Overview of Lasers

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Laser surgery of skin conditions having cosmetic implications has revealed the profound psychological benefits which are unmatched by any other modality of treatment either with or without a knife. An increasingly sophisticated understanding of the biophysics of laser-tissue interactions has lead to a more efficient utilization of the present technology on the clinical side and at the same time is helping the physicists to add more and more highly selective laser systems in the armamentarium of aesthetic laser surgeons.

This article provides a general overview of lasers in skin and cosmetology and discusses its current clinical applications from Plastic Surgeon's point of view.

HISTORY

Laser is an acronym for “Light Amplification by Stimulated Emission of Radiation.” Stimulated emission was based on Einstein's quantum theory of radiation..[1] The first laser was produced by Theodore H. Maiman on 7^th July 1960 using ruby as a lasing medium that was stimulated using high energy flashes of intense light.[2] The Decade of 1960s will always be remembered in the history of lasers as more than ten different lasers were invented using solid, gaseous, semi-conductor as well as liquid lasing media. The refinement of technology along with invention of newer lasers has continued till date and will keep on doing so in the future as well.

A significant understanding of lasers and light sources is required for their optimal use. Also a basic understanding of laser physics is mandatory to carry out an efficient laser treatment.

LASER CHARACTERISTICS

Laser light is monochromatic, bright, unidirectional and coherent.

Monochromaticity: 

The luminous waves emitted come out with the same wavelength and energy. A single wavelength or a narrow band of wavelengths emitted allows precise targeting within tissue, while sparing adjacent structures.

Brilliancy: 

The light beam emitted is extremely intense and angularly well centred. The brightness or intensity is one of the important properties and can be enhanced by techniques like pulsing and Q-switching where extremely high peak power can be delivered in nanoseconds.

Coherency: 

All the photons emitted vibrate in phase agreement both in space and time. Coherence is a measure of precision of the waveform. Highly coherent laser beam can be more precisely focused.

Directionality: 

All the photons travel in Uni direction. Directionality of the laser correlates with the emission of an extremely narrow beam of light that spreads slowly. Within the laser apparatus, efficient collimation of photons into a narrow path results in a divergence factor of approximately 1 mm for every metre travelled. Directionality allows the laser beam to be focused on a very small spot size.

Terminology and measurements

1. IPL: Intense Pulsed Light where peak optical power per pulse is up to 20,000 watts achieved with capacitor banks. All bright light sources are not called IPL, they are just light sources. Wavelengths emitted range usually from 400 nm to 1200 nm and the lower wavelengths can be eliminated by various cut off filters which usually range from 515 to 755 nm.

2. I²PL: Second generation Intense Pulsed Light where wavelengths from 900 to 1200 nm are eliminated.

3. Chromophore: Chromophore is a material, present either endogenous in the tissues or exogenous i.e. brought from outside, which absorbs particular wavelengths depending on its absorption coefficient. Examples of endogenous chromophores are melanin, haemoglobin, (oxy haemoglobin, de-oxyhaemoglobin and meth haemoglobin), water, protein, peptide bonds, aromatic amino acids, nucleic acid, urocanic acid and bilirubin.[3] Exogenous compounds like different colors of tattoo ink also act as chromophores.

4. Parameters: Parameters are the values of wavelength, fluence (see below), number of pulses, pulse duration, pulse delay, repetition rate and spot size which are set on laser or IPL systems to treat a particular condition.

5. Lasing is the process of treating a lesion or a condition with lasers or light.

6. Wavelength: The distance between two subsequent peaks or troughs of a light wave. Usually it is expressed in nm (nanometre i.e. 10⁻⁹ metres)

7. Hertz (Hz): A unit of frequency equal to one cycle per second.

8. Frequency (V or f) ∝ (1/ wavelength (Hz) Therefore shorter the wavelength, higher is the frequency and longer the wavelength, lower is the frequency.

9. Photon: Photon is an elementary particle responsible for electromagnetic phenomena. It is the carrier of electromagnetic radiation of all wavelengths, including in decreasing order of energy, gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. The photon differs from many other elementary particles, such as the electron and the quark, in that it has zero rest mass; therefore, it travels (in a vacuum) at the speed of light.

10. Energy: Each photon carries a ‘quantum’ of energy (E), whereby: E=hV (h – Planck's constant) Therefore:

    Short wavelength = high frequency = high energy photons

    Long wavelength = low frequency = low energy photons

Measurements used routinely in laser applications include wavelength, frequency, energy, fluence, power, and irradiance.

1. Energy: Energy is measured in joules (J) and is proportional to the number of photons.

2. Power: Power is the rate of delivery of the energy. It is measured in watts (W) where 1 W = 1 J/sec.

3. Fluence: Fluence is the energy delivered per unit area. It is measured in J/cm²

4. Irradiance: Irradiance is the power per unit area. It is measured in W/cm²

LASER TISSUE INTERACTION[3]

Laser beam that encounters skin surface may be reflected, transmitted, scattered or absorbed at each layer. Once the laser beam falls on the skin, from this point onwards, we should think about it not as a light but as a continuous or pulsed source of photons. Photon as a particle can only interact with matter by transferring the amount of energy. Therefore, only absorbed photons can... View Full Printable Article