Laser Technology in the Medical and Cosmetic Industry

Abstract: This paper and discusses the use of Lasers and similar technologies (such as Intense Pulsed Light)

in the Medical and Cosmetic Industry. Their theory of operation is detailed along with their power

requirements (which vary for different types of laser technologies). Finally, power solutions to specific

applications that utilize Advanced Energy's low voltage power supply products are described.

1.0 What is a Laser:

To answer the question what is a Laser, we

must first consider what light is. Light is

electromagnetic radiation (fluctuations of the

electro and magnetic fields) which can transfer

energy from one location to another. White light

such as sunlight or the light from an ordinary bulb

is actually made up of many different light rays that

have different wavelengths, phases and directions

that all mix together to form white light, but also

result very diffuse energy transfer. This is known

as incoherent light.

A Laser is an example of coherent light, which is

light of a single wavelength (which means a single

colour), and whose waveforms are all in step, or in

phase. Coherent light can be focused in a

very small area and at a very high intensity. At a

high enough intensity this Laser Light can be used

to cut (essentially vaporise) or weld materials by

means of superheating. At lower intensity levels

lasers can be used to heat localised areas

such as hair follicles, tattoos or melanin, which

are broken up and then reabsorbed by the body.

This ability to superheat, cut, weld (or

cauterize tissue) means that lasers can be

used in a wide range of medical and

cosmetic applications including (but not limited

to):

Cosmetic Procedures

The localised heating ability of Lasers can be used

to breakup tattoos, birthmarks and sunspots or

can be used to remove hairs or whiten teeth.

Lasers are also used to treat scars, stretch

marks, wrinkles or spider veins by making tiny

“micro-cuts” which then promotes collagen

formation.

Surgery

Lasers are used in eye surgery (to reshape or

repair the cornea of the eye to improve vision),

cataract removal, dental surgery, tumour

removal, breast surgery, plastic surgery (etc).

Other Applications

Other medical applications include

medical imaging, microscopy, lithotripsy and

diagnostic applications.

2.0 How do Lasers Work?

All lasers are made up of three parts:

1. A pum

p source (or external power

source)

2. A gain medium (or laser medium)

3. An optical resonator

Pump S

ource

The pump source is used to deliver energy to

the gain medium. There are many types of

pump sources including electrical discharge,

electric current, flashlamps, arc lamps, light from

another laser, chemical reactions and even

explosive devices. The choice of pump source

is generally dictated by the gain medium being

used.

Whether the laser is pulsed or continuous

wave (CW) can also impact the choice of pump

source. For example, flashlamps are pulsed,

while an arc lamp is CW.

Gain Medium

The atoms in the gain medium absorb energy from

the pump source and this results in the emission of

light (photons) at a wavelength that is

determined by the type of gain medium used.

Types of gain media include liquids (dyes),

gases (carbon dioxide, argon etc.), solids

(crystals, glasses) and semiconductors.

Optical Resonator

The optical resonator keeps photons inside the

gain medium which results in amplification of the

Laser light, and focuses the laser energy into a

narrow beam. The simplest form of optical

resonator is two mirrors placed around the

medium with a small hole in one to focus the

Laser beam.

3.0 Laser Example - Red Laser

As an example of a laser design, we will look at

the first laser ever invented, the ruby laser.

As previously discussed, the ruby laser is made

up of a pump source (a flashlamp), a gain

medium (a ruby crystal) and an optical

resonator (two mirrors).

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©Advanced Energy Industries, Inc.

Application Note AN1605

Figure 1. Ruby Laser

A high voltage power supply is used to power

the flashlamp, which flashes with an

intense, incoherent light. These flashes “pump”

energy into the ruby crystal gain medium. Atoms

in the ruby absorb this energy, become

excited and spontaneously emit photons which

travel up and down the gain medium, reflected at

each end by the gain medium. These photons hit

other atoms that are already excited, and they

give off two photons (stimulated emission). The

light in the gain medium has now been

amplified. This is where the name “Laser” comes

from, an acronym for Light Amplification by

Stimulated Electromagnetic Radiation.

A mirror at one end of the ruby crystal continues to

reflect the photons back into the gain medium

to continue the light amplification. Another mirror

at the opposite end of the ruby crystal has the

same function, but this mirror has a small hole in

it and allows some light to escape. This

results in a narrow, powerful beam of powerful

laser light.

4.0 Other Laser Technologies

The Ruby red laser was invented in the 60’s, but in

the following years many other lasers

were invented using different gain media.

Solid and Liquid Lasers

The Ruby Laser is an example of a solid

laser, where a glass or crystal gain medium is

excited by a flashlamp (as described above) or

by another laser (like a diode laser) in place of

the flashlamp. Using a different gain medium

results in a Laser light of a different wavelength.

Liquid lasers are made up of organic

compounds (or dyes) that been dissolved in

solvents such as alcohol and water. Lasing has

been observed in a wide range of organic dyes,

and even dyes that are colourless may absorb light

and emit in the visible, ultra-violet and near infra-

red spectra. Dye lasers are again pumped by

flashlamps or other lasers.

The most exciting feature of dye lasers is the

ability to “tune” the output wavelength over the

entire visible spectrum from ultra-violet to infra-red.

This

is achieved by

mixing several dyes together to form

the gain medium. The ability to deliver tunable,

coherent light is extremely useful in the fields of

spectroscopy and in biomedical applications.

Gas Lasers

In Gas Lasers, the gain medium is a gas at a

very low pressure (a few milli-torr), and the

pump source is a power supply designed to

provide an electric discharge. Collisions between

the electrons in the electric discharge and the

gas molecules produce the photons in this

Laser. The gas is at such a low pressure to

allow an electric discharge over a long path

(provided by a long tube with electrodes at

both ends). Most elements can be made to

lase when in the gas state.

The electrical discharge in a gas is

generally characterised by the voltage / current

curve shown below (with the actual voltage and

current values dictated by the nature of the gas,

its pressure and the length and diameter of the

gain medium).

Figure 2. Current/Voltage Curve for Gas

Discharge Lasers

At low v

oltages, there is no current flow. As the

voltage is increased, a voltage is eventually

reached where a small amount of

current flows (prebreakdown current) due to a

small amount of ionisation that is always

present due to natural radioactivity and cosmic

rays.

As voltage increases further, current

slowly increases until a point where a large

number of molecules are ionised (the peak of

curve). Now the conductivity of the gas increases

and the voltage required to sustain discharge

decreases with increasing current (negative

resistance). Current control (possibly in the form

of a ballast resistor) is required to prevent this

rapid increase of current.

The basic design of a DC supply pump source for

a Gas Laser is the same whatever Gas is

being pumped (although the voltage-current

requirements will depend upon the Gas choice

and the Laser configuration - length of tube etc).

Three essential elements are required:

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Application Note AN1605