GB2228344A

GB2228344A - Ophthalmological lasers

Info

Publication number: GB2228344A
Application number: GB9003449A
Authority: GB
Inventors: Svyatoslav Nikolaevich Fedorov; Albina Ivanovna Ivashina; Leonid Feodosievich Linnik; Alexandr Dmitrievich Semenov; Vladimir Stepanovich Tjurin; Mikhail Jurievich Orlov; Efim Natanovich Beilin; Evgeny Naumovich Lekhtsier; Igor Anatolievich Skvortsov
Original assignee: MEZHOTRASLEVOI NT KOMPLEX MIKR
Current assignee: MEZHOTRASLEVOI NT KOMPLEX MIKR
Priority date: 1989-02-17
Filing date: 1990-02-15
Publication date: 1990-08-22
Also published as: DE4004423A1; IT9041529A0; IT1238825B; DE4004423C2; FR2643258A1; IT9041529A1; GB9003449D0; CN1045227A; HU900836D0; JPH02252453A; JPH0753167B2

Description

- I - Ophthalmoloz-4--9l Lasers The invention relates generally to

medicine, more specifically to ophthalmology and has particular reference to a device for surgical treatment of ametropia.

At present laser methods of treatment are being adopted in worldwide ophthalmosurgical practice, in par-. ticular. laser-assisted methods for correction of eye refraction anomalies by virtue of radiation emitted by, UV excimer lasers. The most urgent problem encountered jo in engineering laser ophthalmosurgical units aimed at the aforesaid purposes is to attain a required profile of irradiation applied to the cornea. for which prupose the output laser radiation should feature a smooth symmetrical distribution of energy density over the beam crosssectional area, predominantly a rectangular (uniform).P distribution. However, the distribution energy density in excimer lasers is not such which readers the problem of how to transform nonuniform and unsymmetrical distribution of laser radiation into a uniform distribution the most urgent one.

One state-of-the-art device for surgical treatment of ametropia Is known to comprise the following components arranged on a common optical axis: a UV pulsed laser, a unit for uniform distribution of laser radiation energy density over the beam cross-sectional area, a shaper of a required distribution of laser radiation energy density over the beam cross-sectional area, and a projecting lens (o:t.SPIE, vol.908, Laser Interaction with Tissue, 1988, by P.R.Ioder at al.q "Beam delivery system for UY laser ablation of the corneall pp.77-82).

In said device the unit for uniform distribution of laser radiation energy density is made as a rotating system of mirrors similar, as to its effect, to the Dove prism known in optics. Uniform distribution of laser radiation energy density over the beam cross-sectional area is attained by rotating the beam as a whole round its optical axis. In this case nonunifortn distribution remains in each separate radiation pulse and uniformity occurs in time as a result of averaging a train of consecutive radiation pulses. Such uniformity of distribution with the aid of the known system is effective only for lasers which feature energy density distribution over the beam cross-sectional area as smooth and monotonous. Thus, to provide uniform density distribution. in the presence of drastic excursions in distribution, which is practically the case in all actual lasers. can be only by cutting off part of the laser beam, wherein energy 13 distribution is loose and monotonous. However, this results in bad energy losses. affected accuracy and 25 prolonged time of the operative procedure.

1 1k 3 Thus the present invention provides a device for ophthalmological laser surgery comprising a laser source, a beam distributor and a projecting lens, wherein the distributor comprises a waveguide. The waveguide preferably has a rectangular cross-section, and will be in the form of a parallelipiped, or a pyramidal frustum with the greater base facing the laser source.

In particular, it is preferred that the distributor further comprises a lens to focus the laser emission into the the waveguide, and preferably which can oscillate normal to the path of laser emission. Generally, the focussing lens used will have different focal lengths in the meridional and sagittal planes.

Alternatively. where the waveguide is in the form of a pyramidal frustum, the apex of the waveguide may be adapted to oscillate about the path of laser emission. Such oscillation of lens or frustum is advantageous to provide even distribution of the laser emission over the surface of the eye.

The device according to the invention will generally further comprise a beam shaper.

L+ The invention also provides that in a device for surgical treatment of ametropia, comprising the following components arranged in succession.qn a common optical axis: a UV pulsed laser, a unit for uniform distribution of laser radiation energy density over the'beam cross-sectional area. a shaper of a required distribution of laser radiation energy density over the beam cross- sectional area,, and a projecting lens, according to the invention, the unit for uniform distribution of laser radiation energy density is shaped as a rectangular cross-section waveguide.

The waveguide may be also be shaped as a square crosssection parallelepiped and an additional lens may be placed ahead of it as along the pathway of laser radiation.

In this case it is also desirable that the additional lens be capable of oscillating in a plane square with the optical axis.

The waveguide may be shaped also as a frustum of 2J.a pyramid which faces with its greater base the laser.

In this caseitisexpedient that the pyramid be capable of oscillating round the geometric centre of the lesser base thereof in two mutually square directions normal to the optical axis.

I.;) The device for surgical treatment of ametropia, ac cording to the present invention features practically complete utilization of laser energy and arbitrary distribution of radiation energy density at its output and enables one to substantially enhance accuracy of surgery and to curtail the operatiT time at least two times.

Furthermore, the device, according to the invention, is constructionally simpler than the heretofore-known device of similar application.

In what follows the invention is illustrated in a detailed description of some specific exemplary embodiments thereof with reference to the accompanying drawings, wherein:

FIG.1 is a schematic side view of an embodiment of a device for surgical treatment of ameturopia provided wit-h a waveguide shaped as a parallelepiped, according to the invention; FIG.2 is a plan view of FIG.1; FIG.3 is as a side view of a device of FIG.1 provided 20 with a waveguide shaped as a frustum, of a pyramid; FIG.4 is a plan view of FIG-3; FIG.5 shows a pattern of a laser beam split into separate portions in the waveguide; FIG.6 is a distribution curve of laser radiation energy density E (plotted as ordinate) along a direction X (appearing as abscissa) square with the axis of the beam emergent from the laser; and FIG.7 is a view of FIG.5 at the waveguide exit.

(0 A J C) 4_ j The device for surgical treatment of ametropia as shown in PIGS 1 and 2 comprises the following components arranged in succession on a common optical axis: a UV pulsed laser 1, a rectangular cross-section diaphragm 2. a unit 3 for uniform distribution of energy density of radiation emitted by the laser 1 over the beam crosssectional area, said unit incorporating an additional lens 4 placed past the diaphragm 2 as along the pathway of radiation emitted by the laser 1, a rectangular crosssection waveguide 5 situated past said additional lens 4, a shaper 6 of a required distribution of laser radiation energy density over the beam cross-sectional area, and a projecting lens 7 which directs the laser radiation onto a patient's cornea 8.

The additional lens is capable of oscillating in a plane square with the optical axis in two mutually square directions independently. for which purpose the lens mount is associated with the output member of a mechanical vibrator 9.

The lens 4 has different focal lengths fl and f2 in the meridional and sagittal planes. respectively (appearing as focal spots P 1 and F2 in the Drawings) and its curved surfaces are in fact crossed cylinders.

The waveguide 5 in a given embodiment is in effect 2_.a square cross-section hollow parallelepiped, the inner 1 1 -7 surfaces 10 of the walls of said parallelepiped having a mirror reflecting coating.

Used as the shaper 6 may be a variable-section circular diaphragm, or a rotary disk with a preset-configuration slit, or else an optic cell featuring variable radiation absorption as over the cross-sectional area thereof.

The projecting lens 7 constructs the image of the plane P of the exit end of the waveguide 5 on the cornea 8.

Unlike the embodiment discussed above,the one presen- ted in PIGS 3 and 4 features a unit 3' for uniform distribution of laser radiation energy density, being essentially a waveguide 11 shaped as a pyramid frustum which faces the laser 1 with its greater base. The pyramid is capable of oscillating round the geometric centre 101 of the pyramid lesser base in two mutually square directions normal to the optical axis, for which purpose the greater pyramid base is associated with the output member of the mechanical vibrator 9. The pyramid (i.e., the waveguide 1 a solid structure made of a material transparent to laser radiation. e.g., magnesium fluoride. while the outer pyramid surfaces are given a high optical quality finish by fine polishing.

1) The embodiment of the device for surgical treatment of ametropia as shown in PIGS 19 2, according to the invention, operates as follow.

A radiation beam 12 emerging from the laser 1 passes through the rectangular cross-section diaphragm 2 having the following adjustable dimensions: height (a) and width (b). The diaphragm 2 cuts off a desirable portion of the radiation from the laser beam 12. Then a beam 13 passes through the lens 4 for its cross-sectional area 1-and angular aperture to change. Having passed through the lens 4 the beam 13 is focussedin two focal planes at the focal distances f 1 and f2 The beam 13 past the lens 4 features a variable rectangular cross-section whose dimensions depend on the distance S from the plane of the 151ens 4 to the plane H of observation. The cross-sectional height alof the radiation beam 13 at a distance S > fl and the cross-sectional width bl thereof at a distance S > f 2 are as follows:

at = (S - f) a 1 771 bl = (S - f2) f b 2 In a given embodiment of the device a'= bl = c.

In this case the extreme rays of the radiation beam 13 entering the paveguide 5 in the plane H of observation 9 1 at the distance S = f 1 (.2 + 1 f a 2 (2 + 1), are incident b upon the walls of the mirror waveguide 5 having a cross section of C x C, the exit end of said waveguide lying in the plane P spaced the distance apart from the plane H which equals the length of the working portion of the waveguide 5.

Provided that n(s - f 1) = m(S - f2)l where n, m = 29 41 6.... arbitrary even numbers, the radiation beam 13 entering the waveguide 5 is split into a system Ij (n + 1), (m + 1) of elementary beams which experience different numbers of reflections from the walls of the waveguide 5. Each of said elementary beams fills the entire exit end of the waveguide 5.

An exemplary splitting of the incoming beam 13 into 35 elementary beams is shown in FIG.5, where n = 6, m = 49 while curves 14 present the areas of equal intensity of the beam 12 at the output of the laser 1, a line 15 indicates the boundaries of the beam 13 past the diaphragm 29 dotted lines 16 exhibit 35 elementary beams of the radiation beam 13. each of them being projected onto the exit end of the waveguide 5 (the plane P), thus filling said exit end completely.

Energy density distribution in the plane P of the exit end of the waveuide 5 is in effect an interference C> 23 pattern resulting from interference of the radiation beams (n + 1), (m + 1).

IG Radiation intensity 1 2 effective at the point having the coordinates (X, Y) at the exit end of the waveguide 5 is equal to _rl 2 + 12 2 + (n+l).(m+1) + iioi + ij where E 11 E2 is the intensity of radiation at the where 8 13 point (X. Y) of the corresponding waves; are interference terms. each being ij directly proportional t 1 03 Sij 2 X X- A ij Aij denotes an optical difference between the runs of the waves i and j; X is the radiation wavelength.

Jith the intensity distribution averaged with respect to the period t of the interference pattern, one obtains:

-12 2 2 +0 Q+ti 1 + 'r2 (m+l) (n+l), since cos sii 00 Thus. energy density distribution at the exit of the waveguide 5. after having been averaged with respect to the period t of the interference pattern is in effect the sum of distribution values of the beams (n + 1). (m + 1), which results in uniform distribution of radiation energy density. For instance, root-mean 0 square deviation of the energy density is reduced by 2- v kn + 1) km +.1) times for random splitting of the 1 -v 1 11 incoming beam 13 into (n + 1) - beams.

13 (m + 1) equal elementary Now let us estimate the period of an interference pattern.

For the sake of simplicity let us consider interference in a-hollow waveguide of two beamsg i.e., the beam that has passed through the device without being reflected from the walls and the other beam that has experienced one reflection. A distance between the adjacent intensity IJ maxima, i.e.g the period t It+ (S f 1)1A with 300 mm, (S - f 50 mm. C 7 mm,;k - 0. 2 the period t is /- 10,m. An actual distance between the adjacent intensity maxima (minima) is substantially lower than the above value due to interference of a great many radiation beams featuring a broad range of differences in the run. An accurate calculatiion of interference pattern is very difficult. therefore let us assume the value of ti= 10).im as the upper estimation of the scale of interference intiomogeneity of radiation intensity distribution at the waveguide exit. The required averaging with respect to the low period t in the course an ophthalmosurgical procedure occurs automatically, since such a surgery consists of about 500 to 1000 pulses of the radiation emitted by the laser 1, during which there occurs complete smearing of the interference pattern due to accidental eye movements on account of intrinsic eyeball oscillatio-ns at a frequency of up to 300 Hz (ocular tremor) which are controlled neither by the Q_ doctor nor by the patient. as well as due to patient's heart beats, respiration. vibrations of the unit itself. etc.

Regardless of the aforesaid factors complete averaging of the interference pattern occurs at the exit end of the waveguide 5 due to oscillations performed by the lens 4 in two mutually square directions.

For the abovesaid parameters of the unit 3 oscillations of the lens 4 with an amplitude exceeding 10 to 20.,A= will lead to complete averaging of the interference patterns corresponding to consecutive radiation pulses and, besides. to smearing of acute radiation intensity outbursts (hot spots) of the laser. l. PIGS 6 and 7 illustrates operation of the unit 3 for uniform distribution of radiation energy density, PIG.6 showing distribution of laser radiation energy density over the cross-sectional area of the be'a'm 12 in the meridional plane. while FIG.7 shows such distribution at the exit of the waveguide 5 (in the plane P). 20 The radiation beam emergent from the exit end of the wavegulde 5 passes through the shaper 6 of a required distribution of laser radiation energy density over the beam cross-sectional area, where the energy density of the radiation beam uniform in the plane P is transformed obeying the law necessary for carrying a given surgery. Used as the shaper 6 may be a variable-section circular diaphragm, a rotary disk having a preset-configuration slit, or else an optic cell featuring variable absorption -1 of radiation of the laser 1 as over the cross-sectional beam area. Further on the radiation beam 17 passes through the lens 7 and is projected onto the cornea 8 of the eye operated upon. The lens 7 is so disposed that the image of the plane P is constructed on the cornea 8.

An embodiment of the device presented in PIGS 39 4 operates similarly to an embodiment shown in PIGS ly 20 the sole difference residing in that the beam 13 after having passed the diaphragm 2 arnives immediately at the Ii entrance of the waveguide 11.

Further on the radiation beam 13 passes through the waveguide 11 shaped as a square pyramid frustum having an entrance end measuring a'. bl, a' >, a and bl > bt and an exit end measuring all. b11, where all..: a I and b11,-' b I z in particulars all = b11 and a' = bl.

The central portion of the radiation beam 12 entering the waveguide 11 passes therethrough without being reflected, whereas the peripheral portions of the beam 13 experience 1, 2, 3... P reflections in one plane W and 1, 2, 3... q reiflections in the other plane square with the former one. As a result, (2p + 1). (2q + 1) radiation beams passes through the exit end of the waveguide, each of said beams filling the entire exit end, , thus attaining uniform distribution of radiation energy density. The angular aperture (,x' 11 -"-/ 2) of the radiation beam emereent from the pyramid-shaped waveguide 11 equals C> to--,/- 2 P anCICA, = 2qA in the meridional and sagittal Lt planes, respectively, where J'19A2 denote the apex angle of the pyramid in the meridional and sagittal planes, respectively.

The length of the waveguide 11 should obey both of the following conditions:

a 1 - all 2 tg 2 b 1 - b'I 2 tg Q2 2 As a result of an angular turn of the greater pyramid base in two mutually square directions round the centre 101. the incoming beam undergoes a certain new splitting into (2p + 1) (2q + 1) elementary beams for every particular laser pulse, whereby there occurs additional equalization of radiation energy density in time.

Distribution of radiation intensity at the exit end of the pyramid-shaped waveguide results from interference of (2p + 1) - (2q + 1) light beams.

Now let as assess the period of.the interference pattern. in view of which let us consider an interference of a radiation beam that has passed through the waveguide without being reflected from the walls thereof and that of a beam that has undergone one reflection from the lateral surface of a cone having an apex angle ofA A distance between the adjacent maxima (minima) of 1< radiation intensity, i.e., the period t = A 2sin 26g, For the typical values of 0.02 to 0.04 and 193 um the period t will be as follows:

t 0.2 to 0.4 2. 0.04 Like in an embodiment shown in FrGS 1 2t in the embodiment under consideration the scale of interference in-homogeneity of such an order is quite negligible for conducting ophthalmosurgical procedures.

An angular turn through a small angle of the order l)of 0.01 rad performed by the greater pyramid base round the point 101 results in a linear displacement of the edge of the exit end by a length of Z1(all) A(all) all cos ir all:z.: all)P2 7 where all stands for the size of the exit end. With a"= 7 mm 10-4 7 2 0 - 3,n.

The above value is quite negligible for ophthalmosurgery It is easily demonstrable that such oscillations of the waveguide entrance end will result in complete smearing 20of the effect of interference patterns from consecutive radiation pulses and, which is much more significant, in complete smearing of the effect of macroscopic inhomogeneity of the incoming laser beam.

0 o

Claims

1. A device for ophthalmological laser surgery comprising a laser source, a beam distributor and a projecting lens, wherein the distributor comprises a waveguide.

2. A device according to claim 1, wherein the waveguide has-a rectangular cross-section.

3. A device according to claim 1 or 2, wherein the distributor further comprises a lens to focus the laser emission into the the waveguide.

4. A device according to claim 3, wherein the focussing lens is adapted to oscillate normal to the path of laser emission.

5. A device according to claim 3 or 4, wherein the focussing lens has different focal lengths in the meridional and sagittal planes.

6. A device according to any preceding claim, wherein the waveguide is in the form of a parallelipiped.

7. A device according to any of claims 1 to 5, wherein the waveguide is in the form of a pyramidal frustum, the greater base facing the laser source.

8. A device according to claim 7, wherein the apex of the waveguide is adapted to oscillate about the path of laser emission.

4 (7

9. A device according to any preceding claim further comprising a beam shaper.

10. A device"for surgical treatment of ametropia, comprising the following components arranged on a common optical axis: a UV pulsed laser, a unit for uniform distribution of laser radiation energy density over the beam cross-sectional area, a shaper of required distribution of laser radiation energy density over the beam cross-sectional area, and a projecting lens, wherein the unit for uniform distribution of laser radiation energy density is a rectangular cross-section waveguide, the device further comprising any, or any combination of, features as defined in any preceding claim.

11. A device for laser surgery comprising a waveguide, substantially as described hereinbefore, with particular reference to the accompanying Figures 1 and 2.

12. A device for laser surgery comprising a waveguide, substantially as described hereinbefore, with particular reference to the accompanying Figures 3 and 4.

Published 1990atThe Patent Oflice, State House. 6671 High Holborn. London WC1R4TP. Further copies mkvbe obtained from The Patent Office. Sales Branch. St Mary Cray. Orpington. Kent BR5 3RD Printed by M1.2tiplex techniques ltd. St Maiy Cray. Kent, Con. 1'87