Basic Science of Lasers by mzq79210



                                                        l    Participants should:
    Basic Science of Lasers                                  » Be familiar with the basic principles for
                                                               the generation of laser light
                                                             » Learn the properties of laser light
      Stephen R. Tan, MD FRCPC                               » Understand the biologic response of
                                                               tissue to laser impact
Director of the Division of Dermatologic Surgery
   HealthPartners Medical Groups & Clinics
                 November 2007

                                                             Absorption & Spontaneous
                    History                                          Emission
l   Laser concept postulated by Einstein in             l    Atoms & molecules are normally in the
    1917:                                                    “resting state”
    » “The Quantum Theory of Radiation”                      » This is the most stable position for a nucleus
                                                               and surrounding electrons
l   First laser developed by Maiman in 1960:            l    Light can be absorbed by atoms and move
    » Stimulated visible light emission with ruby            electrons from a resting state to an excited
      crystals                                               state
l   Called the device a LASER:                          l    Frequency & wavelength of emission and
                                                             absorption are proportional to the change
    » Light Amplification by the Stimulated                  in energy
      Emission of Radiation

       Excitation of Electrons                                 Generation of Photons
                                                    l   The “excited state” is unstable
                                                            » Electrons spontaneously return to the
                                                              “resting state”
                                                            » During this process, they re-emit the energy
                                                              used to push them to the “excited state”
                                                    l   Released energy (“radiation”) is the
                                                        “spontaneous emission” of LASER
                                                    l   May be released as EM radiation (light)
                                                        which travels in packets called photons
         Amplification of Photon                     Amplification of Photon
              Generation                                  Generation

    l   When already excited atoms are
        irradiated with the same energy
        again, it generates two waves of light
        energy with:
        » Same frequency and wavelength
        » Traveling in the same direction
        » Perfect spatial and temporal phase

        How does this happen in a
                 laser?                              Excitation in the Laser
    l   Atoms are placed into a cavity called
        the “optical cavity” or “resonator”
        » Lasers are named for the medium in the
          cavity (ex. CO2, pulsed “rhodamine” dye,
          ruby, Nd:YAG etc)
        » The cavity is electrically charged when
          you turn the laser on
    l   Atoms become excited or “pumped”
        to an excited state by the charge

                                                     Photons Generating more
          Population Inversion                               Photons
l   When the majority of the atoms in the
    resonator are excited, there is a
    “population inversion”:
    » Increasing likelihood of spontaneous emitted
      photons traveling along the long axis of the
      cavity will collide with other atoms
l   Remember that these other atoms are
    already in the “excited state”
    » Stimulated emission produces even more
      photons of the same frequency traveling
      along the same axis
             Amplification of Photon
                  Generation                                                 All Smoke & Mirrors…
l       Reflecting mirrors are placed at either end of the
        » One is totally reflecting, the other partially so
l       Light travels back and forth within the cavity:
        » Promotes further stimulated emission
        » Amplifies the whole process
l       When you press the trigger:
        » The partially reflective mirror allows a small portion
          (~5-10%) of the light out of the cavity
        » Emitted through the handpiece as laser light

           Properties of Laser Light                                      Electromagnetic Spectrum
    l   Monochromaticity: Lasers produce pure
        band(s) of light:
         » Emissions are spectrally very narrow
         » Wavelength depends on the medium used to
           generate the light
         » Also called “temporal coherence”
    l   Monochromaticity allows selective targeting of
        specific chromophores in the skin
         » Different chromophores absorb light of different
         » Allows us to choose the right laser to target the right

         Lasers in the EM Spectrum                                          Properties of Laser Light
                                                                     l   Spatial Coherence: Lasers are highly directional
                                                                         and orderly
                                                                         » Light waves are in phase
                                                                         » All crests & troughs of the light are synchronous
                                                                     l   This is why laser light has a low degree of
                                                                         » Allows laser light to travel great distances while
                                                                           maintaining intensity
                                                                         » Typical laser diverges ~1mm for every meter traveled
                                                                         » The smaller the spot size, the greater the divergence
               Coherence                               Properties of Laser Light

                                                   l   Brightness: Lasers generate
                                                       tremendously high powers
                                                       » Results from the amplification process
                                                   l   High powers with low beam
                                                       divergence = VERY BRIGHT LIGHT!

      Laser Power in Clinical                            Laser Power in Clinical
              Terms                                              Terms
                                               l   Power density: Power at the tissue level
l   Energy: Energy is measured in joules           » Measured as W/cm2 [or (joule/sec)/cm2]
    » Directly proportional to the number of       » The spot size selected determines the area of
      photons in the laser beam → more               the laser striking the skin
      photons = more energy                        » Power density determines the rate of thermal
l   Rate of energy delivery: AKA power               damage → increasing power density increases
                                                     rate of tissue damage
    output is measured in watts
                                               l   Fluence: When the pulse duration is
    » 1 Watt = 1 Joule/second
                                                   known, the laser can multiply this by the
                                                   power density
                                                   » W/cm2 * sec = J/cm2

           Pulse Duration                              Continuous Wave Lasers
                                               l   CW lasers
l   Pulse duration may be set or variable
                                                   produce a
    » May be set by the laser design
                                                   continuous beam
    » May be varied to achieve different
                                                   of laser light
                                               l   No variation over
l   Produced by various optical,
    mechanical, and electrical designs
                                               l   Ex. Original CO2
                       Pulsed Lasers            Superpulsed Lasers
    l    Produce individual                                        l   Specific term for
         pulses of laser                                               CO2 lasers
         light                                                         » Used in the
    l    Energy builds,                                                  Ultrapulse laser
         peaks, and tapers                                         l   Laser produces
         off                                                           very short pulses
    l    Higher power than                                             with a very high
         CW lasers                                                     peak power
    l    Ex. PDL                                                       » Reduces amount
                                                                         of collateral
         » Millisecond domain
                                                                         thermal damage

           Quality (Q)-Switched Lasers              Laser Delivery
l       Further shortens
        pulse duration
        » Uses extremely                 l   Either through an articulated arm or
          fast EM or
          chemical switch                    an optical fiber
l       Allows buildup of
        excessive energy
        in laser cavity
        before discharge
        » Q-switched Ruby
          produces power of
          1,000,000 W/cm2!
l       Delivers very short
        single pulses of
        extremely high
        » Nanosecond

                    Articulated Arm                  Optical Fiber
     Beam Intensity Profiles                           Beam Intensity Profiles

l   Energy distribution over a cross-
    section of the beam
    » “Top hat”: Uniform intensity across the
      diameter → theoretically ideal, but not
      necessarily the most practical
    » Gaussian: Peak energy in the center of
      the spot, energy decreases towards the

                                                          Overlapped vs. Non-
      Why does this matter?                                Overlapped Pulses

l   You can get away with overlapping
    the pulses with a Gaussian
    distributed beam
l   You can burn the patient if you
    overlap the pulses with a “top hat”

       Lasers and the Skin                            Laser Tissue Penetration
                                                l   Generally, the depth of penetration
                                                    increases with longer wavelengths
                                                l   The exceptions are lasers that target water
                    Theory of Selective                                             Choose a Specific
                     Photothermolysis                                                Chromophore
    l   Postulated in 1983 by Anderson & Parish
                                                                       l    Light is absorbed in tissue by
    l   Selective tissue damage is possible if:                             chromophores
         » A specific wavelength is chosen which is
           absorbed by the target chromophore
                                                                       l    Major chromophores in the skin are
                                                                            water, melanin, and hemoglobin
         » Pulse duration is shorter than the thermal
           relaxation time (i.e. cooling time) of the                        » Each has wavelengths where the
           target                                                              chromophore more efficiently absorbs
                                                                               laser light
    l   The goal is selective thermal damage to                              » Modern lasers use this fact to generate
        the target WITHOUT non-specific thermal                                light that will be absorbed by a specific
        damage to surrounding structures                                       chromophore

              Hemoglobin & Melanin                                     Choose a Laser with the Right
                Absorption Curves                                            Pulse Duration
                                                                   l   Thermal relaxation time of cutaneous
                                                                       vessels is between 1 – 10ms
                                                                           » Pulsed dye and KTP lasers have adjustable
                                                                             pulse durations within and beyond this range
                                                                   l   Thermal relaxation time of melanin is 1µs
                                                                           » Q-switched Nd:YAG, Ruby and Alexandrite all
                                                                             have pulse durations on the order of a few

                       Pulse Stacking                                               Vascular Lasers
l       Newer pulsed dye lasers have pulse widths well
        beyond the thermal relaxation time of vessels
l       The idea is to deliver higher cumulative fluence
        through pulse stacking of lower energy pulses
        » Effective fluence = fourth root of # of stacked pulses
                                     fluence used
l       Ex. If a given area is treated with 16 pulses, the
        effective fluence is two times the actual fluence
        » Stacked pulses at a lower fluence can cause
          equivalent thermal damage to a single pulse at high
           Selective Targeting of
              Chromophores                                                 Laser-Tissue Interaction
                                                                  l   When laser light is absorbed by a target
                                                                      chromophore, there are three possible
                                                                      biologic effects:
                                                                       » Photothermal: Effects resulting directly from
                                                                         heat and thermal damage
                                                                       » Photomechanical: Rapid temperature change
                                                                         of the chromophore results in sudden thermal
                                                                         expansion, tissue vaporization, shock wave,
                                                                         or pressure wave formation
                                                                       » Photochemical: Light reacts with an
                                                                         endogenous or exogenous photosensitizer
                                                                         (ex. PDT)

         Photothermal Damage                                                     Heating the Skin
    l   Absorption of laser light results in                           l   The goal of laser therapy is to heat up the
        transformation of light to heat                                    specific target that you want to destroy
                                                                           » < 50oC: Reversible thermal damage
    l   The site and rate of absorption are                                » 50o – 100oC: Most tissues undergo denaturation
        how laser damage is targeted:                                        or irreversible coagulation of proteins
                                                                           » > 100oC: Tissue vaporization occurs
        » The site of heating is determined by the
                                                                       l   Heating targets beyond the reversible
          laser wavelength and absorbing                                   damage threshold results in coagulative
          chromophore                                                      necrosis of epithelial or connective tissue
        » The rate of heating is determined by the                     l   Tissue repairs itself with granulation tissue
          fluence and pulse duration                                       at the site of the original damage

         Heating < 100oC: Dermal                                                   Heating > 100oC:
               Remodeling                                                           Vaporization
l   Used as “non-ablative resurfacing”                        l       Tissue water is heated above the boiling
l   Uses 1320nm Nd:YAG or 1450nm diode lasers                         point → vaporizes into a plume of water
    » Infrared lasers targeting tissue water                          vapor and tissue
l   Initiates and enhances dermal collagen
    » Uses an epidermal cooling device to minimize
      epidermal water heating (kept below 50oC)
    » Thermal injury localized to papillary and upper
      reticular dermis
    » Induces dermal collagen remodeling, with clinically
      mild or moderate atrophic scar or wrinkle improvement
      Fractionated Resurfacing                                            Photomechanical Effects
l   Newer lasers selectively ablate multiple small              l   Produces immediate disruption of viable
    targets within a larger field                                   tissue at the cellular level
l   Speeds healing time versus conventional
                                                                l   May be thermal or non-thermal:
    » Multiple small areas to heal as opposed to complete
                                                                        » Thermal: Sudden heating causes steam
      epidermal + dermal loss                                             formation → PDL causes a 100oC
    » Surrounding skin is intact to provide cells for wound               temperature increase in 450µs, rupturing
      healing                                                             vessel walls and causing purpura
l   May have decreased incidence of side effects                        » Non-thermal: Laser pulses of short duration
l   Multiple treatments potentially needed                                and high peak power (Q-switched) causes
                                                                          shock waves, cavitation, and vaporization

       Photomechanical Effects                                      What else has chromophores?
                                       l   How fast does
                                           the Q-switched           l    What’s the difference between these
                                           laser heat the                fancy sunglasses?
                                       l   10 billion oC per
                                           second!!! (but
                                           the laser only
                                           fires for 20-40
                                       l   At that rate, the
                                           tattoo or
                                           granules literally

     The covered wavelengths!                                                          Conclusion
                                                                    l    Dermatologic laser therapies all have a
                                                                         scientific rationale behind them
                                                                    l    The proper laser medium generates light
                                                                         with the proper:
                                                                          »   Wavelength
                                                                          »   Intensity
                                                                          »   Pulse duration
                                                                          »   Depth
                                                                    l    Allows us to SELECTIVELY damage
                                                                         targets in the skin
                                                                    l    Know what you’re settings mean and what
                                                                         you’re shooting at before pulling the

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