CA2642892A1

CA2642892A1 - Apparatus and method for the detection of the focused position of an optical system and ophthalmological treatment apparatus

Info

Publication number: CA2642892A1
Application number: CA002642892A
Authority: CA
Inventors: Olaf Kittelmann; Peter Triebel
Original assignee: Wavelight Ag; Olaf Kittelmann; Peter Triebel; Wavelight Gmbh; Alcon Inc.
Current assignee: Alcon Inc
Priority date: 2006-02-20
Filing date: 2007-02-20
Publication date: 2007-08-30
Anticipated expiration: 2027-02-20
Also published as: EP1986582A1; EP1986582B1; MX2008010678A; RU2008133868A; US20090127429A1; JP2009527265A; CA2642892C; JP5027163B2; US7982169B2; DE102006007750A1; KR101310045B1; CN101389296B; BRPI0708057A2; BRPI0708057B1; RU2440084C2; CN101389296A; ES2393518T3; KR20080108474A; BRPI0708057B8; WO2007096136A1

Abstract

An apparatus and a method are presented for detecting the focal position of an optical system (10) with a radiation source (12), a focusing imaging system (16), an at least partially reflective surface (18) on the focus (18a), a digital camera (24) for recording an image reflected by said surface (18), a computer (C) for evaluating the image recorded by the camera (24), and with an optical element (34; 36) in the beam path of the optical system (10) upstream of the focusing imaging system (16), which element influences said image depending on the focal position.

Description

Apparatus and method for detecting the focal position of an optical system and ophthalmological treatment apparatus The invention relates to an apparatus and a method for de-tecting the focal position of an optical system. In par-ticular, the invention relates to an apparatus and a method for detecting the depth of focus of an imaging optical sys-tem and moreover also to an apparatus and a method for con-trolling the focal position and in particular the depth of focus. Furthermore, the invention also relates to an oph-1o thalmological treatment and/or diagnosis apparatus using said apparatus and/or said method.

In the case of the optical systems under discussion here, the system in question is in particular an imaging optical system in a material processing installation using light sources, such as lasers and LEDs in particular. Material processing should be understood here to mean also material structuring in the microrange, e.g. for dielectric materi-als, such as biological tissue, or also metallic materials.
In particular, the invention can be used in ophthalmologi-cal optical systems, especially in refractive corneal sur-gery, such as LASIK, for example. A particularly suitable application area for the present invention in this case is fs-LASIK, thus refractive corneal surgery using a femtosec-ond laser.

In the aforesaid optical imaging systems, achieving highly precise material processing operations depends inter alia on exact control of the focal position. "Focal position"
is understood here above all to mean not only the location of the focus in the direction of the optical axis (so-called depth of focus), however, but more generally also the position and orientation of the focused radiation, thus e.g. an offset in relation to the ideal optical axis of the

2 system or angularity of the actual axis of the optical ra-diation in relation to the ideal (desired) optical axis.
In fs-LASIK it is particularly important to adhere to the calculated depth of focus and this is a particular applica-tion of the present invention.

In DE 10 2004 009 212 Al, an optical contact element for laser material processing is presented. This contact ele-ment is used in the preferred embodiment for fs-LASIK. In this case this contact element consists of a diffractive io optical structure. These structures are intended to mini-mize the incidence angles occurring due to high numerical apertures of the lens. The diffractive optical element (DOE) consists here of a grating structure with radially adjusted grating period. The grating periods in this case are between 2001/mm and 5001/mm. Values in the um range are indicated as spot sizes. Due to optical limits, only one numerical aperture of approx. 0.3 is possible.
Enlargement of the aperture is achieved by using a second diffractive element in the beam path of the lens. This DOE
is likewise executed as a circular grating structure with a grating period that becomes larger towards the optical axis. Achieving higher numerical apertures is indicated here as an advantage of this execution. Furthermore, the contact element is executed curved. The radius of curva-ture corresponds to the radius of curvature of the eye, approx. 8 mm. Material processing is carried out with this uniformly preset radius of curvature. The suction attach-ment is carried out similar to WO 03/002008 Al and EP 1 159 986 A2. Focus control is not carried out with this pre-sented method.

In EP 0 627 675 Al, a diffractive optical device is pre-sented for the mapping of one or more space points from a beam. Here the diffractive structure likewise consists of

3 a segment-like arrangement of arbitrary binary or multi-stage diffractive elements. The arrangement can be a hex-agonal or hexangular arrangement in particular. Thus mapping of a light beam is achieved. However, only an in-s tensity or/and phase transformation is undertaken.

In US 2002/0171028, an apparatus for focus control is de-scribed. Here the returning light is brought by an imaging beam path to interference with a second bundle of rays and thus interferometric wave control is carried out.

Focus control by means of interferometric wavefront control is carried out likewise in US 6,666,857 B2. The active wavefront control during the photoablation process on the human eye is then achieved by a combination of adaptive mirrors. No active wavefront control is to be undertaken.

In US 2004/0051976 Al, an optical arrangement of a confocal microscope, consisting of a laser source emitting predomi-nantly in the UV spectral range, a beam expander, a dif-fractive pinhole array and an objective lens is described.
A diffractive pinhole array is not described in its exact zo embodiment. The increase in efficiency can be seen as one advantage of this technical embodiment, as amplitude pin-hole arrays have a typical transmission of between 4% and 10% depending on the aperture ratio. With a diffractive pinhole array, on the other hand, transmission values of such an optical element of up to 80% are possible, depend-ence on the aperture ratio or the number of pinholes only being conditional on the manufacturing here.

In US 2004/0021851, an optical arrangement consisting of a laser and subsequent beam shaping optics is used to measure the focal length of an unknown lens. Measurement of the focal length is carried out in this case by focusing on a

4 reference surface at various distances. The portion of the radiation that is reflected back is detected. The spot diameters are then evaluated at the respective distances.
The focal length is determined by means of the "Newton's"
s relation Z Z'=f2. An optical grating, which is not de-scribed in greater detail, is used to decouple the portion of the radiation reflected back. The Jones matrix formal-ism is likewise drawn on to calculate the focal length.
The accuracy of the method is near 1%.

io In US 6,909,546 B2, an optical arrangement consisting of a light source (Nd:YAG2w) and subsequent beam shaping optics is described. In this case two diffractive optical ele-ments are used to homogenize the laser radiation. The first of the two DOEs is used here for homogenization and 15 spatial frequency filtering. A subsequent pinhole carries out the spatial frequency filtering. Located inside the 2f system of spatial frequency filtering is the second DOE, which produces the desired intensity distribution in the far field. The far field is produced either by the field 20 lens or by the 2nd DOE. The desired intensity distribution is produced in the focus. Focus control is not carried out in this method.

The object of the invention accordingly is to provide an apparatus and a method with which the focal position of an 25 optical system can be determined precisely.

To this end the invention provides an apparatus for detect-ing the focal position of an optical system with a radia-tion source, a focusing imaging system, an at least partially reflective surface on the focus, a suitable digi-30 tal sensor system (e.g. CCD camera, CMOS camera or the li-ke) for recording an image that is reflected by said surface, a computer for evaluating the image recorded by the camera, and an optical element in the beam path of the optical system upstream of the focusing image system, which optical element influences said image depending on the fo-cal position.

5 In this case said focusing optical imaging system is pref-erably focusing optics with adjustable (variable) focal position, thus in particular a system with which the loca-tion of the focus is adjustable in a direction parallel to the optical axis of the image (thus the depth of focus).
io In addition, in such a system the focal position is usually also adjustable in a direction perpendicular to the optical axis of the radiation, e.g. in fs-LASIK.

The apparatus according to the invention and the corre-sponding method thus serve in particular for the initial setting and alignment of an optical system such that imme-diately prior to material processing in relation to a pre-determined plane, the so-called surface, the focus is adjusted precisely, in particular so that it lies exactly on this surface. When used in LASIK, said null plane is preferably a surface that arises due to the fact that the cornea is attached by suction in the area of interest to a reference surface (this is known as such to the LASIK ex-pert). The flattening disc, which is transparent for the radiation used, is coated on its side facing the cornea and lying close to this such that a small percentage of the incident radiation is reflected. This reflection then pro-duces said image of the radiation focused onto this null plane, which image is measured using said camera and evalu-ated. In ideal focusing, the focus should therefore lie exactly on this null plane (thus essentially on the flat-tened cornea surface in the example shown) and according to the evaluation of the reflected image the optical system is then adjusted so that the focusing is optimal, thus the

6 position of the focus is exactly in this null plane. The optical system is thus set and aligned and can be used for the subsequent material processing. In the subsequent ma-terial processing the position of the focus is usually changed in relation to said null plane. Thus in fs-LASIK, for example, when cutting the so-called flap the focus is placed in the stroma and the focus positions are varied successively at right angles to the optical axis to produce the flap. This is known as such. The initial setting of lo the system as described above guarantees exact positioning of the foci at the desired target points.

In other material processing operations the null plane, which can also be described as the reference plane, can be defined differently and does not necessarily have to coin-cide with the surface of the material to be processed. The radiation focused onto the null plane and the measurement of the image reflected in this plane supply calibration of the optical system such that the setting of the optical imaging properties of the optical system for the ideal sta-te of focusing exactly in the null plane is known due to the image measurement, so that then, starting out from the-se settings of the optical system, the focal position can be changed according to the desired material processing, e.g. into the inside of the cornea.

According to one configuration, said optical element, which influences the focus image to be measured depending on the focal position, is a diaphragm matrix (so-called pinhole array).

The optical element can also be a so-called diffractive optical element (DOE), which produces a dot pattern in the far field distribution (known as such to the person skilled in the art and not explained in greater detail here).

7 Said optical element can be arranged in the beam path of the reflected image between the reflective surface and the camera, or also outside this beam path. Advantages result in each case according to the type of application.

The amplitude (intensity) or the phase (wavefront) of the reflected image can preferably be influenced locally with the optical element and the defocus portions of the wave-front can be rendered visible.

It is also possible to provide said optical element in the io beam path both phase-sensitively and amplitude-sensitively, in particular a combination thereof.

According to a preferred configuration, the optical element produces a dot pattern, in particular a regular dot pattern in the form of a matrix.

The invention also provides a method for detecting the fo-cal position of an optical system, in which the radiation of a radiation source is mapped via a focusing imaging sys-tem in a focal plane and wherein to determine the focal position of the optical system including the imaging system zo an image is produced on the focus, which image is reflected there and is recorded by a camera, wherein an optical ele-ment influences the recorded image depending on the focus-ing of the radiation, and depending on said influencing of the image, information about the focal position of the fo-cused radiation at the envisaged focal point is derived.
Practical examples of the invention are described in grea-ter detail below with reference to the drawing.

8 Fig. 1 shows schematically a first practical example of an optical system with an apparatus for detecting a focal po-sition;

Fig. 2 shows a second practical example of an optical sys-tem with an apparatus for detecting the focal position;
Fig. 3 shows schematically a practical example of an ar-rangement according to figure 2 with schematic representa-tion of phase distributions of the radiation in the system and with a hole matrix;

Fig. 4 shows a practical example of an arrangement accord-ing to figure 2 with a diffractive optical element; and Figs. 5, 6 show practical examples of images recorded by a camera with focusing mapping in the manner of a hole matrix with exact focusing and/or focusing errors.

According to figure 1, an optical system 10 has a light source 12, which can be e.g. a laser (such as an fs-laser, for example) or an LED etc. The radiation emitted by the light source 12 passes through an output mirror 14 and is focused via a focusing imaging system 16 onto a plane 18.
The focusing imaging system 16 is only indicated schemati-cally in the figures by a single lens. Normally the focus-ing imaging system 16 has a plurality of lenses, of which one or more can be actuated for setting and changing the focus. Such optical imaging systems are known as such.

In figure 1, areas (points) are marked by the reference signs 20a and 20b at which an optical element described in greater detail below is optionally to be positioned. Exam-ples of such optical elements are the optical elements 34 and 36 shown in figures 3 and 4.

9 Radiation reflected by the reflective surface 18 passes via the optical imaging system 16 and if applicable the optical element arranged in area 20a and described in further de-tail below to the output mirror 14 and is deflected upwards from there in figure 1 via imaging optics 22 to a digital camera 24, e.g. a so-called CCD camera with high local re-solution. The digital image recorded by the camera 24 is entered into a computer C and evaluated there, as described in greater detail further below.

io Figure 2 shows a modified practical example, with compo-nents and features having the same or similar functions being provided with the same reference signs. In the exam-ple according to figure 2, a beam expander (telescope) con-sisting of the optical elements 26, 28 is provided to expand the beam prior to its focusing with the imaging sys-tem 16. Instead of the Keppler telescope shown in the fig-ure, another beam shaping system can also be used in its place. Generally the optical system designated a "beam expander" in figure 2 can also be a beam shaping system.

As already mentioned above, an optical element can be ar-ranged in the areas 20a and/or 20b according to figures 1 and 2 that, depending on the more or less optimal focusing by means of the imaging system 16 onto the reflective sur-face 18, influences the image described above, which was produced by reflection and recorded by the camera 24, and so facilitates a conclusion as to whether the focusing onto the plane corresponding to the surface 18 is precisely that which is desired or whether the focal position is displaced in relation to this plane, e.g: lies too far forward or too far back in the direction of the optical axis (so-called depth of focus).

According to figure 3, a shadow mask 34 is arranged as an optical element in the present sense in the beam path up-stream of the focusing imaging system 16.

In the ideal case, the optical imaging system 16 is thus s set such that the radiation coming from the light source 12 is focused precisely in the plane 18 at a predetermined point. The focus is marked in figure 3 by the reference sign 18a. The practical example according to figure 3 cor-responds to the example according to figure 2 with a beam io expander in the area indicated by reference sign 32. The phase distributions are also marked symbolically there by reference signs 30a, 30b, 30c.

The optical element 34 is a hole matrix with N x M individ-ual holes in the regular arrangement shown. The optical element can be executed in this practical example as a pure amplitude-related element, thus influencing intensities of the radiation. Typical hole diameters in the shadow mask lie between 1~im and 100 um. The holes can be in particu-lar hexangular, square, hexagonal or also circular. The 2o arrangement of the individual holes is oriented to the beam profile used and the requirements in respect of accuracy with regard to the focal position. With the system de-scribed, focal positions can be determined accurate to a few lim. Since the radiation on the path to the plane 18 and the image reflected in the plane 18 each pass through the optical element 34, the image measured by the camera 24 is influenced depending on the accuracy of the focusing in the plane 18. A change in the focal position in relation to the plane 18 (which is the null plane defined above) of 3o a few micrometres can be detected by evaluation of the im-age recorded by the camera 24 in the computer C.

It is also possible to determine the radiation output oc-curring in the focus by integration of the intensities mea-sured by the camera 24 at the individual image points.

Fig. 5 shows, by way of example and schematically, reflec-tion images obtained and evaluated in this manner. In this case figure 5 shows in the middle the matrix-like hole im-age obtained in the event that the optical system including the focusing imaging system 16 is set such that the focus-ing lies exactly on the desired point in the null plane 18.
As stated, the reflective surface for producing the meas-ured image also lies in this plane 18. As the hole image in figure 5, middle, shows, in the reflected image the in-dividual holes are illuminated entirely homogeneously with-out a spherical portion, according to the input beam profile.

In the left-hand hole image, figure 5 shows a displacement of the focal position backwards by approx. 100 ~im in rela-tion to the null plane 18. Compared with exact focusing (figure 5, middle), the image evaluation yields a modifica-tion of the individual image dots in the matrix and the computer C is calibrated for the evaluation such that it "recognizes" this deviation. The calibration of the com-puter can take place e.g. experimentally in such a way that using a known optical imaging system changes in the re-flected image produced are recorded and stored specifically depending on the focal position, so that then the focal position can be determined by comparison with actually mea-sured images.

On the right, figure 5 shows defocusing by -100 pm with a 3o lens focal length of 50 mm with corresponding modification of the hole image compared with ideal focusing. Generally speaking, the asymmetry of the image, as shown to the left and right in figure 5, permits analysis of the focusing.
If, on the basis of image evaluation using the computer C, this analysis results in an asymmetrical brightness distri-bution in the image, then elements of the focusing imaging system 16 can be changed until the image evaluation shows that the focus lies exactly in the plane 18.

Figure 4 shows a practical example of the apparatus for detecting the focal position of an optical system 10, in which the optical element is arranged in area 20b in the io practical example according to figure 2, thus in such a way that the image reflected on the plane 18 does not pass through the optical element 36 on its way to the camera 24.
The optical element 36 in this case is a diffractive opti-cal element (DOE), which forms e.g. a"1 to N" beam split-ter, thus splits an incident single beam into N single beams, wherein N can vary e.g. between 2 and 50. The di-vergence caused by the diffractive element 36 can be cor-rected refractively or diffractively by a second structure (not shown). Several diffractive optical elements can also Zo be arranged one behind another, depending on the beam pro-file and desired analysis. An advantage of an arrangement with diffractive optical elements is the possibility of correction of the incident phase distribution. The phase distribution can be influenced by both the light source and the following optical elements, thus in particular the beam expander. In this practical example also, analogous to the description with reference to figure 3, the image reflected in the plane 18 is recorded by the camera 24 and evaluated in the computer C. Figure 6 shows three images recorded by the camera 24 in the event that the diffractive optical element produces a matrix-like radiation distribution, whe-rein the image on the right in figure 6 shows the case of ideal focusing with relatively uniform illumination of the individual image dots. In figure 6, left, the case is shown in which the focal position deviates laterally from the ideal imaging point 18a, to be precise by several hun-dred micrometres. The individual image dots are illumi-nated asymmetrically. Figure 6, middle, shows a focal position displaced laterally in another direction, wherein the individual matrix-like light dots are likewise illumi-nated less symmetrically than in the case of ideal focusing according to the image in figure 6, right.

io An optical element 36 in the form of a DOE has the advan-tage compared with a hole matrix of high transmission.
With a diffractive element, efficiency of between 80 and 90% can typically be achieved. Such an arrangement also facilitates very high dynamics in the evaluation of the is focal position, i.e. deviations of the focus from the ideal target position can be established over a wide range.

It is also possible to arrange the diffractive optical ele-ment 36 in the areas 20a according to figures 1 and 2.

The diffractive optical element can also be executed as a 2o binary element or also as a so-called multi-level grating structure. The grating structures can be one-dimensional or also two-dimensional.

If an arrangement according to figures 1, 2, 3 or 4 is used in fs-LASIK, then the reflective surface 18, which defines 25 the null plane explained above, can be e.g. the rear of a transparent disc in a suction apparatus known as such, which is constructed (coated or uncoated) such that a small percentage of the incident radiation is reflected to obtain the image to be recorded by the camera 24.

The following are used in particular as diffractive optical elements: gratings, Fresnel zone lenses, so-called beam-shaping elements etc. So-called refractive optical compo-nents can also be used as element (36): e.g. micro-lens arrays, beam-shaping elements etc. If the optical element 34 is used for amplitude analysis, then shadow masks or also arrangements of holes in any geometry such as square, hexangular, hexagonal etc. are particularly suitable, de-pending on the beam type and analysis aim.

The optical element can also be formed as a slot or as an arrangement of several slots.

Using the arrangements described, not only can the focal position be determined and controlled, but beam diver-gences, laser outputs, deviations of the radiation from the optical axis, deviations in the so-called beam product M2 or changes in the output beam profile of the light source 12 can also be detected, since all these beam parameters can have an influence on the reflected image recorded by the camera 24. With regard to all these beam parameters the computer C can be provided experimentally beforehand with a database through targeted trials, which database assigns deviations from the ideal target values, each of which cor-respond to image alterations, to individual beam parame-ters, so that the system is adjustable to ideal values by intervention with corresponding correcting variables. The use of diffractive optical elements here facilitates com-pensation of any phase alterations possibly occurring in the beam path that can also influence the focal position.
The Hartmann Shack sensor, known as such, does not facili-tate such an analysis.

Claims

1. Apparatus for detecting the focal position of an optical system (10) with a radiation source (12), a focusing imaging system (16), an at least partially reflective surface (18) on the focus (18a), a digital sensor system (24) for recording an image reflected by said surface (18), a computer (C) for evaluating the image recorded by the digital sensor system (24), and with an optical element (34; 36) in the beam path of the optical system (10) upstream of the focusing imaging sys-tem (16), characterized in that the optical system (10) is a LASIK arrangement and that the optical element (34; 36) in the beam path influences the phase or amplitude of said image de-pending on the focal position, wherein the partially reflec-tive surface (18) reflects a small percentage of the incident radiation to obtain the image to be recorded using the digital sensor system.

2. Apparatus according to claim 1, characterized in that the optical element (34; 36) is a hole matrix.

3. Apparatus according to claim 1, characterized in that the optical element (34; 36) is a diffractive optical element.

4. Apparatus according to one of the preceding claims, char-acterized in that the optical element (34) is arranged in the beam path of said reflected image.

5. Apparatus according to one of claims 1 to 3, characterized in that the optical element (36) is arranged outside the beam path of the reflected image.

6. Apparatus according to one of the preceding claims, char-acterized in that the optical element (34; 36) has a grating structure.

7. Apparatus according to claim 3, characterized in that the diffractive optical element (34; 36) produces a dot pattern, in particular a dot pattern in the form of a matrix.

8. Apparatus according to one of the preceding claims, char-acterized in that the radiation source (12) is an fs-laser.

9. Apparatus according to one of the preceding claims with means for setting the imaging of the optical system (10) de-pending on the evaluation of the computer.

10. Method for detecting the focal position of an optical sys-tem (10) immediately prior to material processing, in which the radiation of a radiation source (12) is mapped via a fo-cusing imaging system (16) in a focal plane (18) and wherein to determine the focal position of an optical system including the imaging system (16) by means of an optical element (34;
36) in the beam path an image is produced on the focus (18a), which is reflected there and is recorded by a camera (24), wherein said optical element (34; 36) influences the image re-corded depending on the focusing of the radiation and wherein depending on said influencing of the image a conclusion is de-rived about the focal position of the focused radiation in re-lation to an envisaged focal point (18a), characterized in that by means of the optical element (34; 36) the phase or am-plitude of the image is influenced depending on the focal po-sition and that the optical system is a LASIK arrangement, wherein the partially reflective surface (18) reflects a small percentage of the incident radiation to obtain the image to be recorded using the digital sensor system.

11. Method according to claim 10, wherein by means of said derived conclusion about the focal position an optical element of the optical system (10) is set to change the focal posi-tion.

12. Apparatus for carrying out an ophthalmological treatment or diagnosis with femtosecond laser radiation using an appara-tus according to one of claims 1 to 9.