LASER stands for ‘light amplification by the stimulated emission of radiation’. Quantum physics provides the principle on which LASER emission is produced.

Quantum theory says that sometimes an atom absorbs energy and goes into an excited state. This state is short-lived and the atom returns to its resting state, releasing spontaneously a bundle of energy or photon which travels in a random direction. This is called the spontaneous emission of energy. Examples are light emitted by the sun or a bulb. The theory further says that photon is the basic unit that constitutes light.

When millions of atoms in a small container are simultaneously stimulated, what results is a laser made up of photons, all of which have the same wave length and direction.

Laser properties

Laser light, unlike ordinary light, is collimated, coherent and monochromatic.

Collimated means all laser waves travel in the same direction with very low divergence over long distances. Ordinary light waves diverge widely and become dimmer as they travel further. Laser beams remain pencil thin and retain their intensity even after traveling far.

Coherence means all laser waves always travel together in the same direction. This means all laser waves have same wave length. Coherence increases the amplitude or power, increasing the radiance of a laser beam.

Monochromatic means laser light is of one color or wave length or composed of a marrow range of colors or wave lengths. For laser to be coherent it must be monochromatic.

Lasers come in different wave lengths. They can be visible and invisible and belong to the infrared, visible or ultra violet sections of the electro-magnetic spectrum.

Properties of laser energy

In laser applications, two properties are critical. They are power and energy density also known as irradiance and fluence respectively.

Power density or irradiance is energy per area of application and is expressed as watt/sq.cm. A small area increases power density and bigger area lowers it. Tissue cutting needs high irradiance and coagulation low irradiance.

Energy density or fluence is the power produced at any time on unit area. It is measured as watts x time and expressed as joules/sq.cm. Quantity of joules is a meaningless statistic since 100 joules of energy can be obtained by 100 watts applied for 1 second or by applying 1 joule for 100 seconds. The first application will produce marked tissue response while the second would produce gradual tissue warming.

Laser devices and types

All medical lasers consist of some basic components like the housing unit or optical resonator which contain the medium to be excited, a fully and a partially reflective mirror at both ends of the housing unit, an excitation source and a power source like, electricity or another laser.

Laser types are named after the material, whose atoms are excited to produce the laser. This material is called the laser medium. Laser mediums can be solid, liquid or gas.

Solid mediums include certain crystals like, ruby, doped with rare earth or metal ions like, ytterbium and titanium. Liquid mediums are certain dye solutions used in pulse dyed lasers. Gas mediums include CO2, helium and neon mixture, argon and metal vapors. For safety reasons the invisible lasers like, CO2 are linked to visible lasers like, HeNe laser, to make them visible.

Transmission, reflection, absorption and scattering

Laser like light interacts with tissues in four ways; transmission, reflection, absorption and scattering. The laser chosen depends on the outcome desired. Argon laser is used in eye surgery because of its high transmissibility. It reaches the retina without damaging the intervening tissues.

In surgery, absorption and scattering by the tissues are the more desired outcomes. If tissues need cutting then lasers which are absorbed are used. If bleeding has to be controlled, then lasers which are scattered are used. Scattering reduces power intensity and allowing coagulation rather than cutting.

CO2 laser is an efficient cutting tool but ineffective in fluid containing cavities since it is absorbed by water. It is also ineffective in controlling bleeding since most of the energy is absorbed.

Mode of delivery and applications

Laser is delivered in the continuous, and single or repeat pulse mode. Continuous mode delivers laser, without interruptions, at selected wattage for as long as wanted. In single pulse, laser at a selected wattage and duration is delivered once. In repeat pulse, the single delivery is repeated. The pulse mode is generally preferred in precision surgery because it causes less thermal damage.

The highly collimated laser beam, which can be focused to a microscopic dot of extremely high energy density, finds applications in many fields including surgery, telecommunication, the welding and cutting industry, laser printing, CDs, optical discs, and laser heating.