The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation.
The Unique Properties of Laser Light
Natural light is composed of various electromagnetic waves traveling in disoriented fashion, known as incoherent light.
Laser light has three unique properties. First, it is collimated, which means it travels in a single direction with very little divergence even over long distances. In contrast, ordinary light waves spread and lose intensity quickly. Second, laser light is monochromatic, consisting of one color or a narrow range of colors. Ordinary light has much wider range of wavelengths or colors. Third, laser light is coherent, which means all light waves move in phase together in both time and space. Ordinary incoherent light from a light bulb or flashlight is composed of a mixture of frequencies, al out of step with each other, and raveling in different directions.
Main Components of a Medical Laser
In a laser device, laser energy is generated within the laser cavity, which consists of three basic components. To generate laser light in the laser cavity a reliable and high-performance power supply in the form of a power generator is necessary.
The first component is the active medium – the source of the laser energy – which can be solid, liquid, or gas. A solid active medium consists of a cylindrical laser crystal. Popular laser crystals for medical laser applications are Nd:YAG and Er:YAG (neodymium:yttrium-aluminum-garnate and erbium:yttrium-aluminum-garnate). The active medium determines the specific wavelength of light at which the laser operates (e.g. 1.06µm for Nd:YAG and 2.94µm for Er:YAG).
The second component is the incident energy source used to stimulate the atoms of the active medium. A pulsed low-pressure xenon flash lamp is most commonly used.
The third component is the optical resonator – two highly polished mirrors placed at either end of the laser cavity, which redirect the escaping incoherent light of the active medium, producing a very bright, unidirectional, monochromatic, coherent form of light.
Once laser light has been generated in a laser device it first travels through the laser beam delivery system (an articulated arm or optical fiber) and then through the handpiece to reach the target tissue.
Laser – Tissue Interaction
Laser treatments are defined by the laser beam parameters, the target surface area, the light contact pattern and the speed at which the beam moves across the tissue.
The wavelength and power density of a laser beam determine its effect on tissue. Laser light is absorbed, transmitted, reflected, and scattered by the tissue; the relative proportions of these phenomena depend on the optical characteristics of the tissue. The only absorption produces significant tissue effects. Absorption can result in photochemical, photothermal, and photomechanical interactions between the beam and the tissue.
In general: more energy equals more effect. The energy and power of the laser beam can be controlled directly. The speed at which the beam moves across tissue also affects how much energy is absorbed by the tissue. Slower beam movement across an area will result in more energy delivered to that area.
Sometimes the pattern of the light is important. The pattern can be controlled manually or by an automated scanner.
Chemicals that absorb light are called chromophores. The most important chromophores in aesthetic laser therapy are water (which absorbs at 2940nm), melanin, oxyhemoglobin and desoxyhemoglobin (which absorbs at 1064nm).
The table below provides information on ER:YAG and ND:YAG laser wavelength, active medium and typical absorbing chromophores.
|Laser Type||Wavelength (nm)||Active Medium||Absorbing Chromophores|
|Er:YAG||2940||Erbium in yttrium-aluminum-garnet solid-state crystal (Er:YAG)||water, hydroxyapatite|
|Nd:YAG||1064||Neodymium in yttrium-aluminum-garnet solit-state||water, melanin, hemoglobin|
Laser light can be thought of as periodic waves of energy traveling through space. Wavelength refers to the physical distance between crests of successive waves in the laser beam. Typical laser wavelengths are: 1064nm (near-infrared), 2940nm (mid-infra-red) etc. Only laser wavelengths between 400nm and 700nm are visible to the human eye. Wavelength is directly related to the frequency of the light through the speed of light constant. The speed of light divided by the wavelength is the frequency of the light. The energy of each photon in the light beam is related to the wavelength; at a shorter wavelength, the energy of the photon is higher.
Laser power refers to the rate at which energy is generated by the laser. Laser power of 1 Watt means that 1 Joule of energy is emitted in 1 second.
Frequency – Pulse Repetition Rate (Hz)
Medical lasers are usually operated in a repetitive pulse mode. Laser pulses are emitted periodically at a pulse repetition rate, for example, 10 pulses per second. Hertz (Hz) is the most commonly used unit for pulses per second.
Pulse width – Pulse Duration (ns, µs or ms)
Pulse duration and pulse width are synonymous terms, which refer to the temporal length of the laser pulse; that is, the time during which the laser actually emits energy.
Pulse Energy (J)
Pulse energy implies the radiant energy in a laser pulse. When the laser is working in pulsed mode, the pulse energy, measured in Joules, is a more frequently used parameter than laser power, because some of its clinical effects are not directly influenced by the frequency or repetition rate of the laser pulses.
Peak Power (W)
Peak power refers to the power level during an individual laser pulse.
Peak power = Pulse energy / Pulse duration
For a laser operating in a pulsed mode with an energy of 1 Joule and a pulse duration of 100 µs, the peak power is 10W.
Spot Size (mm)
Laser beam spot size refers to the diameter of the laser beam on the target. By changing the laser beam spot size while keeping the laser pulse energy constant, the fluence can be changed substantially and thus the basic mechanism
Fluence refers to the amount of laser energy delivered to the treated surface area (in square centimeters). It is also called a dose of energy or energy density.
Fluence = Energy/Area
In other words, the fluence increases at the same energy settings if the spot size decreases. Vice versa, the fluence decreases at the same energy settings if the spot size increases. Fluence is a very useful parameter for the laser practitioner since it eliminates the need to consider spot size when determining the clinical effect the laser will have. For example, when setting fluence at 60 J/cm2 the clinical effect will be the same (if all other parameters are identical) regardless whether a 6mm or 8mm spot size is being used. Note that this is a theoretical example in which the scattering effect is not taken into account.
Optical and Thermal Tissue Parameters
Absorption Coefficient & Penetration Depth
One of the most important optical features of a target tissue is its ability to absorb laser light. The light absorption coefficient is usually referred to as the µa coefficient and is expressed in units of 1/cm or cm-1. When light with fluence F0 falls on tissue, without scattering, with an absorption coefficient µa the fluence decreases exponentially with depth z in the tissue, according to:
F=F0 * e – µa*z
At a depth of 1/µa the fluence decreases to a value of F = Fo*e -1 (F = 0.367* F0) The absorption coefficient depends strongly on the laser wavelength and tissue type.
Thermal Relaxation Time
The thermal relaxation time (TRT) is a fundamental tissue parameter in laser applications in dermatology and aesthetic medicine. In practice, the TRT is defined as the time it takes for the target to dissipate approximately 63% of the radiant pulse energy.
As a rule, smaller objects cool faster than larger objects of the same material and shape, which means that smaller objects have shorter TRT. This is important when the tissue needs to be heated to a specific temperature at a certain fluence setting to induce a desired clinical effect. If the pulse width is too long, the tissue will start cooling itself via thermal conduction prior to the end of the pulse which may negatively influence the desired clinical outcome. To target smaller skin structures, shorter pulse widths and higher fluence settings are necessary.