SYSTEM AND METHOD FOR MEASURING INTERNAL DIMENSIONS OF AN OBJECT BY OPTICAL COHERENCE TOMOGRAPHY

TECHNICAL FIELD

The present invention relates to a system and a method for optically measuring internal dimensions of an object by means of optical coherence tomography, wherein said object comprises internal interfaces at which the (optical) refraction index changes, so that a portion of incident light is backreflected and/or backscattered and can be detected. The object can generally be any sample object that is at least partially transparent in an at least internal partial volume for wavelengths in a wavelength range of operating wavelength used by an optical coherence tomography (OCT) device for measuring internal dimensions in said volume. The object may comprise relatively complex external and internal structures associated with refraction index changes, and may for example be an object made of transparent plastics having complex internal structures made from modifications of the plastics associated with differing refraction indices, or samples of biological tissue, such as an eye, in particular a human eye.

BACKGROUND

Applications of optical coherence tomography (OCT) to the characterization of geometrical and optical characteristics of notably human eyes are known, e.g. in the diagnosis of the eye, when measuring the geometrical and optical characteristics of different sections of the eye and of the eye as a whole which are relevant to obtain a model of the individual eye of a patient as a basis for developing an optimum treatment plan for the patient's eye for refractive surgery including e.g. laser-based refraction corrections. At present, different diagnosis devices based on different measurement principles must be involved to obtain a precise diagnostic of the geometrical and optical characteristics of different sections of the eye, including e.g. the corneal and anterior segment (CAS) of the eye, and of the eye as a whole including the eye's length and the geometric structure of rear portions of the eye including the retina. The precision required, i.e. an axial resolution Δz and a lateral resolution Δx, is different for the afore-mentioned sections of the eye. For example, the axial resolution obtained with conventional devices in characterizing the topography and thickness of the CAS is between approximately 5 and 10 μm, while a precision resp. measurement accuracy or resolution of less than 3 μm, preferably less than 1 μm, would be desirable for an optimum planning and a priori calculation of a refractive correction treatment. On the other hand, the length, notably the axial length, of the eye and positions of major refraction index interfaces distributed along the length require only an accuracy resp. resolution Δz of approximately 50 μm or better. Conventionally, treatment plannings in (optical) refractive surgery of an eye are based on individual measurements with different diagnosis devices, which may use different measurement and evaluation principles. This renders problems when integrating the measurement data obtained from the different devices into a single model of an individual eye and attempting establishing a single integrated treatment, e.g. refractive surgery, planning. Also, the use of different diagnosis devices is time consuming because the devices are used sequentially and may require device-specific adjustments between the device and the eye to be characterized.

As an example, conventional treatment plannings in refractive surgery of the eye may use different diagnosis devices manufactured by the applicant, which include the so-called Allegro Topolyzer (Trademark) which is used to obtain cornea topography, notably of the cornea front surface and registration of post chamber surfaces (PCS), iris, pupil, limbus and apex; the Allegro Oculyzer (Trademark) for obtaining the topography of front and rear surfaces of the cornea, the thickness of the cornea as well as some geometric data of the anterior chamber of the eye (e.g. anterior chamber depth); Allegro Analyzer (Trademark) for obtaining integral wavefront data and perturbations of the eye as a whole resulting from individual aberrations of e.g. the cornea, lens and vitreous body as well as for obtaining a registration of the iris, pupil, limbus and blood vessels; the Allegro Biograph for determining the thickness of the cornea, the axial length of the eye as a whole and the length resp. thicknesses of further sections and elements of the eye including e.g. the anterior chamber and the lens as well as a registration of the pupil, apex, iris, limbus and blood vessels; and the Pachymeter for local, i.e. pointwise, measurement of the thickness in the center of the cornea and determining the depth of cuts and flap thickness e.g. in Laser in-situ Kerato-milieusis (LASIK). Comparable devices with corresponding properties and limitations are produced by other manufacturers and used in state of the art diagnostics of, and (refractive correction) treatment planning for, the human eye.

Conventional diagnosis devices aiming to precisely measure the frontal section of the eye, including the cornea, anterior chamber, iris, post chamber and the frontal surface of the lens (see FIG. 8) with the required accuracy resp. resolution are not capable to also measure e.g. the total length of the eye and the topography resp. geometry of the rear surface of the lens, while the latter data are required to calculate the total refraction of the eye. Conventionally, the data required to calculate the total refraction of the eye are determined iteratively by calculation on the basis of general model of the eye, whereby calculated data are compared with measured characteristics of wavefronts that have propagated in and through the whole eye.

An example of a high quality in vivo imaging OCT device for anterior segment imaging is disclosed in the article “Anterior segment imaging with Spectral OCT system using a high-speed CMOS camera” by I. Grulkowski et al., published on 12 Mar. 2009 in OPTICS EXPRESS 4842, Vol. 17, No. 6. Another example is disclosed in the article “Extended in vivo anterior eye-segment imaging with full-range complex is spectral domain optical coherence tomography” by J. Jungwirth et al., published in the Journal of Biomedical Optics Letters, Vol. 14(5), September/October 2009. A further example of a measurement of the anterior segment is a device CASIA SS-1000 manufactured by the company TOMEY and described in the system specifications issued therewith.

A first example of the state of the art for measuring the full axial length of an eye, as used to generate a 3D tomogram model, is disclosed in the article “Three-dimensional ophthalmic optical coherence tomography with a refraction correction algorithm”, by R. J. Zawadzki et al., published in SPIE proceedings, Vol. 5140 and in the article “Iterative Berechnung von Ablationsprofilen in der Refraktiven Chirurgie” by Dr. H. P. Iseli et al., published in Augenspiegel, Vol. 20, July-August 2008.

While the discussion of the state of the art of optical measurements of internal dimensions of a sample object comprising internal interfaces at which the refraction index changes focused on particular applications related to the diagnosis of the eye, similar constraints and limitations are also encountered in optical investigations of other types of objects as mentioned above.

SUMMARY OF EXAMPLE EMBODIMENTS

In view of the afore-mentioned problems of the state of the art related to the use of different devices for obtaining the different characteristics in different internal partial volumes of an object, such as the eye, it is a general object of the present invention to save diagnosis time and cost associated with the use of various diagnosis devices, and a particular technical goal to achieve adequate accuracy of the measurement (resolution) for the different sections of the eye to enable a precise individual (customized) treatment for visual correction of a patient's eye.

The object is achieved, according to the invention, in general by providing a single system that allows to measure and obtain the different data in a practically single measurement operation, for example a single diagnostic investigation. In other words, the patient experiences (suffers) one measurement activity only even if more than one parameter is measured. The invention involves the integration of different optical coherence tomography (OCT) devices dedicated to different measurement tasks, to the measurement of different internal partial volumes of the object to be investigated with different appropriate (axial and lateral) resolution resp. accuracy.

According to a first aspect of the present invention, as claimed, there is provided a system for optically measuring internal dimensions of an object comprising internal interfaces at which the refraction index changes so that a portion of incident light is backreflected and/or backscattered and can be detected, by means of optical coherence tomography (OCT), the system comprising at least one first OCT device adapted to measure internal dimensions in a first partial volume of the object.

According to the invention, the system is characterized by a combination with at least one second OCT device adapted to measure internal dimensions in a second partial volume of the same object, wherein said second partial volume is at least partially different from the first partial volume.

The combination of a first and a second OCT device into a single system allows to measure internal dimensions in different internal volumes of the sample object with different appropriate resp. required accuracy using a single system, in a shorter time as compared to using two separate OCT devices each in a single measurement operation.

The first partial volume may be located near or at a front side of the sample object. The front side may essentially face the system. And the second partial volume may be located near or at a rear side of the object or may extend essentially from the front side to the rear side of the object. The object may for example be an eye, particularly a human eye. Measuring the internal dimensions in different partial volumes of the object, notably the eye, with a single integrated system saves time and measurement effort, and in the case of investigating an eye reduces the suffering experienced by a patient.

The first OCT device may comprise a first reference arm and a first sample arm and the second OCT device may comprise a second reference arm and a second sample arm, wherein at least a section of the first sample arm and a section of the second sample arm are directed toward the same said object. Preferably, said section of the second sample arm is at least partially superimposed spatially with said section of the

first sample arm. More preferably, said section of the second sample arm and said section of the first sample arm are directed through a common lens system. Directing the first and second sample arm toward the same object, wherein both sample arms are preferably spatially superimposed and eventually directed through a common lens system, allows to measure different features of an object involving only one single mechanical adjustment of the object with respect to the claimed system.

The first OCT device may be adapted to measure the first partial volume located near or at a front side of the object, such as the corneal and anterior section (CAS) of an eye. The second OCT device may be adapted to measure a length, as measured e.g. along a depth direction, resp. the second partial volume of the object, e.g. the total length from the anterior surface of the cornea to the retina of an eye. Combining the first and second OCT device with their different measurement targets (different partial volumes to be measured) allows reducing the cost, time and measurement effort as compared to adjusting and using different measurement devices sequentially to measure the object. In addition, the combined OCT diagnostic device provides a complete data set necessary to calculate the imaging properties of the eye as a whole with suitable accuracy in one procedure (“in one shot”).

The first and second OCT devices may be adapted, respectively, to emit a first and second beam each focused with a pre-determined first and second focal length, respectively, wherein the first focal length may be shorter than the second focal length. This allows to measure different target internal volumes located at different depth with respect to a front surface of the object.

The first OCT device may be adapted to emit a first beam of first radiation having wavelengths in a first wavelength range defined by a first operating wavelength and a first bandwidth, thus defining a first axial resolution. The second OCT may be adapted to emit a second beam of second radiation having wavelengths in a second wavelength range defined by a second operating wavelength and a second bandwidth, thus defining a second axial resolution. Then, the first axial resolution may be higher than the second axial resolution. Preferably, the first axial resolution may be less than 5 μm and the second axial resolution may be greater than 15 μm. More preferably, the first bandwidth may be greater than approximately 100 nm and the second bandwidth may be smaller than approximately 20 nm. Still more preferably, the first operating wavelength may be in a range from about 700 to about 1350 nm, preferably about 700 to about 900 nm, more preferably about 750 to about 850 nm, and in particular approximately 820 nm; the first bandwidth may be in the range between about 100 nm and about 200 nm. The second operating wavelength may be in a range from about 600 nm to about 1000 nm, preferably about 620 to about 750 nm, alternatively about 800 to about 1000 nm, in particular approximately 700 nm; the second bandwidth may be in the range between approximately 5 nm and 10 nm. Providing different axial resolutions in the measurement of different internal dimensions and partial volumes of the object allows to save measurement time and reduces data volumes and data volume storage requirements, where a lower high resolution over smaller dimensions is required, whereas a lower resolution over larger dimensions is sufficient, resulting in less data to be processed as compared to a system measuring in both volumes with the same high resolution.

The first OCT device may be adapted to emit a first beam of focused radiation having wavelengths in a first wavelength range defined by a first operating wavelength and a first numerical aperture, thus defining a first lateral resolution. The second OCT device may be adapted to emit a second beam of focused radiation having wavelengths in a second wavelength range having a second operating wavelength and a second numerical aperture, thus defining a second lateral resolution. Then, the first lateral resolution may be different from the second lateral resolution. Preferably, the first lateral resolution may be higher than the second lateral resolution. More preferably, the first lateral resolution is approximately 10 μm to 20 μm (and still more preferably in combination with an axial resolution of 1 μm to 3 μm) and the second lateral resolution is approximately 50 μm to 200 μm (and still more preferably in combination with an axial resolution of 10 μm to 50 μm). Providing different lateral resolutions in different beams of focused radiation allows to adapt the resolution to different application requirements and to save measurement time, data amounts and data storage requirements.

The first OCT device may be a spectral-domain OCT device and the second OCT device may be a time-domain OCT device. Alternatively, both the first OCT device and the second OCT device may be a spectral-domain OCT device. Still alternatively, both the first and the second OCT device may be a time-domain OCT device. Adapting the type of the OCT device (spectral-domain or time-domain) to the different partial volumes of the object to be investigated allows optimizing measurement accuracy, minimizing measurement time and adapting/optimizing the speed of data acquisition according to the application of investigating an object.

The first OCT device may have a first sample arm comprising a first lens system and a common lens system, wherein the first lens system and the common lens system are arranged on a first optical axis and in combination form a first focused portion of a first beam in the first sample arm, wherein the first focused beam portion has a first focal length. The second OCT device may have a second sample arm comprising a third lens system, said common lens system and a spectrally partially reflecting mirror arranged between the first lens system and the common lens system so as to direct a second beam passing along a direction of a second optical axis through the third lens system into the direction of the first optical axis and passing through said common lens system, wherein the third lens system and the common lens system in combination form a second focused portion of a second beam in the second sample arm, wherein the second focused beam portion has a second focal length. In this configuration, the first focal length may be different from the second focal length. Preferably, the first focal length is smaller than the second focal length. The focal length determines the depth range (range of measurement). Accordingly, more preferably, the second focal depth is designed so (i.e. sufficiently long) that the whole axial length of the eye can be measured by the second OCT device. Such arrangements, wherein the second sample arm emerges from a second direction along a second optical axis different from the direction of the first optical axis of the first beam, and is then re-directed into the direction of the first optical axis, and then passes through a common lens system together with the first beam, allows to design the first OCT device to be different from the second OCT device e.g. as concerns the type (spectral-domain or time-domain) of the OCT device, the axial and lateral resolution, the selection of the wavelength range of the radiation, the intensity of the radiation and the modulation in time of the radiation generated by the first and second OCT device. —In an alternative embodiment, e.g. intended for applications different from measurements of an eye, the second focal length may be smaller than the first focal length.

The first OCT device may comprise a first light source having a first operating wavelength and a first bandwidth and the second OCT device may comprise a second light source having a second operating wavelength and a second bandwidth. In this configuration, the first bandwidth may be greater than approximately 100 nm and the second bandwidth may be smaller than approximately 20 nm. Preferably, the first operating wavelength may be approximately 820 nm, the first bandwidth may be in the range between approximately 100 nm and about 250 nm (preferably between approximately 100 nm and about 200 nm), and the second operating wavelength may be approximately 700 nm and the second bandwidth may be smaller than 20 nm, and preferably in the range between about 5 nm and about 10 nm. Such spectral configuration of the first and second OCT devices allows the first partial volume to be investigated with a different axial resolution and preferably at a different operating wavelength as compared to the second partial volume.

The first OCT device may have a first sample arm and the second OCT device may have a second sample arm that is at least partially superimposed spatially on the first sample arm. The first and second sample arm may pass through a bi-focal common optical lens system comprising a first focussing portion acting in the first sample arm and having a first focal length and a second focussing portion acting in the second sample arm and having a second focal length. In this configuration, the first focal length may be smaller than the second focal length. —In a preferred first embodiment, the first focussing portion is a circular central portion of the bi-focal length system and the second focussing portion is an annular portion surrounding the first focussing portion. More preferably, the first and second focussing portions may have different spectral transmittance characteristics, each adapted to define an appropriate wavelength range as defined by a respective operating wavelength and bandwidth, according to the need of the investigation of the respective partial volumes, which may be at different distances resp. depths in the object, to which the respective focal lengths of the first and second focusing portion of the common lens system is adapted. —In an preferred alternative second embodiment, the bi-focal length system is embodied as a suitably designed Diffractive Optical Element (DOE) having at least two complementary regions, the first region being designed to render the first focal length and the second region being designed to render the second focal length.

The first OCT device and the second OCT device may comprise a common light source. This further reduces system costs and increases the degree of integration of the first and second OCT device.

The first OCT device may comprise a first reference arm and the second OCT device may comprise a second reference arm which is at least partially superimposed spatially on the first reference arm. The first reference arm may have an optical path length corresponding substantially to the optical path length of the first sample arm and may comprise a first mirror and a first reference arm lens system forming a first reference arm portion that is focused onto the first mirror. The second reference arm may have an optical path length corresponding substantially to the optical path length of the second sample arm and may comprise a second mirror, a second reference arm partially reflecting mirror arranged in the first reference arm in front of the first reference arm lens system and a second reference arm lens system arranged outside of the first reference arm and substantially between the second reference arm partially reflecting mirror and the second reference arm lens system, wherein the partially reflecting mirror re-directs a beam of light having a wavelength in a second wavelength range defined e.g. by a second operating wavelength and a second bandwidth and passing through the first reference arm lens system along a first reference arm direction into a second reference arm direction and through the second reference arm lens system, and wherein the second reference arm partially reflecting mirror and the second reference arm lens system form in combination a second reference arm portion that is focused onto the second mirror. Such configuration allows an at least partial integration resp. superposition of the first and second reference arms of, respectively, the first and second OCT device, while allowing the optical path lengths of the first and second reference arms to correspond substantially to the optical path lengths of the corresponding first and second sample arm.

In an alternative embodiment of the reference arms, the first OCT device comprises the first focussing portion being adapted to act on the first reference arm passing through a first focussing portion of a bi-focal reference arm common lens system and the second OCT device comprises a second reference arm which is at least partially superimposed spatially on the first reference arm and passes through a second focussing portion of said bi-focal reference arm common lens system, wherein the second focussing portion is adapted to act on the second reference arm. In this embodiment, the first reference arm further comprises a first mirror that is spectrally partially reflecting light having wavelengths in a first wavelength range defined e.g. by a first operating wavelength and a first bandwidth, and the second reference arm further comprises a second mirror that is spectrally reflecting light having wavelengths in a second wavelength range defined e.g. by a second operating wavelength and a second bandwidth. The focal length of the first focussing portion may be adapted such that the optical path length of the first reference arm corresponds substantially to the optical path length of the first sample arm and the focal length of the second focussing portion may be adapted such that the optical path length of the second reference arm corresponds substantially to the optical path length of the second sample arm. Preferably, the first focussing portion of the bi-focal reference arm common lens system is a circular central portion and the second focal portion is an angular portion surrounding the first focussing portion. In one configuration, the first and the second focussing portion of the bi-focal reference arm common lens system have different spectral transmission characteristics adapted to the application requirements of first and second beams targeting resp. first and second partial volumes of the object to be investigated. In an alternative configuration, a spectral filter having a selected spectral transmittance characteristic may be arranged behind the bi-focal reference arm common lens system.

According to a second aspect of the invention, as claimed, there is provided a method for optically measuring internal dimensions of an object comprising internal interfaces at which the refraction index changes so that a portion of incident light is backreflected and/or backscattered and can be detected. The object may for example be an eye.

According to the invention, the method comprises a step of measuring internal dimensions in a first partial volume of the object and internal dimensions in a second partial volume of the object by means of optical coherence tomography (OCT) in a single measurement operation, wherein the second partial volume is at least partially different from the first partial volume. This method achieves the same technical effect and advantages as the claimed system defined hereinbefore.

When performing the claimed method, a system as described above may be used.

Further embodiments, advantages and technical effects of the invention may become apparent from the following detailed description of particular embodiments, which is not intended to imposing restrictions on the scope of the invention and which provided with reference to the appended drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a conventional spectral-domain OCT device;

FIG. 2 illustrates an embodiment of a conventional time-domain OCT device;

FIG. 3 illustrates a first embodiment of a system according to the invention, wherein a first OCT device and a second OCT device different from the first OCT device is combined by superimposing only portions of a first and second sample arm directed through a common lens system toward a same object to be investigated;

FIG. 4 illustrates a second embodiment of a system according to the invention, wherein the first and second OCT device are further integrated to have a combined common sample arm;

FIG. 5 illustrates a spectral design of a system according to the invention, providing radiation comprising wavelengths in a first wavelength range as defined by a first operating wavelength and a first bandwidth and radiation comprising wavelengths in a second wavelength range defined by a second operating wavelength and a second bandwidth;

FIG. 6 illustrates a third embodiment of a system according to the invention, wherein both the first and second OCT device are a spectral-domain OCT devices and have partially integrated reference arms;

FIG. 7 illustrates a fourth embodiment of a system according to the invention, wherein both the first and second OCT device are a spectral-domain OCT devices and wherein the design of the reference arm differs from that of the embodiment shown in FIG. 6; and

FIG. 8 is a cross-section through a human eye for illustrating the different partial volumes and internal interfaces of the eye to be investigated.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplifying conventional optical coherence tomography (OCT) device of the spectral-domain type (SD-OCT). The SD-OCT, referenced 100, comprises a preferably broadband light source 102, a light source optical fibre 104, an optical fibre coupler 106, a bi-directionally used optical fibre 108, a beam splitter 112, a sample arm comprising a first common lens system 110 (common for both sample and reference arm), a beam splitter 112 and a sample arm lens system 114, and a reference arm comprising a beam splitter 112, a reference arm lens system 116 and a reference arm mirror 117; a detection arm comprising the fibre coupler 106, a detection arm optical fibre 118, a first collimation lens system 120, an optical grating 122, a second spectrum imaging lens system 124, and a spectrometer detector array 126 comprising a plurality of detector cells 128-1 to 128-n for measuring a spectrally resolved interference pattern. The SD-OCT 100 further comprises a calculation unit 132 for performing a fast Fourier transformation of said spectrally resolved interference pattern 130 to calculate a depth distribution 134 of refractive index interfaces 14, 14′, 14″ in a sample object 10.

In operation of the SD-OCT 100, the light source 102 generates broadband light radiation, i.e. light radiation comprising radiation of wavelength distributed in a relatively broad spectral wavelength range. The generated radiation is transmitted through the light source optical fibre 104 via the fibre coupler 106 through the bi-directionally used optical fibre 108, from a distal end of which the radiation is emitted in the form of a divergent beam B1 passing through the first sample arm lens system 110, which reforms the beam B1 into a beam of essentially parallel light (as shown in FIG. 1) passing through the beam splitter 112. In the beam splitter 112, a portion of the beam of parallel light is transmitted into the sample arm SA1 of the SD-OCT 100 toward a second sample arm lens system 114, which focuses the beam into a focused beam portion having its focus located in the object 10.

The object 10 comprises in its internal volume a plurality of internal interfaces 14, 14′, 14″ at which the refraction index changes and which therefore cause partial reflections of focused beam illuminating the object 10. The radiation reflected from the plural internal interfaces 14, 14′, 14″ is collected by the second sample arm lens system 114, transmitted therethrough as a beam of essentially parallel light, transmits through the beam splitter 112 and is focused by the first sample arm lens system 110 into the distal end of the bi-directionally used optical fibre 108.

Another portion of the radiation transmitted from the bi-directionally used optical fibre 108 through the first sample arm lens system 110 as a beam of essentially parallel radiation is partially reflected by an internal substantially plane surface, which is oblique, preferably at an angle of substantially 45° oriented with respect to the incoming beam of essentially parallel radiation, so as to form the reference arm RA1 directed toward the reference arm lens system 116, which focuses the beam of essentially parallel radiation onto the reference arm mirror 117. The reference arm mirror 117 is arranged stationary and reflects the beam of focused radiation, so that the reflected diverging radiation is collected by the reference arm lens system 116 which transmits the reflected radiation as a beam of essentially parallel radiation from the reference arm. The radiation returning from the reference arm is directed by said plane internal surface of the beam splitter 112 toward the first sample arm lens system 110, which transmits and focuses the light returning from the reference arm RA1 onto the distal end of the bi-directionally used optical fibre 108. The optical fibre 108 thus transmits both the radiation returning from the sample arm SA1 as reflected from the internal interfaces 14, 14′, 14″ of the object 10 and the radiation returning from the reference arm RA1 as reflected from the reference arm mirror 117, allowing these radiation beams to interfere. The interfering radiation is transmitted through the optical fibre 108, via the fibre coupler 106 into and through the detection arm optical fibre 118, from a distal end of which the interfering radiation emerges as a diverging beam, which is collected and transmitted by the first detection lens system 120 into a beam of essentially parallel light toward the optical grating 122. The grating 122 reflects the incoming beam of interference light into a plurality of beams of essentially parallel light with different reflection angles according to the different wavelengths of the radiation impinging on the grating 122. The structure and function of the grating 122 as a spectrally resolving element reflecting impinging radiation at different reflection angles according to the wavelength of the radiation, is known to the skilled person, so that a description thereof is omitted here.

The plurality of spectrally resolved beams of radiation reflected from the grating 122 is collected by the second detection lens system 124 and focused, according to the reflection angle from the grating 122, onto the spectrometer detector array, on which the focused, spectrally resolved beams impinge on, and are detected by, respective ones of the plurality of detector cells 128-1 to 128-n.

According to this arrangement of the first detection lens 120, the optical grating 122, the second detection lens system 124 and the spectrometer detector array 126, a particular position along the spectrometer detector array 126 resp. a particular detector cell 128-i corresponds to a respective particular wavelength of the interference radiation originating from the interference of the radiation returning from the sample arm SA1 and from the reference arm RA1. The spectrometer detector array 126 thus detects the spectrally resolved interference pattern 130, which is essentially a spectral distribution of the intensity of the interference radiation. The spectral distribution is submitted to a Fourier transformation, implemented for example in the fast Fourier transformation calculation unit 132, to yield the depth distribution 134 of refractive index interfaces illustrated in FIG. 1. The distribution 134 comprises essentially a distribution of the intensity resp. amplitude a(z) of the interference radiation as a function of the length of the optical path z as measured in the sample arm SA1 for the contributions of the radiation reflected by the internal interfaces 14, 14′, 14″ in the object 10. As illustrated in FIG. 1, the distribution 134 comprises three peaks corresponding to the three internal interfaces 14, 14′, 14″ in the object 10 as depicted in FIG. 1.

In other words, the broadband spectral distribution of radiation emitted from the light source 102 interferes, after reflection from the refractive index discontinuities resp. internal interfaces 14, 14′, 14″ in the object 10 in the sample arm SA1, with the broadband spectral distribution of radiation reflected in the reference arm RA1. The respective interfering spectral intervals corresponding to the spectral resolution achieved by the optical grating 122 in combination with the particular detector cells 128-i, correspond to information from different depths of the internal interfaces 14, 14′, 14″ in the object 10. The calculated Fourier transformation of the spectrum registered by the spectrometer detector array 126 then yields information on the depth position of the interfaces along the depth direction z within the object 10.

In case the object 10 is a human eye, the refractive index differences of the different portions of the eye 20 (as illustrated in FIG. 8) results from the different refraction indices of the materials traversed by the radiation in the sample arm SA1, including air (refractive index 1.003), the film of tears (refractive index 1.3335), the epithel (refractive index 1.401) and the stroma (refractive index 1.3771). The afore-mentioned values of refractive indices of segments of a human eye 20 are taken from the specification of the afore-mentioned device manufactured by the company TOMEY.

FIG. 2 depicts schematically an example of a conventional OCT device of the time-domain type (TD-OCT). The TD-OCT 150 comprises a preferably low-coherence light source 152, a first light source optical fibre 154, an optional circulator 155, a bi-directionally used second light source optical fibre 156, an optical fibre coupler 158, a sample arm SA2 comprising a bi-directionally used sample arm optical fibre 160, a first sample arm lens system 162, a second sample arm lens system 164 and a sample object 10 comprising internal interfaces 14, 14′, 14″ at which the refraction index changes. The TD-OCT device 150 further comprises a reference arm RA2 comprising a bi-directionally used reference arm optical fibre 166, a reference arm lens system 168, a position-modulated reference arm mirror 170 and a notably high-speed delay scanner 172. The OCT device 150 still further comprises a detection arm comprising a first detection optical fibre 174 and a detector 178. Optionally, as a means to increase the signal-to-noise ratio, the OCT device 150 further comprises the circulator 155, a second detection optical fibre 176 and a difference forming portion of the detector 178, e.g. a dual balanced signal detection (DBSD) unit. As a means for obtaining a depth information on the internal interfaces 14, 14′, 14″ in the object 10, the OCT device 150 still further comprises a band pass filter 180, a demodulator 182 and a computer 184 for receiving a demodulated signal and for calculating the depth information of the internal interface 14, 14′, 14″.

In the operation of the TD-OCT 150, the light source 152 emits radiation which suffices to be of relatively low coherence and which comprises a relatively narrow wavelength range. The radiation emitted by the light source 152 is transmitted through the first light source optical fibre 154 via the optional circulator 155, through the second light source optical fibre 156, via the optical fibre coupler 158 in which it is split into a first radiation portion propagating into the sample arm SA2 and a second radiation portion propagating into the reference arm RA2.

The first radiation portion is transmitted through the sample arm optical fibre 160, from a distal end of which it emerges as a diverging beam which is collected by the first sample arm lens system 162 transmitting the diverging beam as a beam of essentially parallel light toward the second sample arm lens system 164. The lens system 164 transmits and focuses the beam into a focused beam, the focus of which is located in the object 10. Respective internal interfaces 14, 14′, 14″ partially reflect portions of the incoming light back toward the second sample arm lens system 164 which collects the plurality of radiation portions reflected from the plurality of internal interfaces 14, 14′, 14″ and transmits these toward the first sample arm lens system 162, which focuses the reflected radiation portions returning from the sample arm SA2 onto the distal end of the sample arm optical fibre 160, which transmits this radiation via the fibre coupler 158 into the first detection optical fibre 174.

The second radiation portion split by the fiber coupler 158 is transmitted in the reference arm RA2 through the reference arm optical fibre 166, from a distal end of which it emerges as a diverging beam. This is collected by the reference arm lens system 168 and transmitted as a beam of essentially parallel radiation towards the modulated reference arm mirror 170. The reference arm mirror 170 is moved at high speed in a periodic manner to and fro along an axial direction of this portion of the reference arm RA2 by the high-speed delay scanner 172 (as indicated by the double arrow shown in FIG. 2). The radiation reflected from the position-modulated reference arm mirror 170 is transmitted and focused by the reference arm lens system 168 onto the distal end of the reference arm optical fibre 166, which transmits the reflected reference arm radiation via the fibre coupler 158 into the first detection optical fibre 174, where it interferes with the radiation returning from the sample arm SA2 as reflected by the internal interfaces 14, 14′, 14″ of the object 10.

The interference light is transmitted through the first detection optical fibre 174 to an input port (−) of an entrance stage of the detector 178, where a time dependency of the intensity of the interference radiation is detected and registered.

As an optional means for improving the signal-to-noise ratio and e.g. by performing a background subtraction, a portion of the radiation emitted by the light source 152 is transmitted by the circulator 155 into and through the second detection optical fibre 176 to another input port (+) of the entrance stage of the detector 178. The detector 178 subtracts from a signal from the interference radiation a signal from the radiation emitted by the light source 152 and “tapped” by the circulator 155. Due to this configuration of the detector 178 having the (+) and (−) entrance ports, excess noise from the signal of the light source 152 is subtracted from the signal of the interference radiation, thereby improving the signal-to-noise ratio. The so obtained signal is fed through the band pass filter 180 and to the demodulator 182 to remove a high-frequency component resulting from the high-speed modulation of the delay scanner 172 in the reference arm RA2. The so obtained signal is fed to, and registered in, the computer 184, which calculates from the received signal the desired depth information of the internal interfaces 14, 14′, 14″ in the object 10.

In the TD-OCT 150, the narrow band interference radiation is reduced by the interference of radiation reflected from the internal interfaces 14, 14′, 14″ in the object 10 in the sample arm SA2 with radiation returning from the reference arm RA2, the optical path length of which is scanned resp. varied by means of the periodic movement of the mirror 170 as generated by the delay scanner 172.

OCT devices of the spectral-domain type (as exemplified in FIG. 1) have as advantages over the commercially mostly used OCT devices of the time-domain type (as exemplified in FIG. 2) a better resp. higher signal-to-noise ratio and a simultaneously obtainable depth information of the internal interfaces 14, 14′, 14″ without involving mechanically moving parts such as the reference arm mirror 170 of the time-domain OCT device 150.

From fundamental principles of optics it can be derived, and is known to the skilled person, that the axial resolution Δz of an OCT device, hence the accuracy for obtaining depth positions of the internal interfaces 14, 14′, 14″, is determined essentially by a bandwidth (Δλ) and a center wavelength (λ₀) of the radiation used according to:

$\begin{matrix} Δ z = \frac{2 \ln 2}{π n} \times \frac{λ_{o}^{2}}{Δ λ}, & (1) \end{matrix}$

wherein n is the refractive index of a medium presenting the partially reflecting interface. In case that the object 10 is a human eye 20 illustrated in FIG. 8, the relevant refractive index of the foremost interface is that of the cornea having n=1.3771.

The accuracy, by which the depth information is obtained in a lateral direction with respect to the axial direction (z), i.e. the lateral resolution Δx is essentially determined by:

$\begin{matrix} Δ x = \frac{2}{π} \times \frac{λ_{0}}{NA} and NA \propto \frac{1}{f}, & (2) \end{matrix}$

wherein NA is the numerical aperture of a focusing lens system, f is the focal length of the lens system which focuses the radiation in the sample arm on the object 10.

The axial range, from which a sufficiently intensive portion of radiation is reflected resp. scattered back in the object 10, is of the order of magnitude of the depth of focus (DOF) of the lens system which focuses the radiation into the object 10, and is determined by the focal length f respectively the numerical aperture NA of the lens system according to:

DOF∝1/NA²∝f² (3)

When evaluating equations (1), (2) and (3) for an axially extending object for which the second partial volume extends substantially along the total length of the object and the first partial volume is near or at the front side 16 of the object 10 and extends from the front side over only e.g. one tenth of the total axial length, it becomes clear that an OCT device adapted to measure internal surfaces within the first partial volume with adequate accuracy cannot also measure internal interfaces distributed along the total length, i.e. the second partial volume, with the same resolution as in the first partial volume of the object 10. In particular, when the object 10 is a human eye 20 (as shown in FIG. 8), it is not possible to obtain with one single OCT device adapted to measure the frontal section of the eye including the cornea and anterior section (CAS) with high accuracy resp. axial resolution of Δz=1 μm to 3 μm also the length of the eye 20 from the cornea to the retina 26.

In conventional practice, the intra-ocular structures of the CAS of an eye 20 are measured using an OCT device of the spectral-domain type having a relatively high axial resolution of less than 10 μm, where the axial resolution is in the range from about 1 μm to 3 μm. For a precise measurement of the various interfaces of the CAS of the eye, it is possible within the scope of the invention, and highly desirable if not necessary, to employ an SD-type OCT device of the latest state of the art, where the axial resolution is less than 1 μm.

On the other hand, the eye length is conventionally measured e.g. by devices based on the principle of optical low coherence reflectometry (OLCR) or using OCT devices of the time-domain type, wherein the length of the reference arm must be varied (scanned) over a length corresponding to the length of the eye, wherein this is achieved by axially scanning a mirror over such a length equivalent or by laterally moving a prism having a corresponding basis, as implemented e.g. in an OLCR type of device manufactured by the company Haag-Streit.

As stated above, according to the invention, in order to enable simultaneous or quasi simultaneous measurement of both a first partial volume extending only over a relatively small portion of the total length of an object with sufficiently high resolution and a second partial volume extending e.g. along the total length or being axially spaced from the first partial volume by an axial distance of e.g. more than one half of the total length of the object, there is proposed to combine (integrate) a first and a second OCT device adapted to measure, respectively, internal dimensions in the first and the second partial volume of the object. Particular embodiments thereof are described in the following with reference to FIGS. 3, 4, 6 and 7.

In the embodiments described in the following, it is assumed that the first partial volume 17 is located near or at the front side 16 of the object 10 and is measured by a first OCT device OCT1 having a focal range DOF1 extending substantially across the first partial volume 17, and that the second partial volume extends from the front side 16 to the rear side 18 of the object 10 and is measured by a second OCT device OCT2 having a corresponding depth of focus DOF2 extending thereacross.

In the first embodiment shown in FIG. 3, the first OCT device OCT1 is combined with the second OCT device OCT2 by superimposing a portion of the sample arm SA2 of the second OCT device 2 with a portion of the sample arm SA1 of the first OCT device OCT1, and by letting portions of both the sample arm SA2 of the second OCT device OCT2 and the sample arm SA1 of the first OCT device OCT1 extend through a common lens L12 and onto the same object 10. The sample arm SA1 of the first OCT device OCT1 is designed to pass through a first lens system L1 and the common lens system L12, which in combination form a focused sample arm beam portion B1 corresponding approximately to the distance of the first partial volume 17 from the common lens system L12 and a depth of focus DOF1 extending substantially throughout the first partial volume 17 of the object. The sample arm SA2 of the second OCT device OCT2 is designed to comprise a third lens system L3, a partially reflecting mirror M and the common lens system L12, whereby the third lens system L3 is arranged outside of, and the partially reflecting mirror M is arranged in, the first sample arm SA1 of the first OCT device OCT1 between the first lens system L1 and the common lens system L12, so as to deflect a portion of the sample arm SA2 of the second OCT system OCT2 into the direction of the first sample arm SA1 of the first OCT device OCT1. In particular, the portion of the sample arm SA2 of the second OCT device OCT2 is substantially perpendicular to the sample arm SA1 of the first OCT device OCT1, and the partially reflecting mirror M is arranged at an angle of substantially 45° with respect to the direction of the first sample arm SA1 of the first OCT device OCT1.

The arrangement of the partially reflecting mirror M is not limited to the aforementioned arrangement. The partially reflecting mirror M may be arranged at an angle θ different from 45°, e.g. in a range θ from 20° to 70°, and the portion of the sample arm SA2 including the third lens system L3 and the components of the second OCT device OCT2 except the sample arm lens SA2, may be arranged at an angle of 2θ with respect to the sample arm SA1.

The third lens system L3 in combination with the common lens system L12 form a second focusing portion B2 having a focal length f2 corresponding substantially to a distance of a rearward half portion of the object 10 from the common lens system L12, and the depth of focus DOF2 of the second focusing portion FP2 extends substantially throughout the second partial volume 19.

The first OCT device OCT1 is a spectral-domain OCT device, e.g. of the configuration of the SD-OCT 100 shown in FIG. 1, whereby the first sample arm lens system 110 and the second sample arm lens system 114 of the SD-OCT 100 of FIG. 1 correspond, respectively, to the first lens system L1 and the common lens system L12 of the combined system shown in FIG. 3.

The second OCT system OCT2 is a time-domain OCT system, e.g. of the configuration of the TD-OCT 150 shown in FIG. 2, whereby the first sample arm lens system 162 and the second sample arm lens system 164 of the TD-OCT 150 of FIG. 2 correspond, respectively, to the third lens system L3 and the common lens system L12 of the system shown in FIG. 3, and wherein the sample arm SA2 of the device 150 of FIG. 2 is modified by “folding” the sample arm SA2 in the portion between the first and second sample arm lens systems 162 and 164 by inserting the partially reflecting mirror M as shown in FIG. 3.

The first resp. second OCT device OCT1 resp. OCT2 has a first resp. second light source (not shown) that generate first resp. second radiation comprising respective spectra having wavelengths in a first resp. second wavelength range defined by a first resp. second operating wavelength λ1 resp. λ2 and a first bandwidth Δλ1 resp. Δλ2 as illustrated in FIG. 5.

When the first OCT device OCT1 is to be adapted to measure the CAS 22, 24 of a human eye 20 as shown in FIG. 8, a suitable first operating wavelength is λ1≈1.300 nm, however, λ1 could be a wavelength in the range from about 700 nm to about 950 nm, e.g. about 850 nm as in the example of FIG. 5 (see below). The first bandwidth Δλ1 can be in the range from about 100 nm to about 200 nm, e.g. about 100 nm. The first OCT device OCT1 could be a spectral-domain OCT device such as the one manufactured by the company TOMEY mentioned above, wherein the light source comprises a swept-source laser having an central output wavelength λ1≈1310 nm and an output power of 5 mW or less. The partially reflecting mirror M of the configuration shown in FIG. 3 can be implemented as a dichroitic mirror in a conventional way.

When the second OCT device OCT2 is to be adapted to measure the total length of a human eye 20 as shown in FIG. 8, a suitable second wavelength λ2 is in the range from about 800 nm to about 1000 nm. In the past, anterior eye OCT devices used a wavelength of about 1300 nm, but this may be changing toward shorter wavelengths because of an improved availability of suitable light detectors, which operate in the range of less than about 950 nm, such as light detectors using Si-CMOS technology. The second wavelength λ2, including the range defined by the second spectral bandwidth Δλ2, should be different from the first wavelength λ1 and preferably outside of range defined by the first spectral bandwidth Δλ1 comprising the first wavelength λ1, for example to reduce mutual cross-talk between the first and second spectral bands, and notably to ease a spectral design of a dichroitic beam splitter for splitting the first spectral band from the second spectral band (though this is not obligatory).

A preferred example of the spectral arrangement of the first and second spectral band is shown in FIG. 5. In the example of FIG. 5, the first operating wavelength λ1 is about 850 nm and the first bandwidth Δλ1 is 100 nm so that the first spectral band covers the range from about 750 nm to about 950 nm, which corresponds with the spectral sensitivity characteristic of Si-CMOS technology based detectors and Si-CCD detectors; the second operating wavelength λ2 is about 700 nm and the second bandwidth Δλ2 is considerably smaller than the first bandwidth Δλ1, notably less than about 20 nm. In this case, the second OCT device OCT2 can be replaced by a device based on the principle of optical low coherence reflectometry (OLCR), such as in the device manufactured by the company Haag-Streit.

The combination of the third lens system L3 and the common lens system L12 in the sample arm SA2 of the second OCT system OCT2 has a focal length f2 that is relatively long, so as to allow measuring the second partial volume 19 extending across the total axial length of the object 10 and also has a depth of focus DOF2 that is suitably designed to be relatively long so as to extend across the second partial volume 19. On the contrary, the combination of the first lens system L1 and the common lens system L12 in the first sample arm SA1 of the first OCT device OCT1 has a relatively short focal length f1 and a relatively short depth of focus DOF1, respectively, located in and extending only through the first partial volume 17 located at or near the front surface 16 of the object 10.

FIG. 4 illustrates a second embodiment of an integrated system OCT12′, wherein the combination of the first OCT device with the second OCT device is achieved by at least partially superimposing the sample arm SA2 of the second OCT device spatially with the sample arm SA1 of the first OCT device as shown in FIG. 4, and further by integrating the first and the second OCT devices in a combined system OCT12.

In the combined system OCT12′, the detection arms of the first and the second OCT devices are shared in an integrated detection arm (not shown), and the reference arms of the first and the second OCT devices are integrated to an integrated reference arm (not shown) as implemented e.g. in the third and fourth embodiment shown respectively in FIGS. 6 and 7, and further the light sources of the first and the second OCT device are integrated into a common light source LS12 of the combined system OCT12.

The system OCT12′ of the second embodiment shown in FIG. 4 comprises an integration of the sample arms SA1 and SA2, respectively, of the first and second OCT device into a common sample arm, wherein a beam B1 of the first sample arm SA1 of the first OCT device and a beam B2 of the second sample arm SA2 of the second OCT device are largely spatially superimposed over each other.

As illustrated in FIG. 4, radiation associated with the first OCT device emitting at wavelengths in a first wavelength range (as shown in FIG. 5), and radiation associated with the second OCT device emitting wavelengths in a second wavelength range are guided in an integrated sample arm SA12, by guiding the radiation through a common optical fibre SOF12, from a distal end of which it emerges as a diverging common beam B12, which is collected by a first lens L1 and transmitted as a common beam B12′ of essentially parallel light propagating toward a common lens system embodied as a bi-focal common lens system BFL12 and providing a first focusing portion FP1, which acts for the radiation of the first wavelength range and a second focusing portion FP2 which acts for the radiation of the second wavelength range. The first focusing portion FP1 provides a first focal length f1 and a first depth of focus DOF1 extending through the first partial volume 17, and the second focusing portion FP2 provides a second focal length f2 and a second depth of focus DOF2 extending along the second partial volume 19 of the object 10, as shown in FIG. 4.

The common lens system BFL12 is designed such that the first and second focusing portions FP1, FP2 may be arranged one beside another, e.g. in the form of two half planes. Alternatively, as shown in FIG. 4, they are arranged one surrounding the other, whereby the first focusing portion FP1 is a central circular portion and the second focusing portion FP2 is an annular portion surrounding the central circular first focusing portion.

As an alternative to embodying the common lens system as a bi-focal common lens system, the common lens system may be embodied as a bi-focal Fresnel lens or a bi-focal diffraction optical element (DOE) having two different focal lengths, e.g. having a design similar to that of a bi-focal intra-ocular lens (IOL).

In a first sub-embodiment, the radiation of the common beam B12, B12′ of radiation comprises a continuous spectrum of radiation, covering both the first and the second wavelength ranges shown in FIG. 5, and the first and second focusing portions FP1 and FP2 of the lens system BFL12, respectively, have a spectrally filtering transmission characteristics adapted to provide a high transmission coefficient, preferably greater than 90% and preferably near to or approximately 100%, in respectively the first and second wavelength range (as defined by the first resp. second operating wavelengths U, λ2 and the first resp. second bandwidth Δλ1, Δλ2 to as shown in FIG. 5.

In a second sub-embodiment, the spectral composition of the common beam B12, B12′ in the sample arm as shown in FIG. 5 is obtained by providing respective first and second spectral filters, which are respectively congruent with the first FP1 and second FP2 focusing portions of the bi-focal common lens system BFL12.

In both the first and the second sub-embodiment of the embodiment shown in FIG. 4, upon passing through the bi-focal common lens system BFL12, the common beam B12′ of radiation is divided in a first beam B1 of radiation having passed through the first focusing portion FP1 and comprising wavelength in the first wavelength range (defined by λ1 and Δλ1 of FIG. 5), and provides a first focal length f1 and a first depth of focus DOF1 extending through the first partial volume 17 of the object, and a second beam B2 of radiation having passed through the second focusing portion FP2 and comprising wavelengths in the second wavelength range (defined by λ2 and Δλ2 of FIG. 5) and provides a second focal length f2 and a second depth of focus DOF2 extending through the second partial volume 19 extending essentially through the total length 12 of the object 10. By arranging the first focal portion FP1 to surround the second focusing portion FP2 in the bi-focal common lens system BFL12, the first focusing portion FP1 has a greater diameter than the second focusing portion FP2 and accordingly the first beam B1 has a greater numerical aperture than the second beam B2. Consequently, according to equation (2), the first beam B1 in the first partial volume 17 achieves a smaller lateral resolution Δx than the second beam B2 in the second partial volume 19. According to equation (3), the depth of focus DOF1 of the first beam B1 is smaller than the depth of focus DOF2 of the second beam B2, which is adapted to measure the second partial volume 19. In addition, since the first bandwidth Δλ1 of the first beam B1 is relatively broad, e.g. in the range of about 100 nm to 200 nm, and the first operating wavelength λ1 is in the range between about 700 nm and 950 nm, e.g. 850 nm, and the bandwidth Δλ2 of the second beam B2 is relatively narrow band e.g. less than about 20 nm, according to equation (1), the axial resolution ∝1/Δz1 of the first beam B1 is considerably higher than the axial resolution ∝1/Δz2 of the second beam B2, or stated otherwise, Δz1<<Δz2.

When both the first and the second OCT systems are SD-OCT type devices, they can have an integrated sample arm (as shown in FIG. 4, 6 or 7), can share a common light source LS12 (as shown in FIGS. 6 and 7), and can further integrate their reference arms (e.g. as in the embodiments shown in FIGS. 6 and 7).

Such configurations can be particularly adapted to measure the CAS section 17 and the total length 19 of a human eye 20 (see FIG. 8), when the light source LS12 is a broadband light source suitable for SD-OCT and has an emission spectrum ranging from e.g. less than about 700 nm to more than about 950 nm. This range is well adapted to the spectral sensitivity of modern high-speed silicon (SI)-based detectors. The first wavelength can be spectrally filtered out from the emission spectrum of the shared light source LF12 e.g. by a spectrally filtering element designed to transmit radiation having wavelengths around a central first operating wavelength λ1 of about 820 nm and a first bandwidth Δλ1 in the range 100 nm to 200 nm. Also the second wavelength range can be filtered out from the emission spectrum of the shared light source LS12 e.g. by a filter designed to transmit wavelengths around a central second operating wavelength 22 of about 700 nm and a second bandwidth Δλ2 in the range of about 5 nm to about 20 nm.

Respective spectral filters can be provided separately in the integrated sample arm SA12 so as to be congruent with the first and second focussing portions FP1, FP2 of the bi-focal common lens system BFL12, or can be applied directly on the first and second focussing portions FP1, FP2 of the bi-focal common lens system BFL12, e.g. by respective suitable spectral filter coatings, notably edge filter coatings, where the edge of a first edge filter applied in the first focussing portion FP1 is designed to be positioned between the first and second wavelength ranges shown in FIG. 5.

FIG. 6 illustrates a third embodiment of a combined system OCT12″. The combined system OCT12″ has a configuration essentially as a spectral-domain OCT device shown in FIG. 1, however with the following modifications with respect to the embodiment shown in FIG. 4.

Firstly, the sample arm SA12 comprising a sample optical fiber SOF12, the first length system L1 and the bi-focal common lens system BFL12 is configured as in the second embodiment shown in FIG. 4. Secondly, the light source LS12 is a broadband source adapted for SD-OCT application and a spectral filtering of the broadband radiation spectrum is provided either by applying respective first and second spectral filters, on the first and second focussing portions FP1, FP2 of the bi-focal common lens system BFL12 for filtering out the first and second wavelength ranges (defined as shown in FIG. 5). Thirdly, the detector arm is integrated by using a common detection arm optical fiber DOF12 for guiding the interference radiation from the first and second beams B1 and B2, a first detection arm optical lens system DL1, a common detection arm grating DG12, a second detection arm optical lens system DL2 and a common detection arm spectrometer detector array SDA12, which in combination form a similar configuration as the first detection lens system 120, the optical grating 122, the second detection lens system 124 and the spectrometer detector array 126 of the SD-OCT device 100 shown in FIG. 1. However, the common detector arm grating DG12 in combination with the common detector arm spectrometer detector array SDA12 is adapted to detect and spectrally resolve radiation comprising both the first and second wavelength ranges as produced by the spectral filters as described above and as illustrated in FIG. 5.

Fourthly, the reference arm is integrated by at least partially superimposing spatially a first and a second reference arm RA1 and RA2 corresponding, respectively, to the first and second sample arms SA1 and SA2. The first reference arm RA1 comprises a common beam splitter BS12, a first reference arm lens LR1 and a first reference arm mirror MR1 arranged stationary and at a position (distance) with respect to the common beam splitter BS12 so that the optical path length for the radiation RAD1 in the first reference arm RA1 corresponds to the optical path length of the radiation in the first beam B1 focused into the first partial volume 17. The second reference arm RA2 comprises said common beam splitter BS12, a second reference arm partially reflecting mirror MRA, a second reference arm lens system LR2 and a second reference arm mirror MR2, wherein the mirror MRA is arranged in the optical path of the first reference arm RA1 between the common beam splitter BS12 and the first reference arm lens system LR1 and is adapted to partially reflect (deflect) radiation RAD2 comprising wavelengths in the second wavelength range (as defined by λ2 and Δλ2, see FIG. 5) away from the direction of the first reference arm RA1 and towards the second reference arm lens system LR2 and the second reference arm mirror MR2. Also, the second reference arm mirror MR2 is arranged stationary and at a distance with respect to the common beam splitter BS12 so that the optical path length of the second radiation RAD2 in the second reference arm RA2 corresponds to the optical path length of the radiation of the second beam B2 of the second sample arm SA2 focused into the second partial volume 19 of the object 10. The partially reflecting mirror MRA is adapted to be selectively transmissive for the wavelengths of the first radiation RAD1 in the first spectral range and selectively reflective for the wavelengths of the second radiation RAD2 in the second wavelength range.

In order to increase the signal-to-noise ratio and/or to improve the interference signal of the first beam B1 returning from the refractive index interfaces in the first partial volume 17, an additional third reference arm mirror MR3, see FIG. 6, is provided, which is designed to be partially transmissive for wavelengths in the second wavelength range, by providing a transmission coefficient in the range of about 10% to 50%, and reflective for wavelengths in the first wavelength range. The mirror MR3 is provided in the second reference arm RA2 at a position so that the optical path between the beam splitter BS12 and the mirror MR3 corresponds to the optical path between the beam splitter BS12 and the mirror MR1.

In the embodiment shown in FIG. 6, the second reference arm lens system LR2 may have the same focal length as the second focussing portion FP2 of the bi-focal common optical lens system BFL12. Accordingly, the lens system LR2 images on the second and third reference arm mirrors MR2 and MR3 similar beam diameters over a relatively long depth of focus corresponding to the depth of focus DOF2 of the second focussing portion FP2 in the sample arm SA2, so that a sufficient reference signal is obtained.

FIG. 7 shows a fourth embodiment of a combined system OCT12′″, which is similar to the third embodiment of the combined system OCT12″ shown in FIG. 6 as concerns the integration of the sample arm SA12, the light source LS12 and the detection arm, and which differs only as concerns the configuration and degree of integration of the reference arm.

In FIG. 7, the integrated reference arm RA12 comprises a bi-focal common reference arm lens system BFLRA or a suitable bi-focal diffraction optical element (DOE) designed to perform in an equal manner as the bi-focal lens system BFLRA, both of which comprise a first reference arm focussing portion FPR1 and a second reference arm focussing portion FPR2 surrounding the first reference arm focussing portion FPR1. The first reference arm portion FPR1 has a focal length adapted so as to transmit and focus radiation having wavelengths in the first wavelength range (defined by λ1 and Δλ1, see FIG. 5) on a first reference arm mirror MR1 and wherein the second reference arm focussing portion FPR2 is adapted to transmit and focus radiation comprising wavelength in the second wavelength range (defined by λ2 and Δλ2, see FIG. 5) on a second reference arm mirror MR2. The first and second reference arm mirrors MR1 and MR2 are arranged at distances with respect to the common beam splitter BS12 so that the optical path length of the first reference arm RA1 generated by the first reference arm focussing portion FPR1 corresponds to the optical path length of the first beam B1 in the sample arm SA12, and the optical path length of the second reference arm RA2 produced by the second reference arm focussing portion FPR2 corresponds to the optical path length of the second beam B2 of the sample arm SA12. The bi-focal common reference arm common lens is system BFLRA can thus be configured similarly as the bi-focal common lens system BFL12 in the sample arm SA12.

In the second, third and fourth embodiments of the integrated systems OCT12′, OCT12″ and OCT12′″ shown in FIGS. 4, 6 and 7, there may be provided a bi-focal Fresnel lens system respectively instead of the bi-focal common lens system BFL12 in the sample arms SA12 and the bi-focal reference arm common lens system BFLRA in the reference arm.

Furthermore, a distortion correction for the chromatic aberration can be provided in the reference arms RA12 of these integrated systems in order to approximate the chromatic distortion of the first and second beams B1 and B2 in the first and second partial volume 17 and 19 of the object 10 and to improve the signal-to-noise ratio of the integrated system.

SYSTEM AND METHOD FOR MEASURING INTERNAL DIMENSIONS OF AN OBJECT BY OPTICAL COHERENCE TOMOGRAPHY

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