WO2008127204A1

WO2008127204A1 - Optical system for selective laser trabeculoplasty

Info

Publication number: WO2008127204A1
Application number: PCT/SI2008/000024
Authority: WO
Inventors: Grisa Mocnik; Gregor Tavcar; Boris Vedlin; Matjaz Zalar
Original assignee: Optotek doo
Current assignee: Optotek doo
Priority date: 2007-04-11
Filing date: 2008-04-10
Publication date: 2008-10-23
Anticipated expiration: 2009-10-11
Also published as: SI22509A

Abstract

A special optical system for selective laser trabeculoplasty provides a homogenous distribution of power density and a sharp edge of laser shot dot on the target by using a planar source of laser light, having suitable dimension and numerical aperture to emit the light distributed uniformly over the cross section surface and over the angle, and by using a lens system to forward the light from the planar source to the depth of the target in such a way that the light spot on the target has a uniform energy density and that the convergence of the light beam is not greater than 3°. For irradiation of the trabecular meshwork a laser light with a wavelength that ensures good absorption in the melanin is used, wherein the typical wavelength is 532 nm. The light with a 532 nm wavelength is produced by passing the light with a 1064 nm wavelength generated in a laser light source through one nonlinear crystal or through two nonlinear crystals. The laser source can be either the Q-switched Nd:YAG laser or a fiber laser.

Description

OPTICAL SYSTEM FOR SELECTIVE LASER TRABECULOPLASTY

Field of the invention

The present invention relates to the field of medical apparatus for treatment of eye diseases, and more precisely to optical systems designed specifically for performing selective laser trabeculoplasty in therapy of certain types of glaucoma.

Description of the technical problem

Glaucoma is a term for a group of eye disorders in which the optic nerve is damaged. The most frequently used classification of glaucoma is based on the pathophysiology of the disease according to which the many forms of glaucoma are grouped into open-angle glaucoma and closed-angle glaucoma. The latter occur as a consequence of anatomical predisposition, inflammation or neovascularization, and their clinical course is generally acute. A risk factor for development of open-angle glaucoma on the other hand is an increased pressure within the eye which builds up most often as a result of decreased outflow of aqueous humor through trabecular meshwork and the canal of Schlemm into the collector channels that join the venous circulation. The resulting increased intraocular pressure can be reduced by applying the selective laser trabeculoplasty which is a minimally invasive surgical method. In the selective laser trabeculoplasty procedure the laser light shots are absorbed in the tissue, or more precisely, in the epithelial cells containing melanin in the trabecular meshwork located in the anterior chamber angle of the eye. The duration of laser light shots is much shorter than the thermal relaxation time of the target tissue, hence, the temperature rise is confined solely to the laser light irradiated area and the coagulation effect that could occur in the tissue due to absorbed laser light is prevented. In modern solutions of devices for selective laser trabeculoplasty the duration of the laser light shots is typically some nanoseconds, while the wavelength is 532 nm to ensure good absorption of the laser light in the melanin. A typical transverse dimension of the laser beam, specifically of the laser light spot on the trabecular meshwork, is in the order of some tenths of a millimeter, while typical energy of the laser light shot does not exceed 2 mJ. The technical problem which is solved by the optical system according to the present invention is to configure a special optical system for use in a laser device for ophthalmologic surgery which ensures a well defined and uniform distribution of the laser light intensity over the surface of the laser light spot projected onto the target in order to achieve a uniform effect on the radiated tissue. It is a further object of the present invention to provide that the wavelength of the laser light shots is such that good absorption of the laser light in melanin is ensured. Yet another object of the present invention is to create a laser light spot with a sharp edge on the target so that irradiation can be confined solely to a distressed tissue.

Description of the related art

The advanced devices for selective laser trabeculoplasty are typically composed of: - the Nd:YAG Q-switch laser, which includes a compact resonator and generates laser light shots with a wavelength of 1064 nm and a pulse duration of about 5 ns;

- a nonlinear crystal, having its geometry optimized for frequency doubling of laser light with a wavelength of 1064 nm, that is for converting laser light with a wavelength of 1064 nm into laser light with a wavelength of 532 nm. An example of such crystal is KTiOPO₄.

- a lens system, designed for producing a laser light spot of adequate size on the target as well as for ensuring that the laser light beam convergence is small enough so that the beam reaches the eye angle.

The energy and the length of the shots produced by the Nd:YAG laser are well defined by the dimensions and optical characteristics of the laser optical elements, consequently, there is very little variation of pulse energy and pulse length from one shot to another. Because the ratio of resonator length versus beam diameter is relatively small, to wit -10 cm/~1 mm, the distribution of power density over the cross section of the laser light beam is not well defined since this ratio varies from one shot to another. The laser operates namely in a mixture of several transverse modes and the participation of a particular mode in this mixture varies from one laser shot to another. The behavior of the nonlinear crystal is sensitive to power density of the incident laser light; consequently, frequency doubling of such laser shots does not yield a stable outcome. As a result, energy fluctuation in laser light shots with a wavelength of 532 nm produced in a nonlinear crystal is much larger than energy fluctuation in laser light shots with a wavelength of 1064 nm. The surgical treatment with selective laser trabeculoplasty is performed in trabecular meshwork on epithelial melanin containing cells. In executing this operation the damage inflicted to surrounding tissue should be minimal. The laser light spot described above has not sharply defined edge in the depth of the target, because the energy density gradually decreases with the distance from the spot center due to bending of light on its path from its source to the target. With such energy profile of the spot, it is hard to ensure that the laser light would not harm also the adjacent tissue regions which should not be irradiated. The problem of uniform distribution of energy within sharp edges of the laser light spot is solved by an illumination system described in the patent US6532244. The illumination system includes a multimode diode-laser and two optical fibers. Light from the diode-laser is directed into the first optical fiber, and the outgoing light from the first optical fiber is directed by an optical system into a second optical fiber, having a core diameter greater than the first optical fiber and a numerical aperture greater than the numerical aperture of the optical system. A light beam exiting the second optical fiber has more uniform intensity across the profile than the light beam exiting from the first optical fiber. This system solves above all the problem of the oval shape of the spot produced by the light beam from a diode laser.

Description of the solution of the technical problem To solve the technical problem of ensuring a homogenous density of energy as well as a sharp edge of irradiation with the laser light at a selected depth of the tissue, the present invention includes the following essential parts:

- a planar laser light source, having suitable dimension and numerical aperture and emitting light that is uniformly distributed over the planar surface area and over the angle and that has a wavelength that ensures good absorption of light in the melanin;

- an arrangement of lenses, passing on the light beam from said planar source of laser light to the target's depth in such a way that the light has a uniform energy density on the target, as well as ensuring that the said light beam has a very small convergence angle.

The planar source of laser light in the optical system according to the present invention can be made: a) by using an optical fiber which has adequate dimension and numerical aperture; or b) by using a diaphragm with a circular aperture, having a correspondingly smaller diameter than the light beam projected onto the aperture. Laser light exiting from the planar source can be projected onto a target: a) in the form of overlapping collimated beams when the planar source is a light outgoing surface of optical fiber; or b) as an image of the planar source when the planar source is a light outgoing surface of optical fiber or a circular aperture in a diaphragm.

In the following detail description the optical system for selective laser trabeculoplasty according to the present invention will be presented with reference to the accompanying figures showing:

Figure 1 : a schematic depiction of the course of the laser beam propagating from the light outgoing surface of the optical fiber through a converging lens.

Figure 2: a schematic depiction of the course of the laser beam as well as of individual rays in the laser beam propagating from the light outgoing plane of the optical fiber through the arrangement of lenses of the optical system. Figure 3: a schematic depiction of the transposition of aperture image onto the target by uniformly illuminating the aperture in a diaphragm with a laser beam. Figure 4a: a distribution of energy over the laser spot surface before frequency doubling. Figure 4b: a distribution of energy over the laser spot surface after frequency doubling by means of one nonlinear crystal.

Figure 4c: a distribution of energy over the laser spot surface after frequency doubling by means of two nonlinear crystals.

In one embodiment of the optical system according to the present invention, shown schematically in Figure 1 , the laser beam is conveyed to the optical system through the optical fiber 1. The light outgoing flat surface 2 of the optical fiber 1 is positioned at the left focus of the converging lens 3 situated in the plane G3¹. At the right focus of the lens 3 on the other side of the lens, a beam waist of the laser beam is created having a homogenous light intensity profile over its cross section and a well defined sharp edge. The light leaving the outgoing surface 2 of the optical fiber 1 has a rather uniform distribution over the cross section as well as over the angle, which is due to numerous reflections of the laser beam in the optical fiber. The edge of the light beam is defined by the outgoing surface 2 and by the numerical aperture of the optical fiber 1. The light cones emerging from individual points on the outgoing surface 2, which is placed exactly at the focal length distance from the lens 3, are transformed into collimated rays by the lens 3. The axes of all beams intersect at the focal point on the other side of the lens where the intensity profiles of individual beams coincide. The key parameters for good coincidence of the profiles are the diameter and the numerical aperture of the optical fiber, since the product of the divergence of individual beam, having a shape of a light cone, and the distance of the beam focus point from the optical axis on one side of the lens equals to the product of the diameter and the angle of incidence of the same beam on the other side of the lens.

In Figure 2, another embodiment of the optical system according to the present invention is shown schematically. The laser beam is conveyed towards the optical system through the optical fiber 1. The outgoing flat surface 2 of the optical fiber 1 represents a homogenous planar source which is projected through an optical system or a lens system, typically through two converging lenses 3 and 4, as an optical image onto the target 5. Like in the embodiment presented previously, the diameter and the numerical aperture of the optical fiber 1 are the key parameters, since in optical imaging the product of the beam's divergence and the distance between beam's focus and optical axis remains unchanged. The smallest angle of beam's convergence is achieved when the symmetry axes of individual rays within a beam are parallel, which means that the geometry of incidence of light onto the target mirrors the geometry of release of light out of the planar light source. Such geometry is achieved when the outgoing surface of the optical fiber is positioned in the focus of the first lens and the optical image of the outgoing surface is created in the focus of the second lens.

In another embodiment of the optical system, shown schematically in Figure 3, the laser beam 8 is screened by a diaphragm 9 having an aperture 10 which catches only that portion of a laser beam where the planar intensity profile is most homogenous. By means of a lens system, typically with two converging lenses 3 and 4, an optical image of the aperture 10 in the diaphragm 9 is created, with suitable magnification or reduction, on the target 5. The laser beam, which has typically a wavelength of 532 nm and has a diameter size as needed to ensure the entry into the eye with a very small divergence, creates an optical image of the aperture 10 of the diaphragm 9 exactly in the depth of the target 5. The aperture 10 of the diaphragm 9 must be small enough to catch only the portion of the beam with uniform energy density, and at the same time big enough to forward sufficient energy. The lenses 3 and 4 must be converging lenses, as a real optical image can be created only by converging lenses.

The optical system according to the present invention has the following essential parts for generation of laser shots that have stable energy profile and can be absorbed well in melanin: - A pulsed laser light source, emitting near infrared light and producing shots of a few nanoseconds length.

- Two nonlinear crystals, designed for frequency doubling, with dimensions and orientation chosen to efficiently convert near infrared light into the light with a wavelength that ensures good absorption of light in melanin.

To stabilize the energy of laser pulses, which have a specific wavelength, typically 532 nm, to ensure good absorption of light in melanin, a type Il of quadrature frequency doubling method, proposed by D. Eimerl (D. Eimerl, Quadrature frequency conversion, IEEE J. Quantum Electron. 23, 1361-1371 (1987)), is applied in the optical system according to the present invention. Two nonlinear crystals are used, normally having different lengths. Their main planes, i.e. the planes defined by the vector of the laser light beam direction and by the optical axis of the first and of the second crystal, respectively, are perpendicular to one another. In an ideal case of frequency doubling with nonlinear crystal, phase matching of incoming and outgoing laser light waves is perfect. This happens when the incoming light is a monochromatic electromagnetic wave, which is not limited in space and time, and when orientation of the nonlinear crystal is absolutely correct. In such situation, the efficiency of typical conversion of the light with a 1064 nm wavelength into the light with a 532 nm wavelength increases with the density of the intensity of the incoming wave until saturation is achieved, i.e. until 100 %, or the said efficiency increases with the length of the nonlinear crystal, i.e. with the dimension along which travels the light of the laser shot. In such ideal situation, with a given density of the incoming wave intensity, the efficiency of the conversion can be increased by increasing the length of the crystal. In real circumstances, increasing of conversion efficiency as described above is not possible, since a digression from perfect phase matching is always present which only increases with crystal length. So, beyond a given length of the crystal, further lengthening only leads to a counter effect: the conversion efficiency begins to fall, because due to mismatch of phases the already produced light with a 532 nm wavelength is converted back into the light with a 1064 nm wavelength.

Therefore, to achieve maximum conversion efficiency, crystals of different lengths must be applied for different incoming power densities.

For light beams that do not have a uniform distribution of power over the cross section, a very high and constant efficiency of frequency conversion can be achieved by using two nonlinear crystals of different lengths oriented at right angle to each other. Due to right angle positioning of crystals the resulting frequency conversion is called quadrature frequency doubling. The shorter nonlinear crystal performs efficient doubling in regions with high power density, and the longer one in the regions with lower power density. Their arrangement at right angle prevents the light with a 532 nm wavelength that leaves the first crystal from instigating unwanted frequency conversion back to a 1064 nm wavelength in the second crystal, as a 532 nm wavelength light has not the right polarization for such frequency conversion in backward direction.

Due to high stability and high efficiency of conversion, the energy of pulses stays within the range of allowed deviation from nominal energy and the fluctuations of energy density across the intensity profile of the beam in the depth of the target are minimal. The light with a 1064 nm wavelength incoming into the nonlinear crystal in general does not have an even distribution of power density over its cross section, as shown in Figure 4a. If only one nonlinear crystal is used, in the regions with lower energy density frequency doubling is not fully effected, so within the profile of the light with a 532 nm wavelength, captured by the aperture 10 in the diaphragm 9, the regions with insufficient power density occur. The energy density in these regions does not exceed the threshold value required for successful trabeculoplasty, for that reason the region of successful trabeculoplasty is smaller than the laser dot on the target, as illustrated in Figure 4b.

In order to attain a homogenous energy density within a sharp edge of the dot on the target, in all regions of the dot the density of the energy delivered to the target must be higher than the threshold value which ensures efficient trabeculoplasty. By using two nonlinear crystals arranged for quadrature frequency doubling, the frequency doubling is efficient for the light in all the regions of the planar cross section of the beam. As a result, within the profile of the laser beam captured by the aperture 10 in the diaphragm 9, there are no areas with unsatisfactory energy density, as shown in Figure 4c.

A stable energy of a laser shot which typically has a wavelength of 532 nm can be achieved also by using a fiber laser. A fiber laser has a beam quality superior to that of the Nd: YAG laser. Hence, if a fiber laser is used as a source of light with a wavelength of 1064 nm, then the energy density within the beam does not vary from one shot to another, consequently frequency doubling in the nonlinear crystal is constant over time and space. A fiber laser thus ensures a highly homogenous energy density within the sharp edge of the dot on the target, moreover, in all the regions of the dot it provides the energy density level that is above the threshold value needed for efficient trabeculoplasty.

Claims

1. Optical system for selective laser trabeculoplasty, characterized in that it comprises a planar laser light source and an arrangement of lenses; that the mentioned planar laser light source has suitable dimension and numerical aperture that ensure a uniform distribution of intensity of the outgoing light over the planar surface area and over the angle within the confines of the numerical aperture; that the mentioned arrangement of lenses conveys the light from the mentioned planar laser light source to the depth of the target, where the convergence of the light beam is within the region of a few degrees and where the laser dot has a uniform energy density over its entire surface.

2. Optical system of claim 1 , wherein the laser light that exits from the mentioned planar light source has a wavelength that ensures good absorption in melanin.

3. Optical system of claims 1 and 2, wherein the laser light that exits from the mentioned planar light source has a wavelength of 532 nm.

4. Optical system as in claims from 1 to 3, wherein the mentioned planar laser light source is the outgoing light surface (2) of the optical fiber (1 ).

5. Optical system as in claims from 1 to 3, wherein the mentioned planar laser light source is realized by means of a diaphragm (9) with a circular aperture (10) that is suitably smaller, typically approximately twice smaller than the diameter of the laser light beam that falls on the diaphragm (9).

6. Optical system as in claims from 1 to 5, wherein the laser light exiting from the planar laser light source is projected through the lens system (3, 4) onto the target (5) in the form of collimated beams that overlap in the depth of the target.

7. Optical system as in claims 1 , 2, 3, 4 and 6, wherein the laser light exiting from the planar laser light source, which is the outgoing light surface (2) of the optical fiber (1 ), is projected onto the target (5) as an optical image of the planar laser light source.

8. Optical system as in claims 1 , 2, 3, 5 and 6, wherein the laser light exiting from the planar laser light source, which is a circular aperture (10) of the diaphragm (9), is projected onto the target (5) as an optical image of the planar laser light source.

9. Optical system as in claims from 1 to 8, characterized in that it receives laser light for its planar laser light source from the laser system in which a laser having a crystal as active medium and including Q-switching is used as a source of light with a wavelength in the near infrared region around 1000 nm and with the pulse length in the nanosecond region.

10. Optical system as in claim 9, wherein the laser light, which has a wavelength that ensures good absorption in melanin and which enters the optical system, is generated in a laser system by frequency doubling in one nonlinear crystal.

11. Optical system as in claim 9, wherein the laser light, which has a wavelength that ensures good absorption in melanin and which enters the optical system, is generated in a laser system by quadrature frequency doubling in two nonlinear crystals.

12. Optical system as in claims from 1 to 8, characterized in that it receives the laser light for its planar laser light source from the laser system in which a fiber laser is used as a source of light with a wavelength in the near infrared region around 1000 nm and with the pulse length in the nanosecond region.

13. Optical system as in claim 12, wherein the laser light, which has a wavelength that ensures good absorption in melanin and which enters the optical system, is generated in a laser system by frequency doubling in one nonlinear crystal.

14. Optical system as in claim 12, wherein the laser light, which has a wavelength that ensures good absorption in melanin and which enters the optical system, is generated in a laser system by quadrature frequency doubling in two nonlinear crystals.