WO2016068707A2

WO2016068707A2 - Interferometer, in particular for optical coherence tomography, comprising a reference arm having optical elements in a fixed positional relationship

Info

Publication number: WO2016068707A2
Application number: PCT/NL2015/050754
Authority: WO
Inventors: Haaije Rimmer De Vries; Enrique GAMBRA
Original assignee: Cassini B.V.
Priority date: 2014-10-29
Filing date: 2015-10-29
Publication date: 2016-05-06
Also published as: NL2013701B1; WO2016068707A3

Abstract

The invention relates to an interferometer for optical tomography, comprising a sample arm configured to direct a sample beam to and receive a reflected sample beam from a sample structure, in particular an eye,and a reference arm comprising at least one reference mirror and configured to direct a reference beam to and receive a reflected reference beam from the at least one reference mirror. All optical elements of the reference arm are arranged in a fixed positional relationship with respect to each other, and the optical elements of the reference arm comprise an adjustable optical element having an adjustable focal length. The interferometer of the invention allows a fast reference plane selection in optical coherence tomography by using a fast dynamic confocal system.

Description

Title: Interferometer, in particular for optical coherence tomography, comprising a reference arm having optical elements in a fixed positional relationship

The present invention relates to an interferometer for use in optical coherence tomography. The invention also relates to a method using such interferometer.

Optical Coherence Tomography (OCT) is a high resolution (typically around 1 -10 microns) non-invasive imaging technique, based on an interferometer, for example a Michelson interferometer. In OCT a coherent light source -such as a laser or a

superluminiscent diode (SLD)- is typically used. A light beam of the light source is directed towards a beam splitter that divides it in a sample beam and a reference beam. The sample beam is directed towards the object to be imaged, which partially reflects and or scatters light back to the optical system. The reference beam is directed toward a reference mirror, which also reflects light back into the optical system. A beam coupler, which may be formed by the beam splitter, recombines the reflected sample beam and the reflected reference beam, and this combination of reflected sample beam and reflected reference beam is guided to a detector. The detector registers an interferogram if the optical path difference between the two beams is less than the coherence length of the source.

Two main groups of OCT modalities have been developed in the last two decades: Time Domain OCT (TD-OCT) and Fourier Domain OCT (FD-OCT), which is subdivided in Spectral Domain OCT (SD-OCT) and Swept Source OCT (SS-OCT). These techniques provide a 1-D image of the object along the direction of the sample beam. By inserting two optical scanners in the sample arm a 3-D image of the object can be obtained.

In TD-OCT, the reference mirror is moved along a certain depth, scanning the sample along the direction of the sample beam. From the series of interferograms recorded with a photodetector, an A-scan is obtained that provides an 1-D image of the sample. The geometric distance between the different structures can be assessed considering the movement of the reference mirror divided by the refractive index of the sample.

In SD-OCT, the reference mirror is fixed and the interferogram is registered with an optical spectrometer. After a Fourier Transform of the spectral signal, an A-scan is obtained that provides an 1-D image of the sample up to a maximum depth according to the expression: where λ is the central wavelength of the source, n is the average refractive index of the sample and δλ is the spectral resolution of the spectrometer.

SD-OCT is faster and more sensitive than TD-OCT and does not require any movement of the reference mirror to image structures between the reference plan and zmax. A major drawback of SD-OCT is the sensitivity roll-off towards zmax decreasing the visibility and hence measurement capabilities of deeper laying structures.

SS-OCT is also a Fourier Domain technique, but it uses a tunable laser source to encode the spectral information in time instead of in space. SS-OCT devices are more complex, but the measurement speed and sensitivity even at large depths is generally better compared to the other two configurations.

Complex embodiments with moving parts are commonly used to focus the light on the sample and synchronize the sample and reference arms.

Measuring long samples has different implications depending on the OCT modality, In TD-OCT, it would require the use of a longer linear stage in the reference mirror, while in FD-OCT it would for example require the implementation of a configuration of multiple reference mirrors. Alternatively, the measurement range can be increased by reducing the spectral resolution of the spectrometer, but at the cost of losing axial resolution.

US2012/0200859 A1 discloses a device and a method for establishing geometric values at least from a first region and from a second region, distanced from the first region, of a transparent or diffusive object, comprising a coherence tomograph with an object arm, a reference arm, a detector arm, and a light source for emitting light.

WO2013/078447 A1 describes an OCT imaging system capable of imaging curved samples, such as the retina, cornea, plastic tubes, and others. The OCT system comprises a sample arm and a reference arm. In an embodiment, the system of WO2013078447 A1 comprises adjusting the path length of a reference arm during an OCT scan of a curved sample or between OCT scans. The reference arm path length is adjustable using a motorized translation stage supporting a focusing lens and a mirror or by using a liquid lens with or without an additional focusing lens.

It is an object of the invention to provide a high resolution interferometer having sufficient measurement range, for example the length of an eye along an optical axis, and being capable of measuring over this measurement range in a relatively short time.

The invention provides an interferometer for optical tomography, comprising:

a sample arm configured to direct a sample beam to and receive a reflected sample beam from a sample structure, in particular an eye,

a reference arm comprising a first reference mirror and a second reference mirror and configured to direct a reference beam to and receive a reflected reference beam from one or more of the first reference mirror and the second reference mirror, characterized in that the first reference mirror and the second reference mirror are spaced at a distance such that a first measurement range associated with the first reference mirror and a second measurement range associated with the second reference mirror at least partially overlap.

By making the measurement range associated with the first reference mirror and the measurement range associated with the second reference mirror overlap, the measurement range can be increased.

The full measurement range associated with a reference mirror comprises two areas at opposite sides of the reference mirror. These two areas are also referred to as the real part and complex conjugate (or imaginary) part hereinafter referred to as the complex conjugate of the full measurement range. The detector that is used to receive a combined signal of the reference arm and the sample arm cannot determine whether a sample structure determined by said detector is arranged at the real part or complex conjugate of the reference mirror. This is also called the complex conjugate ambiguity. If this complex conjugate ambiguity is removed, the full measurement range can be used, doubling the initial measurement range depth of zmax.

By making the first measurement range associated with the first reference mirror and the second measurement range associated with the second reference mirror overlap, the sample structure may be determined by measurements with both reference mirrors. By comparison of the location of the sample structure in the first measurement range and of the location of the sample structure in the second measurement range, it can be determined whether the sample structure is present in the real part or complex conjugate of the first measurement range and/or second measurement range.

For example, an interferometer may comprise two reference mirrors, wherein the real part of a first measurement range associated with the first reference mirror and the complex conjugate of a second measurement range associated with the first reference mirror overlap. This means that the complex conjugate of the first measurement range has no overlap with the second measurement range and the real part of the second measurement range has no overlap with the first measurement range.

If in such embodiment, a sample structure is detected both in the measurement range of the first reference mirror and the measurement range of the second measurement mirror, it is evident that the sample structure is virtually located between the first and second mirrors. The detected signal appears in the real part of the first measurement range and in the complex conjugate of the second measurement range. If the sample structure is only detected in the first measurement range, the sample structure is located in the complex conjugate of the first measurement range, and, correspondingly, if the sample structure is only detected in the second measurement range, the sample structure is located in the real part of the second measurement range.

In this way the location of the sample structure can be resolved in a total

measurement range which is approximately 3 times zmax (that is, three times the

measurement range associated with a single reference mirror in which the complex ambiguity has not been removed), and at the same time the complex conjugate ambiguity has been taken into account.

In an embodiment, the interferometer is configured to determine length

characteristics of a complete eye and comprises four reference mirrors, wherein

a first reference mirror and a second reference mirror are arranged to determine a position of the retina in the eye,

a third reference mirror arranged to determine a posterior surface of the lens in the eye, and

a fourth reference mirror arranged to determine a position of the cornea and an anterior surface of the lens in the eye.

The increased measurement range of two reference mirrors can advantageously be used to have a relatively large measurement range that is suitable to determine a location of the retina of an eye. The position of the retina can vary substantially between different eyes.

A third reference mirror can be used to determine the posterior surface of the crystalline lens and a fourth reference mirror can be used to determine the cornea and anterior surface of the crystalline lens. Preferably, the third reference mirror is located such that the posterior position of the lens is always located in the real part or the complex conjugate of the third measurement range associated with the third reference mirror.

Similarly, the fourth reference mirror is positioned such that the cornea and anterior surface of the crystalline lens are always located in the complex conjugate of the fourth

measurement range associated with the fourth reference mirror.

By assuring that the relevant sample structure is always located at one side, i.e. the real part or the complex conjugate of the measurement range associated with a respective reference mirror, the complex conjugate ambiguity with respect to the location of the determined sample structure may be avoided.

The invention further provides an interferometer for optical tomography, comprising: a sample arm configured to direct a sample beam to and receive a reflected sample beam from a sample structure, in particular an eye,

a reference arm comprising at least one reference mirror and configured to direct a reference beam to and receive a reflected reference beam from the at least one reference mirror, characterized in that all optical elements of the reference arm are arranged in a fixed positional relationship with respect to each other, and

in that the optical elements of the reference arm comprise an adjustable optical element having an adjustable focal length.

In an interferometer according to the invention, all the optical elements of the reference arm are arranged in a fixed positional relationship with respect to each other, i.e. the optical elements are not intended to be moved with respect to each other during use of the interferometer. The fixed positional relationship or static configuration does not comprise translatable optical elements such as translatable reference mirrors in the reference arm and/or translatable lenses to adjust the focal length in the reference arm or sample arm, that move parallel to the direction of the optical path of a light beam in the interferometer, in particular to adjust focal lengths or light beam path lengths during use of the interferometer.

The configuration further comprises in the sample arm and the reference arm an adjustable optical element having an adjustable focal length.

By arranging an adjustable optical element having an adjustable focal length in the configuration the focal length of a light beam can be adjusted to a desirable focal length.

An example of an adjustable optical element having an adjustable focal length is an electrically tunable lens. The focal length of an electrically tunable lens can be adjusted, for example by adjusting the current that is applied to the tunable lens. The advantage of electrically tunable lenses is that the focal length of the lens can be adjusted in a short time, for example in less than 100 ms, or even less than 50 ms. As a result, the focal length can be quickly switched between different focal modes, for example between focusing on the front side of the eye, e.g. lens and/or cornea and the back side of the eye, e.g. the retina.

It is remarked that tunable lenses may change shape when switching between different focal lengths. Such shape change does however not change the fixed positional relationship between the tunable lens and another optical element of the reference arm.

It is further remarked that in this application optical elements are elements used to reflect or transmit a light beam, such as lenses, reference mirrors, beam splitters, beam couplers and such. Elements used to block a light beam, such as a shutter device, are not regarded to be an optical element.

In an embodiment, all optical elements of the sample arm are arranged in a fixed positional relationship with respect to each other, and the optical elements of the sample arm comprise an adjustable optical element having an adjustable focal length, preferably an electrically tunable lens. By arranging all optical elements of the sample arm in a fixed positional relationship, these optical elements also do not have to be moved with respect to each other during the use of the interferometer. In particular, the focal length in the reference arm and the focal length in the sample arm can be quickly adjusted such that the interferometer can be quickly adapted to measure a sample structure at a desired focal plane.

In an alternative embodiment, the sample arm comprises a scanning element configured to make a scanning movement at least partially perpendicular to a propagation direction of the sample light beam. A scanning element is an element, usually a movable element, that can be used to direct light beam in different directions to scan over a sample plane. Such scanning element can be used to create 2D or 3D images of sample structures.

In an embodiment, the interferometer comprises a control device configured to control the electrically tunable lens of the sample arm and the electrically tunable lens of the reference arm. In order to synchronise control of the electrically tunable lenses in the sample arm and the reference arm a control device may be provided which is adapted to control input signals sent to the tunable lens of the sample arm and the tunable lens of the reference arm in order to synchronize the adjustment of focal lengths in the sample arm and the reference arm.

In an embodiment, the reference arm comprises multiple reference mirrors. Multiple reference mirrors may be provided to have multiple reference beam path lengths in the reference arm corresponding to beam path lengths of locations where sample structure are to be determined, i.e. sample planes. In particular in FD-OCT, the measurement range of a static reference mirror may be limited to for example 4-10 mm in air. The length of an eye is typically larger than this distance. It is therefore desirable to obtain a total measurement range which is larger than the measurement range of a static reference mirror. By arranging multiple reference mirrors at different beam path lengths, each reference mirror may be associated with a sample plane and a measurement range. The combination of multiple reference mirrors may thus provide an increased overall measurement range or combination of measurement ranges.

For example, one or more reference mirrors may be arranged to determine a location of the retina in the eye. Further reference mirrors may be provided to determine a location of the lens and/or cornea of the eye. This information may for example be used to determine the length of an eye along an optical axis.

When multiple reference mirrors are used in combination with a single detector, the reflected light beam should only comprise light that is reflected by a selected one of the reference mirrors to avoid overlapping signals coming from different corresponding structures in the sample beam. In an embodiment, this may be realized by providing a confocal optical system such that in the reference arm only the light that is reflected by the selected reference mirror enters back into the optical system. The reference mirror may be selected by focusing the light beam on this selected mirror. Reflections from other mirrors are not coupled back, or minimally coupled back, into the optical fiber and hence do not result into unwanted peaks in the Fourier Transform of the spectrum.

Similarly, the reflected light of a sample plane in the sample arm may be optically distinguished from other reflections in the sample arm by focusing the sample beam on the selected sample plane and using a confocal system, such that only light reflected by the sample structure at or near the sample plane is reflected back into the optical system.

Such confocal system can for instance be created by providing a pinhole in the optical light path. In another embodiment, an optical fiber is used to emit the light beam to the reference mirror and receive the reflected light beam back into the optical fiber. Any light which is reflected outside the measurement range will be minimally reflected into the tip of the optical fiber, since the optical fiber end is out of focus with respect to these reflections.

In addition or as an alternative, the reference arm comprises one or more shutter devices to selectively block the reference beam towards one or more of the multiple reference mirrors. By providing shutter devices, the light beam to one or more reflective mirrors may be blocked such that only the light reflected by the selected reference mirror is reflected back towards the detector.

Also, the sample arm is preferably a confocal system, wherein only the light reflected by the sample structure is in focus. Also in the sample arm such confocal system may be realized by a pinhole or an optical fiber, or any other suitable device creating a confocal system.

In an embodiment, a first reference mirror and a second reference mirror of the multiple reference mirrors are spaced at a distance such that a first measurement range associated with the first reference mirror and a second measurement range associated with the second reference mirror at least partially overlap.

In this way the location of the sample structure can be resolved in a total

measurement range which is approximately 3 times zmax (that is, three times the

In an embodiment, the interferometer is configured to determine length

characteristics of a complete eye and comprises four reference mirrors, wherein

measurement range associated with the fourth reference mirror.

In an embodiment, the interferometer comprises:

a low coherent light source configured to provide a light beam;

a beam splitter configured to split the light beam in the sample beam for the sample arm and the reference beam for the reference arm,

a beam coupler configured to combine the reflected sample beam and the reflected reference beam, and

a light detector arranged to receive the reflected sample beam and the reflected reference beam.

The invention further provides a method to determine the length of an eye along an optical axis, comprising:

providing an interferometer according to any of the claims 1-24, and

determining at least a location of the cornea of the eye and a location of the retina of the eye along the optical axis, and

determining the length of the eye on the basis of the location of the cornea and the location of the retina.

Further characteristics and advantages of the invention will now be described, whereby reference will be made to the accompanying drawings, in which:

Figure 1 schematically depicts an embodiment of an interferometer according to the invention;

Figure 2 schematically depicts an embodiment of the confocal principle;

Figure 3 schematically depicts an alternative embodiment of a reference arm according to the invention; and

Figure 4 schematically depicts a configuration of reference mirrors and associated measurement ranges in an eye. Figure 1 shows an embodiment of an interferometer IF according to an embodiment of the invention. The interferometer IF is configured to determine at least a relative position of a cornea and retina along an optical axis OA of an eye, such that the length of the eye along this optical axis OA can be measured. The interferometer IF comprises a general part GP, a sample arm SA and a reference arm RA. The interferometer IF further comprises a positioning device to position a head of a person in such a way that the eye is substantially stabilized in a measurement position. The interferometer IF is configured to detect structures of the eye amongst others cornea, crystalline lens, retina and allows to measure the distance between these structures, when the eye is held in the measurement position.

The general part GP of the interferometer IF comprises a low coherent light source

LS, such as a laser or a superluminiscent diode (SLD) configured to provide a light beam that is emitted into an optical fiber OF. The optical fiber OF runs to a beam splitter/coupler BS that splits the light beam into a sample beam and a reference beam.

The sample beam is guided into the sample arm SA by an optical fiber SOF, exits the optical fiber SOF at the optical fiber tip OFT is being collimated by a fixed lens in front of the optical fiber tip OFT (not shown in the figure) and directed towards an optical module composed of at least an electrically tunable lens TLS having an adjustable focal length. The sample beam is emitted from the reference arm RA towards a sample structure, in particular eye E where it may be at least partially reflected by the structure of the eye E, for example by the cornea, lens or retina of the eye. The eye E is placed in a measurement position.

Dependent on the selected focal length of the tunable lens TLS, the sample beam will be focused on a first sample plane P1 or a second sample plane P2. This selected focal length of the tunable lens TLS is controlled by a controller CON which sends a control signal to the tunable lens TLS which sets the focal length of the tunable lens TLS. The first sample plane P1 is selected to detect the location of the cornea of an eye on the optical axis located in a measurement position defined by the interferometer (as shown in Figure 1). The second sample plane P2 is selected to detect the location of the retina of an eye on the optical axis when the eye is positioned in the measurement position.

When the sample beam is focused on the first sample plane P1 at least a part of the reflected sample beam on the sample structure at or near the sample plane P1 will be received by the optical fiber tip OFT of the optical fiber SOF and will run back through the optical fiber SOF towards the beams splitter/coupler BS.

Light of the sample beam which is reflected by the sample structure at or near the second sample plane P2 will mainly not be reflected into the optical fiber tip OFT since the sample beam is not focused on the second sample plane P2. Thus, the optical fiber tip OFT functions as a pinhole of a confocal optical system which substantially only receives light back which is reflected by the sample structure at or near the sample plane on which the sample beam is focused.

It will be clear that when the focal length of the tunable lens TLS is adjusted by the control device CON such that the sample beam emitted by the optical fiber SOF is focused on the second sample plane P2, the light reflected by the sample structure at or near the second sample plane P2 will be reflected back into the optical fiber tip OFT, and the light reflected at or near the first sample plane P1 will mainly not be reflected into the optical fiber tip OFT and therefore not be received back by the optical fiber SOF.

The above approach allows for a fast and elegant way to switch between two focal planes in the eye and to maximize the back-scattered signal from both ocular structures. It is remarked that fixed configurations dividing the light beam for simultaneous focusing in two sample planes, for example by using a lens with a hole, are less suitable as this dividing reduces the energy of the light beam focusing on a particular ocular structure (P1 or P2), therewith also reducing the sensitivity of the device. At the same time, optical safety limitations do not allow to compensate the energy loss by increasing the intensity of the emitted beam (SOF). Moreover, since there is a great variability in the refractive error of the human eye, there is a strong demand on the dynamic range of a sample arm focusing system.

Furthermore, focusing the sample beam on a sample plane not only maximizes the signal from the sample structure, but also allows to select the sample plane that is been imaged. Making use of the confocal principle as depicted in Figure 2, the sample plane that wants to be imaged can be conjugated to a confocal pinhole that blocks the light coming from other sample planes.

It is remarked that in an alternative embodiment of the confocal optical system, any other device functioning as a pinhole may also be used to distinguish between the first sample plane P1 and the second sample plane P2.

Now again referring to Figure 1 , the reference beam is guided into the reference arm RA by an optical fiber ROF and it leaves the optical fiber ROF at an optical fiber tip ROFT. From there the reference beam propagates towards another optical module composed of at least one electrically tunable lens TLR having an adjustable focal length.

From the tunable lens TLR the reference beam propagates to a reference beam splitter RBS, which splits the reference beam in a first reference beam part which propagates towards a first reference mirror M1 and a second reference beam part which propagates towards a second reference mirror M2. The first reference beam part and the second reference beam part are reflected on the first reference mirror M1 and the second reference mirror M2, respectively, back to the reference beam splitter RBS.

The control device CON is configured to control the focal length of the tunable lens TLR in such a way that the reference beam is selectively focused on the first reference mirror M1 or the second reference mirror M2. Similarly to the sample arm SA the optical fiber tip ROFT of the optical fiber ROF of the reference arm RA functions as a pin hole of a confocal system. This means that when the reference beam is focused on the first reference mirror M1 , the optical fiber tip ROFT of the optical fiber ROF will substantially only receive reflected light which is reflected by the first reference mirror M1 and when the reference beam is focused on the second reference mirror M2, the optical fiber tip ROFT of the optical fiber ROF will substantially only receive reflected light which is reflected by the second reference mirror M2.

The reflected sample beam and the reflected reference beam are combined in the beam splitter/coupler BS and guided by a detector optical fiber DOF to a light detector DET, in particular a spectrometer.

The control device CON is configured to control the focal length of the tunable lenses TLS and TLR in such a way that when the sample beam is focused by the tunable lens TLS on the first sample plane P1 , the tunable lens TLR focuses the reference beam on the first reference mirror M1. Similarly, when the sample beam is focused by the tunable lens TLS on the second sample plane P2, the tunable lens TLR focuses the reference beam on the second reference mirror M2. Further, the light path length difference from the beam splitter/coupler BS to the first sample plane P1 and from the beam splitter/coupler BS to the first reference mirror M1 is within the measurement range. Correspondingly, the light path length difference from the beam splitter/coupler BS to the second sample plane P2 and from the beam splitter/coupler BS to the second reference mirror M2 is within the measurement range.

As a result, the combination of the reflected sample beam and the reflected reference beam as received by the detector can be used to determine an interferogram. After a Fourier transform of the spectral signal of this interferogram, an image of the sample structure can be created.

The advantage of the interferometer of Figure 1 , is that to switch between a first measurement in the first sample plane P1 and a second measurement in the second sample plane P2, only the focal length of the tunable lens TLS in the sample arm has to be changed from focusing on the first sample plane P1 to focusing on the second sample plane P2 and the focal length of the tunable lens TLR in the reference arm has to be changed from focusing on the first reference mirror M1 to focusing on the second reference mirror M2. Such change in focal length of the tunable lenses TLS and TLR can be realized within a short time interval of less than 100 ms.

Furthermore, the optical elements of the sample arm SA and the reference arm RA are arranged in a fixed positional relationship. This means that the sample arm SA and the reference arm RA do not comprise any optical elements that have to be moved, in particular translated parallel to the light beam, such as translatable reference mirrors or focus elements, during use of the interferometer IF, in order to obtain measurements at the opposite ends of the eye, i.e. at the cornea and the retina. The presence of translatable optical elements not only makes switching between the different measurement locations relatively slow, but also makes the interferometer more vulnerable for malfunctioning.

Figure 3 shows an alternative embodiment of a reference arm of an interferometer IF according to the invention. The reference arm RA comprises an optical fiber ROF, a tunable lens TLR, and a reference beam splitter RBS. The reference arm RA further comprises four reference mirrors M1 , M2, M3, and M4. The other parts of the interferometer IF may be configured the same as the interferometer IF of Figure 1.

Similar to the embodiment of Figure 1 , the reference beam is guided into the reference arm RA by an optical fiber ROF. The reference beam leaves the optical fiber ROF at the optical fiber tip ROFT. From there the reference beam propagates towards the electrically tunable lens TLR having an adjustable focal length and the reference beam splitter RBS. The reference beam splitter RBS splits the reference beam in a first reference beam part and a second reference beam part.

The first reference beam part propagates into a first sub reference arm comprising a first shutter device SD1 , a first reference mirror M 1 and a third reference mirror M3. The second reference beam propagates into a second sub reference arm comprising a second shutter device SD2, a second reference mirror M2 and a fourth reference mirror M4.

The third reference mirror M3 and the fourth reference mirror M4 are formed by plate beam splitters that are arranged perpendicular to first reference beam part and second reference beam part, respectively. The plate beam splitters partially reflect and partially transmit the respective reference beam part such that light of the reference beam part is reflected by both the first/second reference mirror M1 , M2 and the third/fourth reference mirror M3, M4.

The four reference mirrors M 1-M4 are arranged in a position relevant for

measurement of sample structure of an eye E positioned in a measurement position of the interferometer IF. With each reference mirror M 1-M4 a corresponding position, i.e. a position at the same beam length distance from the beam splitter/coupler BS, of a respective sample plane is defined. Figure 4 shows schematically the positions of the reference mirrors M1-M4 and associated sample planes P1 , P2, P3 and P4. For each combination of reference mirror and sample plane an associated measurement range is indicated. Each measurement range comprises two sides at opposite sides of the respective sample plane. These opposite sides are indicated herein as the real part and the complex conjugate.

Thus, associated with the first reference mirror Ml/first sample plane P1 is a measurement range MR1 having a real part MR1 r and a complex conjugate MR1 cc;

associated with the second reference mirror M2/second sample plane P2 is a second measurement range MR2 having a real part MR2r and a complex conjugate MR2cc;

associated with third reference mirror M3/third sample plane P3 is a third measurement range MR3 having a real part MR3r and a complex conjugate MR3cc; and associated with the fourth reference mirror M4/fourth sample plane P4 is a fourth measurement range MR4 having a real part MR4r and a complex conjugate MR4cc.

Since each of the four measurement ranges comprises a complex conjugate and a real part, the measurements are affected by the "so called" complex conjugate ambiguity. This means that from the interferogram signal that is provided by the detector DET, it cannot be determined if a detected sample structure is placed after or before the respective sample plane, i.e. at the real part or the complex conjugate of the measurement range.

To avoid that the detector signal is misunderstood, the sample plane can be arranged in a position that only one side of the real part and complex conjugate can be used for measurement, such that it is assured that no sample structure is going to appear in the other one.

For example, the fourth sample plane P4 is arranged at a location in front of the cornea so that the cornea can only be detected at the complex conjugate MR4cc of the fourth measurement range. Since the real part MR4r of the fourth measurement range is not used, it is shown in dashed lines in Figure 4.

Similarly, the third sample plane P3 which is arranged to determine the location of the back lens is positioned at a back side of the lens such that it is assured that the sample structure of the lens will only appear in the real part MR3r of the third measurement range. The complex conjugate MR3cc of the third measurement range is typically arranged in the interior of the eye E where typically no sample structure to be imaged will be found. Since the complex conjugate MR3cc of the third measurement range is not used for

measurements, it is also shown in dashed lines.

Another way to deal with the complex conjugate ambiguity is shown with respect to the first sample plane P1 and the second sample plane P2. The positions of the first sample plane P1 and the second sample plane P2 are selected such that the real part MR1 r of the first measurement range and the complex conjugate MR2cc of the second measurement range overlap.

By combining the results of the measurements using the first sample plane P1 and the second sample plane P2, it can be determined in which sub-measurement range of the first and second measurement range a sample structure is detected.

When a sample structure is only detected in a measurement associated with the first sample plane P1 , but not in a measurement associated with the second sample plane P2, it can be concluded that the detected sample structure is located at the complex conjugate MR1 cc of the first measurement range. Similarly, when a sample structure is only detected in a measurement associated with the second sample plane P2, but not in a measurement associated with the first sample plane P1 , it can be concluded that the detected sample structure is located at the real part MR2r of the second measurement range.

Only when the sample structure is detected in both the measurement associated with the first sample plane P1 and the measurement with the second sample plane P2, the detected sample structure is located in the overlapping part of the first measurement range and the second measurement range, i.e. in the real part MRI r of the first measurement range and in the complex conjugate MR2cc of the second measurement range.

An advantage of this arrangement of the first sample plane P1 and the second sample plane P2 is that the measurement range of the interferometer is enlarged to three times the measurement range (zmax) that can be used with a single sample plane, assuming that only one side of the measurement range can be used. The first sample plane P1 and the second sample plane P2 are positioned at locations to cover the whole range of possible positions of the retina of the eye. Since the arrangement of the first sample plane P1 and the second sample plane P2 as shown in Figure 4 allow a substantially larger measurement range, this arrangement is in particular suitable to determine a retina location of the eye arranged in the measurement position, as the location of the retina may substantially vary in different eyes.

The interferometer IF of which the reference arm is shown in Figure 3, is configured to be quickly switched between different modes in which sample structure of different parts in the eye E can be determined.

The control device CON is configured to control the focal length of the tunable lenses TLR and TLS in such a way that the reference beam is focused on a selected one of the reference mirror M1-M4 and simultaneously the sample beam is focused on an associated one of the sample planes P1-P4. Thus, when the reference beam is focused on the first reference mirror M 1 , the sample beam is focused on the first sample plane P1 , when the reference beam is focused on the second reference mirror M2, the sample beam is focused on the second sample plane P2, etc. Since the optical elements of the sample arm SA and the reference arm RA are arranged in a fixed positional relationship, and tunable lenses TLS, TLR are used to change the focal length in the sample arm SA and reference arm RA, the interferometer IF can relatively quickly switch between the different focal modes, for example in less than 100 ms.

Further, the optical fiber tips OFT, ROFT of the optical fibers SOF, ROF will substantially only receive reflected light which is reflected by the respective sample plane/reference mirror on which the sample beam/reference beam is focused.

However, since the first sample plane P1 and the second sample plane P2 and correspondingly, the first reference mirror M 1 and the second mirror M2 are optically close together, the optical fiber tip OFT, ROFT of the respective optical fiber, may not be able to distinguish between light reflected by the first reference mirror M 1 and light reflected from the second reference mirror M2.

To avoid that light reflected by the first reference mirror M 1 and the second mirror M2 will be commingled, the first shutter device SD1 and the second shutter device SD2 are arranged in the embodiment shown in Figure 3.

The first shutter device SD1 is configured to selectively block or let pass the first reference beam part towards the first reference mirror M1 and the third reference mirror M3. The second shutter device SD2 is configured to selectively block or let pass the second reference beam part towards the second reference mirror M2 and the fourth reference mirror M4. The first shutter device SD1 and the second shutter device SD2 are mechanically movable elements that can be moved between a pass position allowing the respective reference beam part to pass the shutter device, and a blocking position, in which the shutter device blocks the light beam. In Figure 3, the first shutter device SD1 is schematically shown in the pass position, and the second shutter device SD2 is schematically shown in the block position. Arrows indicate movement towards the other position. In alternative embodiments, the shutter device may be any suitable mechanical or other element configured to block a light beam.

Since, in the position of Figure 3 the reference beam part towards the second reference mirror M2 is blocked by the second shutter SD2, no light can be reflected by the second reference mirror M2 and commingle with light reflected from the first reference mirror M1 during a measurement associated with the first reference mirror M1. Similarly, light towards the first reference mirror M1 can be blocked by the first shutter device SD1 when a measurement is carried out with respect to the measurement range associated with the second reference mirror M2.

Finally, it is remarked that OCT is a useful technique to image structures and quantify their geometry with for example a micrometer resolution. Thus, if a multi-reference arm is implemented, the distances between the different reference mirrors must be known with a comparable accuracy. Since these distances can vary due to handling, environmental conditions, etc., a calibration procedure is needed before taking a measurement.

In the embodiment shown in Figure 3, the optical distance between the first reference mirror M1 and the second reference mirror M2 can be easily assessed. By opening both shutter devices SD1 and SD2 and blocking the sample beam, for example by a shutter device, the beams reflected in the first reference mirror and the second reference mirror interfere and a peak appears in the results of the measurement at the end of the

measurement range. This peak can be used to determine the optical distance between the first reference mirror M 1 and the second reference mirror M2.

Claims

1. Interferometer for optical tomography, comprising:

a reference arm comprising a first reference mirror and a second reference mirror and configured to direct a reference beam to and receive a reflected reference beam from one or more of the first reference mirror and the second reference mirror,

characterized in that the first reference mirror and the second reference mirror are spaced at a distance such that a first measurement range associated with the first reference mirror and a second measurement range associated with the second reference mirror at least partially overlap.

2. Interferometer as claimed in claim 1 , wherein the first measurement range has a real part and a complex conjugate and wherein the second measurement range has a real part and a complex conjugate, and wherein the real part of the first measurement range overlaps with the complex conjugate of the second measurement range.

3. Interferometer as claimed in claim 1 or 2, wherein the interferometer is configured to determine length characteristics of a complete eye and comprises four reference mirrors, wherein:

a first reference mirror and a second reference mirror are arranged to determine a position of a retina of the eye,

a third reference mirror is arranged to determine a posterior surface of the lens in the eye, and

a fourth reference mirror is arranged to determine a position of the cornea and an anterior surface of the lens in the eye.

4. Interferometer as claimed in any of the preceding claims, wherein the optical elements of the reference arm provide a confocal optical system, and wherein the reference arm comprises an adjustable optical element having an adjustable focal length, such as an electrically tunable lens, such that a light beam can be selectively focused on the first reference mirror or the second reference mirror.

5. Interferometer as claimed in any of the preceding claims, wherein all optical elements of the reference arm are arranged in a fixed positional relationship with respect to each other, and wherein the optical elements of the reference arm comprise an adjustable optical element having an adjustable focal length, such as an electrically tunable lens.

6. Interferometer as claimed in any of the preceding claims, wherein all optical elements of the sample arm are arranged in a fixed positional relationship with respect to each other, and wherein the optical elements of the sample arm comprise an adjustable optical element having an adjustable focal length, such as an electrically tunable lens.

7. Interferometer as claimed in any of the claims 1-5, wherein the sample arm comprises a scanning element configured to make a scanning movement at least partially perpendicular to the beam direction.

8. Interferometer as claimed in any of the claims 4-7, wherein the focal length of the adjustable optical element can be adjusted from a first focal length to a second focal length within less than 100 ms.

9. Interferometer as claimed in any of the preceding claims, wherein the reference arm comprises one or more shutter devices to selectively block the reference beam towards one or more of the multiple reference mirrors.

10. Interferometer for optical tomography, comprising:

a reference arm comprising at least one reference mirror and configured to direct a reference beam to and receive a reflected reference beam from the at least one reference mirror,

characterized in that all optical elements of the reference arm are arranged in a fixed positional relationship with respect to each other, and

1 1. Interferometer as claimed in claim 10, wherein all optical elements of the sample arm are arranged in a fixed positional relationship with respect to each other, and wherein the optical elements of the sample arm comprise an adjustable optical element having an adjustable focal length.

12. Interferometer as claimed in claim 10, wherein the sample arm comprises a scanning element configured to make a scanning movement at least partially perpendicular to the beam direction.

13. Interferometer as claimed in any of the claims 10-12, wherein the adjustable optical element of the reference arm is an electrically tunable lens.

14. Interferometer as claimed in claims 1 1 and 13, wherein the adjustable optical element of the sample arm is an electrically tunable lens.

15. Interferometer as claimed in any of the claims 10-14, wherein the interferometer comprises a control device configured to control the adjustable optical element of the sample arm and the adjustable optical element of the reference arm.

16. Interferometer as claimed in any of the claims 10-15, wherein the reference arm comprises multiple reference mirrors.

17. Interferometer as claimed in in any of the claims 10-16, wherein the reference arm comprises one or more shutter devices to selectively block the reference beam towards one or more of the multiple reference mirrors.

18. Interferometer as claimed in claim 16 or 17, wherein a first reference mirror and a second reference mirror of the multiple reference mirrors are spaced at a distance such that a first measurement range associated with the first reference mirror and a second

measurement range associated with the second reference mirror at least partially overlap.

19. Interferometer as claimed in the preceding claim, wherein the first measurement range has a real part and a complex conjugate and wherein the second measurement range has a real part and a complex conjugate, and wherein the real part of the first measurement range overlaps with the complex conjugate of the second measurement range.

20. Interferometer as claimed in any of the claims 10-19, wherein the interferometer is configured to determine length characteristics of a complete eye and comprises four reference mirrors, wherein

a first reference mirror and a second reference mirror are arranged to determine a position of a retina of the eye, a third reference mirror is arranged to determine a posterior surface of the lens in the eye, and

21. Interferometer as claimed in any of the claims 10-20, wherein the focal length of the adjustable optical element can be adjusted from a first focal length to a second focal length within less than 100 ms.

22. Interferometer as claimed in any of the preceding claims, wherein the interferometer comprises:

a low coherent light source configured to provide a light beam;

23. Interferometer as claimed in any of the preceding claims, wherein the optical elements provide a confocal optical system.

24. Interferometer as claimed in any of the preceding claims, wherein at least a part of the sample arm and the reference arm is formed by an optical fiber.

25. A method to determine the length of an eye along an optical axis, comprising:

providing an interferometer according to any of the preceding claims, and