Patent Application
20100302550
This application claims priority to German Patent Application No. 10 2009 022 958.2 filed on May 28, 2009, which is incorporated herein by reference in its entirety.
The invention relates to a device for the optical measurement of relative distances of structures of an object, which device is implemented as an optical time domain coherence tomograph, which has an object beam path, through which measuring radiation is incident on the object, a deflection beam path, which comprises a detector unit and through which sample radiation reflected or backscattered from the object reaches the detector unit, an interferometer unit, which has a first and a second beam path part having different optical path lengths, which further splits radiation and feeds it into the two beam path parts and superimposes it again after passage through the beam path parts and thus generates a dual beam, which dual beam has components axially offset to one another because of the different optical path lengths of the two beam path parts, the interferometer unit either being situated in the object beam path, so that the measuring radiation is incident on the object as a dual beam, or being situated in the detection beam path, so that the sample radiation reaches the detector unit as a dual beam, and the different optical path lengths of the beam path parts influencing the relative distance of such structures sensed on the object, whose sample radiation is capable of interference at the detector unit, a scanning unit for scanning the relative distance of the acquired structures, wherein the scanning unit is implemented to adjust the optical relative path length of the beam path parts, and a control unit, which controls the scanning unit.
Objects are understood here in particular as living objects or samples or parts thereof, such as the human eye. These objects may also be optically dispersive (cf. EP 1587415) or influence the polarization (cf. WO 02051333) in this case.
Furthermore, the invention relates to a method for the optical measurement of relative distances of structures of an object using optical time domain coherence tomography, wherein a measuring beam is directed onto the object, sample radiation reflected or backscattered from the object is detected, either the measuring radiation or the sample radiation is split into a first and a second beam part, the beam parts pass through different optical path lengths, the beam parts are then superimposed and a dual beam is generated, which has components axially offset to one another because of the different optical path lengths, the different optical path lengths influencing the relative distance of such structures acquired on the object, whose sample radiation is capable of interference at the detector unit, and wherein relative distance of the acquired structures is scanned, in that different optical path lengths are adjusted relative to one another.
A set-up which directs a dual beam onto an object is known from Hitzenberger et al., “Measurement of Corneal Thickness by Laser Doppler Interferometry”, Investigative Opthalmology & Visual Science, Volume 33, Issue 1, January 1992. A dual beam is used in the detection beam path in DE 3201801.
Short coherence interferometers, which operate using optical coherence interferometry, are known for the measurement of transparent or partially-transparent samples, such as the human eye, for example, from U.S. Pat. No. 7,400,410 or WO 2007/065670 A1. They are used for the purpose of acquiring the location and size of scattering centers inside a sample, such as miniaturized optical components or biological tissue, for example, the human eye. Reference is made to US 2006/0109477 A1 and U.S. Pat. No. 5,321,501 for an overview of the corresponding literature on coherence interferometry. These patent specifications also describe the fundamental principles of optical coherence tomography (OCT).
The principle of coherence tomography (OCT) comprises both embodiments in which imaging is performed by scanning at various locations transversely to the direction of incidence of the radiation, and also embodiments simplified in relation thereto, in which the irradiation and radiation detection are performed only along an axis which remains unchanged, thus generating axial (i.e., one-dimensional) scattering profiles. The latter embodiment corresponds in regard to the image acquisition to a so-called A-scan of ultrasound image acquisition; it is also referred to as optical coherence domain reflectometry or coherence reflectometry (OCDR) and is used for the purpose of acquiring the location and size of scattering centers inside a sample, such as the human eye (cf. US 2007081166). A variant of an imaging OCT without lateral scanning is the so-called full field OCT according to US 2008/0231807.
In this description, the term coherence tomography (OCT) is used as a generic term. It thus comprises both three-dimensional imaging and also only one-dimensional imaging in the form of OCDR.
For OCT (and its subgroup OCDR), the variants time domain coherence tomography (time domain or TD-OCD or OCDR), having path length difference adjusted for scanning, and Fourier domain coherence tomography (FD-OCT or OCDR), having fixed path length difference and analysis of spectral information, are known. The latter is further differentiated into a variant employing broadband light sources and spectrometer-based detection (spectral domain or SD-OCT or OCDR) and a variant employing swept-source light sources and broadband detectors (swept source or SS-OCT or OCDR).
In the case of coherence tomography, the axial and the lateral resolutions are largely decoupled. The axial resolution is essentially given by the coherence length of the source. In the lateral direction, the achievable resolution is given by the lateral extension of the focus or the beam waist given in the focal range. The scattering signal of a location is thus the superposition of the backscattered radiation from the smallest resolvable volume. In the case of this superposition of backscattered radiation components, destructive or constructive interference may already also occur, so-called speckles.
An application which is of particular interest for OCDR is the length measurement in the eye. Known methods for measurement of distances or axial length operate along the axis of vision of an eye, thus, for example, the device IOL Master developed and sold by Carl Zeiss Meditec AG, whose design is also the subject matter of the patent specification U.S. Pat. No. 6,779,891.
The time domain OCDR system described in U.S. Pat. No. 6,779,891 uses a so-called dual beam approach. Instead of an independent reference beam path, as is described, for example, in EP 581871, the measuring object, the eye in the case of the IOL Master, is illuminated using a dual beam, which has components axially offset relative to one another. This dual beam is generated using an interferometer configuration. The dual beam principle is distinguished by extensive suppression of artifacts which may arise through axial object movements. Relative distances of structures in the eye are acquired, i.e., the dual beam principle provides a statement about the location of backscattering or back-reflecting structures of the object to one another. Referencing is typically performed to a special structure of the sample. In the case of opthalmology, the corneal front face is typically used, which is also obvious in the case of the axial length measurement of the eye.
The dual beam principle is relatively insensitive to axial movements of the object, but it does not compensate for lateral object shifts. If such shifts are a concern or are even unavoidable, as in the field of opthalmology, the most rapid possible measurements are sought in order to keep the influence of lateral object movements as small as possible. This is also advantageous in the case of high-precision measurements, in order to avoid resolution losses through pulsations on the eye (cf. Schmetterer et al., “Topical measurement of fundus pulsations”, Opt. Eng. 34, 711-716, 1995) or to be able to measure axial length measurement including the pulsation effects. In the case of time domain coherence tomography (or its subgroup TD-OCDR), the most rapid possible adjustment of the path length difference between the axial components of the dual beam or the most rapid possible adjustment of the length of the reference beam path is thus desirable.
Rapidly adjustable delay lines are known, for example, in U.S. Pat. No. 6,243,191, U.S. Pat. No. 6,654,127, U.S. Pat. No. 7,227,646, or the publication by M. Hasegawa et al., “Development of high speed and deep scanning optical coherence tomography system”, (IEEE Lasers and Electro-Optics, 2003; CLEO/Pacific Rim 2003; The 5th Pacific Rim Conference; Volume 1, Issue 15-19 Dec. 2003, page 305).
Dual beam methods are also known from U.S. Pat. No. 6,788,421 and DE 102007046507. A sample reflection is used as the interferometry reference signal therein. However, two spatially separate measuring beams are incident on the eye. A double passage occurs through separate delayed beam paths. In particular in the construction of U.S. Pat. No. 6,788,421, the use of partially reflective beam splitters is necessary, which attenuate the backscattered radiation components to be detected, even before the interfering superposition. Furthermore, it is disadvantageous in the concept of DE 102007046507 that the separate measuring beams cannot be oriented exactly co-linearly, because of which spacing along a measuring axis is difficult therein.
For the purpose of spacing, the use of two independent Michelson interferometers is known from Wang et al., “A low coherence “white light” interferometric sensor for eye length measurement”, Review of Scientific Instruments 66 (12); 5464-5468, in each of which interferometers an adjustment means and a detector for the location determination of a surface in the eye is used. It is thus not a dual beam method which would use interference of light backscattered on both surfaces.
The invention is based on the object of refining a device for the optical measurement of relative distances of structures of an object and/or a corresponding method of the type cited at the beginning so that the measuring speed is increased in relation to the prior art. For this purpose, in particular the advantages of the dual beam approach for achieving a high-precision axial length measurement with insensitivity in relation to axial sample movements are to be maintained.
This object is achieved according to the invention in the case of a device of the type cited above in that the scanning unit has a first adjustment means for adjusting the optical path length of the first beam path part and a second adjustment means for adjusting the optical path length of the second beam path part and the first and the second adjustment means adjust the path lengths for the scanning of the object in synchronized manner under control by the control unit so that the coordinated adjustments define the covered range of the relative distances.
The object is further achieved by a method of the type cited above in which both the path length of the first beam part and also the path length of the second beam part are adjusted in synchronized manner, so that the coordinated adjustments define the covered range of the relative distances of the structures acquired on the object.
According to the invention, the coherence tomograph has an interferometer unit, which generates the dual beam of the dual beam method. A dual beam is understood according to the definition given above as a beam which has two components axially offset (i.e., optically delayed) to one another, which are otherwise capable of interference. If such a dual beam is reflected on an object, which has two partially reflective or backscattering structures, which are spaced apart axially by half of the amount of the axial offset or the optical path, corrected by the index of refraction, of the components of the dual beam, interference occurs in the back-reflected or backscattered sample beam within the time coherence length, because the axial offset is sufficiently canceled out by the back-reflection or backscattering. The so-called dual beam approach in coherence tomography uses this principle. The interference indicates that two structures are spaced apart from one another in the object by half of the distance of the axial offset of the components of the dual beam. If it is additionally known, as in the field of opthalmology, which structure is the reference (such as the corneal anterior face here), one automatically has an absolute reference specification about the relative location of the other structure (such as the retina) in relation to the reference.
Radiation can, of course, only interfere within a coherence volume which results as the product of the time coherence length of the radiation and the speed of light. The precision of the relative specification is thus a function of the time coherence length of the radiation used. Therefore, efforts are made to use short coherence radiation as much as possible in short coherence tomography (or its subgroup OCDR). This is understood here as radiation whose coherence length is established suitably for the desired resolution Δz for structure distances, e.g., via the selection of the spectral bandwidth of the radiation Δλ=2 ln(2) λ2/(πΔz) at the central wavelength λ (cf. Fercher et al., “Optical coherence tomography-principles and applications”, Rep. Prog. Phys. 66 (2003) 239-303). Radiation is preferably used whose coherence length is approximately equal to or one-tenth of the desired measurement resolution. Typical measurement resolutions are 3-30 μm in opthalmology. However, methods are also known in which measurement resolutions significantly below the spatial coherence length may sometimes be achieved, for example, by regression methods (fit of the axial point-spread function, PSF) or phase-sensitive measurements (cf. WO 03052345 A1). The coherence length can thus be understood as a window around twice the axial distance of the offset of the components in the dual beam, within which window a back-reflecting or backscattering structure results in interference in the sample beam. In other words, the coherence window corresponds to the boundary of the maximum optical relative delay of two radiation components, up to which noticeable interference can still occur.
For the best possible interference capability of the dual beam components to be measured at the detector, the best possible equalization of their polarization states is preferably also to be sought, but in any case an avoidance of orthogonal polarization states, which are not capable of interference (for example, horizontal and vertical polarization or right and left circular polarization). Deviations in the polarization states of the dual beam components may result through differing double refraction or polarization rotations along the particular covered optical paths of these radiation components, e.g., as a result of curved optical waveguides in interferometer arms or also through polarization-changing samples. For example, for the measuring radiation components which traverse the cornea of an eye, a polarization-changing influence is to be expected through the double refraction of the eye.
The generation of the dual beam is performed in the device according to the invention using an interferometer unit or by interferometry in the method according to the invention. It is both possible to implement the measuring radiation incident on the object as a dual beam, and also to reshape the returning sample radiation into the dual beam. In the first case, one refers to a pre-interferometer, in the second case to a post-interferometer. U.S. Pat. No. 6,779,891 describes the case of a pre-interferometer. Both approaches are within in the scope of this invention.
The adjustment of the dual beam with respect to the axial offset of the radiation components in the dual beam determines the relative distance of the structures from which interference can occur. The interferometer sweeping or relative adjustment of the path lengths of the beam parts of the interferometer thus sweeps the relative distance of the acquired structures.
The invention achieves a significant shortening of measuring time in that a beam path part or beam part is not adjusted with respect to the optical path length, as in the prior art, but rather both beam path parts or beam parts are adjusted in synchronized manner. Significant acceleration of the measurement is achieved using this approach.
In addition to the direct speed increase, in embodiments, the invention provides the use of known statistical distributions of biometric variables, in order to achieve an acceleration of the measuring procedures on average; for example, in that small measuring ranges are measured in sequence in accordance with an incidence distribution of the measuring variable to be measured until the measuring result has been obtained.
In an embodiment, the invention provides that signal-interfering effects introduced by individual adjustment elements, such as the chromatic dispersion (wavelength-dependent optical delay) in notable glass paths of rotational prisms or rotating glass cubes, are eliminated with little effort. The use of two preferably identical adjustment means in various interferometer arms also causes compensation with little effort of the chromatic dispersion caused by the adjustment means, i.e., a minimization of the dispersion difference caused in the interferometer arms. If a dispersion compensation is nonetheless necessary, optical components having different ratios between group speed index and dispersion are preferably used in the interferometer arms, for example, different fiber types in the interferometer arms (cf. U.S. Pat. No. 7,330,270). In a refinement, a residual dispersion difference is additionally set between the interferometer arms, for example, to compensate for dispersion differences between the dual beam components backscattered or back-reflected at various sample zones. The shorter interferometer arm is to have the higher dispersion for this purpose, because the dual beam component corresponding thereto covers the greater pathway in the dispersive sample, before it can interfere with the other dual beam component.
The invention uses a path length adjustment for the beam parts in the case of the interferometric generation of the dual beam and/or for the beam path parts in the respective interferometer. Such a path length adjustment is also referred to in the prior art as an adjustable delay line (optical delay line). The terms delay and adjustment are interchangeable in the meaning of this description. A plurality of delay lines are known for the adjustment of the optical path lengths in the prior art. The invention achieves an acceleration in relation to all known or future delay lines.
Furthermore, delay lines or adjustment means may now be used which were desirably rapid up to this point for specific measuring tasks, for example, in the field of opthalmology, but could not be used because of an excessively small adjustment travel. Therefore, in one variant, the invention uses adjustment means which are short-travel per se, but are very rapid, for one beam part or beam path part and a very much slower adjustment means operating in discrete adjustment steps for the other beam part or in the other beam path part. This discretely operating adjustment means predefines the individual measuring ranges, within which very rapid measurement is performed using the other adjustment mechanism, which operate continuously and comparatively more rapidly. This approach is of interest in particular if various subareas are to be sensed in an object. In the field of opthalmology, this is the case upon the acquisition of biometry data for an intra-ocular lens adaptation in the context of a cataract operation. Specifications about the axial length of the eye are required here, as well as further detail specifications, such as anterior chamber depth or corneal thickness.
An addition of the operating speeds of known adjustment means for the optical path length is achieved if the first adjustment means is operated to shorten the optical path length and the second adjustment means is operated to lengthen the optical path length. This coordinated cooperation delay means achieves, for example, in the case of identically acting adjustment means, a doubling of the adjustment speed in relation to a set-up having only one adjustment means.
Adjustment means which contribute to a low measuring time operating particularly rapidly are preferred for the invention, such as fiber stretchers (cf. U.S. Pat. No. 4,609,871), lattice-based adjustment means (rapid-scanning optical delay lines, cf. WO 02071117 A3), helicoid reflectors (cf. U.S. Pat. No. 5,907,423), stepped reflectors (cf. WO 2005033624 A1), piezoelectric and electromagnetic translationally moved reflectors, and rotational reflectors (cf. Xinan et al., “Fast-scanning auto correlator with 1-ns scanning range for characterization of mode-locked ion lasers”, Rev. Sci. Instr. 59 (9), 1988). Further adjustment means come into consideration in particular, such as those described in U.S. Pat. No. 6,343,191 and DE 10005696 A1, the above-mentioned publication of Hasegawa et al., U.S. Pat. No. 7,227,646, or U.S. Pat. No. 6,654,127. The content of the disclosure of all these publications is thus expressly incorporated by reference in this regard.
It has been shown in the prior art that a particularly rapid adjustment can be achieved using a rotating disc which has reflectors, the optical path length being a function of the rotational position of the disc.
The above-mentioned opposing adjustment of the optical path lengths can be achieved particularly advantageously using an apparatus which adjusts both path lengths simultaneously. The apparatus can be implemented in such a manner that the rotating disc carries multiple retroreflectors, which each reflect back radiation, which is incident within a sector lying around a main reflection axis along a direction of incidence, wherein the back-reflection occurs parallel to the direction of incidence and offset to the direction of incidence, and wherein the retroreflectors are combined to multiple opposing reflecting retroreflector pairs and the retroreflector pairs are attached to the disc so that the main reflection axes are tangential to the rotating disc, the beam path parts irradiating the radiation on the retroreflectors tangentially and opposite to the disc and a terminal mirror being mounted fixed for each beam path part outside the disc, which reflects the radiation reflected back by one of the retroreflectors parallel to the direction of incidence and offset to the direction of incidence back to the particular retroreflector, so that the first and the second adjustment means are formed by the rotating disc having the retroreflectors and the terminal mirror. The prisms belonging to one prism pair may also be mounted on various sides of the disc, which offers savings in space and greater positioning freedom.
However, it is to be expressly noted that the invention is not restricted to adjustment means which use rotational means. The invention also achieves an acceleration of the measurement by using typical linear displacements, as are described, for example, in above-mentioned U.S. Pat. No. 6,779,891, or also by the use of fiber stretchers.
Furthermore, in an advantageous embodiment, the invention controls the two adjustments in a signal-dependent manner, i.e., as a function of the detected signal. This makes it easier to find desired structures or to set favorable scanning ranges for averaging, which improves the signal-to-noise ratio. Optionally, different sweep speeds for the path length changes in the beam (path) parts are used by the two independently operable adjustment means, which are operated in coordinated manner, so that in the case of repeated measurements, specific relative delays occur for different positions of the adjustment means. Systematic disturbances in the adjustment means are reduced if one averages over the identical relative displacements or distances then predefined in the case of different adjustment means positions.
Because in many cases the adjustment means do not implement uniform adjustment of the path lengths, but rather sinusoidal oscillations, for example, the use of a path and/or speed measurement is advantageous in order to sense the adjustment path of the relative distances and thus achieve a correction of the depth profiles thus set. Suitable measuring units may be based on electrical, magnetic, electromagnetic, or optical principles. Examples to be listed are: capacitive, inductive, resistive, or magnetic measurements (Hall sensors), optical or magnetic coding (encoder), incremental transmitters (cf. U.S. Pat. No. 5,719,673), differential measurements, position measurement, triangulation measurements (for example, using reflective couplers), and also interferometric measurements, for example, by counting interference modulations. For this purpose, the signal radiation itself can be used for the speed determination in the relevant parts of the adjustment path (cf. DE 19810980), but alternatively also long coherent radiation of a narrowband laser diode can additionally be coupled into the interferometer and its interferences can be detected and analyzed over the entire adjustment path. Speed and location measuring units in connection with suitable controls are particularly also suitable for the coordinated operation of multiple adjustment unit. Alternatively, mechanical couplings also come into consideration for this purpose.
The interferometer unit generation of the dual beam can employ any suitable interferometer principle, in particular a Michelson construction or a Mach-Zehnder construction. For this purpose, the latter has the advantage in particular of operation free of back-reflection, which particularly saves the use of optical isolators for protecting the light source in the case of use in a pre-interferometer. If the light source contains a light amplifier (for example, in the case of lasers or super luminescent diodes, for example), back reflections into the source are problematic in particular.
When method steps or features relating to a method have been or are explained in this description, it can also be provided for the device explained that the control unit is implemented in such a manner that it causes an operation which realizes these method steps or features in the device and/or a corresponding function. Vice versa, a described functional feature or an explained mode of operation of the device is also usable as a corresponding method step or corresponding method feature for the described method.
When reference has been made to prior publications in this description with respect to individual aspects, principles, and/or elements, they are incorporated herein in their entirety by reference.
It is obvious that the above-mentioned features and the features to be explained hereafter are usable not only in the specified combinations, but rather also another combinations or alone without leaving the scope of the present invention.
The invention is explained in greater detail hereafter for exemplary purposes on the basis of the appended drawings, which also disclose features essential to the invention. In the figures:
a and b show possible mechanisms for the path length adjustment for one of the interferometers described here,
In a Michelson interferometer 4 the radiation of the source 3 is converted into a dual beam, which has the radiation of the source 3 in two components, which are offset to one another in the radiation direction, i.e., axially, and are thus optically delayed to one another.
In the construction of
The delay lines 8, 9 cause a differing delay of the radiation supplied thereto, and the radiation is conducted back to the beam splitter 5 in the particular beam path 6, 7, where the radiation from the beam path parts is superimposed again. As a result, after the beam splitter 5, which then acts as a combiner, a measuring radiation 10 is provided, which is the described dual beam. This measuring radiation is directed onto the object, in this case the eye 2, by a further beam splitter 11. Structures in the eye 2 back-reflect or backscatter the measuring radiation, so that sample radiation 12 returns from the eye 2. It is coupled out at the beam splitter 11 to a detector unit, which comprises a photoreceiver 14 and an amplifier 15, which prepares the signals of the photoreceiver. The signals at the output of this amplifier 15 are analyzed in an analysis unit (not shown here for the sake of clarity) with incorporation of the instantaneous setting of the adjustment means 8 and 9 with respect to spacing of structures to be measured and optionally also displayed as a depth-resolved scattering profile (A scan). The instantaneous locations of the adjustment means may be measured for this purpose using measuring units (not shown here, but previously explained), or determined from the control signals of the control unit 16.
As already previously explained in the general part of the description, interference occurs in the sample radiation 12 if structures are present in the eye 2 which have a distance along the direction of incidence of the measuring radiation 10 which corresponds to half of the axial offset of the components in the dual beam. This distance parameter applies within the coherence length of the radiation of the source 3, of course, which therefore determines the axial resolution.
In order to adjust the relative distance in which structures are sensed in this way and thus to record a depth profile of the sample, the delay lines 8, 9 are adjustable with respect to their optical path length. They are controlled accordingly by a control unit 16.
Rapid adjustment is achieved in one embodiment of the invention in that the delay lines 8, 9 are driven in opposite directions. For example, when the delay line 8 lengthens the optical path length of the beam path 6, the delay line 9 simultaneously shortens the optical path length of the beam path part 7. The path length difference and thus the axial offset of the components in the dual beam thus varies with the sum of the adjustment speeds of the delay lines 8, 9. A measuring speed is achieved in this way, which is significantly above the speed at which the delay lines 8, 9 may be adjusted. Furthermore, the adjustment range is greater than that of one of the delay lines 8, 9. The total adjustment range is also added up from the individual adjustment ranges of the delay lines 8, 9. An eye length measurement can thus be achieved at higher speed using delay lines which would each alone not allow an acquisition of the eye length, which is typically between 22 and 25 mm.
The delay lines function as the settable adjustment means for the optical path length. An opposing action of these adjustment means is not the only advantageous way of operating the adjustment means or delay lines so they are adapted to one another. Alternatives thereto will be discussed hereafter.
In
Replacing the mirror 19 and optionally the mirror 20 with a retroreflective prism, such as a triple prism, is also known for adjustable delay lines in the prior art. For this purpose, reference is made to U.S. Pat. No. 7,227,646, which is incorporated in its entirety in this context, or the above-mentioned publication of Hasegawa et al.
The beam guiding is implemented by bulk optics in
The delay lines of the beam path parts 6 and 7 are implemented in the construction of
A rotation of the disc 30 in the direction of the arrow shortens the optical path length for the beam path part 7 and lengthens the optical path length for the beam path part 6 by the same amount. It is essential for this purpose that the triple prisms 32, 33 each re-output radiation, inciding on the triple prism in a certain sector around a main direction of incidence, parallel to the direction of incidence of the radiation. Furthermore, the triple prisms 32, 33, situated having coincident main direction of incidence and offset to one another, are combined to form the triple prism pair 31.
Finally, the collimators 29, 28 and/or the terminal mirrors 34, 35 are oriented so that they can radiate or collect (in the case of the collimators) or collect for reflection and reflect back (in the case of the terminal mirrors) radiation inciding within the sector in which the particular triple prisms 32, 33 work. In the construction of
Furthermore, the rotating disc 30 preferably carries multiple triple prism pairs 31, so that a rotation of the rotating disc 30 results in a repeated shortening of one beam path part with simultaneous lengthening of the other. In one rotational direction, as indicated by an arrow in
The figures do not show optional means for partial or complete polarization equalization of the sample beam components to be detected, i.e., to be brought to interference, at the detector. These means may be situated in the interferometer arms and implemented as so-called “polarization paddles”, for example, i.e., fixed or mobile polarization-changing fiber loops employing bending double refraction. Alternatively, wave plates may also be used in the free beam areas. In particular, reference is made to the possibility that a polarization state equalization between the dual beam component which is reflected directly from the cornea and the dual beam component which traverses the generally polarization-changing cornea and is backscattered by deeper structures can be performed.
The superimposed guiding of the dual beam components in the fibers 10 and 12 has the effect that polarization-optical disturbances act jointly and in the same way on both dual beam components in the fibers, in particular if the polarization states have already been equalized correspondingly upon the superposition in the coupler 23. Flexible, i.e., mobile fiber connections may thus also be used, for example, in order to implement a hand-guided axial length measuring device, similar to known ultrasonic devices (cf. DE 4235079).
A further synchronization of the adjustments of the adjustment elements 36 and 37 comprises the travel of the rapid, continuously operated adjustment element 37 being passed through completely at least once before a changeover of the discrete state occurs on the adjustment element 37.
Fundamentally, however, it is also possible to implement multiplex operation using a combination of a discrete element 36, which can be changed over very rapidly, and a slower adjustment of the continuous adjustment element 37, in the case of which all discrete adjustments of the adjustment element 36 are always implemented for each state of the continuous adjustment element 37.
In both variants, it is very advantageous to perform a time-saving ending of the measurement upon establishment of a sufficient or clear measuring signal, i.e., if it can be seen that no further useful information could be obtained by further adjustment of the adjustment elements 36 and 37.
The position of the stepped mirror 36 predefines the range in which the rapid adjustment of the longitudinally oscillating mirror 37, which continuously adjusts the path length, senses back-reflecting or backscattering points in the object, i.e., the eye 2. The measurement is therefore preferably performed, under control of the control unit 16 (which is not shown in
If one implements the continuous delay lines in the beam path part 7, as is the case in the example of the construction of
a shows a prism wheel variant having double adjusting action, which can be used in one or both interferometer arms. An incident beam 53 is deflected at a triple prism 32 being fastened on the turntable 30 and is conducted via mirrors 54 through 57 to a triple prism 33 which is opposite in relation to the turntable axis. It exits therefrom as an outgoing beam 58. The path length is a function of the turntable position. The construction of
b shows a combination of the principle of
The generation of the dual beam was described for exemplary purposes up to this point on the basis of a pre-interferometer, which is implemented as a Michelson interferometer 4. Of course, other interferometer structures are also suitable, such as a Mach-Zehnder interferometer, as is schematically shown in
An embodiment in which the monitoring terminal 46 can be used for signal acquisition is shown in
The delay lines 8 and 45 thus act, on the one hand, as a pre-interferometer and generate the dual beam at the fiber coupler 47. On the other hand, they also act as a post-interferometer for the sample radiation returning at the fiber coupler 47. A double delay is thus achieved and the measuring speed rises accordingly, because short-travel delay lines are possible. Because of the double passage, its travel is de facto doubled. Suitably rapid delay lines are the above-mentioned piezoelectric fiber stretchers, for example.
Finally,
The coordinated change of the optical path lengths in the beam path parts provided in all explained variants of the OCT 1, which generates the dual beam for the measuring beam 10 or from the sample radiation 12, may be used not only for accelerating the measurement. If one uses the independent delay lines having different sweeping speeds, specific relative delays result from different locations of the delay lines upon repeated measurements. This allows systematic disturbances of the delay lines, for example, as a result of mechanical deformations on delay lines or path measurement systems, to be suppressed, for example, by averaging. Of course, a suppression can also be performed in another way, for example, by suitable filtering, etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102009022958.2 | May 2009 | DE | national |
