Properties of Laser Light

A laser generates a beam of very intense light. The major difference between laser light and light generated by white light sources (such as a light bulb) is that laser light is monochromatic, directional and coherent.

Coherent refers to the synchronized phase of the light waves.

Incoherent light bulb vs. coherent laser

Collimated refers to the parallel nature of the laser beam. Laser light is emitted in a very thin beam, with all the light rays parallel. By focusing and defocusing this beam, a surgeon can vary its effect on tissue.

Monochromatic refers to the single (wavelength) color of a laser beam. Ordinary white light is a mixture of colors, as you can demonstrate by shining sunlight through a prism. Because the wavelength of laser light determines its effect on tissue, the monochromatic property of laser light allows energy to be delivered to specific tissues in specific ways.

 

Electromagnetic Spectrum and Wavelengths

The word laser will be limited to electromagnetic radiation-emitting devices using light amplification by stimulated emission of radiation at wavelengths from 180 nanometers to 1 millimeter. The electromagnetic spectrum includes energy ranging from gamma rays to electricity. Figure 1 illustrates the total electromagnetic spectrum and wavelengths of the various regions.

Figure 1 - Electromagnetic Spectrum

Ultraviolet radiation for lasers consists of wavelengths between 180 and 400 nanometers (nm). The visible region consists of radiation with wavelengths between 400 and 700 nm. This is the portion we call visible light. The infrared region of the spectrum consists of radiation with wavelengths between 700 nm and 1 mm.
 

Common Lasers and their Wavelengths

The color or wavelength of light being emitted depends on the type of lasing material being used. For example, if a Neodymium:Yttrium Aluminum Garnet (Nd:YAG) crystal is used as the lasing material, light with a wavelength of 1064 nm will be emitted. Table 1 illustrates various types of material currently used for lasing and the wavelengths that are emitted by that type of laser. Note that certain materials and gases are capable of emitting more than one wavelength. The wavelength of the light emitted in this case is dependent on the optical configuration of the laser.

Table 1. Common Lasers and Their Wavelengths

 

Continuous Wave and Pulsed Lasers
Lasers may be operated in Continuous Wave (CW) or Pulsed modes.

With Continuous Wave Lasers, energy is continuously applied, or "pumped" into a lasing medium, producing a continuous laser output. With pulsed lasers, the pump energy is applied in pulses, usually with a flash lamp (similar to a camera strobe light) in the case of solid state lasers, pulsed radiofrequency or electrical energy in the case of gas lasers. In a Continuous Wave Laser, this process essentially stabilizes into a "steady state", resulting in true continuous output. The output of Continuous Wave Lasers, like that of a light bulb or electric heater, is measured as power in Watts, referring to the rate at which work is performed, or the energy applied per unit time.

In a typical Pulsed Laser, intense pumping at the beginning of the energy pulse causes a population inversion, with high gain and creation of a standing wave in the optical cavity, which depletes the population inversion, and essentially stops the laser output. This process repeats itself until pumping ceases, thus, the laser output consists of a series of intense overlapping energy spikes.

Because of the spiking output of pulsed lasers, the precise output power of a given laser pulse may be difficult to determine although the energy and pulse duration usually remain constant. For this reason, the output of pulsed lasers is more conveniently expressed as energy in Joules. Peak power can then be calculated = Output energy/pulse duration.
 

Power Density

Power density, or Irradiance refers to the power of the laser per unit area. Energy density, or Fluence, is the irradiance multiplied by the exposure time, measured in Joules/square centimeter.

Wavelengths and Absorptions

Infrared light is absorbed primarily by water, while visible and ultraviolet light are absorbed mainly by hemoglobin and melanin, respectively. As the wavelength decreases toward the blue-violet, and ultraviolet, scatter, which limits the depth that light may penetrate into tissue, becomes more significant.

When light is absorbed, it delivers energy to tissue, and the tissue's reaction depends on the intensity and exposure time of the light. An extremely intense, but extremely short pulse of laser light will usually cause an explosive expansion of tissue, or photomechanical (photodisruptive, photoacoustic) reaction. A less intense, longer pulse will cause a rapid heating, or photothermal, effect. Lower intensities applied for longer durations with cause a photochemical change, either by a slow transfer of energy as heat or by a specific chemical reaction as used in photodynamic therapy. and in LASIK vision correction. In actual practice, all of these interactions coexist, although by selecting the proper wavelength, intensity, and pulse duration, the desired effect can be maximized.

 

Light - Tissue Interactions

Laser light's monchromaticity is responsible for its selective effect on biologic tissue. Whenever light hits tissue, it can be transmitted, scattered, reflected, or absorbed, depending on the type of tissue and the wavelength (color) of the light. However, light absorption must take place for there to be any biologic effect, and a given wavelength of light may be strongly absorbed by one type of tissue, and be transmitted or scattered by another. Each type of tissue has its specific absorption characteristics depending on its specific components (i.e., skin is composed of cells, hair follicles, pigment, blood vessels, sweat glands, etc.) The main absorbing components, or chromophores, of tissue are:

• Hemoglobin in blood
• Melanin in skin, hair, moles, etc.
• Water (present in all biologic tissue)
• Protein or "Scatter" (covalent bonds present in tissue)
   

Definitions of tissue interaction terms:

Electromechanical causes dielectric breakdown in tissue caused by shock wave plasma expansion resulting in localized mechanical rupture.

Photoablative causes photodissociation or breaking of the molecular bonds in tissue.

Photothermal converts light energy into heat energy. This causes the tissue to heat up and vaporize.

Photochemical causes target cells to start light-induced chemical reactions.
 

Effect of Laser Spot Size on Tissue Distribution of Light Energy

A beam of light incident on tissue may be reflected, absorbed, or scattered. Scattering in tissue broadens the incident beam, decreasing the effective fluence in the intended target area. Doubling the spot size will increase the effective volume by a factor of eight.

A larger spot size usually enables faster and more effective treatment in dermatologic applications such as treatment of vascular lesions, laser hair removal, etc. However, more photons must be supplied by more complex and expensive power supplies, components, and delivery devices.

As a general rule, doubling the spot size and halving the fluence will yield an equivalent effective fluence at a given depth. This effect become more pronounced with increasing depth.