Note: Descriptions are shown in the official language in which they were submitted.
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System for ophthalmic laser surgery
The invention relates to a system for ophthalmic laser surgery.
s In refractive ophthalmic surgery the refractive properties of the eye are
changed
by interventions in respect of the eye of a patient for the purpose of
correcting
sight defects. In this connection the so-called LASIK process (LASer In-situ
Keratomileusis) has great importance, wherein firstly a planar corneal
incision is
made, as a result of which a small cover disc ¨ the so-called flap ¨ arises.
Said
cal microkeratome used formerly has recently been replaced by an fs laser ¨
that
is to say, a laser that generates pulsed laser radiation with pulse durations
within
the femtosecond range. For an intra-tissue incision, the laser radiation has
to lie
within the transmissive wavelength range of the cornea ¨ that is to say, above
The advantages of a laser incision, in comparison with a corneal incision that
is
made mechanically with a microscalpel, are resulting in an increasing spread
of
When carrying out a flap incision by means of a femtosecond laser, in most
cases the incision is obtained by a precisely defined alignment of closely
adja-
guided, for example, along a meandering, serpentine path in the plane of the
flap incision to be produced (so-called line scan). This cuts the so-called
bed of
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the flap. Subsequently a final marginal incision is made along the desired
edge
of the flap. In this way the edge of the flap is defined.
The individual laser pulses are positioned precisely at the desired point in a
plane (ordinarily designated as the x-y direction) that is normal to the beam
direction, for example by means of a mirror scanner. As an alternative to a
mirror scanner, use may be made of a crystal scanner, for example, in order to
bring about the desired x-y deflection of the laser beam.
The quality of an incision to be made with fs laser radiation is influenced by
the
precise compliance with relevant parameters such as the pulse energy, the
focus
diameter, the focal plane and also the spacing of adjacent focal locations
(spots). These parameters can be separately optimised well for various types
of
incision guidance. In the case of a flap incision, for example, a distinction
can
be made between two forms of incision guidance, namely the flap-bed incision -
which cuts the flap bed and covers the latter, for example, by means of linear
scan paths, largely arranged in parallel with alternating direction of motion -
and
the peripheral marginal incision which is frequently necessary for the detach-
ment of the flap from the stroma.
The course of a scan path along which the laser beam is moved may sometimes
not be optimal for the desired generation of an athermal (cold)
photodisruption
at each point along the scan path. Depending on the course of the path, local
concentrations of the laser spots may occur. For example, in the case of a me-
andering line scan with which the bed of a flap is to be cut, in the region of
the
reversing bends of the individual line segments an accumulation of the spots
per
unit of length or unit of surface area may arise in comparison with the number
of spots in the region of the rectilinear path segments. This accumulation or
concentration is due to the inertia of the scanner - particularly when use is
being
made of a mirror scanner - at the turning-points where the scan direction is
reversed. Adjacent focal points are then possibly no longer clearly separated
from one another but are situated so closely together that thermal damage to
the corneal tissue as a consequence of excessive local radiation of energy can
no
longer be ruled out. Nevertheless, for the remaining region of the flap - that
is
to say, the actual bed ¨ the result of the incision with the chosen beam
parame-
ters may be optimal.
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It is consequently an object of the present invention to create a solution in
terms
of apparatus that, when making incisions in ocular tissue by means of short-
pulse laser radiation, enables the risk of undesirable thermal damage to the
ocular tissue to be reduced.
For the purpose of achieving this object, the invention provides a system for
ophthalmic surgery, comprising a source of pulsed laser radiation with
radiation
parameters matched to the making of an incision in an ocular tissue,
particularly
in the cornea, a scanner for deflecting the laser radiation, an electronic
control
unit which has been set up to control the scanner in accordance with a prede-
termined incision geometry, and a modulator unit for modulating the laser
pulses
emitted from the source. The invention provides that the control unit has been
set up to control the modulator unit in accordance with a beam-deflection pat-
tern established for the incision geometry, in such a manner that in predeter-
mined parts of the beam-deflection pattern at least some of the laser pulses
have a reduced pulse energy or are suppressed. The invention consequently
takes as its starting-point the perception that along the scan path of a laser
beam there may be regions in which, due to the course of the path, an in-
creased area-specific energy input may arise, with otherwise constant
radiation
parameters. The invention counters the risk of thermal damage resulting from
this by purposefully lowering, in predetermined regions of the scan path, the
area-specific energy input by means of suitable energy modulation or blanking
of
selected laser pulses. The energy modulation or blanking may be applied to
each pulse or only to some of the pulses in the path region in question. For
example, it is possible to blank only every second, every third or generally
every
nth pulse in the path region in question. Blanking means that the laser pulse
in
question is totally blocked or suitably deflected and absorbed, so that
substan-
tially nothing from it reaches the ocular tissue. But, instead of a blanking
(mask-
ing), an energy attenuation of selected pulses may also be undertaken, so that
although the pulses in question reach the ocular tissue they do this with pur-
posefully lowered pulse energy in comparison with the energy of the pulses
that
are situated in the remaining parts of the scan path. Such an energy reduction
may be equally strong for all the pulses concerned in the path region ¨ i.e.
all
the laser pulses concerned are substantially lowered to the same energy level -
or the laser pulses concerned may be at least partly energy-modulated to
differ-
ing degrees.
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Irrespective of whether selected pulses are blanked or energy-modulated, along
the entire scan path such radiation parameters as the repetition-rate of the
laser
pulses emitted from the source or/and the spot size (focus diameter) preferen-
tially remain unchanged.
The control of the modulator unit by the control unit is expediently effected
in
location-dependent manner ¨ i.e. depending on the location or the region along
the scan path or along the beam-deflection pattern where the beam focus is
presently located. Alternatively or in addition, the control may take place in
connection with a velocity of the laser radiation relative to the ocular
tissue, with
a change of the stated velocity ¨ that is to say, the acceleration ¨ or with a
pulse
energy of the laser radiation.
In this way it is possible, depending on information concerning the laser-beam
focus, to adapt the pulse energy emitted to the ocular tissue suitably. As men-
tioned, this may be done in location-dependent or position-dependent manner.
Alternatively or in addition, a suitable modulation may be imposed, depending
on a pattern of motion assigned to the beam-deflection pattern ¨ such as, for
instance, a velocity profile of the laser-beam focus ¨ or, for example,
depending
on information that is made available by the scanner unit or other system com-
ponents.
According to a preferred embodiment, the beam-deflection pattern includes a
serpentine pattern with a plurality of line paths extending rectilinearly side
by
side and with a plurality of reversing bends each terminally connecting a pair
of
adjacent line paths. In this connection the control unit has been set up to
con-
trol the modulator unit in the region of at least some of the reversing bends
for
an energy reduction and/or a blanking of at least some of the laser pulses.
In the case of a beam-deflection pattern of such a type, which is composed of
a
plurality of straight line paths extending side by side substantially in
parallel, a
reversal of the direction of motion by about 1800 takes place at the marginal
regions of the incision geometry. At these points of the beam-deflection
pattern,
which are designated here as reversing bends, a retardation of the scan
velocity
arises by reason of an inertia which is inherent in the scanner. Given a
substan-
tially constant repetition-rate of the laser source ¨ i.e. given a
substantially con-
stant pulse-rate of the laser radiation ¨ in the case of a reduction of the
scan
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velocity an increased energy input into the ocular tissue per unit of surface
area
results. A blanking of individual pulses or of entire pulse trains, brought
about
by the control unit, and/or a reduction in the energy of the individual pulses
in
the region of the reversing bends, may counteract harmful thermal loading pos-
sibly resulting from the increased energy input.
A flat incision can be produced not only with a meandering line scan but also
with a so-called spiral scan. In this case the focus is moved along a spiral
path.
Given constant pulse repetition rate and constant angular velocity of the rota-
beam deflection, the path spacing between consecutive focal locations is
reduced towards radially inner branches of the spiral path. This corresponds
to
an increased energy input per unit surface area. For the purpose of avoiding
any possible thermal damage which may arise by virtue of such an increased
energy input, another preferred embodiment provides that the beam-deflection
pattern includes a spiral pattern, the control unit having been set up to
control
the modulator unit for an energy reduction and/or a blanking of at least some
of
the laser pulses towards radially interior branches of the spiral pattern. By
suit-
able energy reduction or blanking of pulses, in the interior parts of the
spiral
scan it is possible to avoid an excessive increase in the energy input per
unit of
surface area, so that a purely non-thermal photodisruption of the ocular
tissue
continues to be possible without concomitant thermal damage. It will be under-
stood that a variation of the pulse repetition rate is not intended to be
ruled out,
and may be implemented in addition to an energy modulation of the pulses.
Overall, a meandering linear incision guidance offers the advantage of a
consid-
erably more freely selectable incision geometry in comparison with a spiral
scan.
The preparation of an elliptical flap incision - as is indicated, for example,
in the
case of an astigmatism - can be realised with a spiral-shaped incision
guidance
with approximately uniform surface density of the microdisruptions only with
increased control effort.
One embodiment provides that the modulator unit includes an optical grating
component with variable diffraction efficiency. The diffraction brought about
by
the grating component either may blank the laser beam completely - by, for
example, completely deflecting it into a beam dump which is optionally present
-
or may diffract only parts of the beam out of the beam path and in this way
reduce the energy brought onto or into the ocular tissue by the beam.
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The modulator unit preferably includes an acousto-optical or electro-optical
modulator. With a modulator of such a type, the laser radiation can be inter-
rupted, for example, very quickly and over a defined short time-interval, in
order
to avoid an undesirable local superposition of several laser-radiation pulses
at
the same location. Alternatively, instead of an interruption of the laser
radiation
or a blanking of individual or several laser pulses, a purposeful adaptation
of the
laser-radiation power or pulse power may be undertaken. In other words, in-
stead of an (idealised) keying/blanking, corresponding to a switch with two
posi-
tions, by virtue of the variation of the diffraction efficiency by means of
the
modulator a plurality of control positions as regards the diffraction
efficiency,
and hence ultimately also the energy emitted to the ocular tissue, can be
taken
up. In this connection, various functional linkages may be provided, for
example
between the diffraction efficiency and the location of the beam focus, the
instan-
taneous velocity of the beam focus or the change in the beam-focus velocity -
i.e. the acceleration.
The control unit may have been set up to control the modulator unit in such a
manner that in at least one predetermined section of the beam-deflection pat-
tern said modulator unit blanks each of several laser pulses situated in this
sec-
tion or reduces the pulse energy of each of these pulses compared with the
pulse energy in other sections of the beam-deflection pattern. Alternatively
or in
addition, the control unit may have been set up to control the modulator unit
in
such a manner that in at least one predetermined section of the beam-
deflection
pattern said modulator unit blanks, alternately in succession, at least one
first
laser pulse or reduces the pulse energy thereof and leaves unchanged the pulse
energy of at least one second laser pulse compared with the pulse energy in
other sections of the beam-deflection pattern.
According to an embodiment of the present application there is provided a sys-
tem for ophthalmic laser surgery, comprising a source of pulsed laser
radiation
with radiation parameters matched to the photodisruptive generation of an inci-
sion in the cornea, a scanner for deflecting the laser radiation, an
electronic
control unit which has been set up to control the scanner in accordance with a
predetermined incision geometry designed for production of a corneal flap,
wherein the incision geometry comprises a bed incision defining the bed of the
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flap as well as a marginal incision defining the edge of the flap, wherein a
ser-
pentine beam-deflection pattern with a plurality of line paths extending
rectiline-
arly side by side and with a plurality of reversing bends each terminally
connecting a pair of adjacent line paths is established for the bed incision,
wherein the reversing bends lie outside the edge of the flap, and a modulator
unit for modulating the laser pulses emitted from the source. The control unit
is
further set up to control the modulator unit in such a manner that in parts of
the
serpentine beam-deflection pattern lying outside the edge of the flap some of
the laser pulses are suppressed.
According to another embodiment there is provided a system for ophthalmic
laser surgery, comprising a source of pulsed laser radiation with radiation pa-
rameters matched to the making of an incision in an ocular tissue,
particularly in
the cornea, a scanner for deflecting the laser radiation, an electronic
control unit
The invention will be elucidated in more detail in the following on the basis
of
the appended drawings. Represented are:
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Fig. 1: a schematic exemplary embodiment of a system according to the
invention for ophthalmic laser surgery,
Fig. 2: a first exemplary scan pattern for a flap incision,
Fig. 3: a second exemplary scan pattern for a flap incision and
O
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Fig. 4: a third exemplary scan pattern for a flap incision.
The system shown in Fig. 1 in schematic block representation, denoted
generally
by 100, is a laser system that is suitable for the production of an intra-
tissue
incision in the eye of a patient. An intracorneal flap incision for producing
a
LASIK flap is one possible and preferred example of an incision for which the
laser system 100 is suitable. However, it is not excluded to produce other
forms
of a tissue incision in the eye with the laser system 100.
The laser system 100 includes a laser oscillator 110 which, in free-running
man-
ner, emits laser pulses with a duration within the femtosecond range and at a
defined repetition-rate. The laser oscillator 110 may, for example, be a solid-
state-laser oscillator, in particular a fibre-laser oscillator. The pulses
emitted by
the laser oscillator 110 pass through a preamplifier arrangement 120 which
increases the power of the pulses. At the same time, the preamplifier arrange-
ment 120 brings about a temporal stretching of the pulses. The laser pulses
that have been pretreated in such a way are then reduced in their repetition-
rate
by means of a so-called pulse picker 130. The laser oscillator 110 provides,
for
example, pulses at a rate of 10 MHz or more. This rate is reduced to, for exam-
ple, 200 kHz with the aid of the pulse picker 130. The pulses that have been
reduced in their repetition-rate in such a way are input to a power amplifier
140
which generates the pulse energy of the still temporally extended pulses that
is
needed for the application. Before the pulses that have been amplified in this
way are supplied to a final pulse compressor 150, they ordinarily have a pulse
length of over one picosecond, which is again compressed by the final pulse
compressor 150 to the short fs pulse width, made possible by the bandwidth of
the oscillator 110 and of the amplifier media, of, for example, below 500 fs.
In
the case of the final pulse compressor 150, it may be a question, for example,
of
a grating compressor.
Components 110, 120, 130, 140 and 150 may be regarded, taken together, as a
laser source in the sense of the invention.
The succession of fs laser pulses generated in this way subsequently passes
through a pulse modulator 170 which, for example, takes the form of an
acousto-optical modulator or an electro-optical modulator. Generally the pulse
modulator 170 may contain arbitrary optically active elements which enable a
=
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rapid blanking or modulation of the energy of the laser pulses. An acousto-
optical modulator may, for example, offer switching-times from less than 10 ps
down to, for example, 2 ps, with an off-time of approximately 10 ps to 100 ps.
Assigned to the pulse modulator 170 in Fig. 1 is a beam dump 180 which serves
to absorb any possible pulses to be blanked that are not to reach the target
to
be treated. Such pulses to be blanked can be deflected by the pulse modulator
170 onto the beam dump 180, so that they are no longer contained in the fur-
ther beam path of the laser beam directed onto the target.
Downstream of the modulator 170 the laser beam reaches a scanning and focus-
ing arrangement 160 which is represented schematically here as a common
block and which deflects the laser beam in a plane (x-y plane) perpendicular
to
the beam direction in accordance with a predetermined scan pattern or beam-
deflection pattern and focuses it onto the desired destination in the beam
direc-
tion (z-direction). In the case of an eye treatment, the destination is
situated in
the ocular tissue and, in particular, in the corneal tissue. For the
consecutive
laser pulses the beam-deflection pattern defines the position of each pulse in
the
x-y plane. In other words, it establishes a path (or several paths), along
which
the laser beam is to be moved, in order ultimately to obtain the desired
incision.
The scanning and focusing arrangement 160 may, for example, include an x-y
mirror scanner with two galvanometrically operated deflecting mirrors, which
are
capable of swivelling about mutually perpendicular axes, for the beam
scanning,
and an f-theta objective for the purpose of beam focusing.
The pulse modulator 170 and the scanning and focusing arrangement 160 are
coupled with a program-controlled control unit 190. The latter contains, in a
program memory which is not represented in any detail, a control program
which upon execution by the control unit 190 brings about such a control of
the
pulse modulator 170 and of the scanning and focusing arrangement 160 that the
laser beam is focused in the desired target plane, is moved over the target
plane
in a manner corresponding to the desired beam-deflection pattern, and, in pre-
determined parts of the beam-deflection pattern which are defined in the
control
program, at least some of the laser pulses are attenuated in energy by the
pulse
modulator 170 or are blanked completely.
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In the exemplary case that is shown, the laser beam that is output by the scan-
ning and focusing arrangement 160 is directed onto a cornea 300 of a human
eye 302 and is guided there with its focus in an intracorneal (planar or non-
planar) incision plane 304. This incision plane 304 is represented as a line
in the
present stylised sectional representation of the eye 302. A detailed
elucidation
of the incision guidance and also of the mode of operation of the modulator
170
in connection with the incision guidance results from the following
description of
Fig. 2.
Fig. 2 shows a detail of the human cornea 300 on which a flap incision
according
to a first flap-incision schema 305 is to be carried out. The flap-incision
schema
305 is only represented schematically; in particular, under certain
circumstances
the size ratios do not correspond to the real ratios. In addition, the flap-
incision
schema 305 is indicated only partially, in order to keep the representation as
a
whole clearly comprehensible.
For the purpose of carrying out the flap incision, laser pulses are focused at
points 310, 315 of the cornea 300 which are illustrated by circles, so that
micro-
disruptions arise. The laser radiation generated by the system 100 is guided
over the surface of the cornea 300 by means of a high-speed scanner. As a
rule, the cornea exhibits a surface curvature which may be designated, in a
first
approximation, as spherical. For the implementation of a flap incision, it is,
for
example, conventional to level the surface of the cornea to be treated by
press-
ing on or suctioning on an attachment. The focusing of the femtosecond laser
radiation is effected within a plane 304 (see Fig. 1) which extends
substantially
perpendicular to the visual axis of the eye, so that a substantially uniform
flap
thickness arises. The laser beam is guided within this plane along defined
path
curves.
In a first part of the incision schema 305 the planar flap-bed incision is pro-
duced. To this end, the laser beam is guided along a substantially straight
scan
path 320 in a first direction of motion 335 and, upon exceeding the desired
flap-
incision radius, changes its direction of motion to a second direction of
motion
345 and is subsequently again guided along a straight line parallel to, and
with a
defined spacing 325 from, the first scan line 320, so that the entire surface
of
the flap incision is scanned in the form of a grid or in meandering form with
alternating directions of motion 335, 345.
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Within the individual scan lines 320 the focal locations 315 are aligned with
one
another in virtually equidistant manner with a spacing 327, since pulse-rate
and
scan velocity along the lines 320 are kept constant. The individual scan lines
320 are provided with a spacing 325 from one another in such a way that, to-
gether with the spacing 327 of the individual focal locations 315 within the
scan
line, on the whole a two-dimensional incision arises. At the margins within
the
reversing bends 330 of the flap-incision pattern the direction of motion of
the
laser beam changes, for example by approximately 180 . At these reversing
bends 330 a retarded relative velocity between laser beam and corneal surface
lo results by reason of the inertia of the scanner, so that many of the
focal loca-
tions 315 are situated locally close together or coincide. This is evident in
a
distinctly smaller focal-location spacing 322 within the reversing bends 330
com-
pared with the focal-location spacing 327 along the scan-route sections 320.
These regions 330 are consequently subject to potential thermal damage.
For the purpose of completing the flap incision, after the surface incision
repre-
sented by the lines 320 a marginal incision along a, for example,
substantially
circular path 340 is carried out. For the marginal incision a different focus
den-
sity may be required or advantageous, compared, for example, with that of the
zo flap-bed incision. Correspondingly, the spacing 324 of the focal
locations 310
along the marginal-incision path curve 340 in the exemplary embodiment shown
in Fig. 2 is smaller than the spacing 327 of the focal locations along the
substan-
tially linear path curves 320. The marginal incision 340 is interrupted at a
point
350 which serves as a (flap) hinge in the course of detaching the severed cor-
neal region and folding it upwards. In the course of folding upwards, the
poten-
tially thermally damaged regions 330 along the line 340 are severed and are
then situated outside the flap.
A first possibility, according to the invention, in order to reduce the
aforemen-
tioned thermal damage at the reversing-points 330 consists in interrupting the
emission of the laser radiation to the cornea by means of a suitable drive of
the
acousto-optical modulator 170 if the focal locations fall outside the
(initially
imaginary) marginal-incision line 340.
This situation is represented at the reversing bend 334. Those focal locations
315 and the associated microdisruptions which in region 334 would,
respectively,
fall on the cornea 300 and be triggered are represented as circles that have
not
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been filled in. In this exemplary embodiment the laser-beam path through the
modulator 170 is blocked in a region outside the edge of the flap, so that no
pulses impinge on the cornea 300. But it is also conceivable that only single
laser pulses or entire series of pulses are blocked. This blanking of laser
pulses
may be effected, for example, in a manner depending on a location signal, ve-
locity signal or acceleration signal made available by the scanner unit 160.
But,
where appropriate, the generation and/or provision of signals may also be ef-
fected by other modules or components which are independent of the scanner
unit. Furthermore, the blanking may, where appropriate, also be effected by a
lo purely temporal control or programming of the laser-beam guidance or by
taking
other suitable signals into account. By virtue of this measure, as can be dis-
cerned in Fig. 2, the marginal region 334 is kept totally free from
microdisrup-
tions induced by the laser beam, and thermal damage in this region is ruled
out.
One strategy for avoiding thermal damage - which may be employed alterna-
tively or, where appropriate, in combination with the possibility presented
above
- consists in a modulation of the energy of individual femtosecond pulses in
the
course of guidance of the incision in the cornea. This is represented in the
re-
versing region 332 in Fig. 2. Instead of, as in region 334, keeping the local
density of the individual focal locations on statistical average substantially
ap-
proximately within a desired range, in region 332 the energy is reduced that
is
emitted to the cornea by the individual laser pulses in the form of the focal
loca-
tions 317 by way of microdisruptions. For the purpose of representation, the
circles that represent the focal locations of the laser radiation are
represented as
circles 317 with a smaller radius. For the purpose of obtaining a lower
emission
of energy, the acousto-optical modulator 170 is not switched from an on-state
into the absolute off-state. Rather, in principle, for each pulse of the train
of
femtosecond pulses individual pulse energies are capable of being set which
can
be adapted to the concrete application in magnitude and succession. In this
connection, switching-times can be realised that can modulate individual im-
pulses at a repetition-rate of up to about 1 MHz. In the present case, for
pulses
that lie outside the flap-incision region a constantly lower pulse energy is
set or
adjusted. But a pulse-energy progression is also conceivable that is adapted
to
the presumable or actual velocity progression or acceleration progression.
Moreover, it is conceivable to arrange the reversing bend 332 not outside the
flap bed but rather within the marginal incision, and in this way to obtain a
tem-
poral shortening of the entire flap-incision procedure by dispensing with scan-
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fling beyond the actual marginal-incision region. With the flap-incision
schema
305 represented in Fig. 2 it is possible for arbitrary flap shapes to be
realised,
which may be an advantage, in particular, in the case of higher-order aberra-
tions of the corneal geometry, such as astigmatism for instance.
Another alternative form of production of a flap incision is represented in
Fig. 3.
Instead of a linear, meandering scanning of the flap-incision region, in the
case
of the flap-incision schema 400 shown in Fig. 3 a spiral scan guidance is pro-
vided. The representation of the incision schema is again only schematic ¨
i.e.
the size ratios and spacing ratios are, as in Fig. 2, not true to scale and
may
differ in reality from the schema that is represented. Furthermore, also as in
Fig. 2 the incision guidance is incomplete. In particular, in the peripheral
region
of the spiral incision yet further pulses have to be positioned in the course
of a
real incision guidance.
In the present exemplary embodiment the incision guidance is effected along a
spiral path 420 evolving outwards from the central region 405 of the cornea
300
to peripheral regions 430 - in the present case, clockwise along the direction
of
motion indicated by an arrow 407 in Fig. 3. The individual focal locations 415
are placed along the spiral path 420 with continuous pulse-rate. The velocity
profile generated by the scanner along the spiral path 420 is composed of a
linear radial component as well as a rotational-speed component. In the case
of
a constant rotational component (i.e. constant angular velocity) and a
constant
radial component, given a constant pulse-rate in the central region 405 a dis-
tinctly higher focal-location density prevails along the path curve 420 than
in
peripheral regions 430, since in the peripheral region 430 the path velocity
is
higher by reason of the constant rotational speed. This is evident in a
smaller
focal-location spacing 432 in the central region 405 compared with the focal-
location spacing 434 in the peripheral region 430.
Although the flap-incision schema 400 has the advantage that - in the case of
the direction of motion 407 that has been described, from the centre 405 to
the
peripheral regions 430 - the flat flap-bed incision can be transformed continu-
ously into the flap-edge incision, on the other hand there is the risk of
thermal
damage in the central region of the cornea 300, which may be a particular dis-
advantage there. Also in the case of an evolution of the spiral path in the
oppo-
site direction of motion ¨ i.e. from the peripheral edge region 430 of the
flap
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inwards into the central region 405 - there is the same risk, since here too
use
has to be made of a mixed pulse power which has a tendency to be too low for
the peripheral region 430, given temporally fixed pulse frequency, and which
in
the central region 405 of the cornea 300 is possibly too high.
In order to achieve a more uniform energy input per unit of surface area, ac-
cording to one embodiment of the invention the energy emitted to the ocular
tissue of the cornea 300 at the focal locations 415 is modulated in such a man-
ner that the energy input is lower in the central region 405 of the cornea
than in
io the peripheral region 430. This is indicated in Fig. 3 by a radius of
the circles
representing the focal locations 415 which increases from the central region
405
to the peripheral region 430. Consequently, although the focal-location
density
is reduced from the inside 405 to the outside 430, by virtue of the increasing
pulse power the energy input per focal location which is brought about by the
triggered microdisruptions is higher and consequently compensates the reducing
focal-location density to give an energy input per unit of surface area that
re-
mains substantially constant within a desired range. This compensation by
means of the modulator 170 can be temporally controlled by a control unit in
accordance with a previously established mathematical function; but a control
loop may also be set up which regulates the pulse power, for example in a man-
ner depending on the radial position of the scan device 160.
As an alternative to a control or regulation of the pulse power along the
spiral-
path curve 420, in the case of a spiral-path-scan schema a constant pulse den-
sity along a path curve can be adjusted by blanking of laser pulses. This is
rep-
resented schematically in Fig. 4. For the purpose of avoiding repetition, in
the
description of Figure 4 only the essential differences from the embodiments
already described, shown in Figs. 2 and 3, will be considered. In Fig. 4 a
flap-
incision schema 500 comparable to the schema of the embodiment shown in
Fig. 3 is represented. By means of a spiral beam-deflection pattern along a
path
curve 520, this schema produces a flap-bed incision by applying laser pulses
515. In order to keep the focal spacing of the laser pulses 515 in the path
curve
520 of the spiral scan substantially constant, by blanking of individual laser
pulses 525 (instead of a variation of the pulse-repetition frequency of the
laser
source, or instead of a variation of the laser-pulse energy) the pulse-rate of
the
laser radiation impinging on the ocular tissue is changed continuously in
accor-
dance with the following equation
=
CA 02763562 2011-11-25
.=
_
-14-
d d,
= con,s1 _____________ j, <<fe as d, do
Joii
where
fl= pulse-rate in the inner spiral region;
fo= pulse-rate in the outer spiral region;
sf= spot spacing in the path curve;
d,= diameter of the path curve in the central region;
do = diameter of the path curve in an outer region.
Consequently, an approximately uniform focal-location density over the entire
flap-bed-incision region of the cornea 300 results by virtue of a blanking of
three
out of four pulses in a central region 505 and by blanking every second pulse
in
a peripheral region 530. The numerical values and size ratios represented here
are, under certain circumstances, not true to reality or true to scale and
serve
only for schematic representation. In a concrete embodiment the actual pulse-
to-blanking ratios may differ considerably from the values that are
represented
in simplified manner.
Overall, the local accumulation or even superposition - which is associated
with
zo negative consequences - of several fs laser pulses in the region of the
reversing-
points in the case of a linear grid-like flap-incision process or a too dense
suc-
cession of fs laser pulses in the case of a spiral scan process can
consequently
be avoided by a program-correlated blanking or by a purposeful modulation of
the pulse power of the laser radiation. In all cases the laser source
continues to
run undisturbed with fixed and optimised beam parameters such as pulse en-
ergy, pulse duration as well as divergence and beam-parameter product, as a
result of which the incision quality remains uniformly optimised.
The invention may also be utilised for other fs laser applications in
ophthalmol-
ogy. For example, similar incision schemata may be employed for lamellar and
penetrating keratoplasty, such as, for instance, in the case of a lenticular
extrac-
tion or similar.
