The Excimer Laser andApplications to Corneal

Ablative SurgeryPeter M. Albrecht

November 28, 2001ECE 355 Final Project

University of Illinois at Urbana-Champaign

Introduction• Excimer – “Excited Dimer”

– A diatomic molecule in which the constituent atoms arebound only in an excited state, while exhibiting mutualrepulsion at all distances in the ground state.

– Excimer molecules critical to laser applications consistof a halogen atom and a noble gas atom.

– Excimer lasers emit photons in the UV (ultraviolet)region of the electromagnetic spectrum

ArF (λ = 193 nm)KrF (248 nm)XeCl (308 nm)XeF (353 nm)

ArF Excitation Mechanism• Pre-Ionization: pulse of light from a UV flashboard; provides the

“seed” electrons in the gain medium which enhance conductivity.

• Electrical discharge: excitation pulse that produces the ionsand excited atoms that form the ArF excited state.

Ar+ + F- → ArF*Ar* + F* → ArF*

ArF* → hυ + Ar + FUpper level lifetime for ArF* ~ 10 nsLower level lifetime for ArF ~ 0 (rapid dissociation)λ = 193 nm ⇒ Ephoton = hc/λ = 6.437 eV

Laser Focus World, 34(7) July 1998

R = rare gas atom

H = halogen atomHorizontal lines containedwithin the potential wellrepresent the variousvibrational modes for theRH* excimer molecule.

Usually have a positive noblegas ion (high ionizationenergy) with a negativehalogen ion (high electronaffinity).

r0

r0 = equilibrium interatomic spacing = 2.2

ArF Laser Characteristics• Pulsed operation: pulse length ranging from several ns to tens

of ns; on the order of the upper level (excited state) lifetime.

• Pulse Energy: 0.1 – 1.0 Joules per pulse

• Fluence: ratio of deposited energy to cross-sectional area of laserbeam, a measure of the energy density per pulse (units J/cm2)

• Repetition rate: tens to hundreds Hz.

Average power = (pulse energy)(repetition rate)Initially, <P> increases linearly with repetition rateEventually, <P>max is obtained, and further increases in repetition rate will

decrease <P>, due to the more rapid decrease in the pulse energy athigher repetition rates.

Hardware and Cavity Parameters

GAIN MEDIUM

lgain = 50 – 100 cm

Rear reflector Output coupler

R ~ 100% R ~ 5%

1) Steel structure with polyvinyl and Teflon coated components –fluorine and other halogen gases are highly corrosive.

2) The high internal gain and nearly “superradiant” nature of the ArFexcimer laser allows for the normal reflectivity of an uncoated opticalsurface to be sufficient for laser oscillation.

3) Windows typically made of MgF2 or CaF2, both of which have ahigher transmission coefficient relative to quartz for 193 nm operation.

Al-MgF2 thin film reflective coating

Uncoated optical surface

Gas Parameters• Total chamber pressure: typically less than 5 atm• Sample composition of an ArF chamber at 2 atm:1.90 atm (95%) buffer gas (e.g., He or Ne)0.09 atm (4.5%) Ar0.01 atm (0.5%) F2 (halogen donor – dissociates to atomic fluorine)

• Degradation of laser gas during operation• Many cavities equipped with a vacuum pump for gas

recirculation and purification• Mixture Lifetime w/o cleaning and purification:

105 – 106 pulses = 103 – 104 s @ 100 Hz. (continuous operation) = 0.278 – 2.78 hours

Excimer Laser Discovery and Development• 1977: German manufacturer Lambda Physik introduces the

first commercial excimer laser.• 1981: Rangaswamy Srinivasan of IBM Research observed

that an excimer laser is capable of etching a smooth groove,compared to simply burning the material. Because of theirability to precisely ablate material, excimer lasers areexcellent tools for corneal reshaping.

Ultraviolet photons emitted by excimer lasers havesufficient kinetic energy to sever intermolecularbonds, resulting in the subsequent vaporization ofthe solid material.

Because the interaction time between the laser andthe substrate is on the order of nanoseconds,detrimental heat conduction to unablated regions isminimal.

Human Hair

100 µm

Canadian Medical Association Journal, 160(9) May 1999

Ophthalmological Problems

Canadian Medical Association Journal, 160(9) May 1999

Canadian Medical Association Journal, 160(9) May 1999

Near-SightednessThe radius of curvature for thecornea is too small, and thisincreased refractive powerfocuses the light in front of theretina.

Far-SightednessThe radius of curvature for thecornea is too large, and thisdecreased refractive powerfocuses the light behind theretina.

Laser Refractive Surgery• By selectively ablating corneal material using a pulsed excimer laser

beam, it is possible to alter the radius of curvature of the cornea.• Refractive Power: 60 - 70 % from cornea, 30 - 40 % from lens

• PRK: photorefractive keratectomy.– After removal of the epithelium (surface layer), the pulsed ArF laser beam

is applied to the cornea.– The pulse energy is deposited in a shallow region, and a combination of

photochemical and thermal processes occurring in the interaction regionresults in the ablation of corneal tissue.

– The ablated material is expelled by a recoil action.– Criteria for Ablation: If the absorbed fluence Φ exceeds the threshold

energy density Φ0 for bond breakage at a distance d below the cornealsurface, the material at that depth will be ablated. Φ0 ~ 50 - 75 J/cm2

– <∆T> = +80°C over the ablation volume– ArF: least damage to residual tissue among various excimer wavelengths

PRK Correction of Myopia (near-sightedness)

Material is removed from the central region of the cornea, resulting ina flatter profile and a larger radius of curvature.

Broad beam lasers: Ablation begins at the center of the cornea, and adiaphragm is used to widen the beam during the surgical procedure

Scanning and Flying-Spot Lasers: Laser pulses are applied in a morespatially random manner, with the goal of producing a smootherablation profile

Canadian Medical Association Journal, 160(9) May 1999

PRK Correction of Hyperopia (far-sightedness)

Material is removed from an outer ring of the corneal periphery,resulting in a steeper profile and a smaller radius of curvature.

This increases the refractive power of the eye, allowing the patient toexperience an enhanced degree of short-range vision (operating acomputer terminal, reading a book, etc.)

Canadian Medical Association Journal, 160(9) May 1999

LASIK• Laser-Assisted in situ keratomileusis

– Prior to the application of the ArF laser beam, a microkeratome is usedto cut a thin, hinged flap away from the corneal surface.

– Microkeratome: a motorized cutting blade utilizing a vacuum suctionmechanism to adhere to the cornea and produce a partial thickness cut.

– The cut for the surface flap is approximately 130 - 160 µm deep.– The laser beam is applied directly to the sub-surface stromal tissue,

preserving the integrity of the outer corneal layer.– Absorption depth for pulsed laser energy: approximately 2 µm– Φ = 205 mJ/cm2 will remove stromal tissue at 0.55 µm per pulse.– The stromal surface layer is heated to ~500°C over the multiple

nanosecond pulse duration.– First LASIK procedure conducted in 1990, 3 years after the clinical

introduction of PRK in 1987.

• What are the advantages of the LASIK procedure?faster recovery time relative to PRK (several days instead of 1-2 weeks)less physical discomfortdecreased post-operative haze and regression

Canadian Medical Association Journal, 160(9) May 1999

LASIK Procedure

• What about the disadvantages? Contaminant particles trapped underneath corneal flap upon replacement Misalignment and/or wrinkling of the flap

Bausch & Lomb Technolas 217A Excimer LaserFlying Spot technology - successive pulses are applied to different regions of the cornea,resulting in a more spatially uniform ablation profile.

Technical Specifications:

λ = 193 nm Repetition Rate = fr = 50 Hz Pulse duration = tp = 18 ns

Fluence @ eye = Φ = 120 mJ/cm2 Beam Diameter = d = 2 mm

Calculations:

Cross-sectional area of laser beam = Abeam = 0.25πd2 = 3.14*10-2 cm2

Energy per pulse = Ep = Φ• Abeam = (120 mJ/cm2)(3.14*10-2 cm2) = 3.768 mJ

Average Power = <P> = Ep•fr = (3.768 mJ)(50 Hz) = 0.1884 W

Additional Laser Characteristics:

- Internal Cooling

- Power Requirements: 220-230 VAC/16A 50/60 Hz

- System weight = 600 kg System dimensions: 2.6m W x 1.2m D x 1.5m H

PlanoScan 2000 software: Implements a flying spot LASIK algorithm, which ensuresthat two consecutive laser pulses will not intercept the same region of the cornea. Thisdecreases the thermal load to the cornea, minimizing the risk of post-operative trauma andedema.

http://www.bausch.com/us/resource/surgical/refractive/technolas.jsp

Lambda Physik COMPex 205 Excimer LaserBroad beam laser system, suitable for low to medium power applications.

Technical Specifications:

λ = 193 nm Max. Repetition Rate = fr = 50 Hz Pulse duration = tp = 20 ns

Maximum Pulse Energy (measured at low repetition rate) = Ep,max = 400 mJ

Average Power @ 50 Hz = <P> = 15 W

Calculate the Pulse Energy for Operation at the Max. Repetition Rate:

Ep(50 Hz) = <P>/fr = (15W)/(50 Hz) = 300 mJ

Thus, the energy per pulse at 50 Hz operation is 75% of the maximum pulse energy.

NovaTube technology- Innovative metal/ceramic laser tube design,which helps to negate the corrosive effect ofthe halogen species.

- Increases the number of pulses for a specificgas mixture.

- 1995 Best Innovation of German Industry

http://www.lambdaphysik.com/productspectrum/index.asp

Specific Advantages of Lasers in Ophthalmic Applications

• Acute response by means of narrow lines and high spatial coherence(the ability to narrowly focus the beam to a small spot).

• Minimal thermal effects by matching of the frequency spectra forabsorption.

• Commercial excimer lasers have precisely specified output fluences(J/cm2), so it is possible to effectively calibrate the thickness of ablatedcorneal material per laser pulse.

• Control the time duration of the pulse.• Minimal Operative Time: Typically, the laser beam is in operation

for only 1-2 minutes per eye. The exact time required for excimerlaser surgery is proportional to the degree of visual impairment,because a larger volume of ablated material is required to properlyreshape the cornea.