SI22509A - Optical system for selective laser trabeculoplastics - Google Patents

Abstract

The optical system for selective laser trabeculoplastics ensures a homogeneous density of energy and a sharp edge of the laser shot dot on the target by way of flat laser light source which features the corresponding dimensions and numerical aperture so that the originating light is within the limits of the numerical aperture distributed uniformly across the plane and angle, and with the help of a lens system, which conveys the light from the planar source to the target depth in such a way that it features a uniform energy density on the target, and which takes care that the convergence by the beam of light does not exceed 3 degrees. For irradiation, laser light of such a wavelength is used which is well absorbed in melanin, the typical wavelength being 532 nm which is produced after the light passes the laser source generating a light of 1064 nm wavelength through a non-linear crystal or through two non-linear crystals. The laser source can be a Nd:YAG laser with quality switch or fibre laser.

Description

OPTICAL SYSTEM FOR SELECTIVE LASER TRABECULOPLASTY

The field of technology

The invention belongs to the field of medical devices for the treatment of eye diseases, more specifically to the field of laser treatment of certain types of glaucoma with selective laser trabeculoplasty.

A technical problem

Glaucoma is a group of eye diseases that affect the optic nerve. The most commonly used classification of glaucoma is based on pathophysiology and divides glaucoma into open-angle and closed-angle glaucoma. The latter are due to anatomical predisposition, inflammation, or neovascularization and usually have an acute course. In open-angle glaucoma, a risk factor is elevated ocular pressure, which is most often the result of decreased flow of ventricular fluid through the trabecular meshwork and the Schlemm channel into the collecting ducts associated with venous plaques. This increase in eye pressure can be reduced by selective laser trabeculoplasty, which is a minimally invasive procedure.

In laser trabeculoplasty, laser shocks are absorbed in the tissue of the trabecular meshwork in the anterior anterior chamber angle, in melanin-stained epithelial cells. Laser shocks are much shorter than the thermal relaxation time of the tissue that is the target of the laser light, so that the temperature rise is limited to the irradiated area, so the absorbed laser light has no coagulation effect on the tissue. Modern solutions for selective laser trabeculoplasty devices typically use a few nanoseconds of long-wave laser light with a wavelength that ensures good absorption in melanin, typically a wavelength of 532 nm. The transverse dimension of the laser beam (dot) on the trabecular mesh is typically a few tens of millimeter in size, and the typical laser shock energy does not exceed 2 mJ.

A technical problem solved by the optical system of the invention is the construction of the optical composition in a laser eye surgery device, which provides a well-defined and uniform distribution of light intensity across the surface of the laser shock dot on the target, in order to achieve a uniform laser light effect on the irradiated tissue. . In doing so, laser shocks must have a wavelength that is well absorbed in melanin.

The object of the system of the invention is also to create a dot on the target with a sharp edge, so that the irradiation can be restricted to the affected tissue only.

The prior art

Modern selective laser trabeculoplasty apparatus typically consists of:

- a quality-switching Nd: YAG laser having a compact resonator and generating sunlight with a wavelength of 1064 nm and a length of about 5 ns;

- a nonlinear crystal with a geometry optimized for frequency doubling of laser light with a wavelength of 1064 nm, that is, it converts laser light with a wavelength of 1064 nm to light with a wavelength of 532 nm. An example of such a crystal is ΚΤ1ΟΡΟ4.

- lenses that ensure that the laser beam dot is at the correct size target and that the beam convergence is sufficiently small to allow the beam to reach the eye corner.

The energy and the length of the shock generated by the Nd: YAG laser are well defined by the dimensions and optical properties of the optical elements of the laser, meaning that they differ very little from shock to shock. Since the ratio of the resonator length to beam diameter is relatively small, namely ~ 10cm / ~ 1 mm, the power density distribution of the laser beam after the beam cross-section is not well defined, as it varies from shock to shock, namely, the laser operates in a mixture of several transverse modes. the individual mode in this blend differs from gust to gust. The behavior of the nonlinear crystal is sensitive to the power density of the incident laser light, and therefore the conversion efficiency is not constant at the frequency doubling of such beams. Thus, energy fluctuations are much larger for beams with a wavelength of 532 nm generated in a nonlinear crystal than for beams with a wavelength of 1064 nm.

In selective laser trabeculoplasty, the intervention is performed on melanin-stained epithelial cells of the trabecular meshwork. In doing so, we want to affect as little adjacent tissue as possible. The dot of the laser beam described above has very sharp edges at the target's depth, meaning that the energy density declines sharply with distance from the center of the dot, as a result of the inevitable deflection of light on the path from the source to the target. With such a dot energy profile, it is difficult to ensure that the laser dot does not affect areas of tissue that we do not want to irradiate.

The problem of uniform energy distribution within the sharp edges of a dot is solved by the illumination system described in US6532244. The illumination system has a multifaceted diode laser and two optical fibers. The light from the laser is directed to the first optical fiber, and the light output from the first optical fiber is directed through the optical system to a second optical fiber having a diameter larger than the first fiber and having a numerical aperture greater than the numerical aperture of the optical system. The light emitting from the second optical fiber has a more uniform intensity in profile than the light from the first fiber. The system described in patent US6532244 primarily solves the problem of the ovality of the light beam dot from a diode laser.

Description of solution to a technical problem

The solution of a technical problem according to the invention has the following essential functional parts to ensure a homogeneous energy density and a sharp irradiation edge at a selected depth:

- a planar laser light source having an appropriate dimension and numerical aperture, from which the resulting light is uniformly distributed over the plane and angle and has a wavelength that ensures good absorption in melanin;

- a lens which brings the light from said planar origin to the depth of the target so that it has a uniform energy density on the target, and which ensures that the beam of this light has very little convergence.

The planar origin in the optical system of the invention may be made:

a) an optical fiber having an appropriate dimension and numerical aperture; or

b) by means of an aperture with a circular aperture which is suitably smaller than the diameter of the laser light incident on the aperture.

Out-of-plane laser light can be projected onto a target:

a) the principle of overlapping collimated rays where the plane origin is the exit plane of the optical fiber; or

b) as a mapping of planar origin when the planar origin is the optical fiber exit plane or the aperture circular aperture.

The embodiments of the optical system for selective laser trabeculoplasty according to the invention are described in more detail below by means of the following figures:

Figure 1: Schematic representation of the laser beam beam path from the optical fiber exit plane through the collecting lens.

Figure 2: Schematic representation of the laser beam or individual beam paths from the optical fiber exit plane through the lens of the optical system. Figure 3: Schematic representation of the aperture mapping, evenly illuminated by the laser beam, to the target.

Figure 4a: Diagram of the energy distribution across the dot plane before frequency doubling. Figure 4b: Diagram of the energy distribution across the dot plane after frequency doubling using a single nonlinear crystal.

Figure 4c: Energy distribution diagram of the dot plane after frequency doubling using two nonlinear crystals.

In the embodiment shown schematically in Figure 1, the laser beam is guided to the optical system by optical fiber 1. The exit surface 2 of optical fiber 1 is located in the left center of the collecting lens 3, which is located in the plane G3 '. In the right focus on the other side of lens 3, located in the G3 plane, a laser beam throat is created which has a homogeneous planar profile of strength and a sharp edge. Due to the many reflections inside the fiber, the light leaving the exit surface 2 of the optical fiber 1 is rather evenly distributed along the profile and angle and is limited by the edge defined by the exit surface 2 and the numerical aperture of the optical fiber 1. The light cones emanating from individual points on the exit surface 2, which is distant from the lens 3 just behind the focal length, the lens 3 transforms into collimated rays. The axes of all rays intersect at the focal length on the other side of the lens, where the plane profiles of the strengths of the individual rays coincide. The diameter and numerical aperture of the fiber are the key parameters, since the product of the divergence of a single beam (as a light cone) and the distance of the focal point of the beam from the optical axis on one side of the lens are equal to the diameter and incidence angle of the same beam on the other side of the lens.

In the embodiment shown schematically in Figure 2, the laser beam is guided to the optical system by optical fiber 1. The output surface 2 of optical fiber 1 is a homogeneous plane origin, which is typically optically combined with an optical or lens system, typically with two collecting lenses 3 and 4. In this embodiment, the diameter and the numerical aperture of the optical fiber 1 are key parameters, since in the case of lens imaging the product of the beam divergence and the focal length of the beam from the optical axis is maintained. The smallest convergence angle of the laser beam is reached when the symmetry of the individual rays within the beam is parallel, that is, the light strikes the target in the mirror-like manner as it leaves the plane origin. Such geometry is achieved when the exit plane of the fiber is placed in the focus of the first lens and an image of the exit plane is created in the focus of the second lens.

In the embodiment shown schematically in Figure 3, the laser beam 8 is screened so that the portion of the beam 9 is trapped in the aperture 10, where the planar strength profile is most homogeneous. The lens is mapped to the target 5 by a suitable zoom in or out using a lens system, typically two collection lenses 3 and 4. A laser beam having a wavelength typically of 532 nm and a diameter required for the application to enter the eye with very small divergence has the property that, at the depth of target 5, it produces an image of the aperture 10 of the aperture 9. The aperture 10 of the aperture 9 must be sufficiently small, that the beam area it captures has a uniform energy density and large enough to allow sufficient energy. Lenses 3 and 4 must be collectable, since only collecting lenses can create a realistic image.

The solution of a technical problem according to the invention has the following essential functional parts for the production of energy-stable shocks of light, which is well absorbed in melanin:

- Light source of laser light in the near infrared region, producing a few nanoseconds in length.

- Two non-linear frequency-doubling crystals whose dimensions and orientation are chosen to effectively convert near-infrared light into wavelength light that provides good absorption in melanin.

To stabilize the shock energy of a wavelength well absorbed in melanin (typically 532 nm), the solution of the invention uses the principle of quadrature doubling of type II frequency (D. Eimerl, Ouadrature frequency conversion, IEEE J. Q. Electr QE23 (1987) 1361-1371). According to this principle, we use two, as a rule, differently different nonlinear crystals, whose principal planes are the planes determined by the laser beam direction vector and the optical axis of the first or the other crystals are perpendicular to each other.

For ideal frequency duplication with a nonlinear crystal, the matching of the incoming and outgoing wave phases is perfect. This occurs when the incident light is a monochromatic EM wave that is spatially and temporally unrestricted and the nonlinear crystal is perfectly oriented correctly. In such a situation, the efficiency of typical conversion of light with a wavelength of 1064 nm into light with a wavelength of 532 nm with a density of incident wave power up to saturation, that is, up to 100%, or with a length of a nonlinear crystal, that is, with a dimension along which the laser light travels along, increases . In such an ideal situation, at a given power density of the incident wave, the conversion efficiency can be increased by extending the length of the crystal.

In the real world, increasing the conversion efficiency described above is impossible, since there is always a deviation from perfect phase matching, which only grows with the lengthening of the crystal. Thus, at a given crystal length, the lengthening has the opposite effect: the conversion efficiency begins to fall as phase mismatches cause the already generated light with a wavelength of 532 nm to be converted back into light with a wavelength of 1064 nm.

In practice, this means that different lengths of nonlinear crystals are required to achieve maximum conversion efficiency at different power densities.

For light beams that do not have a uniform distribution of power density across the cross section, it is possible to achieve a very high and constant frequency conversion efficiency by successively placing two differently long nonlinear crystals oriented perpendicularly to each other. Due to the rectangular arrangement of the crystals, such a frequency conversion is called quadrature frequency doubling. The shorter nonlinear crystal effectively doubles the areas with high power density, and the longer ones areas with lower power density. In doing so, we take advantage of their perpendicular arrangement, which ensures that light with a wavelength of 532 nm leaving the first crystal cannot cause unwanted conversion back to light with a wavelength of 1064 nm in the second crystal, as it has the wrong polarization.

The high stability and high conversion efficiency ensure that the energy of the shock is always within the tolerance of the nominal energy and that the fluctuations of the energy density within the beam intensity profile at the target depth are minimal.

Light with a wavelength of 1064 nm entering a nonlinear crystal typically does not have a constant power density distribution across the profile, as can be seen from Figure 4 a. When only one nonlinear crystal is used, regions with lower energy densities are doubled in frequency, and therefore areas with insufficient power density appear within the light profile with a wavelength of 532 nm captured by the aperture 2. This means that the energy density in those regions does not exceed the threshold 1 for successful trabeculoplasty, that is, the area of successful rabeculoplasty 3 is smaller than the dot on the target, as illustrated in Figure 4b.

In order to achieve the greatest possible homogeneity within the sharp edge of the dot on the target, the density per target of the energy delivered to the target must be above the threshold in all areas of the dot, providing effective trabeculoplasty. When using two nonlinear crystals positioned for quadrature frequency doubling, light from all areas of the planar beam section is effectively frequency doubled, so no unwanted areas with an unsatisfactory energy density are obtained in the laser beam profile captured by the aperture 2, as shown in Figure 4c .

A stable shock energy with a typical wavelength of 532 nm can also be achieved using a fiber laser. The quality of the fiber laser beam is far superior to that of the Nd: YAG laser beam. If a fiber laser is used as a light source with a wavelength of 1064 nm, the energy density inside the beam does not change from shock to shock and therefore the frequency doubling in the nonlinear crystal is constant over time and space. This results in high homogeneity of the energy density within the sharp edge of the dot on the target and such energy density over all the areas of the dot above the threshold that provides effective trabeculoplasty.

Claims (14)

PATENT APPLICATIONS An optical selective laser trabeculoplasty system characterized by a planar laser light source and lens; yes there is 5 the said laser plane light source having an appropriate dimension and numerical aperture to provide a uniform distribution of the light output over the plane and angle within the limits of the numerical aperture; that said lens of light from said planar origin is brought to the target depth, where the beam of light has convergence in the range of a few degrees and has a laser dot 10 uniform energy density throughout the profile. The optical system of claim 1, wherein the laser light exiting the said planar origin has a wavelength that is well absorbed in the melanin. An optical system according to claims 1 and 2, wherein the laser light has an outgoing light 15 from said planar origin, wavelength 532 nm. An optical system according to claims 1 to 3, wherein said plane origin is the exit surface (2) of the optical fiber (1). Optical system according to claims 1 to 3, wherein said planar origin is realized by means of an aperture (9) with a circular opening (10), which is suitable 20 smaller, typically about twice smaller, than the diameter of the laser beam incident on the aperture (9). Optical system according to claims 1 to 5, in which laser-emitting laser light is projected through the lens (3, 4) onto the target (5) in the form of collimated rays that overlap at the target depth. Optical system according to claims 1,2, 3, 4 and 6, in which the exiting laser light is projected from the surface source, which is the exit surface (2) of the optical fiber (1), onto the target (5) as a mapping of the surface origin. Optical system according to claims 1, 2, 3, 5 and 6, in which the exiting laser light is projected from the planar source, which is a circular opening (10) of the aperture (9), onto the target (5) as a mapping of the planar origin. An optical system according to claims 1 to 8, characterized in that it receives 5 laser light for its planar laser light source from a laser system in which a laser with a crystal as an active agent and a quality switch is used as a light source with a wavelength in the near infrared region around a wavelength of 1000 nm and a beam length in the nanosecond range. 10. The optical system of claim 9, wherein the laser wavelength, which is well absorbed in melanin and enters the optical system, is produced in the laser system by doubling the frequency in a single nonlinear crystal. The optical system of claim 9, wherein the laser light is wavelength, 15, which is well absorbed in melanin and enters the optical system, produces in the laser system a quadrature frequency doubling in two nonlinear crystals. Optical system according to claims 1 to 8, characterized in that it receives laser light for its planar laser light source from the laser 20, in which a fiber laser is used as the light source with a wavelength in the near infrared region around a wavelength of 1000 nm and a beamlength in the nanosecond region. The optical system of claim 12, wherein the laser wavelength, which is well absorbed in melanin and enters the optical system, is 25 produces a laser system by doubling the frequency in a single nonlinear crystal. The optical system of claim 12, wherein the laser wavelength, which is well absorbed in melanin and entering the optical system, is produced in the laser system by quadrature doubling of frequency in two 30 nonlinear crystals.