History

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  Laser is an acronym for “light amplification by stimulated emission of radiation.”

  
  
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  In 1917, Albert Einstein described the theoretical basis of lasers in his paper Zur Quantentheorie der Strahlung.¹ The first functional laser was constructed in 1957 by Theodore Maiman, a physicist at Hughes Research Laboratories in Malibu, California.

Physics of Lasers

A laser is a resonant cavity flanked by two mirrors and filled with an active medium which can be gas, liquid, or solid. One of the mirrors is 100% reflective and the other is partially reflective (slightly leaky). A laser also has a pump or external energy source. A laser is pumped by passing current through the active medium or by using a flash lamp. When a laser is pumped, energy is absorbed by the atoms of the active medium, raising electrons to higher energy levels. The high-energy electrons then spontaneously decay to their lower energy “ground state,” emitting a photon in a random direction. This process is called spontaneous emission. Most of these spontaneously emitted photons are absorbed and decay; however, the photons emitted in the direction of the long axis of the resonant cavity are retained as they bounce between the two mirrors of the laser. When these photons encounter an atom in the excited state, they stimulate an excited electron to decay to its ground state and emit another photon of the same wavelength in the same direction. This process is called stimulated emission. When more than half of the atoms in the active medium reach the high-energy state, population inversion occurs. This is a necessary condition for a laser to start working. As light is amplified in the active medium through the process of stimulated emission, the partially reflective mirror begins emanating light which is uniform in wavelength, direction, phase, and polarization. This creates the familiar laser beam (Figure 3-1).

Properties of Laser Light

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  Laser light is monochromatic, unidirectional, and uniform in phase and polarization.

  
  
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  Laser beam spreads over distances, and can be focused with lenses.

  
  
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  Once laser light exits the main resonance chamber, it has to be delivered to tissue via one of two major delivery mechanisms, optical fiber or a waveguide. Light in the visible spectrum easily travels through an optical fiber and can be delivered directly to target tissues using a handpiece without significant energy loss. Even the near-infrared light of the Nd:YAG laser (1.06 µm) can be delivered through a fiber, however, infrared light of the CO₂ laser (10.6 µm) cannot be delivered via optical fiber, a major shortcoming of this highly popular laser. CO₂ laser light is delivered through waveguides which are essentially a series of articulated hollow tubes and mirrors. OmniGuide (Lexington, MA) developed a flexible delivery system for the CO₂ laser, resembling the flexibility of an optical fiber, which allows the surgeon to deliver CO₂ laser light through a handpiece directly to target tissues, though this system can be lossy, especially if bent.

Laser–Tissue Interaction

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  Tissue interacts only with laser light that is absorbed, not reflected, transmitted, or scattered.

  
  
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  In general, lasers with longer wavelengths have deeper tissue penetration. This rule holds for lasers in the visible spectrum, but not for infrared lasers such as CO₂ and Er:YAG. These lasers in the infrared range (3-10 µm) are easily absorbed by water and therefore have shallow tissue penetration.

  
  
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  Ultraviolet (UV) lasers, currently used in ophthalmology, work by tissue heating and photodissociation of molecular bonds. Visible and infrared (IR) lasers, commonly used in otolaryngology, work by heating tissue only. Laser energy is absorbed and converted to heat.

  
  
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  Thermal relaxation time—time needed for tissue to dissipate half of its heat.

  
  
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  A laser characteristically produces a wound with the following layers: tissue vaporization, necrosis, and thermal conductivity and repair (reversible damage) (Figure 3-2).

  
  
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  High laser energy delivered in short pulses minimizes thermal injury by allowing time for heat to diffuse between pulses.

  
  
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  Laser parameters under surgeon’s control are power, spot size, and exposure time.

  
  
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  Tissue effect depends on the amount of energy deposited into tissue (J/cm²).

  
  
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  The surgeon can change the spot size and energy delivered per unit area by changing the lens strength or simply working in and out of focus.

Chromophores and Their Spectral Characteristics

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  Chromophore—the target molecule that absorbs laser light. In laser skin resurfacing, the chromophore is water contained in the skin. When ablating a vascular lesion, the chromophore is hemoglobin. When removing a tattoo, the chromophore is the dye contained within the skin. When removing a pigmented skin lesion or dark hair, the chromophore is melanin.

  
  
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  Spectral analysis of hemoglobin shows the highest absorption peak at 405 to 420 nm (blue) and two secondary peaks at 538 nm (green) and 578 nm (yellow). Absorption drops off abruptly at wavelengths longer than 600 nm. Due to shallow tissue penetration of blue light and competitive absorption of blue light by melanin, medical lasers targeting vascular lesions exploit the secondary absorption peaks (530-600 nm) (Figure 3-3). Examples of lasers targeting hemoglobin in vascular lesions are KTP 532 nm (green), copper vapor 578 nm (yellow), and tunable dye 585 nm (yellow).

  
  
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  The melanin absorption curve is more smooth. Melanin absorption is best at UV and blue wavelengths and gradually decreases as wavelength increases, still absorbing even infrared light. This is why melanin is so great at protecting skin. Lasers that target melanin are used for hair, pigmented lesion, and tattoo removal. They operate in the red part of the visible spectrum where melanin still absorbs, but there is no competing absorption by hemoglobin. Examples are ruby 694 nm (red) and the more versatile alexandrite 755 nm (red), appropriate for a wider range of Fitzpatrick skin types.

  
  
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  Water absorbs infrared light. Water absorption starts in the 800 to 900 nm range and increases as the wavelength increases, peaking at 2.94 µm which corresponds to the Er:YAG laser output.

  
  
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  Lasers can also be used without targeting a specific chromophore, but simply for their general tissue ablative properties, such as the CO₂ or KTP lasers in a variety of otologic, larynx, or sinus surgery applications.