pulse is essentially the duration of time of active emission of light. The pulse duration plays a very important role in the clinical outcome. We can get completely different clinical results using the same wavelength and same power but different pulse durations. The lasers that emit the shortest pulses are termed "Q-switched" lasers, with pulses in the nanosecond range. These lasers are mostly used for pigment removal for such things as lentigos and tattoos. The most commonly used Q-switched lasers are the Q-switched ruby (694 nm), alexandrite (755 nm), Nd:YAG (1,064 nm) and frequency-doubled Nd:YAG (532 nm).
Longer-pulsed lasers (millisecond range) are used for hair removal and treatment of vascular lesions, emitting light in a "smoother" fashion and resulting in less inflammation. Pulse width is a more important component of the laser settings than was originally thought. Precise laser manipulation requires fine-tuning of the pulse width. Inexperienced users rely on fluence as their primary adjustment; however, the more versed the operator becomes, the more the pulse width comes into play.
In true selective photothermolysis terms, the pulse duration should be shorter than or equal to the thermal relaxation time of the chromophore. This thermal relaxation time (Trt) is the time it takes for the target to release two-thirds of its heat to the surrounding tissue. The Trt is directly proportional to its physical size. For example, longer pulse durations are required more for larger vessels than they are for small vessels. Heating of the tissue much beyond this limit can result in damage to the adjacent tissues. This is especially true with ablative lasers, which require short pulse durations to provide true ablation. Longer durations result in more coagulative effects.