The Excimer Laser and Applications to Corneal Ablative Surgery

Transcription

1 The Excimer Laser and Applications to Corneal Ablative Surgery Peter M. Albrecht November 28, 2001 ECE 355 Final Project University of Illinois at Urbana-Champaign

2 Introduction Excimer Excited Dimer A diatomic molecule in which the constituent atoms are bound only in an excited state, while exhibiting mutual repulsion at all distances in the ground state. Excimer molecules critical to laser applications consist of 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)

3 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 ions and excited atoms that form the ArF excited state. Ar + + F - ArF* Ar* + F* ArF* ArF* hυ + Ar + F Upper level lifetime for ArF* ~ 10 ns Lower level lifetime for ArF ~ 0 (rapid dissociation) λ = 193 nm E photon = hc/λ = ev

4 R = rare gas atom H = halogen atom Horizontal lines contained within the potential well represent the various vibrational modes for the RH* excimer molecule. r 0 Laser Focus World, 34(7) July 1998 Usually have a positive noble gas ion (high ionization energy) with a negative halogen ion (high electron affinity). r 0 = equilibrium interatomic spacing = 2.2

5 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: Joules per pulse Fluence: ratio of deposited energy to cross-sectional area of laser beam, a measure of the energy density per pulse (units J/cm 2 ) Repetition rate: tens to hundreds Hz. Average power = (pulse energy)(repetition rate) Initially, <P> increases linearly with repetition rate Eventually, <P> max is obtained, and further increases in repetition rate will decrease <P>, due to the more rapid decrease in the pulse energy at higher repetition rates.

6 Hardware and Cavity Parameters Rear reflector Al-MgF 2 thin film reflective coating Uncoated optical surface GAIN MEDIUM Output coupler l gain = cm 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 ArF excimer laser allows for the normal reflectivity of an uncoated optical surface to be sufficient for laser oscillation. 3) Windows typically made of MgF 2 or CaF 2, both of which have a higher transmission coefficient relative to quartz for 193 nm operation.

7 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%) Ar 0.01 atm (0.5%) F 2 (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: pulses = Hz. (continuous operation) = hours

8 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 their ability to precisely ablate material, excimer lasers are excellent tools for corneal reshaping. 100 µm Human Hair Canadian Medical Association Journal, 160(9) May 1999 Ultraviolet photons emitted by excimer lasers have sufficient kinetic energy to sever intermolecular bonds, resulting in the subsequent vaporization of the solid material. Because the interaction time between the laser and the substrate is on the order of nanoseconds, detrimental heat conduction to unablated regions is minimal.

9 Ophthalmological Problems Near-Sightedness The radius of curvature for the cornea is too small, and this increased refractive power focuses the light in front of the retina. Canadian Medical Association Journal, 160(9) May 1999 Canadian Medical Association Journal, 160(9) May 1999 Far-Sightedness The radius of curvature for the cornea is too large, and this decreased refractive power focuses the light behind the retina.

10 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: % from cornea, % 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 region results 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 Φ 0 for bond breakage at a distance d below the corneal surface, the material at that depth will be ablated. Φ 0 ~ J/cm 2 < T> = +80 C over the ablation volume ArF: least damage to residual tissue among various excimer wavelengths

11 PRK Correction of Myopia (near-sightedness) Canadian Medical Association Journal, 160(9) May 1999 Material is removed from the central region of the cornea, resulting in a flatter profile and a larger radius of curvature. Broad beam lasers: Ablation begins at the center of the cornea, and a diaphragm is used to widen the beam during the surgical procedure Scanning and Flying-Spot Lasers: Laser pulses are applied in a more spatially random manner, with the goal of producing a smoother ablation profile

12 PRK Correction of Hyperopia (far-sightedness) Canadian Medical Association Journal, 160(9) May 1999 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 to experience an enhanced degree of short-range vision (operating a computer terminal, reading a book, etc.)

13 LASIK Laser-Assisted in situ keratomileusis Prior to the application of the ArF laser beam, a microkeratome is used to cut a thin, hinged flap away from the corneal surface. Microkeratome: a motorized cutting blade utilizing a vacuum suction mechanism to adhere to the cornea and produce a partial thickness cut. The cut for the surface flap is approximately 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/cm 2 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.

14 LASIK Procedure Canadian Medical Association Journal, 160(9) May 1999 What are the advantages of the LASIK procedure? faster recovery time relative to PRK (several days instead of 1-2 weeks) less physical discomfort decreased post-operative haze and regression What about the disadvantages? Contaminant particles trapped underneath corneal flap upon replacement Misalignment and/or wrinkling of the flap

15 Bausch & Lomb Technolas 217A Excimer Laser Flying 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 = f r = 50 Hz Pulse duration = t p = 18 ns eye = Φ = 120 mj/cm 2 Calculations: Beam Diameter = d = 2 mm Cross-sectional area of laser beam = A beam = 0.25πd 2 = 3.14*10-2 cm 2 Energy per pulse = E p = Φ A beam = (120 mj/cm 2 )(3.14*10-2 cm 2 ) = mj Average Power = <P> = E p f r = (3.768 mj)(50 Hz) = W Additional Laser Characteristics: - Internal Cooling - Power Requirements: 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 ensures that two consecutive laser pulses will not intercept the same region of the cornea. This decreases the thermal load to the cornea, minimizing the risk of post-operative trauma and edema.

16 Lambda Physik COMPex 205 Excimer Laser Broad beam laser system, suitable for low to medium power applications. Technical Specifications: λ = 193 nm Max. Repetition Rate = f r = 50 Hz Pulse duration = t p = 20 ns Maximum Pulse Energy (measured at low repetition rate) = E p,max = 400 mj Average 50 Hz = <P> = 15 W Calculate the Pulse Energy for Operation at the Max. Repetition Rate: E p (50 Hz) = <P>/f r = (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 of the halogen species. - Increases the number of pulses for a specific gas mixture Best Innovation of German Industry

17 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 for absorption. Commercial excimer lasers have precisely specified output fluences (J/cm 2 ), so it is possible to effectively calibrate the thickness of ablated corneal 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 excimer laser surgery is proportional to the degree of visual impairment, because a larger volume of ablated material is required to properly reshape the cornea.