L.A.S.E.R.s

L.A.S.E.R. Light

A laser controls the way that excited atoms release photons. "Laser" is an acronym for light amplification by stimulated emission of radiation, which describes how a laser works.

It is necessary to have a large collection of atoms in the excited state for the laser to work efficiently. In a laser, the lasing medium is “pumped” to get the atoms into an excited state. Intense flashes of light or electrical discharges ‘pump’ the lasing medium and create a large collection of excited-state atoms.

The atoms are usually excited to a level that is two or three levels above the ground state. This increases the degree of population inversion. The population inversion is the number of atoms in the excited state versus the number in ground state.

Once the lasing medium is ‘pumped’, it has a lot of atoms with some of their electrons promoted to excited levels. When the electrons ‘relax’ and return to their ground state they themselves of the excess energy. This emitted energy comes in the form of photons of electromagnetic radiation of very specific wavelengths. (E=hf)

The light released is:

• monochromatic - it contains only one wavelength of light (one colour). The wavelength of light is determined by the amount of energy released when the electron drops to a lower orbit. (E=hf)
• coherent. It is “organized” -- each photon moves in step with the others. This means that all of the photons have wave fronts that launch in unison.
• very directional. A laser light has a very tight beam and the energy is very concentrated. A torch beam releases light in many directions in comparison, and the light is very weak and diffuse.

Stimulated emission

To make the emitted light have these three properties we need stimulated emission. In ordinary light all of the atoms release their photons randomly. In stimulated emission, photon emission is organized.

The photon that any atom releases has a certain wavelength that is dependent on the energy difference between the excited state and the energy level the electron goes down to. If this photon (possessing a certain energy and phase) should interact with another atom’s electron in the same excited state, stimulated emission can occur.

The first photon can stimulate (or induce) atomic emission so that the photon that has been induced (the one from the second atom) vibrates with the same frequency and direction as the incoming photon.

Mirrors

An important part of a laser is a pair of mirrors, one at each end of the lasing medium. Photons, with a very specific wavelength and phase, reflect off the mirrors to travel back and forth through the lasing medium. In the process, they stimulate other electrons to make the downward energy jump and can cause the emission of more photons of the same wavelength and phase. A cascade effect occurs, and soon we have propagated many, many photons of the same wavelength and phase. The mirror at one end of the laser is "half-silvered," meaning it reflects some light and lets some light through. The light that makes it through is the laser light.

(see ‘How stuff works’ for details of the different types of laser and their uses – not needed for the A2 level medical paper but very interesting!)

Laser Classifications

Lasers are classified into four broad areas depending on the potential for causing biological damage.

When you see a laser, it should be labelled with one of the following four class designations…

• Class I - These lasers cannot emit laser radiation at known hazard levels.
• Class I.A. - This is a special designation that applies only to lasers that are "not intended for viewing," such as a supermarket laser scanner. The upper power limit of Class I.A. is 4.0 mW.
• Class II - These are low-power visible lasers that emit above Class I levels but at a radiant power not above 1 mW. The concept is that the human aversion reaction to bright light will protect a person.
• Class IIIA - These are intermediate-power lasers (cw: 1-5 mW), which are hazardous only for intrabeam viewing. Most pen-like pointing lasers are in this class.
• Class IIIB - These are moderate-power lasers.
• Class IV - These are high-power lasers (cw: 500 mW, pulsed: 10 J/cm2 or the diffuse reflection limit), which are hazardous to view under any condition (directly or diffusely scattered), and are a potential fire hazard and a skin hazard.
• Significant controls are required of Class IV laser facilities.

Medical Uses of lasers

The highly collimated beam of a laser can be further focused to a microscopic dot of extremely high energy density. This makes it useful as a cutting and cauterizing instrument. Lasers have been used to make incisions half a micron wide, compared to about 80 microns for the diameter of a human hair.A focused laser can act as an extremely sharp scalpel for delicate surgery, cauterizing as it cuts. "Cauterizing" refers to long-standing medical practices of using a hot instrument or a high frequency electrical probe to singe the tissue around an incision, sealing off tiny blood vessels to stop bleeding. The cauterizing action is particularly important for surgical procedures in blood-rich tissue such as the liver. Because heat from lasers cauterizes blood vessels, there is less bleeding compared to scalpel use.
Medical lasers have been used for dermatology applications such as removal of port wine stains, dark spots, tattoos, acne scars and other blemishes.
Studies conducted by the manufacturers showed that the laser is as safe and effective as ahigh-speed drill for removing dental decay and preparing a cavity for a filling. A manufacturer's study indicated that fewer patients needed anesthetic for pain.

Lasers may be used to remove tissue in eye surgery as well. This may include removing tumors, cataracts, or proliferating blood vessels common to diabetic retinopathy.Higher power lasers are used after cataract surgery if the supportive membrane surrounding the implanted lens becomes milky. Photodisruption of the membrane often can cause it to draw back like a shade, almost instantly restoring vision.Lasers are used for photocoagulation of the retina to halt retinal hemorrhaging and for the tacking of retinal tears.

Avoid using the following three examples on an exam paper!

Biostimulation lasers, are also called low level laser therapy (LLLT), cold lasers, soft lasers, or laser acupuncture devices. These devices are used for the temporary relief of pain. There are numerous clinical investigations being conducted to determine safety and efficacy with these devices for the intended uses that are proposed. –

Hair removal - Manufacturers may not claim that laser hair removal is either painless or permanent unless they provide sufficient data to demonstrate such results. Several manufacturers have received permission to claim, "permanent reduction," NOT "permanent removal" for their lasers. This means that although laser treatments with these devices will permanently reduce the total number of body hairs, they will not result in a permanent removal of all hair. The specific claim granted is "intended to effect stable, long-term, or permanent reduction" through selective targeting of melanin in hair follicles.

Lasers are also being used to treat wrinkles. Several manufacturers have received clearance to claim treatment of wrinkles, while others may claim skin resurfacing. Patients have reported reddening of the skin, which lasted from one to four months. Pain was mild and could be treated with over-the-counter analgesics. People considering this procedure should consult a dermatologist or the manufacturer to determine whether or not they would be good candidates.

Click here for a video on a medical use of laser light and optical fibres.