Laser Wavelength (nm)

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 medical laser wavelengths are; 1064 nm (near infra-red), 2940 nm (mid infra-red), etc. Only laser wavelengths between 400 nm and 700 nm are visible.

Laser Pulse Energy (J)

If the laser is working in pulsed mode, the energy of the laser pulse is a more reliable parameter than laser power. The energy is measured in Joules.

Laser Power (W)

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 in one second.

Pulse Duration (ms) or Pulse Width

This refers to the temporal length of the laser pulse; that is, the time during which the laser emits energy.

Pulse Duration (ms) or Pulse Width

This refers to the temporal length of laser pulse; that is, the time during which the laser actually emits energy.

Peak Power

Peak power refers to the power level during an individual laser pulse. Peak Power = Pulse energy/ Pulse duration

Laser Beam 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

Fluence (J/cms2)

Fluence refers to the energy (J) delivered to the treated area (in square centimeters). It is called the dose of energy or energy density. Fluence = Energy/Area

Repetition Rate (Frequency) (Hz)

Medical lasers are usually operated in a repetitive pulse mode. Laser pulses are emitted periodically at a pulse rate, such as 10 pulses per second. A commonly used term for pulses per second is Hertz (Hz)

Absorption coefficient

One of the most important optical features of the target tissue is its ability to absorb laser light. The amount of absorbed energy versus the total used energy is called the absorption coefficient. Absorption Coefficient = Absorbed energy / Laser energy.

Thermal Relaxation Time (TRT)

TRT can be defined as the time it takes for an object to cool down from 100º C to 50ºC. It is a rule that a smaller object cools faster than a larger object of the same material and shape which means that the smaller target has a shorter thermal relaxation time. This fact is important when the tissue needs to be heated to a desired temperature at a certain fluence setting. If the pulse width is too long, the tissue will start cooling itself via thermal conduction prior to the completion of a pulse causing a negative clinical effect. The second parameter that should be taken into consideration when estimating the TRT is the shape of the target tissue. A sphere (skin cells) having 360 degrees of cooling surface area cools faster than a cylinder (hair follicles).This allows the hair follicle to retain its heat while the skin cells can cool itself much more efficiently.For this reason, parameters can be selected to destroy the follicle without causing damage to the skin. For targeting smaller structures, a shorter pulse duration and higher fluence are recommended.

Laser safety

The following steps should be adhered to:

  • EYE PROTECTION – All persons in the operating room must wear safety eyewear. Light from the laser can cause severe corneal and retinal damage to the unprotected eye. Eyewear must have side shields and be worn over prescription glasses.
  • REFLECTION – Laser light is easily reflected and care must be taken to ensure the beam is not directed toward shiny surfaces.
  • ELECTRICAL HAZARD – The interior of the laser machine contains high voltage and exposed invisible laser radiation. Only technicians trained in electrical and laser safety are authorized to perform internal maintenance.

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. In the case of a solid active medium, it consists of a cylindrical laser crystal. Popular laser crystals for medical laser applications are Nd:YAG and Er:YAG (neodimium:yttrium-aluminum-garnate and erbium:yttriumaluminum-garnate). The active medium determines the specific wavelength of light at which the laser operates (e.g. 1.06 IJm for Nd:YAG and 2.94 IJm for Er:YAG).
The second component is an incident energy source used to stimulate the atoms of the active medium. A pulsed low pressure xenon flashlamp 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.
Finally, the laser parameters are controlled by the practitioner using an interface.

Laser-Tissue Interaction

There are four primary interactions of laser light with tissue. As the light first strikes the tissue a small fraction is reflected from the surface. Of the light that continues into the tissue some are reflected from features in the tissue such as cell walls nuclei etc. This process is called “scattering”. Whether light is scattered or continues in a straight line, it is generally absorbed by the tissue. The absorption of laser energy and the resulting heating of the tissue produce the major clinical effect of the laser. Large differences in absorption characteristics account for the major clinical differences between the large varieties of lasers. If the light is not fully absorbed in the tissue, some of it may be transmitted through the tissue to the structures on the other side.

Absorption of Laser Light in Target Tissues

The absorption coefficient is a measure of how strongly light is absorbed in a particular material. A large number indicates stronger absorption. As we know from common experience there are great differences between materials. Water is transparent to visible light, but opaque in the mid-infrared portion of the spectrum. Melanin and haemoglobin absorb visible light strongly. The absorption coefficient determines the absorption length of various lasers in human tissue. The absorption length is the distance the laser light travels in the tissue before it is 63% absorbed. The penetration depth of the laser light into the tissue correlates to the absorption coefficient. The higher the absorption in certain chromophores (pigments, water) the lower the penetration depth.