Laser Physics There are 3 essential components to a laser system. First is a Lasing medium, which may be a gas, crystal, liquid, or semiconductor. Second is a Source of Excitation for the lasing medium, for example, flash lamps or continuous light, radiofrequency, high voltage discharge, diodes, and in some cases another laser. Finally, Mirrors are needed to reflect the excited photons back into the resonant cavity containing the lasing medium.
Generation of Laser Radiation. As the lasing medium is excited, molecules are "pumped" to a higher energy level. Some of the excited molecules will spontaneously decay to back to the original lower energy , or ground state, releasing a photon, and if the photon is emitted in the right direction, it will hit one of the mirrors at the ends of the resonant cavity and be reflected back into the excited lasing medium. This excited photon can "stimulate" another excited molecule to decay back to the ground state, releasing another photon which travels in the same direction, which in turn is reflected back into the resonant cavity, stimulating emission of even more photons. If most of the molecules in the lasing medium are in the excited state, termed a "population inversion", there is a net amplification of light energy, hence the name Light Amplification by the Stimulated Emission of Radiation.
By making one of the mirrors at either end of the resonant cavity partially transparent, laser light can escape from the resonant cavity, and will be emitted as long as the lasing medium is excited.
Properties of Laser Light. Unlike ordinary light, laser light is coherent, collimated, and monochromatic
• Coherent refers to the synchronized phase of the light waves. Imagine 100 soldiers marching in step, and compare to 100 pedestrians at a mall. The soldiers are moving in a coherent manner, compared to the more random motion of the shoppers.
• Collimated refers to the parallel nature of the laser beam. Laser light is emitted in a very thin beam, with all the light rays parallel. By focusing and defocusing this beam, a surgeon can vary its effect on tissue.
• Monochromatic refers to the single (wavelength) color of a laser beam. Ordinary white light is a mixture of colors, as you can demonstrate by shining sunlight through a prism. Because the wavelength of laser light determines its effect on tissue, the monochromatic property of laser light allows energy to be delivered to specific tissues in specific ways.
These properties allow a laser of a given power to be immensely more powerful than ordinary light of the same power-for example, compare a powerful CO2 surgical laser at 30 watts to an ordinary light bulb at 60-100 watts. For comparison, a laser pointer is 5mW (5 one thousandths of a watt), and a light show laser is from 1 to 5 watts. The light from an typical light source consists of many wavelengths ("broadband" light), and dispersed non-coherently in all directions. In contrast, laser light is a single wavelength with all of the energy collimated into a thin beam.
Types of lasers
• Solid State Lasers: Solid State lasers use a rod of solid material as the laser medium
CW and Pulsed Lasers: Lasers may be operated in Continuous Wave (CW) or Pulsed modes. With CW lasers, energy is continuously applied, or "pumped" into a lasing medium, producing a continuous laser output. With pulsed lasers, the pump energy is applied in pulses, usually with a flash lamp (similar to a camera strobe light) in the case of solid state lasers, pulsed radiofrequency or electrical energy in the case of gas lasers.
In a typical pulsed laser, intense pumping at the beginning of the energy pulse causes a population inversion, with high gain and creation of a standing wave in the optical cavity, which depletes the population inversion, and essentially stops the laser output. This process repeats itself until pumping ceases, thus, the laser output consists of a series of intense overlapping energy spikes. In a CW laser, this process essentially stabilizes into a "steady state", resulting in true continuous output. For maximum efficiency, the composition and concentration of the lasing medium, as well as the construction of the laser cavity and mirrors must be optimized for pulsed or CW operation.
The output of CW lasers, like that of a light bulb or electric heater, is measured as power in Watts, referring to the rate at which work is performed, or the energy applied per unit time. Because of the spiking output of pulsed lasers, the precise output power of a given laser pulse may be difficult to determine although the energy and pulse duration usually remain constant. For this reason, the output of pulsed lasers is more conveniently expressed as energy in Joules. Peak power can then be calculated = Output energy/pulse duration.
Power density, or Irradiance refers to the power of the laser per unit area. Energy density, or Fluence, is the irradiance multiplied by the exposure time, measured in Joules/square centimeter.
Pulsed Laser Operation: In a typical pulsed solid-state laser, pump energy is delivered to the solid state medium via a flash lamp, which is essentially a very bright strobe light. Usually, the flash lamp and the solid state medium are rod-shaped and of equal diameters, and are positioned at the twin foci of a reflective elliptical cavity (called a resonator or pump chamber), thus ensuring that all photons emitted from the flash lamp will find their way to the rod for maximum efficiency. Photons created by stimulated emission "resonate" between the HR (highly reflective) mirror and the OC (Output coupler, or partially reflective) mirror, and on to the delivery device.
Laser flash lamps are typically filled with xenon or krypton to about 400 torr (mm Hg), with an anode and cathode across which current flows to ionize the gasfill. Recombination of electrons with the ionized gas emits photons, which in turn induce a population inversion of the dopant atoms (Nd in Nd:YAG, Cr+3 in Ruby, etc,) in the laser rod.
Power to the flash lamp is supplied by a pulse forming or LC network consisting of inductors and capacitors, which store energy supplied by the Capacitor Charging Power Supply. A Trigger is used to release the stored energy across the arc of the flash lamp. Once the lamp is ignited, a simmer power supply is used to maintain the arc when not being pulsed by the pulse forming network. This prolongs the useful life of the flash lamp and significantly decreases power requirements.
Although flashlamps are very efficient at converting electrical energy to light (typically >50%, compare to an ordinary light bulb's ~4%), enormous amounts of heat are produced by the flash lamp's operation, and almost all high-power lasers use direct water cooling. Typically, the entire pump chamber is flooded with deionized water which contacts the rod and flash lamp (including the electrodes, hence the deionization) directly. High capacity (>10-20 liters/minute) pumps recirculate the cooling water through a heat exchanger.
CW Lasers operate continuously, using an arclamp rather than a flash lamp, and the continuous duty cycle has significantly higher power and cooling requirements than pulsed lasers. For maximum efficiency, CW lasers also use rods, HRs, and OCs that differ significantly than those used for pulsed lasers.
Some lasers use "quasi-CW" operation, in which pulses are repeated at a high rate (kHz and up), simulating continuous operation. The lower duty cycle of the laser components allow much higher peak powers than true continuous operation.
Light - Tissue Interactions. Laser light's monchromaticity is responsible for its selective effect on biologic tissue. Whenever light hits tissue, it can be transmitted, scattered, reflected, or absorbed, depending on the type of tissue and the wavelength (color) of the light. However, light absorption must take place for there to be any biologic effect, and a given wavelength of light may be strongly absorbed by one type of tissue, and be transmitted or scattered by another. Each type of tissue has its specific absorption characteristics depending on its specific components (i.e., skin is composed of cells, hair follicles, pigment, blood vessels, sweat glands, etc.) The main absorbing components, or chromospheres, of tissue are:
• Hemoglobin in blood
• Melanin in skin, hair, moles, etc.
• Water (present in all biologic tissue)
• Protein or "Scatter" (covalent bonds present in tissue)
Infrared light is absorbed primarily by water, while visible and ultraviolet light are absorbed mainly by hemoglobin and melanin, respectively. As the wavelength decreases toward the blue-violet, and ultraviolet, scatter, which limits the depth that light may penetrate into tissue, becomes more significant.
When light is absorbed, it delivers energy to tissue, and the tissue's reaction depends on the intensity and exposure time of the light. An extremely intense, but extremely short pulse of laser light will usually cause an explosive expansion of tissue, or photomechanical (photodisruptive, photoacoustic) reaction. A less intense, longer pulse will cause a rapid heating, or photothermal, effect. Lower intensities applied for longer durations with cause a photochemical change, either by a slow transfer of energy as heat or by a specific chemical reaction as used in photodynamic therapy. and in LASIK vision correction. In actual practice, all of these interactions coexist, although by selecting the proper wavelength, intensity, and pulse duration, the desired effect can be maximized.
Photomechanical reactions: Lasers may be continuous wave (CW) or pulsed. A CW laser emits a continuous stream of light as long as the medium is excited. A pulsed laser will emit light only in pulses, which may vary from femtoseconds (quadrillionths of a second) to seconds. The simplest way to pulse a laser is to use a mechanical shutter, similar to that in a camera, which works down to the millisecond range. Flashlamps (similar to those used in photographic strobe lights) can also be used to produce low millisecond range pulses. Pulses in the micro- to nanosecond range are produced using Q-Switching.
In Q- (or Quality) Switching, a crystal which rotates the polarization of light with very short pulses of applied high-voltage, called a Pockels Cell, is placed in the laser cavity in front of the reflective mirror, with a suitable polarizing filter to block excited photons. The laser medium can be maximally excited, and when voltage is applied to the Pockels Cell, its polarization rotates to match that of the polarizing filter, and photons pass through to the mirror to stimulate a very short, very high energy laser pulse.
Q-switching a pulsed laser increases its peak power (P=E/t), shortens the output pulse width, and improves the consistency of the ouput power from pulse to pulse. However, there is almost always a net reduction in the average output energy compared to free-running pulsed mode. In free-running pulsed laser operation, lasing starts as soon as there's enough of a population inversion to trigger stimulated emission and resonance within the laser cavity (resonator). Each pulse will be somewhat different in total power and shape than every other (which is why pulsed laser power output is expressed in joules rather than watts). In effect, a q-switch disables the laser resonator until the population inversion is complete.
The easiest way to disable a laser resonator is to block the path to one of the mirrors-in this instance, the medium is pumped, but there can be no stimulated emission until the Q of the resonant cavity is restored, resulting in a brief but intense pulse of laser energy. Q-Switching can be accomplished simply by taking a mirror (typically the HR) out of alignment, usually by mechanically rotating the mirror. Most medical lasers use an electro-optical Q-switch (a Kerr or Pocket Cell) placed in the path of the beam within the resonator. In this instance a delay circuit opens the switch a preprogrammed time after the rod is pumped. A third method is to use a storable absorber as a "passive" Q-Switch. Below a certain threshold, these materials block light below a certain threshold, preventing the cavity from resonating. Above that threshold, the material becomes optically transparent to the particular laser wavelength, allowing lasing to occur. This process can repeat itself producing a series of ultra short, high power laser pulses.
High energy, ultra short pulses of laser light cause extremely rapid heating of the target, with formation of a rapidly expanding thermal plasma. As the plasma collapses, the shock wave causes mechanical disruption of the target. This photomechanical disruption is utilized by Q-Switched Lasers to treat tattoos and certain pigmented skin lesions.
Photothermal reactions: When laser energy is absorbed by a chromospheres, heat is dissipated in the target. Depending on the exposure time, tissue vaporization, or coagulation, or both will take place. The best example of a photothermal laser is the CO2 laser, used to cut and vaporize tissue, which mostly consists of water. Water, and thus soft tissue, vaporizes at 100 degrees C. When the laser hits soft tissue, rapid heating causes the water in the tissue to flash into steam, ablating the tissue.
Collateral damage is produced, including charred debris necrosis, and thermal damage in a zone around the ablation crater, caused by sub-vaporizing lateral transfer of energy and beam scatter. To minimize thermal damage, and maximize the ablation, a short exposure time is necessary. This can be done by either pulsing the laser beam, or scanning a continuous beam in such a way that the time it dwells over the tissue is less than the time it takes for 50% of the laser energy to be thermally conducted to surrounding tissue. This thermal relaxation time (about 600-800 microseconds for skin) allows all of the laser energy to be delivered to the area equivalent to the depth of penetration of the beam, minimizing collateral thermal damage. This type of photothermal reaction is utilized in surgical laser applications (such as for LAUP for snoring), laser resurfacing, laser hair removal, and treatment of vascular lesions.
Photochemical reactions: Laser energy can react chemically with specific molecules within tissue. The use of the noble gas-halide, or Excimer, lasers for modifying the shape of the cornea in LASIK procedures is based on this ultraviolet (UV) laser's ability to break covalent bonds in protein.
In Photodynamic Therapy (PDT), a photosensitizing drug is administered, which is selectively absorbed by tumor cells. When irradiated with the appropriate wavelength of laser light, a chemical reaction takes place, releasing a toxic substance (usually the highly reactive singlet oxygen) which selectively destroys the tumor. Until recently, the use of PDT was limited by the lack of easily administered, sufficiently selective photosensitizing drugs, and difficulties with laser delivery and dosimetry. PDT agents and techniques have recently been FDA approved for the treatment of some skin cancers, precancerous lesions, obstructing tumors in the esophagus and bronchi, as well as intractable cases of psoriasis, but widespread application has been slow because of poor insurance reimbursement.
Simple in theory, Photodynamic Therapy (PDT) has a "magic bullet" appeal as a therapy for various disorders. A photosensitizing drug is administered, localizes in the diseased area, and is excited by photons in the presence of molecular oxygen. Toxic singlet oxygen is generated by the excited photosensitizer, and is expended locally, without systemic effects. Most photosensitizers used clinically are precursors or derivatives of porphyrin, a chemical ring widely distributed in nature, found in hemoglobin, cytochromes, chlorophyll, etc. Porphyrins are capable of capturing photons and transferring the energy to oxygen molecules, converting relatively unreactive triplet oxygen to highly reactive singlet oxygen. Peak absorption of porphyrins is in the 400-650nm range, and they may be excited by broadband, narrowband, or monochromatic laser light. The exact parameters (wavelength, exposure time) and delivery method (direct, fiber, endoscopic, etc. are determined by the clinical situation.
The first photosensitizer used clinically over 20 years ago was hematoporphyrin derivative (HPD), a mixture of various porphyrins derived from hemoglobin containing mono-, oligo-, and polymers. Although a powerful sensitizer, HPD was not well localized by tumors. Purified HPD, or Photofrin, contains primarily mono and polymeric material, is well localized by tumors, and is a powerful photosensitizer. The principal disadvantage of Photofrin is that it has a long half-life after administration, and patients so treated must avoid strong light, including indoor fluorescent and sunlight, for weeks after treatment to avoid photosensitivity reactions.
More recently, aminolevulinic acid (ALA) has been used as a photosensitizer. ALA is a natural occurring compound used metabolically as a porphyring precursor. Topically applied ALA induces a rapid synthesis and actual overproduction of protoporphyrin IX, a naturally occurring precursor of heme (as in hemoglobin), and a strong photosensitizer. Protoporphyrin IX is rapidly broken down or converted to heme within a few hours after treatment, so prolonged photosensitivity is avoided.
Delivery of the photosensitizer and the photonic energy remains problematic in many cases.
As of this writing, PDT has FDA-approved indications for the treatment of "wet" macular degeneration, and precancerous skin lesions. Many other applications, including the treatment of acne, and photo aging are currently being investigated.
Selective Photothermolysis is the process in which transfer of laser energy is restricted to a particular site because of the selective absorption of a chromospheres at that site. In other words, proper selection of the wavelength and exposure time damages only the desired target tissue. This principal is what distinguishes lasers from many other tools in the surgeon's armamentarium.
For selective photothermolysis to occur, the surgeon must choose:
• The appropriate Wavelength selectively absorbed by the target tissue
• The appropriate Exposure time which should be less than the thermal relaxation time of the target tissue
• The appropriate Energy density , or Fluence to produce the desired effect, such as vaporization, coagulation, or photodisruption.
The principal of Selective Photothermolysis was proposed in 1983 by R. Rox Anderson and Simon Parrish. They described the selective absorption and thermal injury of a chromospheres target based on the length of the laser pulse. The thermal relaxation time was defined as the time it takes for a target structure to dissipate 50% of the energy absorbed to surrounding tissue, and that this time was roughly equal to the square of the diameter of the target structure.
The thermal containment time is that time in which no heat (and hence no thermal effect) is dissipated to surrounding tissue, and is roughly one-quarter of the thermal relaxation time. This TCT defines the ideal pulse width for treating a given chromospheres.
To perform a laser procedure properly, the surgeon uses the laser with the right wavelength, then selects the appropriate Fluence and Exposure time to achieve a selective photothermolytic, photomechanical, or photochemical effect on the target.
Effect of Laser Spot Size on Tissue Distribution of Light Energy: A beam of light incident on tissue may be reflected, absorbed, or scattered. Scattering in tissue broadens the incident beam, decreasing the effective fluence in the intended target area. Doubling the spot size will increase the effective volume by a factor of eight.
A larger spot size usually enables faster and more effective treatment in dermatologic applications such as treatment of vascular lesions, laser hair removal, etc. However, more photons must be supplied by more complex and expensive power supplies, components, and delivery devices.
As a general rule, doubling the spot size and halving the fluence will yield an equivalent effective fluence at a given depth. This effect become more pronounced with increasing depth.
Monte Carlo modeling is used to predict the distribution of light in tissue, taking into account absorption, incident and external reflection, and scattering.