EP4698849A1

EP4698849A1 - Apparatus and method for polarisation-sensitive optical coherence tomography

Info

Publication number: EP4698849A1
Application number: EP24791596.0A
Authority: EP
Inventors: Grant Andrew FRISKEN; Qingyun LI; Andrew Geoffrey Watterson; Trevor Bruce Anderson; Steven James Frisken
Original assignee: Alcon Inc
Current assignee: Alcon Inc
Priority date: 2023-04-21
Filing date: 2024-04-19
Publication date: 2026-02-25
Also published as: WO2024216344A1; AU2024258104A1; JP2026513053A; CN121241240A

Abstract

Apparatus and methods are presented for performing polarisation-sensitive optical coherence tomography measurements of a sample utilising polarisation-diverse illumination. A plurality of sets of one of more measurements of a volume of a sample, such as the anterior segment or retina of a human eye, are made with the sample volume illuminated with multi-wavelength light of different polarisation states. The plurality of sets of one or more measurements are processed to generate a plurality of tomographic volume images of the sample, each of the images being of the sample illuminated with light of a different polarisation state. The plurality of tomographic volume images may for example be processed to generate a three-dimensional representation of a polarisation property of the sample, or a polarisation-independent image of the sample. In certain embodiments the polarisation of the illumination is controlled using one or more rotatable wave plates, such as a rotatable quarter wave plate.

Description

APPARATUS AND METHOD FOR POLARISATION-SENSITIVE OPTICAL

COHERENCE TOMOGRAPHY

Field of the Invention

The invention relates to apparatus and methods for polarisation-sensitive optical coherence tomography, in particular for 3-D imaging of structures in the anterior segment or retina of the human eye. However it will be appreciated that the invention is not limited to this particular field of use.

Related Applications

The present application claims priority from Australian Provisional Patent Application No 2023901193 filed on 21 April 2023, the contents of which are incorporated herein by reference.

Background of the Invention

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

Optical coherence tomography (OCT) is a widely used interferometric technique for studying biological samples including in-vivo tissue such as the human eye, with lateral and depth resolution, using information contained in the amplitude and phase of light reflected or scattered from the sample. Most current OCT systems utilise spectral domain techniques where the depth information is encoded in the spectral response of the interference signal, which can be recorded as a time-varying function of wavelength (swept source OCT) or by dispersing the interference signal and detecting the different wavelengths simultaneously along a detector array (spectrometer-based OCT).

Polarisation-sensitive OCT (PS-OCT) is an extension of conventional OCT that measures the polarisation state of light that has interacted with a sample, providing an additional contrast channel such as phase retardation caused by birefringence in the sample, compared with conventional OCT, i.e. intensity-based OCT. PS-OCT has been shown to be of value in several biological applications because certain types of tissue, such as collagen, have a degree of birefringence. Several studies of PS-OCT being applied to in-vivo imaging of the anterior or posterior segments of the human eye, dermatology or tumour detection have been reviewed in De Boer et al ‘Polarization sensitive optical coherence tomography - a review’, Biomedical Optics Express 8(3), 1838-1873 (2017). It is also known, as reported for example by Yasuno et al ‘Visibility of trabecular meshwork by standard and polarization-sensitive optical coherence tomography’, Journal of Biomedical Optics 15(6), 061705 (2010), that the birefringence of certain tissues can cause artefacts in conventional OCT. This is because OCT signal intensity can be affected by a relative difference between the polarisation states of the sample and reference beams.

Many reported PS-OCT apparatus utilise polarisation-diverse detection schemes where the interference signal is split with a polarisation beam splitter into orthogonal polarisation states that are detected in parallel. In some systems the orthogonal polarisation states are detected with separate detectors, as disclosed for example in Gbtzinger et al ‘High speed spectral domain polarization sensitive optical coherence tomography of the human retina’, Optics Express 13(25), 10217-10229 (2005) and Bonesi et al ‘High-speed polarization sensitive optical coherence tomography scan engine based on Fourier domain mode locked laser’, Biomedical Optics Express 3(11), 2987-3000 (2012). In other systems the orthogonal polarisation states are detected at different portions of a single detector. For example Baumann et al ‘ Single camera based spectral domain polarization sensitive optical coherence tomography’, Optics Express 15(3), 1054-1063 (2007) discloses a scanning beam apparatus where the spectra from the orthogonally polarised channels are imaged onto adjoining portions of a line camera, while published US patent application No 2016/0345820 Al entitled ‘High resolution 3-D spectral domain optical imaging apparatus and method’ discloses an OCT apparatus having a multi-beam spectrometer with a polarisation walk-off element for projecting pairs of wavelength-dispersed lines onto separate sets of pixels of a 2-D detector array.

In other PS-OCT schemes orthogonal polarisation channels are detected sequentially with a single detector. For example Park et al ‘ Single-camera polarization-sensitive full-field optical coherence tomography with polarization switch’, Journal of Biomedical Optics 18(10), 100504 (2013) discloses a PS-OCT apparatus with a bi-stable polarisation switching device in the detector arm.

In general, existing PS-OCT apparatus require additional optical components compared to conventional intensity-based OCT apparatus, adding cost and complexity. It would be desirable to be able to add polarisation-sensitive functionality to an intensity -based OCT apparatus without the inclusion of additional optical components. Unless the context clearly requires otherwise, throughout the description and the claims the words ‘comprising’, ‘comprises’ and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense. That is, they are to be construed in the sense of ‘including, but not limited to’. Similarly, unless the context clearly requires otherwise, the word ‘or’ is to be construed in an inclusive sense rather than an exhaustive sense. That is, the expression ‘A or B’ is to be construed as meaning ‘A, or B, or both A and B’.

Object of the Invention

It is an object of the present invention to overcome or ameliorate at least one of the limitations of the prior art, or to provide a useful alternative. It is an object of the present invention in a preferred form to provide an OCT apparatus that can be made polarisation-sensitive without the inclusion of additional optical components.

Summary of the Invention

According to a first aspect of the present invention there is provided a polarisation-sensitive optical coherence tomography apparatus comprising:

(i) an illumination system including a multi -wavelength optical source and an adjustable polarisation retardation system for illuminating a volume of a sample with light of a polarisation state controllable by the adjustable polarisation retardation system;

(ii) a polarisation beam splitter for splitting light from the optical source into a sample beam and a reference beam, and for recombining the sample and reference beams after the sample beam has interacted with the sample;

(iii) a measurement system for making a plurality of sets of one or more simultaneous measurements over a range of wavelengths, each set of one or more simultaneous measurements over a range of wavelengths being of a signal of light reflected or scattered from the sample illuminated with light of a different polarisation state; and

(iv) a computer for processing the plurality of sets of one or more simultaneous measurements to generate a plurality of tomographic volume images of the sample, each of the tomographic volume images being of the sample illuminated with light of a different polarisation state.

The illumination system is preferably configured to illuminate the volume of the sample with an array of beamlets, wherein the illumination system comprises a scanning element for translating the array of beamlets across the sample between simultaneous measurements within a set of two or more simultaneous measurements, so as to increase the density of the tomographic volume images.

In certain embodiments the computer is configured to process the plurality of tomographic volume images to generate a three-dimensional representation of a polarisation property of the sample, or to generate a polarisation-independent image of the sample.

In preferred embodiments the adjustable polarisation retardation system comprises a rotatable quarter wave plate. In certain embodiments the adjustable polarisation retardation system comprises two rotatable quarter wave plates, or a rotatable quarter wave plate and a rotatable half wave plate.

The adjustable polarisation retardation system preferably comprises one or more wave plates each composed of a birefringent material.

In certain embodiments the adjustable polarisation retardation system comprises a liquid crystal variable retarder.

Preferably, the apparatus is configured for in-vivo polarisation-sensitive optical coherence tomography of a sample comprising the anterior segment or retina of an eye.

According to a second aspect of the present invention there is provided a method for performing polarisation-sensitive optical coherence tomography measurements of a sample, the method comprising the steps of:

(a) illuminating a volume of a sample with multi -wavelength light of a polarisation state controllable by an adjustable polarisation retardation system;

(b) making a plurality of sets of one or more simultaneous measurements over a range of wavelengths, each set of one or more simultaneous measurements over a range of wavelengths being of light reflected or scattered from the sample illuminated with light of a different polarisation state; and

(c) processing the plurality of sets of one or more simultaneous measurements to generate a plurality of tomographic volume images of the sample, each of the tomographic volume images being of the sample illuminated with light of a different polarisation state.

The volume of the sample is preferably illuminated with an array of beamlets, wherein the array of beamlets is translated across the sample between simultaneous measurements within a set of two or more simultaneous measurements, so as to increase the density of the tomographic volume images. Preferably, the number of simultaneous measurements within a set of two or more simultaneous measurements is sufficient to generate a dense tomographic volume image.

In certain embodiments the plurality of tomographic volume images are processed to generate a three-dimensional representation of a polarisation property of the sample, or to generate a polarisation-independent image of the sample.

Preferably, the method is applied to a sample comprising the anterior segment or retina of an eye.

According to a third aspect of the present invention there is provided an article of manufacture comprising a non-transitory computer usable medium having a computer readable program code configured to operate the apparatus according to the first aspect, or the implement the method according to the second aspect.

Brief Description of the Drawings

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates in schematic plan view an OCT apparatus configured for in-vivo snapshot imaging of the anterior segment of a human eye;

Figure 2A illustrates in schematic plan view a polarisation-sensitive OCT apparatus according to an embodiment of the present invention, with an adjustable polarisation retardation system in the sample arm;

Figures 2B, 2C, 2D and 2E illustrate in schematic plan view adjustable polarisation retardation systems according to various embodiments of the present invention; Figures 3 A and 3B respectively show an en face image and a horizontal volume slice of the anterior segment of a sample eye extracted from a tomographic volume image acquired using the apparatus of Figure 2A in a standard configuration;

Figures 3C, 3D, 3E, 3F and 3G show the signal strength at the posterior corneal surface of a sample eye extracted from tomographic volume images acquired using the apparatus of Figure 2A with a rotatable quarter wave plate at -22.5°, 22.5°, 45°, 67.5° and 112.5° respectively; Figures 4A and 4B respectively show a polarisation-independent signal from the posterior corneal surface and a polarisation-independent volume slice of the anterior segment of a sample eye;

Figures 5A and 5B respectively show an image of cumulative phase retardation and variation in birefringence axis across the posterior corneal surface of a sample eye;

Figure 5C shows a volume slice of cumulative phase retardation in the anterior segment of a sample eye;

Figures 6A and 6B respectively show close-ups of the anterior angle region from the volume slices of Figures 3B and 4B;

Figures 7A and 7B respectively show a volume slice acquired with a rotatable quarter wave plate at 45° and degree of polarisation uniformity (DOPU) of a portion of the retina of a sample eye; and

Figure 8 illustrates in schematic plan view a polarisation-sensitive OCT apparatus according to an embodiment of the present invention, with an adjustable polarisation retardation system in the sample arm.

Detailed Description of the Invention

Figure 1 shows in schematic plan view a spectral domain optical coherence tomography (OCT) apparatus 100, similar to apparatus described in published US patent application No 2019/0365220 Al entitled ‘Optical coherence metrology and tomography with improved registration’, the contents of which are incorporated herein by reference. The apparatus 100 depicted in Figure 1 is configured for in-vivo snapshot imaging of the anterior segment 102 of a human eye 104 at a plurality of points, with the snapshot nature of the acquisition ameliorating the effect of patient movement. The anterior segment 102 is generally understood to include all parts of the eye in front of the vitreous humour 106, including the cornea 108, iris 110 and lens 112. We will firstly describe the operation of this OCT apparatus 100, before explaining how it can be modified in surprisingly simple fashion to provide polarisation-sensitive functionality. Light 114 from a multi -wavelength optical source 116 in the form of a broadband source such as a superluminescent diode with centre wavelength 840 nm and bandwidth 40 nm is collimated by a collimating element 118 such as a lens or a parabolic mirror, linearly polarised by a polariser 120 then split by a polarisation beam splitter (PBS) 122 into reference and sample beams 124, 126. The polariser 120 may be omitted if the optical source 116 emits suitably polarised light. The reference arm 128 includes a mirror 130 and a quarter wave plate 132 for polarisation transformation of the reference beam 124 so that light reflected from the reference mirror 130 passes through the PBS 122 and into the detection arm 134. The reference arm 128 may also include relay elements as well as dispersion-matching components for compensating for the dispersion of optics in the sample arm 136, or of the sample 104. The mirror 130 may be moved axially to adjust the path length of the reference arm 128 relative to the sample arm 136, e.g. to match different eye positions or to obtain information from structures at different depths in the eye 104, such as the retina 138. Alternatively or additionally, the entire apparatus 100 may be moved axially with respect to the eye 104 to adjust the eye-to-apparatus distance and therefore the path length of the sample arm 136. In another embodiment, described in US 2019/0365220 Al, the reference mirror 130 is replaced by or is interchangeable with a compound reflector having two axially separated reflective surfaces. This modification extends the imaging depth range of the apparatus 100, facilitating for example simultaneous depth-resolved imaging of the anterior segment 102 and retina 138.

The sample arm 136 comprises a quarter wave plate 140, a spatial sampling element 144 in the form of a two-dimensional (2-D) lenslet array to generate from the sample beam 126 a 2-D array of sample beamlets 146 which are relayed to the anterior segment 102 via a 4F lens system 148, thereby illuminating a volume of the anterior segment at a grid of points. For simplicity of illustration the lenses in the relay lens system 148 are depicted as simple single element lenses, although in preferred embodiments they have multiple elements to reduce distortions of the relayed beamlets over the wavelength range of the light 114. In general the number of beamlets 146 depends on the design of the 2-D lenslet array 144. In certain embodiments there may be of order 100 or 1000 beamlets in a square or rectangular pattern with a density of, say, 4 to 100 beamlets per square millimetre at the nominal focal surface 150, i.e. at the waists of the beamlets 146, which may be positioned within the anterior segment 102 as shown. In one particular embodiment the 2-D lenslet array 144 is designed to generate 1008 beamlets 146 on a 24x42 grid, with beamlet waists of approximately 30 pm. Generally, the lateral extent of the illuminated volume will be determined by the on-eye size of the area encompassed by the array of beamlets 146, which may for example be 4 mm x 8 mm, while the lateral resolution will be determined by the diameter of the beamlet waists.

Light 152 from the grid of beamlets 146 scattered or reflected from one or more structures in the anterior segment 102, such as the anterior or posterior surfaces of the cornea 108, passes back through the relay lens system 148, then is re-focused by the lenslet array 144 and directed by the PBS 122 away from the optical source 116 into the detection arm 134 following polarisation transformation at the quarter wave plate 140. The reference beam 124 is combined with the scattered or reflected light 152 at the PBS 122 and the resulting combined beamlets 154 analysed by a polariser 156 to interfere the light from the sample and reference arms 136, 128. The resultant interference pattern is relayed by a system of lenses 158, and an optional aperture 160 to remove stray light, for spectral analysis in a measurement system comprising a spectrometer 162 at a grid of spatial positions determined by a spatial sampling element 164 in the form of a 2-D lenslet array, and a corresponding 2-D aperture array 166. Generally, the lenslet arrays 144, 164 will be aligned such that the combined beamlets 154 are directed into the spectrometer 162.

The spectrometer 162 is able to analyse a plurality of grid points, beams or beamlets simultaneously, or at least within a single frame of a 2-D sensor array 168, for snapshot acquisition. Each snapshot acquisition is a simultaneous measurement over a range of wavelengths of light reflected or scattered from the volume of the eye 104 illuminated by the array of beamlets 146, with the range of wavelengths generally corresponding to the bandwidth of the broadband optical source 116. After entering the spectrometer 162 the interfered beamlets are redirected by a PBS 170 to a lens 172 that collimates the beamlets for dispersion by a wavelength dispersive element 174 in the form of a transmissive grating, followed by double passage through a quarter wave plate 176 via reflection from a mirror 178 to rotate the polarisation state by 90°. The dispersed spectral components are imaged by the lens 172 onto a 2-D sensor array 168 such as a CMOS camera after passing through the PBS 170. The interferogram detected by the 2-D sensor array 168 is read out in a single frame for subsequent analysis by a computer 180 equipped with suitable computer readable program code. In preferred embodiments the grating 174 is oriented with respect to the grid of spatial positions determined by the 2-D lenslet array 164 and the corresponding 2-D aperture array 166 such that each of the combined beamlets 154 entering the spectrometer 162 is dispersed onto a separate set of pixels of the 2-D sensor array 168, as described in published US patent application No 2016/0345820 Al. The computer 180 may for example apply well-known Fourier transform techniques to obtain a sparse tomographic image of the illuminated volume of the anterior segment 102 of the eye 104, containing depth-resolved information from those points in the eye illuminated by the beamlets 146.

To obtain a more complete, i.e. denser tomographic image of the illuminated volume, one or more further snapshot acquisitions, i.e. simultaneous measurements over a range of wavelengths, may be made with the array of beamlets 146 translated incrementally across the eye 104 in one or two lateral dimensions to ‘fill in’ the on-eye gaps between the beamlets. The resulting set of snapshot acquisitions can then be processed by the computer 180 to yield a denser tomographic image of the illuminated volume. In preferred embodiments the number of snapshot acquisitions in a set is sufficient for every point in the target volume of the eye 104 to have been illuminated at least once, to provide a complete or dense tomographic volume image. The snapshot acquisitions within a set can be registered by virtue of the substantially overlapping areas encompassed by the array of beamlets 146. In the illustrated embodiment the array of beamlets 146 can be translated incrementally across the eye 104 using a scanning element 182 in the form of a MEMS mirror located in the sample arm relay system 148. The scanning element 182 may also be used for more substantial movements of the beamlet array 146, e.g. for obtaining another set of snapshot acquisitions at an adjacent volume of the eye 104. For simplicity of illustration the scanning element 182 is shown as being transmissive rather than reflective.

The combination of a PBS 122 and two quarter wave plates 132, 140 advantageously provides efficient usage of light in the apparatus 100. To explain, the PBS 122 splits appropriately polarised source light 114 into orthogonal linear polarisation states. When the reference arm quarter wave plate 132 is oriented such that a birefringent axis is at 45° with respect to the linear polarisation state directed by the PBS 122 into the reference arm 128, the reference beam 124 becomes circularly polarised. After the handedness of the circular polarisation has been flipped by reflection off the mirror 130, the quarter wave plate 132 converts the reference arm light to the orthogonal linear polarisation that passes through the PBS 122 into the detection arm 134 with minimal loss. Similarly, when the sample arm quarter wave plate 140 is oriented such that a birefringent axis is at 45° with respect to the linear polarisation state passing through the PBS 122 into the sample arm 136, and for the case of polarisation-maintaining reflections or scattering from the sample 104, the reflected or scattered light 152 will be redirected by the PBS 122 into the detection arm 134 with minimal loss. As noted in the Background section, however, birefringence or depolarisation in the sample 104 may significantly modify the polarisation state of the returning light 152, which may contain important information but which can also be interpreted as artefacts in OCT images acquired by the apparatus 100.

The inventors have realised that the apparatus 100 can be modified to provide polarisationsensitive imaging by making the sample arm quarter wave plate 140 adjustable, so that measurements of the eye 104 can be obtained using different polarisation states of the sample arm light 126. No additional or different optical components are required in this embodiment, enabling polarisation-sensitive functionality in a simple and cost-effective manner.

Figure 2A shows in schematic plan view a polarisation-sensitive spectral domain optical coherence tomography (OCT) apparatus 200 according to an embodiment of the present invention. In general, elements of the apparatus 200 labelled 2xx in Figure 2A correspond to elements of the apparatus 100 labelled Ixx in Figure 1, except that the apparatus 200 has an adjustable polarisation retardation system 284 for controlling the polarisation state of the light 226 that illuminates the eye 104. In certain embodiments the adjustable polarisation retardation system 284 is proximate to the PBS 222 as shown, although it may be placed at other positions within the sample arm 236. In certain embodiments the adjustable polarisation retardation system 284 is the last-placed optical element in the sample arm 236, i.e. between the relay lens system 248 and the eye 104 when the apparatus 200 is in use. This has the advantage of suppressing reflections from the lenses of the relay system 248 and ensuring that the residual background from these surfaces remains constant as the polarisation of the on-eye illumination is adjusted.

In a preferred embodiment shown in Figure 2B the adjustable polarisation retardation system 284 is in the form of a quarter wave plate 240 that is similar to the quarter wave plate 140 in the apparatus 100 of Figure 1, but configured such that its fast axis can be rotated 286 about an axis 288 corresponding to the propagation direction of light to the sample. In the particular embodiment shown in Figure 2B a quarter wave plate 240 composed for example of crystalline quartz is placed in a rotatable mount 290 actuated by a motor 292 controlled by the computer 280 (not shown). In other embodiments the rotation 286 may be performed manually.

Operation of the PS-OCT apparatus 200 differs from operation of the conventional OCT apparatus 100 in that a plurality of sets of snapshot acquisitions of a sample eye 104 are made with the eye illuminated with light 226 of different polarisation states determined by the adjustable polarisation retardation system 284. For a selected illumination polarisation state, light 252 from the grid of beamlets 246 reflected or scattered from one or more structures in the anterior segment 102 passes back through the adjustable polarisation retardation system 284, then is combined with the reference beam 224 at the PBS 222. The polarisation state of the reference beam 224 is unaffected by the adjustable polarisation retardation system 284. Each set of snapshot acquisitions is a set of simultaneous measurements over a range of wavelengths of light 252 reflected or scattered from a volume of the eye 104 illuminated with light 226 of a different polarisation state. The computer 280 is configured to read out the plurality of sets of simultaneous measurements and process them to generate a plurality of tomographic volume images of the eye, with each of the tomographic volume images being of the eye 104 illuminated with light 226 of a different polarisation state. As will be explained below, the plurality of tomographic volume images may be processed to generate a three-dimensional representation of a polarisation property of the eye 104 or a polarisation-independent image of the eye.

Within a set of simultaneous measurements, the on-eye position of the array of beamlets 246 is moved incrementally using the scanning element 282, with the number of simultaneous measurements within a set chosen according to the required density of the resulting tomographic volume image. In preferred embodiments the number of simultaneous measurements within a set is sufficient to provide a complete or dense tomographic volume image. With the scanning element 282 in the form of a fast-settling MEMS mirror and the 2-D sensor array 268 in the form of a CMOS camera with a 300 Hz frame rate, of order 100 simultaneous measurements over a range of wavelengths can be acquired in 0.3 seconds.

Operation of the PS-OCT apparatus 200 will firstly be described for an embodiment where the adjustable polarisation retardation system 284 is in the form of a rotatable quarter wave plate 240 as shown in Figure 2B. This embodiment is advantageous in its simplicity, requiring no additional optical components compared to the conventional apparatus 100 shown in Figure 1. In the following examples the rotatable quarter wave plate 240 will frequently be referred to as being at a certain angle, e.g. at 45°. This is a shorthand way of saying that the rotatable quarter wave plate 240 is oriented such that its fast axis is oriented at that angle with respect to the polarisation axis, i.e. with respect to the linear polarisation state passing through the PBS 222 into the sample arm 236. A ‘horizontal volume slice’ of a sample is a volume slice oriented in the horizontal plane of the sample. Figures 3 A and 3B respectively show an en face image and a horizontal volume slice of the anterior segment of a sample eye extracted from a dense tomographic volume image acquired using the apparatus 200 with the rotatable quarter wave plate 240 at 45°, which is the standard configuration for intensity -based OCT using the apparatus 100 shown in Figure 1. Among several features of the anterior segment visible in Figure 3B are the anterior surface 301 and posterior surface 303 of the cornea, the iris 310 and the anterior surface 305 of the lens.

Figures 3C, 3D, 3E, 3F and 3G show the signal strength across the curved posterior corneal surface 303 extracted from dense tomographic volume images acquired with the rotatable quarter wave plate 240 at -22.5°, 22.5°, 45°, 67.5° and 112.5° respectively. It can be seen from Figures 3C to 3G that spatial variations in the intensity pattern across the posterior corneal surface 303 differ with the polarisation state of the light that illuminates the eye, as determined by the orientation of the rotatable quarter wave plate 240. Intensity variations within and between these images are an indication that there is some birefringence in the sample eye.

Volume images acquired with the rotatable quarter wave plate 240 at -22.5°, 22.5°, 45°, 67.5° and 112.5° form a dataset from which polarisation-insensitive images and phase retardance can be extracted, as described below, generally after the individual tomographic volume images have been registered, i.e. aligned, by the computer 280 to remove the effect of eye movement. In this context we note that adjustment of the polarisation retardation system 284 may be relatively slow, e.g. 0.5 seconds for rotation 286 of a quarter wave plate 240, so that significant eye movement may occur between the sets of snapshot acquisitions.

With z representing the axial direction, i.e. depth in the sample 104, the OCT signal amplitude S(z), and therefore the intensity, 7(z) = |S(z) | ² , can be expressed in term of Jones matrices as follows: is the Jones matrix of a quarter wave plate with fast axis at angle (p, ^T represents the transpose of a matrix and J_ret > ^z) is the Jones matrix describing the sample birefringence with round trip retardation of 26 and axis 0. The sample reflectivity amplitude at depth z is denoted by r(z).

From equation (1) it can be shown that the single pass phase retardation is given by: where I_totai is the polarisation-independent signal which is given by:

Here, we take advantage of the fact that the 7₄₅o signal strength is independent of the birefringence axis . I<p represents the signal intensity measured with the rotatable quarter wave plate at angle cp. Itotai may be calculated at one or more selected points, or across a set of tomographic volume images to produce a polarisation-insensitive image.

If an additional tomographic volume image is acquired with the rotatable quarter wave plate 240 at 0°, the signal intensity dependence of 7o° on the birefringence axis can be used to give the following expression for 0 : if /₁₁₂.5° + I-22.5⁰ ~ ^22.5° ^— ?.5° > 0, then otherwise

Figure 4A shows a polarisation-independent image from the posterior corneal surface, i.e. a polarisation-independent version of the images shown in Figures 3C to 3G, while Figure 4B shows a polarisation-independent horizontal volume slice of the anterior segment. There are subtle differences between the intensity patterns of the polarisation-independent volume slice of Figure 4B and the volume slice of Figure 3B acquired with the standard configuration, i.e. with the rotatable quarter wave plate 240 at 45°, which can yield useful information as explained below.

Figures 5A and 5B respectively show an image of cumulative phase retardation and variation in birefringence axis ( across a 5 mm radius of the posterior corneal surface, while Figure 5C shows a horizontal volume slice of cumulative phase retardation. In Figures 5A and 5C phase retardation is expressed as -101og(cos²(6)), with cos²(5) calculated using equation (2). Regions that appear brighter in the volume slice of Figure 5C, including the iris and the outer portions of the posterior corneal surface, are indicative of higher birefringence tissue. A diamond-shaped contour indicative of a n/2 single pass phase retardance is seen in Figure 5A, with a similar shape evident in the intensity image of Figure 3E acquired with our apparatus in the standard configuration, i.e. with the rotatable quarter wave plate 240 at 45°. Deviations from this regular shape have been shown to be indicative of early onset of keratoconus, as reported in Fukuda et al ‘The comeal phase retardation measured by a prototype of anterior segment polarization-sensitive OCT’, Investigative Ophthalmology & Visual Science 61, 4312 (2020).

Figure 6A shows a close-up of the anterior angle region from the volume slice of Figure 3B, which was acquired with the standard configuration, while Figure 6B shows the corresponding close-up from the polarisation-insensitive volume slice of Figure 4B. Birefringent anterior angle structure adjacent to the trabecular meshwork is evident in the standard configuration image of Figure 6A, as well as in the phase retardation volume slice of Figure 5C. Improved contrast for Schlemm’s canal due to birefringence artefact reduction is visible in the polarisation-insensitive image of Figure 6B, highlighted with a white arrow.

Although embodiments with the adjustable polarisation retardation system 284 in the form of a rotatable quarter wave plate 240 as shown in Figure 2B are advantageously simple and inexpensive with regards to the optical componentry, a relatively large number of separate sets of acquisitions may be required to obtain certain polarisation parameters. The number of separate measurements required can be reduced if we are not restricted to embodiments where a single quarter wave plate is used to determine the incident polarisation state.

For example, in an embodiment where the adjustable polarisation retardation system 284 is in the form of an active wave plate element such as a liquid crystal variable retarder having adjustable phase retardation (|) and axis angle <p, three captures with /^q, = /90,4s, /iso, 22.5 and /i8o,67.5 (i.e. one capture with quarter wave plate functionality at 45° and two captures with half wave plate functionality at 22.5° and 67.5°) would be sufficient to determine the polarisationindependent signal I_totai using the equation:

Figure 2C shows in schematic form an adjustable polarisation retardation system 284 in the form of a liquid crystal variable retarder 294 comprising a voltage-controlled liquid crystal cell 296 placed in a rotatable mount 290 actuated by a motor 292. The phase retardation c|) and axis angle <p can be independently controlled by adjustment of the applied voltage 298 and mechanical rotation 286 respectively.

Similar functionality could be obtained if the adjustable polarisation retardation system 284 were in the form of a series of two independently rotatable quarter wave plates 240A, 240B as shown in Figure 2D, or in the form of a rotatable quarter wave plate 240 and a rotatable half wave plate 241 as shown in Figure 2E. In addition to measurements of birefringence of samples such as biological tissue, PS-OCT can be used to characterise depolarising properties of a sample. As described in the above- mentioned PS-OCT review by De Boer et al, a metric applicable to OCT systems known as Degree of Polarisation Uniformity (DOPU) can be evaluated using a Stokes vector representation of polarisation state averaged over small regions of a sample. In regions where there is significant depolarisation the local polarisation state will change rapidly over speckle points and give rise to DOPU values less than 1.

Expressions for the Stokes vector and hence DOPU describing the single pass polarisation state output from a sample, given a nominal input polarisation state, can be given in terms of a basis set of measurements acquired with the sample illuminated with light of different polarisation states. For example when the adjustable polarisation retardation system 284 is in the form of a rotatable quarter wave plate 240 as shown in Figure 2B, the DOPU describing the single pass polarisation state output from an illuminated volume of an eye 104 can be calculated from a dataset of six volume images acquired with the rotatable quarter wave plate 240 at -22.5°, 0°, 22.5°, 45°, 67.5° and 112.5°. With the polarisation-independent signal Itotai calculated using equation (3), the Stokes vector components Q, U and V can be calculated according to:

DOPU can then be calculated as:

With the apparatus 200 configured with a different lens relay 248 to image an illuminated volume of the retina 128 of an eye 104 rather than the anterior segment 102, a plurality of sets of tomographic volume images with different illumination polarisations were acquired and processed. Figures 7A and 7B respectively show a horizontal volume slice acquired with the rotatable quarter wave plate at 45° and DOPU of a retina. Noting that darker regions in Figure 7B are indicative of a higher DOPU, the retinal pigment epithelium (RPE) is revealed as a relatively bright layer 702, consistent with the known depolarising effect of the RPE. Figure 8 shows in schematic plan view a polarisation-sensitive spectral domain OCT apparatus 800 according to another embodiment of the invention. This apparatus 800 differs from the apparatus 200 depicted in Figure 2A in that there is no spatial sampling element 244 in the sample arm 236, so that a selected volume 850 of the eye 104 is illuminated with an unstructured beam 886. An objective lens 888 is included to control the lateral extent of the illuminated volume 850, which may be in the anterior segment 102 as shown or at the retina 128 depending on the details of the optics 248, 888. As with the apparatus 200 depicted in Figure 2A, the adjustable polarisation retardation system 284 may be proximate to the PBS 222 as shown, or placed at other positions in the sample arm 236. Light 252 reflected or scattered from the illuminated volume 850 collected by the objective 888 passes back through the adjustable polarisation retardation system 284 then is combined with the reference beam 224 at the PBS 222. The combined beam 854 is spatially sampled with the 2-D lenslet array 264 and the resulting beamlets directed into the spectrometer 262 for spectral analysis as described previously, with snapshot acquisitions, i.e. simultaneous measurements over a range of wavelengths, at the 2-D sensor array 268 followed by computation of tomographic volume images in the computer 280.

In this embodiment the cross-sectional area of the illuminated volume 850 generally needs to be relatively small, of order 100 pm x 100 pm, to reduce the impact of multiple scattering and because of the limited number of sampling points offered by commercially available 2-D lenslet arrays. This is considerably smaller than the cross-sectional areas accessible with the beamlet approach of the apparatus 200 depicted in Figure 2A, which may for example be 4 mm x 8 mm as mentioned previously. If data from only a single illuminated volume 850 is required, then for polarisation-sensitive analysis it suffices for only one snapshot acquisition to be taken per setting of the adjustable polarisation retardation system 284. On the other hand if larger volumes of a sample are to be measured, then for each setting of the adjustable polarisation retardation system 284 a set of two or more snapshot acquisitions can be made with the illuminated volume 850 moved laterally using the scanning element 282, preferably with some overlap for registration of the snapshot acquisitions.

We have described how a spectral domain OCT apparatus configured for snapshot acquisition of volume data from a sample can be modified in surprisingly simple fashion to provide polarisation-sensitive imaging by capturing a plurality of volume images with the sample illuminated with light of different polarisation states. The ability of the apparatus to quickly acquire and register the plurality of volume images is highly advantageous for this application. The polarisation sensitivity is provided by including an adjustable polarisation retardation system 284 in the illumination optics. In many of the described embodiments the adjustable polarisation retardation system 284 is in the form of one or more rotatable wave plates, which may for example be composed of crystalline quartz. In alternative embodiments the adjustable polarisation retardation system 284 may be in the form of one or more voltage-controlled liquid crystal retarders or other electro-optic devices such as Pockels cells. A combination of rotatable wave plates and electro-optic devices could also be used.

In each of the illustrated embodiments, focusing of light is performed with optical power elements in the form of lenses. However other forms of optical power elements such as parabolic or ellipsoidal mirrors could be used. In the apparatus 200 depicted in Figure 2A a sample 104 is illuminated with a 2-D array of beamlets 246 generated with a spatial sampling element 244 in the form of 2-D lenslet array, to enable spatially resolved snapshot imaging of the illuminated volume of the sample. Alternatively, a 1-D lenslet array could be used to illuminate a sample 104 with a 1-D grid of beamlets. In yet other embodiments a 1-D or 2-D array of beamlets 246 could be generated with a spatial sampling element 244 in the form of an appropriately designed aperture array, diffractive optical element or micromirror array. In another variation, the transmissive grating 274 in the spectrometer 262 could be replaced by another form of wavelength dispersive element, such as a reflective grating, prism, wedge pair or grating/prism pair.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

The claims defining the invention are as follows:

1. A polarisation-sensitive optical coherence tomography apparatus comprising:

2. The apparatus according to claim 1, wherein the illumination system is configured to illuminate the volume of the sample with an array of beamlets, and wherein the illumination system comprises a scanning element for translating the array of beamlets across the sample between simultaneous measurements within a set of two or more simultaneous measurements, so as to increase the density of the tomographic volume images.

3. The apparatus according to claim 1 or claim 2, wherein the computer is configured to process the plurality of tomographic volume images to generate a three-dimensional representation of a polarisation property of the sample.

4. The apparatus according to claim 1 or claim 2, wherein the computer is configured to process the plurality of tomographic volume images to generate a polarisation-independent image of the sample.

5. The apparatus according to any one of claims 1 to 4, wherein the adjustable polarisation retardation system comprises a rotatable quarter wave plate.

6. The apparatus according to claim 5, wherein the adjustable polarisation retardation system comprises two rotatable quarter wave plates.

7. The apparatus according to claim 5, wherein the adjustable polarisation retardation system comprises a rotatable quarter wave plate and a rotatable half wave plate.

8. The apparatus according to any one of claims 5 to 7, wherein the adjustable polarisation retardation system comprises one or more wave plates each composed of a birefringent material.

9. The apparatus according to any one of claims 1 to 4, wherein the adjustable polarisation retardation system comprises a liquid crystal variable retarder.

10. The apparatus according to any one of claims 1 to 9, wherein the apparatus is configured for in-vivo polarisation-sensitive optical coherence tomography of a sample comprising the anterior segment or retina of an eye.

11. A method for performing polarisation-sensitive optical coherence tomography measurements of a sample, the method comprising the steps of:

12. The method according to claim 11, wherein the volume of the sample is illuminated with an array of beamlets and wherein the array of beamlets is translated across the sample between simultaneous measurements within a set of two or more simultaneous measurements, so as to increase the density of the tomographic volume images.

13. The method according to claim 12, wherein the number of simultaneous measurements within a set of two or more simultaneous measurements is sufficient to generate a dense tomographic volume image.

14. The method according to any one of claims 11 to 13, wherein the plurality of tomographic volume images are processed to generate a three-dimensional representation of a polarisation property of the sample.

15. The method according to any one of claims 11 to 13, wherein the plurality of tomographic volume images are processed to generate a polarisation-independent image of the sample.

16. The method according to any one of claims 11 to 15, wherein the adjustable polarisation retardation system comprises a rotatable quarter wave plate.

17. The method according to claim 16, wherein the adjustable polarisation retardation system comprises two rotatable quarter wave plates.

18. The method according to claim 16, wherein the adjustable polarisation retardation system comprises a rotatable quarter wave plate and a rotatable half wave plate.

19. The method according to any one of claims 16 to 18, wherein the adjustable polarisation retardation system comprises one or more wave plates each composed of a birefringent material.

20. The method according to any one of claims 11 to 15, wherein the adjustable polarisation retardation system comprises a liquid crystal variable retarder.

21. The method according to any one of claims 11 to 20, wherein the method is applied to a sample comprising the anterior segment or retina of an eye.

22. An article of manufacture comprising a non-transitory computer usable medium having a computer readable program code configured to operate the apparatus according to any one of claims 1 to 10, or the implement the method according to any one of claims 11 to 21.