Surgery – Instruments – Light application
Reexamination Certificate
1999-08-10
2001-05-15
Cohen, Lee (Department: 3739)
Surgery
Instruments
Light application
C606S002000, C606S010000, C606S017000, C219S121800
Reexamination Certificate
active
06231566
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to scan a pulsed laser beam for surface ablation. In particular, the present invention relates to scan a pulsed laser beam of high repetition rate and small spot size to achieve smooth and uniform ablation.
BACKGROUND OF THE INVENTION
To achieve smooth and uniform surface ablation with a pulsed laser beam of high repetition rate and small spot size, a fast and smooth scanning and a proper scanning pattern are crucial. When an intense UV laser pulse impinges a corneal surface, for instance, a plume of decomposed tissue is ejected from the surface. This ejected material may affect the energy disposition of the next pulse. Besides, the stress and heat generated from the ablation process may build up if the pulsed laser beam is not scanned fast enough. Each pulse creates an ablated pit having a typical depth of a fraction of a micron. A uniform ablation profile can be expected only when these pits are arranged in a proper disposition pattern.
There are some 500 U.S. patents associated to scanning a laser beam. The present invention relates specifically to scan a pulsed laser beam for surface ablation. In particular, the present invention relates to scan a pulsed laser beam with a high repetition rate (about a kilohertz) and a small spot size (a fraction of a millimeter) for smooth and uniform surface ablation. A direct application of the present invention is to scan a pulsed laser beam for photo-refractive surgery on a cornea to correct vision disorders.
A few scanning methods have been proposed for photo-refractive surgery. In U.S. Pat. Nos. 4,665,913 and 4,718,418, L'Esperance Jr. presented a method to scan laser pulses with a uniform power over a squared cross section. The scanner is synchronized with the pulses to achieve precise disposition of the pulses. Lin demonstrated in U.S. Pat. No. 5,520,679 a method to achieve smooth ablation by accurately controlling the beam spot size and carefully overlapping the pulses in a single layer. A 100 Hz-pulsed UV laser beam was scanned linearly to show a desirable result. Simon and Wuang disclosed in U.S. Pat. No. 5,599,340 a method of disposing the laser pulses over a programmed pattern in a random process. For a pulsed laser of low repetition rate, this programmed random process can generate a similar result as that of continuous scanning.
When the pulse repetition rate is increased to the kilohertz level, however, the above mentioned scanning methods become non-practical. At a kilohertz repetition rate, the time interval between the pulses is only a millisecond. This is too fast for today's scanner to synchronize precisely the scanner mirror position with the laser pulses. Uniform disposition of the pulses becomes impossible with linear scanning because of the slowdown of the scanning when the beam turns around. Close overlapping between the pulses is not desirable because the plume from the previous pulse will affect the energy disposition of the next pulse. Besides, an accurate spot size on the target is practically impossible to defined and to maintain when the pulse energy varies with time.
SUMMARY OF THE INVENTION
The present invention contemplates a fast and smooth scanning so that the consecutive pulses in each pass of scanning are well separated and uniformly disposed. The scanning does not rely on any synchronization between the laser pulses and the scanner mirror positions. Instead, the scanning takes a series of close loops and the scanning speed on each loop is fine-tuned according to the perimeter of the loop. A uniform and close pulse disposition along the loop is achieved by multiple successive scans along the loop. The scanning pattern is designed such that the energy distribution is uniform for every layer and the smoothness of the ablated surface remains an acceptable level as the number of the layers increases.
In a preferable embodiment, the scanning takes a pattern of concentric rings in each layer. The laser beam is scanned from one ring to another in a spiral fashion. The scanning is approximately at a constant speed, which is set according to the pulse repetition rate and a predetermined disposition space between consecutive pulses. The scanning speed is then fine tuned for each individual ring according to the perimeter of the ring so that the pulse disposition can be uniformly filled in each ring precisely. The pulse disposition on each ring can be accomplished in one or more successive scans along the ring. The scanning deposits the pulses along one ring at a time and then swings smoothly from ring to ring and from layer to layer.
The scanning is spirally inward and outward alternately to generate multiple layers. The diameters of the rings are uniformly increased or decreased in each layer and are slightly varied from layer to layer. Thus, a controllable average can be obtained over the layers and the roughness of the energy disposition will not be built up as the number of layer increases. In the case of UV photo-refractive surgery, the ablation depth of each layer is typically a fraction of a micron and the surface is expected to remain rather smooth after many layers of ablation.
Accordingly, an objective of the present invention is to provide a new and improved method for scanning a pulsed laser beam of high repetition rate and small spot size to achieve a smooth scanning and a uniform ablation.
Another objective of the present invention is to provide a new and improved method to eliminate the effect of the ablation plume on the energy disposition of a pulsed laser beam of high repetition rate.
A further objective of the present invention is to provide a new and improved method to obtain uniform ablation without the synchronization between the laser pulses and the scanner mirror positions.
Another further objective of the present invention is to provide a new and improved method to conduct photo-refractive surgeries with a deep UV laser beam of high repetition rate and small spot size.
These and other objectives and advantages of the invention will become more apparent in the following drawings, detailed description and claims.
REFERENCES:
patent: 4665913 (1987-05-01), L'Esperance, Jr.
patent: 4718418 (1988-01-01), L'Esperance, Jr.
patent: 5284477 (1994-02-01), Hanna et al.
patent: 5520679 (1996-05-01), Lin
patent: 5599340 (1997-02-01), Simon et al.
patent: 5618285 (1997-04-01), Zair
patent: 5634920 (1997-06-01), Hohla
patent: 5743902 (1998-04-01), Trost
patent: 5782822 (1998-07-01), Telfair et al.
patent: 5971978 (1999-10-01), Mukai
patent: 6010497 (2000-01-01), Tang et al.
Cohen Lee
Farah Ahmed
Fish & Richardson P.C.
Katana Research, Inc.
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