Excimer Laser Technology: How It Works and Its Role in LASIK

What Is an Excimer Laser?

An excimer laser (short for "excited dimer") is a type of gas laser that produces ultraviolet light in the deep UV spectrum. The word "excimer" refers to a diatomic molecule that exists only in an excited energy state — the argon fluoride (ArF) gas mixture used in ophthalmic excimer lasers is one such excited dimer. When an electrical pulse is applied to the ArF gas mixture, the excited dimer forms briefly and emits UV light at a wavelength of exactly 193 nanometers as it returns to the ground state. The laser operates in pulses at rates typically between 200 and 1,050 pulses per second depending on the platform.

The 193 nm wavelength is uniquely suited to corneal surgery because it is absorbed by the corneal stroma's peptide bonds — the chemical bonds holding the collagen amino acid chains together — with exceptional precision and without penetrating to deeper tissues or generating significant heat.

Photoablation: How Tissue Is Removed Without Heat

The excimer laser removes corneal tissue through a process called photoablation (or photodecomposition). Rather than burning or melting tissue as an infrared laser would, the 193 nm UV photons carry enough energy to directly break the carbon-carbon and carbon-nitrogen bonds in the collagen molecules. Each photon disrupts a molecular bond; the broken molecular fragments are ejected from the surface at supersonic velocity as a plasma plume visible as a tiny flash of ultraviolet light. The entire process occurs in nanoseconds with no heat diffusion to surrounding tissue — adjacent cells are undamaged and the procedure produces essentially no thermal injury.

This precision is why the excimer laser is uniquely capable of reshaping tissue to the required curvature changes — it can remove quantifiable amounts of tissue layer by layer to micron-scale accuracy.

Precision: 0.25 Microns per Pulse

Each excimer laser pulse removes approximately 0.20–0.25 microns (200–250 nanometers) of corneal stroma — less than 1/200th of a millimeter per pulse. The total ablation for a typical LASIK treatment involves thousands to tens of thousands of pulses, depending on the prescription and optical zone size. Key specifications:

• Tissue removal per pulse: ~0.25 microns (platform-dependent)
• Pulse duration: 10–30 nanoseconds
• Repetition rate: 200–1,050 Hz (pulses per second)
• Treatment time: 20–60 seconds per eye (prescription-dependent)
• Spot size: typically 0.9–1.0 mm per pulse (flying-spot scanning systems)
• Total tissue removal: 10–15 microns per diopter of myopic correction (approximate)

Eye Tracking Systems

During LASIK, the eye can move — small involuntary microsaccades, drift, and occasional larger movements occur even with a cooperative patient looking at a fixation target. Modern excimer lasers address this with sophisticated real-time eye tracking systems:

• Speed: most modern trackers operate at 500–1,050 Hz (measurements per second), capturing eye movements faster than the laser pulse rate
• Tracking dimensions: X-Y lateral tracking (horizontal and vertical movement) and torsional tracking (rotational movement around the visual axis — critical for astigmatism treatment)
• Response: the laser beam's delivery position is adjusted with each pulse to compensate for detected movement, or the laser pauses if movement exceeds safe thresholds
• Cyclotorsion correction: torsional eye tracking corrects for the eye rolling slightly during LASIK flap suction, which would otherwise misalign the astigmatism axis

FDA-Approved Excimer Laser Platforms

Platform	Manufacturer	Key Features
STAR S4 IR / iDESIGN	Abbott/Johnson & Johnson	Wavefront-guided; 1,050 Hz tracker; iris registration
WaveLight EX500 / Allegretto	Alcon	1,050 Hz tracker; topography-guided (Contoura); fast treatment
MEL 90 / MEL 80	Carl Zeiss Meditec	CRS-Master wavefront; Flying-Spot 250 Hz
Technolas 217z / Teneo	Bausch + Lomb	PlanoScan; 400 Hz tracker
Schwind Amaris	Schwind	1,050 Hz tracker; 7D eye tracking including cyclotorsion

How Treatment Type Affects the Ablation

The same excimer laser hardware can deliver different ablation profiles depending on the treatment planning software:

• Standard (manifest refraction): uniform circular ablation based on glasses prescription; treats sphere and cylinder
• Wavefront-optimized: adds pre-programmed adjustment to minimize spherical aberration induction; better than standard
• Wavefront-guided: individualized ablation from Hartmann-Shack measurement of patient's specific aberrations; highest customization
• Topography-guided: individualized ablation from corneal surface map data; addresses corneal irregularities

See wavefront-guided LASIK and topography-guided LASIK.

History: From IBM Lab to LASIK

The excimer laser's application to corneal surgery originated with IBM researcher Rangaswamy Srinivasan, who in 1981 discovered that the ArF excimer laser could ablate organic tissue with extraordinary precision. Ophthalmologist Stephen Trokel recognized the potential for corneal surgery and collaborated with Srinivasan to publish the first description of excimer laser corneal ablation in 1983. The first human PRK procedures were performed in 1987–1988, leading to FDA approval of PRK in 1995. LASIK using the excimer laser for the stromal treatment received FDA approval in 1999. Today, over 700,000 LASIK procedures are performed annually in the United States using descendants of the same ArF excimer laser technology Srinivasan and Trokel pioneered.