Surgery – Instruments – Light application
Reexamination Certificate
2000-02-04
2003-06-10
Shay, David (Department: 3739)
Surgery
Instruments
Light application
C606S003000, C606S013000, C607S089000
Reexamination Certificate
active
06575964
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of medical lasers. More particularly, the present invention relates to the field of medical lasers for effecting incisions, tissue ablation and coagulation.
BACKGROUND OF THE INVENTION
A laser beam is formed when a material capable of lasing, such as a solid-state crystal or gas, is excited by incident light energy. In response, ions within the material are pumped to a high energy level and, then, energy is dissipated when the ions return to a ground state. In transitioning from the high energy level to the ground state, the ions each emit a photon in addition to heat energy. The emitted photons have a uniform wavelength (&lgr;) and eventually form the laser beam.
FIG. 1
schematically illustrates a solid state laser in accordance with the prior art. A cylindrical rod-shaped crystal
10
is disposed between two reflective surfaces
12
,
14
. The surfaces
12
,
14
are aligned parallel to one another and perpendicular to the longitudinal axis of the crystal
10
. While light can be emitted from the crystal
10
in various different directions, only a coherent beam of light
16
which travels along the axis of the crystal
10
is reflected between the surfaces
12
,
14
. The surface
12
has a reflectivity of nearly 100% and, thus, reflects all of the beam
16
. The surface
14
, however, has a reflectivity of less than 100% and a transmissivity of greater than zero. Thus, a portion
18
of the beam
16
passes through the surface
14
. The emitted beam
18
can be utilized for industrial or medical applications. For example, the beam
18
can be directed to a target surface.
A property of a laser beam is that the beam is continually diffracting. This diffraction is evidenced by convergence (narrowing) to a waist or divergence from a waist.
FIG. 2
schematically illustrates a laser beam
50
converging to a waist
52
and, then, diverging from the waist
52
. The waist
52
, whose radius is given as &ohgr;
0
, is the narrowest portion of the beam
52
. A distance known as the Rayleigh Range (RR), is a distance from the waist
52
that the beam
50
achieves a radius given by the square root of two times &ohgr;
0
(1.414&ohgr;
0
). Thus, the Rayleigh Range is a measure of the convergence and divergence of the beam
50
. An important relationship which holds for the beam
50
is given by:
RR
=
πω
0
2
M
2
λ
where M is a constant which characterizes the number of times greater than the diffraction limit is the beam
52
. As can be seen from this equation, for a given waist, the Rayleigh Range is longest when M is equal to one. Similarly, for a given Rayleigh Range, the waist is at its most narrow when M is equal to one. The constant M, however, can be greater than one.
As mentioned, when the excited ions of the crystal
10
(
FIG. 1
) return from an excited state to the ground state, heat is dissipated, in addition to a photon. To avoid excessive heat from building up in the crystal
10
, this heat is typically removed by a cooling jacket which surrounds the crystal
10
. As a result of removing heat from the crystal
10
, the temperature of the crystal is higher in the center than near the outer edges. As shown in
FIG. 3
a,
when the temperature (T) of the crystal
10
is plotted along a vertical axis and distance (X) from the center of the crystal
10
is plotted along a horizontal axis, a parabolic curve results. The refractive index of the crystal
10
, however, varies with temperature. As a result, the crystal
10
behaves as a lens (a thermal lens). This thermal lens tends to narrow the beam
16
′ at its ends, as schematically shown in
FIG. 3
b,
thereby counter-acting the natural tendency for the beam to diverge, as illustrated in FIG.
2
.
FIG. 3
b
illustrates the effects of a thermal lens on the solid state laser of
FIG. 1
where the constant M for the beam
16
′ is one. When the constant M is equal to one, the beam
16
′ is considered to be of mode TEM
00
. For the emitted beam
18
, M is also equal to one.
Under operating conditions where the temperature in the center of the crystal
10
is increased, as shown in
FIG. 4
a,
the effect of the thermal lens is to further narrow the mode TEM
00
beam
16
″.
FIG. 4
b
schematically illustrates the effects of increasing temperatures in the solid state laser of FIG.
1
. Note that in
FIG. 4
b
, the TEM
00
beam
16
″ does not pass through the outermost portions of the crystal
10
. Light emitted from these regions, forms a beam
20
of another mode (e.g., TEM
01
). The constant M for the beam
20
of mode TEM
01
is 1.4. A beam
22
which emits from the laser shown in
FIG. 4
b
has a constant M which is between 1 and 1.4. Further increases in the strength of the thermal lens results in increasing values for the constant M of the beam
22
.
Lasers are used in medical procedures to rejuvenate, restore and resurface skin damaged due to many causes including prolonged exposure to the sun. Laser energy is delivered to the surface of the skin in a controlled pattern in order to ablate or burn away layers of the skin. As the layers of skin grow back within the area of skin exposed to the laser, the skin is effectively resurfaced. To avoid excessive bleeding, it is important that a zone of thermal necrosis or coagulation is formed within the newly exposed tissue. In addition, lasers are used for vision correction by reshaping the lens in the eye. Lasers are also used for forming incisions by ablating a narrow band of tissue.
For each of these functions, the intensity and distribution of the laser beam, its wavelength, and the duration of exposure, all must be understood and/or appropriately controlled so that the desired results are achieved. Advances in laser apparatus have been directed to the difficulty in controlling these factors.
U.S. Pat. No. 4,791,927 discloses a dual-wavelength laser scalpel. A short wavelength blue light cuts the target tissue and a longer-wavelength red light cauterizes. The two wavelengths are formed as the fundamental frequency and second harmonic of a single laser source.
U.S. Pat. No. 5,651,784 discloses a rotatable aperture apparatus and methods for selective photoablation of surfaces. The intensity distribution of a beam of radiation is modified by inserting a rotatable mask into the beam. The mask is formed with one or more apertures that have a geometric spiral shape originating substantially from the center of rotation of the mask. A beam of radiation incident on the rotating mask is transmitted therethrough with intensity that varies as a function of radial position with respect to the rotation point.
U.S. Pat. No. 4,887,019 discloses a device for the generation of a laser beam spot of adjustable size on an object, in particular, in the human eye. A focusing device projects the laser beam onto the eye with a small focusing spot and a large aperture cone. A deflector device moves the focusing spot over the desired beam spot in a predetermined scanning pattern.
U.S. Pat. No. 4,941,093 discloses surface erosion using lasers for eroding a surface, such as a patient's cornea. A laser beam exits an optical system and is incident on the surface. An iris is placed in the beam between the optical system and the surface which can be opened while pulsing the beam so as to erode the center of the surface to a greater extent than the surrounding area. However, this patent teaches that iris diaphragms are undesirable because the shape of the opening can change along with its size. Thus, an alternative system is disclosed in U.S. Pat. No. 4,941,093 in which a beam shaping stop is placed between the optical system and the surface. The beam shaping stop is arranged to move along the beam axis in a direction of convergence or divergence of the beam.
What is needed is a method and apparatus for controlling the delivery of laser energy. What is further needed is a method and apparatus for controlling the delivery of laser energy for performing medical procedures.
SUMMARY OF THE INVENT
Gott Ken
Hobart James L.
Hugues Rene
Negus Daniel K.
Haverstock & Owens LLP
Sciton, Inc.
Shay David
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